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Nelson Hua: Ultrafast Electronic Domain Fluctuations Detected in a Nonequilibrium X-Ray Speckle Visibility Experiment
Since the advent of X-ray free electron lasers, a standard method to study metal-insulator transitions is in a nonequilibrium pump-probe diffraction experiment to disentangle how different order parameters evolve at ultrafast timescales. However, this technique is blind to domain fluctuations of the order parameter that may play a critical role in driving these nonequilibrium transitions. To directly couple to these domain fluctuations at ultrafast timescales requires coherent X-ray probes following a laser excitation. Here we employed a novel coherent X-ray technique that uses a split-and-delay line in a pump-probe-probe experimental scheme to measure ultrafast domain fluctuations for the first time. This experiment was carried out at the X-ray Correlation Spectroscopy (XCS) beamline of the Linac Coherent Light Source (LCLS) where we accessed the speckle pattern of a resonant charge order peak in Fe3O4 to quantify domain fluctuations with 1-picosecond temporal resolution. A complementary, standard X-ray pump-probe experiment on the same charge order peak was carried out at the Bernina end station of SwissFEL to reveal ultrafast melting of the electronic order parameter. Together these two experiments reveal the nonequilibrium evolution of the charge order at picosecond and nanometer length scales.
Gregor Humar: False vacuum decay in the transverse field Ising model on a quantum annealer
This presentation deals with using quantum annealing for observing false vacuum decay in the transverse field Ising model. False vacuum decay is one of the central ideas in quantum field theory. It describes a scenario where a system in a metastable false vacuum state transitions to the true vacuum state. The transition happens by creation of bubbles of true vacuum that expand over the whole system. The timescales and the dynamics of this phenomenon are difficult to observe and describe analytically. The process of transition is analogue to first order phase transitions in condensed matter physics. Metastable states with analogous dynamics can be observed on measurable timescales. An example of such a system is the transverse field Ising model, where false vacuum decay appears in nonequilibrium dynamics following a sudden change in the direction of the external field. Numerical studies have shown the existance of a set of parameters for which false vacuum decay can be observed. Transverse field Ising model is implemented in the D-Wave quantum annealer. Measurements on this device are used to simulate the dynamics of the transverse Ising model. In general, results of simulations show decay dynamics that do not match the theoretical description of the false vacuum decay exactly. This implies additions effects on the dynamics. In the specific limit of low transverse fields and high longitudinal fields the measured dynamics approach the theoretically expected dynamics. Possible explanations for the observed deviations include the open nature of the system in the quantum annealer, the slow change of the field direction and poor validity of the approximations used for theoretical predictions for magnitudes of fields used in the simulations.
Daniele Fausti: Cavity-Mediated Thermal Modulation of the Metal-to-Insulator Transition in 1T-TaS2
This seminar will focus on the potential of cavity electrodynamics in shaping material properties, opened by on our recent investigation into cavity-mediated thermal control of the metal-to-insulator transition in 1T-TaS2, which demonstrated the feasibility of reversible cavity manipulation of a phase transition in a correlated solid-state material.By immersing the charge density wave material 1T-TaS2 into cryogenic tunable terahertz cavities, we unveil a remarkable shift between conductive states. This transition, triggered by a substantial alteration in sample temperature, is controlled by mechanical adjustments of the distance between cavity mirrors and their alignment.
Martin Eckstein: Light-matter hybrids made from strongly correlated electron systems
Enhancing the light-matter coupling in cavities provides an intriguing avenue to control properties of matter, from chemical reactions to transport and thermodynamic phase transitions. In this talk, I discuss two mechanisms in which quantum light can influence extended condensed matter systems, in particular strongly correlated electron systems: (i) The hybridization of light and matter can affect first-order metal insulator transitions, because light selectively modifies the free energy of the metallic phase . This mechanism has been discussed in relation to the cavity-controlled metalinsulator transition in 1T-TaS2. While it most likely is not the relevant mechanism in this case, we discuss other situations where it can be decisive. (ii) We discuss the possibility to induce photon mediated long-range interactions between spin and orbital degrees of freedom, which rely on the nonlinear light matter interaction (Raman scattering or two-photon absorption and emission).
Federico Cilento: Ultrafast Dynamics of the Charge Density Wave in Kagome Metals
The Charge Density Wave (CDW) order, descending from a metallic parent state, offers an intriguing playground to study the interplay of structural and electronic degrees of freedom in complex materials. Recently, this phenomenology has been discovered also in Kagome metals. With dispersive and correlation features including topological Dirac-like itinerant states, van-Hove singularities, correlated flat bands, and magnetic transitions at low temperature, kagome metals are located in the interesting regime where both phonon and electronically mediated couplings are significant. In particular, the van-Hove singularities, which are intrinsic to the kagome tiling, have been conjectured to play a key role in mediating the CDW instability. However, to date, the origin and the main driving force behind this charge order is elusive. Here, we use the topological bilayer kagome metal ScV6Sn6 as a platform to investigate this puzzling problem, and combine time-resolved optical spectroscopy, to unveil the ultrafast dynamics of its CDW phase, with angle-resolved photoelectron spectroscopy and density functional theory. We identify the structural degrees of freedom to play a fundamental role in the stabilization of charge order. In particular, we find ScV6Sn6 to feature a charge density wave order that predominantly originates from phonons, as odd with other recent findings on other kagome metals like those from the AV3Sb5 (A = K, Rb, Cs) family, where the CDW originates from an electronic instability. As we shed light on the lattice-mediated low-temperature ordered phase, our findings pave the way for a deeper understanding of ordering phenomena in CDW kagome metals.
