97 research outputs found
Hydrodynamic approach to coherent nuclear spin transport
We develop a linear response formalism for nuclear spin diffusion in a
dipolar coupled solid. The theory applies to the high-temperature,
long-wavelength regime studied in the recent experiments of Boutis et al.
[Phys. Rev. Lett. 92, 137201 (2004)], which provided direct measurement of
interspin energy diffusion in such a system. A systematic expansion of Kubo's
formula in the flip-flop term of the Hamiltonian is used to calculate the
diffusion coefficients. We show that this approach is equivalent to the method
of Lowe and Gade [Phys. Rev. 156, 817 (1967)] and Kaplan [Phys. Rev. B 2, 4578
(1970)], but has several calculational and conceptual advantages. Although the
lowest orders in this expansion agree with the experimental results for
magnetization diffusion, this is not the case for energy diffusion. Possible
reasons for this disparity are suggested.Comment: 7 pages, REVTeX4; Published Versio
Nuclear spin diffusion in semiconductor nanostructures: Effects of inhomogeneous hyperfine interactions
We study the effect of contact hyperfine interaction on the nuclear spin
diffusion coefficients in semiconductor quantum dots. The diffusion
coefficients are calculated with both the method of moment and density matrix.
We show that nuclear spin diffusion is strongly suppressed by the nonuniform
hyperfine coupling resulting from the confined electron wavefunction. Our
results agree with the observed suppression of nuclear spin diffusion in these
structures in recent experiments, and clarify the degree of validity of the
method of moment in an inhomogeneous system.Comment: 7 pages, 4 figure
Statistical Theory of Spin Relaxation and Diffusion in Solids
A comprehensive theoretical description is given for the spin relaxation and
diffusion in solids. The formulation is made in a general
statistical-mechanical way. The method of the nonequilibrium statistical
operator (NSO) developed by D. N. Zubarev is employed to analyze a relaxation
dynamics of a spin subsystem. Perturbation of this subsystem in solids may
produce a nonequilibrium state which is then relaxed to an equilibrium state
due to the interaction between the particles or with a thermal bath (lattice).
The generalized kinetic equations were derived previously for a system weakly
coupled to a thermal bath to elucidate the nature of transport and relaxation
processes. In this paper, these results are used to describe the relaxation and
diffusion of nuclear spins in solids. The aim is to formulate a successive and
coherent microscopic description of the nuclear magnetic relaxation and
diffusion in solids. The nuclear spin-lattice relaxation is considered and the
Gorter relation is derived. As an example, a theory of spin diffusion of the
nuclear magnetic moment in dilute alloys (like Cu-Mn) is developed. It is shown
that due to the dipolar interaction between host nuclear spins and impurity
spins, a nonuniform distribution in the host nuclear spin system will occur and
consequently the macroscopic relaxation time will be strongly determined by the
spin diffusion. The explicit expressions for the relaxation time in certain
physically relevant cases are given.Comment: 41 pages, 119 Refs. Corrected typos, added reference
Krylov-Bogoliubov-Mitropolsky Averaging Used to Construct Effective Hamiltonians in the Theory of Strongly Correlated Electron Systems
We show that the Krylov-Bogoliubov-Mitropolsky averaging in the canonical
formulation can be used as a method for constructing effective Hamiltonians in
the theory of strongly correlated electron systems. As an example, we consider
the transition from the Hamiltonians of the Hubbard and Anderson models to the
respective Hamiltonians of the t-J and Kondo models. This is a very general
method, has several advantages over other methods, and can be used to solve a
wide range of problems in the physics of correlated systems.Comment: 9 page
Ratchet-Like Solitonic Transport in Quantum Hall Bilayers
The pseudo-spin model for double layer quantum Hall system with total landau
level filling factor is discussed. Unlike the "traditional" one where
interlayer voltage enters as static magnetic field along pseudo- spin hard
axis, in our model we consider applied interlayer voltage as a frequency of
precessing pseudo-magnetic field lying into the easy plane. It is shown that a
Landau-Lifshitz equation for the considered pseudo magnetic system well
describes existing experimental data. Besides that, the mentioned model
predicts novel directed intra-layer transport phenomenon in the system:
unidirectional intra-layer energy transport is realized due to interlayer
voltage induced motion of topological kinks. This effect could be observed
experimentally detecting counter-propagating intra-layer inhomogeneous charge
currents which are proportional to the interlayer voltage and total topological
charge of the pseudo-spin system.Comment: 4 pages, 4 figure
Two-pulse stimulated echo in magnets
The results of experimental study of two-pulse stimulated echo in
ferromagnets of two types are presented. Ferromagnet Co and half-metal Co MnSi
2, in which a single-pulse echo formed by the distortion mechanism of the
fronts of exciting pulse is also observed, are classified among the first type.
Lithium ferrite and intermetal compound MnSb characterized by the absence of
single-pulse echo in them - belong to the second type. For signals of two-pulse
stimulated echo in the materials of the first type a short time and a long time
of relaxations are observed. The short time is about the order of value shorter
less than the spin-spin relaxation time. The long time is close to the
transverse relaxation time of single-pulse echo formed by the distortion
mechanism. The mechanisms that provide the possible interpretations of the
peculiarities of the processes of nuclear magnetic relaxation are discussed.Comment: 10 pages, 7 figure
Theory of Sound Propagation in Superfluid Solutions Filled Porous Media
A theory of the propagation of acoustic waves in a porous medium filled with
superfluid solution is developed. The elastic coefficients in the system of
equations are expressed in terms of physically measurable quantities. The
equations obtained describe all volume modes that can propagate in a porous
medium saturated with superfluid solution. Finally, derived equations are
applied to the most important particular case when the normal fluid component
is locked inside a highly porous media (aerogel) by viscous forces and the
velocities of two longitudinal sound modes are calculated.Comment: 13 pages, 0 figure
Saturation of the Inhomogeneously Broadened EPR Line at Low Temperatures under Bottleneck Conditions
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