Publikationen

@article{b635f0072b6b4335ad089b078426f3bb,

title = "Perturbative solution of fermionic sign problem in quantum Monte Carlo computations",

abstract = "We have developed a strong-coupling perturbation scheme for a general doped Hubbard model around a particle-hole-symmetric reference system, which is free from the fermionic sign problem. Our approach is based on the lattice determinantal Quantum Monte Carlo (QMC) method in both continuous and discrete time versions for large periodic clusters in a fermionic bath. By considering the first-order perturbation in the shift of the chemical potential and the second-neighbor hopping, we are able to obtain an accurate electronic spectral function for a range of parameters that correspond to optimally doped cuprate systems at temperatures of up to T = 0.1t, which are challenging to access using straightforward lattice QMC calculations. We also discuss the formation of the pseudogap and the nodal-antinodal dichotomy for a doped Hubbard system with the interaction parameter U equal to the bandwidth and the optimal value of the next-nearest-neighbor hopping parameter t′ for high-temperature superconducting cuprates.",

author = "Sergei Iskakov and Katsnelson, \{Mikhail I.\} and Lichtenstein, \{Alexander I.\}",

note = "Publisher Copyright: {\textcopyright} The Author(s) 2024.",

year = "2024",

month = dec,

doi = "10.1038/s41524-024-01221-w",

language = "English",

volume = "10",

journal = "npj Computational Materials",

issn = "2057-3960",

publisher = "Nature Publishing Group",

number = "1",

@article{d941a2d4e156442595324ab153080d51,

title = "Prerelaxation in quantum, classical, and quantum-classical two-impurity models",

abstract = "We numerically study the relaxation dynamics of impurity-host systems, focusing on the presence of long-lived metastable states in the nonequilibrium dynamics after an initial excitation of the impurities. In generic systems, an excited impurity coupled to a large bath at zero temperature is expected to relax and approach its ground state over time. However, certain exceptional cases exhibit metastability, where the system remains in an excited state on timescales largely exceeding the typical relaxation time. We study this phenomenon for three prototypical impurity models: a tight-binding quantum model of independent spinless fermions on a lattice with two stub impurities, a classical-spin Heisenberg model with two weakly coupled classical impurity spins, and a tight-binding quantum model of independent electrons with two classical impurity spins. Through numerical integration of the fundamental equations of motion, we find that all three models exhibit similar qualitative behavior: complete relaxation for nearest-neighbor impurities and incomplete or strongly delayed relaxation for next-nearest-neighbor impurities. The underlying mechanisms leading to this behavior differ between models and include impurity-induced bound states, emergent approximately conserved local observables, and exact cancellation of local and nonlocal dissipation effects.",

author = "Michael Elbracht and Michael Potthoff",

year = "2024",

month = sep,

day = "10",

doi = "10.1103/PhysRevResearch.6.033275",

language = "English",

volume = "6",

journal = "Physical Review Research",

issn = "2643-1564",

publisher = "American Physical Society",

number = "3",

@article{62333f8e232140238cd99783fd8bca9e,

title = "Kondo screening and indirect magnetic exchange through a conventional superconductor studied by the density matrix renormalization group",

author = "Cassian Plorin and Michael Potthoff",

year = "2024",

month = aug,

day = "9",

doi = "10.1103/PhysRevB.110.085119",

language = "English",

volume = "110",

journal = "Phys. Rev. B",

issn = "2469-9950",

publisher = "American Physical Society",

@article{77e495be8f004e18a0f36506a990fd65,

title = "Bound states and local topological phase diagram of classical impurity spins coupled to a Chern insulator",

abstract = "The existence of bound states induced by local impurities coupled to an insulating host depends decisively on the global topological properties of the host's electronic structure. In this context, we consider magnetic impurities modeled as classical unit-length spins that are exchange coupled to the spinful Haldane model on the honeycomb lattice. We investigate the spectral flow of bound states with the coupling strength J in both the topologically trivial and Chern-insulating phases. In addition to conventional k-space topology, an additional, spatially local topological feature is available, based on the space of impurity-spin configurations forming, in case of R impurities, an R-fold direct product of two-dimensional spheres. Global k-space and local S-space topology are represented by different topological invariants, the first (k-space) Chern number and the Rth (S-space) spin-Chern number. We demonstrate that there is a local S-space topological transition as a function of J associated with a change in the spin-Chern number and work out the implications of this for the J-dependent local electronic structure close to the impurities and, in particular, for in-gap bound states. The critical exchange couplings' dependence on the parameters of the Haldane model, and thus on the k-space topological state, is obtained numerically to construct local topological phase diagrams for systems with R=1 and 2 impurity spins.",

author = "Simon Michel and Axel F{\"u}nfhaus and Robin Quade and Roser Valent{\'i} and Michael Potthoff",

