arXiv subjects

Philipp Werner

Publications and source records attributed to Philipp Werner.

At least 55 records · Page 3Linked to original sources

arXiv2024

Hidden Kondo lattice physics in single-orbital Hubbard models

Single-orbital Hubbard models exhibit remarkably nontrivial correlation phenomena, even on nonfrustrated bipartite lattices. Some of these, like non-Fermi-liquid metal states, or the coexistence of heavy and light quasi-particles, are reminiscent of the properties of more complex multi-orbital or Kondo-lattice systems. Here, we use basis transformations to map single-orbital models to effective multi-orbital descriptions and clarify how a ferromagnetic Kondo-lattice-like behavior emerges in prototypical models with flat bands or van Hove singularities in the density of states: the Hubbard model on the diamond chain, square-lattice, Lieb lattice and honeycomb lattice. In particular, this mapping explains the non-Fermi-liquid states and pseudo-gaps found in the correlated metal regime.

cond-mat.str-el↗

arXiv2024

Terahertz harmonic generation across the Mott insulator-metal transition

We demonstrate terahertz (THz) harmonic generation across the Mott insulator-metal transition in rare-earth nickelates (RNiO$_3$, R = rare-earth atom). The THz harmonic generation is observed in all the three different phases with distinct behaviors: the intensity of harmonics increases upon cooling in both the low-temperature antiferromagnetic (AFM) insulating and high-temperature paramagnetic (PM) metallic phases, while this trend is reversed in the intermediate PM insulating phase. Using single- and two-band Hubbard models, we find different dominant origins of THz harmonics in different phases: strong spin-charge and orbital-charge couplings in the AFM insulating phase, intraband currents from renormalized quasi-particles with frequency-dependent scattering rate in the PM metallic phase, and the reduction of the charge carrier density due to the opening of the Mott gap in the PM insulating phase. Our study offers strategies for efficient THz harmonic generation from Mott and other strongly correlated systems and insights into the fundamental physics of complex materials.

cond-mat.str-el↗

arXiv2024

Measuring the ultrafast screening of $U$ in photo-excited charge-transfer insulators with time-resolved X-ray absorption spectroscopy

Recent seminal experiments have utilized time-resolved X-ray absorption spectroscopy (XAS) to investigate the ultrafast photo-induced renormalization of the electron interaction (''Hubbard $U$'') in Mott and charge transfer insulators. In this paper, we analyze the change of interactions due to dynamical screening as it is encoded in the XAS signal, using the non-equilibrium GW+EDMFT formalism. Our study shows that XAS is well-suited for measuring this change, but two aspects should be kept in mind if the screening processes are not substantially faster than the valence electron dynamics: (i) Screening in a photo-excited system can affect both the position and the lineshape of the absorption lines. (ii) In general, the effect cannot be captured by the modification of a single interaction parameter. Specifically, an estimate for $ΔU$ extracted from the shift of the XAS lines does not necessarily describe the related shift of the the upper Hubbard band. We clarify these aspects using a minimal cluster model and the three-band Emery model for a charge transfer insulator.

cond-mat.str-el↗

arXiv2024

Photo-induced charge, spin, and orbital order in the two-orbital extended Hubbard model

Nonequilibrium control of electronically ordered hidden phases may lead to the development of ultrafast switches and memory devices. In this study, we demonstrate tunable hidden orders in the photo-doped two-orbital extended Hubbard model. Using steady-state nonequilibrium dynamical mean field theory, we clarify the coexistence and interplay of nonthermal charge, spin, and orbital order. The hidden state at low effective temperature and sufficiently high photo-doping is reminiscent of Kugel-Khomskii order in the two-orbital Hubbard model at $\frac{1}{4}$ and $\frac{3}{4}$ filling, but it emerges out of a nonequilibrium charge ordered state and exhibits a different magnetic structure. A low-energy effective Hamiltonian is used to analyze the exchange processes which stabilize the nonthermal order.

cond-mat.str-el↗

arXiv2024

High harmonic generation in altermagnets

We study high harmonic generation in altermagnetic metals with and without spin-orbit coupling. The altermagnetism manifests itself in the magnetic field dependence of the low harmonics associated with intra-band dynamics. Spin-orbit coupling leads to additional higher energy peaks and plateau structures originating from inter-band transitions. While the pure altermagnet or spin-orbit system exhibits no circular dichroism in the high-harmonic response, an altermagnetic system with spin-orbit coupling shows such a dichroism. We also analyze the spin currents and their high harmonic spectrum.

