342 research outputs found
Transport and conservation laws
We study the lowest order conservation laws in one-dimensional (1D)
integrable quantum many-body models (IQM) as the Heisenberg spin 1/2 chain, the
Hubbard and t-J model. We show that the energy current is closely related to
the first conservation law in these models and therefore the thermal transport
coefficients are anomalous. Using an inequality on the time decay of current
correlations we show how the existence of conserved quantities implies a finite
charge stiffness (weight of the zero frequency component of the conductivity)
and so ideal conductivity at finite temperatures.Comment: 6 pages, Late
Colloidal brazil nut effect in sediments of binary charged suspensions
Equilibrium sedimentation density profiles of charged binary colloidal
suspensions are calculated by computer simulations and density functional
theory. For deionized samples, we predict a colloidal ``brazil nut'' effect:
heavy colloidal particles sediment on top of the lighter ones provided that
their mass per charge is smaller than that of the lighter ones. This effect is
verifiable in settling experiments.Comment: 4 pages, 4 figure
Numerical Evidence of Luttinger and Fermi Liquid Behaviour in the 2D Hubbard Model
The two dimensional Hubbard model with a single spin-up electron interacting
with a finite density of spin-down electrons is studied using the quantum
Monte Carlotechnique, a new conjugate gradient method for the evaluation of
the Edwards wavefunction ansatz, and the standard second order perturbation
theory. We performed simulations up to 242 sites at reaching the zero
temperature properties with no ``fermion sign problem'' and found a
surprisingly good accuracy of the Edwards wavefunction ansatz at low density or
low doping. The conjugate gradient method was then applied to system up to 1922
sites and infinite for the Edwards state. Fermi liquid theory seems to
remain stable in 2D for all cases studied with the exception of the half
filling case where a ``Luttinger like behavior'' survives in the Hubbard model
, yielding a vanishing quasiparticle weight in the thermodynamic limit.Comment: 10 pages + 4 pictures, RevTex, SISSA 121/93/CM/M
Spin conductance, dynamic spin stiffness and spin diffusion in itinerant magnets
We discuss analogies between the charge- and spin response functions of
itinerant magnets. We show that the spin-analog of the charge stiffness is not
given by the usual spin stiffness rho_s, but by the dynamic spin stiffness D_s,
which is obtained from the dynamic spin conductance G_s (omega) in the limit of
vanishing frequency omega. The low-frequency behavior of G_s (omega) is used to
define ideal spin conductors, normal spin conductors, and spin insulators.
Assuming diffusive spin dynamics, we show that the spin diffusion coefficient
is proportional to lim_{omega rightarrow 0} {Re} {G}_s (omega). We exploit this
fact to develop a new extrapolation scheme for the spin diffusion coefficient
in the paramagnetic phase of the Hubbard model.Comment: Discussion of gauge invariance and difference between transverse and
longitudinal response added. Accepted for publication in Phys. Rev.
Normal transport properties for a classical particle coupled to a non-Ohmic bath
We study the Hamiltonian motion of an ensemble of unconfined classical
particles driven by an external field F through a translationally-invariant,
thermal array of monochromatic Einstein oscillators. The system does not
sustain a stationary state, because the oscillators cannot effectively absorb
the energy of high speed particles. We nonetheless show that the system has at
all positive temperatures a well-defined low-field mobility over macroscopic
time scales of order exp(-c/F). The mobility is independent of F at low fields,
and related to the zero-field diffusion constant D through the Einstein
relation. The system therefore exhibits normal transport even though the bath
obviously has a discrete frequency spectrum (it is simply monochromatic) and is
therefore highly non-Ohmic. Such features are usually associated with anomalous
transport properties
Quantum Transport in a Nanosize Silicon-on-Insulator Metal-Oxide-Semiconductor
An approach is developed for the determination of the current flowing through
a nanosize silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect
transistors (MOSFET). The quantum mechanical features of the electron transport
are extracted from the numerical solution of the quantum Liouville equation in
the Wigner function representation. Accounting for electron scattering due to
ionized impurities, acoustic phonons and surface roughness at the Si/SiO2
interface, device characteristics are obtained as a function of a channel
length. From the Wigner function distributions, the coexistence of the
diffusive and the ballistic transport naturally emerges. It is shown that the
scattering mechanisms tend to reduce the ballistic component of the transport.
