Condensed Matter

We have carried out a microscopic study of the dynamic structure factor of liquid 4 He across the normal-superfluid transition temperature using the path integral Monte Carlo method. The ill-posed problem of the inverse Laplace transform, from the imaginary-time intermediate scattering function to the dynamic response, is tackled by stochastic optimization. Our results show a quasi-particle peak and a small and broad multiphonon contribution. In spite of the lack of strength in the collective peaks, we clearly identify the rapid dropping of the roton peak amplitude when crossing the transition temperature T λ . Other properties such as the static structure factor, static response, and one-phonon contribution to the response are also calculated at different temperatures. The changes of the phonon-roton spectrum with the temperature are also studied. An overall agreement with available experimental data is achieved.

We consider a clean quantum system subject to strong periodic driving. The existence of a dominant energy scale, h x D , can generate considerable structure in an effective description of a system which, in the absence of the drive, is non-integrable, interacting, and does not host localization. In particular, we uncover points of freezing in the space of drive parameters (frequency and amplitude). At those points, the dynamics is severely constrained due to the emergence of an almost exact local conserved quantity, which scars the {\it entire} Floquet spectrum by preventing the system from heating up ergodically, starting from any generic state, even though it delocalizes over an appropriate subspace. At large drive frequencies, where a naïve Magnus expansion would predict a vanishing effective (average) drive, we devise instead a strong-drive Magnus expansion in a moving frame. There, the emergent conservation law is reflected in the appearance of an `integrability' of an effective Hamiltonian. These results hold for a wide variety of Hamiltonians, including the Ising model in a transverse field in {\it any dimension} and for {\it any form of Ising interactions}. The phenomenon is also shown to be robust in the presence of {\it two-body Heisenberg interactions with any arbitrary choice of couplings}. Further, we construct a real-time perturbation theory which captures resonance phenomena where the conservation breaks down, giving way to unbounded heating. This opens a window on the low-frequency regime where the Magnus expansion fails.

Quantum interference lies at the heart of several surprising equilibrium and non-equilibrium phenomena in many-body Physics. Here we discuss two recently explored non-equilibrium scenarios where external periodic drive applied to closed (i.e., not attached to any external bath) quantum many-body systems have apparently opposite effects in respective cases. In one case it freezes/localizes a disorder free system dynamically, while in the other it delocalizes a disordered many-body localized system, and quantum interference is responsible for both the effects. We review these in the perspective of more general questions of ergodicity, energy absorption, asymptotic behavior, and finally the essential role of quantum mechanics in understanding these issues in periodically driven closed many-body systems. In this article we intend to deliver a non-technical account of some recent developments in this field in a manner accessible to a broad readership.

Both applied electric currents and magnetization dynamics modify the Dzyaloshinskii-Moriya interaction (DMI), which we call current-induced DMI (CIDMI) and dynamical DMI (DDMI), respectively. We report a theory of CIDMI and DDMI. The inverse of CIDMI consists in charge pumping by a time-dependent gradient of magnetization ∂ 2 M(r,t)/∂r∂t , while the inverse of DDMI describes the torque generated by ∂ 2 M(r,t)/∂r∂t . In noncollinear magnets CIDMI and DDMI depend on the local magnetization direction. The resulting spatial gradients correspond to torques that need to be included into the theories of Gilbert damping, gyromagnetism, and current-induced torques (CITs) in order to satisfy the Onsager reciprocity relations. CIDMI is related to the modification of orbital magnetism induced by magnetization dynamics, which we call dynamical orbital magnetism (DOM), and spatial gradients of DOM contribute to charge pumping. We present applications of this formalism to the CITs and to the torque-torque correlation in textured Rashba ferromagnets.

The zero-temperature dynamical structure factor S(q,ω) of one-dimensional hard rods is computed using state-of-the-art quantum Monte Carlo and analytic continuation techniques, complemented by a Bethe Ansatz analysis. As the density increases, S(q,ω) reveals a crossover from the Tonks-Girardeau gas to a quasi-solid regime, along which the low-energy properties are found in agreement with the nonlinear Luttinger liquid theory. Our quantitative estimate of S(q,ω) extends beyond the low-energy limit and confirms a theoretical prediction regarding the behavior of S(q,ω) at specific wavevectors Q n =n2π/a , where a is the core radius, resulting from the interplay of the particle-hole boundaries of suitably rescaled ideal Fermi gases. We observe significant similarities between hard rods and one-dimensional 4 He at high density, suggesting that the hard-rods model may provide an accurate description of dense one-dimensional liquids of quantum particles interacting through a strongly repulsive, finite-range potential.

