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I will discuss the interplay between many-body localization and spin-symmetry. I will present the time evolution of several observables in the anisotropic t − J model. Like the Hubbard chain, the studied model contains charge and spin degrees of freedom. Yet, it has a smaller Hilbert space and thus allows for numerical studies of larger systems. I will compare the field disorder that breaks the Z2 spin symmetry and a potential disorder that preserves the latter symmetry. In the former case, sufficiently strong disorder leads to localization of all studied observables, at least for the studied system sizes. However, in the case of symmetry-preserving disorder, we observe that odd operators under the Z2 spin transformation relax towards the equilibrium value at relatively short time scales that grow only polynomially with the disorder strength. On the other hand, the dynamics of even operators and the level statistics within each symmetry sector are consistent with localization. Our results indicate that localization exists within each symmetry sector for symmetry preserving disorder. Odd operators’ apparent relaxation is due to their time evolution between various symmetry sectors.

In this presentation, we study single-particle properties of quantum-chaotic quadratic models, which are identified with quadratic models having random matrix theory correlations in single-particle spectra, i.e., exhibiting single-particle quantum chaos. We analyze matrix elements of local and nonlocal operators in two paradigmatic Hamiltonians, i.e., the quadratic Sachdev-Ye-Kitaev model and the three-dimensional Anderson model below the localization transition. We demonstrate that their matrix elements display single-particle eigenstate thermalization. Specifically, we show that the diagonal matrix elements exhibit vanishing eigenstate-to-eigenstate fluctuations, and the variance proportional to the inverse Hilbert space dimension. We also demonstrate that the ratio between the variance of diagonal and off-diagonal matrix elements agrees with the prediction of random matrix theory. We also study distributions of matrix elements, and establish the conditions under which they are (not) Gaussian.

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By means of time- and angle-resolved photoemission spectroscopy (tr-ARPES), we investigate the effect of the charge density wave (CDW) phase transition on the equilibrium and out-ofequilibrium electronic properties of the transition metal dichalcogenide VSe2. The electronic band structure of VSe2 has recently been subject of investigation ranging from the bulk to the monolayer regime, in search for the manifestation of the opening of the band gap in its CDW phase . However, at present, only a few studies on the effect of an ultrafast optical excitation are available . In our contribution we present a study on the bulk material. By selecting the polarization of the probe pulses, tr-ARPES allows us to disentangle states with different orbital character, originating from the V and Se valence bands. When moving across the critical temperature of the CDW phase transition, our tr-ARPES data show indication for a change in the fast relaxation dynamics and for a different filling of novel photoinduced states near the Fermi level, lasting for several picoseconds after photoexcitation.

IrTe2, a Van-der-Waals transition metal dichalcogenide, is known to host a complicated phase diagram: two first order phase transitions into charge ordered stripe phases of different periodicity are observed in bulk undoped samples : superconductivity emerges in bulk doped samples resulting in the familiar superconducting dome behavior : superconductor – normal metal switching was demonstrated in un-doped quenched nanoflakes : and superconductivity has been shown to emerge in thin flakes resulting in a superconducting dome as a function of flake thickness . Most interestingly however, fast cooling of undoped bulk samples has resulted in the formation of superconducting hexagonal-like surface patches appearing at three-fold stripe phase intersections . We have studied the possibility to control the low temperature charge order in IrTe2 with ultrafast pulses using optical time domain spectroscopy. High fluence photoexcitation gives rise to non-thermal behavior of transient reflectivity on sub-picosecond timescales, indicative of a transient photoinduced phase transition thought to be associated with Ir dimer breaking and recovery . Following the trajectory of the system in double-pump measurements we observe definite evidence of a transition to a mixture of higher temperature equilibrium phases several picoseconds after the arrival of the strongly perturbing pulse, as well as the material’s subsequent relaxation into the low temperature equilibrium state some 100 to 150 picoseconds after excitation. We conclude that the phase transitions at these timescales are driven by the increase in the transient lattice temperature imposed by laser excitation. On the other hand, by tracking the response to increasing intensity of the pump beam at a fixed delay in the double-pump experiment we observe threshold-like behavior in the temporal as well as in the spectral domain, thus shedding light on the connection between the dimer breaking transition at sub-picosecond timescales and the transient lattice temperature driven transitions at longer timescales.

