TY - CHAP A1 - Seibold, Götz ED - Hokamp, Sascha ED - Gulyas, Lazlo ED - Koehler, Matthew ED - Wijesinghe, Sanith T1 - From Spins to Agents: An Econophysics Approach to Tax Evasion T2 - Agent-based Modeling of Tax Evasion: Theoretical Aspects and Computational Simulations N2 - The only single-source guide to understanding, using, adapting, and designing state-of-the-art agent-based modeling of tax evasion A computational method for simulating the behaviour of individuals or groups and their effects on an entire system, agent-based modeling has proven itself to be a powerful new tool for detecting tax fraud. While interdisciplinary groups and individuals working in the tax domain have published numerous articles in diverse peer-reviewed journals and presented their findings at international conferences, until Agent-based Modeling of Tax Evasion there has been no authoritative, single-source guide to state-of-the-art agent-based tax evasion modeling techniques and technologies. Featuring contributions from distinguished experts in the field from around the globe, Agent-Based Modeling of Tax Evasion provides in-depth coverage of an array of field tested agent-based tax evasion models. Models are presented in a unified format so as to enable readers to systematically work their way through the various modeling alternatives available to them. Three main components of each agent-based model are explored in accordance with the Overview, Design Concepts, and Details (ODD) protocol, each section of which contains several sub elements that help to illustrate the model clearly and that assist readers in replicating the modeling results described. Presents models in a unified and structured manner to provide a point of reference for readers interested in agent-based modeling of tax evasion Explores the theoretical aspects and diversity of agent-based modeling through the example of tax evasion Provides an overview of the characteristics of more than thirty agent-based tax evasion frameworks Functions as a solid foundation for lectures and seminars on agent-based modeling of tax evasion The only comprehensive treatment of agent-based tax evasion models and their applications, this book is an indispensable working resource for practitioners and tax evasion modelers both in the agent-based computational domain and using other methodologies. It is also an excellent pedagogical resource for teaching tax evasion modeling and/or agent-based modeling generally. KW - General & Introductory Statistics KW - Applied Probability & Statistics Statistics for Finance" Business & Economics KW - Statistics for Finance" Business & Economics Y1 - 2018 SN - 978-1-119-15570-6 SP - 315 EP - 336 PB - Wiley CY - Weinheim ER - TY - GEN A1 - Seibold, Götz A1 - Caprara, Sergio A1 - Grilli, Marco A1 - Raimondi, Roberto T1 - On the Evaluation of the Spin Galvanic Effect in Lattice Models with Rashba Spin-Orbit Coupling T2 - Condensed matter N2 - The spin galvanic effect (SGE) describes the conversion of a non-equilibrium spin polarization into a charge current and has recently attracted renewed interest due to the large conversion efficiency observed in oxide interfaces. An important factor in the SGE theory is disorder which ensures the stationarity of the conversion. Through this paper, we propose a procedure for the evaluation of the SGE on disordered lattices which can also be readily implemented for multiband systems. We demonstrate the performance of the method for a single-band Rashba model and compare our results with those obtained within the self-consistent Born approximation for a continuum model. KW - spintronics KW - spin-galvanic effect KW - lattice models Y1 - 2018 UR - http://www.mdpi.com/2410-3896/3/3/22 U6 - https://doi.org/10.3390/condmat3030022 SN - 2410-3896 VL - 3 IS - 33 SP - 1 EP - 10 ER - TY - CHAP A1 - Seibold, Götz A1 - Caprara, Sergio A1 - Raimondi, Roberto ED - Drouhin, Henri-Jean ED - Wegrowe, Jean-Eric ED - Razeghi, Manijeh ED - Jaffrès, Henri T1 - Theory of charge-spin conversion at oxide interfaces: the inverse spin-galvanic effect T2 - Spintronics XI, SPIE Nanoscience + Engineering, 19-23 August 2018, San Diego, California, United States N2 - We evaluate the non-equilibrium spin polarization induced by an applied electric field for a tight-binding model of electron states at oxides interfaces in LAO/STO heterostructures. By a combination of analytic and numerical approaches we investigate how the spin texture of the electron eigenstates due to the interplay of spin-orbit coupling and inversion asymmetry determines the sign of the induced spin polarization as a function of the chemical potential or band filling, both in the absence and presence of local disorder. With the latter, we find that the induced spin polarization evolves from a non monotonous behavior at zero temperature to a monotonous one at higher temperature. Our results may provide a sound framework for the interpretation of recent experiments. Y1 - 2018 UR - https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10732/107322P/Theory-of-charge-spin-conversion-at-oxide-interfaces--the/10.1117/12.2319592.short U6 - https://doi.org/10.1117/12.2319592 PB - Spie CY - Bellingham, USA ER - TY - GEN A1 - Martens, Christian A1 - Bill, Andreas A1 - Seibold, Götz T1 - Phase separation and proximity effects in itinerant ferromagnet/superconductor heterostructures T2 - Physical Review B N2 - Heterostructures made of itinerant ferromagnets and superconductors are studied. In contrast to most previous models, ferromagnetism is not enforced by an effective Zeeman field but induced in a correlated single-band model (CSBM) that displays itinerant ferromagnetism as a mean-field ground state. In this model superconductivity and magnetism are both calculated self-consistently. We calculate the magnitude of the magnetization, the superconducting correlations, and variations of the charge density self-consistently