cond-mat.mes-hall↗

arXiv2024

Many-body effects on high-harmonic generation in Hubbard ladders

We show how many-body effects associated with background spin dynamics control the high-harmonic generation (HHG) in Mott insulators by analyzing the two-leg ladder Hubbard model. Spin dynamics activated by the interchain hopping $t_y$ drastically modifies the HHG features. When two chains are decoupled ($t_y=0$), HHG originates from the dynamics of coherent doublon-holon pairs because of spin-charge separation. With increasing $t_y$, the doublon-holon pairs lose their coherence due to their interchain hopping and resultant spin-strings. Furthermore, the HHG signal from spin-polarons -- charges dressed by spin clouds -- leads to an additional plateau in the HHG spectrum. For large $t_y$, we identify unconventional HHG processes involving $three$ elementary excitations -- two polarons and one magnon. Our results demonstrate the nontrivial nature of HHG in strongly correlated systems, and its qualitative differences to conventional semiconductors.

cond-mat.str-el↗

arXiv2024

Photo-induced charge-transfer renormalization in NiO

Photo-doped states in strongly correlated charge transfer insulators are characterized by $d$-$d$ and $d$-$p$ interactions and the resulting intertwined dynamics of charge excitations and local multiplets. Here we use femtosecond x-ray absorption spectroscopy in combination with dynamical mean-field theory to disentangle these contributions in NiO. Upon resonant optical excitation across the charge transfer gap, the Ni $L_3$ and O $K$ absorption edges red-shift for $>10$ ps, associated with photo-induced changes in the screening environment. An additional signature below the Ni $L_3$ edge is identified for $<1$ ps, reflecting a transient nonthermal population of local many-body multiplets. We employ a nonthermal generalization of the multiplet ligand field theory to show that the feature originates from $d$-$d$ transitions. Overall, the photo-doped state differs significantly from a chemically doped state. Our results demonstrate the ability to reveal excitation pathways in correlated materials by x-ray spectroscopies, which is relevant for ultrafast materials design.

cond-mat.str-el↗

arXiv2024

Photo-induced insulator-metal transition in paramagnetic (V$_{1-x}$Cr$_{x}$)$_2$O$_3$

Pump-probe experiments with femtosecond time resolution allow to disentangle the electronic dynamics from the lattice response and thus provide valuable insights into the non-equilibrium behavior of correlated materials. In Cr-doped V$_2$O$_3$, a multi-orbital Mott-Hubbard material which has been intensively investigated for decades, time-resolved experiments reported a photo-induced insulator-metal transition leading to a transient metal state with nonthermal properties. Here, we combine non-equilibrium dynamical mean-field theory with realistic first principles modeling to simulate the ultrafast response of this material to a laser excitation. Our calculations reproduce the insulating initial state, with orbital occupations in agreement with experiment, and reveal an ultrafast pump-induced gap filling associated with a charge reshuffling between the $e_g^π$ and $a_{1g}$ orbitals. However, in contrast to the related compound VO$_2$, the electronic system thermalizes within a few tens of femtoseconds and we find no evidence for the existence of a metastable nonthermal metal. This suggests that the reported nonthermal behavior in the experiments may be associated with the mismatch between the electronic and lattice temperatures.

cond-mat.str-el↗

arXiv2024

Photo-induced Ferromagnetic and Superconducting Orders in Multi-orbital Hubbard Models

The search for hidden orders in photoexcited lattice systems is an active research field driven by experimental reports of light-induced or light-stabilized phases. In this study, we investigate hidden electronic orders in strongly correlated two-orbital Hubbard models with orbital-dependent bandwidths. In equilibrium, the half-filled systems are antiferromagnetically ordered. Using non-equilibrium dynamical mean field theory we demonstrate the appearance of nonthermal ferromagnetic order in the photo-doped state, if the two bandwidths are sufficiently different, and its coexistence with spin-singlet $η$-superconductivity in the high photo-doping region. Spin-triplet $η$-superconducting order appears instead if the two bandwidths are comparable. The rich nonequilibrium phasediagram uncovered in this work shows that Mott insulating multi-orbital systems provide an interesting platform for the realization of nonthermal electronic orders.

