9 research outputs found
Nonlocality of Majorana modes in hybrid nanowires
Spatial separation of Majorana zero modes distinguishes trivial from topological midgap states and is key to topological protection in quantum computing applications. Although signatures of Majorana zero modes in tunneling spectroscopy have been reported in numerous studies, a quantitative measure of the degree of separation, or nonlocality, of the emergent zero modes has not been reported. Here, we present results of an experimental study of nonlocality of emergent zero modes in superconductor-semiconductor hybrid nanowire devices. The approach takes advantage of recent theory showing that nonlocality can be measured from splitting due to hybridization of the zero mode in resonance with a quantum dot state at one end of the nanowire. From these splittings as well as anticrossing of the dot states, measured for even and odd occupied quantum dot states, we extract both the degree of nonlocality of the emergent zero mode, as well as the spin canting angles of the nonlocal zero mode. Depending on the device measured, we obtain either a moderate degree of nonlocality, suggesting a partially separated Andreev subgap state, or a highly nonlocal state consistent with a well-developed Majorana modeThis research was supported by Microsoft, the Danish National Research Foundation, the European Commission, and the Spanish Ministry of Economy and Competitiveness through Grants No. FIS2015-65706-P, No. FIS2015-64654-P, and No. FIS2016-80434-P (AEI/FEDER, EU), the Ramón y Cajal programme Grant No. RYC-2011-09345, and the María de Maeztu Programme for Units of Excellence in R&D (Grant No. MDM-2014-0377). C.M.M. acknowledges support from the Villum Foundation. M.-T.D. acknowledges support from State Key Laboratory of High Performance Computing, Chin
Majorana bound states in a coupled quantum-dot hybrid-nanowire system
Hybrid nanowires combining semiconductor and superconductor materials appear
well suited for the creation, detection, and control of Majorana bound states
(MBSs). We demonstrate the emergence of MBSs from coalescing Andreev bound
states (ABSs) in a hybrid InAs nanowire with epitaxial Al, using a quantum dot
at the end of the nanowire as a spectrometer. Electrostatic gating tuned the
nanowire density to a regime of one or a few ABSs. In an applied axial magnetic
field, a topological phase emerges in which ABSs move to zero energy and remain
there, forming MBSs. We observed hybridization of the MBS with the end-dot
bound state, which is in agreement with a numerical model. The ABS/MBS spectra
provide parameters that are useful for understanding topological
superconductivity in this system.Comment: Article and Supplementary Materia
Relaxation lifetimes of plasmonically enhanced hybrid gold-carbon nanotubes systems
Recently, we introduced a novel hybridization route for carbon nanotubes using
gold nanoparticles, whose close proximity neatly enhances their radiative
emission. Here we investigate the mechanisms behind the enhancement by
monitoring the de-excitation dynamics of our π-hybrids through two-color pump-
probe time-resolved spectroscopy. The de-excitation process reveals a fast
component and a slow component. We find that the presence of gold prominently
affects the fast processes, indicating a stronger influence of the gold
nanoparticle on the intra-band non-radiative relaxation than on the inter-band
recombination of the single-walled carbon nanotube. By evaluating the de-
excitation times, we estimate the balance between near-field pumping and the
faster metal-induced de-excitation contributions, proving the enhanced pumping
to be the leading mechanism
Anomalous metallic phase in tunable destructive superconductors
Multiply connected superconductors smaller than the coherence length show
destructive superconductivity, characterized by reentrant quantum phase
transitions driven by magnetic flux. We investigate the dependence of
destructive superconductivity on flux, transverse magnetic field, temperature,
and current in InAs nanowires with a surrounding epitaxial Al shell, finding
excellent agreement with mean-field theory across multiple reentrant
transitions. Near the crossover between destructive and nondestructive regimes,
an anomalous metal phase is observed with temperature-independent resistance,
controlled over two orders of magnitude by a millitesla-scale transverse
magnetic field
Zero-bias peaks at zero magnetic field in ferromagnetic hybrid nanowires
We report transport measurements and tunneling spectroscopy in hybrid
nanowires with epitaxial layers of superconducting Al and the ferromagnetic
insulator EuS, grown on semiconducting InAs nanowires. In devices where the Al
and EuS covered facets overlap, we infer a remanent effective Zeeman field of
order 1 T, and observe stable zero-bias conductance peaks in tunneling
spectroscopy into the end of the nanowire, consistent with topological
superconductivity at zero applied field. Hysteretic features in critical
current and tunneling spectra as a function of applied magnetic field support
this picture. Nanowires with non-overlapping Al and EuS covered facets do not
show comparable features. Topological superconductivity in zero applied field
allows new device geometries and types of control.Comment: Nature Physics (2020
Flux-induced topological superconductivity in full-shell nanowires
We present a novel route to realizing topological superconductivity using
magnetic flux applied to a full superconducting shell surrounding a
semiconducting nanowire core. In the destructive Little-Parks regime, reentrant
regions of superconductivity are associated with integer number of phase
windings in the shell. Tunneling into the core reveals a hard induced gap near
zero applied flux, corresponding to zero phase winding, and a gapped region
with a discrete zero-energy state around one applied flux quantum, {\Phi}_0 =
h/2e, corresponding to 2{\pi} phase winding. Theoretical analysis indicates
that in the presence of radial spin-orbit coupling in the semiconductor, the
winding of the superconducting phase can induce a transition to a topological
phase supporting Majorana zero modes. Realistic modeling shows a topological
phase persisting over a wide range of parameters, and reproduces experimental
tunneling conductance data. Further measurements of Coulomb blockade peak
spacing around one flux quantum in full-shell nanowire islands shows
exponentially decreasing deviation from 1e periodicity with device length,
consistent with Majorana modes at the ends of the nanowire.Comment: NBI QDEV CMT 2020. Supersedes previous separate theory
(arXiv:1809.05512) and experiment (arXiv:1809.05513) version
Epitaxial Pb on InAs nanowires for quantum devices
Semiconductor-superconductor hybrids are used for realizing complex quantum phenomena but are limited in the accessible magnetic field and temperature range. Now, hybrid devices made from InAs nanowires and epitaxially matched, single-crystal, atomically flat Pb films present superior characteristics, doubling the available parameter space. Semiconductor-superconductor hybrids are widely used to realize complex quantum phenomena, such as topological superconductivity and spins coupled to Cooper pairs. Accessing new, exotic regimes at high magnetic fields and increasing operating temperatures beyond the state-of-the-art requires new, epitaxially matched semiconductor-superconductor materials. One challenge is the generation of favourable conditions for heterostructural formation between materials with the desired properties. Here we harness an increased knowledge of metal-on-semiconductor growth to develop InAs nanowires with epitaxially matched, single-crystal, atomically flat Pb films with no axial grain boundaries. These highly ordered heterostructures have a critical temperature of 7 K and a superconducting gap of 1.25 meV, which remains hard at 8.5 T, and therefore they offer a parameter space more than twice as large as those of alternative semiconductor-superconductor hybrids. Additionally, InAs/Pb island devices exhibit magnetic field-driven transitions from a Cooper pair to single-electron charging, a prerequisite for use in topological quantum computation. Semiconductor-Pb hybrids potentially enable access to entirely new regimes for a number of different quantum systems