A novel ferroelectric Rashba semiconductor

Fast, reversible, and low-power manipulation of the spin texture is crucial for next generation spintronic devices like non-volatile bipolar memories, switchable spin current injectors or spin field effect transistors. Ferroelectric Rashba semiconductors (FERSC) are the ideal class of materials for the realization of such devices. Their ferroelectric character enables an electronic control of the Rashba-type spin texture by means of the reversible and switchable polarization. Yet, only very few materials are established to belong to this class of multifunctional materials. Here, Pb$_{1−x}$Ge$_{x}$Te is unraveled as a novel FERSC system down to nanoscale. The ferroelectric phase transition and concomitant lattice distortion are demonstrated by temperature dependent X-ray diffraction, and their effect on electronic properties are measured by angle-resolved photoemission spectroscopy. In few nanometer-thick epitaxial heterostructures, a large Rashba spin-splitting is exhibiting a wide tuning range as a function of temperature and Ge content. This work defines Pb$_{1−x}$Ge$_{x}$Te as a high-potential FERSC system for spintronic applications

Persistence of structural distortion and bulk band Rashba splitting in SnTe above its ferroelectric critical temperature

The ferroelectric semiconductor $\alpha$-SnTe has been regarded as a topological crystalline insulator and the dispersion of its surface states has been intensively measured with angle-resolved photoemission spectroscopy (ARPES) over the last decade. However, much less attention has been given to the impact of the ferroelectric transition on its electronic structure, and in particular on its bulk states. Here, we investigate the low-energy electronic structure of $\alpha$-SnTe with ARPES and follow the evolution of the bulk-state Rashba splitting as a function of temperature, across its ferroelectric critical temperature of about $T_c\sim 110$ K. Unexpectedly, we observe a persistent band splitting up to room temperature, which is consistent with an order-disorder contribution to the phase transition that requires the presence of fluctuating local dipoles above $T_c$. We conclude that no topological surface state can occur at the (111) surface of SnTe, at odds with recent literature.Comment: 26 pages, 8 figure

High Electron Mobility in Epitaxial Trilayer Graphene on Off-axis SiC(0001)

International audienceThe van de Waals heterostructure formed by an epitaxial trilayer graphene is of particular interest due to its unique tunable electronic band structure and stacking sequence. However, to date, there has been a lack in the fundamental understanding of the electronic properties of epitaxial trilayer graphene. Here, we investigate the electronic properties of large-area epitaxial trilayer graphene on a 4° off-axis SiC(0001) substrate. Micro-Raman mappings and atomic force microscopy (AFM) confirmed predominantly trilayer on the sample obtained under optimized conditions. We used angle-resolved photoemission spectroscopy (ARPES) and Density Functional Theory (DFT) calculations to study in detail the structure of valence electronic states, in particular the dispersion of π bands in reciprocal space and the exact determination of the number of graphene layers. Using far-infrared magneto-transmission (FIR-MT), we demonstrate, that the electron cyclotron resonance (CR) occurs between Landau levels with a (B)1/2 dependence. The CR line-width is consistent with a high Dirac fermions mobility of ~3000 cm2·V−1·s−1 at 4 K

Atomically Sharp Interface in an h-BN-epitaxial graphene van der Waals Heterostructure

International audienceStacking various two-dimensional atomic crystals is a feasible approach to creating unique multilayered van der Waals heterostructures with tailored properties. Herein for the first time, we present a controlled preparation of large-area h-BN/graphene heterostructures via a simple chemical deposition of h-BN layers on epitaxial graphene/SiC(0001). Van der Waals forces, which are responsible for the cohesion of the multilayer system, give rise to an abrupt interface without interdiffusion between graphene and h-BN, as shown by X-ray Photoemission Spectroscopy (XPS) and direct observation using scanning and High-Resolution Transmission Electron Microscopy (STEM/HRTEM). The electronic properties of graphene, such as the Dirac cone, remain intact and no significant charge transfer i.e. doping, is observed. These results are supported by Density Functional Theory (DFT) calculations. We demonstrate that the h-BN capped graphene allows the fabrication of vdW heterostructures without altering the electronic properties of graphene

