519 research outputs found
Temperature Dependence of Interlayer Magnetoresistance in Anisotropic Layered Metals
Studies of interlayer transport in layered metals have generally made use of
zero temperature conductivity expressions to analyze angle-dependent
magnetoresistance oscillations (AMRO). However, recent high temperature AMRO
experiments have been performed in a regime where the inclusion of finite
temperature effects may be required for a quantitative description of the
resistivity. We calculate the interlayer conductivity in a layered metal with
anisotropic Fermi surface properties allowing for finite temperature effects.
We find that resistance maxima are modified by thermal effects much more
strongly than resistance minima. We also use our expressions to calculate the
interlayer resistivity appropriate to recent AMRO experiments in an overdoped
cuprate which led to the conclusion that there is an anisotropic, linear in
temperature contribution to the scattering rate and find that this conclusion
is robust.Comment: 8 pages, 4 figure
Hot electrons in low-dimensional phonon systems
A simple bulk model of electron-phonon coupling in metals has been
surprisingly successful in explaining experiments on metal films that actually
involve surface- or other low-dimensional phonons. However, by an exact
application of this standard model to a semi-infinite substrate with a free
surface, making use of the actual vibrational modes of the substrate, we show
that such agreement is fortuitous, and that the model actually predicts a
low-temperature crossover from the familiar T^5 temperature dependence to a
stronger T^6 log T scaling. Comparison with existing experiments suggests a
widespread breakdown of the standard model of electron-phonon thermalization in
metals
The holographic spectral function in non-equilibrium states
We develop holographic prescriptions for obtaining spectral functions in
non-equilibrium states and space-time dependent non-equilibrium shifts in the
energy and spin of quasi-particle like excitations. We reproduce strongly
coupled versions of aspects of non-equilibrium dynamics of Fermi surfaces in
Landau's Fermi-liquid theory. We find that the incoming wave boundary condition
at the horizon does not suffice to obtain a well-defined perturbative expansion
for non-equilibrium observables. Our prescription, based on analysis of
regularity at the horizon, allows such a perturbative expansion to be achieved
nevertheless and can be precisely formulated in a universal manner independent
of the non-equilibrium state, provided the state thermalizes. We also find that
the non-equilibrium spectral function furnishes information about the
relaxation modes of the system. Along the way, we argue that in a typical
non-supersymmetric theory with a gravity dual, there may exist a window of
temperature and chemical potential at large N, in which a generic
non-equilibrium state can be characterized by just a finitely few operators
with low scaling dimensions, even far away from the hydrodynamic limit.Comment: revtex; 43 pages, 2 figures; typos corrected, accepted for
publication in PR
Solution of the Boltzmann equation in a random magnetic field
A general framework for solving the Boltzmann equation for a 2-dimensional
electron gas (2DEG) in random magnetic fields is presented, when the random
fields are included in the driving force. The formalism is applied to some
recent experiments, and a possible extension to composite fermions at
is discussed.Comment: 15 pages, Revtex 3.0. The 5 postscript figures can be obtained from
our WWW-server: http://roemer.fys.ku.dk/randbolt.htm , or on request from the
author
The Permafrost and Organic LayEr module for Forest Models (POLE-FM) 1.0
Climate change and increased fire are eroding the resilience of boreal forests. This is problematic because boreal vegetation and the cold soils underneath store approximately 30 % of all terrestrial carbon. Society urgently needs projections of where, when, and why boreal forests are likely to change. Permafrost (i.e., subsurface material that remains frozen for at least two consecutive years) and the thick soil-surface organic layers (SOLs) that insulate permafrost are important controls of boreal forest dynamics and carbon cycling. However, both are rarely included in process-based vegetation models used to simulate future ecosystem trajectories. To address this challenge, we developed a computationally efficient permafrost and SOL module that operates at fine spatial (1 ha) and temporal (daily) resolutions. The module mechanistically simulates daily changes in depth to permafrost, annual SOL accumulation, and their complex effects on boreal forest structure and functions. We coupled the module to an established forest landscape model, iLand, and benchmarked the model in interior Alaska at spatial scales of stands (1 ha) to landscapes (61,000 ha) and over temporal scales of days to centuries. The coupled model could generate intra- and inter-annual patterns of snow accumulation and active layer depth (portion of soil column that thaws throughout the year) consistent with independent observations in 17 instrumented forest stands. The model was also skilled at representing the distribution of near-surface permafrost presence in a topographically complex landscape. We simulated 34.6 % of forested area in the landscape as underlain by permafrost; a close match to the estimated 33.4 % from the benchmarking product. We further determined that the model could accurately simulate moss biomass, SOL accumulation, fire activity, tree-species composition, and stand structure at the landscape scale. Modular and flexible representations of key biophysical processes that underpin 21st-century ecological change are an essential next step in vegetation simulation to reduce uncertainty in future projections and to support innovative environmental decision making. We show that coupling a new permafrost and SOL module to an existing forest landscape model increases the model’s utility for projecting forest futures at high latitudes. Process-based models that represent relevant dynamics will catalyze opportunities to address previously intractable questions about boreal forest resilience, biogeochemical cycling, and feedbacks to regional and global climate. </p
