Program
All talks will be in English and will take place at the Maresias Beach Hotel Convention Hall
Posters should be printed in A0 size or 120 x 90 cm (portrait).
(Click on the speaker's name to read the abstract)
- Day 1 | Sunday - August 13, 2017
Opening
- Day 2 | Monday - August 14, 2017
A single electron spin in silicon is dressed by a microwave field to create a new qubit with tangible advantages for quantum computation and nanoscale research. Coherent dressing of a quantum two-level system has been demonstrated on a variety of systems, including atoms, self-assembled quantum dots, and superconducting quantum bits. It is used to gain access to a new quantum system with improved properties - a different and tuneable level splitting, faster and easier control, and longer coherence times. Here, we present coherent dressing of a single electron spin bound to a 31P donor in isotopically purified silicon. The electron spin constitutes a two level quantum system with extremely long coherence times of $T_{2}^{\rm CPMG}=0.5 s$ and excellent control fidelities of $99.95 \%$, figures of merit that are on a par with the best solid-state quantum bits realized. In our work we investigate the properties of the dressed, donor-bound electron spin in silicon, and probe its potential for the use as quantum bit in scalable architectures. Here, the two dressed spin-polariton levels constitute the quantum bit. We observe a Mollow triplet in the excitation spectrum, and demonstrate full two-axis control of the driven qubit in the dressed frame with a number of different control methods. We present coherent control with an oscillating magnetic field, an oscillating electric field, by frequency modulating the driving field, or by a simple detuning pulse. We measure coherence times of $T_{2\rho}^* = 2.4 ms$ and $T_{2\rho}^{\rm Hahn} = 9 ms$, one order of magnitude longer than those of the undressed qubit. Furthermore, we demonstrate that the dressed spin can be driven at Rabi frequencies as high as its transition frequency, making it a model system for the breakdown of the rotating wave approximation. This research was funded by the Australian Research Council via CQC2T (CE110001027) and the US Army Research Office (W911NF-13-1-0024).
The modern tendencies of development of infrared detectors based on HgCdTe heterostructures are fabricating photoconductors and photodiodes type focal plane arrays (FPA). For producing of high quality FPA’s it is necessary to grow HgCdTe with high uniformity properties over the large surface area. The growth of HgCdTe hetero- and nanostructures on GaAs and Si substrates allows to decrease the cost of HgCdTe material and essentially to simplify the technological process of FPA fabricating. The investigations of development of HgCdTe hetero- and nanostructures growth on GaAs substrates are presented. Multi-chamber MBE installation for growth of HgCdTe epilayers on GaAs and Si substrates up to 4” in diameter with precise control of films quality in situ allows to solve many problems connected with the producing high uniformity MCT layer composition over the surface area and control the MCT composition throughout the thickness. The defects formation mechanisms, its nature, the parameters characterization allows are presented.The growth of HgCdTe heterostructures with different composition distribution throughout the thickness allows to prepare material with unique properties that lead to simplification of fabricating high quality IR detectors on their basis. The new fields of science are connected with different new physical phenomena studied on HgCdTe nanostructures such as single or multilayer quantum wells (QW). The results of HgCdTe based QW mostly HgTe QW growth with ellipsomentric control and parameters measurement are presented. The application of HgCdTe heterostructures for different IR detector type is presented. We presented the results of study of different HgTe QW in field of carrier transport, interaction with THz radiation, for laser radiation, 2D and 3D TI etc. This work were partially supported by grants RBFR “15-52-16017 NTSIL_a”, “15-52-16008 NTSIL_a” and “Volkswagen Stiftung”.
Electronic transport in the quantum spin Hall state due to the presence of adatoms in graphene
Heavy adatoms, even at low concentrations, are predicted to turn a graphene into a topological insulator with a substantial gap. The adatoms mediate the spin-orbit coupling that is fundamental to the quantum spin Hall effect. The adatoms act as local spin-orbit scatterer inducing hopping processes between distant carbon atoms giving origin to transverse spin currents. Although there are effective models that describe spectral properties of such systems with great detail, quantitative theoretical work for the transport counterpart is still lacking. We developed a multiprobe recursive Green's function technique with spin resolution to analyze the transport properties for large geometries. We use an effective tight-binding Hamiltonian to describe the problem of adatoms randomly placed at the center of the honeycomb hexagons, which is the case for most transition metals. We introduce a model for current and voltage probes that mimics an ideal voltmeter, that is, it draws no current, giving accurate readings of the non-local voltage. We also discuss the electronic propagation in the system by imaging the local density of states and the electronic current densities.
Long-range exchange interactions between magnetic impurities in Dirac materials
Dirac materials, where electronic carriers have non-trivial orbital and spin multicomponent spinor states, have opened a fascinating new area of materials research. The availability of 2D materials with strong spin-orbit effects, such as transition metal dichalcogenides (TMD) or bismuthene, as well as their 3D analogues (such as Na3Bi), provide for unique playgrounds to study different properties in many labs across the world. The presence of strong spin-orbit coupling allows for interesting exchange effects between magnetic impurities (MIs) embedded in these materials. Through the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, carriers in metallic (or doped semiconducting) hosts mediate long-range interactions between MIs which moreover have strong non-collinear character. Such sizable Dzyaloshinskii-Moriya (DM) interactions between MIs has been shown to exist in 2D [1], and 3D materials [2], and to give rise to strongly anisotropic interactions. I will describe how states at the edges of crystallites or lateral interfaces of 2D materials result in unusually long-range RKKY and DM interactions between MIs adsorbed or hybridized in these regions. We use a tight-binding description of the materials and study differences between different boundary geometries. The boundary states are shown to mediate interactions between MIs that may give rise to interesting magnetic phases. The combination of long range interactions and DM terms leads to helical and strongly frustrated impurity interactions in chains of MIs, with remarkable phase transitions as the range and relative signs of the different interactions is varied. We show that the magnetic configurations depend on the impurity concentration and doping levels in the host, opening an interesting experimental approach to study these phase transitions.[1] F. Parhizgar et al., PRB 87, 125401 (2013); D. Mastrogiuseppe et al., PRB 90, 161403(R) (2014); [2] D. Mastrogiuseppe et al., PRB 93, 094433 (2016).
Interband Cascade Lasers: Current Status and Future Challenges
The Interband Cascade Laser (ICL) combines the interband transition as in a conventional diode laser with the cascading scheme of a Quantum Cascade Laser. ICLs allow for an external quantum efficiency greater than which is enabled because of the special band alignment of GaInSb/AlAs/InAs-interfaces that separates hole and electron injector and internally feed each cascade with carriers. This makes ICLs a unique with great design flexibility. By changing the InAs layer thickness of the typically used W-shaped quantum well (W-QW) the emission wavelength can be tuned within the entire mid infrared region which is known as the fingerprint region of a variety of industrially relevant molecules. Absorption spectra of two prominent ones (acetone and nitric oxide) are shown in Fig. 1 together with room temperature spectra of broad area ICLs driven under pulsed condition. Meanwhile GaSb-based ICLs that are operational in continuous wave mode at room temperature have been realized from 2.8 µm to 5.6 µm. The greatest advantage over the QCL is the low threshold power. Lasing at a temperature of 25 °C could be achieved at an input power as low as 29 mW which is especially beneficial for battery powered sensing systems. In the talk wavelength dependent performance characteristics will be discussed as well as different technologies that enable operation on a single longitudinal mode.
The spin orbit interaction in semiconductors underlies many topological phenomena such as the quantum spin Hall effect in topological insulators, skyrmions in chiral magnets and crossed persistent spin helices, and Majorana bound states in quantum wires and dots. In this tutorial, I will first (i) present a brief yet rigorous derivation of the spin-orbit Hamiltonian in quantum wells, wires and dots and then (ii) discuss the main role of this interaction in the context of some topological systems. More specifically, I will discuss the Rashba spin-orbit interaction in wires with proximity induced superconductivity and a magnetic field leading to Majorana bound states (‘Kitaev model’), the canonical Bernevig-Hugues-Zhang (BHZ) model for topological insulators and its possible realization in ordinary III-V quantum wells with only electrons, skyrmionic textures in ordinary two-dimensional electron gases and stretchable persistent spin helices. Many of these topics are subjects of ongoing research in my group in São Carlos and in collaboration with colleagues abroad.
