16,179 research outputs found
Steady states and edge state transport in topological Floquet-Bloch systems
We study the open system dynamics and steady states of two dimensional
Floquet topological insulators: systems in which a topological Floquet-Bloch
spectrum is induced by an external periodic drive. We solve for the bulk and
edge state carrier distributions, taking into account energy and momentum
relaxation through radiative recombination and electron-phonon interactions, as
well as coupling to an external lead. We show that the resulting steady state
resembles a topological insulator in the Floquet basis. The particle
distribution in the Floquet edge modes exhibits a sharp feature akin to the
Fermi level in equilibrium systems, while the bulk hosts a small density of
excitations. We discuss two-terminal transport and describe the regimes where
edge-state transport can be observed. Our results show that signatures of the
non-trivial topology persist in the non-equilibrium steady state.Comment: 9 pages, 4 figures + supplementary materia
Do the surface Fermi arcs in Weyl semimetals survive disorder?
We theoretically study the topological robustness of the surface physics
induced by Weyl Fermi-arc surface states in the presence of short-ranged
quenched disorder and surface-bulk hybridization. This is investigated with
numerically exact calculations on a lattice model exhibiting Weyl Fermi-arcs.
We find that the Fermi-arc surface states, in addition to having a finite
lifetime from disorder broadening, hybridize with nonperturbative bulk rare
states making them no longer bound to the surface (i.e. they lose their purely
surface spectral character). Thus, we provide strong numerical evidence that
the Weyl Fermi-arcs are not topologically protected from disorder. Nonetheless,
the surface chiral velocity is robust and survives in the presence of strong
disorder, persisting all the way to the Anderson-localized phase by forming
localized current loops that live within the localization length of the
surface. Thus, the Weyl semimetal is not topologically robust to the presence
of disorder, but the surface chiral velocity is.Comment: Single column; 24 pages, 12 figure
The anomalous Floquet-Anderson insulator as a non-adiabatic quantized charge pump
Periodically driven quantum systems provide a novel and versatile platform
for realizing topological phenomena. Among these are analogs of topological
insulators and superconductors, attainable in static systems; however, some of
these phenomena are unique to the periodically driven case. Here, we show that
disordered, periodically driven systems admit an "anomalous" two dimensional
phase, whose quasi-energy spectrum consists of chiral edge modes that coexist
with a fully localized bulk - an impossibility for static Hamiltonians. This
unique situation serves as the basis for a new topologically-protected
non-equilibrium transport phenomenon: quantized non-adiabatic charge pumping.
We identify the bulk topological invariant that characterizes the new phase
(which we call the "anomalous Floquet Anderson Insulator", or AFAI). We provide
explicit models which constitute a proof of principle for the existence of the
new phase. Finally, we present evidence that the disorder-driven transition
from the AFAI to a trivial, fully localized phase is in the same universality
class as the quantum Hall plateau transition
A Gravitational Wave Background from Reheating after Hybrid Inflation
The reheating of the universe after hybrid inflation proceeds through the
nucleation and subsequent collision of large concentrations of energy density
in the form of bubble-like structures moving at relativistic speeds. This
generates a significant fraction of energy in the form of a stochastic
background of gravitational waves, whose time evolution is determined by the
successive stages of reheating: First, tachyonic preheating makes the amplitude
of gravity waves grow exponentially fast. Second, bubble collisions add a new
burst of gravitational radiation. Third, turbulent motions finally sets the end
of gravitational waves production. From then on, these waves propagate
unimpeded to us. We find that the fraction of energy density today in these
primordial gravitational waves could be significant for GUT-scale models of
inflation, although well beyond the frequency range sensitivity of
gravitational wave observatories like LIGO, LISA or BBO. However, low-scale
models could still produce a detectable signal at frequencies accessible to BBO
or DECIGO. For comparison, we have also computed the analogous gravitational
wave background from some chaotic inflation models and obtained results similar
to those found by other groups. The discovery of such a background would open a
new observational window into the very early universe, where the details of the
process of reheating, i.e. the Big Bang, could be explored. Moreover, it could
also serve in the future as a new experimental tool for testing the
Inflationary Paradigm.Comment: 22 pages, 18 figures, uses revtex
Controlled Population of Floquet-Bloch States via Coupling to Bose and Fermi Baths
External driving is emerging as a promising tool for exploring new phases in
quantum systems. The intrinsically non-equilibrium states that result, however,
are challenging to describe and control. We study the steady states of a
periodically driven one-dimensional electronic system, including the effects of
