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Complex Quantum Systems

We consider complexity and associated phenomena emerging in quantum matter: from low-dimensional (topological) materials to quantum gases, from classical to quantum, from integrability to many-body quantum chaos, from few to many particles.

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2021: News and Events ________________________________________________________________________

Second funding period for CRC 1277

DFG has approved our application for a second funding period from July 2021 until June 2025.

Press release here.

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Upcoming: Workshop at MPI-PKS Dresden (October 11 – 15, 2021)

Probing complex quantum dynamics through out-of-time-ordered correlators Scientific Coordinators: Rodolfo Jalabert, Klaus Richter (more information here)

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Upcoming: Symposium within the meeting of the Condensed Matter Division of the German Physical Society (September 26 -- October 1, 2021)

Facets of Many-Body Quantum Chaos; organized by Markus Heyl and Klaus Richter (more information will follow).

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Generating high harmonics in topological insulators

In a collaboration of the experimental groups of Rupert Huber (UR) and  Ulrich Höfer (University of Marburg) with the group of Ferdinand Evers (UR) and us on the theory side, a novel mechanism for generating high-order harmonics in the surface states of a topological insulator has been discovered. When quasi-relativistic electrons are accelerated through the Dirac point by an intense lightwave, they abruptly reverse their velocity allowing for purely non-integer high-harmonics.

This work is published in Nature (May 2021)
Press release here.

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Very weak bonds to artificial atoms formed by quantum corrals

Employing numerical simulations we have accompanied and explained experiments of the Giessibl group (UR), who explored the bonding properties of the quantum corral (a circle of 48 iron atoms placed on a copper surface) as an artificial atom using an atomic force microscope (AFM). The confined electronic states can form a bond to the front atom of the AFM. The measured forces are about 1/1000 of typical forces in atomically resolved AFM. The confined electrons showed covalent attraction to metal tips and Pauli repulsion to CO-terminated tips.

This work is published in Science (May 2021)
Press release here.
The work is also feautured in ChemistryWorld.

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PD Dr. Cosimo Gorini

Congratulations: Cosimo Gorini has finished his habilitation with the thesis “Pseudospin- and spin-orbit coupled dynamics: from normal metals to Dirac systems” and becomes Privatdozent at University of Regensburg (April 2021).

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Klaus Richter receives DFG funding for a Reinhart Koselleck project

DFG has recently approved his application for the project "Many-Body Quantum Processes at the Edge of Chaos: From Non-Equilibrium Thermodynamics towards Quantum Gravity" and supports his research with 750.000 € for 5 years. According to DFG the Reinhart Koselleck programme enables outstanding researchers to pursue exceptionally innovative, higher-risk projects (April 2021).

Press release here.

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A Renormalized Semiclassical Expansion in Quantum Critical Many-Body Systems

We have developed a general framework that shows how in the quasiclassical regime (N ≫ 1) the dynamics of an N-particle quantum system at a critical point is governed by an effective Planck constant, renormalized through the leading Lyaponov exponent. For scrambling in many-body hyperbolic systems, our results provide formal grounds to a conjectured multiexponential form of so-called out-of-time-ordered correlators.

This work is published in Physical Review Letters (March 2021).

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2020: News and Events ________________________________________________________________________

Cosimo Gorini receives researcher position at CEA Saclay

Cosimo Gorini has accepted an offer for a permanent researcher position at the Service de Physique de l'Etat Condensé (Condensed Matter Physics Laboratory) of CEA Saclay, France, starting December 2020. __________________________________________________________________________________

Probing Dirac Surface States via Quantum Capacitance

We model the electronic density of states probed by capacitance experiments of the Weiss group at UR in thin layers of the topological insulator BiSbTeSe2. The results point at the coexistence and intimate coupling of Dirac surface states with a bulk many-body phase (a Coulomb glass) in 3D-TIs.


This work is published in Nanoletters (November 2020).

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Hexagonal Cyclotron Motion in Graphene Superlattices

In collaboration with the groups of R. Danneau (Karlsruhe) and M.-H- Liu (Tainan) we found that Dirac quasiparticles originating from graphene superlattice minibands do not undergo conventional cyclotron motion but follow more subtle trajectories. In particular, dynamics at low magnetic fields is characterized by peculiar, straight trajectory segments. Our results open up novel possibilities to use periodic potentials in electron optics experiments.

