221 research outputs found
Heat Transport in low-dimensional systems
Recent results on theoretical studies of heat conduction in low-dimensional
systems are presented. These studies are on simple, yet nontrivial, models.
Most of these are classical systems, but some quantum-mechanical work is also
reported. Much of the work has been on lattice models corresponding to phononic
systems, and some on hard particle and hard disc systems. A recently developed
approach, using generalized Langevin equations and phonon Green's functions, is
explained and several applications to harmonic systems are given. For
interacting systems, various analytic approaches based on the Green-Kubo
formula are described, and their predictions are compared with the latest
results from simulation. These results indicate that for momentum-conserving
systems, transport is anomalous in one and two dimensions, and the thermal
conductivity kappa, diverges with system size L, as kappa ~ L^alpha. For one
dimensional interacting systems there is strong numerical evidence for a
universal exponent alpha =1/3, but there is no exact proof for this so far. A
brief discussion of some of the experiments on heat conduction in nanowires and
nanotubes is also given.Comment: 78 pages, 25 figures, Review Article (revised version
Observation of spin Coulomb drag in a two-dimensional electron gas
An electron propagating through a solid carries spin angular momentum in
addition to its mass and charge. Of late there has been considerable interest
in developing electronic devices based on the transport of spin, which offer
potential advantages in dissipation, size, and speed over charge-based devices.
However, these advantages bring with them additional complexity. Because each
electron carries a single, fixed value (-e) of charge, the electrical current
carried by a gas of electrons is simply proportional to its total momentum. A
fundamental consequence is that the charge current is not affected by
interactions that conserve total momentum, notably collisions among the
electrons themselves. In contrast, the electron's spin along a given spatial
direction can take on two values, "up" and "down", so that the spin current and
momentum need not be proportional. Although the transport of spin polarization
is not protected by momentum conservation, it has been widely assumed that,
like the charge current, spin current is unaffected by electron-electron (e-e)
interactions. Here we demonstrate experimentally not only that this assumption
is invalid, but that over a broad range of temperature and electron density,
the flow of spin polarization in a two-dimensional gas of electrons is
controlled by the rate of e-e collisions
Chiral tunneling and the Klein paradox in graphene
The so-called Klein paradox - unimpeded penetration of relativistic particles
through high and wide potential barriers - is one of the most exotic and
counterintuitive consequences of quantum electrodynamics (QED). The phenomenon
is discussed in many contexts in particle, nuclear and astro- physics but
direct tests of the Klein paradox using elementary particles have so far proved
impossible. Here we show that the effect can be tested in a conceptually simple
condensed-matter experiment by using electrostatic barriers in single- and
bi-layer graphene. Due to the chiral nature of their quasiparticles, quantum
tunneling in these materials becomes highly anisotropic, qualitatively
different from the case of normal, nonrelativistic electrons. Massless Dirac
fermions in graphene allow a close realization of Klein's gedanken experiment
whereas massive chiral fermions in bilayer graphene offer an interesting
complementary system that elucidates the basic physics involved.Comment: 15 pages, 4 figure
Observation of Magnon Polarization
We measure the mode-resolved direction of the precessional motion of the magnetic order, i.e., magnon polarization, via the chiral term of inelastic polarized neutron scattering spectra. The magnon polarization is a unique and unambiguous signature of magnets and is important in spintronics, affecting thermodynamic properties such as the magnitude and sign of the spin Seebeck effect. However, it has never been directly measured in any material until this work. The observation of both signs of magnon polarization in Y3Fe5O12 also gives direct proof of its ferrimagnetic nature. The experiments agree very well with atomistic simulations of the scattering cross section
Spin entropy as the likely source of enhanced thermopower in $\rm\bf Na_xCo_2O_4
In an electric field, the flow of electrons in a solid produces an entropy
current in addition to the familiar charge current. This Peltier effect
underlies all thermoelectric refrigerators. The upsurge in thermoelectric
cooling applications has led to a search for more efficient Peltier materials
and to renewed theoretical interest in how electron-electron interaction may
enhance the thermopower of materials such as the transition-metal oxides
\cite{Mahan,Beni,Kotliar,Chaikin}. An important factor in this enhancement is
the electronic spin entropy, which is predicted \cite{Chaikin,Kwak,KwakChaikin}
to dominate the entropy current. Here we report evidence for such suppression
in the layered oxide , and present evidence that it is a
strong-correlation effect.Comment: 5 pages, 5 figures, already publishe
From Luttinger to Fermi liquids in organic conductors
This chapter reviews the effects of interactions in quasi-one dimensional
systems, such as the Bechgaard and Fabre salts, and in particular the Luttinger
liquid physics. It discusses in details how transport measurements both d.c.
and a.c. allow to probe such a physics. It also examine the dimensional
crossover and deconfinement transition occurring between the one dimensional
case and the higher dimensional one resulting from the hopping of electrons
between chains in the quasi-one dimensional structure.Comment: To be published In the book "The Physics of Organic Conductors and
Superconductors", Springer, 2007, ed. A. Lebe
Thermal Properties of Graphene, Carbon Nanotubes and Nanostructured Carbon Materials
Recent years witnessed a rapid growth of interest of scientific and
engineering communities to thermal properties of materials. Carbon allotropes
and derivatives occupy a unique place in terms of their ability to conduct
heat. The room-temperature thermal conductivity of carbon materials span an
extraordinary large range - of over five orders of magnitude - from the lowest
in amorphous carbons to the highest in graphene and carbon nanotubes. I review
thermal and thermoelectric properties of carbon materials focusing on recent
results for graphene, carbon nanotubes and nanostructured carbon materials with
different degrees of disorder. A special attention is given to the unusual size
dependence of heat conduction in two-dimensional crystals and, specifically, in
graphene. I also describe prospects of applications of graphene and carbon
materials for thermal management of electronics.Comment: Review Paper; 37 manuscript pages; 4 figures and 2 boxe
Physics of Neutron Star Crusts
The physics of neutron star crusts is vast, involving many different research
fields, from nuclear and condensed matter physics to general relativity. This
review summarizes the progress, which has been achieved over the last few
years, in modeling neutron star crusts, both at the microscopic and macroscopic
levels. The confrontation of these theoretical models with observations is also
briefly discussed.Comment: 182 pages, published version available at
<http://www.livingreviews.org/lrr-2008-10
- …