for a superconducting-magnetic bilayer by solving the Bogoliubov–de Gennes equations on a two-dimensional lattice. We determine all three quantities as a function of the Coulomb repulsion U and the ferromagnetic exchange interaction J. The CSBM displays a variety of features not present in the Zeeman exchange model—for example, the occurrence of electronic phase separation and the competition of magnetic and superconducting orders far away from the interface. KW - charge order KW - Exchange interaction KW - Ferromagnetism KW - Local density of states KW - Microphase separation KW - Proximity effect KW - Superconducting order parameter Y1 - 2018 U6 - https://doi.org/10.1103/PhysRevB.98.174513 SN - 2469-9969 SN - 1095-3795 VL - 98 IS - 17 ER - TY - GEN A1 - Miao, Hu A1 - Fumagalli, Roberto A1 - Rossi, M. A1 - Lorenzana, José A1 - Seibold, Götz A1 - Yakhou-Harris, Flora A1 - Kummer, K. A1 - Brookes, Nicholas B. A1 - Gu, G. D. A1 - Braicovich, Lucio A1 - Ghiringhelli, Giacomo Claudio A1 - Dean, M. P. M. T1 - Formation of Incommensurate Charge Density Waves in Cuprates T2 - Physical Review X N2 - Although charge density waves (CDWs) are omnipresent in cuprate high-temperature superconductors, they occur at significantly different wave vectors, confounding efforts to understand their formation mechanism. Here, we use resonant inelastic x-ray scattering to investigate the doping- and temperature-dependent CDW evolution in La₂₋ₓBaₓCuO₄ (x=0.115–0.155). We discover that the CDW develops in two stages with decreasing temperature. A precursor CDW with a quasicommensurate wave vector emerges first at high temperature. This doping-independent precursor CDW correlation originates from the CDW phase mode coupled with a phonon and “seeds” the low-temperature CDW with a strongly doping-dependent wave vector. Our observation reveals the precursor CDW and its phase mode as the building blocks of the highly intertwined electronic ground state in the cuprates. KW - Research Areas Charge density waves KW - Electrical Properties KW - Cuprates KW - Strongly correlated systems KW - Resonant inelastic x-ray scattering KW - Superconductivity Y1 - 2019 UR - https://journals.aps.org/prx/abstract/10.1103/PhysRevX.9.031042 U6 - https://doi.org/10.1103/PhysRevX.9.031042 SN - 2160-3308 VL - 9 IS - 3 SP - 031042-1 EP - 031042-11 ER - TY - RPRT A1 - Noell, M. A1 - Seibold, Götz A1 - Krebs, Andreas T1 - Excitations in disordered superconductors: Inclusion of long-range Coulomb interactions Y1 - 2019 UR - https://www.hlrn.de/HLRN-Report-2019.pdf SN - 978-3-00-063360-7 SP - 269 PB - HLRN CY - Berlin ER - TY - GEN A1 - Noatschk, Katharina A1 - Martens, Christian A1 - Seibold, Götz T1 - Time-Dependent Gutzwiller Approximation: Theory and Applications T2 - Journal of Superconductivity and Novel Magnetism N2 - The time-dependent Gutzwiller approximation is rederived on the basis of a variational wave function which is a direct product of a Slater determinant and a coherent state. The latter can be related to the slave-boson formulation by Kotliar and Ruckenstein, and our approach offers a convenient way to generalize the theory towards the implementation of symmetry-broken states and the study of non-equilibrium phenomena. We discuss the concept with regard to applications in the linear response limit and for quenched antiferromagnetic order for which we evaluate the optical conductivity in non-equilibrium situations. KW - Hubbard model KW - Gutzwiller approximation KW - Non-equilibrium Y1 - 2020 UR - https://link.springer.com/article/10.1007/s10948-019-05406-z#Sec1 U6 - https://doi.org/10.1007/s10948-019-05406-z VL - 33 IS - 8 SP - 2389 EP - 2393 ER - TY - GEN A1 - Seibold, Götz A1 - Lorenzana, Jose T1 - Nonequilibrium dynamics from BCS to the bosonic limit T2 - Physical Review B N2 - Within the time-dependent Gutzwiller approximation applied to the negative-U Hubbard model, we investigate the dynamics of a superconductor after an interaction quench for different values of the final interaction from weak to strong coupling. The equilibrium BCS-BEC crossover becomes a sharp transition in the out-of-equilibrium dynamics between weak- and strong-coupling dynamical phases. Two different frequencies (ΩJ<ΩU) dominate the order parameter dynamics. In the weak coupling phase, ΩJ follows twice the asymptotic average value of the gap (ΩJ≈2Δ∞) and ΩU is much larger but hardy visible in the anomalous density. At long times the BCS dynamics is recovered but surprisingly differences remain during the transient phase. In the strong coupling phase, the dynamics decouples from the asymptotic value of the order parameter except at exactly half filling and close to the dynamical transition where ΩU≈2Δ∞. The out-of-equilibrium transient spectral density and optical conductivity are presented and discussed in relation to pump probe experiments. Both ΩJ and ΩU give rise to a complex structure of self-driven slow Rabi oscillations which are visible in the nonequilibrium optical conductivity where also sidebands appear due to the modulation of the double occupancy by superconducting amplitude oscillations. Analogous results apply to CDW and SDW systems. Our results show that in systems with long coherence times, pump-probe experiments allow us to characterize the regime (BCS vs preformed pairs) through very specific out-of-equilibrium fingerprints. KW - BEC-BCS crossover KW - Superconductivity fluctuations KW - Superconductivity Y1 - 2020 UR - https://journals.aps.org/prb/abstract/10.1103/PhysRevB.102.144502 U6 - https://doi.org/10.1103/PhysRevB.102.144502 SN - 2469-9969 VL - 102 IS - 14 ER - TY - GEN A1 - Bill, Andreas A1 - Hizhnyakov, Vladimir A1 - Kremer, Reinhard K. A1 - Seibold, Götz A1 - Shelkan, Aleksander A1 - Sherman, Aleksei T1 - Phase Separation and Pairing Fluctuations in Oxide Materials T2 - Condensed Matter N2 - The microscopic mechanism of charge instabilities and the formation of inhomogeneous