Events

Charge excitations across electronic band gaps are a key ingredient for transport in optoelectronics and light-harvesting applications. In contrast to conventional semiconductors, studies of above-band-gap photo-excitations in correlated materials are still in their infancy. The new idea presented in this talk is that lifetime changes after a photo-excitation in correlated systems carry essential information about the competition between active degrees of freedom. We will exemplify the concept by a comparative analysis between theoretical many-body simulations and time-resolved ARPES on the excitonic insulator candidate Ta2NiSe5, where the interplay between electronic and lattice degrees of freedom is a matter of hot debate. We will employ photo-induced changes in the lifetime to quantify the competition between electronic and lattice interactions . In particular, the distinction between electron-driven versus lattice-driven situation is apparent as in the former, the photoinduced lifetime changes are substantial, while in the latter, they are strongly suppressed. The quantitative comparison between experiment and theory demonstrates the pivotal (but not necessarily sole) contribution of electron-electron interactions to stabilizing the electronic gap in the material.

Intermediate-scale quantum technologies provide unprecedented opportunities for scientific discoveries while posing the challenge of identifying important problems that can take advantage of them through algorithmic innovations. A major open problem in quantum many-body physics is the table-top generation and detection of emergent excitations analogous to gravitons -- the elusive mediators of gravitational force in a quantum theory of gravity. In solid state materials, fractional quantum Hall phases are one of the leading platforms for realizing graviton-like excitations, however their direct observation remains an experimental challenge. Here, we generate these excitations on the IBM quantum processor. We first identify an effective one-dimensional model that captures the geometric properties and graviton dynamics of fractional quantum Hall states. We then develop an efficient, optimal-control-based variational quantum algorithm to simulate geometric quench and the subsequent graviton dynamics, which we successfully implement on the IBM quantum computer. Our results open a new avenue for studying the emergence of gravitons in a new class of tractable models that lend themselves to direct implementations on the existing quantum hardware.

The ordering of systems emerging through non-equilibrium symmetry breaking transitions is inevitably accompanied by domain formation. The underlying microscopic physics that defines the system's energy landscape for tunneling between domain configurations is of interest in many different areas of physics, ranging from cosmology to solid state quantum matter . Domains may reconfigure by thermally-driven microscopic processes , or - in quantum systems - by macroscopic quantum tunneling. Here, we report quantum domain melting dynamics in two embodiments: an electronic crystal 1T-TaS2, and its matching simulation on a quantum computer . We use scanning tunneling microscopy to measure the time-evolution of electronic domain reconfiguration dynamics in real time, and compare this with the time evolution of domains in an ensemble of entangled correlated electrons in simulated quantum domain melting. The domain reconfiguration is found to proceed by tunneling between minima in a self-configuring energy landscape. A quantum charged lattice gas model is set up in a quantum annealer, that closely matches the experiment. Both are seen to exhibit characteristic ragged time evolution and temperature-dependence observed macroscopically averaged over the ensemble. Understanding the quantum processes involved in electronic domain melting opens the way to understanging non-equilibrium interacting many-body quantum systems at the microscopic level.

We investigate the quench and real-time formation of the Mott state and photoexcited carrier relaxation dynamics in the Mott insulator κ-(BEDT-TTF)2CuCl (κ-Cl) and the superconductor κ-(BEDT-TTF)2CuBr (κ-Br) using three-pulse femtosecond optical spectroscopy. In both salts, we find that transient reflectivity amplitude recovers on 2 ps timescale after a strong near-infrared pulse quench. The transient reflectivity relaxation time is nearly constant throughout indicating that the energy gap for charge excitations is filled rather than closed, by photoinduced carriers of only 0.5% per dimer site. The Mott state is re-formed on a few-picosecond timescale with the disappearance of the in-gap photo-doping induced states near the Fermi energy. In κ-Br, a similar behavior to that in κ-Cl is observed and attributed to the disorder induced phase-separated Mott insulating regions .

