Search

Inspired by concepts developed for fermionic systems in the framework of condensed matter physics, topology and topological states are recently being explored also in bosonic systems. Recently, some of these concepts have been successfully applied to acoustic phonons in nanoscale multilayered systems. The reported demonstration of confined topological phononic modes was based on Raman scattering spectroscopy [M. Esmann et al., Phys. Rev. B 97, 155422 (2018)], yet the resolution did not suffice to determine lifetimes and to identify other acoustic modes in the system. Here, we use time-resolved pump-probe measurements using an asynchronous optical sampling (ASOPS) technique to overcome these resolution limitations. By means of one-dimensional GaAs/AlAs distributed Bragg reflectors (DBRs) used as building blocks, we engineer high frequency (∼200 GHz) topological acoustic interface states. We are able to clearly distinguish confined topological states from stationary band edge modes. The generation/detection scheme reflects the symmetry of the modes directly through the selection rules, evidencing the topological nature of the measured confined state. These experiments enable a new tool in the study of the more complex topology-driven phonon dynamics such as phonon nonlinearities and optomechanical systems with simultaneous confinement of light and sound., This work was supported by the European Commission in the form of the H2020 FET Proactive project TOCHA (No. 824140). The authors acknowledge funding by the European Research Council Starting Grant No. 715939, Nanophennec; by the French RENATECH network, and through a public grant overseen by the ANR as part of the “Investissements d’Avenir” program (Labex NanoSaclay Grant No. ANR-10-LABX-0035). ICN2 was supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706) and the project PHENTOM (Fis 2015-70862-P), as well as by the CERCA Programme/Generalitat de Catalunya, and by the European Commission in the form of the H2020 FET Open project PHENOMEN (No. 713450 to G.A.). G.A. was supported by a BIST Ph.D. fellowship and P.D.G. by a Ramon y Cajal Fellowship No. RyC-2015-18124. M.E. acknowledges funding from the German Research Foundation DFG (Forschungsstipendium ES 560/1-1).

International audience; We report the analysis of the electrical properties of In x-1 Ga x P nanowires (NWs) grown by template-assisted selective epitaxy (TASE). The individual NW properties are investigated by means of electron beam induced current microscopy (EBIC) and current-voltage curves acquired on single nano-objects. First, a set of samples containing n-doped InGaP NWs grown on a p-doped Si substrate are investigated. The electrical activity of the hetero-junction between the NWs and the substrate is demonstrated and the material parameters are analyzed, namely the n-doping level is determined in relation with the dopant flow used during the growth. These results were used to design and elaborate InGaP NWs containing a p-n homo-junction. The electrical activity of the homo-junction is evidenced using EBIC mapping on single NWs and material parameters (namely, the doping and the minority carrier diffusion lengths) for the p-and n-doped InGaP segments are estimated. Finally, the first proof of a photovoltaic effect from the NW homo-junctions is obtained by photocurrent measurements of a contacted NW array under white light irradiation.

Electronic properties of low dimensional superconductors are determined by many-body-effects. This physics has been studied traditionally with superconducting thin films, and in recent times with two-dimensional electron gases (2DEGs) at oxide interfaces. In this work, we show that a superconducting 2DEG can be generated by simply evaporating a thin layer of metallic Al under ultra-high vacuum on a SrTiO3 crystal, whereby Al oxidizes into amorphous insulating alumina, doping the SrTiO3 surface with oxygen vacancies. The superconducting critical temperature of the resulting 2DEG is found to be tunable with a gate voltage with a maximum value of 360 mK. A gate-induced switching between superconducting and resistive states is demonstrated. Compared to conventionally-used pulsed-laser deposition (PLD), our work simplifies to a large extent the process of fabricating oxide-based superconducting 2DEGs. It will make such systems accessible to a broad range of experimental techniques useful to understand low-dimensional phase transitions and complex many-body-phenomena in electronic systems., Comment: 15 pages, 5 figures

Silicene is an emerging 2D material with advantages of high carrier mobility, compatibility with the silicon-based semiconductor industry, and the tunable gap by a vertical electrical field due to the buckling structure. In this work, we report a first-principles investigation on the spin injection system, which consists of a Fe(111)/silicene stack as the spin injector and pure silicene as the spin channel. An extremely high spin injection efficiency (SIE) close to 100% is achieved. The partial density of states of Fe layers in the Fe(111)/silicene stack shows that spin-down states dominate above the Fermi level, resulting in a negligible spin-up current and high SIE. The transmission spectra have been investigated to analyze the spin-resolved properties. The spin injection system based on silicene is promising for the efficient silicon-based spintronics devices such as switching transistors.

International audience; The monolithic integration of III-V semiconductors on silicon and particularly of GaAs has aroused great interest since the 1980s. Potential applications are legion, ranging from photovoltaics to high mobility channel transistors. By using a novel integration method, we have shown that it is possible to achieve heteroepitaxial integration of GaAs crystals (typical size 1 lm) on silicon without any structural defect such as antiphase domains, dislocations, or stress, usually reported for direct GaAs heteroepitaxy on silicon. However, concerning their electronic properties, conventional free carrier characterization methods are impractical due to the micrometric size of GaAs crystals. In order to evaluate the GaAs material quality for optoelectronic applications, a series of indirect analyses such as atom probe tomography, Raman spectroscopy, and micro-photoluminescence as a function of temperature were performed. These revealed a high content of partially electrically active carbon originating from the trimethylgallium used as the Ga precursor. Nevertheless, the very good homogeneity observed by this doping mechanism and the attractive properties of carbon as a dopant once controlled to a sufficient degree are a promising route to device doping.