Search results for "diffusion"

The exact solution of the diffusion trapping model of defect profiling with variable energy positrons

2013

We report an exact analytical solution of so-called positron diffusion trapping model. This model have been widely used for the treatment of the experimental data for defect profiling of the adjoin surface layer using the variable energy positron (VEP) beam technique. Hovewer, up to now this model could be treated only numerically with so-called VEPFIT program. The explicit form of the solutions is obtained for the realistic cases when defect profile is described by a discreet step-like function and continuous exponential-like function. Our solutions allow to derive the analytical expressions for typical positron annihilation characteristics including the positron lifetime spectrum. Latter …

Spin and rotational symmetries in unrestricted Hartree–Fock states of quantum dots

2007

Ground state energies are obtained using the unrestricted Hartree Fock method for up to four interacting electrons parabolically confined in a quantum dot subject to a magnetic field. Restoring spin and rotational symmetries we recover Hund first rule. With increasing magnetic field, crossovers between ground states with different quantum numbers are found for fixed electron number that are not reproduced by the unrestricted Hartree Fock approximation. These are consistent with the ones obtained with more refined techniques. We confirm the presence of a spin blockade due to a spin mismatch in the ground states of three and four electrons.

Anomalous diffusion: Summary

2007

Coulomb interaction in disordered metals and HTSC

2001

Abstract We consider underdoped or overdoped cuprates as disordered conductors. The diffusion coefficient D can be as low as 10 −5 m 2 s −1 . Under these conditions Coulomb interaction between electrons must be taken into account. The main effect is to open a dip in the density of state near the Fermi level. We show that this model explains most of the observed features of the so-called “pseudogap” in the normal state including its value, anisotropy and variation with doping. Such a model applied to disordered metals explains the dips observed in conductance measurements.

Relaxation of the field-induced structural anisotropy in a rotating magnetic fluid

2009

The relaxation of field-induced anisotropy in a magnetic fluid with dominant repulsion is theoretically modeled and experimentally measured by small angle neutron scattering on a sample rotating at angular velocity ω. The scattered pattern distortion scales as the Mason number Mn=ω·τq, τq being the q-dependent diffusion time of nanoparticles. The model accounts for the magnetophoretical drift in the non-homogeneous self-magnetic field of the assembly, continuously created by the thermal noise. The Mn-dependence of the pattern distortion is well described without any adjustable parameter.

Testing Mode-Coupling Theory for a Supercooled Binary Lennard-Jones Mixture II: Intermediate Scattering Function and Dynamic Susceptibility

1995

We have performed a molecular dynamics computer simulation of a supercooled binary Lennard-Jones system in order to compare the dynamical behavior of this system with the predictions of the idealized version of mode-coupling theory (MCT). By scaling the time $t$ by the temperature dependent $\alpha$-relaxation time $\tau(T)$, we find that in the $\alpha$-relaxation regime $F(q,t)$ and $F_s(q,t)$, the coherent and incoherent intermediate scattering functions, for different temperatures each follows a $q$-dependent master curve as a function of scaled time. We show that during the early part of the $\alpha$-relaxation, which is equivalent to the late part of the $\beta$-relaxation, these mast…

Theory of Kondo suppression of spin polarization in nonlocal spin valves

2016

We theoretically analyze contributions from the Kondo effect to the spin polarization and spin diffusion length in all-metal nonlocal spin valves. Interdiffusion of ferromagnetic atoms into the normal metal layer creates a region in which Kondo physics plays a significant role, giving discrepancies between experiment and existing theory. We start from a simple model and construct a modified spin drift-diffusion equation which clearly demonstrates how the Kondo physics not only suppresses the electrical conductivity but even more strongly reduces the spin diffusion length. We also present an explicit expression for the suppression of spin polarization due to Kondo physics in an illustrative …

Instantaneous diffusion in spin-echo dynamics: A frequency-domain experimental investigation.

1992

We report an experimental investigation on the spectral content of the echo radiation emitted by an electron spin system excited by a two-pulse sequence. The measurements are carried out at liquid-helium temperature in a sample of quartz containing a very low concentration of [${\mathrm{AlO}}_{4}$${]}^{0}$ centers, in which instantaneous diffusion mechanism makes the main contribution to the phase relaxation. The experimental spectral shapes reproduce the theoretical ones, as calculated for an ideal system of noninteracting spins. However, the agreement is only qualitative, as the experimental spectral widths are found to be less than expected by a factor that depends on the exciting sequen…

Dynamics of the rotational degrees of freedom in a supercooled liquid of diatomic molecules

1997

Using molecular dynamics computer simulations, we investigate the dynamics of the rotational degrees of freedom in a supercooled system composed of rigid, diatomic molecules. The interaction between the molecules is given by the sum of interaction-site potentials of the Lennard-Jones type. In agreement with mode-coupling theory (MCT), we find that the relaxation times of the orientational time correlation functions C_1^(s), C_2^(s) and C_1 show at low temperatures a power-law with the same critical temperature T_c, and which is also identical to the critical temperature for the translational degrees of freedom. In contrast to MCT we find, however, that for these correlators the time-tempera…

Microscopic theory for the glass transition in a system without static correlations

2002

We study the orientational dynamics of infinitely thin hard rods of length L, with the centers-of-mass fixed on a simple cubic lattice with lattice constant a.We approximate the influence of the surrounding rods onto dynamics of a pair of rods by introducing an effective rotational diffusion constant D(l),l=L/a. We get D(l) ~ [1-v(l)], where v(l) is given through an integral of a time-dependent torque-torque correlator of an isolated pair of rods. A glass transition occurs at l_c, if v(l_c)=1. We present a variational and a numerically exact evaluation of v(l).Close to l_c the diffusion constant decreases as D(l) ~ (l_c-l)^\gamma, with \gamma=1. Our approach predicts a glass transition in t…