Structure and magnetism of orthorhombic epitaxial FeMnAs

The molecular beam epitaxy growth of Fe on MnAs/GaAs(001) leads to the formation of an epitaxial FeMnAs phase at the Fe/MnAs interface. The investigation of the structure by high angle annular dark field imaging in a scanning transmission electron microscope reveals an unusual orthorhombic structure, with vacancy ordering. Ab initio calculations show an antiferromagnetic ground state for this orthorhombic FeMnAs.Fil: Demaille, Dominique. Laboratorio Internacional Franco-Argentino en Nanociencias; Francia. Universite Pierre et Marie Curie. Institut des Nanosciences de Paris; FranciaFil: Patriarche, Gilles. Centre National de la Recherche Scientifique; FranciaFil: Helman, Christian. Comisión Nacional de Energía Atómica; Argentina. Laboratorio Internacional Franco-Argentino en Nanociencias; FranciaFil: Eddrief, Mahmoud. Laboratorio Internacional Franco-Argentino en Nanociencias; Francia. Universite Pierre et Marie Curie. Institut des Nanosciences de Paris; FranciaFil: Etgens, Hugo. Laboratorio Internacional Franco-Argentino en Nanociencias; Francia. Universite Pierre et Marie Curie. Institut des Nanosciences de Paris; FranciaFil: Sacchi, Maurizio. Laboratorio Internacional Franco-Argentino en Nanociencias; Francia. Universite Pierre et Marie Curie. Institut des Nanosciences de Paris; Francia. L’Orme des merisiers Saint-Aubin. Synchrotron SOLEIL; FranciaFil: Llois, Ana Maria. Comisión Nacional de Energía Atómica; Argentina. Laboratorio Internacional Franco-Argentino en Nanociencias; Francia. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Marangolo, Massimiliano. Laboratorio Internacional Franco-Argentino en Nanociencias; Francia. Universite Pierre et Marie Curie. Institut des Nanosciences de Paris; Franci

Spin-Orbit induced phase-shift in Bi2_{2}Se3_{3} Josephson junctions

The transmission of Cooper pairs between two weakly coupled superconductors produces a superfluid current and a phase difference; the celebrated Josephson effect. Because of time-reversal and parity symmetries, there is no Josephson current without a phase difference between two superconductors. Reciprocally, when those two symmetries are broken, an anomalous supercurrent can exist in the absence of phase bias or, equivalently, an anomalous phase shift $\varphi_0$ can exist in the absence of a superfluid current. We report on the observation of an anomalous phase shift $\varphi_0$ in hybrid Josephson junctions fabricated with the topological insulator Bi$_2$Se$_3$ submitted to an in-plane magnetic field. This anomalous phase shift $\varphi_0$ is observed directly through measurements of the current-phase relationship in a Josephson interferometer. This result provides a direct measurement of the spin-orbit coupling strength and open new possibilities for phase-controlled Josephson devices made from materials with strong spin-orbit coupling

Ultrafast Structural Dynamics along the β − γ Phase Transition Path in MnAs

International audienceWe investigate the orthorhombic distortion and the structural dynamics of epitaxial MnAs layers on GaAs(001) using static and time-resolved x-ray diffraction. Laser-induced intensity oscillations of Bragg reflections allow us to identify the optical phonon associated with orthorhombic distortion and to follow its softening along the path towards an undistorted phase of hexagonal symmetry. The frequency of this mode falls in the THz range, in agreement with recent calculations. Incomplete softening suggests that the $\beta-\gamma$ transformation deviates from a purely second-order displacive transition

Low-temperature Raman fingerprints for few-quintuple layer topological insulator Bi2Se3 films epitaxied on GaAs

International audienceTopological insulators (Bi2Se3) of single-and few-quintuple-layer (few-QLs) films were investigated by Raman spectroscopy and epitaxied on a GaAs substrate. At a measurement temperature of 80 K, we observed the emergence of additional A(2u) and E-u modes (Raman inactive in the bulk crystal) below 9-QLs film thicknesses, assigned to the crystal-symmetry breakdown in ultrathin films. Furthermore, the out-of-plane A(1g) modes changed in width, frequency, and intensity for decreasing numbers of QL, while the in-plane E-g mode split into three Raman lines, not resolved in previous room temperature experiments. The out-of-plane Raman modes showed a strong Raman resonance at 2.4 eV for around 4-QLs film thickness, and the resonant position of the same modes shifted to 2.2 eV for 18-QLs-thick film. The film thickness-dependence of the phonons frequencies cannot solely be explained within models of weak van der Waals interlayer coupling. The results are discussed in terms of stacking-induced changes in inter-and intralayer bonding and/or the presence of long-range Coulombic interlayer interactions in topological insulator Bi2Se3. This work demonstrates that Raman spectroscopy is sensitive to changes in film thickness over the critical range of 9-to 4-QLs, which coincides with the transition between a gapless topological insulator (occurring above 6-QLs) to a conventional gapped insulator (occurring below 4-QLs)