An elegant approach to circumvent the decoherence problem, present e.g. for quantum dot spin-qubit realizations, are topologically protected systems. The quantum Hall effect (QHE) provides such a topological protection. It turns out that quasiparticle excitations of certain fractional QHE states, i.e. the 5/2 and 12/5 states, are expected to exhibit anyonic exchange statistics and are, thus, interesting for quantum computing. However, the experimentally observed gap energies in ultrahigh mobility two-dimensional electron systems based on the AlGaAs/GaAs material system, corresponding to these states are still small and inconsistent with theoretical predictions. I will outline current and new heterostructure design concepts, including buried gates fabricated by local ion implantation, in order to solve this problem. Another way to achieve topological protection is by the combination of a two-dimensional topological insulator (2DTI), the combined InAs/GaSb quantum well system in our case, with a superconductor. As a result of the broken band-alignment between InAs and GaSb a hybridization gap forms and for a Fermi level alignment within this gap a 2DTI should result. I will give an overview on the current status of this material system and the experimental signatures for the predicted edge channel transport consistent with the 2DTI state. Finally, I will present results on InAs and InSb quantum well systems. These semiconductors, exhibiting pronounced spin-orbit coupling, could, when interfaced with a superconductor, provide a scalable platform for the formation of Majorana bound states. In collaboration with: C. Reichl, T. Tschirky, C. Lehner, S. Fält, M. Berl. W. Dietsche, L. Tiemann, K. Ensslin, T Ihn, S. Müller, M. Karalic, C. Mittag, V. Pribiag, L. Kouwenhoven
Molecular Beam Epitaxial Growth of the Topological Insulator $Bi_{2}Te_{3}$
Bismuth telluride has been recently established as a simple model system for the three-dimensional topological insulator with a single Dirac cone on the surface, as determined experimentally from angle-resolved photoemission spectroscopy [1]. The conductivity measurement of the metallic surface states in $Bi_{2}Te_{3}$ is hindered by the bulk conductivity due to intrinsic defects, like vacancies and anti-sites. Counter doping (Ca, Sn or Pb) is a way to control the Fermi level and suppress the bulk contribution. Intrinsic conduction through topological surface states has been also obtained in very thin insulating $Bi_{2}Te_{3}$ epitaxial films [2].The small lattice mismatch (< 0.04 %) to bismuth telluride makes $BaF_{2}$ (111) a suitable substrate to grow high-quality thin films. The molecular beam epitaxial (MBE) growth of $Bi_{2}Te_{3}$ layers on $BaF_{2}$ (111) has recently been reported using either separate Bi and Te solid sources [2] or $Bi_{2}Te_{3}$ and additional Te cells [3]. Depending on the growth parameters, other $Bi_{x}Te_{y}$ phases are obtained or mixed BixTey phases coexist in the same epitaxial film [4]. In this work, we report on a systematic study of the MBE growth of bismuth telluride films on $BaF_{2}$ (111). The substrate temperature, the $Bi_{2}Te_{3}$ source temperature and the additional Te flux were varied in a wide range to determine the optimum growth conditions for $Bi_{2}Te_{3}$ single phase films. The structural properties of the films were investigated in situ by reflection high-energy electron diffraction and ex situ by high-resolution x-ray diffraction, x-ray reflectivity, atomic force microscopy (AFM), X ray photoelectron Spectroscopy (XPS) and Angle Resolved Photoelectron Spectroscopy (ARPES). [1] Y.L. Chen et al., Science 325, 178 (2009); [2] K. Hoefer et al., PNAS 111, 14979 (2014); [3] O. Caha et al., Cryst. Growth Des. 13, 3365 (2013); [4] H. Steiner et al., J. Appl. Cryt. 47, 1889 (2014).
Topological nonsymmorphic ribbons out of symmorphic bulk
States of matter with nontrivial topology have been classified by their bulk symmetry properties. However, by cutting the topological insulator into ribbons, the symmetry of the system is reduced. By constructing effective Hamiltonians containing the proper symmetry of the ribbon, we find that the nature of topological states is dependent on the reduced symmetry of the ribbon and the appropriate boundary conditions. We apply our model to the recently discovered two-dimensional topological crystalline insulators (TCIs) composed by IV-VI monolayers, where we verify that the edge terminations play a major role on the Dirac crossings. Particularly, we find that some bulk cuts lead to nonsymmorphic ribbons, even though the bulk material is symmorphic. The nonsymmorphism yields a new topological protection, where the Dirac cone is preserved for arbitrary ribbon width. In this talk we will present a group theory approach to obtain the effective Hamiltonians for TCI ribbons with different terminations, and how to properly account for the boundary conditions in each case. We show that the model reproduce the ab initio calculations. We acknowledge support from CNPq, CAPES, and FAPEMIG, and the computational facilities from CENAPAD.
Energy transfer to rare earth ions in amorphous semiconductor alloys
Rare earth luminescence has been observed in amorphous silicon and alloys for many years. When excited through light absorption by the host, the rare earth ions normally present luminescence efficiency higher than in the crystalline counterparts. This requires a specific efficient energy transfer mechanism. We have prepared Tb-doped $a-Si_3N_4:H$ by reactive RF-sputtering and by ECR-PECVD. $Tb^{3+}$ photoluminescence and its correlation with the dangling bond density, Tb concentration and Tb chemical neighborhood can be well explained by the defect-related Auger excitation (DRAE) model, originally proposed for Er in a-Si:H [1]. In this model, rare earth ions are excited through Coulomb interaction when an electron and a hole recombine in a deep defect nearby. Previous results for Er and Nd-doped $a-SiO_x:H$ also support the model. By modifying the bandgap of the alloys and fine tuning the density of defects we show that the maximum excitation rate occurs when (1) there is an intra 4-f transition in the rare earth ion that corresponds to half the bandgap and (2) the density of rare earth ions and the density of dangling bonds match. The DRAE mechanism combined with a favorable non-centrosymmetric chemical environment are probably the reasons for the highly efficient luminescence of rare earth ions in amorphous silicon and alloys. [1] W. Fuhs et al. PRB 56 (1997), 9545.
- Day 3 | Tuesday - August 15, 2017
Topological superconductors with Majorana fermions
Topological superconductors with Majorana fermion quasiparticles form a newly discovered class of matter. The Majorana fermion can be seen as half an electron, or more accurately, the electron wave function has split up into two separate parts. This non-local property is currently been intensively explored for implementing fault-tolerant quantum computation. I will explain where and why Majorana fermions appear, in particular focusing on systems where very standard components are combined to achieve the required non-trivial topology: spin-orbit coupled semiconductors, magnetism, and conventional s-wave superconductivity. I will also present some of our recent results in modeling topological superconductors with Majorana fermions, focusing on a simple mean to detect Majorana fermions, their robustness against disorder, and how they often appear in conjunction with spontaneous currents.
Topological superconductors with Majorana fermion quasiparticles form a newly discovered class of matter. The Majorana fermion can be seen as half an electron, or more accurately, the electron wave function has split up into two separate parts. This non-local property is currently been intensively explored for implementing fault-tolerant quantum computation. I will explain where and why Majorana fermions appear, in particular focusing on systems where very standard components are combined to achieve the required non-trivial topology: spin-orbit coupled semiconductors, magnetism, and conventional s-wave superconductivity. I will also present some of our recent results in modeling topological superconductors with Majorana fermions, focusing on a simple mean to detect Majorana fermions, their robustness against disorder, and how they often appear in conjunction with spontaneous currents.
Energy scaling and magnetic properties of sub-gap excitations in a hybrid superconductor-semiconductor nanowire quantum dot
The interaction of a magnetic impurity and a superconductor results in localized states known as Andreev levels or Yu-Shiba-Rusinov (YSR) states. In recent years, there has been a growing interest in this type of system, largely motivated by theoretical work that suggests that Andreev levels are precursors of Majorana zero modes and that, as a result, chains of such impurities could be engineered into a topological superconductor. A semiconductor quantum dot coupled to a superconductor represents a versatile platform to investigate, in a controllable and quantitative manner, the physics of the corresponding single-impurity limit. Here, we have employed nanowire-based quantum dots, coupled strongly to a superconductor and weakly to a normal metal probe, to spectroscopically study the energy scaling of Andreev levels, as well as their magnetic properties. Towards the first goal, we have exploited the ability to tune the hybridization of the quantum dot and the superconductor by adjusting gate electrodes. We demonstrate that the sub-gap excitations scale with the ratio of the Kondo temperature and the superconducting gap. We further exploit the electrical control over device parameters to obtain an experimental phase diagram of the possible ground states, which shows a remarkable agreement with numerical renormalization group calculations. In parallel, we have studied the magnetic properties of the Andreev levels. We demonstrate that the Zeeman effect results in a splitting of the sub-gap states only when the ground state is a spin singlet. In this case, the applied magnetic field can also lead to a quantum phase transition to a spin-polarized ground state. The herein demonstrated electrical tuning of Andreev levels as well as their spin-polarization could be harnessed to pursue proposals of realizing a topological superconductor using quantum dot arrays.
