Holger Fehske

I’m a theoretical physicist, with research interests mostly in quantum matter exhibiting  strong correlations and/or a nontrivial geometry/topology.  Thereby the focus is on (partly exotic) ground-state, spectral and dynamical properties of many-particle quantum systems, such as solids,  optical lattices, photonic crystals, spin chains,  quantum liquids and condensates, nanostructures, quantum dots, clusters, and bounded plasmas,  with potential applications in the rapidly developing fields of nano electronics, spintronics, plasmonics and photonics.

In particular, in our group we investigate

• the interplay of spin, charge, lattice and orbital degrees of freedom in strongly correlated materials materials (heavy fermions, high-temperature superconducting cuprates, colossal magnetoresistive manganites, charge-ordered nickelates,...)
• order, quantum phase transitions and criticality in  low-dimensional electron/spin-phonon coupled systems (Peierls & spin-Peierls instabilties, phase separation, intrinsic localized vibrational modes, Luttinger liquid behaviour, spin-charge-separation, metal-insulator & insulator-insulator transitions,...)
• transport properties in coupled fermion-boson systems
• symmetry-protected states in topological materials
• electronic and optical properties of graphene-based nanostructures
• disorder effects (Anderson localization, percolation, frustration, ,…)
• magnetism, magnetic short-range order, and spin glas behaviour
• polaron, bipolaron & exciton formation (self-trapping transition, optical response, polaronic superlattices, excitonic insulators, Bose-Einstein condensation,...)
• cavity quantum electrodynamics (strong light-matter interaction, development of non-classical light and entanglement)
• optomechanics (route to chaos)
• equlibration and thermalisation
• the plasma surface interface. 

In all projects, we try to gain insight from a (rather) microscopic point of view. To this end, we use, develop and exploit analytical and numerical techniques of modern quantum statistical physics, such as

• functional integral (slave-field) methods,
• Green's-function projection approaches,
• unitary transformation schemes,
• exact Lanczos & Jacobi-Davidson diagonalizations,
• Chebyshev/kernel-polynomial expansion & maximum entropy methods,
• density matrix renormalisation group techniques,
• quantum state diffusion and Markov approaches.

Various present-day supercomputers are used  in the numerical evaluation, with modern programming concepts making allowance for scalability, numerical reliability, fault tolerance, holistic performance, and power engineering.

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