1,550 research outputs found
Fermi Edge Resonances in Non-equilibrium States of Fermi Gases
We formulate the problem of the Fermi Edge Singularity in non-equilibrium
states of a Fermi gas as a matrix Riemann-Hilbert problem with an integrable
kernel. This formulation is the most suitable for studying the singular
behavior at each edge of non-equilibrium Fermi states by means of the method of
steepest descent, and also reveals the integrable structure of the problem. We
supplement this result by extending the familiar approach to the problem of the
Fermi Edge Singularity via the bosonic representation of the electronic
operators to non-equilibrium settings. It provides a compact way to extract the
leading asymptotes.Comment: Accepted for publication, J. Phys.
The trajectory of loneliness in UK young adults during the summer to winter months of COVID-19
Current research has shown that young adults are at the greatest risk of loneliness during the pandemic. Drawing a sample from the Understanding Society COVID-19 survey, this study investigated the trajectory of loneliness in young adults (aged 18-25) from June to November 2020 and its association with emotional support, demographic and health factors. The analytic sample included 419 young adults (296 females; 123 males). Growth curve modelling revealed a U-shape longitudinal trend in self-reported loneliness, with a sharp rise during the winter months under the national lockdown. Young adults with long-standing physical or mental health conditions were more likely to report feeling lonely. Those with a lower household income, who were unemployed or not in school reported higher levels of loneliness. Gender was found to moderate the association between emotional support and loneliness. While greater emotional support was associated with less loneliness in males, no association was shown for females. The current findings add to our understanding of how the pandemic has affected the mental health of young adults and the differential influences of emotional support as a potential coping strategy for males and females
Coherent perfect absorption and reflection in slow-light waveguides
We identify a family of unusual slow-light modes occurring in lossy
multi-mode grating waveguides, for which either the forward or backward mode
components, or both, become degenerate. In the fully-degenerate case, by
varying the wave amplitudes in a uniform input waveguide, one can modulate
between coherent perfect absorption (zero reflection) and perfect reflection.
The perfectly-absorbed wave has anomalously short absorption length, scaling as
the inverse 1/3 power of the absorptivity
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The structure theory of nilspaces I
This paper forms the first part of a series by the authors [GMV2,GMV3]
concerning the structure theory of nilspaces of Antol\'in Camarena and Szegedy.
A nilspace is a compact space together with closed collections of cubes
, satisfying some natural axioms.
Antol\'in Camarena and Szegedy proved that from these axioms it follows that
(certain) nilspaces are isomorphic (in a strong sense) to an inverse limit of
nilmanifolds. The aim of our project is to provide a new self-contained
treatment of this theory and give new applications to topological dynamics.
This paper provides an introduction to the project from the point of view of
applications to higher order Fourier analysis. We define and explain the basic
definitions and constructions related to cubespaces and nilspaces and develop
the weak structure theory, which is the first stage of the proof of the main
structure theorem for nilspaces. Vaguely speaking, this asserts that a nilspace
can be built as a finite tower of extensions where each of the successive
fibers is a compact abelian group.