Kondo effect and quantum criticality of magnetic vacancies and adatoms in graphene
We discuss the effects of dilute concentrations of magnetic adatoms or single vacancies on the electronic transport properties of graphene, within the linear-response regime. We used simple impurity models that capture the basic symmetry properties of top and hollow-site adsorbates (TOP and HS), and bond-reconstructed (REC) and symmetric vacancies (VAC). At low energies, these impurity problems map onto $|\varepsilon|$ and $|\varepsilon|^3$ pseudo-gap Anderson models (TOP/REC and HS/VAC, respectively), with distinct critical behaviors in charge-neutral graphene. For finite carrier densities, Kondo correlations are ubiquitous for all impurity types considered, and we predict experimentally accessible Kondo temperatures for realistic parameters. Our results indicate that electronic transport is determined by normal impurity scattering with, and bound state formation by TOP and REC impurities, which locally break the inversion and/or $C_{3v}$ symmetry of the graphene honeycomb lattice. By contrast, VAC (HS) impurities preserve $C_{3v}$ ($C_{3v}$ and inversion) symmetry, and remain decoupled from electronic states at symmetry points (full branches) throughout the Brillouin zone, leading to a vanishing contribution to the sample resistivity. References: Phys. Rev. B 94, 085425 (2016); Phys. Rev. B 95, 115408 (2017).
Observation of thermally activated hysteresis in graphene/hBN devices: the role of the hBN-SiO2 interface
Graphene research is a very active field involving basic science and with promising application on nanoelectronic. In basic science, hexagonal boron nitride (hBN) has been considered the best substrate as a platform for graphene devices, reducing charge scattering and allowing observation of new quantum phenomena at low temperatures. However, little attention is been given to graphene/hBN devices operating at high temperature, or working at operating temperatures of sensors or integrate circuits. In this work, we report on gate hysteresis of resistance in high quality graphene/hBN devices [1]. We observe a thermally activated hysteretic behavior in resistance as a function of the applied gate voltage at temperatures above 375 K. In order to investigate the origin of the hysteretic phenomenon, we compare graphene/h-BN heterostructure devices with SiO2/Si back gate electrodes to devices with graphite back gate electrodes. The gate hysteretic behavior of the resistance is present only in devices with an h-BN/SiO2 interface and is dependent on the orientation of the applied gate electric field and sweep rate. We describe a phenomenological model which captures all of our findings based on charges trapped at the h-BN/SiO2 interface. Such hysteretic behavior in graphene resistance must be considered in high temperature applications for graphene devices and may open new routes for applications in digital electronics and memory devices [1] A.R. Cadore et. al., Appl. Phys. Lett. 108, 233101 (2016). Acknowledgements: FAPEMIG, CAPES, CNPQ, INCT/Nanocarbono, Rede de Nano-Instrumentação and Pós-graduação em Física da UFMG.
Avik Ghosh
Nano-electronics and Moore’s Law - what comes next?
With the current slow-down of Moore's law and the abolition of the ITRS roadmap, there is a pressing need to explore various material, architectural and physical solutions for low-power electronics, ranging from spintronics to 2D materials to subthermal switching. I will summarize the opportunities and challenges for various material, device and circuit level solutions in addressing the future of electronics. Digital electronics is based on the control of charge current. Over the last decade or two, we have made enormous progress both in understanding the quantum flow of charge in nano-structures at their molecular scales, as well as translating this understanding into practical, predictive simulation tools. I will start with a multiscale model that couples bandstructure and quantum transport for physics based compact models for charge, spin and heat flow, ranging from ballistic to diffusive, quantum to classical, non-interacting to many-body. I will then show how we can convert these into 'first principles' simulation platforms that take into account details of the interfacial chemistry, bonding, spin and topological indices to provide atomistic insights into non-equilibrium properties. Finally, I will show how we can use these tools to explore emerging low-power devices ranging from nano-magnetic switches for all spin logic, to metal insulator transitions in complex oxides, to 'phase transition switches' such as relays, tunnelFETs and 2D chiral tunneling FETs that attempt to bypass the fundamental Boltzmann tyranny limiting today's silicon devices. I will also outline possible architectural solutions such as neuromorphic and approximate computing schemes that maybe able to utilize existing CMOS hardware in innovative ways.
Victor Lopez-Richard
A quantum dot arquitechture for memristive and memcapacitive functionalities
The memory and dynamical functionalities of memcapacitors and memristors would enable not only high density integration but also pave the way for new computational schemes and the emulation of neural networks. We have engineered a quantum-dot based transistor with controllable counting ability based on its intrinsic memcapacitive bistability. The conductance is tuned by charging a quantum dot which was precisely positioned in the center of a narrow quantum wire. The Coulomb interaction of the localized charges with a nearby transistor channel results in a wide maximum to minimum conductance ratio. This allows producing periodic super-cycles of defined periods and predetermined reliability. Being an intrinsic behavior of our device, it may seem, at first glance, a perplexing electric response. It is not. In the two terminal configuration, our device can be set to its memristive mode. Applying voltage pulses, the input signal can be integrated in a way that the memristor state is reset with periods that depend on the amplitude or the frequency of the input signal. In general, the control of the rate solely with the input signal requires a feedback and to realize very dense artificial neural networks it would be beneficial to implement this feedback without the need of additional circuitry. Our protocol delivers a state-dependent threshold voltage for the reset with a single memristor. This memdevice can emulate key functionalities of neurons (integrate-and-fire) and synapses (synaptic plasticity). Learning rules can be reproduced by tuning the shapes of pre- and post-synaptic voltage pulses. In this case, the conductance is controlled by the time difference between the pre- and post-synaptic voltage pulses and the corresponding shapes. The presented memristor is also optically active and its state can be controlled by the pulse wavelength and width. So beyond the electrical excitation, light-sensitive synapses or optically tunable memories can also be foreseen.
Inter-valley Auger recombination in InGaAs/InP quantum wells
Auger recombination is one of the most important non-radiative processes which affects the efficiency of optoelectronic devices particularly at high excitation power or high injection, when a high density of carriers is generated. In the presented work the electron transport and recombination processes of photoexcited electron-hole pairs were studied in InGaAs/InP single quantum wells. Comprehensive transport data analysis reveals asymmetric shape of the quantum well potential where the electron mobility was found to be dominated by interface-roughness scattering. The low-temperature time-resolved photoluminescence was employed to investigate recombination kinetics of photogenerated electrons. Remarkable modification of Auger recombination was observed with variation of the electron mobility. In high mobility quantum wells the increasing pump power resulted in a new and unexpected phenomenon: a considerably enhanced Auger non-radiative recombination time. We propose that the distribution of the photoexcited electrons over different conduction band valleys might account for this effect. Such phonon-assisted inter-valley Auger recombination process is important in direct band gap semiconductors when the difference between the energies of the $\Gamma$ conduction band minimum and a lateral (X or L) conduction band minimum is close to the gap between the conduction band and valence band extrema. This condition favors transference of the energy of recombining electron-hole pair to a third electron excited into the lateral conduction band valley. In low mobility quantum wells, disorder-induced relaxation of the momentum conservation rule causes inter-valley transitions to be insignificant, resulting in decreasing of non-radiative recombination time with the increasing pump power. Thus, the disorder driven transition between two types of Auger processes (intra and inter-valley) was observed.
Christiano J. S. de Matos
2D materials and their application to optoelectronics and photonics
Since the isolation of graphene in 2004, a wide range of other atomically thin (2D) materials have been obtained and studied. 2D conductors, insulators, semiconductors and even superconductors have been identified, with properties that are different from their bulk (3D) counterparts. Additionally, 2D materials can be stacked to yield 2D heterostructures, allowing for a new generation of thin and flexible electronic, optoelectronic and photonic devices. This tutorial will review the recent advances in the science and technology of 2D materials, discussing the methods to synthesise and characterise them, as well as some of their applications in optoelectronics and photonics.
