115 research outputs found
Nonzero temperature effects on antibunched photons emitted by a quantum point contact out of equilibrium
Electrical current fluctuations in a single-channel quantum point contact can
produce photons (at frequency omega close to the applied voltage V x e/hbar)
which inherit the sub-Poissonian statistics of the electrons. We extend the
existing zero-temperature theory of the photostatistics to nonzero temperature
T. The Fano factor F (the ratio of the variance and the average photocount) is
1 for T>T_c (bunched photons). The
crossover temperature T_c ~ Deltaomega x hbar/k_B is set by the band width
Deltaomega of the detector, even if hbar x Deltaomega << eV. This implies that
narrow-band detection of photon antibunching is hindered by thermal
fluctuations even in the low-temperature regime where thermal electron noise is
negligible relative to shot noise.Comment: 4 pages, 2 pages appendix, 3 figure
Scattering formula for the topological quantum number of a disordered multi-mode wire
The topological quantum number Q of a superconducting or chiral insulating
wire counts the number of stable bound states at the end points. We determine Q
from the matrix r of reflection amplitudes from one of the ends, generalizing
the known result in the absence of time-reversal and chiral symmetry to all
five topologically nontrivial symmetry classes. The formula takes the form of
the determinant, Pfaffian, or matrix signature of r, depending on whether r is
a real matrix, a real antisymmetric matrix, or a Hermitian matrix. We apply
this formula to calculate the topological quantum number of N coupled dimerized
polymer chains, including the effects of disorder in the hopping constants. The
scattering theory relates a topological phase transition to a conductance peak,
of quantized height and with a universal (symmetry class independent) line
shape. Two peaks which merge are annihilated in the superconducting symmetry
classes, while they reinforce each other in the chiral symmetry classes.Comment: 8 pages, 3 figures, this is the final, published versio
Scattering theory of topological insulators and superconductors
The topological invariant of a topological insulator (or superconductor) is
given by the number of symmetry-protected edge states present at the Fermi
level. Despite this fact, established expressions for the topological invariant
require knowledge of all states below the Fermi energy. Here, we propose a way
to calculate the topological invariant employing solely its scattering matrix
at the Fermi level without knowledge of the full spectrum. Since the approach
based on scattering matrices requires much less information than the
Hamiltonian-based approaches (surface versus bulk), it is numerically more
efficient. In particular, is better-suited for studying disordered systems.
Moreover, it directly connects the topological invariant to transport
properties potentially providing a new way to probe topological phases.Comment: 11 pages, 7 figures, 1 table, 3 ancilla videos. v2: updated figures
and references. v3: added appendix (published version). v4: fixed typos,
updated reference
Topological quantum number and critical exponent from conductance fluctuations at the quantum Hall plateau transition
The conductance of a two-dimensional electron gas at the transition from one
quantum Hall plateau to the next has sample-specific fluctuations as a function
of magnetic field and Fermi energy. Here we identify a universal feature of
these mesoscopic fluctuations in a Corbino geometry: The amplitude of the
magnetoconductance oscillations has an e^2/h resonance in the transition
region, signaling a change in the topological quantum number of the insulating
bulk. This resonance provides a signed scaling variable for the critical
exponent of the phase transition (distinct from existing positive definite
scaling variables).Comment: 6 pages, 9 figure
FOXN1 forms higher-order nuclear condensates displaced by mutations causing immunodeficiency
The transcription factor FOXN1 is a master regulator of thymic epithelial cell (TEC) development and function. Here, we demonstrate that FOXN1 expression is differentially regulated during organogenesis and participates in multimolecular nuclear condensates essential for the factor’s transcriptional activity. FOXN1’s C-terminal sequence regulates the diffusion velocity within these aggregates and modulates the binding to proximal gene regulatory regions. These dynamics are altered in a patient with a mutant FOXN1 that is modified in its C-terminal sequence. This mutant is transcriptionally inactive and acts as a dominant negative factor displacing wild-type FOXN1 from condensates and causing athymia and severe lymphopenia in heterozygotes. Expression of the mutated mouse ortholog selectively impairs mouse TEC differentiation, revealing a gene dose dependency for individual TEC subtypes. We have therefore identified the cause for a primary immunodeficiency disease and determined the mechanism by which this FOXN1 gain-of-function mutant mediates its dominant negative effect
Treating the placenta to prevent adverse effects of gestational hypoxia on fetal brain development.
Some neuropsychiatric disease, including schizophrenia, may originate during prenatal development, following periods of gestational hypoxia and placental oxidative stress. Here we investigated if gestational hypoxia promotes damaging secretions from the placenta that affect fetal development and whether a mitochondria-targeted antioxidant MitoQ might prevent this. Gestational hypoxia caused low birth-weight and changes in young adult offspring brain, mimicking those in human neuropsychiatric disease. Exposure of cultured neurons to fetal plasma or to secretions from the placenta or from model trophoblast barriers that had been exposed to altered oxygenation caused similar morphological changes. The secretions and plasma contained altered microRNAs whose targets were linked with changes in gene expression in the fetal brain and with human schizophrenia loci. Molecular and morphological changes in vivo and in vitro were prevented by a single dose of MitoQ bound to nanoparticles, which were shown to localise and prevent oxidative stress in the placenta but not in the fetus. We suggest the possibility of developing preventative treatments that target the placenta and not the fetus to reduce risk of psychiatric disease in later life
Transport in topological insulator nanowires
In this chapter we review our work on the theory of quantum transport in
topological insulator nanowires. We discuss both normal state properties and
superconducting proximity effects, including the effects of magnetic fields and
disorder. Throughout we assume that the bulk is insulating and inert, and work
with a surface-only theory. The essential transport properties are understood
in terms of three special modes: in the normal state, half a flux quantum along
the length of the wire induces a perfectly transmitted mode protected by an
effective time reversal symmetry; a transverse magnetic field induces chiral
modes at the sides of the wire, with different chiralities residing on
different sides protecting them from backscattering; and, finally, Majorana
zero modes are obtained at the ends of a wire in a proximity to a
superconductor, when combined with a flux along the wire. Some parts of our
discussion have a small overlap with the discussion in the review [Bardarson
and Moore, Rep. Prog. Phys., 76, 056501, (2013)]. We do not aim to give a
complete review of the published literature, instead the focus is mainly on our
own and directly related work.Comment: 22 pages, 8 figures; Chapter in "Topological Matter. Springer Series
in Solid-State Sciences, vol 190. Springer
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