arXiv subjects

T. M. Rice

Publications and source records attributed to T. M. Rice.

At least 73 records · Page 4Linked to original sources

arXiv2001

A Bosonic Model of Hole Pairs

We numerically investigate a bosonic representation for hole pairs on a two-leg t-J ladder where hard core bosons on a chain represent the hole pairs on the ladder. The interaction between hole pairs is obtained by fitting the density profile obtained with the effective model to the one obtained with the \tj model, taking into account the inner structure of the hole pair given by the hole-hole correlation function. For these interactions we calculate the Luttinger liquid parameter, which takes the universal value $K_ρ=1$ as half filling is approached, for values of the rung exchange $J'$ between strong coupling and the isotropic case. The long distance behavior of the hole-hole correlation function is also investigated. Starting from large $J'$, the correlation length first increases as expected, but diminishes significantly as $J'$ is reduced and bound holes sit mainly on adjacent rungs. As the isotropic case is approached, the correlation length increases again. This effect is related to the different kind of bonds in the region between the two holes of a hole pair when they move apart.

cond-mat↗

arXiv2000

Localization in ruthenates: magnetic and electronic properties of Ca(2-x)Sr(x)RuO(4)

The electronic structures of the metallic and insulating phases of Ca(2-x)Sr(x)RuO(4) (0<= x <= 2) are calculated using LDA, LDA+U and Dynamical Mean-Field Approximation methods. For x=0 the ground state is an orbitally non-degenerate antiferromagnetic insulator. For 0.2 0.5 the observed Curie-Weiss paramagnetic metallic state possessing a local moment with the unexpected spin S=1/2 is explained by the localization of only two of the three Ru-4d-orbitals.

cond-mat.str-el↗

arXiv2000

Multiexciton molecules in the hexaborides

We investigate multiexciton bound states in a semiconducting phase of divalent hexaborides. Due to three degenerate valleys in both the conduction and valence bands the binding energy of a 6-exciton molecule is greatly enhanced by the shell effect. The ground state energies of multiexciton molecules are calculated using the density functional formalism. We also show that charged impurities stabilize multiexciton complexes leading to condensation of localized excitons. These complexes can act as nucleation centers of local moments.

cond-mat.mtrl-sci↗

arXiv2000

Successive opening of the Fermi surface in doped N-leg Hubbard ladders

We study the effect of doping away from half-filling in weakly (but finitely) interacting N-leg Hubbard ladders using renormalization group and bosonization techniques. For a small on-site repulsion U, the N-leg Hubbard ladders are equivalent to a N-band model, where at half-filling the Fermi velocities are v_{1}=v_{N}<v_{2}=v_{N-1}<... We then obtain a hierarchy of energy-scales, where the band pairs (j,N+1-j) are successively frozen out. The low-energy Hamiltonian is then the sum of N/2 (or (N-1)/2 for N odd) two-leg ladder Hamiltonians without gapless excitations (plus a single chain for N odd with one gapless spin mode), similar to the N-leg Heisenberg spin-ladders. The energy-scales lead to a hierarchy of gaps. Upon doping away from half-filling, the holes enter first the band(s) with the smallest gap: For odd N, the holes enter first the nonbonding band (N+1)/2 and the phase is a Luttinger liquid, while for even N, the holes enter first the band pair (N/2,N/2+1) and the phase is a Luther-Emery liquid, similar to numerical treatments of the t-J model, i.e., at and close to half-filling, the phases of the Hubbard ladders for small and large U are the same. For increasing doping, hole-pairs subsequently enter at critical dopings the other band pairs (j,N+1-j) (accompanied by a diverging compressibility): The Fermi surface is successively opened by doping, starting near the wave vector (pi/2,pi/2). Explicit calculations are given for the cases N=3,4.

cond-mat.str-el↗

arXiv2000

Comment on "Electronic fine structure in the electron-hole plasma of SrB6"

In this comment we point out several misleading conclusions reached by Rodriguez et al. [PRL 84, 3903 (2000) and cond-mat/0003201] about the details of the band structure in SrB_6 and their role in interpreting the high-temperature weak ferromagnetism, which develops in this material upon doping.

cond-mat.str-el↗

arXiv2000

Orbitally Degenerate Spin-1 Model for Insulating V2O3

Motivated by recent neutron, X-ray absorption and resonant scattering experiments, we revisit the electronic structure of V2O3. We propose a model in which S=1 V3+ ions are coupled in the vertical V-V pairs forming two-fold orbitally degenerate configurations with S=2. Ferro-orbital ordering of the V-V pairs gives a description which is consistent with all experiments in the antiferromagnetic insulating phase.