states in systems with strong electron correlations is investigated. We demonstrate that within a strong coupling expansion the single-band Hubbard model shows an instability towards phase separation and extend the approach also for an analysis of phase separation in the Hubbard-Kanamori hamiltonian as a prototypical multiband model. We study the pairing fluctuations on top of an inhomogeneous stripe state where superconducting correlations in the extended s-wave and d-wave channels correspond to (anti)bound states in the two-particle spectra. Whereas extended s-wave fluctuations are relevant on the scale of the local interaction parameter U, we find that d-wave fluctuations are pronounced in the energy range of the active subband which crosses the Fermi level. As a result, low energy spin and charge fluctuations can transfer the d-wave correlations from the bound states to the low energy quasiparticle bands. Our investigations therefore help to understand the coexistence of stripe correlations and d-wave superconductivity in cuprates. KW - phase separation KW - cuprate superconductors KW - electronic correlations Y1 - 2020 UR - https://www.mdpi.com/2410-3896/5/4/65/htm U6 - https://doi.org/10.3390/condmat5040065 SN - 2410-3896 VL - 5 IS - 4 ER - TY - GEN A1 - Seibold, Götz A1 - Arpaia, Riccardo A1 - Ying Ying, Peng A1 - Fumagalli, Roberto A1 - Braicovich, Lucio A1 - Di Castro, Carlo A1 - Grilli, Marco A1 - Ghiringhelli, Giacomo Claudio A1 - Caprara, Sergio T1 - Strange metal behaviour from charge density fluctuations in cuprates T2 - Communications Physics N2 - Besides the mechanism responsible for high critical temperature superconductivity, the grand unresolved issue of the cuprates is the occurrence of a strange metallic state above the so-called pseudogap temperature T*. Even though such state has been successfully described within a phenomenological scheme, the so-called Marginal Fermi-Liquid theory, a microscopic explanation is still missing. However, recent resonant X-ray scattering experiments identified a new class of charge density fluctuations characterized by low characteristic energies and short correlation lengths, which are related to the well-known charge density waves. These fluctuations are present over a wide region of the temperature-vs-doping phase diagram and extend well above T*. Here we investigate the consequences of charge density fluctuations on the electron and transport properties and find that they can explain the strange metal phenomenology. Therefore, charge density fluctuations are likely the long-sought microscopic mechanism underlying the peculiarities of the metallic state of cuprates. KW - Electronic properties and materials KW - Superconducting properties and materials Y1 - 2021 UR - https://www.nature.com/articles/s42005-020-00505-z U6 - https://doi.org/10.1038/s42005-020-00505-z SN - 2399-3650 VL - 4 SP - 1 EP - 6 ER - TY - GEN A1 - Seibold, Götz A1 - Udina, Mattia A1 - Castellani, Claudio A1 - Benfatto, Lara T1 - Third harmonic generation from collective modes in disordered superconductors T2 - Physical Review B N2 - Recent experiments with strong THz fields in both conventional and unconventional superconductors have clearly evidenced a marked third-harmonic generation below the superconducting temperature Tc. Its interpretation challenged substantial theoretical work aimed at establishing the relative efficiency of quasiparticle excitations and collective modes in triggering such a resonant response. Here we compute the nonlinear current by implementing a time-dependent Bogoljubov–de Gennes approach, with the twofold aim to account nonperturbatively for the effect of local disorder, and to include the contribution of all collective modes, i.e., superconducting amplitude (Higgs) and phase fluctuations, and charge fluctuations. We show that, in agreement with previous work, already at small disorder the quasiparticle response is dominated by paramagnetic effects. We further demonstrate that paramagnetic processes mediate also the response of all collective modes, with a substantial contribution of charge/phase fluctuations. These processes, which have been overlooked so far, turn out to dominate the third-order current at strong disorder. In addition, we show that disorder strongly influences the polarization dependence of the nonlinear response, with a marked difference between the clean and the disordered case. Our results are particularly relevant for recent experiments in cuprates, whose band structure is in a first approximation reproduced by our lattice model. KW - High-order harmonic generation KW - Impurities in superconductors KW - Optical & microwave phenomena KW - Quasiparticles & collective excitations KW - Superconductivity KW - Disordered systems KW - Higgs bosons KW - Bogoliubov-de Gennes equations Y1 - 2021 UR - https://journals.aps.org/prb/abstract/10.1103/PhysRevB.103.014512 U6 - https://doi.org/10.1103/PhysRevB.103.014512 VL - 103 IS - 1 ER - TY - GEN A1 - Noatschk, Katharina A1 - Hofmann, E. V. S. A1 - Dabrowski, Jarek Marek A1 - Curson, Neil J. A1 - Schröder, Thomas A1 - Klesse, Wolfgang Matthias A1 - Seibold, Götz T1 - Ge(001) surface reconstruction with Sn impurities T2 - Surface Science N2 - Defects play an important role for surface reconstructions and therefore also influence the substrate growth. In this work we present a first principle calculation for the Ge(001) surface without and with tin impurities incorporated into the top surface layer. By mapping the system onto an Ising-type model, with interaction constants taken from density functional theory, the stability of the surface reconstructions under the influence of different concentrations of tin impurities is explored. This approach allows us to simulate the possible phase transitions for the different surface reconstructions including the local structure around the tin impurity atoms. In addition, we compare our