138 research outputs found
Near-zero-energy end states in topologically trivial spin-orbit coupled superconducting nanowires with a smooth confinement
A one-dimensional spin-orbit coupled nanowire with proximity-induced pairing
from a nearby s-wave superconductor may be in a topological nontrivial state,
in which it has a zero energy Majorana bound state at each end. We find that
the topological trivial phase may have fermionic end states with an
exponentially small energy, if the confinement potential at the wire's ends is
smooth. The possible existence of such near-zero energy levels implies that the
mere observation of a zero-bias peak in the tunneling conductance is not an
exclusive signature of a topological superconducting phase even in the ideal
clean single channel limit.Comment: 4 pages, 4 figure
Shot noise in Weyl semimetals
We study the effect of inelastic processes on the magneto-transport of a
quasi-one dimensional Weyl semi-metal, using a modified Boltzmann-Langevin
approach. The magnetic field drives a crossover to a ballistic regime in which
the propagation along the wire is dominated by the chiral anomaly, and the role
of fluctuations inside the sample is exponentially suppressed. We show that
inelastic collisions modify the parametric dependence of the current
fluctuations on the magnetic field. By measuring shot noise as a function of a
magnetic field, for different applied voltage, one can estimate the
electron-electron inelastic length .Comment: 7 pages, 1 figur
Superconductor insulator transition in thin films driven by an orbital parallel magnetic field effect
We study theoretically orbital effects of a parallel magnetic field applied
to a disordered superconducting film. We find that the field reduces the phase
stiffness and leads to strong quantum phase fluctuations driving the system
into an insulating behavior. This microscopic model shows that the critical
field decreases with the sheet resistance, in agreement with recent
experimental results. The predictions of this model can be used to discriminate
spin and orbital effects. We find that experiments conducted by A. Johansson
\textit{et al.} are more consistent with the orbital mechanism.Comment: 4 pages, 2 figure
Multiple Particle Scattering in Quantum Point Contacts
Recent experiments performed on weakly pinched quantum point contacts, have
shown a resistance that tend to decrease at low source drain voltage. We show
that enhanced Coulomb interactions, prompt by the presence of the point
contact, may lead to anomalously large multiple-particle scattering at finite
bias voltage. These processes tend to decrease at low voltage, and thus may
account for the observed reduction of the resistance. We concentrate on the
case of a normal point contact, and model it by a spinfull interacting
Tomonaga-Luttinger liquid, with a single impurity, connected to non interacting
leads. We find that sufficiently strong Coulomb interactions enhance
two-electron scattering, so as these dominate the conductance. Our calculation
shows that the effective charge, probed by the shot noise of such a system,
approaches a value proportional to e* = 2e at sufficiently large backscattering
current. This distinctive hallmark may be tested experimentally. We discuss
possible applications of this model to experiments conducted on Hall bars.Comment: 5 pages, 2 figure
Endstates in multichannel spinless p-wave superconducting wires
Multimode spinless p-wave superconducting wires with a width W much smaller
than the superconducting coherence length \xi are known to have multiple
low-energy subgap states localized near the wire's ends. Here we compare the
typical energies of such endstates for various terminations of the wire: A
superconducting wire coupled to a normal-metal stub, a weakly disordered
superconductor wire and a wire with smooth confinement. Depending on the
termination, we find that the energies of the subgap states can be higher or
lower than for the case of a rectangular wire with hard-wall boundaries.Comment: 10 pages, 7 figure
Pumped heat and charge statistics from Majorana braiding
We examine the heat and charge transport of a driven topological superconductor. Our particular system of interest consists of a Y-junction of topological superconducting wires, hosting non-Abelian Majorana zero modes at their edges. The system is contacted to two leads which act as continuous detectors of the system state. We calculate, via a scattering matrix approach, the full counting statistics of the driven heat transport, between two terminals contacted to the system, for small adiabatic driving and characterize the energy transport properties as a function of the system parameters (driving frequency, temperature). We find that the geometric, dynamic contribution to the pumped heat statistics results in a correction to the Gallavotti-Cohen type fluctuation theorem for quantum heat transfer. Notably, the correction term to the fluctuation theorem extends to cycles which correspond to topologically protected braiding of the Majorana zero modes. This geometric correction to the fluctuation theorem differs from its analogs in previously studied systems in that (i) it is nonvanishing for adiabatic cycles of the system's parameters, without the need for cyclic driving of the leads and (ii) it is insensitive to small, slow fluctuations of the driving parameters due to the topological protection of the braiding operation
Dynamic changes in optical and chemical properties of tar ball aerosols by atmospheric photochemical aging
Following wood pyrolysis, tar ball aerosols were laboratory generated from
wood tar separated into polar and nonpolar phases. Chemical information of
fresh tar balls was obtained from a high-resolution time-of-flight aerosol
mass spectrometer (HR-ToF-AMS) and single-particle laser desorption/resonance
enhanced multiphoton ionization mass spectrometry (SP-LD-REMPI-MS). Their
continuous refractive index (RI) between 365 and 425 nm was retrieved using
a broadband cavity enhanced spectroscopy (BBCES). Dynamic changes in the
optical and chemical properties for the nonpolar tar ball aerosols in
NOx-dependent photochemical process were investigated in an
oxidation flow reactor (OFR). Distinct differences in the chemical
composition of the fresh polar and nonpolar tar aerosols were identified.
Nonpolar tar aerosols contain predominantly high-molecular weight
unsubstituted and alkyl-substituted polycylic aromatic hydrocarbons (PAHs),
while polar tar aerosols consist of a high number of oxidized aromatic
substances (e.g., methoxy-phenols, benzenediol) with higher O : C ratios
and carbon oxidation states. Fresh tar balls have light absorption
characteristics similar to atmospheric brown carbon (BrC) aerosol with higher
absorption efficiency towards the UV wavelengths. The average retrieved RI is
1.661+0.020i and 1.635+0.003i for the nonpolar and polar tar aerosols,
respectively, with an absorption Ångström exponent (AAE) between 5.7
and 7.8 in the detected wavelength range. The RI fits a volume mixing rule
for internally mixed nonpolar/polar tar balls. The RI of the tar ball
aerosols decreased with increasing wavelength under photochemical oxidation.
Photolysis by UV light (254 nm), without strong oxidants in the system,
slightly decreased the RI and increased the oxidation state of the tar balls.
Oxidation under varying OH exposure levels and in the absence of
NOx diminished the absorption (bleaching) and increased the
O : C ratio of the tar balls. The photobleaching via OH radical initiated
oxidation is mainly attributed to decomposition of chromophoric aromatics,
nitrogen-containing organics, and high-molecular weight components in the
aged particles. Photolysis of nitrous oxide (N2O) was used to
simulate NOx-dependent photochemical aging of tar balls in
the OFR. Under high-NOx conditions with similar OH exposure,
photochemical aging led to the formation of organic nitrates, and increased
both oxidation degree and light absorption for the aged tar ball aerosols.
These observations suggest that secondary organic nitrate formation
counteracts the bleaching by OH radical photooxidation to eventually regain
some absorption of the aged tar ball aerosols. The atmospheric implication
and climate effects from tar balls upon various oxidation processes are
briefly discussed.</p
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