Recent experiments provide possible evidence for Majorana bound states in chains of magnetic adatoms on a conventional superconductor. The formation of topological superconductivity in this system relies on ferromagnetic order of the magnetic moments and spin-orbit coupling of the substrate superconductor. In this talk, I will discuss the physical picture underlying these experiments which starts with the physics of individual magnetic adatoms and includes a possible explanation of the unexpectedly strong localization of the observed end states.
Optical properties of large area tungsten disulfide monolayers
The direct-gap monolayer transition metal dichalcogenides (TMDs) are very attractive two dimensional (2D) systems for optical applications and for the investigation of fundamental physics such as spin-valley coupled physics and strong excitonic effects. However, their optical properties are not well understood and can be affected by laser irradiation, type of substrate/ambient conditions, which can be a critical issue for effective implementation of TMDs as optoelectronic devices. In this work, we have investigated the nature of emission bands and laser irradiation effects on optical properties and optical stability of large area WS2 monolayer grown by the Van der Waals Epitaxy using Chemical Vapor Deposition (CVD) on SiO2/Si substrates. We have performed macro- and micro-photoluminescence measurements as a function of laser intensity and temperature. Remarkably, drastic changes on photoluminescence (PL) spectra are induced by laser irradiation on WS2 monolayers even by using low laser intensities. At low temperatures, the exciton and the broad localized state emission intensities decrease on time scale of minutes accompanied by a relative enhancement of both trions and biexciton /bound exciton bands. For higher laser intensities, the lower emission band becomes dominant and its PL peak energy presents a red shift/blue shift with increasing time for low /high temperature respectively. Furthermore, this dominant PL band undergo to irreversible changes at lower temperatures. Sharp PL peaks were also observed for lower temperatures (T< 50K). These sharp PL peaks have linewidth of ~2 meV, are unstable on time scale of seconds and strongly depends on the temperature, substrate and on the laser position on the sample. The physical origin of observed emission bands and laser induced effects on its properties are discussed.
Vacancy and magnetism in graphene: DFT modeling
There has been extensive study of the vacancy in graphene in the past decade, from the experimental and theoretical side, with different results concerning the magnetic moment induced by the defect. The values coming from different theoretical simulations performed using Density Functional Theory DFT vary from $1.04 – 1.75 \mu_B$, and also with other important divergences on the reason for the variation [1,2]. We investigate the defect with different theoretical approaches, cluster and periodic boundary conditions PBC, using the same computational code [3]. We use DFT based formalisms, PBE and hybrid PBE0 where a fraction $\alpha$ of Exact Exchange XC is included [4]. The value of $\alpha$ is chosen to reproduce the properties of graphene in the Fermi energy region [5]. We choose symmetric hydrogen-terminated cluster models, with arm-chair and zig-zag edges; we also studied three different supercell sizes for the periodic models. We find that a serious point to be taken into account when treating this defect is the self-interaction error present in bare DFT, which can give rise to fractional occupation of bands near the Fermi energy for periodic conditions, and leads to the previously reported fractional spins. When using PBE0, our results point to one and the same integer magnetic moment $2 \mu_{B}$ for the vacancy in graphene. Concerning the periodic array of defects generated in PBC, we find that the defect-related states in different supercells can show different character, indicating possible patterns for spin-spin interaction. [1] O. V. Yazyev, L. Helm, PRB 75, 125408 (2007); [2] J.J. Palacios, F. Ynduráin, PRB 85, 245443 (2012); [3] V.Blum, R. Gehrke, F. Hanke, P. Havu, V. Havu, X. Ren, K. Reuter, M. Scheffler, CPC 180, 2175 (2009); [4] J. P.Perdew, K. Burke, M. Ernzerhof PRL 77, 3865 (1996); J. P. Perdew, M. Ernzerhof, K. Burke JCP 105, 9982 (1996); [5]M. Pinheiro, M. J. Caldas, P. Rinke, V. Blum, and M. Scheffler, PRB 92, 195134 (2015).
The Effect of Growth Techniques on the Electrical Active defects in Indium doped TiO2 Thin Films
Transparent conducting oxides (TCOs) materials have wide applications in terms of absorbing, transparency of the visible light and electrical conductivity.They are used in optoelectronics,photovoltaic,photocatalytic and photoelectrochemical water splitting applications. Titanium dioxide (TiO2) is an oxide semiconductor which has been extensively studied and is considered as a suitable material for solar cell and photovoltaic applications due to its remarkable electrical and optical properties, which can be even improved by proper doping. TiO2 can form many different phases,most commonly anatase and rutile. It is a wide band gap material with an energy of 3.2 eV and 3.0 eV for anatase and rutile, respectively. This wide band gap energy makes TiO2 only sensitive to UV light and therefore the efficiency of TiO2-based solar cells is limited.Incorporating Indium (In) into TiO2 can tune its band gap for absorbing visible light, which will make it a suitable photocatalytic and solar cell material.In this work, TiO2 thin films have been grown on silicon substrates using pulse laser deposition (PLD) and sputtering techniques.These were followed by a thermally evaporated Indium layer of 50 nm thickness.The structures were then annealed using Rapid Thermal Annealing process in order to incorporate the Indium into the TiO2 host lattice. The aim was to investigate the effect of the growth techniques on the electrically active defects in the Indium doped TiO2.It was observed that at -4V reverse bias the PLD samples have lower leakage currents(~1.4 x 10-7 A) as compared to the sputtering samples(~5.9 x 10-7 A).In addition,PLD samples exhibited lower ideality factors and higher barrier heights as compared to those of the sputtering samples. The DLTS measurements revealed only one defect in the PLD samples whereas five defects have been detected in the sputtering samples.Due to the lower leakage current and less number of defects,PLD technique is better suited for the growth of TiO2.
- Day 4 | Wednesday - August 16, 2017
Understanding and controlling spin relaxation is central for spin qubits, setting an upper limit to the coherence time T2. The spin-orbit (SO) and hyperfine interactions are the most important mechanisms, giving rise to spin relaxation by emitting the Zeeman energy into a phonon. In presence of moderate magnetic fields, it has been shown that spin relaxation is primarily caused by the SO interaction. Here, we present measurements of the spin relaxation rate W in a gate defined single-electron GaAs quantum dot at electron temperatures down to 60 mK as a function of both direction as well as strength of magnetic field, spanning an unprecedented range from 0.6 T to 14 T applied in the plane of the 2D electron gas. Due to the interplay of Rashba and Dresselhaus SO contributions, W shows strong anisotropy when varying the direction of the applied in-plane magnetic field B with a piezoelectric rotator. Along the crystal axis where SOI coupling is weak, a spin relaxation time T1 of 57+/-10 s has been obtained at a magnetic field of 0.6 T. However, quite surprisingly, this is still more than one order of magnitude shorter than the expected value based on SO mediated spin relaxation. Further, W shows a B3 dependence and becomes isotropic at the lowest magnetic fields. These observations thus indicate hyperfine interaction mediated spin relaxation (non flip-flop) via phonons at the lowest magnetic fields used here. Command of the dot orbitals, control of the B-field direction and low-B-field measurements -- made possible by a low electron temperature -- reveal hyperfine spin relaxation and allow comprehensive modeling, giving excellent agreement between experiment and theory.
We present the analysis of electronic band structure of InSe and (other III-VI semiconductors) films, from the stoichiometric mono-layer to N-layer films, and we describe the resulting optical properties of these 2D materials [1,2]. This study is based on the ab initio DFT and related multi-orbital tight-binding model analysis of the electronic band structure and wave functions in the two-dimensional N-layer InSe crystals, and it is compared to the results of luminescence spectroscopy of this material. We show [1-3] that the band gap in InSe (and GaSe) strongly depend on the number of layers, with a strong (more than twice) reduction from the monolayer to crystals with N>6. We find that the conduction-band-edge electron mass in few-layer InSe is quite light (comparable to Si), which suggests opportunities for high-mobility devices and the development of nanocircuits. In contrast, the valence band in mono-, bi- and trilayer InSe is flat, opening possibilities for strongly correlated hole gases in p-doped films. We also propose a model to describe electronic properties of misaligned layers of InSe. Using the band structure and wave functions, we analyse optical transitions in thin films of InSe, identify their polarisation and compare the results of modelling to the measurements performed on hBN-encapsulated atomically thin InSe crystals. [1] D. Bandurin, et al, Nature Nanotechnology (2016); doi:10.1038/nnano.2016.242; [2] Magorrian, S., Zolyomi, V. & Falko V. , Phys. Rev. B 94, 245431 (2016); [3] Mudd, G. W., Molas, M. R., Chen, X., Zólyomi, V., Nogajewski, K., Kudrynskyi.