theoretical results with experimental STM images on clean and Sn-doped Ge(100) surfaces. KW - Gesn KW - DFT KW - Monte carlo simulations KW - Surface reconstruction KW - Impurities Y1 - 2021 UR - https://www.sciencedirect.com/science/article/pii/S0039602821001163?dgcid=author U6 - https://doi.org/10.1016/j.susc.2021.121912 VL - 713 SP - 1 EP - 8 ER - TY - GEN A1 - Wahlberg, Eric A1 - Arpaia, Riccardo A1 - Seibold, Götz A1 - Rossi, Matteo A1 - Fumagalli, Roberto A1 - Trabaldo, Edoardo A1 - Brookes, Nicholas B. A1 - Braicovich, Lucio A1 - Caprara, Sergio A1 - Lombardi, Floriana A1 - Gran, Ulf A1 - Ghiringhelli, Giacomo Claudio A1 - Bauch, Thilo T1 - Restored strange metal phase through suppression of charge density waves in underdoped YBa2Cu3O7–δ T2 - Science N2 - The normal state of optimally doped cuprates is dominated by the “strange metal” phase that shows a linear temperature (T) dependence of the resistivity persisting down to the lowest T. For underdoped cuprates, this behavior is lost below the pseudogap temperature T*, where charge density waves (CDWs), together with other intertwined local orders, characterize the ground state. We found that the T-linear resistivity of highly strained, ultrathin, underdoped YBa2Cu3O7–δ films is restored when the CDW amplitude, detected by resonant inelastic x-ray scattering, is suppressed. This observation suggests an intimate connection between the onset of CDWs and the departure from T-linear resistivity in underdoped cuprates. Our results illustrate the potential of using strain control to manipulate the ground state of quantum materials. KW - Cuprate superconductors KW - Charge-Density Wave Y1 - 2021 UR - https://www.science.org/doi/10.1126/science.abc8372 U6 - https://doi.org/10.1126/science.abc8372 VL - 373 IS - 6562 SP - 1506 EP - 1510 ER - TY - GEN A1 - Martens, Christian A1 - Seibold, Götz T1 - Non-equilibrium dynamics of stripes in cuprate superconductors T2 - Journal of Magnetism and Magnetic Materials N2 - We use the time-dependent Gutzwiller approximation for the single-band Hubbard model to analyze the non-equilibrium dynamics for stripe ground states. In particular, we are interested in the interplay between spin and charge dynamics which is analyzed by quenching the system either in the charge or spin sector. This allows us to investigate the coupled relaxation dynamics as a function of the supplied energy. Further insight is provided by mapping the Gutzwiller dynamics onto a time-dependent Landau approach which lacks the double occupancy contribution to the time-evolution but allows to tune the coupling between spin- and charge degrees of freedom. KW - Hubbard model KW - Gutzwiller approximation KW - Landau KW - Non-equilibrium KW - Charge order KW - Spin order Y1 - 2022 UR - https://www.sciencedirect.com/science/article/pii/S030488532100812X U6 - https://doi.org/10.1016/j.jmmm.2021.168565 SN - 1873-4766 VL - 543 SP - 1 EP - 4 ER - TY - GEN A1 - Caprara, Sergio A1 - Di Castro, Carlo A1 - Mirarchi, Giovanni A1 - Seibold, Götz A1 - Grilli, Marco T1 - Dissipation-driven strange metal behavior T2 - Communications Physics N2 - Anomalous metallic properties are often observed in the proximity of quantum critical points, with violation of the Fermi Liquid paradigm. We propose a scenario where, near the quantum critical point, dynamical fluctuations of the order parameter with finite correlation length mediate a nearly isotropic scattering among the quasiparticles over the entire Fermi surface. This scattering produces a strange metallic behavior, which is extended to the lowest temperatures by an increase of the damping of the fluctuations. We phenomenologically identify one single parameter ruling this increasing damping when the temperature decreases, accounting for both the linear-in-temperature resistivity and the seemingly divergent specific heat observed, e.g., in high-temperature superconducting cuprates and some heavy-fermion metals. KW - Electronic properties and materials KW - Superconducting properties and materials Y1 - 2022 UR - https://www.nature.com/articles/s42005-021-00786-y U6 - https://doi.org/10.1038/s42005-021-00786-y SN - 2399-3650 IS - 5 SP - 1 EP - 7 ER - TY - GEN A1 - Mirarchi, Giovanni A1 - Seibold, Götz A1 - Di Castro, Carlo A1 - Grilli, Marco A1 - Caprara, Sergio T1 - The Strange-Metal Behavior of Cuprates T2 - Condensed Matter N2 - Recent resonant X-ray scattering experiments on cuprates allowed to identify a new kind of collective excitations, known as charge density fluctuations, which have finite characteristic wave vector, short correlation length and small characteristic energy. It was then shown that these fluctuations provide a microscopic scattering mechanism that accounts for the anomalous transport properties of cuprates in the so-called strange-metal phase and are a source of anomalies in the specific heat. In this work, we retrace the main steps that led us to attributing a central role to charge density fluctuations in the strange-metal phase of cuprates, discuss the state of the art on the issue and provide an in-depth analysis of the contribution of charge density fluctuations to the specific heat. KW - high-temperature superconductors KW - cuprates KW - charge density fluctuations KW - strange metal KW - dynamical quantum criticality Y1 - 2022 UR - https://www.mdpi.com/2410-3896/7/1/29/htm U6 - https://doi.org/10.3390/condmat7010029 SN - 2410-3896 VL - 7 IS - 1 SP - 1 EP - 17 ER - TY - GEN A1 - Seibold, Götz A1 - Castellani, Claudio A1 - Lorenzana, José T1 - Adiabatic transition from a BCS superconductor to a Fermi liquid and phase dynamics T2 - Physical Review B N2 - We investigate the physics of an adiabatic transition from a BCS superconductor to a Fermi liquid for an exponentially slow decreasing pairing interaction. We show that, depending on the order of the thermodynamic limit and large times, a situation can arise in which the Fermi liquid keeps