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
- Day 2 | Monday - August 14, 2017
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).
- Day 3 | Tuesday - August 15, 2017
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.
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.
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.
- Day 4 | Wednesday - August 16, 2017
- Day 5 | Thursday - August 17, 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.
Electrical generation and manipulation of electron and nuclear spin polarization in semiconductors
- 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.