111 research outputs found
Diameter-dependent conductance of InAs nanowires
Electrical conductance through InAs nanowires is relevant for electronic
applications as well as for fundamental quantum experiments. Here we employ
nominally undoped, slightly tapered InAs nanowires to study the diameter
dependence of their conductance. Contacting multiple sections of each wire, we
can study the diameter dependence within individual wires without the need to
compare different nanowire batches. At room temperature we find a
diameter-independent conductivity for diameters larger than 40 nm, indicative
of three-dimensional diffusive transport. For smaller diameters, the resistance
increases considerably, in coincidence with a strong suppression of the
mobility. From an analysis of the effective charge carrier density, we find
indications for a surface accumulation layer.Comment: 9 pages, 5 figure
Anisotropic Pauli spin blockade in hole quantum dots
We present measurements on gate-defined double quantum dots in Ge-Si
core-shell nanowires, which we tune to a regime with visible shell filling in
both dots. We observe a Pauli spin blockade and can assign the measured leakage
current at low magnetic fields to spin-flip cotunneling, for which we measure a
strong anisotropy related to an anisotropic g-factor. At higher magnetic fields
we see signatures for leakage current caused by spin-orbit coupling between
(1,1)-singlet and (2,0)-triplet states. Taking into account these anisotropic
spin-flip mechanisms, we can choose the magnetic field direction with the
longest spin lifetime for improved spin-orbit qubits
Spin transport in ferromagnet-InSb nanowire quantum devices
Signatures of Majorana zero modes (MZMs), which are the building blocks for
fault-tolerant topological quantum computing, have been observed in
semiconductor nanowires (NW) with strong spin-orbital-interaction (SOI), such
as InSb and InAs NWs with proximity-induced superconductivity. Realizing
topological superconductivity and MZMs in this most widely-studied platform
also requires eliminating spin degeneracy, which is realized by applying a
magnetic field to induce a helical gap. However, the applied field can
adversely impact the induced superconducting state in the NWs and also places
geometric restrictions on the device, which can affect scaling of future
MZM-based quantum registers. These challenges could be circumvented by
integrating magnetic elements with the NWs. With this motivation, in this work
we report the first experimental investigation of spin transport across InSb
NWs, which are enabled by devices with ferromagnetic (FM) contacts. We observe
signatures of spin polarization and spin-dependent transport in the
quasi-one-dimensional ballistic regime. Moreover, we show that electrostatic
gating tunes the observed magnetic signal and also reveals a transport regime
where the device acts as a spin filter. These results open an avenue towards
developing MZM devices in which spin degeneracy is lifted locally, without the
need of an applied magnetic field. They also provide a path for realizing
spin-based devices that leverage spin-orbital states in quantum wires.Comment: 30 pages, 12 figure
Strain engineering in Ge/GeSn core/shell nanowires
Strain engineering in Sn-rich group IV semiconductors is a key enabling
factor to exploit the direct band gap at mid-infrared wavelengths. Here, we
investigate the effect of strain on the growth of GeSn alloys in a Ge/GeSn
core/shell nanowire geometry. Incorporation of Sn content in the 10-20 at.%
range is achieved with Ge core diameters ranging from 50nm to 100nm. While the
smaller cores lead to the formation of a regular and homogeneous GeSn shell,
larger cores lead to the formation of multi-faceted sidewalls and broadened
segregation domains, inducing the nucleation of defects. This behavior is
rationalized in terms of the different residual strain, as obtained by
realistic finite element method simulations. The extended analysis of the
strain relaxation as a function of core and shell sizes, in comparison with the
conventional planar geometry, provides a deeper understanding of the role of
strain in the epitaxy of metastable GeSn semiconductors
Exfoliated hexagonal BN as gate dielectric for InSb nanowire quantum dots with improved gate hysteresis and charge noise
We characterize InSb quantum dots induced by bottom finger gates within a
nanowire that is grown via the vapor-liquid-solid process. The gates are
separated from the nanowire by an exfoliated 35\,nm thin hexagonal BN flake. We
probe the Coulomb diamonds of the gate induced quantum dot exhibiting charging