Optoelectronics in a two-dimensional nanometer-sized device based on the graphene-hexagonal boron nitride heterostructure
Two-dimensional heterostructures based on layered materials bring the possibility of electrically controlling light confined in polaritons. Here, we demonstrate fine tuning and diode behavior of hyperbolic plasmon-phonon polaritons (HP3) modes in nano-devices composed of the graphene (G) - hexagonal boron nitride (hBN) heterostructure lying on gold (Au) contacts. By gating the G-hBN/Au device, we achieve electrostatic tuning of the HP3 modes residing at the type I band of hBN, which correspond to crystalline out-of-plane vibrations. Remarkably, the experimental maxima of the intensity of the main mode at 815 cm-1 plotted against the applied gate voltages features, albeit in inverted shape, characteristic curves of resistance versus gating for graphene systems. Hence, we conclude that the tuning directly results from the electrostatic influence on the plasmon-phonon coupling. We note that the substrate dramatically affects the dielectric environment, which determines the HP3 momenta. At the transitions between air and Au substrate, it happens a mismatch between the momenta of the HP3 modes existing in G-hBN/air and G-hBN/Au leading to the rectification of HP3 modes analogously to that of electronic diodes. Our findings constitute an important step toward the control of light in the nanoscale, which is one of the primary goals of nanophotonics.
In this work we will show our recent developments on the understanding of electron relaxation pathways and the non-linear optical properties of novel 2D materials. By using two-color pump probe scheme, we have studied the photo-excited electronic cooling in graphene in presence of controlled defect densities. We clearly observe an inverse linear dependence of the electron scattering rate with the mean distance between defects in the samples [1]. This dependence can be explained the defect assisted acoustic phonon supercollision model in graphene. The disorder- assisted scattering process allows for large phonon recoil momentum values and the entire thermal distribution of acoustic phonons can contribute to the scattering process, resulting in efficient carrier energy dissipation. Also we have used both Second Harmonic Generation (SHG) and Coherent Anti Stokes Raman Scattering (CARS) to study the nonlinear optical properties of mono- and few-layers of molybdenum disulfide (MoS_2) and graphene respectively. In the case of the molybdenum disulfide we have observed efficient SHG from odd number of layers due to the absence of inversion symmetry [2]. By using different laser excitation energies, we could probe the resonant effect in the SHG due to the presence of different optical transitions in MoS_2. By analyzing the resonant profile of SHG we can observe the different types of excitons in this material, which are compared with recent theoretical results in the literature. Because graphene have inversion symmetry, the SHG is almost absent, however third order nonlinearities are greatly enhanced in this material due its peculiar band structure. We have studied four wave mixing process in graphene, and in particular we will discuss the CARS process in graphene [3]. We acknowledge FAPEMIG, CNPq, Finep and CAPES. [1] T. V. Alencar et al., Nano Letters 14, 5621 (2014); [2] L. M. Malard et al., Phys. Rev. B 87, 201401 (2013); [3] L. Lafetá L., arXiv1701.09023 (2017).
Twist-controlled tunnelling in vertically stacked heterostructures of graphene/bilayer-graphene and boron-nitride
I shall discuss how careful alignment of the crystallographic orientation of two vertically stacked graphene (or bilayer-graphene) electrodes, separated by a thin layer of hexagonal boron nitride in a transistor device, can achieve resonant tunnelling with conservation of electron energy, momentum and chirality [1]. The crystallographic alignment produces multiple features in the I-V characteristics, including negative differential conductances, which can be traced to specific alignments of the bands and the Fermi levels on the two flakes. Also, the application of a magnetic field in the plane of the two graphene layers can be used to reveal the effects of graphene's chirality on the tunnelling current, and produce valley polarised currents [2]. Finally, I note that the momentum and velocity distribution of the tunnelling electrons is highly anisotropic, which may lead to interesting effects in a ballistic device. [1] Mishchenko, Tu, Cao, Gorbachev, Wallbank, Greenaway, Morozov, Morozov, Zhu, Wong, Withers, Woods, Kim, Watanabe, Taniguchi, Vdovin, Makarovsky, Fromhold, Falko, Geim, Eaves, Novoselov, Nature Nanotechnology 9, 808 (2014) ; [2] Wallbank, Ghazaryan, Misra, Cao, Tu, Piot, Potemski, Pezzini, Wiedmann, Zeitler, Lane, Morozov, Greenaway, Eaves, Geim, Fal'ko, Novoselov, Mishchenko Science 353 575 (2016).
Exciton fine structure of black phosphorus quantum dots
We study the size-dependent exciton fine structure in black phosphorus quantum dots (BPQDs) deposited on different substrates (isolated, Si and SiO$_2$) using a combination of tight-binding method to calculate the single-particle states, and the configuration interaction formalism to determine the excitonic spectrum. We demonstrate that the substrate plays a dramatic role on the excitonic gaps and excitonic spectrum of the QDs. For reasonably high dielectric constants ($\varepsilon_{sub} \sim \varepsilon_{Si} = 11.7 \varepsilon_0$), the excitonic gap can be described by a single power law $E_X(R) = E_X^{(bulk)} + C/R^{\gamma}$. For low dielectric constants $\varepsilon_{sub} \leq \varepsilon_{SiO_2} = 3.9 \varepsilon_0$, the size dependence of the excitonic gaps requires the sum of two power laws $E_X(R) = E_g^{(bulk)} + A/ R^{n} - B/R^{m}$ to describe both strong and weak quantum confinement regimes, where $A$, $B$, $C$, $\gamma$, $n$, and $m$ are substrate-dependent parameters. The exciton states are composed of a linear combination of electron-hole pairs $(e_m,h_n)$ where the participation of each pair in a given state depends on the QD size. The excitonic spectrum is composed of bright and dark excitons. In certain circumstances, the ground state (GS) exciton may be dark. This occurs when the GS composition involves pairs $(e_m,h_n)$ with single-particle states of opposite parities. We also predict that the exciton lifetimes exhibit a strong temperature dependence, ranging between 2-8 ns (Si substrate) and 3-14 ns (SiO$_2$ substrate) for QDs up 10 nm in size.
- Day 5 | Thursday - August 17, 2017
Microcavity exciton-polaritons (MPs) are bosonic quasi-particles resulting from the strong coupling between photons in a microcavity (MCs) and excitons in a quantum well (QW) embedded in it. MPs have a very low effective mass (10$^{-4}$-10$^{-5}$ of the electron mass) and, therefore, de Broglie wavelengths $λ_{dB}≫1~\mu$m. As a result, MPs experience strong confinement effects even for $\mu$m-sized potentials. Furthermore, MPs undergo at high densities a transition to a Bose-Einstein(BE)-like a condensate with extended temporal and spatial coherences.[1] In this talk, we review recent results on the confinement of MP and their condensates in micro-structured MCs grown by Molecular Beam Epitaxy. Micrometric static confinement potentials for MPs can be produced by structuring the thickness of the MC layers in-between growth steps. Spatially resolved photoluminescence shows confined states with discrete energy levels for MP confined in the traps. Dynamic lattices for MPs are be produced by modulating the MC using the strain field of a surface acoustic wave (SAW). A SAW propagating on the surface of an (Al,Ga)As polariton MC induces a periodic energy modulation of both the photonic and excitonic polariton components. For SAW wavelengths $<<\lambda_{dB}$, this lateral modulation forms a one-dimensional (1D) moving MP crystal with period and contrast given by the SAW period and amplitude, respectively. [2] 2D MP moving lattices can be formed by interfering two orthogonal SAW beams.[3,4] The latter are, solid-state analogs of optical lattices of cold atoms. They thus form a prototype system for the investigation of many body interactions in non-equilibrium quantum phases as well as for the implementation of functionalities for quantum information processing. [1] J. Kasprzak et al., Nature 443, 409 (2006); [2] E. A. Cerda-Méndez et al., Phys. Rev. Lett. 105, 116402 (2010), Phys. Rev. Lett. 111, 146401 (2013); [3] J. Buller et al., Phys. Rev. B94, 125432 (2016).
