Université de Fribourg

Gauge theory of topological phases of matter

Fröhlich, Jürg ; Werner, Philipp

In: EPL (Europhysics Letters), 2013, vol. 101, no. 4, p. 47007

We study the response of quantum many-body systems to coupling some of their degrees of freedom to external gauge fields. This serves to understand the current Green functions and transport properties of interacting many-body systems. Our analysis leads to a "gauge theory of states of matter" complementary to the well-known Landau theory of order parameters. We illustrate the power of our...

Université de Fribourg

Extracting spectral properties from Keldysh Green functions

Dirks, Andreas ; Eckstein, Martin ; Pruschke, Thomas ; Werner, Philipp

In: Physical Review E, 2013, vol. 87, no. 2, p. 023305

We investigate the possibility to assist the numerically ill-posed calculation of spectral properties of interacting quantum systems in thermal equilibrium by extending the imaginary-time simulation to a finite Schwinger-Keldysh contour. The effect of this extension is tested within the standard maximum entropy approach to analytic continuation. We find that the inclusion of real-time data...

Université de Fribourg

Spectral Properties of Correlated Materials: Local Vertex and Nonlocal Two-Particle Correlations from Combined GW and Dynamical Mean Field Theory

Ayral, Thomas ; Werner, Philipp ; Biermann, Silke

In: Physical Review Letters, 2012, vol. 109, no. 22, p. 226401

We present a fully self-consistent combined GW and dynamical mean field (DMFT) study of the extended two-dimensional Hubbard model. The inclusion of the local dynamical vertex stemming from the DMFT self-energy and polarization is shown to cure the known problems of self-consistent GW. We calculate momentum-resolved spectral functions, two-particle polarizations, and electron-loss spectra, as...

Université de Fribourg

Efficient implementation of the continuous-time hybridization expansion quantum impurity solver

Hafermann, Hartmut ; Werner, Philipp ; Gull, Emanuel

In: Computer Physics Communications, 2013, vol. 184, no. 4, p. 1280–1286

Strongly correlated quantum impurity problems appear in a wide variety of contexts ranging from nanoscience and surface physics to material science and the theory of strongly correlated lattice models, where they appear as auxiliary systems within dynamical mean-field theory. Accurate and unbiased solutions must usually be obtained numerically, and continuous-time quantum Monte Carlo algorithms,...

Université de Fribourg

Nonthermal symmetry-broken states in the strongly interacting Hubbard model

Werner, Philipp ; Tsuji, Naoto ; Eckstein, Martin

In: Physical Review B - Condensed Matter and Materials Physics, 2012, vol. 86, no. 20, p. 205101

We study the time evolution of the antiferromagnetic order parameter after interaction quenches in the Hubbard model. Using the nonequilibrium dynamical mean-field formalism, we show that the system, after a quench from intermediate to strong interaction, is trapped in a nonthermal state which is reminiscent of a photodoped state and protected by the slow decay of doublons. If the effective...

Université de Fribourg

From the Cooper Problem to Canted Supersolids in Bose-Fermi Mixtures

Anders, Peter ; Werner, Philipp ; Troyer, Matthias ; Sigrist, Manfred ; Pollet, Lode

In: Physical Review Letters, 2012, vol. 109, no. 20, p. 206401

We calculate the phase diagram of the Bose-Fermi Hubbard model on the 3d cubic lattice at fermionic half filling and bosonic unit filling by means of single-site dynamical mean-field theory. For fast bosons, this is equivalent to the Cooper problem in which the bosons can induce s-wave pairing between the fermions. We also find miscible superfluid and canted supersolid phases depending on the...

Université de Fribourg

Rubidium superoxide: A p-electron Mott insulator

Kováčik, Roman ; Werner, Philipp ; Dymkowski, Krzysztof ; Ederer, Claude

In: Physical Review B - Condensed Matter and Materials Physics, 2012, vol. 86, no. 7, p. 075130

Rubidium superoxide, RbO₂, is a rare example of a solid with partially filled electronic p states, which allows us to study the interplay of spin and orbital order and other effects of strong electronic correlations in a material that is quite different from the conventional d or f electron systems. Here we show, using a combination of density functional theory (DFT) and dynamical mean-field...

Université de Fribourg

Relaxation dynamics of the Kondo lattice model

Werner, Philipp ; Eckstein, Martin

In: Physical Review B - Condensed Matter and Materials Physics, 2012, vol. 86, no. 04, p. 045119

We study the relaxation properties of the Kondo lattice model using the nonequilibrium dynamical mean-field formalism in combination with the noncrossing approximation. The system is driven out of equilibrium either by a magnetic field pulse, which perturbs the local singlets, or by a sudden quench of the Kondo coupling. For relaxation processes close to thermal equilibrium (after a weak...

Université de Fribourg

Magnetic susceptibility of cerium: An LDA+DMFT study

Streltsov, S. V. ; Gull, E. ; Shorikov, A. O. ; Troyer, M. ; Anisimov, V. I. ; Werner, Philipp

In: Physical Review B - Condensed Matter and Materials Physics, 2012, vol. 85, no. 19, p. 195109

The magnetic properties of Ce in the α and γ phase are calculated within the local-density approximation and dynamical mean-field theory (LDA+DMFT) approach. The magnetic susceptibility in these two phases shows a similar behavior over a wide temperature range: a Curie-Weiss law at high temperatures, indicating the presence of local moments, followed by a maximum in a crossover regime, and a...