This work is published as an Editor’s suggestion in Physical Review Letters (November 2020)
Press release here. __________________________________________________________________________________

Novel Gate-Tunable Two-Dimensional Superlattices in Graphene

Together with our long-term collaborator, M.-H- Liu (Tainan), we provided theory support for experiments of the Eroms/Weiss group at UR, using a new technique for gate-tunable superlattices in graphene by the combined action of a back gate and a few-layer graphene patterned bottom gate. Thereby, well-pronounced satellite Dirac points and signatures of the Hofstadter butterfly are observed in corresponding transport measurements and theoretically explained.

This work is published in Nanoletters (October 2020)

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Spin relaxation in superconductors: resonant vs. off-resonant spin-flip scatterering

In a joint work with D. Kochan and colleagues from the Fabian group at UR, we investigate the relaxation of quasiparticle spins in graphene proximitized by an s-wave superconductor in the presence of resonant magnetic and spin-orbit active impurities. We show that for resonant scattering the spin relaxation decreases with decreasing temperature, opposite to what is known for off-resonant scattering.

This work is published in Physical Review Letters (August 2020)

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The electronic thickness of graphene

Graphene is often considered the thinnest existing material, but how thick are two atomically close graphene layers? Together with the Ensslin group (Zurich) doing transport measurements and with our theory colleagues, M.-H. Liu (Tainan) and M. Kurpas (Katowice), performing transport calculations, we investigate the electrostatics of two twisted graphene layers. We determine the finite dielectric thickness of each graphene layer to be ≈ 2.6 Å.

This work is published in Science Advances (March 2020)

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Trapping Dirac Fermions in a Quantum Magnetic Bottle

We show that topological insulator nanocones exhibit intriguing mesoscopic surface transport phenomena, including the quantum Hall effect. Furthermore, a nanocone may act as a quantum magnetic bottle, confining surface Dirac electrons, leading to a largely interaction-dominated regime of Coulomb blockade type and, moreover, suggesting that TI nanocone junctions may serve as building blocks for Dirac electron optics setups.

This work is published as an Editor’s suggestion in Physical Review Letters (March 2020)
Press release here.

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2019: News and Events ________________________________________________________________________

Interview with Professor Hans Joachim Schellnhuber

Prof. H. J. Schellnhuber (Director Emeritus of the Potsdam Institute for Climate Impact Research), when visiting the University of Regensburg on the occasion of the UR Dies Academicus 2019, spoke with Klaus Richter about his memories as a student at the Physics Department, his scientific career and about global warming. A written version of this conversation can be found here (in German).

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Many-Body Quantum Interference in Chaotic Bosonic Systems

Quantum interference plays a prominent role in the far-out-of-equilibrium dynamics of many-body systems. Here, we examine a particular interference effect arising from discrete symmetries, which can significantly enhance quantum observables with respect to classical predictions. For the Bose-Hubbard model we further show strong evidence for the presence of dynamical localization.

This work is published in Physical Review Letters (November 2019).

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Reversible quantum information spreading in many-body systems

Quantum chaotic interacting many-body systems are assumed to show fast and irreversible spreading of quantum information on short time scales. Here, we show that, in contrast to this common notion, certain quantum critical many-body systems exhibit oscillatory behavior between reentrant localization and delocalization of information in Hilbert space.

This work is published in Physical Review Letters (October 2019) __________________________________________________________________________________

Spectral Universality in Repulsive Bose Gases

We show that the crossover from an ideal Bose gas to the strongly correlated, fermionized gas exhibits universal behavior: Quantum systems from very few and up to many particles share the same underlying spectral features. Our method provides predictions for excitation spectra that enable access to finite-temperature thermodynamics in large parameter ranges.

This work is published in Physical Review Letters (June 2019)

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Klaus Richter reelected as DPG board member

Klaus Richter has been elected for another two years as member of the board of the German Physical Society (DPG), responsible for DPG’s scientific programs and prizes (March 2019). __________________________________________________________________________________

Klaus Richter receives Physik-Preis Dresden

The prize is jointly awarded by the TU Dresden and the Max Planck Institute for the Physics of Complex Systems (January 2019)

Press releases here and here.
 


  1. Physics Department
  2. Condensed Matter Theory

Richter Group

Complex Quantum Systems


Office:
D. Meier, E. Haushalter,
T. Siegmund

Phys 4.1.34
+49 (0)941 943 2030
richter.office(at)ur.de