NMR spectra under skin-effect conditions at ultralow temperatures

An investigation based on the coupled Maxwell-Bloch equations for a system of equivalent exchange-coupled spins is performed in order to explain a number of features of NMR spectra obtained in metals by Fourier-transforming of the free-induction decay at ultralow temperatures. Small angles of tilting of the nuclear magnetization by the exciting rf field are considered. It is shown that the free precession inherits the nonuniformity in the distribution of the rf field and the magnetization produced at the excitation stage inside the sample on account of the skin effect. As a result, the NMR spectrum is found to consist of a set of peaks—signals due to standing spin waves. However, such a spectrum can be observed only when the detuning of the exciting rf field is sufficiently large relative to the Larmor frequency of the spins. Otherwise, the rf field does not penetrate into the sample because of strong absorption by the spins. If the detuning is large, the dispersion signal and part of the NMR absorption signal are proportional to the equilibrium magnetization to the power 3/2. Such behavior is expected at low temperatures so that the coupling of the magnetization with the rf field is strong. The results obtained qualitatively explain the experimentally observed characteristics of the NMR spectra: the presence of kinks and structure of the NMR lines, the dependence of the shape and intensity of the spectrum on the detuning of the exciting rf field, and the nonlinear dependence of the nuclear susceptibility on the reciprocal of the sample temperature. Zh. éksp. Teor. Fiz. 114, 1836–1847 (November 1998)     

Phononless thermal conduction in glasses at ultralow temperatures

It is shown that the thermal conduction in dielectric glasses at ultralow temperatures is provided by energy transfer over the infinite cluster of resonance pairs of two-level systems. Expressions for the phononless thermal conductivity coefficient (～T ^4/3) and for the temperature of crossover between the phonon and phononless mechanisms of thermal conduction are obtained. The possibility of experimental corroboration of the results obtained is discussed.     

Universal mechanism of dissipation in Fermi superfluids at ultralow temperatures

We show that the vortex dynamics in Fermi superfluids at ultralow temperatures is governed by the local heating of the vortex cores creating the heat flux carried by nonequilibrium quasiparticles emitted by moving vortices. This mechanism provides a universal zero temperature limit of dissipation in Fermi superfluids. For the typical experimental conditions realized by the turbulent motion of ^{3}He-B, the temperature of the vortex cores is estimated to be of the order 0.2 T(c). The dispersion of Kelvin waves is derived, and the heat flow generated by Kelvin cascade is shown to have a value close to that experimentally observed.     

Ordered phase of nuclear spins in uniaxial ferromagnets at ultralow temperatures

Paper investigates the onset of nuclear magnetic ordering caused by the indirect Suhl-Nakamura interaction in ferromagnets. The necessary condition for nuclear spin ordering with definite ordering vector is obtained. Particularly, it is shown that ferromagnetically ordered phase of nuclear spins could be observed only in case of disk shaped samples. The spectrum of the nuclear spin excitations is also found. Received 25 January 1999 and Received in final form 5 May 1999     

Quantum gravity corrections to neutrino propagation

Massive spin-1/2 fields are studied in the framework of loop quantum gravity by considering a state approximating, at a length scale L much greater than Planck length l(P), a spin-1/2 field in flat spacetime. The discrete structure of spacetime at l(P) yields corrections to the field propagation at scale L. Neutrino bursts ( &pmacr; approximately 10(5) GeV) accompanying gamma ray bursts that have traveled cosmological distances L are considered. The dominant correction is helicity independent and leads to a time delay of order (&pmacr;l(P))L/c approximately 10(4) s. To next order in &pmacr;l(P), the correction has the form of the Gambini and Pullin effect for photons. A dependence L(-1)(os) approximately &pmacr;(2)l(P) is found for a two-flavor neutrino oscillation length.     

Evolution of phonon nonequilibrium in single-crystal ZnSe at liquid-helium temperatures

The photoluminescence and nonequilibrium heat transfer observed in ZnSe single crystals grown by the free growth method on an oriented single-crystal ZnSe substrate in a hydrogen atmosphere were investigated. The nonequilibrium heat transfer (or propagation of nonequilibrium phonons) was studied by the heat pulse method. A region of local thermal equilibrium or “a hot phonon spot” was found to form in the material, and the threshold of its formation was determined. The constant of spontaneous anharmonic phonon decay in ZnSe was estimated from an analysis of the propagation of nonequilibrium phonons via comparison of the experimental responses with those calculated by the Monte Carlo method.     

