780 research outputs found
Kinetic simulations of ladder climbing by electron plasma waves
The energy of plasma waves can be moved up and down the spectrum using
chirped modulations of plasma parameters, which can be driven by external
fields. Depending on whether the wave spectrum is discrete (bounded plasma) or
continuous (boundless plasma), this phenomenon is called ladder climbing (LC)
or autoresonant acceleration of plasmons. It was first proposed by Barth
\textit{et al.} [PRL \textbf{115}, 075001 (2015)] based on a linear fluid
model. In this paper, LC of electron plasma waves is investigated using fully
nonlinear Vlasov-Poisson simulations of collisionless bounded plasma. It is
shown that, in agreement with the basic theory, plasmons survive substantial
transformations of the spectrum and are destroyed only when their wave numbers
become large enough to trigger Landau damping. Since nonlinear effects decrease
the damping rate, LC is even more efficient when practiced on structures like
quasiperiodic Bernstein-Greene-Kruskal (BGK) waves rather than on Langmuir
waves \textit{per~se}
Superfluidity and dimerization in a multilayered system of fermionic polar molecules
We consider a layered system of fermionic molecules with permanent dipole
moments aligned by an external field. The dipole interactions between fermions
in adjacent layers are attractive and induce inter-layer pairing. Due to
competition for pairing among adjacent layers, the mean-field ground state of
the layered system is a dimerized superfluid, with pairing only between
every-other layer. We construct an effective Ising-XY lattice model that
describes the interplay between dimerization and superfluid phase fluctuations.
In addition to the dimerized superfluid ground state, and high temperature
normal state, at intermediate temperature, we find an unusual dimerized
"pseudogap" state with only short-range phase coherence. We propose light
scattering experiments to detect dimerization.Comment: 4 pages main text + 3 pages supplemental Appendices, 4 figure
Theory of the striped superconductor
We define a distinct phase of matter, a "pair density wave" (PDW), in which
the superconducting order parameter varies periodically as a function of
position such that when averaged over the center of mass position, all
components of vanish identically. Specifically, we study the simplest,
unidirectional PDW, the "striped superconductor," which we argue may be at the
heart of a number of spectacular experimental anomalies that have been observed
in the failed high temperature superconductor, La BaCuO. We
present a solvable microscopic model with strong electron-electron interactions
which supports a PDW groundstate. We also discuss, at the level of Landau
theory, the nature of the coupling between the PDW and other order parameters,
and the origins and some consequences of the unusual sensitivity of this state
to quenched disorder.Comment: 16 pages, 3 figures, 1 table; Journal ref. adde
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Benchmarking File System Benchmarking: It *IS* Rocket Science
The quality of file system benchmarking has not improved in over a decade of intense research spanning hundreds of publications. Researchers repeatedly use a wide range of poorly designed benchmarks, and in most cases, develop their own ad-hoc benchmarks. Our community lacks a definition of what we want to benchmark in a file system. We propose several dimensions of file system benchmarking and review the wide range of tools and techniques in widespread use. We experimentally show that even the simplest of benchmarks can be fragile, producing performance results spanning orders of magnitude. It is our hope that this paper will spur serious debate in our community, leading to action that can improve how we evaluate our file and storage systems.Engineering and Applied Science
The Fermi Problem in Discrete Systems
The Fermi two-atom problem illustrates an apparent causality violation in
Quantum Field Theory which has to do with the nature of the built in
correlations in the vacuum. It has been a constant subject of theoretical
debate and discussions during the last few decades. Nevertheless, although the
issues at hand could in principle be tested experimentally, the smallness of
such apparent violations of causality in Quantum Electrodynamics prevented the
observation of the predicted effect. In the present paper we show that the
problem can be simulated within the framework of discrete systems that can be
manifested, for instance, by trapped atoms in optical lattices or trapped ions.
Unlike the original continuum case, the causal structure is no longer sharp.
Nevertheless, as we show, it is possible to distinguish between "trivial"
effects due to "direct" causality violations, and the effects associated with
Fermi's problem, even in such discrete settings. The ability to control
externally the strength of the atom-field interactions, enables us also to
study both the original Fermi problem with "bare atoms", as well as correction
in the scenario that involves "dressed" atoms. Finally, we show that in
principle, the Fermi effect can be detected using trapped ions.Comment: Second version - minor change
Violation of the zeroth law of thermodynamics for a non-ergodic interaction
The phenomenon described by our title should surprise no one. What may be
surprising though is how easy it is to produce a quantum system with this
feature; moreover, that system is one that is often used for the purpose of
showing how systems equilibrate. The violation can be variously manifested. In
our detailed example, bringing a detuned 2-level system into contact with a
monochromatic reservoir does not cause it to relax to the reservoir
temperature; rather, the system acquires the reservoir's
level-occupation-ratio
The Kibble-Zurek Problem: Universality and the Scaling Limit
Near a critical point, the equilibrium relaxation time of a system diverges
and any change of control/thermodynamic parameters leads to non-equilibrium
behavior. The Kibble-Zurek problem is to determine the dynamical evolution of
the system parametrically close to its critical point when the change is
parametrically slow. The non-equilibrium behavior in this limit is controlled
entirely by the critical point and the details of the trajectory of the system
in parameter space (the protocol) close to the critical point. Together, they
define a universality class consisting of critical exponents-discussed in the
seminal work by Kibble and Zurek-and scaling functions for physical quantities,
which have not been discussed hitherto. In this article, we give an extended
and pedagogical discussion of the universal content in the Kibble-Zurek
problem. We formally define a scaling limit for physical quantities near
classical and quantum transitions for different sets of protocols. We report
computations of a few scaling functions in model Gaussian and large-N problems
and prove their universality with respect to protocol choice. We also introduce
a new protocol in which the critical point is approached asymptotically at late
times with the system marginally out of equilibrium, wherein logarithmic
violations to scaling and anomalous dimensions occur even in the simple
Gaussian problem.Comment: 19 pages,10 figure
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