Parity transitions in the superconducting ground state of hybrid InSb-Al Coulomb islands

The number of electrons in small metallic or semiconducting islands is quantized. When tunnelling is enabled via opaque barriers this number can change by an integer. In superconductors the addition is in units of two electron charges (2e), reflecting that the Cooper pair condensate must have an even parity. This ground state (GS) is foundational for all superconducting qubit devices. Here, we study a hybrid superconducting-semiconducting island and find three typical GS evolutions in a parallel magnetic field: a robust 2e-periodic even-parity GS, a transition to a 2e-periodic odd-parity GS,and a transition from a 2e- to a 1e-periodic GS. The 2e-periodic odd-parity GS persistent in gate-voltage occurs when a spin-resolved subgap state crosses zero energy. For our 1e-periodic GSs we explicitly show the origin being a single zero-energy state gapped from the continuum, i.e. compatible with an Andreev bound states stabilized at zero energy or the presence of Majorana zero modes

Quantized Majorana conductance

Majorana zero-modes hold great promise for topological quantum computing. Tunnelling spectroscopy in electrical transport is the primary tool to identify the presence of Majorana zero-modes, for instance as a zero-bias peak (ZBP) in differential-conductance. The Majorana ZBP-height is predicted to be quantized at the universal conductance value of 2e2/h at zero temperature. Interestingly, this quantization is a direct consequence of the famous Majorana symmetry, 'particle equals antiparticle'. The Majorana symmetry protects the quantization against disorder, interactions, and variations in the tunnel coupling. Previous experiments, however, have shown ZBPs much smaller than 2e2/h, with a recent observation of a peak-height close to 2e2/h. Here, we report a quantized conductance plateau at 2e2/h in the zero-bias conductance measured in InSb semiconductor nanowires covered with an Al superconducting shell. Our ZBP-height remains constant despite changing parameters such as the magnetic field and tunnel coupling, i.e. a quantized conductance plateau. We distinguish this quantized Majorana peak from possible non-Majorana origins, by investigating its robustness on electric and magnetic fields as well as its temperature dependence. The observation of a quantized conductance plateau strongly supports the existence of non-Abelian Majorana zero-modes in the system, consequently paving the way for future braiding experiments.Comment: 5 figure

Surface and electronic structure of epitaxial PtLuSb (001) thin films

The surface and electronic structure of single crystal thin films of PtLuSb (001) grown by molecular beam epitaxy were studied. Scanning tunneling spectroscopy (STS), photoemission spectroscopy, and temperature dependent Hall measurements of PtLuSb thin films are consistent with a zero-gap semiconductor or semi-metal. STS and photoemission measurements show a decrease in density of states approaching the Fermi level for both valence and conduction bands as well as a slight shift of the Fermi level position into the valence band. Temperature dependent Hall measurements also corroborate the Fermi level position by measurement of p-type carriers. (C) 2014 AIP Publishing LLC

Cross-sectional scanning tunneling microscopy and spectroscopy of semimetallic ErAs nanostructures embedded in GaAs

The growth and atomic/electronic structure of molecular beam epitaxy-grown ErAs nanoparticles and nanorods embedded within a GaAs matrix are examined for the first time via cross-sectional scanning tunneling microscopy and spectroscopy. Cross sections enable the interrogation of the internal structure and are well suited for studying embedded nanostructures. The early stages of embedded ErAs nanostructure growth are examined via these techniques and compared with previous cross-sectional transmission electron microscopy work. Tunneling spectroscopy I(V) for both ErAs nanoparticles and nanorods was also performed, demonstrating that both nanostructures are semimetallic. (C) 2011 American Vacuum Society. [DOI: 10.1116/1.3547713

Epitaxial growth and surface studies of the Half Heusler compound NiTiSn (001)

The Half Heuslers are currently an attractive family of compounds for high temperature thermoelectrics research, and recently, there has been renewed interest since some of these compounds are proposed to be topological insulators. NiTiSn belongs to the family of 18 valence electron Half Heuslers that are predicted to be semiconducting, despite being composed entirely of metallic elements. The growth of the Half Heusler compound NiTiSn by molecular beam epitaxy is demonstrated. The NiTiSn films are epitaxial and single crystalline as observed by reflection high-energy electron diffraction and x-ray diffraction. Temperature dependent transport measurements suggest the films may be semiconducting, but with a high background carrier density indicative of a high density of electrically active defect states. Methods of protecting the sample surface for synchrotron-based photoemission measurements are explored. These methods may be applied to the study of surface electronic structure in unconventional materials. (C) 2013 American Vacuum Society

Controlling n-Type Carrier Density from Er Doping of InGaAs with MBE Growth Temperature

International audienceUnder certain growth conditions in molecular beam epitaxy, erbium, indium, gallium, and arsenic form a two-phase composite, consisting of ErAs nanoparticles embedded in dilute Er-doped In0.53Ga0.47As. This paper further explores the effect of growth conditions, specifically growth temperature, on the nanostructure of this material and the resulting thermal and electrical transport properties. For a set of samples grown with substrate temperatures varying from 430C to 525C, we find that the thermal conductivity decreases slightly with increasing growth temperature (from 4.8 W/m K to 4.1 W/m K) while the electrical conductivity decreases dramatically with increasing growth temperature (from 2100 S/cm to 110 S/cm), which is largely due to decreasing carrier concentration. At higher growth temperatures, more erbium precipitates out of solution and the size and density of the ErAs nanoparticles increase, as characterized by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), while the total erbium concentration does not change with growth temperature, as characterized by Rutherford backscatter spectrometry (RBS). Measurement of the erbium concentration by secondary-ion mass spectrometry suggests that the Er bonding configuration changes with growth temperature. These results indicate that increasing the ratio of solute Er atoms in the In0.53Ga0.47As host to precipitated Er atoms in ErAs particles increases the carrier density and electrical conductivity of the total composite material

Dataset underlying the paper: Parity transitions in the superconducting ground state of hybrid InSb-Al Coulomb islands

Data for the figures in the paper 'Parity transitions in the superconducting ground state of hybrid InSb-Al Coulomb islands