Controling the exciton dynamics in single quantum-dot embeded in a cavity
The strong coupling between quantum dot (QD) and cavity has been subject of intense research in recent years. Many of these studies rely on understanding the avoided crossing seen in the emission spectral of such system, which is usually obtained by pumping the system with a laser above the band gap of the used semiconductor material or in resonance with a higher energy cavity mode. In both cases, the pumping of the cavity and/or QD is treated theoretically as an incoherent pumping [1]. In this work in investigate in more details the coherent pumping of a single QD embed in nanocavity using a external laser. We consider a continuous laser being applied in resonance with the cavity mode and a pulsed laser interacting with the QD. To model our system we use the Jaynes-Cummings model and incoherent losses were take into account by using Lindblad operators. Our results indicate that it is possible to use the continuous laser to prepare the cavity in a coherent state, and use the external laser pulses to control the population inversion of a single QD exciton [2]. The effects of exciton-cavity detuning, the laser-cavity detunings, the pulse area and losses over the QD exciton dynamics are analyzed. We also show how to use a continuous laser pumping in resonance with the cavity mode to sustain a coherent state inside the cavity, providing some protection to the exciton state against cavity loss. We acknowledge financial support of CAPES, CNPQ and FAPEMIG. [1] A. Laucht, N. Hauke, J. M. Villas-Bôas, F. Hofbauer, G. Böhm, M. Kaniber, and J. J. Finley, Phys. Rev. Lett. 103, 087405 (2009); [2] Antonio de Freitas, L. Sanz, and José M. Villas-Bôas, Phys. Rev. B 95, 115110 (2017).
Giant Photoinduced Magnetic Polarons in Europium Chalcogenides
Optical manipulation of the magnetic state of matter is a topic of current interest from the fundamental point of view and due to its high relevance in respect to technological applications. Recently we demonstrated that the light resonant with the EuTe bandgap forms long-living spin polaron states associated with the optical excitations. Europium chalcogenides, which include EuTe, are intrinsic magnetic semiconductors. So far, they have been much less explored in respect to optical manipulation of their magnetic state. A possible reason is that they do not show a near-bandgap photolumenescense that is typical of the dilute magnetic material. Here we demonstrate a new approach to the magnetization of a medium by light irradiation. Using a two-colour pump-probe Faraday rotation technique, we demonstrate that in the EuTe at temperature 5K light generates magnetic polarons with a magnetic moment larger than 600 Bohr magnetons. This is about an order of magnitude larger than magnetic polarons arising in diluted magnetic semiconductors. Because of the giant magnetic moment of a polaron, a modest magnetic field of a few tens of mT leads to a full alignment of polarons. To determine how efficiently the light magnetizes the medium we investigate photoinduced magnetic polarons in several europium chalcogenides as a function of pump intensity and temperature. In EuSe and EuO we detected magnetic polarons with an even greater magnetic moment than in EuTe. Moreover, we prove that the density of photoinduced magnetic polarons in EuSe is much larger. The first observation of giant photoinduced magnetic polarons is extremely important not only from the fundamental point of view but also for practical applications. This work was supported by FAPESP (Project 2016/24125-5), CNPq (projects 401694/2012-7, 307400/2014-0, and 456188/ 2014-2), RFBR (project 16-02-00377) and RSF (project 17-12-01314).
Time-Dependent Spin Precession Frequency in InGaAs/GaAs Quantum Wells with Mn Delta-Doped Heterostructures
Semiconductor spintronics may have a giant impact on the market of storage and reading devices. Nevertheless, the improvement of actual systems still requires spin injection and magnetic ordering at room temperatures. Heterostructures like (Ga,As)Mn, combining the Mn ferromagnetic properties with the well know technology of III-IV semiconductor structures, are being studied with the propose of increasing the number of spin carriers in semiconductors[1]. The initial problem of low critical temperature may have been solved with the demonstration of TC = 250 K in GaAs by growing delta-Mn-layers[2]. Furthermore, the possibility of data memory was shown in InGaAs/GaAs quantum wells (QWs) adjacent to a delta-Mn-layer, due to the wavefunction overlap of spins carrier inside the quantum well and Mn atoms[3]. Here, we studied the spin dynamics in InGaAs QWs with Mn delta-doping in the barrier grown by MOCVD. Time-resolved Kerr rotation was performed using a tunable mode-locked Ti:sapphire laser with pulse duration of 1 ps and repetition rate of 76 MHz. The time delay (∆t) between pump and probe pulses was varied by a mechanical delay line. The pump beam was circularly polarized by means of a photo-elastic modulator and the probe was linearly polarized and modulated by a chopper. The polarization rotation of the reflected probe beam was detected with a balanced bridge using coupled photodiodes. The sample was immersed in the variable temperature insert of a split-coil superconductor magnet in the Voigt geometry. We observed a time-dependent spin precession frequency for the photo-excited electrons, we associated to the dynamical alignment of the internal effective magnetic field produced by the Mn after optical excitation and successive relaxation. The strong dependence and control of the system magnetization with the experimental conditions will be presented. 1 A. Haury, et al. PRL 79, 511; 2 A. M. Nazmul et al. PRL 95, 017201; 3 M. A. G. Balanta et al. Sci. Rep 6, 24537.
Drift and diffusion of spin polarization in a semiconductor two-dimensional electron gas is strongly influenced by spin precession in the effective spin-orbit magnetic field. The non-commuting spin rotations that occur between subsequent scattering events typically lead to rapid spin dephasing, which can be lifted by engineering the spin-orbit interaction to a persistent spin helix (PSH) symmetry [1], or by laterally confining the electron gas to a length scale smaller than the spin-orbit length [2]. If the spin-orbit interaction is linear in momentum, the average precession angle only depends on the distance the electrons travel, irrespective of whether transport occurs by diffusion or by drift. We show that for cubic Dresselhaus spin-orbit interaction, drift and diffusion by same distances lead to spin precession angles that differ by a factor of two. We have measured spin dynamics in a GaAs-based two-dimensional electron gas tuned to the PSH symmetry using spatially and time-resolved Kerr rotation measurements. Spin polarization is locally injected using a focused circularly polarized laser pulse. In absence of an external magnetic field, the spin polarization measured at a fixed position with respect to the injection point is found to precess with time. The precession frequency depends linearly on the drift velocity and can be explained within a simple model [3,4]. This finding highlights the role of nonlinear SOI in spin transport and is relevant for spintronics applications that require spin manipulation in absence of an external magnetic field. [1] J. Schliemann, J. C. Egues, and D. Loss, Phys. Rev. Lett. 90, 146801 (2003); [2] P. Altmann, M. Kohda, C. Reichl, W. Wegscheider and G. Salis, Phys. Rev. B 92, 235304 (2015); [3] P. Altmann, F. G. G. Hernandez, G. J. Ferreira, M. Kohda, C. Reichl, W. Wegscheider and G. Salis, Phys. Rev. Lett. 116, 196802 (2016); [4] G. J. Ferreira, F. G. G. Hernandez, P. Altmann, and G. Salis, Phys. Rev. B 95, 125119 (2017).
Persistent Skyrmion Lattice of Noninteracting Electrons with Spin-Orbit Coupling
A persistent spin helix (PSH) is a robust helical spin-density pattern arising in disordered 2D electron gases with Rashba $\alpha$ and Dresselhaus $\beta$ spin-orbit (SO) tuned couplings, i.e., $\alpha=\beta$. We investigate the emergence of a persistent Skyrmion lattice (PSL) resulting from the coherent superposition of PSHs along orthogonal directions—crossed PSHs—in wells with two occupied subbands $\nu=1,2$. For realistic GaAs wells, we show that the Rashba αν and Dresselhaus βν couplings can be simultaneously tuned to equal strengths but opposite signs, e.g., $\alpha_1=\beta_1$ and $\alpha_2=-\beta_2$. In this regime, and away from band anticrossings, our noninteracting electron gas sustains a topologically nontrivial Skyrmion-lattice spin density excitation, which inherits the robustness against spin-independent disorder and interactions from its underlying crossed PSHs. We find that the spin relaxation rate due to the interband SO coupling is comparable to that of the cubic Dresselhaus term as a mechanism of the PSL decay. Near anticrossings, the interband-induced spin mixing leads to unusual spin textures along the energy contours beyond those of the Rahsba-Dresselhaus bands. Our PSL opens up the unique possibility of observing topological phenomena, e.g., topological and Skyrmion Hall effects, in ordinary GaAs wells with noninteracting electrons.
