41 research outputs found
Specific-heat evidence for strong electron correlations in the thermoelectric material (Na,Ca)Co_{2}O_{4}
The specific heat of (Na,Ca)Co_{2}O_{4} is measured at low-temperatures to
determine the magnitude of the electronic specific-heat coefficient \gamma, in
an attempt to gain an insight into the origin of the unusually large
thermoelectric power of this compound. It is found that \gamma is as large as
48 mJ/molK^2, which is an order of magnitude larger than \gamma of simple
metals. This indicates that (Na,Ca)Co_{2}O_{4} is a strongly-correlated
electron system, where the strong correlation probably comes from the
low-dimensionality and the frustrated spin structure. We discuss how the large
thermopower and its dependence on Ca doping can be understood with the strong
electron correlations.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev.
Effect of high-K dielectrics on charge transport in graphene
The effect of various dielectrics on charge mobility in single layer graphene
is investigated. By calculating the remote optical phonon scattering arising
from the polar substrates, and combining it with their effect on Coulombic
impurity scattering, a comprehensive picture of the effect of dielectrics on
charge transport in graphene emerges. It is found that though high-
dielectrics can strongly reduce Coulombic scattering by dielectric screening,
scattering from surface phonon modes arising from them wash out this advantage.
By comparing the room-temperature transport properties with narrow-bandgap
III-V semiconductors, strategies to improve the mobility in single layer
graphene are outlined.Comment: 6 pages, 4 Figure
Nonequilibrium Approach to Bloch-Peierls-Berry Dynamics
We examine the Bloch-Peierls-Berry dynamics under a classical nonequilibrium
dynamical formulation. In this formulation all coordinates in phase space
formed by the position and crystal momentum space are treated on equal footing.
Explicitly demonstrations of the no (naive) Liouville theorem and of the
validity of Darboux theorem are given. The explicit equilibrium distribution
function is obtained. The similarities and differences to previous approaches
are discussed. Our results confirm the richness of the Bloch-Peierls-Berry
dynamics
Anomalous Hall effect in (In,Mn)Sb dilute magnetic semiconductor
High magnetic field study of Hall resistivity in the ferromagnetic phase of
(In,Mn)Sb allows one to separate its normal and anomalous components. We show
that the anomalous Hall term is not proportional to the magnetization, and that
it even changes sign as a function of magnetic field. We also show that the
application of pressure modifies the scattering process, but does not influence
the Hall effect. These observations suggest that the anomalous Hall effect in
(In,Mn)Sb is an intrinsic property and support the application of the Berry
phase theory for (III,Mn)V semiconductors. We propose a phenomenological
description of the anomalous Hall conductivity, based on a field-dependent
relative shift of the heavy- and light-hole valence bands and the split-off
band
Lorentz shear modulus of fractional quantum Hall states
We show that the Lorentz shear modulus of macroscopically homogeneous
electronic states in the lowest Landau level is proportional to the bulk
modulus of an equivalent system of interacting classical particles in the
thermodynamic limit. Making use of this correspondence we calculate the Lorentz
shear modulus of Laughlin's fractional quantum Hall states at filling factor
( an odd integer) and find that it is equal to ,
where is the density of particles and the sign depends on the direction of
magnetic field. This is in agreement with the recent result obtained by Read in
arXiv:0805.2507 and corrects our previous result published in Phys. Rev. B {\bf
76}, 161305 (R) (2007).Comment: 8 pages, 3 figure
Emission spectra of atoms with non-Markovian interaction: Fluorescence in a photonic crystal
We present a formula to evaluate the spontaneous emission spectra of an atom
in contact with a radiation field with non-Markovian effects. This formula is
written in terms of a two-time correlation of system observables and the
environmental correlation function, and depends on the distance between the
emitting atom and the detector. As an example, we use it to analyze the
fluorescence spectra of a two level atom placed as an impurity in a photonic
crystal. The radiation field within those materials has a gap or discontinuity
where electromagnetic modes cannot propagate in the stationary limit. In that
situation, the atomic emission occurs in the form of evanescent waves which are
detected with less efficiency the farther we place the detector. The
methodology presented in this paper may be useful to study the non-Markovian
dynamics of any quantum open system in linear interaction with a harmonic
oscillator reservoir and within the weak coupling approximation
VO: A Strongly Correlated Metal Close to a Mott- Hubbard Transition
Here we present experimental and computational evidences to support that
rock-salt cubic VO is a strongly correlated metal with Non-Fermi-Liquid
thermodynamics and an unusually strong spin-lattice coupling. An unexpected
change of sign of metallic thermopower with composition is tentatively ascribed
to the presence of a pseudogap in the density of states. These properties are
discussed as signatures of the proximity to a magnetic quantum phase
transition. The results are summarized in a new electronic phase diagram for
the 3d monoxides, which resembles that of other strongly correlated systems.
