Doping Dependence of Polaron Hopping Energies in La(1-x)Ca(x)MnO(3) (0<= x<= 0.15)

Measurements of the low-frequency (f<= 100 kHz) permittivity at T<= 160 K and dc resistivity (T<= 430 K) are reported for La(1-x)Ca(x)MnO(3) (0<= x<= 0.15). Static dielectric constants are determined from the low-T limiting behavior of the permittivity. The estimated polarizability for bound holes ~ 10^{-22} cm^{-3} implies a radius comparable to the interatomic spacing, consistent with the small polaron picture established from prior transport studies near room temperature and above on nearby compositions. Relaxation peaks in the dielectric loss associated with charge-carrier hopping yield activation energies in good agreement with low-T hopping energies determined from variable-range hopping fits of the dc resistivity. The doping dependence of these energies suggests that the orthorhombic, canted antiferromagnetic ground state tends toward an insulator-metal transition that is not realized due to the formation of the ferromagnetic insulating state near Mn(4+) concentration ~ 0.13.Comment: PRB in press, 5 pages, 6 figure

Towards first-principles understanding of the metal-insulator transition in fluid alkali metals

By treating the electron-ion interaction as perturbation in the first-principles Hamiltonian, we have calculated the density response functions of a fluid alkali metal to find an interesting charge instability due to anomalous electronic density fluctuations occurring at some finite wave vector {\bi Q} in a dilute fluid phase above the liquid-gas critical point. Since |{\bi Q}| is smaller than the diameter of the Fermi surface, this instability necessarily impedes the electric conduction, implying its close relevance to the metal-insulator transition in fluid alkali metals.Comment: 11 pages, 5 figure

Epitaxial Growth of La1/3_{1/3}Sr2/3_{2/3}FeO3_3 thin films by laser ablation

We report on the synthesis of high quality La$_{1/3}$Sr$_{2/3}$FeO$_3$ (LSFO) thin films using the pulsed laser deposition technique on both SrTiO$_3$ (STO) and LaAlO$_3$ (LAO) substrates (100)-oriented. From X-Ray diffraction (XRD) studies, we find that the films have an out-of-plane lattice parameter around 0.3865nm, almost independent of the substrate (i.e. the nature of the strains). The transport properties reveal that, while LSFO films deposited on STO exhibit an anomaly in the resistivity vs temperature at 180K (corresponding to the charge-ordered transition and associated with a transition from a paramagnetic to an antiferromagnetic state), the films grown on LAO display a very small magnetoresistance behavior and present an hysteresis around 270K under the application of a 4T magnetic field. The changes in transport properties between both substrates are discussed and compared with the corresponding single crystals.Comment: 9 pages, 4 figure

Mott physics and first-order transition between two metals in the normal state phase diagram of the two-dimensional Hubbard model

For doped two-dimensional Mott insulators in their normal state, the challenge is to understand the evolution from a conventional metal at high doping to a strongly correlated metal near the Mott insulator at zero doping. To this end, we solve the cellular dynamical mean-field equations for the two-dimensional Hubbard model using a plaquette as the reference quantum impurity model and continuous-time quantum Monte Carlo method as impurity solver. The normal-state phase diagram as a function of interaction strength $U$, temperature $T$, and filling $n$ shows that, upon increasing $n$ towards the Mott insulator, there is a surface of first-order transition between two metals at nonzero doping. That surface ends at a finite temperature critical line originating at the half-filled Mott critical point. Associated with this transition, there is a maximum in scattering rate as well as thermodynamic signatures. These findings suggest a new scenario for the normal-state phase diagram of the high temperature superconductors. The criticality surmised in these systems can originate not from a T=0 quantum critical point, nor from the proximity of a long-range ordered phase, but from a low temperature transition between two types of metals at finite doping. The influence of Mott physics therefore extends well beyond half-filling.Comment: 27 pages, 16 figures, LaTeX, published versio

