Energy level dynamics across the many-body localization transition

The level dynamics across the many body localization transition is examined for XXZ-spin model with a random magnetic field. We compare different scenaria of parameter dependent motion in the system and consider measures such as level velocities, curvatures as well as their fidelity susceptibilities. Studying the ergodic phase of the model we find that the level dynamics does not reveal the commonly believed universal behavior after rescaling the curvatures by the level velocity variance. At the same time, distributions of level curvatures and fidelity susceptibilities coincide with properly rescaled distributions for Gaussian Orthogonal Ensemble of random matrices. Profound differences exists depending on way the level dynamics is imposed in the many-body localized phase of the model in which the level dynamics can be understood with the help of local integrals of motion.Comment: version close to that accepted in PR

Fidelity susceptibility in Gaussian Random Ensembles

The fidelity susceptibility measures sensitivity of eigenstates to a change of an external parameter. It has been fruitfully used to pin down quantum phase transitions when applied to ground states (with extensions to thermal states). Here we propose to use the fidelity susceptibility as a useful dimensionless measure for complex quantum systems. We find analytically the fidelity susceptibility distributions for Gaussian orthogonal and unitary universality classes for arbitrary system size. The results are verified by a comparison with numerical data.Comment: 2nd version, 5+5pp, comments welcom

Strong-Field Double Ionization in a Three-Electron System: Momentum Distribution Analysis

We study strong-field double ionization in a three-electron system by applying a simplified, reduced-dimensionality model with three active electrons. The influence of the spin-induced symmetry of the spatial part of the wavefunction on the final two-photoectron momentum distribution is discussed. We identify partial momentum distributions originating from different sets of spins of outgoing electrons providing in this way a quantum support connection between V-structure and direct ionization typically explained classically. Changes in the momentum distribution with increasing field amplitude obtained in our simplified model are shown to be well-correlated with experimental data known from the literature. The possible relation between the observed dependencies and different ionization mechanisms is discussed.Comment: 9 pages, 4 figure

Restricted space ab initio models for double ionization by strong laser pulses

Double electron ionisation process occurs when an intense laser pulse interacts with atoms or molecules. Exact {\it ab initio} numerical simulation of such a situation is extremely computer resources demanding, thus often one is forced to apply reduced dimensionality models to get insight into the physics of the process. The performance of several algorithms for simulating double electron ionization by strong femtosecond laser pulses are studied. The obtained ionization yields and the momentum distributions of the released electrons are compared, and the effects of the model dimensionality on the ionization dynamics discussed

The cooperativity in 3D spin-crossover nanocrystals with ferromagnetic and antiferromagnetic surface

In this paper we focus on the thermal hysteresis exhibited by spin-crossover nanocrystals with inhomogeneous interactions changing from the surface to the bulk of lattice. The model of 3D nanocrystal with ferromagnetic and antiferromagnetic coupling of the sites on the surface has been considered. The properties of size effects for ferromagnetic and antiferromagnetic surface have been analyzed in detail. Most of the features experimentally observed heretofore are reproduced by computational simulations using Monte Carlo method for Ising-like model of spin-crossover solid. The results are interpreted in terms of cooperativity