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.
Our group with guests (Yuriko Baba, Prof. Caio Lewenkopf, Prof. Steven Tomsovic)
Chaotic dynamics has the propensity to be the villain of quantum device control as it leads rapidly to scrambling of quantum information and entropy production, certainly a fundamental hindrance to controlling quantum evolution. We could show that chaos, conversely, can be harnessed to control and efficiently guide the evolution of a complex quantum system toward desired states of matter, i.e. targeting. In effect, is it possible to mimic a quantum Maxwell demon to some extent.
The work has been published in Physical Review Letters.
Klaus Richter has been awarded the title "professor des Jahres" by the UNICUm Foundation.
The Vorstandsrat of the German Physical Society (DPG) has elected Klaus Richter as next DPG president fpr the term 2024-2026.
Our Topical Review has been published in Journal of Physics A.
Dr. Angelika Knothe joins our group as Wissenschaftliche Assistentin.
A public lecture by Klaus Richter given during the "Highlights der Physik" can be found here.
(picture @Highlights der Physik)
Program can be found here.
Camilo Moreno, who received his PhD in our group in 2021, and Tobias Lautenschäger
will be funded for their business idea "Symlab" (see here).
An invited overview article by Klaus Richter has been published in Physik Journal (in German) in March 2022. A preprint is found here. The published version is accessible for DPG members.
(picture @DPG, Physik Journal)
Probing complex quantum dynamics through out-of-time-ordered correlators Scientific Coordinators: Rodolfo Jalabert, Klaus Richter (more information here).
Facets of Many-Body Quantum Chaos; organized by Markus Heyl and Klaus Richter.
More informations can be find here.
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.
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.
DFG has approved our application for a second funding period from July 2021 until June 2025.
Press release here.
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).
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.
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).
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. __________________________________________________________________________________
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).
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. __________________________________________________________________________________
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)
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)
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)
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.
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).
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).
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) __________________________________________________________________________________
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)
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). __________________________________________________________________________________
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.
Complex Quantum Systems
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