cond-mat.str-el↗

arXiv2024

Internal consistency of multi-tier $GW$+EDMFT

The multi-tier $GW$+EDMFT scheme is an ab-initio method for calculating the electronic structure of correlated materials. While the approach is free from ad-hoc parameters, it requires a selection of appropriate energy windows for describing low-energy and strongly correlated physics. In this study, we test the consistency of the multi-tier description by considering different low-energy windows for a series of cubic SrXO$_3$ (X=V,Cr,Mn) perovskites. Specifically, we compare the 3-orbital $t_{2g}$ model, the 5-orbital $t_{2g}+e_g$ model, the 12-orbital $t_{2g}+O_p$ model, and (in the case of SrVO$_3$) the 14-orbital $t_{2g}+e_g+O_p$ model and compare the results to available photoemission and X-ray absorption measurements. The multi-tier method yields consistent results for the $t_{2g}$ and $t_{2g}+e_g$ low-energy windows, while the models with $O_p$ states produce stronger correlation effects and mostly agree well with experiment, especially in the unoccupied part of the spectrum. We also discuss the consistency between the fermionic and bosonic spectral functions and the physical origin of satellite features, and present momentum-resolved charge susceptibilities.

cond-mat.str-el↗

arXiv2023

Spin correlations in the bilayer Hubbard model with perpendicular electric field

We present a nonequilibrium steady-state implementation of the two-particle self-consistent method. This approach respects the Mermin-Wagner theorem and incorporates non-local spatial fluctuations through self-consistent static vertices. The real-frequency implementation allows to compute spectral properties without analytical continuation in both equilibrium and nonequilibrium. As an interesting application, we investigate spin correlations in the bilayer square lattice Hubbard model under a perpendicular static electric field. In equilibrium, the result yields spin correlations which are in good agreement with recent optical lattice experiments. Under a large enough static electric field, the inter-layer spin correlations switch from antiferromagnetic to ferromagnetic. We clarify how this phenomenon is linked to the nonequilibrium modifications of the spin excitation spectrum.

cond-mat.str-el↗

arXiv2023

Nonequilibrium diagrammatic many-body simulations with quantics tensor trains

The nonequilibrium Green's function formalism provides a versatile and powerful framework for numerical studies of nonequilibrium phenomena in correlated many-body systems. For calculations starting from an equilibrium initial state, a standard approach consists of discretizing the Kadanoff-Baym contour and implementing a causal time-stepping scheme in which the self-energy of the system plays the role of a memory kernel. This approach becomes computationally expensive at long times, because of the convolution integrals and the large amount of computer memory needed to store the Green's functions. A recent idea for the compression of nonequilibrium Green's functions is the quantics tensor train representation. Here, we explore this approach by implementing equilibrium and nonequilibrium simulations of the two-dimensional Hubbard model with a second-order weak-coupling approximation to the self-energy. We show that calculations with compressed two-time functions are possible without any loss of accuracy, and that the quantics tensor train implementation shows a much improved scaling of the computational effort and memory demand with the length of the time contour.

cond-mat.str-el↗

arXiv2023

Spin-orbit interaction driven terahertz nonlinear dynamics in transition metals

The interplay of electric charge, spin, and orbital polarizations, coherently driven by picosecond long oscillations of light fields in spin-orbit coupled systems, is the foundation of emerging terahertz spintronics and orbitronics. The essential rules for how terahertz light interacts with these systems in a nonlinear way are still not understood. In this work, we demonstrate a universally applicable electronic nonlinearity originating from spin-orbit interactions in conducting materials, wherein the interplay of light-induced spin and orbital textures manifests. We utilized terahertz harmonic generation spectroscopy to investigate the nonlinear dynamics over picosecond timescales in various transition metal films. We found that the terahertz harmonic generation efficiency scales with the spin Hall conductivity in the studied films, while the phase takes two possible values (shifted by π), depending on the d-shell filling. These findings elucidate the fundamental mechanisms governing non-equilibrium spin and orbital polarization dynamics at terahertz frequencies, which is relevant for potential applications of terahertz spin- and orbital-based devices.

physics.app-ph↗

arXiv2023

Correlated electronic structure of La$_3$Ni$_2$O$_7$ under pressure

Recently, superconductivity with a $T_c$ up to 78 K has been reported in bulk samples of the bilayer nickelate La$_3$Ni$_2$O$_7$ at pressures above 14 GPa. Important theoretical tasks are the formulation of relevant low-energy models and the clarification of the normal state properties. Here, we study the correlated electronic structure of the high-pressure phase in a four-orbital low-energy subspace using different many-body approaches: $GW$, dynamical mean field theory (DMFT), extended DMFT (EDMFT) and $GW$+EDMFT, with realistic frequency-dependent interaction parameters. The nonlocal correlation and screening effects captured by $GW$+EDMFT result in an instability towards the formation of charge stripes, with the $3d_{z^2}$ as the main active orbital. We also comment on the potential relevance of the rare-earth self-doping pocket, since hole doping suppresses the ordering tendency.

