Dynamic Structure Factor of Normal Fermi Gas from Collisionless to Hydrodynamic Regime

The dynamic structure factor of a normal Fermi gas is investigated by using the moment method for the Boltzmann equation. We determine the spectral function at finite temperatures over the full range of crossover from the collisionless regime to the hydrodynamic regime. We find that the Brillouin peak in the dynamic structure factor exhibits a smooth crossover from zero to first sound as functions of temperature and interaction strength. The dynamic structure factor obtained using the moment method also exhibits a definite Rayleigh peak ($/omega /sim 0$), which is a characteristic of the hydrodynamic regime. We compare the dynamic structure factor obtained by the moment method with that obtained from the hydrodynamic equations.Comment: 19 pages, 9 figure

Tunable pinning of a superconducting vortex a by a magnetic vortex

The interaction between a straight vortex line in a superconducting film and a soft magnetic nanodisk in the magnetic vortex state in the presence of a magnetic field applied parallel to the film surfaces is studied theoretically. The superconductor is described by London theory and the nanodisk by the Landau-Lifshitz continuum theory of magnetism, using the approximation known as the rigid vortex model. Pinning of the vortex line by the nanodisk is found to result, predominantly, from the interaction between the vortex line and the changes in the nanodisk magnetization induced by the magnetic field of the vortex line and applied field. In the context of the rigid vortex model, these changes result from the displacement of the magnetic vortex. This displacement is calculated analytically by minimizing the energy, and the pinning potential is obtained. The applied field can tune the pinning potential by controlling the displacement of the magnetic vortex. The nanodisk magnetization curve is predicted to change in the presence of the vortex lineComment: 9 pages, 8 figures. Submitted to Phys. Rev.

Comment on "Universal Decoherence in Solids"

We discuss the formula for the decoherence rate proposed by Eugene M. Chudnovsky [Phys. Rev. Lett. 92, 120405 (2004)]. We show that it has a limited range of validity and is not applicable to evaluation of the width of a low-energy optical mode considered by Chudnovsky as an example. This is due to unjustified use of the Fermi golden rule for calculation of the relaxation rate. We present more general expression for the probability of the phonon-induced transition.Comment: 1 page, no figure

Wounded quarks and diquarks in high energy collisions

Particle production in Au-Au, Cu-Cu, d-Au and p-p collisions at 200 GeV c.m. energy are analyzed in the wounded quark-diquark model. Existing data are well reproduced. Emission functions of wounded and unwounded constituents are determined. Implications for the collective evolution of the system are discussed.Comment: version to be published in Phys. Rev. C, minor changes, discussion extende

Surprises in nonperturbative dynamics in sigma-model at finite density

The linear $SU(2)_L \times SU(2)_R$ sigma-model occupies a unique place in elementary particle physics and quantum field theory. It has been recently realized that when a chemical potential for hypercharge is added, it becomes a toy model for the description of the dynamics of the kaon condensate in high density QCD. We review recent results in nonperturbative dynamics obtained in the ungauged and gauged versions of this model.Comment: Brief review. 16 pages, 5 figure

The effect of a velocity barrier on the ballistic transport of Dirac fermions

We propose a novel way to manipulate the transport properties of massless Dirac fermions by using velocity barriers, defining the region in which the Fermi velocity, $v_{F}$, has a value that differs from the one in the surrounding background. The idea is based on the fact that when waves travel accross different media, there are boundary conditions that must be satisfied, giving rise to Snell's-like laws. We find that the transmission through a velocity barrier is highly anisotropic, and that perfect transmission always occurs at normal incidence. When $v_{F}$ in the barrier is larger that the velocity outside the barrier, we find that a critical transmission angle exists, a Brewster-like angle for massless Dirac electrons.Comment: 4.3 pages, 5 figure

Unified approach to structure factors and neutrino processes in nucleon matter

We present a unified approach to neutrino processes in nucleon matter based on Landau's theory of Fermi liquids that includes one- and two-quasiparticle-quasihole pair states as well as mean-field effects. We show how rates of neutrino processes involving two nucleons may be calculated in terms of the collision integral in the Landau transport equation for quasiparticles. Using a relaxation time approximation, we solve the transport equation for density and spin-density fluctuations and derive a general form for the response functions. We apply our approach to neutral-current processes in neutron matter, where the spin response function is crucial for calculations of neutrino elastic and inelastic scattering, neutrino-pair bremsstrahlung and absorption from strongly-interacting nucleons. We calculate the relaxation rates using modern nuclear interactions and including many-body contributions, and find that rates of neutrino processes are reduced compared with estimates based on the one-pion exchange interaction, which is used in current simulations of core-collapse supernovae.Comment: 16 pages, 4 figures; NORDITA-2008-30; published versio

Relaxation Mechanism for Ordered Magnetic Materials

We have formulated a relaxation mechanism for ferrites and ferromagnetic metals whereby the coupling between the magnetic motion and lattice is based purely on continuum arguments concerning magnetostriction. This theoretical approach contrasts with previous mechanisms based on microscopic formulations of spin-phonon interactions employing a discrete lattice. Our model explains for the first time the scaling of the intrinsic FMR linewidth with frequency, and 1/M temperature dependence and the anisotropic nature of magnetic relaxation in ordered magnetic materials, where M is the magnetization. Without introducing adjustable parameters our model is in reasonable quantitative agreement with experimental measurements of the intrinsic magnetic resonance linewidths of important class of ordered magnetic materials, insulator or metals

Microwave Response and Spin Waves in Superconducting Ferromagnets

Excitation of spin waves is considered in a superconducting ferromagnetic slab with the equilibrium magnetization both perpendicular and parallel to the surface. The surface impedance is calculated and its behavior near propagation thresholds is analyzed. Influence of non-zero magnetic induction at the surface is considered in various cases. The results provide a basis for investigation of materials with coexisting superconductivity and magnetism by microwave response measurements.Comment: 10 pages, 7 figure