Annalen der Physik

Landau quasiparticle (qp) theory may be extended into the non-Fermi liquid regime of metallic compounds near a quantum critical point (QCP), provided the width of the quasiparticle states is less than their energy. Sufficiently far from the QCP, when the critical fluctuations are non-interacting, their influence on the qp properties may be calculated in a controlled way.

The metal-insulator (MI) transition in Si:P can be tuned by varying the P concentration or – for barely insulating samples – by application of uniaxial stress S. On-site Coulomb interactions lead to the formation of localized magnetic moments and the Kondo effect on the metallic side, and to a Hubbard splitting of the donor band on the insulating side.

FeSb₂ has been recently identified as a new model system for studying many-body renormalizations in a d-electron based narrow gap semiconducting system, strongly resembling FeSi. The electron-electron correlations in FeSb₂ manifest themselves in a wide variety of physical properties including electrical and thermal transport, optical conductivity, magnetic susceptibility, specific heat and so on. The authors review some of the properties that form a set of experimental evidences revealing a significant role of correlation effects in FeSb₂.

The authors analyze the scaling behavior at and near a quantum critical point separating a semimetallic from a superfluid phase. To this end they compute the renormalization group flow for a model of attractively interacting electrons with a linear dispersion around a single Dirac point. They study both ground state and finite temperature properties.

Underdoped Mott insulators provide us with a challenge of many-body physics. Recent renewed understanding is discussed in terms of the evolution of pole and zero structure of the single-particle Green's function. Pseudogap as well as Fermi arc/pocket structure in the underdoped cuprates is well reproduced from the recent cluster extension of the dynamical mean-field theory. Emergent coexisting zeros and poles set the underdoped Mott insulator apart from the Fermi liquid, separated by topological transitions. The cofermion proposed as a generalization of exciton in the slave-boson framework accounts for the origin of the zero surface formation. The cofermion-quasiparticle hybridization gap offers a natural understanding of the pseudogap and various unusual Mottness. Furthermore the cofermion offers a novel pairing mechanism, where the cofermion has two roles: It reinforces the Cooper pair as a pair partner of the quasiparticle and acts as a glue as well. It provides a strong insight for solving the puzzle found in the dichotomy of the gap structure.

Using general arguments of an optimization taking place between the pair wave function and the repulsive part of the electron-electron interaction, the authors analyze the superconducting gap in materials with multiple Fermi-surface (FS) pockets, with exemplary application to two proto-type ferropnictide setups. The main point is to show that the SC state, its gap, and, in particular, its anisotropy in momentum space is determined by an optimization which determines and optimizes the interplay between the attractive interaction in the SC-channel and the Coulomb repulsion.

The authors report on polarization dependent reflectivity measurements in KCuF₃ in the far-infrared frequency regime. The observed IR active phonons at room temperature are in agreement with the expected modes for tetragonal symmetry. A splitting of one mode already at 150 K and the appearance of a new mode in the vicinity of the Néel temperature is observed.

Interrelations between global and local structure and magnetism and transport in three-dimensional perovskite manganites is reviewed and compared with recent studies on thin films and superlattices. The concept of correlated Jahn-Teller (JT) polarons is discussed within the phase separation scenario; their role in the local and global structural modifications of manganites is demonstrated. fig.

Recent developments on studies of transport through quantum dots obtained by applying the time-dependent density matrix renormalization group method are summarized. Some new aspects of Kondo physics which appear in nonequilibrium steady states are discussed both for the single dot case and for the serially coupled double-quantum-dot case.

When spatial correlations are short-range, the physics of strongly correlated systems is controlled by local quantum fluctuations. In those regimes, Dynamical Mean-Field Theory can be viewed as a `compass' which provides guidance on the relevant degrees of freedom and their effective dynamics over intermediate energy scales. These intermediate energy scales and associated crossovers play a crucial role in the physics of strongly correlated materials.

The authors review their investigations of electronic properties of strongly correlated materials using the combination of first principles electronic band structures and the dynamical mean-field theory, so called LDA+DMFT method. Their investigations focus on two phenomena, the spin state transitions and their relationship to the metal-insulator transition, and the effect of hybridization between correlated and ligand orbitals in charge-transfer type materials.

The anisotropic two-orbital Hubbard model with different bandwidths and degrees of frustration in each orbital is investigated in the framework of both single-site dynamical mean-field theory (DMFT) as well as its cluster extension (DCA) for clusters up to four sites combined with a continuous-time quantum Monte Carlo algorithm. This model shows a rich phase diagram which includes the appearance of orbital selective phase transitions, non-Fermi liquid behavior as well as antiferromagnetic metallic states.

The authors propose an approach for the ab initio calculation of materials with strong electronic correlations which is based on all local (fully irreducible) vertex corrections beyond the bare Coulomb interaction. It includes the so-called GW and dynamical mean field theory and important non-local correlations beyond, with a computational effort estimated to be still manageable.

In this article the authors formulate the superperturbation theory for the Anderson impurity model on the real axis. The resulting impurity solver allows to evaluate dynamical quantities without numerical analytical continuation by the maximum entropy method or Padé approximants.

The effects of quantum coherence on two-particle vertex functions due to impurity scattering of electrons are studied. The authors use the diagrammatic perturbation expansion in the impurity potential and analyze possible ways how to introduce renormalizations on the two-particle level.

Variational wave functions are very useful for describing the panoply of ground states found in interacting many-electron systems. Some particular trial states are “adiabatically” linked to a reference state, from which they borrow the essential properties. A prominent example is the Gutzwiller ansatz, where one starts with the filled Fermi sea. To describe symmetry breaking, the reference state has to be modified accordingly.

The simple Gutzwiller approximation, applied to the approach to Mott localization in a half-filled band Hubbard model, gives a remarkably good description of the properties of liquid ³He as the liquid-solid transition is approached under pressure. Here the Gutzwiller approximation is extended to incorporate correlations between nearest neighbors.

A technique based on positive semidefinite operator properties has been used in deducing exact ground states for hexagon chains of polyphenylene type (hexagons interconnected by bonds build up the 1D periodic structure), placed in a constant external magnetic field perpendicular to the surface containing the system.

In the Fractional Quantum Hall Effect (FQHE), in the noninteracting limit, only a fraction ν of the Lowest Landau Level (LLL) is occupied, producing a huge degeneracy. Interactions lift this degeneracy and mix in higher LL's. In the limit in which we ignore all but the LLL, the kinetic energy is an irrelevant constant and the ratio of potential to kinetic energy is essentially infinite, making this the most strongly correlated problem imaginable. The Hamiltonian theory of the FQHE deals with this problem with some success.