We also make some modest innovations and extensions to this theory. In
particular, we consider a class of maps that we term fibrations, which are
essentially equivalent to what are termed fiber-surjective morphisms by
Anatol\'in Camarena and Szegedy, and we formulate and prove a relative analogue
of the weak structure theory alluded to above for these maps. These results
find applications elsewhere in the project.Royal Societ
Zero bias anomaly in a two dimensional granular insulator
We compare tunneling density of states (TDOS) into two ultrathin Ag films,
one uniform and one granular, for different degrees of disorder. The uniform
film shows a crossover from Altshuler-Aronov (AA) zero bias anomaly to Efros
Shklovskii (ES) like Coulomb gap as the disorder is increased. The granular
film, on the other hand, exhibits AA behavior even deeply in the insulating
regime. We analyze the data and find that granularity introduces a new regime
for the TDOS. While the conductivity is dominated by hopping between clusters
of grains and is thus insulating, the TDOS probes the properties of an
individual cluster which is "metallic".Comment: 4 pages, 4 figure
Suppression of geometrical barrier in crystals by Josephson vortex stacks
Differential magneto-optics are used to study the effect of dc in-plane
magnetic field on hysteretic behavior due to geometrical barriers in
crystals. In absence of in-plane field a vortex
dome is visualized in the sample center surrounded by barrier-dominated
flux-free regions. With in-plane field, stacks of Josephson vortices form
vortex chains which are surprisingly found to protrude out of the dome into the
vortex-free regions. The chains are imaged to extend up to the sample edges,
thus providing easy channels for vortex entry and for drain of the dome through
geometrical barrier, suppressing the magnetic hysteresis. Reduction of the
vortex energy due to crossing with Josephson vortices is evaluated to be about
two orders of magnitude too small to account for the formation of the
protruding chains. We present a model and numerical calculations that
qualitatively describe the observed phenomena by taking into account the
demagnetization effects in which flux expulsion from the pristine regions
results in vortex focusing and in the chain protrusion. Comparative
measurements on a sample with narrow etched grooves provide further support to
the proposed model.Comment: 12 figures (low res.) Higher resolution figures are available at the
Phys Rev B version. Typos correcte
Dynamics of waves in 1D electron systems: Density oscillations driven by population inversion
We explore dynamics of a density pulse induced by a local quench in a
one-dimensional electron system. The spectral curvature leads to an "overturn"
(population inversion) of the wave. We show that beyond this time the density
profile develops strong oscillations with a period much larger than the Fermi
wave length. The effect is studied first for the case of free fermions by means
of direct quantum simulations and via semiclassical analysis of the evolution
of Wigner function. We demonstrate then that the period of oscillations is
correctly reproduced by a hydrodynamic theory with an appropriate dispersive
term. Finally, we explore the effect of different types of electron-electron
interaction on the phenomenon. We show that sufficiently strong interaction
[ where is the fermionic mass and the relevant spatial
scale] determines the dominant dispersive term in the hydrodynamic equations.
Hydrodynamic theory reveals crucial dependence of the density evolution on the
relative sign of the interaction and the density perturbation.Comment: 20 pages, 13 figure
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Challenging the meaning of the past from below: a typology for comparative research on memory activists
Memory activists have recently received more scholarly and public attention, but the concept lacks conceptual clarity. In this article, we articulate an analytical framework for studying memory activists, proposing a relatively narrow definition: “Memory activists” strategically commemorate the past to challenge (or protect) dominant views on the past and the institutions that represent them. Their goal is mnemonic change or to resist change. We locate scholarship on memory activists at the intersection of memory studies and social movement studies. We introduce a typology for comparative analysis of memory activism according to activist roles, temporality, and modes of interaction with other actors in memory politics, and illustrate this with a diverse set of empirical examples. We contend that the analytical utility of the concept of the “memory activist” is premised on its value-neutrality, and in particular, its application to both pro and anti-democratic cases of activism
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The structure theory of nilspaces ii: Representation as nilmanifolds
This paper forms the second part of a series by the authors [GMV1,GMV3]
concerning the structure theory of nilspaces of Antol\'in Camarena and Szegedy.
A nilspace is a compact space together with closed collections of cubes
, satisfying some natural axioms. From
these axioms it follows that a nilspace can be built as a finite tower of
extensions where each of the successive fibers is a compact abelian group.
Our main result is a new proof of a result due to Antol\'in Camarena and
Szegedy [CS12], stating that if each of these groups is a torus then is
isomorphic (in a strong sense) to a nilmanifold . We also extend the
theorem to a setting where the nilspace arises from a dynamical system .
These theorems are a key stepping stone towards the general structure theorem
in [GMV3] (which again closely resembles the main theorem of [CS12]).
The main technical tool, enabling us to deduce algebraic information from
topological data, consists of existence and uniqueness results for solutions of
certain natural functional equations, again modelled on the theory in [CS12]
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