Electron-Phonon Driven Spin Frustration in Multi-Band Hubbard Models: MX Chains and Oxide Superconductors

We discuss the consequences of both electron-phonon and electron-electron couplings in 1D and 2D multi-band (Peierls-Hubbard) models. After briefly discussing various analytic limits, we focus on (Hartree-Fock and exact) numerical studies in the intermediate regime for both couplings, where unusual spin-Peierls as well as long-period, frustrated ground states are found. Doping into such phases or near the phase boundaries can lead to further interesting phenomena such as separation of spin and charge, a dopant-induced phase transition of the global (parent) phase, or real-space (``bipolaronic'') pairing. We discuss possible experimentally observable consequences of this rich phase diagram for halogen-bridged, transition metal, linear chain complexes (MX chains) in 1D and the oxide superconductors in 2D.Comment: 6 pages, four postscript figures (appended), in regular Te

Photoinduced charge separation in Q1D heterojunction materials: Evidence for electron-hole pair separation in mixed-halide MXMX solids

Resonance Raman experiments on doped and photoexcited single crystals of mixed-halide $MX$ complexes ($M$=Pt; $X$=Cl,Br) clearly indicate charge separation: electron polarons preferentially locate on PtBr segments while hole polarons are trapped within PtCl segments. This polaron selectivity, potentially very useful for device applications, is demonstrated theoretically using a discrete, 3/4-filled, two-band, tight-binding, extended Peierls-Hubbard model. Strong hybridization of the PtCl and PtBr electronic bands is the driving force for separation.Comment: n LaTeX, figures available by mail from JTG ([email protected]

Superconductivity and spin-glass like behavior in system with Pd sheet sandwiched between graphene sheets

Pd-metal graphite (Pd-MG) has a layered structure, where each Pd sheet is sandwiched between adjacent graphene sheets. DC magnetization and AC magnetic susceptibility of Pd-MG have been measured using a SQUID magnetometer. Pd-MG undergoes a superconducting transition at $T_{c}$ ($= 3.63 \pm 0.04$ K). The superconductivity occurs in Pd sheets. The relaxation of $M_{ZFC}$ (aging), which is common to spin glass systems, is also observed below $T_{c}$. The relaxation rate $S(t)$ shows a peak at a characteristic time $t_{cr}$, which is longer than a wait time $t_{w}$. The irreversibility between $\chi_{ZFC}$ and $\chi_{FC}$ occurs well above $T_{c}$. The susceptibility $\chi_{FC}$ obeys a Curie-Weiss behavior with a negative Curie-Weiss temperature ($-13.1 \leq \Theta \leq -5.4$ K). The growth of antiferromagnetic order is limited by the disordered nature of nanographites, forming spin glass-like behavior at low temperatures in graphene sheets.Comment: 21 pages, 15 figures; submitted to J. Phys.: Condensed Matte

Effects of the Lattice Discreteness on a Soliton in the Su-Schrieffer-Heeger Model

In this paper we analytically study the effects of the lattice discreteness on the electron band in the SSH model. We propose a modified version of the TLM model which is derived from the SSH model using a continuum approximation. When a soliton is induced in the electron-lattice system, the electron scattering states both at the bottom of the valence band and the top of the conduction band are attracted to the soliton. This attractive force induces weakly localized electronic states at the band edges. Using the modified version of the TLM model, we have succeeded in obtaining analytical solutions of the weakly localized states and the extended states near the bottom of the valence band and the top of the conduction band. This band structure does not modify the order parameters. Our result coincides well with numerical simulation works.Comment: to be appear in J.Phys.Soc.Jpn. Figures should be requested to the author. They will be sent by the conventional airmai

Fractional Statistics in Three Dimensions: Compact Maxwell-Higgs System

We show that a (3+1)-dimensional system composed of an open magnetic vortex and an electrical point charge exhibits the phenomenon of Fermi-Bose transmutation. In order to provide the physical realization of this system we focus on the lattice compact scalar electrodynamics $SQED_c$ whose topological excitations are open Nielsen-Olesen strings with magnetic monopoles attached at their ends.Comment: 8 page

