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Patrycja Łydżba: Integrals of motion in dipole-conserving models
The Hilbert space fragmentation, for which the Hamiltonian shatters into exponentially many blocks in the site occupation basis, can result in the breakdown of thermalization. In this presentation, we focus on the pair-hopping (PH) model, a paradigmatic model of the Hilbert-space fragmentation. Furthermore, it can be derived as an effective model of the Stark chain, imposing strict conservation of the dipole moment. Notably, the non-vanishing autocorrelation functions in the PH model, as reported in, suggests the existence of local or quasilocal integrals of motion (IOMs). Hence, we propose a numerical algorithm that establishes all IOMs linear in a given set of operators. We employ it to demonstrate that the density modes in the PH model are frozen and become strict IOMs in the thermodynanic limit. Nevertheless, these modes become subdiffusive after incorporating higher-order corrections the PH model. Finally, we make a connection with the Stark model. We demonstrate that although both energy and dipole moment are conserved in the thermodynamic limit, the Stark chain supports only one IOM.
Miroslav Hopjan: Critical quantum dynamics of observables at eigenstate transitions
It is an outstanding goal to unveil the fingerprints of universal quantum dynamics at eigenstate transitions. Focusing on quadratic fermionic Hamiltonians, we identify physical observables that exhibit critical behavior at the transition. Our result is based on two ingredients: (a) A relationship between the observable time evolution in a many-body state and the transition probabilities in single-particle states, and (b) a scale invariance of transition probabilities, which generalizes the recent result for survival probabilities. We then show that these properties give rise to a critical behavior in the quantum quench dynamics of observables, which share the common eigenbasis with the Hamiltonian before the quench. We numerically demonstrate this phenomenon at the localization transition in the three-dimensional Anderson model, for which the critical bahavior can be detected in experimentally relevant observables such as site occupations and particle imbalance.
Hui Zhao: Laser-induced tunnel ionization and femtosecond nonthermal melting in MgO
Laser-induced melting plays a crucial role in advancing manufacturing technology and ultrafast science; however, its atomic processes and microscopic mechanisms remain elusive due to complex interplays between many degrees of freedom within a timescale of ~100 femtoseconds. We employ MgO as an example to investigate the nonequilibrium mechanisms of laser-driven ultrafast nonthermal melting of wide-gap materials. Our study is based on real-time time-dependent density functional theory (rt-TDDFT) molecular dynamics (MD) simulations. We report here that laser melting is greatly accelerated by tunnel ionization processes. The tunneling processes generate a large number of photocarriers and results in intense energy absorption, instantaneously changing the potential energy surface of lattice configuration. The strong electron-phonon couplings and fast carrier relaxation enable the efficient energy transfer between electrons and the lattice. The modulation of melting thresholds and phase boundary demonstrate the possibility of manipulating phase transition on demand. A shock wave curve is also obtained at moderate conditions (P < 50 GPa), extending Hugoniot curve to new regimes. These results account well for the latest ultrafast melting experiments, and provide atomistic details and nonequilibrium mechanism of photoinduced ultrafast phase transitions in wide-gap materials.
Rok Venturini: Nanoscale polytype heterostructures
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.
Yevhenii Vaskivskyi: Electronic Amorphous State of 1T-TaS2: An Order Embedded in the Disorder
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.
Dragan Mihailovic: Tutorial. Trajectories through emergent metastable states in 2D correlated electron systems
Non-equilibrium states of matter have become of great fundamental and practical interest in recent years because of their wide importance in diverse areas of physics. With the rapid development of new time-resolved techniques, the temporal dynamics of competing processes and interactions were recently elucidated in a wide variety of complex condensed matter systems. However, the physics of metastable mesoscopically non-periodic quantum textures emerging from phase transitions has been largely experimentally inaccessible till now: current state of the art time-resolved methods using x-rays, electron diffraction, photoemission, THz and optical spectroscopy all average over multiple transition outcomes. Moreover, they cannot resolve irregular nonperiodic nanoscale structures, but some progress has been made in the use of scanning tunneling microscopy. Here I will outline experimental and theoretical progress in the study of emergent phenomena in metastable states, concentrating on well-known systems that offer insights into complex physics of multicomponent non-equilibrium electronic systems, such as charge fractionalization, jamming and topologically arrested kinetics.
Yuta Murakami: Possible hidden phases in photo-doped Mott insulators
Doping charge carriers into Mott insulators provides a pathway to produce intriguing emergent phenomena. In equilibrium systems, doping can be chemically controlled. On the other hand, photo-doping, where particles are excited across the Mott gap, provides an alternative way. Compared to chemical-doping, photo-doping creates a wider variety of charge carriers, which may lead to the emergence of fascinating nonequilibrium states. In particular, when the gap is large, the lifetime of photo-carriers is exponentially enhanced, leading to quasi-steady states after intraband cooling of photo-carriers.In this talk, we explore possible hidden phases that arise as quasi-steady states of photodoped Mott insulators using the quasi-equilibrium approach. Within this approach, we treat the photo-doped state as an equilibrium state of an effective model for a given photo-doping level. We apply the idea to the 1D extended Hubbard model. In the first part, we present our numerical results obtained with the infinite time-evolving block decimation. We show the emergence of the so-called η-pairing phase and the string charge-density-wave phase, and discuss their physical properties. In the second part, we reveal the analytical aspects of these photo-doped states.
Zala Lenarčič: Tutorial. Flavours of Openness
I will introduce and review different aspects of open quantum systems dynamics, providing some background and motivation for the 2023 NQW talks with a flavour of openness.
Gergely Zarand: Non-equilibrium transport and quantum information jet in the Hubbard model
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 . 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.
Lunch
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Denis Arčon: The pairing symmetry in the quasi-one-dimensional superconductors A2Mo3As3 (A=Rb, Cs)
The A2Mo3As3 (A = K, Rb, Cs) compounds are built of assembled Mo3As3 chains and are thus the potential realisations of quasi-one-dimensional metals. Various experiments on these materials hint to the Tomonaga-Luttinger liquid (TLL) physics from which superconductivity with a relatively high critical temperatures of Tc ≈ 10.5 K emerges. However, TLL and its relevance to emerging superconductivity are not yet resolved for A2Mo3As3 due to their multiband nature.Here we report a combined 75As nuclear quadrupole resonance (NQR), 87Rb and 133Cs nuclear magnetic resonance (NMR) and muon spin relaxation (mSR) study of Rb2Mo3As3 and Cs2Mo3As3 powders. The alkali metal spin-lattice relaxation rates show a characteristic power-law temperature dependence over a broad temperature range, which is a hallmark of the TLL dynamics dominated by the two-quasi-one-dimensional bands. Detailed analysis reveals that these compounds may be in an unusual state of effective attractive interactions, which opens up intriguing possibilities for the unconventional pairing symmetries of the superconducting state. To address the superconducting state we, in addition to 75As and alkali metal relaxation rate studies, employ also the ,transverse-field mSR. The temperature dependence of the muon relaxation rate, s, is in the superconducting state due to the field distribution created by the vortex lattice and is compared against s-, p- , and d-wave scenarios. The penetration depth l = 669 nm and the coherence length z = 3.4 nm are also determined.