Resonant electronic Raman scattering: A BCS-like system
This work investigates the resonant intersubband Raman scattering of two-dimensional electron systems in GaAs-AlGaAs single quantum wells. Self-consistent calculations of the polarized and depolarized Raman cross sections show that the appearance of excitations at the unrenormalized single-particle energy are related to three factors: the extreme resonance regime, the existence of degeneracy in intersubband excitations of the electron gas, and, finally, degeneracy in the interactions between pairs of excitations. It is demonstrated that the physics that governs the problem is similar to the one that gives rise to the formation of the superconducting state in the BCS theory of normal metals. This work was supported by the Brazilian funding agencies CNPq, FAPEMIG, CAPES, Brazil.
In the last decades, nanotechnology has entered our daily life with one of the representatives being semiconductor technology in its various forms. Beside the ability to scale processes down to nanometer sizes, semiconductor technology and science is driven by the capacity to form semiconductor junctions and combinations. This idea, known as heterostructure, was created in the late 50s, after the invention of the transistor in the late 40s. It took some two decades before its realization by a major technological breakthrough – the invention of molecular beam epitaxy (MBE). By now, heterostructures and heterostructure based devices are the backbone of internet communication, mobile telephones and other advanced electronics – many grown by MBE. In this tutorial, I would like to give an overview of MBE - a technique that is important for basic science providing samples for quantum mechanical studies - like the quantum hall effect, single photon sources or Kondo effect – to device structures including high efficient solar cells, quantum cascade lasers or high mobility transistors embedded in cell phones. I will cover the basics of epitaxial growth, do an introduction, what a heterostructure is and its possibilities for band structure design. To illustrate these possibilities, I will take about photoluminescence from heterostructures like quantum wells and quantum dots and discuss, how a 2D electron gas emerge at heterostructure boundaries. I will provide examples from our research to explain basic growth phenomena from flat layer to pattern substrates and luminescence from semiconductor nanostructures fabricated in our labs.
Vanessa Sih
Electrical generation and manipulation of electron and nuclear spin polarization in semiconductors
Electrical generation and manipulation of electron and nuclear spin polarization in semiconductors
Current-induced spin polarization is a phenomenon in which carrier spins in semiconductors are oriented when subjected to current flow. However, the mechanism that produces this spin polarization remains an open question. Existing theory predicts that the spin polarization should be proportional to the spin-orbit splitting yet no clear trend had been observed experimentally. We perform experiments on samples designed so that the magnitude and direction of the in-plane current and applied magnetic field can be varied and use optical techniques to measure the electrical spin generation efficiency and spin-orbit splitting [1]. Contrary to expectation, the magnitude of the current-induced spin polarization is shown to be smaller for crystal directions corresponding to larger spin-orbit splitting. Angle-dependent measurements demonstrate that the steady-state in-plane spin polarization is not along the direction of the spin-orbit field, which we attribute to anisotropic spin relaxation. We show that this electrically-generated electron spin polarization can drive dynamic nuclear spin polarization and measure the nuclear spin polarization as a function of laboratory time and applied electric and magnetic field [2]. In order to understand how different parameters affect the electrical spin generation efficiency, we perform measurements on samples with different indium concentrations and doping densities [3]. [1] B. M. Norman, C. J. Trowbridge, D. D. Awschalom, and V. Sih, "Current-Induced Spin Polarization in Anisotropic Spin-Orbit Fields," Phys. Rev. Lett. 112, 056601 (2014); [2] C. J. Trowbridge, B. M. Norman, Y. K. Kato, D. D. Awschalom, and V. Sih, "Dynamic nuclear polarization from current-induced electron spin polarization," Phys. Rev. B 90, 085122 (2014); [3] M. Luengo-Kovac et al, in preparation (2017).
Ultra-weak Interlayer Coupling in Two-Dimensional Gallium Selenide
The beyond-graphene two-dimensional (2D) materials are envisioned as the future technology for optoelectronics, and the study of the group IIIA metal monochalcogenides (GIIIAMM) in 2D form is an emerging research field. Bulk gallium selenide (GaSe) is a layered material from this family which is widely used in nonlinear optics and is promising as lubricant. The interlayer coupling in few-layer GaSe is currently unknown, and the stability for different polytypes is unclear. Here we use symmetry arguments and first-principles calculations to investigate the phase stability, interlayer coupling, and the Raman and infrared activity of the low-frequency shear and breathing modes expected in few-layer GaSe. Strategies to distinguish the number of layers and the ββ and εε polytypes are discussed. These symmetry results are valid to other isostructural few-layer GIIIAMM materials. Most importantly, by using a linear chain model, we show that the shear and breathing force constants reveal an ultra-weak interlayer coupling at the nanoscale in GaSe. These results suggest that ββ and εε few-layer GaSe show similar lubricant properties to those observed in few-layer graphite. Our analysis opens new perspectives about the study of interlayer interactions and its role in the mechanical and electrical properties in these new 2D materials. The authors acknowledge support from the Brazilian agencies CNPq, FAPEMIG and Capes. R. L. thanks the computational time expended at LCC-UFLA. J. R.-S. gratefully acknowledges support from the Pro-Reitoria de Pesquisa (UFLA). Reference: Phys. Chem. Chem. Phys., 2016, 18, 25401.
Direct probing the electrostatic potential inside unstrained mesoscopic GaAs structures
In this work, we used the Kelvin probe force microscopy (KPFM) to explore the nano-electrostatic properties of strain-free mesoscopic GaAs structures (MGS). The structures are fabricated by overgrowth of a nanohole template using molecular beam epitaxy [1]. Therefore, a combination of Ga assisted deoxidation and local hole etching is used to create initial holes with a depth of ca. 10 to 15 nm, which are covered subsequently with AlGaAs changing the concentration of Al and filled with GaAs of different thicknesses. Atomic force microscopy and transmission electron microscopy shown that the hole shape is preserved during AlGaAs overgrowth than filled with GaAs forming an elongated mount over the hole. We investigate the local potential and charge distribution in these structures using single pass KPFM. We observe a clear potential difference in the middle of the structure, where we expect a filled hole of 10 nm to 15 nm depth. We investigate the dependence of this potential difference as a function of hole. Furthermore, potential maps from the initial structure – showing no charge accumulation – as well as from different shaped MGSs are obtained exhibiting a similar potential distribution. In a first approach, we ascribe this observation to a quantization and discretization of carrier confinement inside the unstrained mesoscopic GaAs structure. This interpretation is supported by the calculation of the band bending and of the electron wave function, demonstrating a relaxation of the free carrier due to background doping and band bending into the MGS. In conclusion, KPFM allows us to directly probe the electronic structure of these an unstrained quantum dot and investigate their electronic properties. [1] S.F.C. da Silva, T. Mardegan, S.R. de Araújo, C.A.O. Ramirez, S. Kiravittaya, O.D.D. Couto, F. Iikawa, C. Deneke, Fabrication and Optical Properties of Strain-free Self-assembled Mesoscopic GaAs Structures, Nanoscale Res. Lett. 12 (2017).
Modeling the Schottky like contacts on nanowires InP-InGaP tunnel diodes
InP-InGaP heterojunction tunnel diode nanowires were grown by use of the VLS method in a MOVPE reactor. The nanowires were processed by mechanically transferring them to new substrates, followed by electron beam lithography and thermal evaporation of metallic contacts to obtain individually contacted nanowires and measure their $IxV$ curves. The measurements revealed the characteristic negative differential region (NDR) of a tunnel diode. The observed valley currents were large when compared to the peak currents, indicating the presence of a significantly high contribution of excess currents, originated possibly from defects close to the PN interface or surface native oxide states. In order to quantify each current contribution on the nanowires (tunneling, thermal and excess) we modeled the system as a contact resistance R in series with a tunnel diode and a parasitic resistance $R_{p}$ connected in parallel to the voltage source. The contact resistance $R$ is the element that limits the current on the diode for large bias voltages and the parasitic resistance $R_{p}$ was included to take into account current leakage effects. We then solved the transcendental system of equations for the resistors and the tunnel diode to fit the theoretical model to the measured $IxV$ curves. The fitting parameters used were the diode ideality factor $n$ and the saturation current $I_0$ (for the thermal current), the peak voltage $V_{p}$ and current $I_{p}$ (for the tunnel contribution), the excess saturation voltage $V_{exc0}$ and current $I_{exc0}$ (for the excess contribution) and, finally, the resistances $R$ and $R_{p}$. The measured curves and the model were in agreement only for the positive high bias voltage region, where the thermal contribution dominates. It was not possible to fit the NDR for any set of parameters. A new model, now including Schottky like contacts was then proposed to model the system. The new fits were in excellent agreement to the measured data.
