Table of contents

If a magnetic polarization excess is locally injected in a crystal of interacting spins in thermal equilibrium, this 'excitation' would spread as consequence of spin–spin interactions. Such an apparently irreversible process is known as spin diffusion and it can lead the system back to 'equilibrium'. Even so, a unitary quantum dynamics would ensure a precise memory of the non-equilibrium initial condition. Then, if at a certain time, say $t/2$, an experimental protocol reverses the many-body dynamics by changing the sign of the effective Hamiltonian, it would drive the system back to the initial non-equilibrium state at time t. As a matter of fact, the reversal is always perturbed by small experimental imperfections and/or uncontrolled internal or environmental degrees of freedom. This limits the amount of signal M(t) recovered locally at time t. The degradation of M(t) accounts for these perturbations, which can also be seen as the sources of decoherence. This general idea defines the Loschmidt echo (LE), which embodies the various time-reversal procedures implemented in nuclear magnetic resonance. Here, we present an invitation to the study of the LE following the pathway induced by the experiments. With such a purpose, we provide a historical and conceptual overview that briefly revisits selected phenomena that underlie the LE dynamics including chaos, decoherence, localization and equilibration. This guiding thread ultimately leads us to the discussion of decoherence and irreversibility as an emergent phenomenon. In addition, we introduce the LE formalism by means of spin–spin correlation functions in a manner suitable for presentation in a broad scope physics journal. Last, but not least, we present new results that could trigger new experiments and theoretical ideas. In particular, we propose to transform an initially localized excitation into a more complex initial state, enabling a dynamically prepared LE. This induces a global definition of the LE in terms of the raw overlap between many-body wave functions. Our results show that as the complexity of the prepared state increases, it becomes more fragile towards small perturbations.

Computational mathematics, physics and engineering form a major constituent of modern computational science, which now stands on an equal footing with the established branches of theoretical and experimental sciences. Computational mechanics solves problems in science and engineering based upon mathematical modeling and computing, bypassing the need for expensive and time-consuming laboratory setups and experimental measurements. Furthermore, it allows the numerical simulations of large scale systems, such as the formation of galaxies that could not be done in any earth bound laboratories. This article is written as part of the 21st Century Frontiers Series to illustrate some state-of-the-art computational science. We emphasize how to do numerical modeling and visualization in the study of a contemporary event, the pulverizing crash of the Germanwings Flight 9525 on March 24, 2015, as a showcase. Such numerical modeling and the ensuing simulation of aircraft crashes into land or mountain are complex tasks as they involve both theoretical study and supercomputing of a complex physical system. The most tragic type of crash involves 'pulverization' such as the one suffered by this Germanwings flight. Here, we show pulverizing airliner crashes by visualization through video animations from supercomputer applications of the numerical modeling tool LS-DYNA. A sound validation process is challenging but essential for any sophisticated calculations. We achieve this by validation against the experimental data from a crash test done in 1993 of an F4 Phantom II fighter jet into a wall. We have developed a method by hybridizing two primary methods: finite element analysis and smoothed particle hydrodynamics. This hybrid method also enhances visualization by showing a 'debris cloud'. Based on our supercomputer simulations and the visualization, we point out that prior works on this topic based on 'hollow interior' modeling can be quite problematic and, thus, not likely to be correct. We discuss the effects of terrain on pulverization using the information from the recovered flight-data-recorder and show our forensics and assessments of what may have happened during the final moments of the crash. Finally, we point out that our study has potential for being made into real-time flight crash simulators to help the study of crashworthiness and survivability for future aviation safety. Some forward-looking statements are also made.

The control of nonlinear drift waves in a magnetized plasmas column has been investigated. The studies are based on the Hasegawa–Mima model, which is solved on a disk domain with radial inhomogeneity of the plasma density. The system is forced by a rotating potential with varying frequency defined on the boundary. To excite and control the waves we apply the autoresonant effect, taking place when the amplitude of the forcing exceeds a threshold value and the waves are phase-locked with the forcing. We demonstrate that the autoresonant approach is applicable for excitation of a range of steady nonlinear waves of the lowest azimuthal mode numbers and for controlling their amplitudes and phases. We also demonstrate the excitation of zonal flows (m = 0 modes), which are controlled via the forced modes.

A reverse Monte Carlo analysis method was employed to extract the structure of CeO₂ from Neutron total scattering (comprising both neutron diffraction (ND) and pair-distribution functions (PDF) and Ce L₃- and K-edge EXAFS data. Here it is shown that there is a noticeable difference between using short ranged x-ray absorption spectroscopy data and using medium-long range PDF and ND data in regards to the disorder of the cerium atoms. This illustrates the importance of considering multiple length scales and radiation sources.