Orbital selective insulator-metal transition in V2O3 under external pressure

We present a detailed account of the physics of Vanadium sesquioxide (${\rm V_2O_3}$), a benchmark system for studying correlation induced metal-insulator transition(s). Based on a detailed perusal of a wide range of experimental data, we stress the importance of multi-orbital Coulomb interactions in concert with first-principles LDA bandstructure for a consistent understanding of the PI-PM MIT under pressure. Using LDA+DMFT, we show how the MIT is of the orbital selective type, driven by large changes in dynamical spectral weight in response to small changes in trigonal field splitting under pressure. Very good quantitative agreement with ($i$) the switch of orbital occupation and ($ii$) S=1 at each $V^{3+}$ site across the MIT, and ($iii$) carrier effective mass in the PM phase, is obtained. Finally, using the LDA+DMFT solution, we have estimated screening induced renormalisation of the local, multi-orbital Coulomb interactions. Computation of the one-particle spectral function using these screened values is shown to be in excellent quantitative agreement with very recent experimental (PES and XAS) results. These findings provide strong support for an orbital-selective Mott transition in paramagnetic ${\rm V_2O_3}$.Comment: 12 pages, 7 figure

Metal-insulator transitions in tetrahedral semiconductors under lattice change

Although most insulators are expected to undergo insulator to metal transition on lattice compression, tetrahedral semiconductors Si, GaAs and InSb can become metallic on compression as well as by expansion. We focus on the transition by expansion which is rather peculiar; in all cases the direct gap at $\Gamma$ point closes on expansion and thereafter a zero-gap state persists over a wide range of lattice constant. The solids become metallic at an expansion of 13 % to 15 % when an electron fermi surface around L-point and a hole fermi surface at $\Gamma$-point develop. We provide an understanding of this behavior in terms of arguments based on symmetry and simple tight-binding considerations. We also report results on the critical behavior of conductivity in the metal phase and the static dielectric constant in the insulating phase and find common behaviour. We consider the possibility of excitonic phases and distortions which might intervene between insulating and metallic phases.Comment: 12 pages, 8 figure

Theory for Gossamer and Resonating Valence Bond Superconductivity

We use an effective Hamiltonian for two-dimensional Hubbard model including an antiferromagnetic spin-spin coupling term to study recently proposed gossamer superconductivity. We formulate a renormalized mean field theory to approximately take into account the strong correlation effect in the partially projected Gutzwiller wavefucntions. At the half filled, there is a first order phase transition to separate a Mott insulator at large Coulomb repulsion U from a gossamer superconductor at small U. Away from the half filled,the Mott insulator is evolved into an resonating valence bond state, which is adiabatically connected to the gossamer superconductor.Comment: 10 pages, 13 figure

Magnetic Anisotropy in Single Crystalline CeAu2_{2}In4_{4}

We have grown the single crystals of LaAu$_{2}$In$_{4}$ and CeAu$_{2}$In$_{4}$ by high temperature solution method and report on the anisotropic magnetic behavior of CeAu$_{2}$In$_{4}$ . The compounds crystallize in an orthorhombic structure with space group \textit {Pnma}. LaAu$_{2}$In$_{4}$ shows a Pauli-paramagnetic behavior. CeAu$_{2}$In$_{4}$ do not order down to 1.8 K. The easy axis of magnetization for CeAu$_{2}$In$_{4}$ is along [010] direction. The magnetization data is analyzed on the basis of crystalline electric field (CEF) model.Comment: 7 figures 4 page

Analytical calculation of the Green's function and Drude weight for a correlated fermion-boson system

In classical Drude theory the conductivity is determined by the mass of the propagating particles and the mean free path between two scattering events. For a quantum particle this simple picture of diffusive transport loses relevance if strong correlations dominate the particle motion. We study a situation where the propagation of a fermionic particle is possible only through creation and annihilation of local bosonic excitations. This correlated quantum transport process is outside the Drude picture, since one cannot distinguish between free propagation and intermittent scattering. The characterization of transport is possible using the Drude weight obtained from the f-sum rule, although its interpretation in terms of free mass and mean free path breaks down. For the situation studied we calculate the Green's function and Drude weight using a Green's functions expansion technique, and discuss their physical meaning.Comment: final version, minor correction

Glassy behavior of electrons near metal-insulator transitions

The emergence of glassy behavior of electrons is investigated for systems close to the disorder and/or interaction-driven metal-insulator transitions. Our results indicate that Anderson localization effects strongly stabilize such glassy behavior, while Mott localization tends to suppress it. We predict the emergence of an intermediate metallic glassy phase separating the insulator from the normal metal. This effect is expected to be most pronounced for sufficiently disordered systems, in agreement with recent experimental observations.Comment: Final version as published in Physical Review Letter