Optical Conductivity and Pseudo-Momentum Conservation in Anisotropic Fermi Liquids

Umklapp scattering determines the conductivity of clean metals. In typical quasi one-dimensional Fermi liquids with an open Fermi surface, certain pseudo-momenta do not decay by 2-particle collisions even in situations where Umklapp scattering relaxes the momentum of the quasi particles efficiently. Due to this approximate conservation of pseudo-momentum, a certain fraction of the electrical current decays very slowly and a well-pronounced low-frequency peak emerges in the optical conductivity. We develop simple criteria to determine under what conditions approximate pseudomomentum conservation is relevant and calculate within in Fermi liquid theory the weights of the corresponding low-frequency peaks and the temperature dependence of the various relevant decay rates. Based on these considerations, we obtain a qualitative picture of the frequency and temperature dependence of the optical conductivity of an anisotropic Fermi liquid.     

Precessing Domain NMR in Normal and Superfluid Fermi Liquids

Coherently precessing two domain spin states, in both normal and superfluid Fermi liquids, can be formed owing to reactive spin currents. A short review of these states is given. Existing and potential applications of coherently precessing domains are discussed. PACS numbers: 67.57.Pq, 67.57.Bc, 67.57.De.     

Zero-Temperature Relaxation in Pure Fermi Liquids and Ferromagnetic Metals

We discuss the effect of the zero-temperature transverse attenuation in spin-polarized Fermi liquids on related phenomena in helium and electron systems. In helium Fermi liquids, the magnetic dipole-dipole interaction leads to a transfer of transverse attenuation to longitudinal processes resulting in finite sound attenuation and effective viscosity even at zero temperature. In Heisenberg ferromagnetic metals, this Fermi-liquid effect affects the attenuation of ferromagnetic magnons as a result of exchange coupling between spins of ferromagnetic and conduction electrons.     

Transport in Fermi Liquids Confined by Rough Walls

I present theoretical calculations of the thermal conductivity of Fermi liquid \(^3\) He confined to a slab of thickness of order \(\sim \) 100 nm. The effect of the roughness of the confining surfaces is included directly in terms of the surface roughness power spectrum which may be determined experimentally. Transport at low temperatures is limited by scattering off rough surfaces and evolves into the known high-temperature limit in bulk through an anomalous regime in which both inelastic quasiparticle scattering and elastic scattering off the rough surface coexist. I show preliminary calculations for the coefficients of thermal conductivity. These studies are applicable in the context of electrical transport in metal nanowires as well as experiments that probe the superfluid phase diagram of liquid \(^3\) He in a slab geometry.     

Temperature Increase of Highly-Polarized Fermi Liquids in Spin-Echo Experiments

We show that there are restrictions on the maximum tipping angle that can be used without significantly raising the temperature of the ³ He distribution in high B/T spin-echo experiments with pure liquid ³ He and ³ He- ⁴ He solutions. The temperature increase occurs during the diffusion process as quasiparticles in mixed-spin states are scattered and converted into thermal excitations at the spin-up and spin-down Fermi surfaces. This temperature increase can mimic the effects of zero temperature attenuation, leading to a higher values of the measured anisotropy temperature T_a. We analyze the dependence of the increase on polarization, initial temperature, ³ He concentration, and tip angle, and estimate the size of the effect in recent experiments.     

Local Polaronic Effects in Compounds with Atoms of Transition Metals

The e-ph coupling may be of the different strength for different electronic bands. If coupling is strong enough for one or more bands, polaronic effects practically decouple these bands from the rest. Single ions being dressed by strong e-ph interactions move in the local potentials having two or more minima, and hence, can be viewed as the system of the Ising spins. Intersite interactions via the RKKY-type exchange by the electron-hole pairs tend to order the latter thus providing the CDW instability mechanism with a structural vector, Q that has nothing in common with the Fermi surfaces parameters. Among the most typical manifestations of such strong e-ph coupling in the system are large characteristic energy scales significantly exceeding the temperatures for the onset of the CDW phase. The available experimental data support interpretation of properties of the transition-atoms-dichalcogenides and the compounds of the A15 group in terms of the local polaronic effects.     

