Nonlinear spin relaxation in strongly nonequilibrium magnets

Abstract

A general theory is developed for describing the nonlinear relaxation of spin systems from a strongly nonequilibrium initial state, when, in addition, the sample is coupled to a resonator. Such processes are characterized by nonlinear stochastic differential equations. This makes these strongly nonequilibrium processes principally different from the spin relaxation close to an equilibrium state, which is represented by linear differential equations. The consideration is based on a realistic microscopic Hamiltonian including the Zeeman terms, dipole interactions, exchange interactions, and a single-site anisotropy. The influence of cross correlations between several spin species is investigated. The critically important function of coupling between the spin system and a resonant electric circuit is emphasized. The role of all main relaxation rates is analyzed. The phenomenon of self-organization of transition coherence in spin motion, from the quantum chaotic stage of incoherent fluctuations, is thoroughly described. Local spin fluctuations are found to be the triggering cause for starting the spin relaxation from an incoherent nonequilibrium state. The basic regimes of collective coherent spin relaxation are studied.Comment: Latex file, 31 page

Cold bosons in optical lattices

“…Therefore it is necessary to invoke a more delicate decoupling procedure. For this purpose, it is convenient to employ the stochastic mean-field approximation suggested and used earlier for other physical systems [241,242,[370][371][372][373]. This approximation in the present case yields…”

Bose systems in spatially random or time-varying potentials

“…To treat the last term in (40), a more delicate decoupling procedure is required. For this purpose, we shall employ the stochastic mean-field approximation suggested and used earlier for other physical systems [38][39][40][41][42][43]. Following the idea of this approximation, we simplify the last term of Eq.…”

“…Exceptions are the articles [22,23], where the stochastic mean-field approximation [38][39][40][41][42][43] was employed allowing for the description of systems with arbitrarily strong atomic interactions and arbitrarily strong disorder. It is necessary to stress that the perturbation theory with respect to the disorder strength may be inapplicable to the Bose systems in random potentials.…”

Optimal conditions for magnetization reversal of nanocluster assemblies with random properties

“…In resonance experiments, it enhances the NMR signals [16][17][18]. For strongly nonequilibrium systems of polarized nuclei, it leads to fast magnetization reversal [23][24][25][26][27] that has been discovered in experiments [23] and later confirmed in other experimental studies (see references in the review article [4]). …”

“…Here Γ 2 is a transverse attenuation parameter, γ 1 is longitudinal attenuation parameter, and S ≡ N j=1 S j /N is average spin value. In the case of strong initial polarization, the transverse attenuation is characterized [4,14,27,29] by the expression…”