Thermalization of magnons in yttrium-iron garnet: nonequilibrium functional renormalization group approach
Using a nonequilibrium functional renormalization group (FRG) approach we
calculate the time evolution of the momentum distribution of a magnon gas in
contact with a thermal phonon bath. As a cutoff for the FRG procedure we use a
hybridization parameter {\Lambda} giving rise to an artificial damping of the
phonons. Within our truncation of the FRG flow equations the time evolution of
the magnon distribution is obtained from a rate equation involving
cutoff-dependent nonequilibrium self-energies, which in turn satisfy FRG flow
equations depending on cutoff-dependent transition rates. Our approach goes
beyond the Born collision approximation and takes the feedback of the magnons
on the phonons into account. We use our method to calculate the thermalization
of a quasi two-dimensional magnon gas in the magnetic insulator yttrium-iron
garnet after a highly excited initial state has been generated by an external
microwave field. We obtain good agreement with recent experiments.Comment: 16 pages, 6 figures, final versio
Counting statistics of interfering Bose-Einstein condensates
A method is presented that is able to predict the probability of outcomes of
snapshot measurements, such as the images of the instantaneous particle density
distribution in a quantum many-body system. It is shown that a gauge-like
transformation of the phase of the many-body wave function allows one to
construct a probability generating functional, the Fourier transform of which
with respect to the "gauge" field returns the joint probability distribution to
detect any given number of particles at various locations. The method is
applied to the problem of interference of two independent clouds of
Bose-Einstein condensates, where the initially separated clouds with fixed
boson numbers expand and the density profile image of the overlapping clouds is
registered. In the limit of large particle numbers, the probability to observe
a particular image of the density profile is shown to be given by a sum of
partial probability distributions, each of which corresponds to a noisy image
of interference of two matter waves with definite phase difference. In
agreement with earlier theoretical arguments, interference fringes are,
therefore, expected in any single shot measurement, the fringe pattern randomly
varying from run to run. These results conform to the physical picture where
the Bose-Einstein clouds are in spontaneously symmetry broken states, the
hidden phases of which are revealed by the density profile measurement via the
position of the interference fringes.Comment: Some changes in presentation, as published, 6 pages, LaTe
Evaluation of a 3-D rockfall module within a forest patch model
Many slopes in the Alps are prone to rockfall and forests play a vital role in protecting objects such as (rail) roads and infrastructure against rockfall. Decision support tools are required to assess rockfall processes and to quantify the rockfall protection effect of forest stands. This paper presents results of an iterative sequence of tests and improvements of a coupled rockfall and forest dynamics model with focus on the rockfall module. As evaluation data a real-size rockfall experiment in the French Alps and two 2-D rockfall trajectories from Austria and Switzerland were used. Modification of the rebound algorithm and the inclusion of an algorithm accounting for the sudden halt of falling rocks due to surface roughness greatly improved the correspondence between simulated and observed key rockfall variables like run-out distances, rebound heights and jump lengths for the real-size rockfall experiment. Moreover, the observed jump lengths and run-out distances of the 2-D trajectories were well within the stochastic range of variation yielded by the simulations. Based on evaluation results it is concluded that the rockfall model can be employed to assess the protective effect of forest vegetation
GW approximations and vertex corrections on the Keldysh time-loop contour: application for model systems at equilibrium
We provide the formal extension of Hedin's GW equations for single-particle
Green's functions with electron-electron interaction onto the Keldysh time-loop
contour. We show an application of our formalism to the plasmon model of a core
electron within the plasmon-pole approximation. We study in detail the
diagrammatic perturbation expansion of the core-electron/plasmon coupling on
the spectral functions of the so-called S-model which provides an exact
solution, concentrating especially on the effects of self-consistency and
vertex corrections on the GW self-energy. For the S-model, self-consistency is
essential for GW-like calculations to obtain the full spectral information. The
second- order exchange diagram (i.e. a vertex correction) is crucial to obtain
a better spectral description of the plasmon peak and side-band peaks in
comparison to GW-like calculations. However, the vertex corrections are well
reproduced within a non-self-consistent calculation. We also consider
conventional equilibrium GW calculations for the pure jellium model. We find
that with no second-order vertex correction, we cannot obtain the full set of
plasmon side-band peaks. Finally, we address the issues of formal connection
for the Dyson equations of the time-ordered Green's function and the Keldysh
Green's functions at equilibrium in the cases of zero and finite temperature.Comment: Published in PRB November 22 201
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