The ballistic component increases with decreasing the channel length.Comment: 21 pages, 8 figures, E-mail addresses: [email protected]
Effect of Finite Impurity Mass on the Anderson Orthogonality Catastrophe in One Dimension
A one-dimensional tight-binding Hamiltonian describes the evolution of a
single impurity interacting locally with electrons. The impurity spectral
function has a power-law singularity
with the same exponent
that characterizes the logarithmic decay of the quasiparticle weight
with the number of electrons , . The exponent
is computed by (1) perturbation theory in the interaction strength and
(2) numerical evaluations with exact results for small systems and variational
results for larger systems. A nonanalytical behavior of is observed in
the limit of infinite impurity mass. For large interaction strength, the
exponent depends strongly on the mass of the impurity in contrast to the
perturbative result.Comment: 26 pages, RevTeX, 7 figures included, to be published in Phys. Rev.
Electron-Phonon Interacation in Quantum Dots: A Solvable Model
The relaxation of electrons in quantum dots via phonon emission is hindered
by the discrete nature of the dot levels (phonon bottleneck). In order to
clarify the issue theoretically we consider a system of discrete fermionic
states (dot levels) coupled to an unlimited number of bosonic modes with the
same energy (dispersionless phonons). In analogy to the Gram-Schmidt
orthogonalization procedure, we perform a unitary transformation into new
bosonic modes. Since only of them couple to the fermions, a
numerically exact treatment is possible. The formalism is applied to a GaAs
quantum dot with only two electronic levels. If close to resonance with the
phonon energy, the electronic transition shows a splitting due to quantum
mechanical level repulsion. This is driven mainly by one bosonic mode, whereas
the other two provide further polaronic renormalizations. The numerically exact
results for the electron spectral function compare favourably with an analytic
solution based on degenerate perturbation theory in the basis of shifted
oscillator states. In contrast, the widely used selfconsistent first-order Born
approximation proves insufficient in describing the rich spectral features.Comment: 8 pages, 4 figure
A rescued dataset of sub-daily meteorological observations for Europe and the southern Mediterranean region, 1877–2012
Sub-daily meteorological observations are needed for input to and assessment of high-resolution reanalysis products to improve understanding of weather and climate variability. While there are millions of such weather observations that have been collected by various organisations, many are yet to be transcribed into a useable format. Under the auspices of the Uncertainties in Ensembles of Regional ReAnalyses (UERRA) project, we describe the compilation and development of a digital dataset of 8.8 million meteorological observations of essential climate variables (ECVs) rescued across the European and southern Mediterranean region. By presenting the entire chain of data preparation, from the identification of regions lacking in digitised sub-daily data and the location of original sources, through the digitisation of the observations to the quality control procedures applied, we provide a rescued dataset that is as traceable as possible for use by the research community. Data from 127 stations and of 15 climate variables in the northern African and European sectors have been prepared for the period 1877 to 2012. Quality control of the data using a two-step semi-automatic statistical approach identified 3.5 % of observations that required correction or removal, on par with previous data rescue efforts. In addition to providing a new sub-daily meteorological dataset for the research community, our experience in the development of this sub-daily dataset gives us an opportunity to share some suggestions for future data rescue projects. All versions of the dataset, from the raw digitised data to data that have been quality controlled and converted to standard units, are available on PANGAEA: https://doi.org/10.1594/PANGAEA.886511 (Ashcroft et al., 2018)
The relative influences of disorder and of frustration on the glassy dynamics in magnetic systems
The magnetisation relaxations of three different types of geometrically
frustrated magnetic systems have been studied with the same experimental
procedures as previously used in spin glasses. The materials investigated are
YMoO (pyrochlore system), SrCrGaO (piled
pairs of Kagom\'e layers) and (HO)Fe(SO)(OH) (jarosite
compound). Despite a very small amount of disorder, all the samples exhibit
many characteristic features of spin glass dynamics below a freezing
temperature , much smaller than their Curie-Weiss temperature .
The ageing properties of their thermoremanent magnetization can be well
accounted for by the same scaling law as in spin glasses, and the values of the
scaling exponents are very close. The effects of temperature variations during
ageing have been specifically investigated. In the pyrochlore and the
bi-Kagom\'e compounds, a decrease of temperature after some waiting period at a
certain temperature re-initializes ageing and the evolution at the new
temperature is the same as if the system were just quenched from above .
However, as the temperature is raised back to , the sample recovers the
state it had previously reached at that temperature. These features are known
in spin glasses as rejuvenation and memory effects. They are clear signatures
of the spin glass dynamics. In the Kagom\'e compound, there is also some
rejuvenation and memory, but much larger temperature changes are needed to
observe the effects. In that sense, the behaviour of this compound is
quantitatively different from that of spin glasses.Comment: latex VersionCorrigee4.tex, 4 files, 3 figures, 5 pages (Proceedings
of the International Conference on Highly Frustrated Magnetism (HFM2003),
August 26-30, 2003, Institut Laue Langevin (ILL), Grenoble, France
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