Between metal and insulator : ultrafast dynamics in the topological insulator Bi2Te3and microscopic domains at the Mott transition in V2O3

Cette thèse présente l'étude de la coexistence métal isolant dans deux systèmes très différents pour la communauté scientifique de la matière condensée : l'isolant topologique 3D Bi2Te3 et le composé prototype de la transition de Mott V2O3. Ces deux systèmes ont été étudiés par des techniques basées sur la spectroscopie de photoélectrons. La première technique utilisée est le TR-ARPES (time and angle resolved photoemission spectroscopy), avec une résolution temporelle de 80 fs, appliquée à l'isolant topologique 3D Bi2Te3 pour distinguer la dynamique ultra-rapide des états métalliques de la surface de celle des états isolants du volume. Cette mesure a permet de comprendre les différents mécanismes de diffusion entre la surface et le volume, ainsi que l'amélioration de la relaxation du cône de Dirac par la préexistence à la sous-surface d'une bande de flexion. La seconde technique utilisée dans cette thèse est le SPEM (scanning photoelectron microscopy), avec une résolution spatiale de 150 nm, permettant d'étudier la coexistence des domaines métalliques et isolants à la transition de Mott dans V2O3 ; cette coexistence a pour origine le caractère 1 er ordre de la transition. La mesure montre une coexistence métal-isolant dans le Cr-dopé : les domaines métalliques sont dus à des centres de nucléations < 150 nm et la forme des domaines est clairement liée à la forme des marches de clivage.This thesis presents the study of metal-insulator coexistence in two very different systems for the scientific community of condensed matter: the 3D topological insulator Bi2Te3 and the prototype compound of the Mott transition V2O3. Both systems were studied by techniques based on photoelectron spectroscopy. The first technique is the TR- ARPES (time and angle resolved photoemission spectroscopy), with a temporal resolution of 80 fs, applied to the 3D topological insulator Bi2Te3 to distinguish the ultrafast dynamics of metallic surface states from that of the insulating bulk states. This allows us to understand the different mechanisms of scattering between the surface and the bulk, as well as the amelioration on the Dirac cone relaxation due to the preexistence of subsurface band bending. The second technique used in this thesis is the SPEM (scanning photoelectron microscopy), with a spatial resolution of 150 nm, which was used to study the coexistence of metallic and insulating domains at the Mott transition on V2O3. This coexistence takes its origin from the first order character of the transition. The measurement shows the metal-insulator coexistence on the Cr-doped: metal domains are due to nucleation centers < 150 nm and the shape of the domains is clearly linked to the shape of the cleavage steps

Entre métal et isolant : dynamique ultrarapide dans l'isolant topologique Bi2Te3 et domaines microscopiques à la transition De Mott Dans V203

This thesis presents the study of metal-insulator coexistence in two very different systems for the scientific community of condensed matter: the 3D topological insulator Bi2Te3 and the prototype compound of the Mott transition V2O3. Both systems were studied by techniques based on photoelectron spectroscopy. The first technique is the TR- ARPES (time and angle resolved photoemission spectroscopy), with a temporal resolution of 80 fs, applied to the 3D topological insulator Bi2Te3 to distinguish the ultrafast dynamics of metallic surface states from that of the insulating bulk states. This allows us to understand the different mechanisms of scattering between the surface and the bulk, as well as the amelioration on the Dirac cone relaxation due to the preexistence of subsurface band bending. The second technique used in this thesis is the SPEM (scanning photoelectron microscopy), with a spatial resolution of 150 nm, which was used to study the coexistence of metallic and insulating domains at the Mott transition on V2O3. This coexistence takes its origin from the first order character of the transition. The measurement shows the metal-insulator coexistence on the Cr-doped: metal domains are due to nucleation centers < 150 nm and the shape of the domains is clearly linked to the shape of the cleavage steps.Cette thèse présente l'étude de la coexistence métal isolant dans deux systèmes très différents pour la communauté scientifique de la matière condensée : l'isolant topologique 3D Bi2Te3 et le composé prototype de la transition de Mott V2O3. Ces deux systèmes ont été étudiés par des techniques basées sur la spectroscopie de photoélectrons. La première technique utilisée est le TR-ARPES (time and angle resolved photoemission spectroscopy), avec une résolution temporelle de 80 fs, appliquée à l'isolant topologique 3D Bi2Te3 pour distinguer la dynamique ultra-rapide des états métalliques de la surface de celle des états isolants du volume. Cette mesure a permet de comprendre les différents mécanismes de diffusion entre la surface et le volume, ainsi que l'amélioration de la relaxation du cône de Dirac par la préexistence à la sous-surface d'une bande de flexion. La seconde technique utilisée dans cette thèse est le SPEM (scanning photoelectron microscopy), avec une résolution spatiale de 150 nm, permettant d'étudier la coexistence des domaines métalliques et isolants à la transition de Mott dans V2O3 ; cette coexistence a pour origine le caractère 1 er ordre de la transition. La mesure montre une coexistence métal-isolant dans le Cr-dopé : les domaines métalliques sont dus à des centres de nucléations < 150 nm et la forme des domaines est clairement liée à la forme des marches de clivage

