@misc{FioreSellatiGabrieleetal., author = {Fiore, Jacopo and Sellati, Niccol{\`o} and Gabriele, Francesco and Castellani, Claudio and Seibold, G{\"o}tz and Udina, Mattia and Benfatto, Lara}, title = {Investigating Josephson plasmons in layered cuprates via nonlinear terahertz spectroscopy}, series = {Physical Review B}, volume = {110}, journal = {Physical Review B}, doi = {10.1103/PhysRevB.110.L060504}, pages = {1 -- 6}, abstract = {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.}, language = {en} } @misc{MirarchiArpaiaWahlbergetal., author = {Mirarchi, Giovanni and Arpaia, Riccardo and Wahlberg, Eric and Bauch, Thilo and Kalaboukhov, Alexei and Caprara, Sergio and Di Castro, Carlo and Grilli, Marco and Lombardi, Floriana and Seibold, G{\"o}tz}, title = {Tuning the ground state of cuprate superconducting thin films by nanofaceted substrates}, series = {Communications Materials}, journal = {Communications Materials}, number = {5}, doi = {10.1038/s43246-024-00582-5}, pages = {1 -- 6}, abstract = {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.}, language = {en} } @misc{LorenzanaSeibold, author = {Lorenzana, Jos{\´e} and Seibold, G{\"o}tz}, title = {Undamped higgs modes in strongly interacting superconductors}, series = {Condens Matter}, volume = {9}, journal = {Condens Matter}, number = {4}, doi = {10.3390/condmat9040038}, pages = {1 -- 12}, abstract = {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.}, language = {en} } @misc{GrilliDiCastroMirarchietal., author = {Grilli, Marco and Di Castro, Carlo and Mirarchi, Giovanni and Seibold, G{\"o}tz and Caprara, Sergio}, title = {Dissipative Quantum Criticality as a Source of Strange Metal Behavior}, series = {Symmetry}, volume = {15}, journal = {Symmetry}, number = {3}, issn = {2073-8994}, doi = {10.3390/sym15030569}, abstract = {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.}, language = {en} } @misc{HizhnyakovSeibold, author = {Hizhnyakov, Vladimir and Seibold, G{\"o}tz}, title = {Nanoscale phase separation in cuprate superconductors}, series = {Physica C: Superconductivity and its Applications}, volume = {612}, journal = {Physica C: Superconductivity and its Applications}, issn = {1873-2143}, doi = {10.1016/j.physc.2023.1354309}, pages = {1 -- 7}, abstract = {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 M{\"u}ller. 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.}, language = {en} } @misc{BenfattoCastellaniSeibold, author = {Benfatto, Lara and Castellani, Claudio and Seibold, G{\"o}tz}, title = {Linear and nonlinear current response in disordered d-wave superconductors}, series = {Physical Review B}, volume = {108}, journal = {Physical Review B}, issn = {2469-9969}, doi = {10.1103/PhysRevB.108.134508}, pages = {134508-1 -- 134508-15}, abstract = {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.}, language = {en} } @misc{BergerHokampSeibold, author = {Berger, Wolfram and Hokamp, Sascha and Seibold, G{\"o}tz}, title = {Dynamic behavioral changes in an agent-based econophysics tax compliance model. Bomb-crater versus target effects and efficient audit strategies}, series = {Journal of Public Finance and Public Choice}, volume = {36}, journal = {Journal of Public Finance and Public Choice}, number = {1}, issn = {2515-6926}, doi = {10.1332/251569120X15840237292628}, pages = {3 -- 24}, abstract = {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.}, language = {en} } @misc{Seibold, author = {Seibold, G{\"o}tz}, title = {On the Evaluation of Higher-Harmonic-Current Responses for High-Field Spectroscopies in Disordered Superconductors}, series = {Condensed Matter}, volume = {8}, journal = {Condensed Matter}, number = {4}, issn = {2410-3896}, doi = {10.3390/condmat8040095}, pages = {1 -- 16}, abstract = {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.}, language = {en} } @misc{LorenzanaSeibold, author = {Lorenzana, Jose and Seibold, G{\"o}tz}, title = {Long-Lived Higgs Modes in Strongly Correlated Condensates}, series = {Physical Review Letters}, volume = {132}, journal = {Physical Review Letters}, number = {2}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.132.026501}, pages = {026501-1 -- 026501-8}, abstract = {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.}, language = {en} } @misc{SchlipfCutoloManganellietal., author = {Schlipf, Jon and Cutolo, Maria Alessandra and Manganelli, Costanza Lucia and Reiter, Sebastian and Seibold, G{\"o}tz and Skibitzki, Oliver and Wenger, Christian and Fischer, Inga Anita}, title = {Fabrication and optical characterization of CMOS-compatible honeycomb-like large-scale lattices of near-field coupled plasmonic TiN nanotriangles}, series = {Advanced optical materials}, volume = {2025}, journal = {Advanced optical materials}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {2195-1071}, doi = {10.1002/adom.202403408}, pages = {1 -- 8}, abstract = {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.}, language = {en} }