Université de Fribourg

Extended mean field study of complex φ4-theory at finite density and temperature

Akerlund, Oscar ; Forcrand, Philippe de ; Georges, Antoine ; Werner, Philipp

In: Physical Review D, 2014, vol. 90, no. 6, p. 065008

We review the extended mean field theory (EMFT) approximation and apply it to complex, scalar φ4 theory on the lattice. We study the critical properties of the Bose condensation driven by a nonzero chemical potential μ at both zero and nonzero temperature and determine the (T,μ) phase diagram. The results are in very good agreement with recent Monte Carlo data for all parameter values...

Université de Fribourg

Dynamical mean field approximation applied to quantum field theory

Akerlund, Oscar ; Forcrand, Philippe de ; Georges, Antoine ; Werner, Philipp

In: Physical Review D, 2013, vol. 88, no. 12, p. 125006

We apply the dynamical mean field theory (DMFT) approximation to the real, scalar φ4 quantum field theory. By comparing to lattice Monte Carlo calculations, perturbation theory and standard mean field theory, we test the quality of the approximation in two, three, four and five dimensions. The quantities considered in these tests are the critical coupling for the transition to the ordered phase...

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

Nonequilibrium dynamical mean-field theory and its applications

Aoki, Hideo ; Tsuji, Naoto ; Eckstein, Martin ; Kollar, Marcus ; Oka, Takashi ; Werner, Philipp

In: Reviews of Modern Physics, 2014, vol. 86, no. 2, p. 779–837

The study of nonequilibrium phenomena in correlated lattice systems has developed into one of the most active and exciting branches of condensed matter physics. This research field provides rich new insights that could not be obtained from the study of equilibrium situations, and the theoretical understanding of the physics often requires the development of new concepts and methods. On the...

Université de Fribourg

Screening and nonlocal correlations in the extended Hubbard model from self-consistent combined GW and dynamical mean field theory

Ayral, Thomas ; Biermann, Silke ; Werner, Philipp

In: Physical Review B, 2013, vol. 87, no. 12, p. 125149

We describe a recent implementation of the combined GW and dynamical mean field method (GW+DMFT) for the two-dimensional Hubbard model with onsite and nearest-neighbor repulsion. We clarify the relation of the GW+DMFT scheme to alternative approaches in the literature, and discuss the corresponding approximations to the free-energy functional of the model. Furthermore, we describe a numerically...

Université de Fribourg

Influence of Fock exchange in combined many-body perturbation and dynamical mean field theory

Ayral, Thomas ; Biermann, Silke ; Werner, Philipp ; Boehnke, Lewin

In: Physical Review B, 2017, vol. 95, no. 24, p. 245130

In electronic systems with long-range Coulomb interaction, the nonlocal Fock- exchange term has a band-widening effect. While this effect is included in combined many-body perturbation theory and dynamical mean field theory (DMFT) schemes, it is not taken into account in standard extended DMFT (EDMFT) calculations. Here, we include this instantaneous term in both approaches and investigate its...

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

Nonthermal Melting of Néel Order in the Hubbard Model

Balzer, Karsten ; Wolf, F. Alexander ; McCulloch, Ian P. ; Werner, Philipp ; Eckstein, Martin

In: Physical Review X, 2015, vol. 5, no. 3, p. 031039

Symmetry-broken states such as magnetic order and superconductivity are the hallmark of complex properties in solids. An intriguing new direction in condensed-matter physics is to manipulate these properties on the fastest possible time scales using ultrashort laser pulses. Theoretically, however, the collective many-particle dynamics that is responsible for the formation and melting of...

Université de Fribourg

Photoenhanced excitonic correlations in a Mott insulator with nonlocal interactions

Bittner, Nikolaj ; Golež, Denis ; Eckstein, Martin ; Werner, Philipp

In: Physical Review B, 2020, vol. 101, no. 8, p. 085127

We investigate the effect of nonlocal interactions on the photodoped Mott insulating state of the two-dimensional Hubbard model using a nonequilibrium generalization of the dynamical cluster approximation. In particular, we compare the situation where the excitonic states are lying within the continuum of doublon-holon excitations to a setup where the excitons appear within the Mott gap. In...

Université de Fribourg

When strong correlations become weak: Consistent merging of $GW$ and DMFT

Boehnke, Lewin ; Nilsson, Fredrik ; Aryasetiawan, Ferdi ; Werner, Philipp

In: Physical Review B, 2016, vol. 94, no. 20, p. 201106

The cubic perovskite SrVO3 is generally considered to be a prototype strongly correlated metal with a characteristic three-peak structure of the d-electron spectral function, featuring a renormalized quasiparticle band in between pronounced Hubbard sidebands. Here we show that this interpretation, which has been supported by numerous “ab initio” simulations, has to be reconsidered. Using...