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.The discovery of many body localization (MBL) phenomenon resulting in non-ergodicity of the dynamics in many body systems [1] restored also the interest in purely ergodic phenomena modeled by Gaussian random ensembles (GRE) [2] and in possible measures to characterize them. The gap ratio between adjacent level spacings [3] was introduced precisely for that purpose as it does not involve the so called unfolding [4] necessary for meaningful studies of level spacing distributions and yet often leading to spurious results [5]. Still, the level spacing distribution belongs to the most popular statistical measures used for single particle quantum chaos studies [6][7][8][9] and also in the transition to MBL [10-13]. A particular place among different measures was taken by those characterizing level dynamics for a Hamiltonian H(λ) dependent on some parameter λ. In Pechukas-Yukawa formulation [14,15] energy levels are positions of a fictitious gas particles, derivatives with respect to the fictitious time λ are velocities (level slopes), the second derivatives describe curvatures of the levels (accelerations). Simons and Altschuler [16] put forward a proposition that the variance of velocities distribution is an important parameter characterizing universality of level dynamics. This led to predictions for distributions of avoided crossings [17] and, importantly, curvature distributions postulated first on the basis of numerical data for GRE [18] and then derived analytically via supersymmetric method by von Oppen [19,20] (for alternative techniques see [21,22]). Curvature distributions were recently addressed in MBL studies [23,24].Apart from quantum chaos studies in the eighties and nineties of the last millennium, another "level dynamics" tool has been introduced in the quantum information area, i.e. the fidelity [25]. It compares two close (possibly mixed) quantum states. If these states are dependent on a parameter λ it is customary to introduce a fidelity susceptibility χ. For sufficiently small λ, in a finite system, one hasFidelity susceptibility is directly related to the quantum Fisher information (QFI), G, being directly proportional to the Bures distance between density matrices at slightly differing values of λ [26,27], with G(λ) = 4χ. Fidelity susceptibility emerged as a useful tool to study quantum phase transitions as at the transition point the ground state changes rapidly leading to the enhancement of χ [27][28][29][30][31][32][33][34]. All of these studies w...

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.arXiv:1904.05057v2 [cond-mat.dis-nn]

Double electron ionisation process occurs when an intense laser pulse interacts with atoms or molecules. Exact 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. arXiv:1803.08364v2 [physics.atom-ph]

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The thermal transition accompanied by the variation of the molecular volume in nanoparticles of spin-crossover materials has been studied on the basis of microscopic Ising-like model solved using Monte Carlo methods. For considered model, we examined the spin-crossover phenomenon with applied hydrostatic pressure and thus was shown the possibility to shift transition temperature toward its room value. The obtained results of numerical simulations are in agreement with the experimental ones.

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Strong field processes involving several active electrons reveal unambiguous dynamical signatures of the Pauli principle importance even in the nonrelativistic regime. We exemplify this statement studying three active electrons model atoms interacting with strong pulsed radiation, using an ab-initio time-dependent Schrödinger equation on a grid. In our restricted dimensionality model we are able to analyze momenta correlations of the three outgoing electrons using Dalitz plots. The different symmetries of the electronic wavefunctions, directly related to the initial state spin components, appear clearly visible.

The spin-crossover (SCO) phenomenon corresponds to the switching of the electronic configuration of a transition metal compound between high spin and low spin states as a result of a perturbation. The latter can be of many kinds including modifications of temperature, pressure, light irradiation, magnetic field or pH. The SCO inevitably induces a change of magnetic, optical and structural properties of the material. Whereas in solution the SCO takes place without cooperativity between metal centres resulting in a conversion that is always very gradual, in the solid state the conversion presents a large diversity of behaviours ranging from very abrupt with hysteresis to multi-step or incomplete reversible SCO for instance. It is therefore in the solid state that the SCO offers the best circumstances to investigate its fundamental aspects and to offer rational molecular engineering through the establishment of structure-properties relationships with applications in view [1]. In molecular solids, the SCO is always associated to structural modifications that start at the atomic level and then propagate from the coordination sphere to the crystal-packing and to the sample scales. As a consequence of these structural modifications, the phase-transition diagrams of SCO molecular solids can appear rather intricate in some cases, as illustrated in the figure below showing the interplay between the crystal symmetry, the spin state and external perturbations in the case of an iron(II) molecular complex (adapted from [1]). We will discuss the structural modifications in molecular SCO solids considering various physical scales. A focus will be made on the amplitude of the multiscale volume variations as well as on the transition mechanisms and the resulting phase diagrams, including unexpected behaviours. Structural movies determined thanks to X-ray diffraction [2] will be used to illustrate the results. Last but not least, discrepancies between observation made on molecular SCO solids in the form of powders and single-crystals will be highlighted and opened to discussions.