184 research outputs found
A topological classification of interaction-driven spin pumps
When adiabatically varied in time, certain one-dimensional band insulators
allow for the quantized noiseless pumping of spin even in the presence of
strong spin orbit scattering. These spin pumps are closely related to the
quantum spin Hall system, and their properties are protected by a time-reversal
restriction on the pumping cycle. In this paper we study pumps formed of
one-dimensional insulators with a time-reversal restriction on the pumping
cycle and a bulk energy gap which arises due to interactions. We find that the
correlated gapped phase can lead to novel pumping properties. In particular,
systems with different ground states can give rise to different
classes of spin pumps, including a trivial class which does not pump quantized
spin and non-trivial classes allowing for the pumping of quantized spin
on average per cycle, where . We discuss an example
of a spin pump that transfers on average spin without transferring
charge.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
Sharp Superconductor-Insulator Transition in Short Wires
Recent experiments on short MoGe nanowires show a sharp
superconductor-insulator transition tuned by the normal state resistance of the
wire, with a critical resistance of . These results
are at odds with a broad range of theoretical work on Josephson-like systems
that predicts a smooth transition, tuned by the value of the resistance that
shunts the junction. We develop a self-consistent renormalization group
treatment of interacting phase-slips and their dual counterparts, correlated
cooper pair tunneling, beyond the dilute approximation. This analysis leads to
a very sharp transition with a critical resistance of . The addition of
the quasi-particles' resistance at finite temperature leads to a quantitative
agreement with the experimental results. This self-consistent renormalization
group method should also be applicable to other physical systems that can be
mapped onto similar sine-Gordon models, in the previously inaccessible
intermediate-coupling regime.Comment: 11 pages, 5 figures. Contribution to the proceedings of "Fluctuations
and phase transitions in superconductors", Nazareth Ilit, Israel, 2007. To be
published in Physica C, vol. 46
Dark states of quantum search cause imperfect detection
We consider a quantum walk where a detector repeatedly probes the system with
fixed rate until the walker is detected. This is a quantum version of
the first-passage problem. We focus on the total probability,
, that the particle is eventually detected in some target
state, for example on a node on a graph, after an arbitrary
number of detection attempts. Analyzing the dark and bright states for finite
graphs, and more generally for systems with a discrete spectrum, we provide an
explicit formula for in terms of the energy eigenstates
which is generically independent. We find that disorder in the
underlying Hamiltonian renders perfect detection: , and
then expose the role of symmetry with respect to sub-optimal detection.
Specifically, we give a simple upper bound for that is
controlled by the number of equivalent (with respect to the detection) states
in the system. We also extend our results to infinite systems, for example the
detection probability of a quantum walk on a line, which is -dependent
and less than half, well below Polya's optimal detection for a classical random
walk.Comment: 18 pages, 8 figures, revised and enlarged version, partly merged with
arxiv:1909.02114v1, corrected a mistake in Eq. (17
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
Gapless excitations in strongly fluctuating superconducting wires
We study the low temperature tunneling density of states of thin wires where
superconductivity is destroyed through quantum phase-slip proliferation.
Although this regime is believed to behave as an insulator, we show that for a
large temperature range this phase is characterized by a conductivity falling
off at most linearly with temperature, and has a gapless excitation spectrum.
This novel conducting phase results from electron-electron interaction induced
pair breaking. Also, it may help clarify the low temperature metallic features
found in films and wires whose bulk realization is superconducting.Comment: 4 pages, 2 figure
Global environmental implications of atmospheric methane removal through chlorine-mediated chemistry-climate interactions
Atmospheric methane is both a potent greenhouse gas and photochemically active, with approximately equal anthropogenic and natural sources. The addition of chlorine to the atmosphere has been proposed to mitigate global warming through methane reduction by increasing its chemical loss. However, the potential environmental impacts of such climate mitigation remain unexplored. Here, sensitivity studies are conducted to evaluate the possible effects of increasing reactive chlorine emissions on the methane budget, atmospheric composition and radiative forcing. Because of non-linear chemistry, in order to achieve a reduction in methane burden (instead of an increase), the chlorine atom burden needs to be a minimum of three times the estimated present-day burden. If the methane removal target is set to 20%, 45%, or 70% less global methane by 2050 compared to the levels in the Representative Concentration Pathway 8.5 scenario (RCP8.5), our modeling results suggest that additional chlorine fluxes of 630, 1250, and 1880 Tg Cl/year, respectively, are needed. The results show that increasing chlorine emissions also induces significant changes in other important climate forcers. Remarkably, the tropospheric ozone decrease is large enough that the magnitude of radiative forcing decrease is similar to that of methane. Adding 630, 1250, and 1880 Tg Cl/year to the RCP8.5 scenario, chosen to have the most consistent current-day trends of methane, will decrease the surface temperature by 0.2, 0.4, and 0.6 °C by 2050, respectively. The quantity and method in which the chlorine is added, its interactions with climate pathways, and the potential environmental impacts on air quality and ocean acidity, must be carefully considered before any action is taken
Photocatalytic chlorine atom production on mineral dustâsea spray aerosols over the North Atlantic
Active chlorine in the atmosphere is poorly constrained and so is its role in the oxidation of the potent greenhouse gas methane, causing uncertainty in global methane budgets. We propose a photocatalytic mechanism for chlorine atom production that occurs when Sahara dust mixes with sea spray aerosol. The mechanism is validated by implementation in a global atmospheric model and thereby explaining the episodic, seasonal, and location-dependent 13C depletion in CO in air samples from Barbados [J.E. Mak, G. Kra, T. Sandomenico, P. Bergamaschi, J. Geophys. Res. Atmos. 108 (2003)], which remained unexplained for decades. The production of Cl can also explain the anomaly in the CO:ethane ratio found at Cape Verde [K. A. Read et al., J. Geophys. Res. Atmos. 114 (2009)], in addition to explaining the observation of elevated HOCl [M. J. Lawler et al., Atmos. Chem. Phys. 11, 7617â7628 (2011)]. Our model finds that 3.8 Tg(Cl) yâ1 is produced over the North Atlantic, making it the dominant source of chlorine in the region; globally, chlorine production increases by 41%. The shift in the methane sink budget due to the increased role of Cl means that isotope-constrained topâdown models fail to allocate 12 Tg yâ1 (2% of total methane emissions) to 13C-depleted biological sources such as agriculture and wetlands. Since 2014, an increase in North African dust emissions has increased the 13C isotope of atmospheric CH4, thereby partially masking a much greater decline in this isotope, which has implications for the interpretation of the drivers behind the recent increase of methane in the atmosphere
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 Ă
ngström 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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