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Quantum Many-Body and Condensed Matter Theory

Prof. Dr. Johann Kroha


News and highlights


Non-Hermitian phase transition at an exceptional point                                                       in a Bose-Einstein condensate of light

Fahri Emre Öztürk, Tim Lappe, Göran Hellmann, Julian Schmitt, Jan Klaers, Frank Vewinger, Johann Kroha and Martin Weitz,

Science 372, Issue 6537, 88 (2021)                                                                         

                                                                     Exceptional point photon BEC

(02 April, 2021)

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Press coverage (examples):    Science Daily


Fast, yet quantized


Topological, quantized transport has been limited to slowly varying potentials. The reason is that any nonvanishing driving frequency changes the topological parameter space from a circle to a torus, with band gaps opening in the band structure of the driven system. Here, in a collaboration of theory and experiment, the authors show that a topologically nontrivial band structure and associated quantized transport can be restored by non-Hermitian Floquet engineering at a driving frequency as large as the band gap of the nondissipative system. They realize this effect experimentally by means of periodically modulated, plasmonic waveguide arrays. 

(27 July, 2020)


 Read article at Nature Communications...

Echoes from the quantum world


A collaboration of experimental and theoretical groups, lead by Johann Kroha of the Physikalische Institut at the University of Bonn and Manfred Fiebig of the Materials Department at the ETH, shows that certain materials emit optical echo pulses that reveal direct information about the quantum-mechanical nature of these systems.        

(10 August, 2018)

HF: THz time-domain spectroscopy


Read article at Nature Physics...


Thermalization of Isolated Quantum Systems: If and How?


A. Posazhennikova, M. Trujillo Martinez, J. Kroha, Ann. Phys. (Berlin) 530, 1700124 (2018)

QuantumThermalizationA quantum system, isolated from the environment and initially in a single, “pure” quantum state, will evolve in time according to the Schrodinger equation, that is, it will remain in a pure state for all times. This is also true for a many-body quantum system. At first sight, this fundamental feature of quantum dynamics appears impossible to reconcile with experimental observations that show a sufficiently complex quantum system typically relaxes fast to thermal behavior if left on its own. This constitutes the quantum thermalization problem.

Read more at Advanced Science News...          ASN


(18 January 2018)                                                                                                      Read review article at Annalen der Physik...


Time-Resolved Collapse and Revival of the Kondo State near a Quantum Phase Transition

Christoph Wetli, Johann Kroha, Kristin Kliemt, Cornelius Krellner, Oliver Stockert, Hilbert von Löhneysen, Manfred Fiebig

One of the most successful paradigms of many-body physics is the concept of quasiparticles: excitations in strongly interacting matter behaving like weakly interacting particles in free space. Quasiparticles in metals are very robust objects. Yet, when a system's ground state undergoes a qualitative change at a quantum critical point (QCP), these quasiparticles can disintegrate and give way to an exotic quantum-fluid state of matter where the very notion of particles comprising the system breaks down. The nature of this breakdown is intensely debated, because the emergent quantum fluid dominates the material properties up to high temperature and might even be related to the occurence of superconductivity in some compounds. In collaboration with the experimental group of Manfred Fiebig at the ETH Zürich, supported by groups at KIT Karlsruhe, MPI-CPfS Dresden and U Frankfurt, the resurgence of heavy-fermion quasiparticles out of a photoexcited nonequilibrium state and their dynamics towards the QCP are monitored in a time-resolved experiment and analyzed by our many-body calculations. A terahertz pulse transforms heavy fermions in CeCu_{5.9}Au_{0.1} into light electrons. Under emission of a delayed, phase-coherent terahertz reflex the heavy-fermion state recovers, with a memory time 100 times longer than the coherence time typically associated with metals. The quasiparticle weight collapses towards the QCP, yet its formation temperature remains almost constant. This suggests a revised view of quantum criticality in between disintegration and preservation of the quasiparticle picture.

(14 March 2017)



Kondo destruction in RKKY-coupled Kondo lattice and multi-impurity systems

Ammar Nejati, Katinka Ballmann and Johann Kroha
Phys. Rev. Lett. 118, 117204 (2017)

Ammar Nejati and Johann Kroha
J. Phys: Conf. Series 807, 082004 (2017); SCES2016 Proc.

In a Kondo lattice, the spin exchange coupling between a local spin and the conduction electrons acquires nonlocal contributions due to conduction electron scattering from surrounding local spins and subsequent RKKY interaction. It leads to a hitherto unrecognized interference of Kondo screening and RKKY interaction beyond the Doniach scenario. We develop a renormalization group theory for the RKKY-modified Kondo vertex. The Kondo temperature, T_K(y), is suppressed in a universal way, controlled by the dimensionless RKKY coupling parameter y. Complete spin screening ceases to exist beyond a critical RKKY strength y_c even in the absence of magnetic ordering. At this breakdown point, T_K(y) remains nonzero and is not defined for larger RKKY couplings, y>y_c. The results are in quantitative agreement with STM spectroscopy experiments on tunable two-impurity Kondo systems. The possible implications for quantum critical scenarios in heavy-fermion systems are discussed.

10 Mar 2017




OSCAR taking off!

Since 1 July 2016, the DFG collaborative research center OSCAR, Open System Control of Atomic and photonic matter with Reservoirs, is up and running. Groups from U Bonn and TU Kaiserslautern are collaborating within this CRC. Our group contributes two projects.


Research area B: Control of quantum many-body systems by environments.
Our group contributes to this area by developing the theory for understanding the dynamics of Bose-Einstein condensates with complex interactions, like photon condensates investigated experimentally in Martin Weitz' group.  The main theoretical tool is non-equilibrium Keldysh quantum field theory.





Research area C: Topological states in atomic and photonic systems.
The main focus lies on the development of basic tools and methods to create topological order in atomic and photonic matter as an alternative approach to control and protect quantum states. We investigate the topological stabilization of transport in Floquet-topological systems, where topological order is induced by time-periodic driving. Floquet-topological states are investigated experimentally in Dieter Meschede's group, using quantum walk techniques.

01 Jul 2016




Inflationary quasiparticle creation and thermalization dynamics in coupled Bose-Einstein condensates

Anna Posazhennikova, Mauricio Trujillo-Martinez and Johann Kroha

Phys Rev. Lett. 116, 225304 (2016)


When a Bose-Einstein condensate (BEC) is trapped in a double-well potential, it can perform quantum coherent oscillations between the wells, so-called Josephson oscillations. It is a fundamental problem of quantum statistical mechanics if and how such a non-equilibrium system can eventually come to rest and reach thermodynamic equilibrium. Since the quantum system is ideally isolated from its environment and, thus, evolves in time in a unitary way (i.e., the initially pure quantum state of the entire systems must remain a pure state for all times). 

We show that the thermalization happens in that single bosonic atoms, so-called quasiparticles, get excited out of the condensate. This cloud of quasiparticles thermalizes by exchanging energy and particles with the BEC, thereby maximizing the entanglement entropy (the entropy associated with the entanglement of the quasiparticle states with the condensate state). That is, the BEC acts as a grand canonical reservoir for the quasiparticle subsystem. The dynamics of this thermalization process exhibits three distinct time scales: (I) an initial period of undamped Josephson oscillations, (II) a period of fast quasiparticle creation out of the BEC due to a dynamically generated, parametric resonance, and (III) a regime of slow thermalization of the quasiparticle gas.

The picture shows the time-dependent population of the first five quasiparticle states as well as the total population. This dynamics bears similarities to the creation of elementary particles, as it is thought to have happened during the inflation and reheating periods of the early universe.

02 Jun 2016     

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