1,068 research outputs found
Quantum magneto-oscillations in a two-dimensional Fermi liquid
Quantum magneto-oscillations provide a powerfull tool for quantifying
Fermi-liquid parameters of metals. In particular, the quasiparticle effective
mass and spin susceptibility are extracted from the experiment using the
Lifshitz-Kosevich formula, derived under the assumption that the properties of
the system in a non-zero magnetic field are determined uniquely by the
zero-field Fermi-liquid state. This assumption is valid in 3D but, generally
speaking, erroneous in 2D where the Lifshitz-Kosevich formula may be applied
only if the oscillations are strongly damped by thermal smearing and disorder.
In this work, the effects of interactions and disorder on the amplitude of
magneto-oscillations in 2D are studied. It is found that the effective mass
diverges logarithmically with decreasing temperature signaling a deviation from
the Fermi-liquid behavior. It is also shown that the quasiparticle lifetime due
to inelastic interactions does not enter the oscillation amplitude, although
these interactions do renormalize the effective mass. This result provides a
generalization of the Fowler-Prange theorem formulated originally for the
electron-phonon interaction.Comment: 4 pages, 1 figur
Collisions of charged black holes
We perform fully nonlinear numerical simulations of charged-black-hole collisions, described by the Einstein-Maxwell equations, and contrast the results against analytic expectations. We focus on head-on collisions of nonspinning black holes, starting from rest and with the same charge-to-mass ratio, Q/M. The addition of charge to black holes introduces a new interesting channel of radiation and dynamics, most of which seem to be captured by Newtonian dynamics and flat-space intuition. The waveforms can be qualitatively described in terms of three stages: (i) an infall phase prior to the formation of a common apparent horizon; (ii) a nonlinear merger phase that corresponds to a peak in gravitational and electromagnetic energy; (iii) the ringdown marked by an oscillatory pattern with exponentially decaying amplitude and characteristic frequencies that are in good agreement with perturbative predictions. We observe that the amount of gravitational-wave energy generated throughout the collision decreases by about 3 orders of magnitude as the charge-to-mass ratio Q/M is increased from 0 to 0.98. We interpret this decrease as a consequence of the smaller accelerations present for larger values of the charge. In contrast, the ratio of energy carried by electromagnetic to gravitational radiation increases, reaching about 22% for the maximum Q/M ratio explored, which is in good agreement with analytic predictions
Entropy and de Haas-van Alphen oscillations of a three-dimensional marginal Fermi liquid
We study de Haas-van Alphen oscillations in a marginal Fermi liquid resulting
from a three-dimensional metal tuned to a quantum-critical point (QCP). We show
that the conventional approach based on extensions of the Lifshitz-Kosevich
formula for the oscillation amplitudes becomes inapplicable when the
correlation length exceeds the cyclotron radius. This breakdown is due to (i)
non-analytic finite-temperature contributions to the fermion self-energy (ii)
an enhancement of the oscillatory part of the self-energy by quantum
fluctuations, and (iii) non-trivial dynamical scaling laws associated with the
quantum critical point. We properly incorporate these effects within the
Luttinger-Ward-Eliashberg framework for the thermodynamic potential by treating
the fermionic and bosonic contributions on equal footing. As a result, we
obtain the modified expressions for the oscillations of entropy and
magnetization that remain valid in the non-Fermi liquid regime.Comment: 20+6 pages, 6 figure
Lectures on holographic non-Fermi liquids and quantum phase transitions
In these lecture notes we review some recent attempts at searching for
non-Fermi liquids and novel quantum phase transitions in holographic systems
using gauge/gravity duality. We do this by studying the simplest finite density
system arising from the duality, obtained by turning on a nonzero chemical
potential for a U(1) global symmetry of a CFT, and described on the gravity
side by a charged black hole. We address the following questions of such a
finite density system:
1. Does the system have a Fermi surface? What are the properties of low
energy excitations near the Fermi surface?
2. Does the system have an instability to condensation of scalar operators?
What is the critical behavior near the corresponding quantum critical point?
We find interesting parallels with those of high T_c cuprates and heavy
electron systems. Playing a crucial role in our discussion is a universal
intermediate-energy phase, called a "semi-local quantum liquid", which
underlies the non-Fermi liquid and novel quantum critical behavior of a system.
It also provides a novel mechanism for the emergence of lower energy states
such as a Fermi liquid or a superconductor.Comment: 70 pages. Based on lectures given by Hong Li
Massive disk formation in the tidal disruption of a neutron star by a nearly extremal black hole
Black hole-neutron star (BHNS) binaries are important sources of
gravitational waves for second-generation interferometers, and BHNS mergers are
also a proposed engine for short, hard gamma-ray bursts. The behavior of both
the spacetime (and thus the emitted gravitational waves) and the neutron star
matter in a BHNS merger depend strongly and nonlinearly on the black hole's
spin. While there is a significant possibility that astrophysical black holes
could have spins that are nearly extremal (i.e. near the theoretical maximum),
to date fully relativistic simulations of BHNS binaries have included
black-hole spins only up to =0.9, which corresponds to the black hole
having approximately half as much rotational energy as possible, given the
black hole's mass. In this paper, we present a new simulation of a BHNS binary
with a mass ratio and black-hole spin =0.97, the highest simulated
to date. We find that the black hole's large spin leads to the most massive
accretion disk and the largest tidal tail outflow of any fully relativistic
BHNS simulations to date, even exceeding the results implied by extrapolating
results from simulations with lower black-hole spin. The disk appears to be
remarkably stable. We also find that the high black-hole spin persists until
shortly before the time of merger; afterwards, both merger and accretion spin
down the black hole.Comment: 20 pages, 10 figures, submitted to Classical and Quantum Gravit
Efficient Dynamic Importance Sampling of Rare Events in One Dimension
Exploiting stochastic path integral theory, we obtain \emph{by simulation}
substantial gains in efficiency for the computation of reaction rates in
one-dimensional, bistable, overdamped stochastic systems. Using a well-defined
measure of efficiency, we compare implementations of ``Dynamic Importance
Sampling'' (DIMS) methods to unbiased simulation. The best DIMS algorithms are
shown to increase efficiency by factors of approximately 20 for a
barrier height and 300 for , compared to unbiased simulation. The
gains result from close emulation of natural (unbiased), instanton-like
crossing events with artificially decreased waiting times between events that
are corrected for in rate calculations. The artificial crossing events are
generated using the closed-form solution to the most probable crossing event
described by the Onsager-Machlup action. While the best biasing methods require
the second derivative of the potential (resulting from the ``Jacobian'' term in
the action, which is discussed at length), algorithms employing solely the
first derivative do nearly as well. We discuss the importance of
one-dimensional models to larger systems, and suggest extensions to
higher-dimensional systems.Comment: version to be published in Phys. Rev.
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