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Quantum material systems upon applying ultrashort laser pulses provide a rich platform to access excited material phases and their transformations that are not entirely like their equilibrium counterparts. The addressability and potential controls of metastable or long- trapped out-of-equilibrium phases have motivated interests both for the purposes of understanding the nonequilibrium physics and advancing the quantum technologies. Thus far, the dynamical spectroscopic probes eminently focus on microscopic electronic and phonon responses. For characterizing the long-range dynamics, such as order parameter fields and fluctuation effects, the ultrafast scattering probes offer direct sensitivity. Bridging the connections between the microscopic dynamics and macroscopic responses is central toward establishing the nonequilibrium physics behind the light-induced phases.Learning from the quantum gases microscope experiments and the nonequilibrium sciences on atomic and molecular systems, we present a path to understand the nonequilibrium many-body dynamics using excited quantum material phases as a platform. We first highlight the synergies based on phenomenology to identify common open questions and potentially different manifestations in hard condensed matter systems. To this end, we give the basic theoretical framework on describing the non-equilibrium scattering problems and describe how such framework relates to the out-of-equilibrium phenomena. On the experimental realizations, the focus will be placed on the short-time behaviors mediated by interaction quench. We give effective models outlining the emergences of nonthermal critical points, and hidden phases setting the initial condition for the long-time relaxation dynamics as the system re-establish the thermal states. In particular, the inhomogeneity embedded in the system formation and their impacts on the dynamical evolutions into especially the long-time trapped states stemming from interaction quench are of particular interests. Following the phenomenology, rich scenarios as those involve competitive broken-symmetry orders, vestigial orders, and the intertwined ground states could be identified, leading to intriguing nonequilibrium phenomena from vacuum- suspended rare-earth tritellurides, tantalum disulfides thin films, and vanadium dioxide nanocrystalline materials upon light excitations re-examined in recent experiments.

Resistance switching between charge ordered phases of the 1T-TaS₂ has shown to be potentially useful for the development of high-speed, energy efficient non-volatile memory devices. While ultrafast switching was previously reported with optical pulses, determination of the intrinsic speed limits of actual devices that are triggered by electrical pulses is technically challenging and hitherto still largely unexplored.
Using an optoelectronic “laboratory-on-a-chip” especially designed for measurements of ultrafast memory switching, we are able to accurately measure the electrical switching parameters with sub-100 fs temporal resolution. A photo-switch is used for ultrashort electrical pulse generation, while its propagation along a coplanar transmission line, and across the memory device, is detected using electro-optical sampling using a purpose-grown highly-resistive electro-optic (Cd,Mn)Te crystal substrate.
We observe non-volatile resistance switching with single 1.9 ps electrical pulses, with a switching energy of 0.47 atto-Joules. This represents a significant advance over existing non-volatile memory device concepts in terms of both parameters. The ground-breaking result suggest that electrical charge manipulation in 1T-TaS2 could become a new technological platform for cryogenic, ultrahigh-speed, energy efficient memory devices.

Tunnelling is one of the most fascinating phenomena in quantum physics, whose implications on the dynamics of many-body systems are still unclear. Recently, it has been argued that the presence of inter-particle interactions may lead to cooperative effects, such as the modification of the single-particle tunnelling between wells or the simultaneous tunnelling of a few particles as a single object through a potential barrier. Under certain conditions, these non-standard Hubbard terms are considerably more important than previously assumed, due to correct account of the Wannier functions, which tails were disregarded in many estimations. In this talk, we will examine some preliminary results about possible cooperative effects shown by two particles in a double-well potential, under the effect of different types of interaction. Our results show that, under certain conditions, the non-standard density-induced tunnelling amplitude may suppress the single-particle tunnelling even for repulsive two-particle interactions. This would correspond to a bound state localized in one well, which cannot decay but only propagate between the wells due to the non-standard pair-tunnelling mechanism.

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Identifying an efficient pathway to change the order parameter via a subtle excitation of the coupled high-frequency mode is the ultimate goal of the field of ultrafast phase transitions . This is an especially interesting research direction in magnetism, where the coupling between spin and lattice excitations is required for magnetization reversal . Despite several attempts however, the switching between magnetic states via resonant pumping of phonon modes has not yet beendemonstrated.To provide resonant excitation of the phonon modes, we use pulses from FELIX (Free Electron Lasers for Infrared eXperiments, Nijmegen, The Netherlands). The IR/THz light with photon energy ranging between 25 meV and 124 meV (wavelength 10-50 μm) is typically focused onto the sample. The pulses of FELIX have been shown to be Fourier-transform limited , with their bandwidth experimentally tunable in the range of 0.5-2.0%, corresponding to the typical pulse width of 1-10 ps, depending onthe wavelength range.And thus we show how an ultrafast resonant excitation of the longitudinal optical phonon modes in magnetic garnet films switches magnetization into a peculiar quadrupolar magnetic domain pattern, unambiguously revealing the magneto-elastic mechanism of the switching . In contrast, the excitation of strongly absorbing transverse phonon modes results in thermal demagnetization effect only. The mechanism appears to be very universal, and is shown to work in samples with very different crystallographic symmetry and magnetic properties.

