Theoretical Solid State Physics

Technical University Munich

James-Franck-Straße 1

85748 Garching

+49 89 289 53760


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We seek to understand how fascinating physical phenomena like superconductivity, magnetism, and topological order can emerge from simple interactions between many microscopic quantum particles.


Research focus: topological phases of matter, quantum many-body entanglement, solid state theory

Matter occurs in various phases with different properties. For example, certain solids become magnetic when cooled to sufficiently low temperatures, and others become superconductors, i.e., they conduct electrical current without dissipation. Both of these phases are examples of collective phenomena, which arise due to the interactions between many electrons in the solid. Our group is interested in systems where the interaction between the electrons yields new phases of matter.

One focus is the investigation of topological phases—which are prime examples of exotic phases with unusual properties. Intriguingly, such phases may be ideal building blocks of fault-tolerant quantum computers. Topological phases are very different from conventional phases of matter and have no classical analogue. They are characterized by their low-energy anyonic excitations, which are highly non-local objects that can sense each other through the medium in which they live even when they are very far apart. In our studies, we develop conceptual frameworks for classifying topological phases and we construct concrete models in order to realize them as a first step towards searching for these phases in nature.


We are also interested in understanding quantum many-body systems far out of equilibrium. Historically, the non-equilibrium evolution of quantum states was limited to extremely short time spans because of dissipation and decoherence effects. Recent experiments on ultra-cold gases in optical lattices as well as in nano structures allowed for the first time to overcome these limitations. At the same time, there has been a huge advance in the development of numerical and analytical techniques, which allow us to get a deeper understanding of the experiments. In particular, highly efficient numerical methods based on matrix-product states allow a direct comparison with theoretical predictions and experimental results.


Emergent Glassy Dynamics in a Quantum Dimer Model

J. Feldmeier, F. Pollmann, and M. Knap.

Physical Review Letters 123, 040601 (2019).

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We consider the quench dynamics of a two-dimensional quantum dimer model and determine the role of its kinematic constraints. We interpret the nonequilibrium dynamics in terms of the underlying equilibrium phase transitions consisting of a Berezinskii-Kosterlitz-Thouless (BKT) transition between a columnar ordered valence bond solid (VBS) and a valence bond liquid (VBL), as well as a first-order transition between a staggered VBS and the VBL. We find that quenches from a columnar VBS are ergodic and both order parameters and spatial correlations quickly relax to their thermal equilibrium. By contrast, the staggered side of the first-order transition does not display thermalization on numerically accessible timescales. Based on the model’s kinematic constraints, we uncover a mechanism of relaxation that rests on emergent, highly detuned multidefect processes in a staggered background, which gives rise to slow, glassy dynamics at low temperatures even in the thermodynamic limit.

DOI: 10.1103/PhysRevLett.123.040601

Avoided quasiparticle decay from strong quantum interactions

R. Verresen, R. Moessner und F. Pollmann.

Nature Physics 15, 750-753 (2019).

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Quantum states of matter—such as solids, magnets and topological phases—typically exhibit collective excitations (for example, phonons, magnons and anyons). These involve the motion of many particles in the system, yet, remarkably, act like a single emergent entity—a quasiparticle. Known to be long lived at the lowest energies, quasiparticles are expected to become unstable when encountering the inevitable continuum of many-particle excited states at high energies, where decay is kinematically allowed. Although this is correct for weak interactions, we show that strong interactions generically stabilize quasiparticles by pushing them out of the continuum. This general mechanism is straightforwardly illustrated in an exactly solvable model. Using state-of-the-art numerics, we find it at work in the spin-1/2 triangular-lattice Heisenberg antiferromagnet (TLHAF). This is surprising given the expectation of magnon decay in this paradigmatic frustrated magnet. Turning to existing experimental data, we identify the detailed phenomenology of avoided decay in the TLHAF material Ba3CoSb2O9, and even in liquid helium, one of the earliest instances of quasiparticle decay. Our work unifies various phenomena above the universal low-energy regime in a comprehensive description. This broadens our window of understanding of many-body excitations, and provides a new perspective for controlling and stabilizing quantum matter in the strongly interacting regime.

DOI: 10.1038/s41567-019-0535-3

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