Frustration effects at finite temperature in the half filled Hubbard model.

We investigate the finite temperature properties of the half filled Hubbard model in two dimensions, with onsite interaction (U), in presence of (frustrating) next nearest neighbor hopping (t') using a semiclassical approximation scheme. We show that introduction of t' results in a finite temperature pseudogapped (PG) phase that separates the small U Fermi liquid and large U Mott insulator. We map out the PG to normal metal crossover temperature scale (T*) as a function of U and t'. We demonstrate that in the PG phase, the quadratic dependence of resistivity on temperature is violated due to thermally induced spin fluctuations. We conclude with exact diagonalization calculations, that complement our finite temperature results, and indicate the presence of a frustration driven PG state between the Fermi liquid and the Mott insulator at zero temperature as well.

Hybridization-Switching Induced Mott Transition in ABO_{3} Perovskites.

We propose the concept of a "hybridization-switching induced Mott transition" which is relevant to a broad class of ABO_{3} perovskite materials including BiNiO_{3} and PbCrO_{3} that feature extended 6s orbitals on the A-site cation (Bi or Pb), and a strong A-O covalency induced ligand hole. Using ab initio electronic structure and slave rotor theory calculations, we show that such systems exhibit a breathing phonon driven A-site to oxygen hybridization-wave instability which conspires with strong correlations on the B-site transition metal ion (Ni or Cr) to trigger a Mott insulating state. This class of systems is shown to undergo a pressure induced insulator to metal transition accompanied by a colossal volume collapse due to ligand hybridization switching.

Non-Fermi Liquid Behavior and Continuously Tunable Resistivity Exponents in the Anderson-Hubbard Model at Finite Temperature.

We employ a recently developed computational many-body technique to study for the first time the half-filled Anderson-Hubbard model at finite temperature and arbitrary correlation U and disorder V strengths. Interestingly, the narrow zero temperature metallic range induced by disorder from the Mott insulator expands with increasing temperature in a manner resembling a quantum critical point. Our study of the resistivity temperature scaling T^{α} for this metal reveals non-Fermi liquid characteristics. Moreover, a continuous dependence of α on U and V from linear to nearly quadratic is observed. We argue that these exotic results arise from a systematic change with U and V of the "effective" disorder, a combination of quenched disorder and intrinsic localized spins.

Parallelized traveling cluster approximation to study numerically spin-fermion models on large lattices.

Lattice spin-fermion models are important to study correlated systems where quantum dynamics allows for a separation between slow and fast degrees of freedom. The fast degrees of freedom are treated quantum mechanically while the slow variables, generically referred to as the "spins," are treated classically. At present, exact diagonalization coupled with classical Monte Carlo (ED + MC) is extensively used to solve numerically a general class of lattice spin-fermion problems. In this common setup, the classical variables (spins) are treated via the standard MC method while the fermion problem is solved by exact diagonalization. The "traveling cluster approximation" (TCA) is a real space variant of the ED + MC method that allows to solve spin-fermion problems on lattice sizes with up to 10(3) sites. In this publication, we present a novel reorganization of the TCA algorithm in a manner that can be efficiently parallelized. This allows us to solve generic spin-fermion models easily on 10(4) lattice sites and with some effort on 10(5) lattice sites, representing the record lattice sizes studied for this family of models.

Charge disproportionation without charge transfer in the rare-earth-element nickelates as a possible mechanism for the metal-insulator transition.

We study a model for the metal-insulator (M-I) transition in the rare-earth-element nickelates RNiO3, based upon a negative charge transfer energy and coupling to a rocksaltlike lattice distortion of the NiO6 octahedra. Using exact diagonalization and the Hartree-Fock approximation we demonstrate that electrons couple strongly to these distortions. For small distortions the system is metallic, with a ground state of predominantly d8L character, where L_ denotes a ligand hole. For sufficiently large distortions (δdNi-O∼0.05-0.10 Å), however, a gap opens at the Fermi energy as the system enters a periodically distorted state alternating along the three crystallographic axes, with (d8L_2)S=0(d8)S=1 character, where S is the total spin. Thus the M-I transition may be viewed as being driven by an internal volume "collapse" where the NiO6 octahedra with two ligand holes shrink around their central Ni, while the remaining octahedra expand accordingly, resulting in the (1/2, 1/2, 1/2) superstructure observed in x-ray diffraction in the insulating phase. This insulating state is an example of charge ordering achieved without any actual movement of the charge.

Theory of strain-controlled magnetotransport and stabilization of the ferromagnetic insulating phase in manganite thin films.

We show that applying strain on half-doped manganites makes it possible to tune the system to the proximity of a metal-insulator transition and thereby generate a colossal magnetoresistance (CMR) response. This phase competition not only allows control of CMR in ferromagnetic metallic manganites but can be used to generate CMR response in otherwise robust insulators at half-doping. Further, from our realistic microscopic model of strain and magnetotransport calculations within the Kubo formalism, we demonstrate a striking result of strain engineering that, under tensile strain, a ferromagnetic charge-ordered insulator, previously inaccessible to experiments, becomes stable.

Theory of half-metallic ferrimagnetism in double perovskites.

Double perovskites such as Sr(2)FeMoO(6) are rare examples of materials with half-metallic ground states and a ferrimagnetic T(c) above room temperature. We present a comprehensive theory of the temperature and disorder dependence of their magnetic properties by deriving and validating a new effective spin Hamiltonian for these materials, amenable to large-scale three-dimensional simulations. We show how disorder, ubiquitous in these materials, affects T(c), the magnetization, and the conduction electron polarization. We conclude with a novel proposal to enhance T(c) without sacrificing polarization.

Distinct effects of homogeneous weak disorder and dilute strong scatterers on phase competition in manganites.

We study the two orbital double-exchange model in two dimensions including antiferromagnetic (AFM) superexchange, Jahn-Teller coupling, and substitutional disorder. At hole doping x = 0.5 we focus on phase competition between the ferromagnetic metal (FMM) and the charge-ordered (CO) and orbital-ordered (OO) CE state and compare the impact of weak homogeneous disorder to that of a low density of strong scatterers. Even moderate homogeneous disorder suppresses the CE-CO-OO phase and leads to a glass with nanoscale correlations, while dilute strong scatterers of comparable strength convert the CE-CO-OO phase to a phase separated state with ferromagnetic metal and AFM-CO-OO clusters.