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To date, excitons have mainly been studied in rigid band semiconductors. Due to the irrelevance of electron-electron correlations in these systems, the same hydrogenic excitonic models apply to many material classes. In Mott insulators, however, strong interactions between electronic, spin, and orbital degrees of freedom create pathways for other excitonic binding mechanisms. These so-called Hubbard excitons are predicted to exhibit novel non-hydrogenic properties that rely critically on the microscopic description of the host material. I will report the spectroscopic signatures of transient exciton formation in the antiferromagnetic spin-orbital Mott insulator Sr2IrO4 obtained by employing an ultrafast terahertz probe that is sensitive to transitions between different excitonic states. A near-infrared photoexcitation is used to generate a conductive particle-hole gas, which decays into insulating excitonic states. Strong spin-exciton coupling is deduced from an analysis of the excitonic recombination dynamics as a function of temperature. To develop a microscopic picture of the excitonic states, I will compare the experimental results against numerical exact diagonalization calculations. These results stimulate the search for novel excitonic states in other Mott insulating systems.

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We consider thermoelectric transport in correlated metals with disorder within a simple phenomenological approach. We allow for a particle-hole asymmetry in the inelastic scattering rate. We find that the effects of this asymmetry disappear at low temperatures in Fermi liquids (where electronic contribution to resistivity r~ T2) but not in non-Fermi liquids (r~ Tn: n*1), which can lead to changes of sign of the Seebeck coefficient at low temperatures with respect to that found in a band theory. We apply the theory to recent measurements of thermoelectric transport in Nd-LSCO cuprate close to the pseudo-gap ending point and show that both the inplane and outofplane measured resistivity and the Seebeck coefficient are consistently described.

Quantum systems evolving unitarily and subject to quantum measurements exhibit various types of non-equilibrium phase transitions, arising from the competition between unitary evolution and measurements. Dissipative phase transitions in steady states of time-independent Liouvillians and measurement induced phase transitions at the level of quantum trajectories are two primary examples of such transitions. Investigating a many-body spin system subject to periodic resetting measurements, we argue that many-body dissipative Floquet dynamics provides a natural framework to analyze both types of transitions. We show that a dissipative phase transition between a ferromagnetic ordered phase and a paramagnetic disordered phase emerges for long-range systems as a function of measurement probabilities. A measurement induced transition of the entanglement entropy between volume law scaling and area law scaling is also present, and is distinct from the ordering transition. The ferromagnetic phase is lost for short range interactions, while the volume law phase of the entanglement is enhanced. An analysis of multifractal properties of wave function in Hilbert space provides a common perspective on both types of transitions in the system. Our findings are immediately relevant to trapped ion experiments, for which we detail a blueprint proposal based on currently available platforms.

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The three-dimensional Anderson model represents a paradigmatic model to understand the Anderson localization transition. Using modern numerical approaches, we investigate the model’s spectral properties, focusing particularly on the quantitative comparison between the level sensitivity statistics and the level statistics. While the former probes the susceptibility of the Hamiltonian eigenlevels towards the introduction of the magnetic flux to the system, the latter deals with the unperturbed energy levels. We define two versions of the dimensionless conductance, the first corresponding to the width of the level curvature distribution relative to the mean level spacing, and the other corresponding to the ratio of Heisenberg and Thouless times obtained from the spectral form factor. We show that both conductances display remarkably similar behaviour around the localization transition, in particular, they predict a nearly identical critical point consistent with other well-established measures of the transition.Motivated by the pioneering work of Edwards and Thouless , we then calculate the characteristic energy, defined as the mean change of the eigenlevels upon switching the hopping along one of the lattice edges from periodic to antiperiodic boundary conditions. Making use of the modern hardware to perform a systematic analysis, we obtain results for system sizes much greater than the ones available to the authors of the original study. By accurately pinpointing the location of the critical point, we establish the latter method as a reliable tool for detecting the localization transition in noninteracting systems. In the context of the spectral form factor, we show that at the critical point it enters a broad time-independent regime, in which its value is consistent with the level compressibility obtained from the level variance. Finally, we test the scaling solution of the average level spacing ratio in the crossover regime using the cost function minimization approach introduced recently in . The latter approach seeks for the optimal scaling solution in the vicinity of the crossing point, while at the same time allowing for the drift of the crossing point due to finite-size corrections. We find that the extracted transition point and the scalling coefficient accurately agree with those obtained from the literature.