Dispersive Effects in the Unitary Fermi Gas

We investigate within density functional theory various physical properties of the zero-temperature unitary Fermi gas which critically depend on the presence of a dispersive gradient term in the equation of state. First, we consider the unitary Fermi superfluid gas confined to a semi-infinite domain and calculate analytically its density profile and surface tension. Then we study the quadrupole modes of the superfluid system under harmonic confinement finding a reliable analytical formula for the oscillation frequency, which reduces to the familiar Thomas-Fermi one in the limit of a large number of atoms. Finally, we discuss the formation and propagation of dispersive shock waves in the collision between two resonant fermionic clouds, and compare our findings with recent experimental results.     

Spin Susceptibility and Strong Coupling Effects in an Ultracold Fermi Gas

We investigate magnetic properties and strong coupling corrections in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas. Within the framework of an extended T-matrix theory, we calculate the spin susceptibility χ above the superfluid phase transition temperature $T_{\rm c}$ . In the crossover region, the formation of preformed Cooper pairs is shown to cause a non-monotonic temperature dependence of χ, which is similar to the so-called spin-gap phenomenon observed in the under-doped regime of high-T _c cuprates. From this behavior of χ, we determine the spin-gap temperature as the temperature at which χ takes a maximum value, in the BCS-BEC crossover region. Since the spin susceptibility is sensitive to the formation of singlet Cooper pairs, our results would be useful in considering the temperature region where pairing fluctuations are important in the BCS-BEC crossover regime of an ultracold Fermi gas.     

Dislocation Inelastic Effects in Zinc Single Crystals

Dislocation internal friction,elastic modulusdefect and their ratio r have been studied in an-nealed and plastically deformed Zn single crystals inthe temperature range of 80 to 300 K in a widerange of oscillation amplitudes.The results ob-tained are discussed within the present notion aboutthe nature of dissipative elastic oscillation losses insolids.     

Electric-Field-Induced Impurity Effects in p-GaSe Single Crystals

Inorganic Materials - Electric-field-induced impurity photoconductivity and spontaneous dark current pulsations have been detected in nominally undoped and rare-earth (gadolinium and erbium) doped...     

Effects of Fermi Surface Anisotropy on Unconventional Superconductivity in UPt3

We discuss the weak-coupling BCS theory of the heavy fermion superconductor UPt ₃ , accounting for that system's anisotropic, multisheeted Fermi surface by expanding the order parameter and pair potential in terms of appropriate basis functions of the irreducible representations of the D _6h crystal point group. Within a phenomenological model for the electronic structure of UPt ₃ chosen to capture the qualitative features of local density functional calculations and de Haas–van Alphen measurements, we show how Fermi surface anisotropy can favor pairing in certain symmetry classes, and influence the phase diagram of unconventional superconductors. We also calculate the Ginzburg–Landau coefficients, focussing on those coefficients relevant to current theories of the UPt ₃ phase diagram.     

The Fermi Arc and Fermi Pocket in Cuprates in a Short-Range Diagonal Stripe Phase

In this paper, we studied the Fermi arc and the Fermi pocket in cuprates in a short-range diagonal stripe phase with wave vectors (7π/8,7π/8), which reproduce with a high accuracy the positions and sizes of the Fermi arc and Fermi pocket and the superstructure in cuprates observed by Meng et al. 2009. The low-energy spectral function indicates that the Fermi pocket results from the main band and the shadow band at the Fermi energy. Above the Fermi energy, the shadow band gradually departs away from the main band, leaving a Fermi arc. Thus, we conclude that the Fermi arc and Fermi pocket can be fully attributed to the stripe phase but has nothing to do with pairing. Incorporating a d-wave pairing potential in the stripe phase the spectral weight in the antinodal region is removed, leaving a clean Fermi pocket in the nodal region.     