cond-mat.str-el↗

arXiv2023

Nature of the photo-induced metallic state in monoclinic VO$_2$

The metal-insulator transition of VO$_2$, which in equilibrium is associated with a structural phase transition, has been intensively studied for decades. In particular, it is challenging to disentangle the role of Mott physics from dimerization effects in the insulating phase. Femtosecond time-resolved experiments showed that optical excitations can induce a transient metallic state in the dimerized phase, which is distinct from the known equilibrium phases. In this study, we combine non-equilibrium cluster dynamical mean-field theory with realistic first principles modeling to clarify the nature of this laser-induced metallic state. We show that the doublon-holon production by laser pulses with polarization along the V-V dimers and the subsequent inter-orbital reshuffling of the photo-carriers leads to a population of orbital-mixed states and the filling of the gap. The photo-induced metal state is qualitatively similar to a hot electronic state in the dimerized structure, and does not involve a collapse of the Mott gap.

cond-mat.str-el↗

arXiv2023

Photo-induced nonequilibrium states in Mott insulators

The study of nonequilibrium phenomena in interacting lattice systems can provide new perspectives on correlation effects, and information on metastable states of matter. Mott insulators are a promising class of systems for nonequilibrium studies, since they exhibit exotic phenomena and complex phase diagrams upon doping, and because a large Mott gap provides protection against fast thermalization and heating after photo-excitations. We can thus expect the emergence of interesting transient states and photo-induced phases in Mott systems. This review presents the current understanding of the mechanisms which control the time evolution of photo-doped charge carriers and the properties of photo-induced metastable states. We focus on recent theoretical progress, identify the relevant underlying concepts, and link them to experimental observations. The review starts with a general discussion of field-induced nonequilibrium setups and an overview of key experiments which revealed characteristic properties of photo-excited Mott states, proceeds with a compact overview of the theoretical tools which have been developed to investigate these strongly correlated nonequilibrium states, and then analyzes Mott insulators driven out of equilibrium by static electric fields, periodic fields, and short laser pulses. We also discuss the appearance of nonthermal electronic orders in photo-excited Mott systems, including nonthermal spin and orbital orders, $η$ pairing states, and novel types of excitonic orders.

cond-mat.str-el↗

arXiv2023

Quaternary borocarbides: a testbed for DFT for superconductors

Using ab-initio density functional theory for superconductors (SCDFT), we systematically study the quaternary borocarbides $RM_2$B$_2$C. Treating the retarded (frequency-dependent) interaction $W(ω)$ within the random-phase approximation (RPA), we find good agreement with experiments for the calculated superconducting critical temperature $T_c$ in the nonmagnetic Ni- and Pd-based compounds. We argue that the problem of accurately placing the $f$-bands within DFT, and possibly the lack of an explicit magnetic pair-breaking mechanism, explain the difficulties of SCDFT in reproducing $T_c$ in members of the magnetic $R$Ni$_2$B$_2$C series ($R$ rare-earth with partially filled $4f$). While the calculated $T_c$ is overestimated, SCDFT qualitatively captures the experimentally observed trend along the rare-earth series, which indicates that the electron-phonon couplings and dynamically screened interactions have a significant effect on $T_c$.

cond-mat.supr-con↗

arXiv2023

Nonequilibrium DMFT approach to time-resolved Raman spectroscopy

Raman spectroscopy uses light scattering to extract information on low-energy excitations of solids. The Raman process is described by diagrams which are fourth order in the light-matter interaction, and in particular the resonant contribution, which involves four different space-time arguments, is difficult to evaluate. If one instead simulates explicitly the incoming (classical) light pulse, the Raman signal is given by the outgoing photon flux and can be determined from a two-point correlation function. Such a formalism can be used to compute the time-resolved Raman spectrum of non-equilibrium systems, as well as nonlinear signals which are higher order in the incoming field, such as hyper Raman scattering. Here we explain how to implement this time-dependent formalism within the dynamical mean field theory framework. The method is illustrated with applications to the Holstein-Hubbard model in the strong electron-phonon coupling regime. We demonstrate hyper Raman scattering in measurements with strong probe fields and frequency mixing signals in the presence of a pump field, and simulate the evolution of Stokes and anti-Stokes features after photo-excitations of metallic and Mott insulating systems.

cond-mat.str-el↗