note = "Publisher Copyright: {\textcopyright} 2024 American Physical Society. ",

year = "2024",

month = apr,

day = "4",

doi = "10.1103/PhysRevB.109.155116",

language = "English",

volume = "109",

journal = "Phys. Rev. B",

issn = "1098-0121",

publisher = "American Physical Society",

number = "15",

@article{322934a53dca4b1eb8e3039aa80eb431,

title = "Unraveling quantum coherences mediating primary charge transfer processes in photosystem II reaction center",

abstract = "Photosystem II (PSII) reaction center (RC) is a unique complex that is capable of efficiently separating electronic charges across the membrane. The primary energy- and charge-transfer (CT) processes occur on comparable ultrafast timescales, which makes it extremely challenging to understand the fundamental mechanism responsible for the near-unity quantum efficiency of the transfer. Here, we elucidate the role of quantum coherences in the ultrafast energy and CT in the PSII RC by performing two-dimensional (2D) electronic spectroscopy at the cryogenic temperature of 20 kelvin, which captures the distinct underlying quantum coherences. Specifically, we uncover the electronic and vibrational coherences along with their lifetimes during the primary ultrafast processes of energy and CT. We construct an excitonic model that provides evidence for coherent energy and CT at low temperature in the 2D electronic spectra. The principles could provide valuable guidelines for creating artificial photosystems with exploitation of system-bath coupling and control of coherences to optimize the photon conversion efficiency to specific functions.",

author = "Ajay Jha and Zhang, \{Pan Pan\} and Vandana Tiwari and Lipeng Chen and Michael Thorwart and Miller, \{R. J.Dwayne\} and Duan, \{Hong Guang\}",

note = "Publisher Copyright: Copyright {\textcopyright} 2024 Th Authors, some rights.",

year = "2024",

month = mar,

day = "8",

doi = "10.1126/sciadv.adk1312",

language = "English",

volume = "10",

journal = "Science Advances ",

issn = "2375-2548",

publisher = "American Association for the Advancement of Science",

number = "10",

@article{c785ac6f629e40f0979b064943293b6e,

title = "Enhancing exotic quantum fluctuations in a strongly entangled cavity BEC system",

abstract = "We show that the strong coupling of a quantum light field and correlated quantum matter induces exotic quantum fluctuations in the matter sector. We determine their spectral characteristics and reveal the impact of the atomic s-wave scattering. In particular, we derive the dissipative Landau and Beliaev processes from the microscopic Hamiltonian using imaginary-time path integrals. By this, their strongly sub-Ohmic nature is revealed analytically. A competition between damping and antidamping channels is uncovered. Their intricate influence on physical observables is quantified analytically and the Stokes shift of the critical point is determined. This illustrates the tunability of the quantum matter fluctuations by exploiting strong light-matter coupling.",

keywords = "quant-ph, cond-mat.quant-gas",

author = "Leon Mixa and Hans Ke{\ss}ler and Andreas Hemmerich and Michael Thorwart",

note = "13 pages, 8 figures, to be published in Physical Review Research",

year = "2024",

month = feb,

day = "2",

doi = "10.1103/PhysRevResearch.6.L012024",

language = "English",

volume = "6",

pages = "602--607",

journal = "Physical Review Research",

issn = "2643-1564",

publisher = "American Physical Society",

number = "1",

@article{08a40cc4af2b48929ea71107ac70f0c4,

title = "Spin wave driven skyrmions in a bipartite antiferromagnetic lattice",

abstract = "We show that a skyrmion in a classical bipartite antiferromagnetic lattice can be spatially displaced in a controlled manner by externally applied spin waves. We reveal the relation between the skyrmion motion and the spin wave properties. To this end, we derive a classical spin wave formalism, which is tailored to the antiferromagnetic two-dimensional square lattice. The antiferromagnetic spin waves can be classified into two types with respect to their polarization, with two modes each. The circularly polarized spin waves oscillate with different amplitudes in the respective sublattices and induce a skyrmion Hall effect. The two modes are symmetric under sublattices exchange and determine the overall sign of the Hall angle. For linearly polarized spin waves, the two sublattices oscillate elliptically in opposite direction, however, with the same amplitude. These accelerate the skyrmion solely into their own propagation direction. The two modes are symmetric under component x-y exchange and impact Bloch or N{\'e}el skyrmions differently. Our results indicate possible technical applications of spin wave driven skyrmion motion. As one example we propose a racetrack where spin waves pump skyrmions along the track in antiferromagnets.",

author = "Michael Lau and Wolfgang H{\"a}usler and Michael Thorwart",

note = "Publisher Copyright: {\textcopyright} 2024 American Physical Society.",

year = "2024",

month = jan,

day = "29",

doi = "10.1103/PhysRevB.109.014435",

language = "English",

volume = "109",

journal = "Physical review B",

issn = "2469-9950",

publisher = "American Physical Society",

number = "1",

@article{70718580351f4bc6aef0bc531a292830,

title = "Microscopic nonlinear optical response: analysis and calculations with the Floquet-Bloch formalism",

