We introduce and discuss dynamical universality of charge fluctuations in charged single-file systems. The full counting statistics of such systems out of equilibrium generically undergoes first and second order dynamical phase transitions, while equilibrium typical fluctuations are non-Gaussian and given by a universal distribution. Similar phenomenology of dynamical criticality is observed in equilibrium in the easy axis and isotropic regimes of an integrable spin chain. While the easy axis regime does not satisfy a single-file kinetic constraint, it nevertheless supports the non-Gaussian distribution of the charged single-file universality class. Fluctuations at the isotropic point are also anomalous and distinct from those of the Kardar-Parisi-Zhang universality class.
Ta2NiSe5 has long been considered as a prominent candidate for realization of the excitonic condensation that was invoked to explain the opening of the gap in the photoemission and to rationalize the existence of short time scales in pump-probe experiments. A structural transition that coincides with the putative excitonic transition leads to eternal chicken-egg debate addressing the dominant mechanism of the transition. We consider a realistic 6 orbital model and discuss its instabilities in Hartree-Fock calculations including the relevant B2g Raman active phonon. The model realizes excitonic transition with experimentally expected symmetry only provided the electron-phonon coupling is taken into account. From time-dependent calculations we evaluate also two-particle response corresponding to Raman and optical spectra. The key feature of the calculated response is a prominent phase mode, and the extent to which it survives in the structurally distorted ground state may help settling the debate.
Excitons are correlated electron-hole pairs in multi-band electron systems, which can condense and form ordered phases of matter called excitonic insulators. These are expected to display novel and technologically highly relevant features like superfluid energy transport. While it is experimentally challenging to identify real materials hosting equilibrium excitonic order, out-of-equilibrium protocols open up an independent route to stabilize excitonic condensates.Ma et.al. proposed a gated semiconductor bilayer architecture, in which an applied voltage bias allows for the continuous creation of interlayer excitons by means of an induced electrical current. We model the setup starting from the quasi-stationary situation within the static Hartree-Fock and second order Born approximations. We compare results from dynamical mean-field theory to simulations in one spatial dimension to shed light on the strong impact of dimensionality on the formation of the excitonic state. To go beyond the quasi-stationary case, we discuss results of time-dependent simulations of a driven four-band model in one spatial dimension, which is coupled to a bosonic bath.
In the last few years there has been great interest in the dynamics of monitored quantum many-body systems. The interplay between unitary evolution and dissipative dynamics leads to many effects, including measurement induced phase transitions of the entanglement scaling. While earlier works all considered Markovian dissipative processes, lately there has been growing interest in non-Markovian dissipation and the effect of memory on entanglement transitions.I will present recent results and developments on the study of entanglement in non-Markovian systems. In particular, I will focus on a free fermions ladder model, where one of the chains is the system of interest and the other chain (bath chain) is subjected to Markovian projective measurements. The global dynamics is Markovian and can be studied through standard Montecarlo quantum jumps methods, but because of the internal dynamics of the bath chains, the reduced dynamics on the system chain is non-Markovian. The introduction of this ancillary chain allows to study the entanglement transition in the presence of memory effects. The studied model exhibits a variety of phases, including transition a from area-law to CFT phase and different regimes where the monogamy of entanglement produces strong counter-intuitive effects.
Localization, which is typically induced by disorder, is an exotic phenomenon where a quantum state fails to spread over the entire Hilbert space. Recently, measurement is utilized as another mechanism to localize a quantum state in nonunitary quantum circuits and continuously monitored systems, which exhibit novel quantum phenomena dubbed measurement-induced phase transitions (MIPTs). However, while both the disorder and the measurement localize the wave function and suppress the entanglement spreading, it is still not clear whether they exhibit the same localization properties.
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.
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.
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.
The transitional metal dichalcogenides crystalize in different polytypes, dictating their charge density wave (CDW) states. It is possible to perform the polytype transformation in situ by either an optical or an electrical pulse, creating new nanoscale structures and polytypes that sometimes do not exist as bulk materials. We explore the polytype transformed structures of bulk materials 2H-NbSe2 and 4Hb-TaSe2.Using the electrical pulse from the STM tip, the surface of the 2H-NbSe2 is transformed to the 1T polytype in which we observe rich CDW physics of domains and domain walls, as well as dynamical switching between different charge ordering configurations. The diverse spectroscopic signature of the CDW state shows a similarity to the hidden state of 1T-TaS2.
We also present experiments on 4Hb-TaSe2 which consists of alternating stacking of the 1T and 1H layers. Similar to the 2H-NbSe2 experiments, we show that it is possible to create a nanoscale polytype transformation of the top surface from the 1H to the 1T polytype with the electrical pulse from the STM tip. Although the transformed 1T polytype in both materials adopts the same √13×√13 CDW superlattice, their electronic structure is different. Other materials from the group of 2H and 4Hb polytypes could likely be explored for nanoscale polytype transformations.
The 1T-TaS2, a peculiar transition metal dichalcogenide with a charge density wave (CDW) at low temperatures, has been studied for the last 10 years since the original discovery of a metastable hidden (H) state in it. Mainly, the research was focused on the properties of this state as well as their possible applications. The H-state can be achieved by driving the material out of equilibrium using an ultrashort laser pulse or a short electrical pulse. Similarly to the H-state, at higher excitation fluences an electronic amorphous (A) state can be reached.In our scanning tunnelling microscopy studies, we are exploring the coexistence of amorphous, commensurate (C) and hidden CDWs; and the dynamical reconfigurations between them. At the same time, we are showing that in addition to these CDWs, the A-state can locally form embedded `islands` of charge order not observed in any other states of the material. In addition, studies of the dynamics in the A-state are showing possible transitions as well as thermal fluctuations of this ordering.
The observed local behaviour of this system with various emergent states is a good example of topologically-constrained dynamics. As a result, the A-state of 1T-TaS2 provides valuable insight into the correlation between topology and kinetics in many-body quantum states and can be considered an accessible experimental model for studying non-equilibrium quantum dynamics.