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Optical control of magnetization in numerous ferro and ferrimagnets has been demonstrated in recent years. While the absence of stray fields, the insensitivity to external magnetic fields and ultra-fast dynamics make antiferromagnets promising candidates for active elements in spintronic devices, optical control has been limited to a few insulating antiferromagnets with specific spin configurations at cryogenic temperatures. Here, we demonstrate optical manipulation of the staggered magnetization in the metallic collinear antiferromagnet Mn2Au by combining tensile strain and excitation with femtosecond optical pulses at room temperature. By applying tensile strain along one of the two orthogonal in-plane easy axes and exciting the sample at room temperature by a train of intense femtosecond pulses, we are able to manipulate the direction of the Néel vector, resulting in a stable magnetically aligned state. The dependence of optically induced Néel vector alignment on excitation density and strain suggests the alignment is a result of induced depinning of 90° domain walls and their montion in the direction of the free-energy gradient, governed by the magneto-elastic energy. Such an approach may be applicable to a wider range of collinear antiferromagnets.

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Our recent numerical results on many-body localization in disordered and quasiperiodic spin chains will be presented. The time dynamics in 1D disordered Heisenberg spin-1/2 chain is studied focusing on a regime of large system sizes and a long time evolution. Performing extensive numerical simulations of the imbalance, a quantity often employed in the experimental studies of MBL, we show that the regime of a slow power-law decay of imbalance persists to disorder strengths exceeding by at least a factor of 2 the current estimates of the critical disorder strength for MBL. Even though we investigate time evolution up to few thousands tunneling times, we observe no signs of the saturation of imbalance that would suggest freezing of system dynamics and provide a smoking gun evidence of MBL. We demonstrate that the situation is qualitatively different when the disorder is replaced by a quasiperiodic potential. In this case, we observe an emergence of a pattern of oscillations of the imbalance that is stable with respect to changes in the system size. This suggests that the dynamics of quasiperiodic systems remain fully local at the longest time scales we reach provided that the quasiperiodic potential is sufficiently strong. The results for time dynamics are further confirmed by a finite-size scaling analysis of eigenstates and spectral statistics across the many-body localization in quasiperiodic systems. The analysis shows the many-body localization transition in quasiperiodic systems belongs to the Berezinskii-Kosterlitz-Thouless class, the same as in the case of uniformly disordered systems. However, the finite size effects are less severe in quasiperiodic systems than in chains with random disorder. Also interestingly, deep in the ergodic regime, we find an unexpected double-peak structure of distribution of onsite magnetizations. Our studies identifies challenges in an unequivocal experimental observation of the phenomenon of MBL .

The competition between unitary evolution that spreads information throughout the manybody system, and the monitoring action of an environment gives rise to dynamical phases separated by measurement-induced phase transitions. The first part of the talk will be devoted to numerical investigations of the structure of many-body wave functions of 1D random quantum circuits with local measurements across a measurement-induced transition between phases with volume-law and area-law scaling of entanglement entropy. The many-body wave functions are investigated by means of the participation entropies. The leading term in system size dependence of participation entropies indicates a multifractal scaling of the wavefunctions at any non-zero measurement rate. The sub-leading term contains universal information about measurement-induced phase transitions and plays the role of an order parameter, being non-zero in the volume-law phase and vanishing in the area-law phase. We provide an analytical interpretation of this behavior expressing the participation entropy in terms of partition functions of classical statistical models in 2D. The second part of the talk will concern the structure of many-body wave functions in systems that undergo localization transitions – both in presence and in absence of interactions. The ensuing measures of localization in the system alongside with the more conventional quantities will be used to discuss the stability of many-body localization in the Kicked Ising model.

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