Floating of Extended States and Localization Transition in a Weak Magnetic Field

We report results of a numerical study of non-interacting electrons moving in a random potential in two dimensions in the presence of a weak perpendicular magnetic field. We study the topological properties of the electronic eigenstates within a tight binding model. We find that in the weak magnetic field or strong randomness limit, extended states float up in energy. Further, the localization length is found to diverge at the insulator phase boundary with the same exponent $\nu$ as that of the isolated lowest Landau band (high magnetic field limit).Comment: RevTex, 4 pages, 3 figures available upon reques

Pressure Tuning of the Charge Density Wave in the Halogen-Bridged Transition-Metal (MX) Solid Pt2Br6(NH3)4Pt_2Br_6(NH_3)_4

We report the pressure dependence up to 95 kbar of Raman active stretching modes in the quasi-one-dimensional MX chain solid $Pt_2Br_6(NH_3)_4$. The data indicate that a predicted pressure-induced insulator-to-metal transition does not occur, but are consistent with the solid undergoing either a three-dimensional structural distortion, or a transition from a charge-density wave to another broken-symmetry ground state. We show that such a transition cacan be well-modeled within a Peierls-Hubbard Hamiltonian. 1993 PACS: 71.30.+h, 71.45.Lr, 75.30.Fv, 78.30.-j, 81.40.VwComment: 4 pages, ReVTeX 3.0, figures available from the authors on request (Gary Kanner, [email protected]), to be published in Phys Rev B Rapid Commun, REVISION: minor typos corrected, LA-UR-94-246

Multifractality of the quantum Hall wave functions in higher Landau levels

To probe the universality class of the quantum Hall system at the metal-insulator critical point, the multifractality of the wave function $\psi$ is studied for higher Landau levels, $N=1,2$, for various range $(\sigma )$ of random potential. We have found that, while the multifractal spectrum $f(\alpha)$ (and consequently the fractal dimension) does vary with $N$, the parabolic form for $f(\alpha)$ indicative of a log-normal distribution of $\psi$ persists in higher Landau levels. If we relate the multifractality with the scaling of localization via the conformal theory, an asymptotic recovery of the single-parameter scaling with increasing $\sigma$ is seen, in agreement with Huckestein's irrelevant scaling field argument.Comment: 10 pages, revtex, 5 figures available on request from [email protected]

Simulation of the Zero Temperature Behavior of a 3-Dimensional Elastic Medium

We have performed numerical simulation of a 3-dimensional elastic medium, with scalar displacements, subject to quenched disorder. We applied an efficient combinatorial optimization algorithm to generate exact ground states for an interface representation. Our results indicate that this Bragg glass is characterized by power law divergences in the structure factor $S(k)\sim A k^{-3}$. We have found numerically consistent values of the coefficient $A$ for two lattice discretizations of the medium, supporting universality for $A$ in the isotropic systems considered here. We also examine the response of the ground state to the change in boundary conditions that corresponds to introducing a single dislocation loop encircling the system. Our results indicate that the domain walls formed by this change are highly convoluted, with a fractal dimension $d_f=2.60(5)$. We also discuss the implications of the domain wall energetics for the stability of the Bragg glass phase. As in other disordered systems, perturbations of relative strength $\delta$ introduce a new length scale $L^* \sim \delta^{-1/\zeta}$ beyond which the perturbed ground state becomes uncorrelated with the reference (unperturbed) ground state. We have performed scaling analysis of the response of the ground state to the perturbations and obtain $\zeta = 0.385(40)$. This value is consistent with the scaling relation $\zeta=d_f/2- \theta$, where $\theta$ characterizes the scaling of the energy fluctuations of low energy excitations.Comment: 20 pages, 13 figure