Bose-Einstein condensation (BEC) has been observed in a dilute gas of sodium atoms. A Bose-Einstein condensate consists of a macroscopic population of the ground state of the system, and is a coherent state of matter. In an ideal gas, this phase transition is purely quantum-statistical. The study of BEC in weakly interacting systems which can be controlled and observed with precision holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles.

Bose-Einstein condensation in a gas of sodium atoms

We propose an experimental scheme to observe spin Hall effects with cold atoms in a light-induced gauge potential. Under an appropriate configuration, the cold atoms moving in a spatially varying laser field experience an effective spin-dependent gauge potential. Through numerical simulation, we demonstrate that such a gauge field leads to observable spin Hall currents under realistic conditions. We also discuss the quantum spin Hall state in an optical lattice.

Spin Hall effects for cold atoms in a light induced gauge potential

We propose an optical lattice scheme which would permit the experimental observation of Zitterbewegung (ZB) with ultracold, neutral atoms. A four-level tripod variant of the setup for stimulated Raman adiabatic passage (STIRAP) has previously been proposed for generating non-Abelian gauge fields. Dirac-like Hamiltonians, which exhibit ZB, are simple examples of such non-Abelian gauge fields; we show how a variety of them can arise, and how ZB can be observed, in a tripod system. We predict that the ZB should occur at experimentally accessible frequencies and amplitudes.

Observing Zitterbewegung with Ultracold Atoms

The structure of molecules, particularly the dynamic changes in structure, plays an essential role in understanding physical and chemical phenomena. Time-resolved (TR) scattering techniques serve as crucial experimental tools for studying structural dynamics, offering direct sensitivity to molecular structures through scattering signals. Over the past decade, the advent of x-ray free-electron lasers (XFELs) and mega-electron-volt ultrafast electron diffraction (MeV-UED) facilities has ushered TR scattering experiments into a new era, garnering significant attention. In this review, we delve into the basic principles of TR scattering experiments, especially focusing on those that employ x-rays and electrons. We highlight the variations in experimental conditions when employing x-rays vs electrons and discuss their complementarity. Additionally, cutting-edge XFELs and MeV-UED facilities for TR x-ray and electron scattering experiments and the experiments performed at those facilities are reviewed. As new facilities are constructed and existing ones undergo upgrades, the landscape for TR x-ray and electron scattering experiments is poised for further expansion. Through this review, we aim to facilitate the effective utilization of these emerging opportunities, assisting researchers in delving deeper into the intricate dynamics of molecular structures. 

A comparative review of time-resolved x-ray and electron scattering to probe structural dynamics

An intense femtosecond-laser excitation of a solid induces highly nonthermal conditions. In materials like silicon, laser-induced bond-softening leads to a highly incoherent ionic motion and eventually nonthermal melting. But is this outcome an inevitable consequence, or can it be controlled? Here, we performed ab initio molecular dynamics simulations of crystalline silicon after multiple femtosecond-laser pulse excitations with total energy above the nonthermal melting threshold. Our results demonstrate an excitation mechanism, which can be generalized to other materials, that pauses nonthermal melting and creates a metastable state instead, which has an electronic structure similar to the ground state. Our findings open the way for a search for metastable structural and/or electronic transitions in the high-excitation regime. In addition, our approach could be used to switch off nonthermal contributions in experiments, which could be used to obtain reliable electron-phonon coupling constants more easily.

pdf/pausing-ultrafast-melting-by-multiple-femtosecond-laser-3cz79rb6.pdf

Pausing ultrafast melting by multiple femtosecond-laser pulses

Ultrafast lattice disordering can be accelerated by electronic collisional forces

We have investigated dynamic characteristics of massless Dirac and hybrid quasiparticles in an optical lattice. Zitterbewegung (ZB) of the quasiparticles has been realized and well manipulated. The results reveal that massless Dirac quasiparticles can also generate ZB oscillation, only with an initial velocity being introduced to the quasiparticles as the price to pay. Besides, we noticed an essential distinction between this momentum-induced ZB and an ordinary ZB generated by massive Dirac particles, i.e., instead of all directions, ZB induced by momentum in a 2D system will emerge exclusively in the direction orthogonal to where the momentum is introduced. On the other hand, momentum-induced ZB will be attenuated more rapidly since the initial momentum causes greater expansion rate of wave packets. Therefore, an optimal range of parameters is necessary in detecting such ZB. We have confidence that momentum-induced ZB for massless Dirac quasiparticles will be observed in the near future since the frequency, amplitude and lifespan of ZB can now be controlled in a detectable range in the cold atom system.

Momentum-induced Zitterbewegung of massless relativistic quasiparticles emreged in an optical lattice

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Ultrafast lattice disordering can be accelerated by electronic collisional forces

Momentum-induced Zitterbewegung of massless relativistic quasiparticles emreged in an optical lattice