Optical parametric oscillations (OPOs) - a non-linear process involving the coherent coupling of an optically excited two particle pump state to a signal and an idler states with different energies - is a relevant mechanism for optical amplification as well as for the generation of correlated photons. OPOs require states with well-defined symmetries and energies: the fine-tuning of material properties and structural dimensions to create these states remains a challenge for the realization of scalable OPO-based functionalities in semiconductor nanostructures. Here, we demonstrate a pathway towards this goal based on the control of confined microcavity exciton-polaritons modulated by the spatially and time varying dynamical potentials produced by a surface acoustic waves (SAW). The exciton-polariton are confined in um-sized intra-cavity traps fabricated by structuring a planar semiconductor microcavity during the epitaxial growth process. OPOs in these structures benefit from the enhanced non-linearities of confined systems. We show that SAW fields induce state-dependent and time-varying energy shifts, which enable the energy alignment of the confined levels with the appropriate symmetry for OPO triggering. Furthermore, the dynamic acoustic tuning, which is fully described by a theoretical model for the modulation of the confined polaritons by the acoustic field, compensates for fluctuations in symmetry and dimensions of the confinement potential thus enabling a variety of dynamic OPO regimes. The robustness of the acoustic tuning is demonstrated by the synchronous excitation of an array of confined OPOs using a single acoustic beam, thus opening the way for the realization of scalable non-linear on-chip systems.

The motion of helium crystals has been experimentally studied when the crystals fall in the superfluid liquid owing to gravity at temperatures above the roughening transitions where the whole crystal surface is in the atomically rough state. The rate of crystal fall at T = 1.25 K is higher than at T = 1.54 K. This is proof of the essential role of the normal component of superfluid helium in the deceleration of crystal motion. The pressure measurements in the container have shown the effect of surface kinetics on the motions of the crystal and its size. The fall of crystals with the low surface mobility at T = 1.54 K does not change the pressure significantly. The high surface mobility at T = 1.25 K results in decreasing the pressure in the container in the course of the fall of a crystal. The pressure drop exceeds the difference in the hydrostatic pressure between the initial and final positions of the crystal. After the stop, the pressure in the container relaxes to the difference mentioned above. This fact demonstrates an additional growth of the crystal in the flow of a superfluid liquid.

Peculiar dynamics of a free surface of the superfluid 4He has been observed experimentally with a newly established technique utilizing a number of electrically charged fine metal particles trapped electrically at the surface by Moroshkin et al. They have reported that some portion of the particles exhibit some irregular motions and suggested the existence of quantized vortices interacting with the metal particles. We have conducted calculations with the vortex filament model, which turns out to support the idea of the vortex-particle interactions. The observed anomalous metal particle motions are roughly categorized into two types; (1) circular motions with specific frequencies, and (2) quasi-linear oscillations. The former ones seem to be explained once we consider a vertical vortex filament whose edges are terminated at the bottom and at a particle trapped at the surface. Although it is not yet clear whether all the anomalous motions are due to the quantum vortices, the vortices seem to play important roles for the motions.

The present high-resolution inelastic neutron scattering experiment on ice XVII, containing molecular hydrogen with different ortho/para ratio, allows to assign the H 2 motion spectral bands to rotational and center-of-mass translational transitions of either {\it para}- or {\it ortho}-H 2 . Due to its structure, ice XVII confines H 2 molecules to move in spiral channels of molecular size. Reported data demonstrate that H 2 molecules rotate almost freely in these nanometric channels, though showing larger perturbation than in clathrate hydrates, and perform a translational motion exhibiting two low frequency excitations. The good agreement between experimental spectra and corresponding molecular dynamics results clearly enables to portray a picture of the confined motions of a hydrophobic guest within a metastable ice framework, i.e. ice XVII.

Describing superfluid turbulence at intermediate scales between the inter-vortex distance and the macroscale requires an acceptable equation of motion for the density of quantized vortex lines L . The closure of such an equation for superfluid inhomogeneous flows requires additional inputs besides L and the normal and superfluid velocity fields. In this paper we offer a minimal closure using one additional anisotropy parameter I l0 . Using the example of counterflow superfluid turbulence we derive two coupled closure equations for the vortex line density and the anisotropy parameter I l0 with an input of the normal and superfluid velocity fields. The various closure assumptions and the predictions of the resulting theory are tested against numerical simulations.