Spatiotemporal correlation functions provide the key diagnostic tool for studying spatially extended complex quantum many-body systems. In ergodic systems scrambling causes initially local observables to spread uniformly over the whole available Hilbert space and causes exponential suppression of correlation functions with the spatial size of the system. In this talk, we present a perturbed free quantum circuit model, in which ergodicity is induced by a unitary impurity placed on the system's boundary and that allows for demonstrating the underlying mechanism governing the asymptotic scaling of correlations with system size. This is achieved by mapping dynamical correlation functions of local operators in a system of linear size L at time t to a partition function with complex weights defined on a two-dimensional lattice of smaller size t/L × L with a helix topology. We evaluate this partition function in terms of suitable transfer matrices. As this drastically reduces the complexity of the computation of correlation functions, we are able to treat system sizes far beyond what is accessible by exact diagonalization. By studying the spectra of transfer matrices numerically and combining our findings with analytical arguments we determine the asymptotic scaling of correlation functions with system size. For impurities that remain unitary under partial transpose, we demonstrate that correlation functions at times proportional to system size L are generically exponentially suppressed with L. In contrast, for generic unitary impurities correlations show persistent revivals with a period given by the system size.

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The discovery of graphene in 2004 has opened a new field of research about two-dimensional (2D) materials. Nowadays, by virtue of the tunability of their electronic and structural properties, 2D materials are applied in a great variety of areas like, for example, sensors, energy storage, and photonic devices. Among the emerging 2D materials, we have focused our attention on the van-der-Waals-layered semiconductor bismuth tri-iodide (BiI3). First of all, BiI3 exhibits a clear and isolated absorption resonance in the visible range, detectable also at room temperature. Besides this, the lattice vibration is dominated by an out-of-plane Ag mode. The co-presence of these two degrees of freedom makes BiI3 an ideal candidate to be used in the investigation of exciton-phonon coupling.However, how exciton-phonon coupling manifests in the time and energy domains is still an open debate between experiment and theory. Through time-resolved broadband reflectivity, we investigate the ultrafast optical response of BiI3 single crystal. Our measurements reveal a multi-step electron relaxation dynamic with time constants that range from a few hundreds of femtoseconds up to tens of nanoseconds, superimposed by a periodic intensity modulation ascribed to the generation of coherent optical phonons. Here, we will focus only on the coherent optical response of BiI3.Our joint theoretical and experimental effort allows uncovering the relationship between the photoinduced periodic excitonic energy modulation and the generation of coherent optical phonons. Moreover, employing ab-initio DFT calculation coupled with a transient analysis of the experimental results, we were able to extrapolate the photoinduced atomic displacement in the real space from the excitonic energy modulation. In conclusion, with our work(), we set the spectral fingerprints for the optical detection of exciton-phonon coupling in layered semiconductors. Moreover, our findings represent a step forward on the road to coherent manipulation of the excitonic properties on ultrafast timescales.

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The local magnetic moment of an interacting quantum dot can be screened by a Bogoliubov quasiparticle from a nearby superconductor. This gives rise to a long lived discrete spin singlet state inside the superconducting gap, known as the Yu-Shiba-Rusinov (YSR) state. We study the nature of the subgap states in a quantum dot coupled to one or two superconducting islands. These are described by a number conserving Richardson model of superconductivity, which allows us to account for the charging energy of the superconducting islands. For charging energy comparable to the superconducting gap, the subgap states are stabilized by a combination of Kondo exchange screening and charge redistribution driven by the Coulomb interaction. The model predictions match experimental results very well. There are two singlet subgap states in the case of a quantum dot embedded between two superconducting channels, corresponding to the formation of a YSR singlet in each channel. The system can be tuned to a regime where the subgap states can be put into a superposition and coherently manipulated by electronic pulses. Such a qubit could be implemented using known technology. In the doublet spin sector we investigate the overscreened state. Overscreening in normal state systems leads to a fixed point in the renormalization flow with non Fermi liquid properties, while for superconducting islands it emerges as a doublet subgap state with curious spin properties.

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