Superfluid Phase Transition and Strong-Coupling Effects in an Ultracold Fermi Gas with Mass Imbalance

We investigate the superfluid phase transition and effects of mass imbalance in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an cold Fermi gas. We point out that the Gaussian fluctuation theory developed by Nozières and Schmitt-Rink and the T-matrix theory, that are now widely used to study strong-coupling physics of cold Fermi gases, give unphysical results in the presence of mass imbalance. To overcome this problem, we extend the T-matrix theory to include higher-order pairing fluctuations. Using this, we examine how the mass imbalance affects the superfluid phase transition. Since the mass imbalance is an important key in various Fermi superfluids, such as ⁴⁰K-⁶Li Fermi gas mixture, exciton condensate, and color superconductivity in a dense quark matter, our results would be useful for the study of these recently developing superfluid systems.     

Effects of Translational Nonequilibrium in the Shock Wave Front in Dense Gases and Liquids

On the basis of the generalized Enskog kinetic equation, the problem of the structure of a shock wave in dense reacting gases and liquids is solved. It is shown that the physicochemical processes in the zone of translational nonequilibrium (ZTN) in the shock wave front do not obey the Arrhenius kinetics. The rates of high-threshold processes in the ZTN in the shock wave front may exceed these rates in equilibrium behind the front by a factor of 10⁶ and more. An analytical expression is obtained for the rate constant of nonequilibrium processes in the front. Comparison of the results of calculation of the yield of products with broken double bond in the benzene molecule with the experimental data reveals that the formation of these products proceeds by the mechanism of nonadiabatic collisions realized under highly nonequilibrium conditions.     

Shell Effects in the First Sound Velocity of an Ultracold Fermi Gas

We investigate the first sound of a normal dilute and ultracold two-component Fermi gas in a cylindrical trap with harmonic radial confinement. We show that the velocity of the sound that propagates along the axial direction strongly varies in the dimensional crossover of the system. In particular, we predict that the first-sound velocity exhibits shell effects: by increasing the density, that is by inducing the crossover from one to three-dimensions, the first-sound velocity shows jumps in correspondence with the filling of harmonic modes. The experimental achievability of these effects is discussed by considering ⁴⁰K atoms.      

Symmetry Splitting of Equivalent Sites in Oxide Crystals and Related Mechanical Effects

Changes in the symmetry of a crystal caused by an applied strain have been used to show in what circumstances an internal friction peak can result from the motion of isolated point defects. General rules are given to make the prediction, and these are applied to several structures of common oxides. The prediction for rutile is compared with experimental results which are interpreted by the movement of titanium ions between interstitial positions in the structure.     

Reversed and Anomalous Doppler Effects in Photonic Crystals and other Time-dependent Periodic Media

We predict that reversed and anomalous non-relativistic Doppler shifts can be observed under some circumstances when light reflects from a shock wave front propagating through a photonic crystal, or material with a periodic modulation of the dielectric. This theoretical prediction is generalizable and applies to wave-like excitations in a variety of periodic media. An experimental observation of this effect has recently been made (Seddon, N. and Bearpark, T. Science, 302 (2003) 1537) and we provide a brief discussion of this experiment.     

Convective Effects During Diffusivity Measurements in Liquids with an Applied Magnetic Field

Convective contamination of self-diffusion experiments with an applied magnetic field is considered using a two-dimensional axisymmetric model. Constant, uniform, and an additional non-uniform heat fluxes are imposed along the sidewall of the cylinder while constant heat loss occurs through the top and bottom. In this model, due to a very small thermal Péclet number, convective heat transfer is neglected, and the flow is steady and inertialess. Time-dependent concentration is solved for various values of the mass Péclet number, Pe _m , (the ratio between the convective transport rate and the diffusive transport rate) and different magnetic field strengths represented by the Hartmann number Ha. Diffusivities are obtained using the same algorithm used to extract diffusivity values from the actual experimental data. Normalized values of these diffusivities vs. effective Pe _m are presented for different imposed temperature profiles. In all cases, the diffusivity value obtained through the simulated measurement increases as the effective Pe _m increases. The numerical results suggest that an additional periodic flux, or hot and cold spots, can significantly decrease the convective contamination in our geometry. The number of periodicity in temperature does not have a significant impact on the diffusivity results. Keeping the top wall slightly warmer than the bottom wall has no effect on the diffusivities for this model.