a memory of the parent BCS state. Furthermore, a time inversion of the interaction, supplemented by a manipulation analogous to a spin-/photon-echo experiment, allows us to recover the parent BCS state. Moreover, we study the evolution of the order parameter phase ϕ in transforming the BCS superconductor to a conventional metal. Since the global phase is the conjugate variable of the density, we explicitly show how to use the dynamics of ϕ together with gauge invariance to build up the noninteracting chemical potential away from particle-hole symmetry. We further analyze the role of ϕ in restoring the gauge-invariant current response when the noninteracting Fermi liquid is approached starting from a BCS superconductor in the presence of an external vector field. KW - Superconductivity KW - Nonequilibrium systems KW - Adiabatic approximation KW - BCS theory KW - Gauge symmetries KW - Variational approach Y1 - 2022 UR - https://journals.aps.org/prb/abstract/10.1103/PhysRevB.105.184513 U6 - https://doi.org/10.1103/PhysRevB.105.184513 SN - 2469-9969 VL - 105 IS - 18 SP - 184513-1 EP - 184513-14 ER - TY - GEN A1 - Udina, Mattia A1 - Fioro, Jacopo A1 - Cea, Tommaso A1 - Castellani, Claudio A1 - Seibold, Götz A1 - Benfatto, Lara T1 - THz non-linear optical response in cuprates: predominance of the BCS response over the Higgs mode T2 - Faraday Discussions N2 - Recent experiments with strong THz fields in unconventional cuprate superconductors have clearly evidenced an increase of the non-linear optical response below the superconducting critical temperature Tc. As in the case of conventional superconductors, a theoretical estimate of the various effects contributing to the non-linear response is needed in order to interpret the experimental findings. Here, we report a detailed quantitative analysis of the non-linear THz optical kernel in cuprates within a realistic model, accounting for the band structure and disorder level appropriate for these systems. We show that the BCS quasiparticle response is the dominant contribution for cuprates, and its polarization dependence accounts very well for the third-harmonic generation measurements. On the other hand, the polarization dependence of the THz Kerr effect is only partly captured by our calculations, suggesting the presence of additional effects when the system is probed using light pulses with different central frequencies. Y1 - 2022 UR - https://pubs.rsc.org/en/content/articlelanding/2022/FD/D2FD00016D U6 - https://doi.org/10.1039/D2FD00016D SN - 1364-5498 VL - 237 SP - 168 EP - 185 ER - TY - GEN A1 - Mirarchi, Giovanni A1 - Grilli, Marco A1 - Seibold, Götz A1 - Caprara, Sergio T1 - The Shrinking Fermi Liquid Scenario for Strange-Metal Behavior from Overdamped Optical Phonons T2 - Condensed Matter N2 - We discuss how the interaction of electrons with an overdamped optical phonon can give rise to a strange-metal behavior over extended temperature and frequency ranges. Although the mode has a finite frequency, an increasing damping shifts spectral weight to progressively lower energies so that despite the ultimate Fermi liquid character of the system at the lowest temperatures and frequencies, the transport and optical properties of the electron system mimic a marginal Fermi liquid behavior. Within this shrinking Fermi liquid scenario, we extensively investigate the electron self-energy in all frequency and temperature ranges, emphasizing similarities and differences with respect to the marginal Fermi liquid scenario. KW - strange-metal behavior KW - marginal Fermi liquid KW - high-temperature cuprate superconductors Y1 - 2024 UR - https://www.mdpi.com/2410-3896/9/1/14 U6 - https://doi.org/10.3390/condmat9010014 SN - 2410-3896 VL - 9 IS - 1 SP - 1 EP - 13 ER - TY - GEN A1 - Katsumi, Kota A1 - Fiore, Jacopo A1 - Udina, Mattia A1 - Romero, Ralph A1 - Barbalas, David A1 - Jesudasan, John A1 - Raychaudhuri, Pratap A1 - Seibold, Götz A1 - Benfatto, Lara A1 - Armitage, N. P. T1 - Revealing novel aspects of light-matter coupling by Terahertz Two-Dimensional Coherent Spectroscopy: the case of the amplitude mode in superconductors T2 - Physical Review Letters N2 - Recently developed terahertz (THz) two-dimensional coherent spectroscopy (2DCS) is a powerful technique to obtain materials information in a fashion qualitatively different from other spectroscopies. Here, we utilized THz 2DCS to investigate the THz nonlinear response of conventional superconductor NbN. Using broadband THz pulses as light sources, we observed a third-order nonlinear signal whose spectral components are peaked at twice the superconducting gap energy 2⁢Δ. With narrow-band THz pulses, a THz nonlinear signal was identified at the driving frequency Ω and exhibited a resonant enhancement at temperature when Ω=2⁢Δ. General theoretical considerations show that such a resonance can arise only from a disorder-activated paramagnetic coupling between the light and the electronic current. This proves that the nonlinear THz response can access processes distinct from the diamagnetic Raman-like density fluctuations, which are believed to dominate the nonlinear response at optical frequencies in metals. Our numerical simulations reveal that, even for a small amount of disorder, the Ω=2⁢Δ resonance is dominated by the superconducting amplitude mode over the entire investigated disorder range. This is in contrast to other resonances, whose amplitude-mode contribution depends on disorder. Our findings demonstrate the unique ability of THz 2DCS to explore collective excitations inaccessible in other spectroscopies. Y1 - 2024 UR - https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.132.256903 U6 - https://doi.org/10.1103/PhysRevLett.132.256903 VL - 132 ER - TY - GEN A1 - Caprara, Sergio A1 - Di Castro, Carlo A1 - Mirarchi, Giovanni A1 - Seibold, Götz A1 - Grilli, Marco T1 - The shrinking fermi liquid scenario for cuprates under the scrutiny of optical