energies of and orbital excitation energies up to
. The gate hysteresis for sweeps covering 5 Coulomb diamonds
reveals an energy hysteresis of only between upwards and
downwards sweeps. Charge noise is studied via long-term measurements at the
slope of a Coulomb peak revealing potential fluctuations of at 1\,Hz. This makes h-BN the dielectric with
the currently lowest gate hysteresis and lowest low-frequency potential
fluctuations reported for low-gap III-V nanowires. The extracted values are
similar to state-of-the art quantum dots within Si/SiGe and Si/SiO
systems
Giant optical birefringence of semiconductor nanowire metamaterials
Semiconductor nanowires exhibit large polarization anisotropy for the
absorption and emission of light, making them ideal building blocks for novel
photonic metamaterials. Here, we demonstrate that a high density of aligned
nanowires exhibits giant optical birefringence, a collective phenomenon
observable uniquely for collections of wires. The nanowire material was grown
on gallium phosphide (GaP) (111) in the form of vertically standing GaP
nanowires. We obtain the largest optical birefringence to date, with a
difference between the in-plane and out-of-plane refractive indices of 0.80 and
a relative birefringence of 43%. These values exceed by a factor of 75 the
natural birefringence of quartz and a by more than a factor of two the highest
values reported so far in other artificial materials. By exploiting the
specific crystallographic growth directions of the nanowires on the substrate,
we further demonstrate full control over the orientation of the optical
birefringence effect in the metamaterial.Comment: 10 pages, 4 figure
Tunable Supercurrent Through Semiconductor Nanowires
Nanoscale superconductor-semiconductor hybrid devices are assembled from InAs
semiconductor nanowires individually contacted by aluminum-based superconductor
electrodes. Below 1 K, the high transparency of the contacts gives rise to
proximity-induced superconductivity. The nanowires form superconducting weak
links operating as mesoscopic Josephson junctions with electrically tunable
coupling. The supercurrent can be switched on/off by a gate voltage acting on
the electron density in the nanowire. A variation in gate voltage induces
universal fluctuations in the normal-state conductance which are clearly
correlated to critical current fluctuations. The ac Josephson effect gives rise
to Shapiro steps in the voltage-current characteristic under microwave
irradiation.Comment: 9 pages, 3 figure
Conductance Quantization at zero magnetic field in InSb nanowires
Ballistic electron transport is a key requirement for existence of a
topological phase transition in proximitized InSb nanowires. However,
measurements of quantized conductance as direct evidence of ballistic transport
have so far been obscured due to the increased chance of backscattering in one
dimensional nanowires. We show that by improving the nanowire-metal interface
as well as the dielectric environment we can consistently achieve conductance
quantization at zero magnetic field. Additionally, studying the sub-band
evolution in a rotating magnetic field reveals an orbital degeneracy between
the second and third sub-bands for perpendicular fields above 1T
Single-crystalline PbTe film growth through reorientation
Heteroepitaxy enables the engineering of novel properties, which do not exist
in a single material. Two principle growth modes are identified for material
combinations with large lattice mismatch, Volmer-Weber and Stranski-Krastanov.
Both lead to the formation of three-dimensional islands, hampering the growth
of flat defect-free thin films. This limits the number of viable material
combinations. Here, we report a distinct growth mode found in molecular beam
epitaxy of PbTe on InP initiated by pre-growth surface treatments. Early
nucleation forms islands analogous to the Volmer-Weber growth mode, but film
closure exhibits a flat surface with atomic terracing. Remarkably, despite
multiple distinct crystal orientations found in the initial islands, the final
film is single-crystalline. This is possible due to a reorientation process
occurring during island coalescence, facilitating high quality heteroepitaxy
despite the large lattice mismatch, difference in crystal structures and
diverging thermal expansion coefficients of PbTe and InP. This growth mode
offers a new strategy for the heteroepitaxy of dissimilar materials and expands
the realm of possible material combinations
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