In this presentation, I focus on the impact of interaction and symmetries on non-equilibrium charge and spin transport in the Hubbard model. The structure of non-equilibrium dynamics is found to change dramatically upon increasing interaction [1-4]. In usual quench experiments, a relatively standard Fermi liquid-like behavior appears for small interactions, while phenomena such as composite particle formation along with negative temperature states emerge for larger interactions.
We investigated temperature dependent ultrafast near-infrared transient reflectivity dynamics in coexisting superconducting (SC) and charge density wave (CDW) phases of layered 2H-NbSe2 using NIR and visible excitations. With visible pump-photon excitation (400 nm) we find a slow high-energy quasiparticle relaxation channel which is present in all phases. In the CDW phase, we observe a distinctive transient response component. The component is marked by the absence of coherent amplitude mode oscillations and a relatively slow, picosecond rise time, which is different than in most of the typical CDW materials, such as 1T-VSe2. In the SC phase, another tiny component emerges that is associated with optical suppression of the SC phase. The transient reflectivity relaxation in the CDW phase is dominated by phonon diffusive processes with an estimated low-T heat diffusion constant anisotropy of ∼ 30. Strong excitation of the CDW phase reveals a weakly non-thermal CDW order parameter (OP) suppression. Unlike CDW systems with a larger gap, where the optical OP suppression involves only a small fraction of phonon degrees of freedom, the OP suppression in 2H-NbSe2 is characterized by the excitation of a large number of phonon degrees of freedom and significantly slower dynamics.
We introduce a Langevin unravelling of the density matrix evolution of an open quantum system over matrix product states, which we term the time-dependent variational principle-Langevin equation. This allows the study of entanglement dynamics as a function of both temperature and coupling to the environment. As the strength of coupling to and temperature of the environment is increased, we find a transition where the entanglement of the individual trajectories saturates, permitting a classical simulation of the system for all times. This is the Hamiltonian open system counterpart of the saturation in entanglement found in random circuits with projective or weak measurements. If a system is open, there is a limit to the advantage in simulating its behaviour on a quantum computer, even when that evolution harbours important quantum effects. Moreover, if a quantum simulator is in this phase, it cannot simulate with quantum advantage.
Quantum materials are characterised by enhanced correlations between their microscopic degrees of freedom that favor the occurence of exciting physical properties that are tunable by optical excitation, including the formation of charge-density waves (CDWs). The investigation of the dynamics following such an optical quench is available in ultrafast transmission electron microscopes (UTEMs), versatile tools that combine nanometer spatial with femtosecond temporal resolution (Fig. 1a).Here, we investigate the transformation between the nearly-commensurate (NC) and the incommensurate (IC) CDW phase in the layered quantum material 1T-TaS2. Therein, selective contrast enhancement by means of ultrafast dark-field microscopy allows us to follow the spatially heterogeneous suppression of the NC phase on the nanoscale, particularly at interphase boundaries. In a complementary approach, the establishment of three-dimensional IC CDW order is accessible by means of ultrafast electron diffraction. Specifically, the high-coherence electron source of the Göttingen UTEM enables collimated diffractive probing of nanometer-sized, spatially homogenous sample regions with high reciprocal-space resolution (Fig. 1c).
When a continuous symmetry is spontaneously broken, two types of collective modes emerge: phase and amplitude mode fluctuations of the order parameter. The latter is a massive excitation living at the edge of the gap. Recently, signatures of the amplitude mode were observed in photo-doped superconductors using third-harmonics generation in the terahertz regime, which opened the field of nonlinear light-amplitude mode coupling.We will show that a weak photo-excitation in superconductors leads to a long-lived prethermal phase with a reduced order parameter and superimposed amplitude oscillations. As we increase the intensity of the pump pulse, the amplitude mode oscillations exhibit chirping; namely, the frequency slows down as a function of time. The chirping gets amplified as we approach the nonthermal critical point – an excitation at which the intensity of the light is large enough to destroy the superconducting order. We identify signatures of the chirped amplitude mode in photo-induced current after a monocycle or Gaussian pulse and propose a transmission line circuit experiment to detect the phenomena. In the last part, we will present recent theoretical advances based on the compressed representation of quantum propagators, which allow the analysis of time scales relevant for the collective mode dynamics.
Driving certain cuprates and organic materials has been shown to induce THz optical properties reminiscent of superconductivity far above the equilibrium transition temperature. However, the magnetic response of these non-equilibrium states remains unexplored. This study investigates whether these states exhibit a Meissner effect, expelling external magnetic fields, and examines their response to changing magnetic fields on sub-picosecond time scales.Our methodology involves studying the ultrafast magnetic response of these materials in static and time-dependent magnetic fields using the Faraday effect in a magneto-optical crystal adjacent to the sample. This provides sub-picosecond time resolution for reconstructing the position-dependent magnetic properties.
Many modern devices are based on the operations with spins. They require the preparation of the initial state: with spin polarization or at least with spin correlations. There is a limited number of conventional methods to achieve this: application of ferromagnetic materials, current to spin conversion due to spin-orbit interaction and static polarization in high magnetic fields. We show the existence of another way to achieve spin polarization in non-magnetic solid-state devices. It requires small magnetic fields, hyperfine interaction between electron and nuclear spins and small exchange interaction between electron spins. All the interaction energies are considered small compared to temperature, but the mechanism requires non-equilibrium conditions.
Description of the relaxation or thermalization of the strongly correlated system close to the integrable point remains a challenge. Although the fate of such systems is ultimately an ergodic dynamic, the road to it could take an extremely long time and can display some exotic (type of integrability breaking-dependent) behavior. On the one hand, many extremely long-time scales – reminiscent of integrals of motions – prevent numerical simulations from reaching an unbiased conclusion on such systems. On the other hand, the nonintegrability hinders the analytical approaches. Recently, a new family of ergodicity-breaking systems was found where the Hilbert space is fragmented into exponentially many parts due to constraints on the possible dynamics. Taking the t-Jz model as an example, we will show how one can control the degree of Hilbert space fragmentation, i.e., the number of disconnected subspaces. We will discuss how various time scales emerge from the breakdown of fragmentation and how they affect the relaxation of such systems.
The ability to switch magnets between two stable bit states is the main principle of digital data storage technologies since the early days of the computer. Since our demonstration of magnetization reversal by a single 40 femtosecond laser pulse, the manipulation of spins by ultra-short laser pulses has developed into an alternative and energy efficient approach to magnetic recording. Though originally thought to be due to an optically induced effective field, later studies demonstrated that the switching occurred via a strongly non-equilibrium state, exploiting the exchange interaction between the spins. Recent work also show how magnetic textures like skyrmions are generated via a non-equilibrium phase. While for a long time, all-optical switching (AOS) was exclusively observed in ferrimagnetic alloys, more recent work demonstrated AOS in a broad range of ferromagnetic multilayer materials, albeit that in those examples a large number of pulses were required. By studying the dynamics of this switching process, we have discovered that this switching is a 2-step process, which led us to the subsequent demonstration that highly efficient AOS can be achieved by using pairs of femto/pico-second laser pulses. By combining optical laser excitation with in situ magnetic force microscopy we recently found that the nucleation and switching process evolves via a stochastic network of domains.
A. Caviglia1 1University of Geneva, Switzerland