conductivity measurements T2 - Materials N2 - In a recent paper [B. Michon et al., Nat. Commun. (2023) 14:3033], optical conductivity experiments in cuprate superconductors were shown to display scaling properties consistent with the Marginal Fermi Liquid theory. Here, we argue that the temperature regime studied in these experiments does not allow for distinguishing between Marginal Fermi Liquid and Shrinking Fermi Liquid. In the latter scenario, which we recently proposed and which applies near a quantum critical point, dynamical fluctuations of the order parameter with a short correlation length mediate a nearly isotropic scattering among the quasiparticles over the entire Fermi surface leading to strange metal behavior. If the damping of these nearly local fluctuations increases by decreasing the temperature, the Fermi liquid regime shrinks and the strange metal behavior is extended to the lowest temperatures. This Shrinking Fermi Liquid scenario has many similarities and some differences with respect to the Marginal Fermi Liquid theory. In particular, we show that the approximate scaling properties of the optical conductivity in some high-frequency regimes predicted by the Shrinking Fermi Liquid scenario account for a very good description of the experimental data. Y1 - 2024 U6 - https://doi.org/10.3390/ma17235849 SN - 1996-1944 VL - 17 IS - 23 PB - MDPI AG ER - TY - GEN A1 - Fiore, Jacopo A1 - Sellati, Niccolò A1 - Gabriele, Francesco A1 - Castellani, Claudio A1 - Seibold, Götz A1 - Udina, Mattia A1 - Benfatto, Lara T1 - Investigating Josephson plasmons in layered cuprates via nonlinear terahertz spectroscopy T2 - Physical Review B N2 - Josephson plasmons in layered superconductors represent a natural source of optical nonlinearity, thanks to their intrinsically anharmonic nature. Here we derive the selection rules behind nonlinear plasmonics showing its dependence on plasmonic branches hidden to other spectroscopies, such as RIXS. We benchmark our results for the case of layered cuprates, showing how in a layered system the combined effect of plasmon dispersion and light polarization can move the resonance of the bilayer system away from the plasma edge measured in linear spectroscopy. Our results demonstrate the dependence of the nonlinear THz response on the convoluted plasmon dispersion in a momentum region complementary to RIXS, and offer a possible perspective for the generation of THz pulses by artificially designed Josephson heterostructures. KW - Research Areas Josephson effect KW - Plasmons KW - Superconducting fluctuations KW - Superconductors KW - Cuprates KW - Terahertz spectroscopy Y1 - 2024 UR - https://journals.aps.org/prb/abstract/10.1103/PhysRevB.110.L060504 U6 - https://doi.org/10.1103/PhysRevB.110.L060504 VL - 110 SP - 1 EP - 6 ER - TY - GEN A1 - Mirarchi, Giovanni A1 - Arpaia, Riccardo A1 - Wahlberg, Eric A1 - Bauch, Thilo A1 - Kalaboukhov, Alexei A1 - Caprara, Sergio A1 - Di Castro, Carlo A1 - Grilli, Marco A1 - Lombardi, Floriana A1 - Seibold, Götz T1 - Tuning the ground state of cuprate superconducting thin films by nanofaceted substrates T2 - Communications Materials N2 - Anisotropic transport properties have been assessed in a number of cuprate superconductors, providing evidence for a nematic state. We have recently shown that in ultra-thin YBa2Cu3O7−δ films, where nematicity is induced via strain engineering, there is a suppression of charge density wave scattering along the orthorhombic a-axis and a concomitant enhancement of strange metal behavior along the b-axis. Here we develop a microscopic model, that is based on the strong interaction between the substrate facets and the thin film, to account for the unconventional phenomenology. Based on the atomic force microscopy imaging of the substrates’ surface, the model is able to predict the absence (presence) of nematicity and the resulting transport properties in films grown on SrTiO3 (MgO) substrates. Our result paves the way to new tuning capabilities of the ground state of high-temperature superconductors by substrate engineering. KW - Electronic properties and materials KW - Surfaces, interfaces and thin films Y1 - 2024 UR - https://www.nature.com/articles/s43246-024-00582-5?utm_source=rct_congratemailt&utm_medium=email&utm_campaign=oa_20240809&utm_content=10.1038/s43246-024-00582-5 U6 - https://doi.org/10.1038/s43246-024-00582-5 IS - 5 SP - 1 EP - 6 ER - TY - GEN A1 - Lorenzana, José A1 - Seibold, Götz T1 - Undamped higgs modes in strongly interacting superconductors T2 - Condens Matter N2 - In superconductors, gauge 𝑈(1) symmetry is spontaneously broken. According to Goldstone’s theorem, this breaking of a continuous symmetry establishes the existence of the Bogoliubov phase mode while the gauge-invariant response also includes the amplitude fluctuations of the order parameter. The latter, which are also termed ‘Higgs’ modes in analogy with the standard model, appear at the energy of the spectral gap 2Δ, when the superconducting ground state is evaluated within the weak-coupling BCS theory, and, therefore, are damped. Previously, we have shown that, within the time-dependent Gutzwiller approximation (TDGA), Higgs modes appear inside the gap with a finite binding energy relative to the quasiparticle continuum. Here, we show that the binding energy of the Higgs mode becomes exponentially small in the weak-coupling limit converging to the BCS solution. On the other hand, well-defined undamped amplitude modes exist in strongly coupled superconductors when the interaction energy becomes of the order of the bandwidth. KW - superconductivity KW - time-dependent Gutzwiller KW - collective modes Y1 - 2024 U6 - https://doi.org/10.3390/condmat9040038 VL - 9 IS - 4 SP - 1 EP - 12 ER - TY - GEN A1 - Grilli, Marco A1 - Di Castro, Carlo A1 - Mirarchi, Giovanni A1 - Seibold, Götz A1 - Caprara, Sergio T1 - Dissipative Quantum Criticality as a Source of Strange Metal Behavior