abstract = "We analyze microscopic nonlinear optical response of periodic structures within the Floquet-Bloch formalism. The analysis is focused on the real-space distributions of optically induced charge and electron current density within the unit cell of a crystal. We demonstrate that the time-reversal symmetry of a crystal determines the phases of the temporal oscillations of these distributions. We further analyze their spatial symmetries and connection to macroscopic optical response. We illustrate our study with ab initio calculations that combine density functional theory with the Floquet-Bloch formalism. The calculations provide time-dependent optically induced charge distributions and electron current densities within the unit cells of a crystal with inversion symmetry MgO and a crystal without inversion symmetry GaAs in response to a strong-field excitation. The real-space, microscopic view on nonlinear optical response provides insightful information about the strong field-matter interaction.",

author = "Daria Popova-Gorelova and Robin Santra",

year = "2024",

month = jan,

day = "1",

doi = "10.1063/4.0000220",

language = "English",

volume = "11",

journal = "Structural Dynamics",

issn = "2329-7778",

publisher = "AIP Publishing",

number = "1",

@article{a3887250a57b43958f96c92f1e2a21c3,

title = "A theory for colors of strongly correlated electronic systems: Nature Communications",

abstract = "Many strongly correlated transition metal insulators are colored, even though they have band gaps much larger than the highest energy photons from the visible light. An adequate explanation for the color requires a theoretical approach able to compute subgap excitons in periodic crystals, reliably and without free parameters—a formidable challenge. The literature often fails to disentangle two important factors: what makes excitons form and what makes them optically bright. We pick two archetypal cases as examples: NiO with green color and MnF 2 with pink color, and employ two kinds of ab initio many body Green{\textquoteright}s function theories; the first, a perturbative theory based on low-order extensions of the G W approximation, is able to explain the color in NiO, while the same theory is unable to explain why MnF 2 is pink. We show its color originates from higher order spin-flip transitions that modify the optical response, which is contained in dynamical mean-field theory (DMFT). We show that symmetry lowering mechanisms may determine how {\textquoteleft}bright{\textquoteright} these excitons are, but they are not fundamental to their existence.",

keywords = "crystal structure, insulation, optical property, transition element, wavelength, ab initio calculation, article, theoretical study",

author = "S. Acharya and D. Pashov and C. Weber and \{van Schilfgaarde\}, M. and A.I. Lichtenstein and M.I. Katsnelson",

note = "Export Date: 11 December 2023; Cited By: 0; Correspondence Address: S. Acharya; Institute for Molecules and Materials, Radboud University, Nijmegen, 6525 AJ, Netherlands; email: swagata.acharya@nrel.gov",

year = "2023",

month = dec,

doi = "10.1038/s41467-023-41314-6",

language = "English",

volume = "14",

journal = "Nature Communications",

issn = "2041-1723",

publisher = "Nature Publishing Group",

number = "1",

@article{7b2815167c9743deb07d43d39f012da1,

title = "Multiple-core-hole resonance spectroscopy with ultraintense X-ray pulses: Nature Communications",

abstract = "Understanding the interaction of intense, femtosecond X-ray pulses with heavy atoms is crucial for gaining insights into the structure and dynamics of matter. One key aspect of nonlinear light–matter interaction was, so far, not studied systematically at free-electron lasers—its dependence on the photon energy. Here, we use resonant ion spectroscopy to map out the transient electronic structures occurring during the complex charge-up pathways of xenon. Massively hollow atoms featuring up to six simultaneous core holes determine the spectra at specific photon energies and charge states. We also illustrate how different X-ray pulse parameters, which are usually intertwined, can be partially disentangled. The extraction of resonance spectra is facilitated by the possibility of working with a constant number of photons per X-ray pulse at all photon energies and the fact that the ion yields become independent of the peak fluence beyond a saturation point. Our study lays the groundwork for spectroscopic investigations of transient atomic species in exotic, multiple-core-hole states that have not been explored previously. {\textcopyright} 2023, Springer Nature Limited.",

keywords = "parameter estimation, resonance, spectroscopy, X-ray analysis, Article, electron, light absorption, nonhuman, photon, X ray spectroscopy",

author = "A. R{\"o}rig and S.-K. Son and Tommaso Mazza and P. Schmidt and T.M. Baumann and Benjamin Erk and M. Ilchen and J. Laksman and V. Music and S. Pathak and Rivas, \{Daniel E.\} and Daniel Rolles and S. Serkez and S. Usenko and R. Santra and M. Meyer and Rebecca Boll",

note = "Export Date: 11 December 2023; Cited By: 0; Correspondence Address: R. Boll; European XFEL, Schenefeld, Germany; email: rebecca.boll@xfel.eu; S.-K. Son; Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany; email: sangkil.son@cfel.de",

year = "2023",

month = sep,

day = "15",

doi = "10.1038/s41467-023-41505-1",

language = "English",

volume = "14",

journal = "Nature Communications",

issn = "2041-1723",

publisher = "Nature Publishing Group",

number = "1",