Structural and transport properties of epitaxial Fe/ZnSe/FeCo magnetic tunnel junctions

The use of a semiconductor barrier in heteroepitaxy with ferromagnetic metal electrodes describes a novel type of magnetic tunnel junctions. In this work, epitaxial Fe/ZnSe/FeCo magnetic tunnel junctions have been grown on ZnSe(001) buffered GaAs(001) substrates by molecular beam epitaxy. Alloying a small amount of Co with Fe in the base electrode provides a better seed epilayer for the growth of the ZnSe barrier compared to pure Fe as seen by transmission electron microscopy. Detailed characterization of the FeCo surface was performed by in situ scanning tunneling microscopy and revealed pyramid-shaped defects of up to 60 AA in height distributed on the FeCo surface. Transport measurements on microfabricated tunnel junctions yielded up to 16% magnetoresistance at 10 K and the effect is attributed to spin polarized tunneling across the semiconductor barrier in the energy range of the gap. The observed anomalous character of the magnetoresistance was attributed to conduction through the pyramidal defects in the base FeCo laye

Surface structures of MnAs grown on GaAs (111)-B substrates.

Atomically flat MnAs epilayers grown on GaAs(111)B substrates by molecular-beam epitaxy present surface structures depending on the sample temperature and on the surface stoichiometry: the known (2×2) reconstruction and two new phases, i.e., a (3×1) reconstruction and a mixed phase of (2×2) and (3×1). By monitoring the reflection high-energy electron-diffraction diagram evolution as a function of the annealing temperature, it was possible to ascribe the (3×1) as being the As richest phase and the (2×2) as the As poorest phase. Two structural models for the observed reconstructions [i.e., (3×1) and (2×2)] are proposed

Thermal conductivity of Bi 2 Se 3 from bulk to thin films: Theory and experiment

International audienceWe calculate the lattice-driven in-plane (κ) and out-of-plane (κ ⊥) thermal conductivities of Bi2Se3 bulk, and of films of different thicknesses, using the Boltzmann equation with phonon scattering times obtained from anharmonic third order density functional perturbation theory. We compare our results for the lattice component of the thermal conductivity with published data for κ on bulk samples and with our room-temperature thermoreflectance measurements of κ ⊥ on films of thickness (L) ranging from 18 nm to 191 nm, where the lattice component has been extracted via the Wiedemann-Franz law. Ab-initio theoretical calculations on bulk samples, including an effective model to account for finite sample thickness and defect scattering, compare favorably both for the bulk case (from literature) and thin films (new measurements). In the low-T limit the theoretical in-plane lattice thermal conductivity of bulk Bi2Se3 agrees with previous measurements by assuming the occurrence of intercalated Bi2 layer defects. The measured thermal conductivity monotonically decreases by reducing L, its value is κ ⊥ ≈ 0.39 ± 0.08 W/m·K for L = 18 nm and κ ⊥ = 0.68 ± 0.14 W/m·K for L = 191 nm. We show that the decrease of room-temperature κ ⊥ in Bi2Se3 thin films as a function of sample thickness can be explained by the incoherent scattering of out-of-plane momentum phonons with the film surface. Our work outlines the crucial role of sample thinning in reducing the out-of-plane thermal conductivity

Ripple rotation in epitaxial growth of MnAs(1(1)over-bar00)

International audienceRippled states formation driven by kinetic instability is evidenced in the case of MnAs(1 (1) over bar 00) hetero- and homoepitaxial growth in a narrow multistable range of growth parameters. The evolution of the surface morphology in this range, comprising slope selection and ripple rotation, maps the kinetic phase diagram recently predicted [A. Levandovsky and L. Golubovic, Phys. Rev. E 76, 041605 (2007)] for growth on rectangular symmetry surfaces, including Ehrlich-Schwoebel instability and effects related to vertical asymmetry