radiative recombination, electron-phonon interactions, and the coupling to an
external fermionic reservoir. Using a kinetic equation for the populations of
the Floquet eigenstates, we show that the steady-state distribution can be
controlled using the momentum and energy relaxation pathways provided by the
coupling to phonon and Fermi reservoirs. In order to utilize the latter, we
propose to couple the system and reservoir via an energy filter which
suppresses photon-assisted tunneling. Importantly, coupling to these reservoirs
yields a steady state resembling a band insulator in the Floquet basis. The
system exhibits incompressible behavior, while hosting a small density of
excitations. We discuss transport signatures, and describe the regimes where
insulating behavior is obtained. Our results give promise for realizing Floquet
topological insulators.Comment: 24 pages, 7 figures; with appendice
Evolution and CNO yields of Z=10^-5 stars and possible effects on CEMP production
Our main goals are to get a deeper insight into the evolution and final fates
of intermediate-mass, extremely metal-poor (EMP) stars. We also aim to
investigate their C, N, and O yields. Using the Monash University Stellar
Evolution code we computed and analysed the evolution of stars of metallicity Z
= 10^-5 and masses between 4 and 9 M_sun, from their main sequence until the
late thermally pulsing (super) asymptotic giant branch, TP-(S)AGB phase. Our
model stars experience a strong C, N, and O envelope enrichment either due to
the second dredge-up, the dredge-out phenomenon, or the third dredge-up early
during the TP-(S)AGB phase. Their late evolution is therefore similar to that
of higher metallicity objects. When using a standard prescription for the mass
loss rates during the TP-(S)AGB phase, the computed stars lose most of their
envelopes before their cores reach the Chandrasekhar mass, so our standard
models do not predict the occurrence of SNI1/2 for Z = 10^-5 stars. However, we
find that the reduction of only one order of magnitude in the mass-loss rates,
which are particularly uncertain at this metallicity, would prevent the
complete ejection of the envelope, allowing the stars to either explode as an
SNI1/2 or become an electron-capture SN. Our calculations stop due to an
instability near the base of the convective envelope that hampers further
convergence and leaves remnant envelope masses between 0.25 M_sun for our 4
M_sun model and 1.5 M_sun for our 9 M_sun model. We present two sets of C, N,
and O yields derived from our full calculations and computed under two
different assumptions, namely, that the instability causes a practically
instant loss of the remnant envelope or that the stars recover and proceed with
further thermal pulses. Our results have implications for the early chemical
evolution of the Universe.Comment: 12 pages, 13 figures, accepted for publication in A&
HS 0139+0559, HS 0229+8016, HS 0506+7725, and HS 0642+5049 : four new long-period cataclysmic variables
We present time-resolved optical spectroscopy and photometry of four relatively bright (V ⌠14.0â15.5) long-period cataclysmic variables(CVs) discovered in the Hamburg Quasar Survey: HS 0139+0559, HS 0229+8016, HS 0506+7725, and HS 0642+5049. Their respective orbital periods, 243.69
± 0.49 min, 232.550 ± 0.049 min, 212.7 ± 0.2 min, and 225.90
± 0.23 min are determined from radial velocity and photometric variability studies. HS 0506+7725 is characterised by strong Balmer and He emission lines, short-period (âŒ10â20 min) flickering, and weak X-ray emission in the ROSAT All Sky Survey. The detection of a deep low state (B 18.5) identifies HS 0506+7725 as a member of the VY Scl
stars. HS 0139+0559, HS 0229+8016, and HS 0642+5049 display thick-disc like spectra and no or only weak flickering activity. HS 0139+0559 and HS 0229+8016 exhibit clean quasi-sinusoidal radial velocity variations of their emission lines but no or very little orbital photometricvariability. In contrast, we detect no radial velocity variation in HS 0642+5049 but a noticeable orbital brightness variation. We identify all three systems either as UX UMa-type novalike variables or as Z Cam-type dwarf novae. Our identification of these four new systems underlines that the currently known sample of CVs is rather incomplete even for bright objects. The four new systems add to the clustering of orbital periods in the 3â4 h range found in the sample of HQS selected CVs, and we discuss the large incidence of magnetic CVs and VY Scl/SW Sex stars found in this period range among the known population of CVs
Photoassisted tunneling from free-standing GaAs thin films into metallic surfaces
The tunnel photocurrent between a gold surface and a free-standing
semiconducting thin film excited from the rear by above bandgap light has been
measured as a function of applied bias, tunnel distance and excitation light
power. The results are compared with the predictions of a model which includes
the bias dependence of the tunnel barrier height and the bias-induced decrease
of surface recombination velocity. It is found that i) the tunnel photocurrent
from the conduction band dominates that from surface states. ii) At large
tunnel distance the exponential bias dependence of the current is explained by
that of the tunnel barrier height, while at small distance the change of
surface recombination velocity is dominant
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