Michael Sentef: Light-Matter Control of Quantum Materials
Advances in time-resolved pump-probe spectroscopies have enabled us to follow the microscopic dynamics of quantum materials on femtosecond time scales. This gives us a glimpse into the inner workings of how complex, emergent functionalities of quantum many-body systems develop on ultrafast time scales or react to external forces. The ultimate dream of the community is to use light as a tuning parameter to create new states of matter on demand with designed properties and new functionalities, perhaps not achievable by other means. In this talk I will discuss recent progress in controlling and engineering properties of quantum materials through light-matter interaction. I will highlight work on Floquet engineering — the creation of effective Hamiltonians by time-periodic drives — on sub-cycle time scales combining theory and pump-probe experiments at the limits of energy and time resolution. I will then showcase recent theories on inducing superconductivity with light by employing enhanced light-matter interaction in the near-field involving polaritonic excitations.
James McIver: Ultrafast control of topological transport in quantum materials
Quantum materials exhibit remarkable non-equilibrium phenomena when driven by the strong fields in femtosecond laser pulses. Recent years have seen a surge of interest in using ultrafast light-matter interaction to create and manipulate photon-dressed Floquet-Bloch states as a strategy for controlling material properties. This excitement is fueled by the predictive power of Floquet theory, which has been used, for example, to correctly predict the formation of topological edge modes in periodically-driven systems that exhibit no topological properties in equilibrium. Many of these proposals have been verified in quantum simulation settings, but are only just beginning to be explored in solids.In this talk, I will present results on the electrical transport properties of quantum materials driven by mid-infrared laser pulses, probed using an ultrafast optoelectronic device architecture. The talk will primarily focus on recent results obtained on the Weyl semimetal Td-MoTe2, where a rectified, helicity-dependent injection current that scales linearly with the applied laser field was observed. This scaling violates the perturbative description of nonlinear optics/transport, which demands a quadratic field scaling for current rectification to occur. The results can be explained using Floquet theory, which predicts that the observed linear scaling arises from the stimulated emission that accompanies the hybridization of Floquet-Bloch states.
Sourav Nandy: Emergent dipole moment conservation and subdiffusion in tilted chains
I plan to talk regarding the transport dynamics of an interacting tilted (Stark) chain, based on our recent work. In this work, we have shown that the crossover between diffusive and subdiffusive dynamics in such a system is governed by F√L, where F is the strength of the field, and L is the wave-length of the excitation. While the subdiffusive dynamics persist for large fields, the corresponding transport coefficient is exponentially suppressed with F so that the finite-time dynamics appear almost frozen. Our work explains the crossover scale between the diffusive and subdiffusive transport by bounding the dynamics of the dipole moment for arbitrary initial state. We also prove its emergent conservation at infinite temperature. Consequently, the studied chain is one of the simplest experimentally realizable models for which numerical data are consistent with the hydrodynamics of fractons.
Lunch
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Andrei Kirilyuk: Shaken, not stirred: a recipe for ultrafast magnetic switching via phononic resonances
Strong light-matter interaction constitutes the bedrock of all photonic applications, empowering material elements to create and mediate interactions of light with light. Among others, phononamplified interactions were shown to bring a specific twist into this, in the infrared (IR) frequency range. Thus, phono-magnetic effects are the low-frequency analogues of inverse Faraday and Cotton-Mouton effects where phonons, not electrons, mediate the interaction between light and spins. In this case, light couples to the spins indirectly by exciting coherent vibrations of the crystal lattice (phonons) that transfer angular momentum to the magnetic ions. The optically driven chiral phonons in materials with strong spin-orbit coupling were shown to produce giant effective magnetic fields that exceed those previously seen by several orders of magnitude. The mechanism allows for bidirectional control of the induced magnetization through phonon chirality that in turn can be controlled by the polarization of the laser pulse.
Philipp Werner: Photo-induced nonthermal metals
Several insulating materials can be switched by laser pulses into transient metal states with apparently nonthermal properties. Often, this has been interpreted as a nonthermal closing of a Mott gap. An alternative mechanism is the generation of in-gap states by the nonthermal population of multiplets (e. g. singlet-triplet excitations in dimerized systems), or the nonthermal reshuffling of charge between orbitals. I will present recent nonequilibrium dynamical mean field theory studies of model systems, which provide insights into the nature of photo-induced nonthermal metal states in 1T-TaS2 and rare earth nickelates, and realistic simulations of the photo-induced dynamics in VO2, which clarify the excitation and charge reshuffling processes leading to the nonthermal monoclinic metal phase.