T2 - Symmetry N2 - The strange metal behavior, usually characterized by a linear-in-temperature (T) resistivity, is a still unsolved mystery in solid-state physics. It is often associated with the proximity to a quantum critical point (a second order transition at temperature T=0, leading to a broken symmetry phase) focusing on the related divergent order parameter correlation length. Here, we propose a paradigmatic shift, focusing on a divergent characteristic time scale due to a divergent dissipation acting on the fluctuating critical modes while their correlation length stays finite. To achieve a divergent dissipation, we propose a mechanism based on the coupling between a local order parameter fluctuation and electron density diffusive modes that accounts both for the linear-in-T resistivity and for the logarithmic specific heat versus temperature ratio CV/T∼log(1/T), down to low temperatures. KW - strange metal behavior KW - cuprates KW - charge density fluctuations KW - diffusive modes Y1 - 2023 U6 - https://doi.org/10.3390/sym15030569 SN - 2073-8994 VL - 15 IS - 3 ER - TY - GEN A1 - Hizhnyakov, Vladimir A1 - Seibold, Götz T1 - Nanoscale phase separation in cuprate superconductors T2 - Physica C: Superconductivity and its Applications N2 - Electronic inhomogeneities are nowadays a well established feature of high-temperature superconductors, most prominently exemplified by the observation of charge-ordered states in a large number of cuprate materials. In this contribution we review the particular contribution of the Stuttgart-Tartu-Cottbus group to this field, which has been largely stimulated by intense discussion with K. Alex Müller. The original scenario, which was based on the formation of a percolative electronic network, is substantiated by a microscopic analysis of the clustering of magnetic polarons within the one-band Hubbard model. KW - Cuprates KW - Superconductors KW - Phase separation KW - Electronic inhomogeneities Y1 - 2023 UR - https://www.sciencedirect.com/science/article/pii/S0921453423001004 U6 - https://doi.org/10.1016/j.physc.2023.1354309 SN - 1873-2143 VL - 612 SP - 1 EP - 7 ER - TY - GEN A1 - Benfatto, Lara A1 - Castellani, Claudio A1 - Seibold, Götz T1 - Linear and nonlinear current response in disordered d-wave superconductors T2 - Physical Review B N2 - We present a detailed theoretical investigation of the linear and nonlinear optical response in a model system for a disordered d-wave superconductor. By evaluating the quasiparticle contribution (BCS response) we show that for both quantities the gap symmetry considerably changes the paradigm of the optical response as compared to the conventional s-wave case. For what concerns the linear response our findings agree with previous work showing that in strongly disordered d-wave superconductors a large fraction of uncondensed spectral weight survives below Tc, making the optical absorption around the gap-frequency scale almost unchanged with respect to the normal state. Our numerical results are in excellent quantitative agreement with experiments in overdoped cuprates. In the nonlinear regime we focus on the third-harmonic generation (THG), finding that, as already established for the s-wave case, in general a large THG is triggered by disorder-activated paramagnetic processes. However, in the d-wave case the BCS response is monotonously increasing in frequency, losing any signature of THG enhancement when the THz pump frequency ω matches the gap maximum Δ, a hallmark of previous experiments in conventional s-wave superconductors. Our findings, along with the mild polarization dependence of the response, provide an explanation for recent THG measurements in cuprates, setting the framework for the theoretical understanding of nonlinear effects in unconventional cuprates. KW - Superconducting order parameter KW - Superfluid density KW - High-temperature superconductors KW - Superconductivity KW - Drude model KW - Numerical techniques KW - Reflectivity KW - Terahertz spectroscopy Y1 - 2023 UR - https://journals.aps.org/prb/abstract/10.1103/PhysRevB.108.134508 U6 - https://doi.org/10.1103/PhysRevB.108.134508 SN - 2469-9969 SN - 2469-9950 VL - 108 SP - 134508-1 EP - 134508-15 ER - TY - GEN A1 - Berger, Wolfram A1 - Hokamp, Sascha A1 - Seibold, Götz T1 - Dynamic behavioral changes in an agent-based econophysics tax compliance model. Bomb-crater versus target effects and efficient audit strategies T2 - Journal of Public Finance and Public Choice N2 - We investigate an inhomogeneous Ising model in the context of tax evasion dynamics where different types of agents are parametrised via local temperatures and magnetic fields. Our work focuses on the dynamic behavioural change of agents after an audit which either corresponds to a temporal reduction or enhancement of compliance and is also known under the terms 'bomb crater' and 'target effect', respectively. We analyse this effect for different types of agents: endogenously non-compliant types, agents that have the tendency to copy (non-)compliant behaviour from their social environment, ethical agents with strong endogenous morale attitudes and random types that show large uctuations between compliant and non-compliant behaviour. Each type in uences the overall tax evasion differently, and interestingly, our model predicts that increasing the audit probability can have the counterintuitive effect of increasing tax evasion under certain circumstances. We analyse audit strategies that can suppress this effect and thus contribute to the burgeoning literature on the actual impact of tax audits. Y1 - 2021 U6 - https://doi.org/10.1332/251569120X15840237292628 SN - 2515-6926 VL - 36 IS - 1 SP - 3 EP - 24 ER - TY - GEN A1 - Seibold, Götz T1 - On the Evaluation of Higher-Harmonic-Current Responses for High-Field Spectroscopies in Disordered Superconductors T2 - Condensed Matter N2 - We discuss a formalism that allows for the