- Day 6 | Friday - August 18, 2017
Ingrid D. Barcelos
Study of structural properties of heterostructures formed from two dimensional materials
Large part of the technological advances that emerged from solid state physics has its origin in the manufacture of semiconductor heterostructures. They currently make up the research object of two-thirds of all research groups working in semiconductor physics. This is due to the fact that new properties arise by changes in the electronic structure of interfaces that occur to put different materials in contact. A natural tendency is the predictable search heterostructure concepts and fabrication methods using new materials. This presentation consists of single/few layer graphene foils produced by chemical vapor deposition (CVD) are rolled with selfpositioned layers of InGaAs/Cr forming compact multi-turn tubular structures that consist on successive graphene/metal/semiconductor heterojunctions on a radial superlattice. Using elasticity theory and Raman spectroscopy, we show that it is possible to produce homogeneously curved graphene with a curvature radius on the 600−1200 nm range. Additionally, the study of tubular structures also allows the extraction of values for the elastic constants of graphene that are in excellent agreement with elastic constants found in the literature. However, our process has the advantage of leading to a well-defined and nonlocal curvature. From the results described in this work, one can assume that curvature effects solely do not modify the Raman signature of graphene and that strain phenomena observed previously may be ascribed to possible stretching due to the formation of local atomic bonds. This implies that the interactions of graphene with additional materials on heterostructures must be investigated in detail prior to the development of applications and devices.
Strain-Gradient Position Mapping of Semiconductor Quantum Dots
Semiconductor quantum dots (QDs) embedded in a mechanical oscillator can exhibit a large coupling between the energy of the confined exciton and the mechanical mode. This coupling is due to the stress field generated as the oscillator moves, and can be very large if the layer containing the QDs coincides with the region of maximal stress for a given structure, as our group has previously shown [1]. Moreover, the stress field is usually not uniform and depends on which mechanical mode is excited in the host structure. We introduce a nondestructive method to determine the position of randomly distributed In(Ga)As QDs embedded at the bottom of a vertical tapered GaAs photonic waveguide. By resonantly driving the first mechanical mode of the structure we generate a large stress gradient across the QDs plane. Therefore, the emission energy of each QD will shift sinusoidally during the mechanical oscillation, with an amplitude and phase that depend on the position of that QD within the waveguide. Additionally, we use the fact that geometrical asymmetries in the waveguide split the mechanical mode into two non-degenerate ones, to extract position information with respect to the orthogonal basis formed by the oscillation directions. This method allowed us to map the position of the QDs with an accuracy ranging from $\pm35$ nm down to $\pm1$ nm, which is comparable to that of techniques only available to QDs very close to the surface. The principle of strain-gradient position mapping can be generalized to different photonic nanostructures embedding any stress-sensitive quantum emitters. [1] I. Yeo et al., Nature Nanotechnology 9, 106–110 (2014).
Realistic Gap and Spin-orbit splitting from hybrid-DFT: determining effective mass parameters comparable to experiments
III-V semiconductors have received great attention in the last decades due to the wide range of technological applications such as light-emitting diodes, infrared detectors, spintronic devices, etc. Thousands of studies have been reported every year. However, several questions remain open, e.g., how is possible to obtain an accurate ab initio description of the band gap and spin-orbit splitting for semiconductor materials? Currently, the state-of-the-art hybrid density functional theory (DFT) calculations provide the possibility to perform systematic studies with great accuracy and reasonable computational cost. In this work, we report a study of the structural and electronic properties employing hybrid-DFT within the HSE functional [1] for 12 III-V zinc-blend semiconductor. As reported in the literature, the band gap and spin-orbit splitting at the $\Gamma$-point of semiconductors systems depend almost linearly on the magnitude of the non-local Fock exchange that replaces part of the semilocal functionals. To obtain accurate band gap and spin-orbit splittings, we performed a simultaneous fitting of both quantities as a function of the magnitude of the non-local Fock exchange, obtaining a different $\alpha$ parameter for each system. The improvement in the electronic properties was substantial and the side effects on the structural parameters almost negligible. To improve our analysis of the electronic properties, we employed the k.p method to obtain the effective masses and g-factors through a fitting done directly to the HSE band structure [2], finding parameters in excellent agreement with the experimental values. In conclusion, we present a robust and versatile methodology to determine the structural and electronic properties of semiconductor systems. [1] Heyd, Scuseria & Ernzerhof, J. Chem. Phys., 118, 8207 (2003); [2] Bastos, Sabino, Faria Junior, Campos, Da Silva & Sipahi, Semicond. Sci. Technol., 31, 105002 (2016).
Many-body electronic structure calculations of europium complexes in ZnO
Doping has been widely used to tailor the electronic, magnetic, and optical properties of semiconductors. Wide band-gap semiconductors such as ZnO are attractive for ultraviolet light-emitting diodes, lasers and high-power photonic applications. In ZnO, rare-earth elements can be incorporated in the material and the long lifetimes of the excited states allow for an easy realization of population inversion with promising applications in optoelectronics. Previous theoretical investigations using density functional theory in the generalized-gradient approximation (GGA) in ZnO:Eu were unable to ascertain the origin of the experimentally observed emission in ZnO. The main challenge here is the correct description of both ZnO band edges and defect states. It is common understanding that the use of local exchange-correlation functionals wrongly described the ZnO band gap, which could lead to misleading conclusions on the location of the impurity rare-earth f states. Besides, intrinsic defects may also play an important role. In this work, the formation energies and electronic structure of europium complexes in zinc oxide have been determined using density-functional theory and the many-body GW technique. We show that a complex containing a single oxygen interstitial defect and an europium atom substituting a zinc atom can explain the observed emission in the red region of the spectrum in europium-doped ZnO nanowires.
Charge-Kondo quantum dots have recently emerged as a new nanoelectronics paradigm [1,2]. A pseudospin qubit is implemented with quasidegenerate macroscopic charge states of a large semiconductor dot, connected to two or more leads via quantum point contacts. Such devices offer unprecedented control over quantum effects and strongly correlated electron physics on the nanoscale. I this talk I show that charge-Kondo quantum dot devices are essentially perfect quantum simulators of nontrivial quantum impurity models, with Majorana-mediated quantum critical transport in two-channel charge-Kondo experiments [1] agreeing with our theory [2] over a remarkable 9 orders of magnitude. I then discuss more exotic physics in three-lead systems and coupled-dot devices, with finally an outlook towards realizing lattice spin models. [1] Z. Iftikhar et al, Nature 526, 233 (2015); [2] A. K. Mitchell et al, Phys. Rev. Lett. 116, 157202 (2016).
Kondo effect in one dimension spin-orbit coupled systems
The Kondo effect (KE) is the paradigmatic many-body phenomena occurring in magnetic alloys as well as in strong correlated nanoscopic systems. The essence the KE is the dynamical screening of localized magnetic moment by itinerant electrons that takes place at temperatures below a characteristic temperature termed Kondo temperature (TK). Recently, the question of whether the Rashba spin-orbit (SO) coupled electrons affect this screening has been addressed. Studies conducted recently on a system composed by an Anderson impurity on a 2DEG with Rashba spin-orbit have been shown that it can enhance or suppress the Kondo temperature (TK), depending on the relative energy level position of the impurity with respect to the particle-hole symmetric point. Here we investigate a system composed by a single Anderson impurity side-coupled to a quantum wire with spin-orbit coupling (SOC). We derive an effective Hamiltonian in which the Kondo coupling is modified by the SOC. In addition, the Hamiltonian contains two other scattering terms, the so called Dzaloshinskyi-Moriya interaction, know to appear in these systems, and a new one describing processes similar to the Elliott-Yafet scattering mechanisms. By performing a renormalization group analysis on the effective Hamiltonian, we find that the correction on the Kondo coupling due to the SOC favors and enhancement of the Kondo temperature even in the particle-hole symmetric point of the Anderson model, agreeing with the NRG results. Moreover, away from the particle-hole symmetric point, TK always increases with the SOC, accordingly with the previous renormalization group analysis.
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