Johannes Bauer

Density wave instabilities in a correlated two-dimensional metal

Department of Physics, Harvard University, Cambridge MA 02138(Dated: February 26, 2014)Motivated by recent experimental evidence of charge order in the pseudogap phase of cuprates,we perform a variational analysis of charge-neutral, spin-singlet ordering in metals on the squarelattice, using a wavefunction with double occupancy projected out. We examine ordering with andwithout time-reversal symmetry, with arbitrary wavevector and tunable form factor. Depending onparameters, we nd d-wave bond density wave ordering with wavevector either parallel to the latticegenerators or diagonally oriented, or a ground state which carries a time reversal-breaking patternof spontaneous currents.

Dynamical mean-field theory and numerical renormalization group study of superconductivity in the attractive Hubbard model

We present a study of the attractive Hubbard model based on the dynamical mean field theory (DMFT) combined with the numerical renormalization group (NRG). For this study the NRG method is extended to deal with self-consistent solutions of effective impurity models with superconducting symmetry breaking. We give details of this extension and validate our calculations with DMFT results with antiferromagnetic ordering. We also present results for static and integrated quantities for different filling factors in the crossover from weak (BCS) to strong coupling (BEC) superfluidity. We study the evolution of the single-particle spectra throughout the crossover regime. Although the DMFT does not include the interaction of the fermions with the Goldstone mode, we find strong deviations from the mean-field theory in the intermediate and strong coupling (BEC) regimes. In particular, we show that low-energy charge fluctuations induce a transfer of spectral weight from the Bogoliubov quasiparticles to a higher-energy incoherent hump.

From Infinite to Two Dimensions through the Functional Renormalization Group

We present a novel scheme for an unbiased, nonperturbative treatment of strongly correlated fermions. The proposed approach combines two of the most successful many-body methods, the dynamical mean field theory and the functional renormalization group. Physically, this allows for a systematic inclusion of nonlocal correlations via the functional renormalization group flow equations, after the local correlations are taken into account nonperturbatively by the dynamical mean field theory. To demonstrate the feasibility of the approach, we present numerical results for the two-dimensional Hubbard model at half filling.

Electronic structure of undoped and potassium-doped coronene investigated by electron energy-loss spectroscopy

We performed electron energy-loss spectroscopy studies in transmission in order to obtain insight into the electronic properties of potassium intercalated coronene, a recently discovered superconductor with a rather high transition temperature of about 15\,K. A comparison of the loss function of undoped and potassium intercalated coronene shows the appearance of several new peaks in the optical gap upon potassium addition. Furthermore, our core level excitation data clearly signal filling of the conduction bands with electrons.

Competition between antiferromagnetic and charge order in the Hubbard-Holstein model

We study the competition between an instantaneous local Coulomb repulsion and a boson mediated retarded attraction, as described by the Hubbard-Holstein model. Restricting to the case of half-filling, the ground-state phase diagram and the transitions from antiferromagnetically ordered states to charge ordered states are analyzed. The calculations are based on the model in large dimensions, so that dynamical mean-field theory can be applied, and the associated impurity problem is solved using the numerical renormalization group method. The transition is found to occur when electron-electron coupling strength

The theory of electron?phonon superconductivity: does retardation really lead to a small Coulomb pseudopotential?

U

Quantitative reliability study of the Migdal-Eliashberg theory for strong electron-phonon coupling in superconductors

and the induced interaction

Interplay between Kondo and Andreev-Josephson effects in a quantum dot coupled to one normal and two superconducting leads

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Microscopic resolution of the interplay of Kondo screening and superconducting pairing

due to electron-phonon coupling approximately coincide,

Retardation effects and the Coulomb pseudopotential in the theory of superconductivity

U\ensuremath{\simeq}\ensuremath{\lambda}