Extremely low-energy ARPES of quantum well states in cubic-GaN/AlN and GaAs/AlGaAs heterostructures

Quantum well (QW) heterostructures have been extensively used for the realization of a wide range of optical and electronic devices. Exploiting their potential for further improvement and development requires a fundamental understanding of their electronic structure. So far, the most commonly used experimental techniques for this purpose have been all-optical spectroscopy methods that, however, are generally averaging in momentum space. Additional information can be gained by angle-resolved photoelectron spectroscopy (ARPES), which measures the electronic structure with momentum resolution. Here we report on the use of extremely low-energy ARPES (photon energy ~ 7 eV) to increase depth sensitivity and access buried QW states, located at 3 nm and 6 nm below the surface of cubic-GaN/AlN and GaAs/AlGaAs heterostructures, respectively. We find that the QW states in cubic-GaN/AlN can indeed be observed, but not their energy dispersion, because of the high surface roughness. The GaAs/AlGaAs QW states, on the other hand, are buried too deep to be detected by extremely low-energy ARPES. Since the sample surface is much flatter, the ARPES spectra of the GaAs/AlGaAs show distinct features in momentum space, which can be reconducted to the band structure of the topmost surface layer of the QW structure. Our results provide important information about the samples’ properties required to perform extremely low-energy ARPES experiments on electronic states buried in semiconductor heterostructures

Self-organized metal-semiconductor epitaxial graphene layer on off-axis 4H-SiC(0001)

International audienceThe remarkable properties of graphene have shown promise for new perspectives in future electronics, notably for nanometer scale devices. Here we grow graphene epitaxially on an off-axis 4H-SiC(0001) substrate and demonstrate the formation of periodic arrangement of monolayer graphene on planar (0001) terraces and Bernal bilayer graphene on (112 — 0) nanofacets of SiC. We investigate these lateral superlattices using Raman spectroscopy, atomic force microscopy/ electrostatic force microscopy (AFM/EFM) and X-ray and angle resolved photoemission spectroscopy (XPS/ARPES). The correlation of EFM and ARPES reveals the appearance of permanent electronic band gaps in AB-stacked bilayer graphene on (112 — 0) SiC nanofacets of 150 meV. This feature is confirmed by density functional theory (DFT) calculations. The charge transfer between the substrate and graphene bilayer results in an asymmetric charge distribution between the top and the bottom graphene layers opening an energy gap. This surface organization can be thus defined as self-organized metal–semiconductor graphene

Evidence for Flat Bands near the Fermi Level in Epitaxial Rhombohedral Multilayer Graphene

The stacking order of multilayer graphene has a profound influence on its electronic properties. In particular, it has been predicted that a rhombohedral stacking sequence displays a very flat conducting surface state: the longer the sequence, the flatter the band. In such a flat band, the role of electron electron correlation is enhanced, possibly resulting in high T-c superconductivity, magnetic order, or charge density wave order. Here we demonstrate that rhombohedral multilayers are easily obtained by epitaxial growth on 3C-S1C(111) on a 2 degrees off-axis 6H-SiC(0001). The resulting samples contain rhombohedral sequences of five layers on 70% of the surface. We confirm the presence of the flat band at the Fermi level by scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy, in close agreement with the predictions of density functional theory calculations