Andreev-Lifshitz Hydrodynamics Applied to an Ordinary Solid under Pressure

We have applied the Andreev-Lifshitz hydrodynamic theory of supersolids to an ordinary solid. This theory includes an internal pressure $P$, distinct from the applied pressure $P_a$ and the stress tensor $\lambda_{ik}$. Under uniform static $P_{a}$, we have $\lambda_{ik} = (P-P_{a})\delta_{ik}$. For $P_{a} \ne 0$, Maxwell relations imply that $P \sim P_{a}^{2}$. The theory also permits vacancy diffusion but treats vacancies as conserved. It gives three sets of propagating elastic modes; it also gives two diffusive modes, one largely of entropy density and one largely of vacancy density (or, more generally, defect density). For the vacancy diffusion mode (or, equivalently, the lattice diffusion mode) the vacancies behave like a fluid within the solid, with the deviations of internal pressure associated with density changes nearly canceling the deviations of stress associated with strain. We briefly consider pressurization experiments in solid $^4$He at low temperatures in light of this lattice diffusion mode, which for small $P_{a}$ has diffusion constant $D_{L} \sim P_{a}^{2}$. The general principles of the theory -- that both volume and strain should be included as thermodynamic variables, with the result that both $P$ and $\lambda_{ik}$ appear -- should apply to all solids under pressure, especially near the solid-liquid transition. The lattice diffusion mode provides an additional degree of freedom that may permit surfaces with different surface treatments to generate different responses in the bulk.Comment: 10 pages. Accepted by Physical Review

Generation Efficiencies for Propagating Modes in a Supersolid

Using Andreev and Lifshitz's supersolid hydrodynamics, we obtain the propagating longitudinal modes at non-zero applied pressure $P_{a}$ (necessary for solid 4He), and their generation efficiencies by heaters and transducers. For small $P_{a}$, a solid develops an internal pressure $P \sim P_{a}^2$. This theory has stress contributions both from the lattice and an internal pressure $P$. Because both types of stress are included, the normal mode analysis differs from previous works. Not surprisingly, transducers are significantly more efficient at producing elastic waves and heaters are significantly more efficient at producing fourth sound waves. We take the system to be isotropic, which should apply to systems that are glassy or consist of many crystallites; the results should also apply, at least qualitatively, to single-crystal hcp 4He.Comment: 10 pages. Accepted by Physical Review

Thermal Equilibration and Thermally-Induced Spin Currents in a Thin-Film Ferromagnet on a Substrate

Recent spin-Seebeck experiments on thin ferromagnetic films apply a temperature difference $\Delta T_{x}$ along the length $x$ and measure a (transverse) voltage difference $\Delta V_{y}$ along the width $y$. The connection between these effects is complex, involving: (1) thermal equilibration between sample and substrate; (2) spin currents along the height (or thickness) $z$; and (3) the measured voltage difference. The present work studies in detail the first of these steps, and outlines the other two steps. Thermal equilibration processes between the magnons and phonons in the sample, as well as between the sample and the substrate leads to two surface modes, with surface lengths $\lambda$, to provide for thermal equilibration. Increasing the coupling between the two modes increases the longer mode length and decreases the shorter mode length. The applied thermal gradient along $x$ leads to a thermal gradient along $z$ that varies as $\sinh{(x/\lambda)}$, which can in turn produce fluxes of the carriers of up- and down- spins along $z$, and gradients of their associated \textit{magnetoelectrochemical potentials} $\bar{\mu}_{\uparrow,\downarrow}$, which vary as $\sinh{(x/\lambda)}$. By the inverse spin Hall effect, this spin current along $z$ can produce a transverse (along $y$) voltage difference $\Delta V_y$, which also varies as $\sinh{(x/\lambda)}$.Comment: 14 pages, 7 figures, 1 tabl

Andreev-Lifshitz Supersolid Hydrodynamics Including the Diffusive Mode

We have re-examined the Andreev-Lifshitz theory of supersolids. This theory implicitly neglects uniform bulk processes that change the vacancy number, and assumes an internal pressure $P$ in addition to lattice stress $\lambda_{ik}$. Each of $P$ and $\lambda_{ik}$ takes up a part of an external, or applied, pressure $P_a$ (necessary for solid 4He). The theory gives four pairs of propagating elastic modes, of which one pair corresponds to a fourth-sound mode, and a single diffusive mode, which has not been analyzed previously. The diffusive mode has three distinct velocities, with the superfluid velocity much larger than the normal fluid velocity, which in turn is much larger than the lattice velocity. The mode structure depends on the relative values of certain kinetic coefficients and thermodynamic derivatives. We consider pressurization experiments in solid 4He at low temperatures in light of this diffusion mode and a previous analysis of modes in a normal solid with no superfluid component.Comment: 8 pages. Accepted by Physical Review

Violation of Quasineutrality in Semiconductor Transport: The Dember Effect

Exact solution of the linearized equations for steady-state transport in semiconductors yields two modes that vary exponentially in space, one involving screening (without entropy production) and one involving diffusion and recombination (with entropy production). Neither mode is quasineutral. For constant surface photoexcitation with generation of electrons and holes, the steady-state response is a linear combination of these modes, subject to global electroneutrality. The resultant charge separation produces a voltage difference across the sample (the Dember effect)

Consistent Asymptotic Expansion of Mott's Solution for Oxide Growth

Many relatively thick metal oxide films grow according to what is called the parabolic law L = 2At + . . . . Mott explained this for monovalent carriers by assuming that monovalent ions and electrons are the bulk charge carriers, and that their number fluxes vary as t^{-1/2} at sufficiently long t. In this theory no charge is present in the bulk, and surface charges were not discussed. However, it can be analyzed in terms of a discharging capacitor, with the oxide surfaces as the plates. The theory is inconsistent because the field decreases, corresponding to discharge, but there is no net current to cause discharge. The present work, which also includes non-monovalent carriers, systematically extends the theory and obtains the discharge current. Because the Planck-Nernst equations are nonlinear (although Gauss's Law and the continuity equations are linear) this leads to a systematic order-by-order expansion in powers of t^{-1/2} for the number currents, concentrations, and electric field during oxide growth. At higher order the bulk develops a non-zero charge density, with a corresponding non-uniform net current, and there are corrections to the electric field and the ion currents. The second order correction to ion current implies a logarithmic term in the thickness of the oxide layer: L = (2At)^{1/2} + B ln t + . . . . It would be of interest to verify this result with high-precision measurements.Comment: 11 pages, 1 figur

Phase Diagram for Magnon Condensate in Yttrium Iron Garnet Film

Recently, magnons, which are quasiparticles describing the collective motion of spins, were found to undergo Bose-Einstein condensation (BEC) at room temperature in films of Yttrium Iron Garnet (YIG). Unlike other quasiparticle BEC systems, this system has a spectrum with two degenerate minima, which makes it possible for the system to have two condensates in momentum space. Recent Brillouin Light scattering studies for a microwave-pumped YIG film of thickness d=5 $\mu$m and field H=1 kOe find a low-contrast interference pattern at the characteristic wavevector $Q$ of the magnon energy minimum. In this report, we show that this modulation pattern can be quantitatively explained as due to non-symmetric but coherent Bose-Einstein condensation of magnons into the two energy minima. Our theory predicts a transition from a high-contrast symmetric phase to a low-contrast non-symmetric phase on varying the $d$ and $H$, and a new type of collective oscillations.Comment: 6 figures. Accepted by Nature Scientific Report