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-$\kappa$ 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 $\nu=1/m$ ($m$ an odd integer) and find that it is equal to $\pm \hbar mn/4$, where $n$ 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 [$\sigma(\omega)$] of the intermediate valence compound YbAl$_3$ has been measured at temperatures 8 K $\leq T \leq$ 690 K to study its microscopic electronic structures. Despite the highly metallic characters of YbAl$_3$, $\sigma(\omega)$ 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 $\sim$ 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$f$ 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