The structural and electronic simplicity of 3d monoxides make them ideal
candidates to progress in the understanding of highly correlated electron
systems.Comment: Submitted to Phys. Rev.
Disorder and excess modes in hard-sphere colloidal systems
The anomalous thermodynamic properties of glasses remain incompletely understood, notably the anomalous peak in the heat capacity at low temperatures; it is believed to be due to an excess of low-frequency vibrational modes and a manifestation of the structural disorder in these systems. We study the thermodynamics and vibrational dynamics of colloidal glasses and (defected) crystals. The experimental determination of the vibrational density of states allows us to directly observe a strong enhancement of low-frequency modes. Using a novel method (Zargar R. et al., Phys. Rev. Lett. 110 (2013) 258301) to determine the free energy, we also determine the entropy and the specific heat experimentally. It follows that the emergence of the excess modes and high values of the specific heat are directly related and are specific to the glass: even for solids containing a very large amount of defects, both the low-frequency density of states and the specific heat are significantly smaller than for the glass
Interplay of charge and orbital ordering in manganese perovskites
A model of localized classical electrons coupled to lattice degrees of
freedom and, via the Coulomb interaction, to each other, has been studied to
gain insight into the charge and orbital ordering observed in lightly doped
manganese perovskites. Expressions are obtained for the minimum energy and
ionic displacements caused by given hole and electron orbital configurations.
The expressions are analyzed for several hole configurations, including that
experimentally observed by Yamada et al. in La_{7/8}Sr_{1/8}MnO_3. We find
that, although the preferred charge and orbital ordering depend sensitively on
parameters, there are ranges of the parameters in which the experimentally
observed hole configuration has the lowest energy. For these parameter values
we also find that the energy differences between different hole configurations
are on the order of the observed charge ordering transition temperature. The
effects of additional strains are also studied. Some results for
La_{1/2}Ca_{1/2}MnO_3 are presented, although our model may not adequately
describe this material because the high temperature phase is metallic.Comment: 12 pages in RevTex, 5 figures in PS files, to appear in Phys. Rev. B
(New paragraphs and references added, typos corrected
Pseudogap Formation and Heavy Carrier Dynamics in Intermediate Valence YbAl3
Infrared optical conductivity [] of the intermediate valence
compound YbAl has been measured at temperatures 8 K 690 K to
study its microscopic electronic structures. Despite the highly metallic
characters of YbAl, exhibits a clear pseudogap (strong
depletion of spectral weight) of about 60 meV below 40 K. It also shows a
strong mid-infrared peak centered at 0.25 eV. Energy-dependent effective
mass and scattering rate of the carriers obtained from the data indicate the
formation of a heavy-mass Fermi liquid state. These characteristic results are
discussed in terms of the hybridization states between the Yb 4 and the
conduction electrons. It is argued, in particular, that the pseudogap and the
mid-infrared peak result from the indirect and the direct gaps, respectively,
within the hybridization state. band.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp