Condensed Matter Theory

Prof. Dr. Johann Kroha

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 arXiv:1703.04443 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.

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

Talk by Nicolas Néel, TU Ilmenau

The properties of magnetic structures on surfaces were investigated using a low temperature scanning tunneling microscope (STM).  On non-magnetic metallic surfaces the interaction of the magnetic moment of a single Co atom with the conduction electrons of the surface leads to the Kondo effect. Modifications of the Kondo effect were investigated using the unique capabilities of the STM, that is, fabrication of artificial clusters containing Cu and Co atoms, spectroscopy with high energy resolution and the controlled contact of the Kondo atom with the tip of the microscope.  Using magnetic electrodes magneto-resistive effects in atomic-scale junctions were investigated. Spin valve effects and anisotropic magneto-resistance were observed in these junctions revealing the crucial role of the electronic orbital symmetry in interpreting these phenomena.

 24 Nov 2016

Lecture Course: Selected Topics in Modern Condensed-Matter Theory

During the winter term 2016/2017, our group will host a lecture on selected topics in condensed-matter physics. Over the past few years, research in this field has witnessed several novel developments, which are revolutionizing our understanding of many-body systems. Among those developments are

• the simulation of many-body problems in ultracold atomic-gas systems;
• quantum phase transitions as a means for realizing exotic states of matter;
• topological aspects of Hilbert spaces.

The course will discuss these developments and provide some of the necessary theoretical techniques.

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