In order to study the temporally distributed energy deposition in the formation of laser-induced periodic surface structures (LIPSS) on single-crystalline zinc oxide (ZnO), two-colour double-fs-pulse experiments were performed. Parallel or cross-polarised double-pulse sequences at 400 and 800 nm wavelength were generated by a Mach–Zehnder interferometer, exhibiting inter-pulse delays up to a few picoseconds between the sub-ablation 50-fs-pulses. Twenty two-colour double-pulse sequences were collinearly focused by a spherical mirror to the sample surface. The resulting LIPSS periods and areas were analysed by scanning electron microscopy. The delay-dependence of these LIPSS characteristics shows a dissimilar behaviour when compared to the semiconductor silicon, the dielectric fused silica, or the metal titanium. A wavelength-dependent plasmonic mechanism is proposed to explain the delay-dependence of the LIPSS on ZnO when considering multi-photon excitation processes. Our results support the involvement of nonlinear processes for temporally overlapping pulses. These experiments extend previous two-colour studies on the indirect semiconductor silicon towards the direct wide band-gap semiconductor ZnO and further manifest the relevance of the ultrafast energy deposition for LIPSS formation.

The possibility of investigating the dynamics of solids on timescales faster than the thermalization of the internal degrees of freedom has disclosed novel non-equilibrium phenomena that have no counterpart at equilibrium. Transition metal oxides (TMOs) provide an interesting playground in which the correlations among the charges in the metal d-orbitals give rise to a wealth of intriguing electronic and thermodynamic properties involving the spin, charge, lattice and orbital orders. Furthermore, the physical properties of TMOs can be engineered at the atomic level, thus providing the platform to investigate the transport phenomena on timescales of the order of the intrinsic decoherence time of the charge excitations. Here, we review and discuss three paradigmatic examples of transient emerging properties that are expected to open new fields of research: (i) the creation of non-thermal magnetic states in spin–orbit Mott insulators; (ii) the possible exploitation of quantum paths for the transport and collection of charge excitations in heterostructures; (iii) the transient wave-like behavior of the temperature field in strongly anisotropic TMOs.

We review our work on the photoinduced dynamics of X[M(dmit)₂]₂ salts in the charge-separated (CS) phase from the viewpoints of both electronic structural change and lattice or molecular structural change. The driving force for the CS phase formation is a strong correlation among the charge distribution, orbital energies, and the molecular structure; this is in contrast to frequently studied charge-ordered systems such as EDOTTF₂PF₆, whose formation is mainly driven by the intersite Coulomb repulsion. Despite the localized nature of the structural component, the cooperativity inherent to the crystal is likely to play an important role in photoinduced phenomena. In this review, we summarize the results obtained by a series of optical pump-probe experiments on this class of materials. We have also recently extended these studies using ultrafast electron diffraction to follow the nuclear motion in one of these systems. Such information is crucial for a full understanding of the photoinduced phenomena and it is anticipated that combining the results of the optical and diffraction studies will lead to more fruitful insight than either technique can offer in isolation.

Phase-conjugate scattering can trigger modulational instabilities in a fluid of exciton-polaritons created in a pumped semiconductor quantum-well microcavity. These instabilities can settle into density patterns, e.g. hexagons and stripes, which produce corresponding patterns in the emitted light. The density patterns can be switched by relatively weak control optical beams. This paper reviews progress in our theoretical understanding of the physical processes that regulate the competitions among various patterns and drive the optical switching. Simulation results of pattern switching using a microscopic model of polariton dynamics are shown, and the mechanisms underlying competitions and switching are analyzed using reduced models that restrict the polariton motions to a limited number of relevant modes. We also briefly indicate the effects of the spin dependence of the polariton dynamics on the patterns.

In this review, we develop the formalism employed to describe charge-density-wave insulators in pump/probe experiments that use ultrashort driving pulses of light. The theory emphasizes exact results in the simplest model for a charge-density-wave insulator (given by a noninteracting system with two bands and a gap) and employs nonequilibrium dynamical mean-field theory to solve the Falicov–Kimball model in its ordered phase. We show how to develop the formalism and how the solutions behave. Care is taken to describe the details behind these calculations and to show how to verify their accuracy via sum-rule constraints.

An analytical model to study the whistler turbulence spectrum and inertial range spectral scalings related with the electric and magnetic field spectra in a weakly non-collisional magnetized plasma is developed. In the present model, the dispersion relation of whistler wave propagating along the background magnetic field is exploited to derive the inertial range scaling laws corresponding to the electric field and magnetic field fluctuations. The model is based on the concept of Iroshnikov–Kraichnan inertial range magnetohydrodynamic turbulence. The present phenomenological turbulence scaling model suggests the energy spectra associated with the whistler wave electric field fluctuations and magnetic field fluctuations pursue −1 and −3 power-laws, respectively. Moreover, the results obtained by the present analytical model reasonably able to explain and compare the observational energy spectra in the wave number domain. The observational and analytical results support the whistler dominated turbulence with the similar spectral index −1 of the electric field spectra, whereas, the magnetic field spectral indices in the wave number domain are found $-13/3$ and −3 for the observational and analytical approach, respectively.

We propose an alternative way for managing near shore surging waves, including extreme waves like tsunamis, going beyond the conventional passive measures like the warning system. We study theoretically the possibility of influencing the nonlinear surface waves through a leakage boundary effect at the bottom. It has been found through analytic result, that the controlled leakage at the bottom might regulate the amplitude of the surface solitary waves. This could lead to a possible decay of the surging waves to reduce its hazardous effects near the shore. Our theoretical results are estimated by applying it to a real coastal bathymetry of the Bay of Bengal in India.