Recent experiments and theoretical studies suggest that electronic orders such as superconductivity or excitonic insulator phases can be realized in highly nonthermal systems. In this talk, I will discuss some examples which have been demonstrated in nonequilibrium dynamical mean field theory simulations of Hubbard-type models. After briefly reviewing results for nonthermal magnetic order and the eta-pairing state in photo-doped Mott insulators, I will discuss in more detail two recent studies on nonthermal excitonic order in a two-orbital Hubbard model with Hund coupling and crystal field splitting and on a nonthermal form of composite order (or odd-frequency orbital order) in a three-orbital Hubbard model which is relevant for the description of A3C60 . The excitonic insulator study reveals a new entropy-trapping mechanism for nonthermal orders, while the fulleride example demonstrates a nonthermal electronic order which is stabilized by the kinetic energy of the photo-excited charge carriers.

The possibility to form excitons in photo-illuminated correlated materials is central from fundamental and application oriented perspectives. We show how the interplay of electron-electron interactions and a magnetic superstructure leads to the formation of a peculiar spinful exciton, which can be detected in ARPES-type experiments and optical measurements. We study this by using matrix product states (MPS) to compute the time evolution of single-particle spectral functions and of the optical conductivity following an electron-hole excitation in a class of one-dimensional correlated band-insulators, simulated by Hubbard models with on-site interactions and alternating local magnetic fields. An excitation in only one specific spin direction leads to an additional band in the gap region of the spectral function only in the spin direction unaffected by the excitation and to an additional peak in the optical conductivity. Recombination of the excitation happens on much longer time scales than the ones amenable to MPS. We discuss implications for experimental studies in correlated insulator systems.

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Wide-band-gap insulators such as NiO offer the exciting prospect of coherently manipulating electronic correlations with strong optical fields . Contrary to metals where rapid dephasing of optical excitations via electronic processes occurs, the sub-gap excitation in charge-transfer insulators has been shown to couple to low-energy bosonic, probably phonon, excitations . Here we use the prototypical charge-transfer insulator NiO to demonstrate that sub-gap excitation leads to a renormalized NiO band-gap in combination with a significant reduction of the antiferromagnetic order. We employ element-specific x-ray absorption spectroscopy at the FLASH free electron laser to demonstrate the reduction of the upper band-edge at the O 1s-2p core-valence resonance whereas the antiferromagnetic order is probed via x-ray magnetic linear dichroism (XMLD) at the Ni 2p-3d resonance. Comparing the transient XMLD spectral lineshape to ground-state measurements allows us to extract a spin temperature rise of more than 60 K for time delays longer than 400 fs. This is accompanied by a band-gap reduction. Before 400 fs a non-equilibrium state is formed characterized by O 2p mid-gap states. We will discuss these results in terms of a transient Ni-O charge transfer during the optical driving field. The energetic proximity of our 800 nm excitation wavelength to phonon-assisted NiO d-d transitions facilitates the bosonic dressing of the laser field. Finally, laser-excited phonons couple to magnons on a ~400 fs timescale enabling the quenching of the NiO magnetic order .

Results will be presented of how the spins, the orbitals and the lattice react to phase stable excitations of low energy phonons, electromagnons or orbitals. Examples will cover soft mode driving of the ferroelectric mode in SrTiO3, electromagnon excitation in hexaferrites and orbital excitation in magnetically frustrated Tb2Ti2O7. These ultrafast changes caused by the excitations are probed by resonant and non-resonant X-ray diffraction to obtain the lattice the spin and/or the orbital dynamics. Recent results from SwissFEL will be presented.

α-GeTe(111) is a bulk ferroelectric Rashba semiconductor which exhibits the largest known Rashba-type spin splitting of so-far known materials, one of the most promising mechanism to reversibly manipulate spin polarization. Its electronic structure in the occupied states has been intensively studied by Angle Resolved Photoemission Spectroscopy (ARPES) and Spin ARPES (SARPES) , the key technique to understand the spin texture of materials. Using operando SARPES, it has been demonstrated that it is possible to reversibly manipulate spin polarization by an external electric field in α-GeTe(111) : a promising behavior for spintronics applications. A stimulating direction of research is to investigate whether it is possible to coherently modify the ferroelectric properties of GeTe upon photoexcitation.Using a 800 nm photoexcitation, we drive α-GeTe(111) out-of-equilibrium and probe its transient low-energy electronic structure with time-resolved ARPES. We reveal that the Rashba splitting of its bulk states is enhanced after 200 fs. By comparison with density functional theory calculations, we show that this change of the electronic structure is driven by a shift of the Ge atomic layer towards the Te atomic layer, meaning an increase of the ferroelectric distortion. A coherent phonon oscillation linked to this ferroelectric distortion is also observed, with a frequency consistent with the amplitude mode of the related polar mode.We identify a surface photovoltage effect as the mechanism responsible for this transient enhancement of the ferroelectricity at the surface of GeTe and link it to a delayed displacive excitation of the coherent phonon of the ferroelectric distortion.

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