calculation of a higher-harmonic-current response to a strong applied electric field for disordered superconducting systems described on the basis of tight-binding models with on- and/or intersite interactions. The theory is based on an expansion of the density matrix in powers of the field amplitudes, where we solve the equation of motion for the individual components. This allows the evaluation of higher-order response functions on significantly larger lattices than one can achieve with a previously used approach, which is based on a direct temporal integration of the equation of motion for the complete density matrix. In the case of small lattices, where both methods can be applied by including also the contribution of collective modes, we demonstrate the agreement of the corresponding results. KW - superconductivity KW - third-harmonic generation KW - disorder Y1 - 2023 UR - https://www.mdpi.com/2410-3896/8/4/95 U6 - https://doi.org/10.3390/condmat8040095 SN - 2410-3896 VL - 8 IS - 4 SP - 1 EP - 16 ER - TY - GEN A1 - Lorenzana, Jose A1 - Seibold, Götz T1 - Long-Lived Higgs Modes in Strongly Correlated Condensates T2 - Physical Review Letters N2 - We investigate order parameter fluctuations in the Hubbard model within a time-dependent Gutzwiller approach. While in the weak coupling limit we find that the amplitude fluctuations are short-lived due to a degeneracy with the energy of the edge of the quasiparticle continua (and in agreement with Hartree-Fock+RPA theory), these are shifted below the edge upon increasing the interaction. Our calculations therefore predict undamped amplitude (Higgs) oscillations of the order parameter in strongly coupled superconductors, cold atomic fermion condensates, and strongly interacting charge- and spin-density wave systems. We propose an experimental realization for the detection of the spin-type Higgs mode in undoped cuprates and related materials where, due to the Dzyaloshinsky-Moriya interaction, it can couple to an out-of-plane ferromagnetic excitation that is visible via the Faraday effect. KW - Magnetism KW - Quasiparticles & collective excitations KW - Spin dynamics KW - Superconducting order parameter KW - Superconductivity KW - Antiferromagnets KW - Atomic gases KW - High-temperature superconductors Y1 - 2024 UR - https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.132.026501 U6 - https://doi.org/10.1103/PhysRevLett.132.026501 SN - 0031-9007 VL - 132 IS - 2 SP - 026501-1 EP - 026501-8 ER - TY - GEN A1 - Wahlberg, Eric A1 - Arpaia, Riccardo A1 - Chakraborty, Debmalya A1 - Kalaboukhov, Alexei A1 - Vignolles, David A1 - Proust, Cyril A1 - Black-Schaffer, Annica M. A1 - Bauch, Thilo A1 - Seibold, Götz A1 - Lombardi, Floriana T1 - Boosting superconductivity in ultrathin YBa₂Cu₃O₇−δ films via nanofaceted substrates T2 - Nature communications N2 - In cuprate high-temperature superconductors the doping level is fixed during synthesis, hence the charge carrier density per CuO2 plane cannot be easily tuned by conventional gating, unlike in 2D materials. Strain engineering has recently emerged as a powerful tuning knob for manipulating the properties of cuprates, in particular charge and spin orders, and their delicate interplay with superconductivity. In thin films, additional tunability can be introduced by the substrate surface morphology, particularly nanofacets formed by substrate surface reconstruction. Here we show a remarkable enhancement of the superconducting onset temperature and the upper critical magnetic field Hc,2 in nanometer-thin YBa2Cu3O7−δ films grown on a substrate with a nanofaceted surface. We theoretically show that the enhancement is driven by electronic nematicity and unidirectional charge density waves, where both elements are captured by an additional effective potential at the interface between the film and the uniquely textured substrate. Our findings show a new paradigm in which substrate engineering can effectively enhance the superconducting properties of cuprates. This approach opens an exciting frontier in the design and optimization of high-performance superconducting materials. KW - Electronic properties and materials KW - Superconducting properties and materials Y1 - 2026 U6 - https://doi.org/10.1038/s41467-025-67500-2 SN - 2041-1723 VL - 17 SP - 1 EP - 8 PB - Springer Nature CY - London ER - TY - GEN A1 - Schlipf, Jon A1 - Cutolo, Maria Alessandra A1 - Manganelli, Costanza Lucia A1 - Reiter, Sebastian A1 - Seibold, Götz A1 - Skibitzki, Oliver A1 - Wenger, Christian A1 - Fischer, Inga Anita T1 - Fabrication and optical characterization of CMOS-compatible honeycomb-like large-scale lattices of near-field coupled plasmonic TiN nanotriangles T2 - Advanced optical materials N2 - Honeycomb-like plasmonic titanium nitride nanotriangle arrays defined by photolithography and fabricated in a modified silicon-germanium electronic–photonic integrated circuit process in a state-of-the-art pilot line. The nanotriangle arrays are characterized in experiments and simulations. The momentum-dependent reflectance spectra exhibit not only features that are consistent with surface lattice resonances in the honeycomb lattice but also minima governed by near-field coupling of the individual nanotriangles. The optical characterization results in combination with simulation-based predictions indicate that such nanotriangle arrays are capable of supporting collective plasmonic resonances that can be described as massless Dirac particles. The fabrication approach opens up the possibility of integrating the structures into device fabrication processes, and avenues toward near-infrared sensing and communication applications are predicted. KW - Plasmonic Nanostructures KW - Optics of Nanostructures Y1 - 2025 UR - https://advanced.onlinelibrary.wiley.com/doi/epdf/10.1002/adom.202403408 U6 - https://doi.org/10.1002/adom.202403408 SN - 2195-1071 VL - 2025 SP - 1 EP - 8 PB - Wiley-VCH CY - Weinheim ER -