Structure and stability of vortices in dilute Bose-Einstein condensates at ultralow temperatures

We compute the structure of a quantized vortex line in a harmonically trapped dilute atomic Bose-Einstein condensate using the Popov version of the Hartree-Fock-Bogoliubov mean-field theory. The vortex is shown to be (meta)stable in a nonrotating trap even in the zero-temperature limit, thus confirming that weak particle interactions induce for the condensed gas a fundamental property characterizing "classical" superfluids. We present the structure of the vortex at ultralow temperatures and discuss the crucial effect of the thermal gas component to its energetic stability.     

Toward a theory of nuclear relaxation in dielectric glasses at ultralow temperatures

The temperature and frequency dependence of the nuclear relaxation rate in dielectric glasses is investigated. It is shown that at low and ultralow temperatures nuclear relaxation is due to an interaction between the nuclear quadrupole moment and fluctuations of the electric field created by dipole moments of two-level systems. Fluctuations of this field can be associated with the background relaxation or are due only to the dipole-dipole interaction between two-level systems. It is shown that at lower temperatures the second relaxation mechanism begins to dominate. Expressions are obtained for the temperature and frequency of crossover between different nuclear relaxation regimes. The possibility of experimental confirmation of our results is discussed. Zh. éksp. Teor. Fiz. 115, 2254–2262 (June 1999) Russian Scientific Center “Kurchatov Institute”     

Some properties of the renormalized harmonic phonon approximation at high temperatures

Using the Einstein and the high temperature approximation the renormalized harmonic phonon approximation yields at constant pressure a first order phase transition similar to the van der Waals phase transition. Calculation of the compressibility by different methods leads to an inconsistency of about 55%.     

Peripheral phonons and phonon conductivity of a metal at low temperatures

The role of the peripheral and non-peripheral phonons in the estimation of the lattice thermal conductivity of a metal has been studied at low temperatures by calculating their separate contributions towards the total lattice thermal conductivity. The study is made in the temperature range 0.4–2.5 K with the help of the Ziman expression for the scattering of phonons by the charge carriers and the Callaway expression of the phonon conductivity, and Sb is taken as an example. The separate percentage contributions due to peripheral and non-peripheral phonons have also been studied and it is found that the percentage contribution due to peripheral phonons increases with increasing temperature while the percentage contribution due to non-peripheral phonons decreases with increasing temperature. The percentage contributions of the lattice thermal resistivities due to electrons and holes towards the total lattice thermal resistivity of Sb have also been reported in the present note.     

The paired phonon model for liquid4He at low temperatures

The paired phonon model for the one-body density matrix is studied for liquid⁴He at low temperatures within the framework of hypernetted chain theory. Analytic expressions are derived for this quantity and the condensate fraction in the uniform limit. The results reproduce correctly theT ²-dependence of the condensate fraction derived earlier by other independent methods.     

Saturation of a nonuniformly broadened line under phonon bottleneck conditions

Using the methods of nonequilibrium statistics, equations are obtained that describe the saturation of a nonuniformly broadened ESR line taking into account the phenomenon of spectral diffusion under phonon bottleneck conditions. It is shown that the width and depth of the holes, burnt out by the saturation in the line, increase as the phonon bottleneck sector increases. The temperature dependence of the width of the holes burnt out in the line is studied.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 70–75, August, 1978.In conclusion, we wish to thank B. I. Kochelaev for discussing the results.     

Quantum friction of micromechanical resonators at low temperatures

Dissipation of micro- and nanoscale mechanical structures is dominated by quantum-mechanical tunneling of two-level defects intrinsically present in the system. We find that at high frequencies-usually, for smaller, micron-scale structures-a novel mechanism of phonon pumping of two-level defects gives rise to weakly temperature-dependent internal friction, Q-1, concomitant to the effects observed in recent experiments. Because of their size, comparable to or shorter than the emitted phonon wavelength, these structures suffer from superradiance-enhanced dissipation by the collective relaxation of a large number of two-level defects contained within the wavelength.     

Calculation of the phonon contribution to the specific heat of an overlayer-film substrate at low temperatures

Using the Green function method, we calculate the vibrational specific heat C_v produced by an overlayer-film substrate interface at low temperatures. A variation law in T³ is found and the dependence of C_v upon the film thickness is determined analytically.