748 research outputs found
Charge Ordering and Spin Dynamics in NaV2O5
We report high-resolution neutron inelastic scattering experiments on the
spin excitations of NaV2O5. Below Tc, two branches associated with distinct
energy gaps are identified. From the dispersion and intensity of the spin
excitation modes, we deduce the precise zig-zag charge distribution on the
ladder rungs and the corresponding charge order (about 0.6). We argue that the
spin gaps observed in the low-T phase of this compound are primarily due to the
charge transfer.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let
Deviations from Matthiessen's Rule for and
We have measured the change in the resistivity of thin films of and upon introducing point defects by electron
irradiation at low temperatures, and we find significant deviations from
Matthiessen's rule. For a fixed irradiation dose, the induced change in
resistivity {\it decreases} with increasing temperature. Moreover, for a fixed
temperature, the increase in resistivity with irradiation is found to be {\it
sublinear}. We suggest that the observed behavior is due to the marked
anisotropic scattering of the electrons together with their relatively short
mean free path (both characteristic of many metallic oxides including cuprates)
which amplify effects related to the Pippard ineffectiveness condition
Weak spin-orbit interactions induce exponentially flat mini-bands in magnetic metals without inversion symmetry
In metallic magnets like MnSi the interplay of two very weak spin-orbit
coupling effects can strongly modify the Fermi surface. In the absence of
inversion symmetry even a very small Dzyaloshinsky-Moriya interaction of
strength delta<<1 distorts a ferromagnetic state into a chiral helix with a
long pitch of order 1/delta. We show that additional small spin-orbit coupling
terms of order delta in the band structure lead to the formation of
exponentially flat minibands with a bandwidth of order exp(-1/sqrt(delta))
parallel to the direction of the helix. These flat minibands cover a rather
broad belt of width sqrt(delta) on the Fermi surface where electron motion
parallel to the helix practically stops. We argue that these peculiar
band-structure effects lead to pronounced features in the anomalous skin
effect.Comment: 7 pages, minor corrections, references adde
Ab-initio study of the thermopower of biphenyl-based single-molecule junctions
Employing ab-initio electronic structure calculations combined with the
non-equilibrium Green's function technique, we study the dependence of the
thermopower Q on the conformation in biphenyl-based single-molecule junctions.
For the series of experimentally available biphenyl molecules, alkyl side
chains allow us to gradually adjust the torsion angle \phi\ between the two
phenyl rings from 0 to 90{\deg} and to control in this way the degree of
\pi-electron conjugation. Studying different anchoring groups and binding
positions, our theory predicts that the absolute values of the thermopower
decrease slightly towards larger torsion angles, following an a+b*cos^{2}\phi\
dependence. The anchoring group determines the sign of Q and a,b,
simultaneously. Sulfur and amine groups give rise to Q,a,b>0, while for cyano
Q,a,b<0. The different binding positions can lead to substantial variations of
the thermopower mostly due to changes in the alignment of the frontier
molecular orbital levels and the Fermi energy. We explain our ab-initio results
in terms of a \pi-orbital tight-binding model and a minimal two-level model,
which describes the pair of hybridizing frontier orbital states on the two
phenyl rings. The variations of the thermopower with \phi\ seem to be within
experimental resolution.Comment: 8 pages, 4 figues, 3 table
The localization transition at finite temperatures: electric and thermal transport
The Anderson localization transition is considered at finite temperatures.
This includes the electrical conductivity as well as the electronic thermal
conductivity and the thermoelectric coefficients. An interesting critical
behavior of the latter is found. A method for characterizing the conductivity
critical exponent, an important signature of the transition, using the
conductivity and thermopower measurements, is outlined.Comment: Article for the book: "50 Years of Anderson Localization", edited by
E. Abrahams (World Scientific, Singapore, 2010
Heat transport in ultra-thin dielectric membranes and bridges
Phonon modes and their dispersion relations in ultrathin homogenous
dielectric membranes are calculated using elasticity theory. The approach
differs from the previous ones by a rigorous account of the effect of the film
surfaces on the modes with different polarizations. We compute the heat
capacity of membranes and the heat conductivity of narrow bridges cut out of
such membranes, in a temperature range where the dimensions have a strong
influence on the results. In the high temperature regime we recover the
three-dimensional bulk results. However, in the low temperature limit the heat
capacity, , is proportional with (temperature), while the heat
conductivity, , of narrow bridges is proportional to , leading
to a thermal cut-off frequency .Comment: 6 pages and 6 figure
Incomplete quantum process tomography and principle of maximal entropy
The main goal of this paper is to extend and apply the principle of maximum
entropy (MaxEnt) to incomplete quantum process estimation tasks. We will define
a so-called process entropy function being the von Neumann entropy of the state
associated with the quantum process via Choi-Jamiolkowski isomorphism. It will
be shown that an arbitrary process estimation experiment can be reformulated in
a unified framework and MaxEnt principle can be consistently exploited. We will
argue that the suggested choice for the process entropy satisfies natural list
of properties and it reduces to the state MaxEnt principle, if applied to
preparator devices.Comment: 8 pages, comments welcome, references adde
Sequential and co-tunneling behavior in the temperature-dependent thermopower of few-electron quantum dots
We have studied the temperature dependent thermopower of gate-defined,
lateral quantum dots in the Coulomb blockade regime using an electron heating
technique. The line shape of the thermopower oscillations depends strongly on
the contributing tunneling processes. Between 1.5 K and 40 mK a crossover from
a pure sawtooth- to an intermitted sawtooth-like line shape is observed. The
latter is attributed to the increasing dominance of cotunneling processes in
the Coulomb blockade regime at low temperatures.Comment: 4 pages, 4 figures, submitted to Phys. Rev.
Kinetic Limit for Wave Propagation in a Random Medium
We study crystal dynamics in the harmonic approximation. The atomic masses
are weakly disordered, in the sense that their deviation from uniformity is of
order epsilon^(1/2). The dispersion relation is assumed to be a Morse function
and to suppress crossed recollisions. We then prove that in the limit epsilon
to 0 the disorder averaged Wigner function on the kinetic scale, time and space
of order epsilon^(-1), is governed by a linear Boltzmann equation.Comment: 71 pages, 3 figure
Theory of helimagnons in itinerant quantum systems II: Nonanalytic corrections to Fermi-liquid behavior
A recent theory for the ordered phase of helical or chiral magnets such as
MnSi is used to calculate observable consequences of the helical Goldstone
modes or helimagnons. In systems with no quenched disorder, the helimagnon
contributions to the specific heat coefficient is shown to have a linear
temperature dependence, while the quasi-particle inelastic scattering rate is
anisotropic in momentum space and depends on the electronic dispersion
relation. For cubic lattices the generic temperature dependence is given by a
non-Fermi-liquid T^(3/2) behavior. The contribution to the temperature
dependence of the resistivity is shown to be T^(5/2) in a Boltzmann
approximation. The helimagnon thus leads to nonanalytic corrections to
Fermi-liquid behavior. Implications for experiments, and for transport theories
beyond the Boltzmann level, are discussed.Comment: 15 pp., 5 figs, includes Erratum to PRB versio
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