Stochastic Dynamics in Quenched-in Disorder and Hysteresis

The conditions under which relaxation dynamics in the presence of quenched-in disorder lead to rate-independent hysteresis are discussed. The calculation of average hysteresis branches is reduced to the solution of the level-crossing problem for the stochastic field describing quenched-in disorder. Closed analytical solutions are derived for the case where the disorder is characterized by Wiener-Levy statistics. This case is shown to be equivalent to the Preisach model and the associated Preisach distribution is explicitly derived, as a function of the parameters describing the original dynamic problem.Comment: 7 pages, 3 figures, MMM Conference, to be published on J.Appl.Phy

Non-equilibrium thermodynamics of the spin Seebeck and spin Peltier effects

We study the problem of magnetization and heat currents and their associated thermodynamic forces in a magnetic system by focusing on the magnetization transport in ferromagnetic insulators like YIG. The resulting theory is applied to the longitudinal spin Seebeck and the spin Peltier effects. By focusing on the specific geometry with one YIG layer and one Pt layer, we obtain the optimal conditions for generating large magnetization currents into Pt or large temperature effects in YIG. The theoretical predictions are compared with experiments from the literature permitting to derive the values of the thermomagnetic coefficients of YIG: the magnetization diffusion length $l_M \sim 0.4 \, \mu$m and the absolute thermomagnetic power coefficient $\epsilon_M \sim 10^{-2}$ TK$^{-1}$.Comment: accepted for publication on Physical Review

Non-equilibrium Thermodynamics of the Longitudinal Spin Seebeck Effect

In this paper we employ non equilibrium thermodynamics of fluxes and forces to describe magnetization and heat transport. By the theory we are able to identify the thermodynamic driving force of the magnetization current as the gradient of the effective field ▿H∗. This definition permits to define the spin Seebeck coefficient ϵM which relates ▿H∗ and the temperature gradient ▿T. By applying the theory to the geometry of the longitudinal spin Seebeck effect we are able to obtain the optimal conditions for generating large magnetization currents. Furthermore, by using the results of recent experiments, we obtain an order of magnitude for the value of ϵM ∼ 10-2 TK-1 for yttrium iron garnet (Y3Fe5O12)

Ground state optimization and hysteretic demagnetization: the random-field Ising model

We compare the ground state of the random-field Ising model with Gaussian distributed random fields, with its non-equilibrium hysteretic counterpart, the demagnetized state. This is a low energy state obtained by a sequence of slow magnetic field oscillations with decreasing amplitude. The main concern is how optimized the demagnetized state is with respect to the best-possible ground state. Exact results for the energy in d=1 show that in a paramagnet, with finite spin-spin correlations, there is a significant difference in the energies if the disorder is not so strong that the states are trivially almost alike. We use numerical simulations to better characterize the difference between the ground state and the demagnetized state. For d>=3 the random-field Ising model displays a disorder induced phase transition between a paramagnetic and a ferromagnetic state. The locations of the critical points R_c(DS), R_c(GS) differ for the demagnetized state and ground state. Consequently, it is in this regime that the optimization of the demagnetized stat is the worst whereas both deep in the paramagnetic regime and in the ferromagnetic one the states resemble each other to a great extent. We argue based on the numerics that in d=3 the scaling at the transition is the same in the demagnetized and ground states. This claim is corroborated by the exact solution of the model on the Bethe lattice, where the R_c's are also different.Comment: 13 figs. Submitted to Phys. Rev.

The two tryptophans of β2-microglobulin have distinct roles in function and folding and might represent two independent responses to evolutionary pressure

We have recently discovered that the two tryptophans of human β2-microglobulin have distinctive roles within the structure and function of the protein. Deeply buried in the core, Trp95 is essential for folding stability, whereas Trp60, which is solvent-exposed, plays a crucial role in promoting the binding of β2-microglobulin to the heavy chain of the class I major histocompatibility complex (MHCI). We have previously shown that the thermodynamic disadvantage of having Trp60 exposed on the surface is counter-balanced by the perfect fit between it and a cavity within the MHCI heavy chain that contributes significantly to the functional stabilization of the MHCI. Therefore, based on the peculiar differences of the two tryptophans, we have analysed the evolution of β2-microglobulin with respect to these residues

Perturbation theory of nonlinear resonators with an application to Kerr-lens mode locking

The beam propagation in nonlinear optical systems and resonators is formulated by means of operators, and a perturbation method is used to solve, to the first order, the problem of calculation of the nonlinear losses caused by intracavity nonlinearities. Steady-state situations and weak nonlinearities are assumed, but no other restrictions concerning the type of nonlinearity and the resonator structure are imposed. The main result is a simple, closed-form expression of the resonator-loss perturbations that can be determined without the preliminary calculation of the nonlinear self-consistent cavity field, and it has a clear physical interpretation. As an example of an application, a formula for the nonlinear losses of resonators used for Kerr-lens mode locking is derived in the context of Gaussian modes. The result, which is valid for any type of resonator, is numerically illustrated in a few cases of practical relevance

Most efficient beams for frequency mixing in second order nonlinear crystals

none1ISBN: 9780819439482V. MagniMagni, Vittori

Multielement stable resonators containing a variable lens

A unified formulation for the analysis of linear stable resonators containing a lens of variable focal length, which represents the rod of a solid-state laser, and other intracavity optical systems is presented. The stability, the mode spot sizes, the dynamical stability, and the misalignment sensitivity are investigated, and general properties that are valid for any resonator are derived. Some important practical consequences for resonator design are discussed