281 research outputs found
Interaction effects in a microscopic quantum wire model with strong spin-orbit interaction
We investigate the effect of strong interactions on the spectral properties
of quantum wires with strong Rashba spin-orbit interaction in a magnetic field,
using a combination of Matrix Product State and bosonization techniques.
Quantum wires with strong Rashba spin-orbit interaction and magnetic field
exhibit a partial gap in one-half of the conducting modes. Such systems have
attracted wide-spread experimental and theoretical attention due to their
unusual physical properties, among which are spin-dependent transport, or a
topological superconducting phase when under the proximity effect of an s-wave
superconductor. As a microscopic model for the quantum wire we study an
extended Hubbard model with spin-orbit interaction and Zeeman field. We obtain
spin resolved spectral densities from the real-time evolution of excitations,
and calculate the phase diagram. We find that interactions increase the pseudo
gap at and thus also enhance the Majorana-supporting phase and
stabilize the helical spin order. Furthermore, we calculate the optical
conductivity and compare it with the low energy spiral Luttinger Liquid result,
obtained from field theoretical calculations. With interactions, the optical
conductivity is dominated by an excotic excitation of a bound
soliton-antisoliton pair known as a breather state. We visualize the
oscillating motion of the breather state, which could provide the route to
their experimental detection in e.g. cold atom experiments
Triple Point Topological Metals
Topologically protected fermionic quasiparticles appear in metals, where band degeneracies occur at the Fermi level, dictated by the band structure topology. While in some metals these quasiparticles are direct analogues of elementary fermionic particles of the relativistic quantum field theory, other metals can have symmetries that give rise to quasiparticles, fundamentally different from those known in high-energy physics. Here, we report on a new type of topological quasiparticles—triple point fermions—realized in metals with symmorphic crystal structure, which host crossings of three bands in the vicinity of the Fermi level protected by point group symmetries. We find two topologically different types of triple point fermions, both distinct from any other topological quasiparticles reported to date. We provide examples of existing materials that host triple point fermions of both types and discuss a variety of physical phenomena associated with these quasiparticles, such as the occurrence of topological surface Fermi arcs, transport anomalies, and topological Lifshitz transitions.National Science Foundation (U.S.) (DMR-1410636)National Science Foundation (U.S.) (DMR-1120901
Automated construction of symmetrized Wannier-like tight-binding models from ab initio calculations
Wannier tight-binding models are effective models constructed from
first-principles calculations. As such, they bridge a gap between the accuracy
of first-principles calculations and the computational simplicity of effective
models. In this work, we extend the existing methodology of creating Wannier
tight-binding models from first-principles calculations by introducing the
symmetrization post-processing step, which enables the production of
Wannier-like models that respect the symmetries of the considered crystal.
Furthermore, we implement automatic workflows, which allow for producing a
large number of tight-binding models for large classes of chemically and
structurally similar compounds, or materials subject to external influence such
as strain. As a particular illustration, these workflows are applied to
strained III-V semiconductor materials. These results can be used for further
study of topological phase transitions in III-V quantum wells
Simultaneously-Measured Mid-Infrared Refractive Indices of GaAs/AlGaAs
We present our results for a simultaneous measurement of the refractive
indices of Gallium Arsenide (GaAs) and Aluminum Gallium Arsenide
(AlGaAs) in the spectral region from to
( to ). These
values are obtained from a monocrystalline thin-film multilayer Bragg mirror of
excellent purity (background doping ), grown via molecular beam epitaxy. To recover the
refractive indices over such a broad wavelength range, we fit a dispersion
model for each material. For that, we measure both a photometrically accurate
transmittance spectrum of the Bragg mirror via Fourier-transform infrared
spectrometry and the individual physical layer thicknesses of the structure via
scanning electron microscopy. To infer the uncertainty of the refractive index
values, we estimate relevant measurement uncertainties and propagate them via a
Monte-Carlo-type method. This method conclusively yields propagated relative
uncertainties on the order of over the measured spectral range for
both GaAs and AlGaAs. The fitted model can also approximate
the refractive index for MBE-grown AlGaAs for . These updated values will be essential in the design and
fabrication of next-generation active and passive optical devices in a spectral
region which is of high interest in many fields, e.g., laser design and
cavity-enhanced spectroscopy.Comment: 20 pages, 5 figures, submitted to PR
Automated construction of symmetrized Wannier-like tight-binding models from ab initio calculations
Wannier tight-binding models are effective models constructed from first-principles calculations. As such, they bridge a gap between the accuracy of first-principles calculations and the computational simplicity of effective models. In this work, we extend the existing methodology of creating Wannier tight-binding models from first-principles calculations by introducing the symmetrization post-processing step, which enables the production of Wannier-like models that respect the symmetries of the considered crystal. Furthermore, we implement automatic workflows, which allow for producing a large number of tight-binding models for large classes of chemically and structurally similar compounds or materials subject to external influence such as strain. As a particular illustration, these workflows are applied to strained III-V semiconductor materials. These results can be used for further study of topological phase transitions in III-V quantum wells
The B-cell inhibitory receptor CD22 is a major factor in host resistance to Streptococcus pneumoniae infection
Streptococcus pneumoniae is a major human pathogen, causing pneumonia and sepsis. Genetic components strongly influence host responses to pneumococcal infections, but the responsible loci are unknown. We have previously identified a locus on mouse chromosome 7 from a susceptible mouse strain, CBA/Ca, to be crucial for pneumococcal infection. Here we identify a responsible gene, Cd22, which carries a point mutation in the CBA/Ca strain, leading to loss of CD22 on B cells. CBA/Ca mice and gene-targeted CD22-deficient mice on a C57BL/6 background are both similarly susceptible to pneumococcal infection, as shown by bacterial replication in the lungs, high bacteremia and early death. After bacterial infections, CD22-deficient mice had strongly reduced B cell populations in the lung, including GM-CSF producing, IgM secreting innate response activator B cells, which are crucial for protection. This study provides striking evidence that CD22 is crucial for protection during invasive pneumococcal disease.info:eu-repo/semantics/publishedVersio
Electric field tunable superconductor-semiconductor coupling in Majorana nanowires
We study the effect of external electric fields on
superconductor-semiconductor coupling by measuring the electron transport in
InSb semiconductor nanowires coupled to an epitaxially grown Al superconductor.
We find that the gate voltage induced electric fields can greatly modify the
coupling strength, which has consequences for the proximity induced
superconducting gap, effective g-factor, and spin-orbit coupling, which all
play a key role in understanding Majorana physics. We further show that level
repulsion due to spin-orbit coupling in a finite size system can lead to
seemingly stable zero bias conductance peaks, which mimic the behavior of
Majorana zero modes. Our results improve the understanding of realistic
Majorana nanowire systems.Comment: 10 pages, 5 figures, supplemental information as ancillary fil
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