cond-mat.str-el↗

arXiv1999

Breakdown of the Landau-Fermi liquid in Two Dimensions due to Umklapp Scattering

We study the renormalization group flow of the interactions in the two-dimensional t-t' Hubbard model near half filling in a N-patch representation of the whole Fermi surface. Starting from weak to intermediate couplings the flows are to strong coupling with different character depending on the choice of parameters. In a large parameter region elastic Umklapp scatterings drive an instability which on parts of the Fermi surface exhibits the key signatures of an insulating spin liquid (ISL), as proposed by Furukawa et al., rather than a conventional symmetry-broken state. The ISL is characterized by both strong d-wave pairing and antiferromagnetic correlations, however it is insulating due to the vanishing local charge compressibility and a spin liquid because of the spin gap arising from the pairing correlations. We find that the ISL is a consequence of a Fermi surface close to the saddle points at the Brillouin zone boundaries which provides an intrinsic and mutually reinforcing coupling between pairing and Umklapp channels.

cond-mat.str-el↗

arXiv1999

Electron-hole liquid in the hexaborides

We investigate the energetics of the electron-hole liquid in stoichiometric divalent metal hexaborides. The ground state energy of an electron-hole plasma is calculated using RPA and Hubbard schemes and compared to the binding energy of a single exciton. Intervalley scattering processes play an important role in increasing this binding energy and stabilizing a dilute Bose gas of excitons.

cond-mat.str-el↗

arXiv1999

Excitonic ferromagnetism in the hexaborides

A ferromagnet with a small spontaneous moment but with a high Curie temperature can be obtained by doping an excitonic insulator made from a spin triplet exciton condensate. Such a condensate can occur in a semimetal with a small overlap or a semiconductor with a small bandgap. We propose that it is responsible for the unexpected ferromagnetism in the doped hexaboride material Ca_{1-x}La_xB_6.

cond-mat.str-el↗

arXiv1999

The electronic structure of the heavy fermion metal $LiV_2O_4$

The electronic structure of the first reported heavy fermion compound without f-electrons LiV_2O_4 was studied by an ab-initio calculation method. In the result of the trigonal splitting and d-d Coulomb interaction one electron of the $d^{1.5}$ configuration of V ion is localized and the rest partially fills a relatively broad conduction band. The effective Anderson impurity model was solved by Non-Crossing-Approximation method, leading to an estimation for the single-site Kondo energy scale T_K. Then, we show how the so-called exhaustion phenomenon of Nozières for the Kondo lattice leads to a remarkable decrease of the heavy-fermion (or coherence) energy scale $T_{coh}\equiv {T_K}^2/D$ (D is the typical bandwidth), comparable to the experimental result.

cond-mat.str-el↗

arXiv1999

Phase diagram of three-leg ladders at strong coupling along the rungs

A phase diagram of the t-J three-leg ladder as a function of hole dopping is derived in the limit where the coupling parameters along the rungs, $t_{\perp}$ and $J_{\perp}$, are taken to be much larger than those along the legs, $t_{||}$ and $J_{||}$ At large exchange coupling along the rungs, $J_{\perp}/t_{\perp}> 3 / \sqrt{2}$, there is a transition from a low-dopping Luttinger liquid phase into a Luther-Emery liquid at a critical hole concentration $n_{crit}\approx 1/3$. In the opposite case, $J_{\perp}/t_{\perp}< 3 / \sqrt{2}$, there as a sequence of three Luttinger liquid phases (LLI, LLII and LLIII) as a function of hole dopping.

cond-mat.str-el↗

arXiv1999

Phenomenology of the superconducting state in Sr2RuO4

The symmetry of the superconducting phase of Sr2RuO4 is identified as the odd-parity pairing state d(k)=\hat{z}(k_x \pm i k_y) based on recent experiments. The experimental evidence for the so-called orbital dependent superconductivity leads to a single-band description of superconductivity based on spin fluctuation mechanism. It is shown that the state \hat{z}(k_x \pm i k_y) can be stabilized by the spin fluctuation feedback mechanism analogous to the A-phase in 3He and by spin-orbit coupling effects.

cond-mat.supr-con↗

arXiv1998

On possible superconductivity in the doped ladder compound La_(1-x)Sr_xCuO_2.5

LaCuO_2.5 is a system of coupled, two-chain, cuprate ladders which may be doped systematically by Sr substitution. Motivated by the recent synthesis of single crystals, we investigate theoretically the possibility of superconductivity in this compound. We use a model of spin fluctuation-mediated superconductivity, where the pairing potential is strongly peaked at πin the ladder direction. We solve the coupled gap equations on the bonding and antibonding ladder bands to find superconducting solutions across the range of doping, and discuss their relevance to the real material.

cond-mat.str-el↗

arXiv1998

Thermodynamics of the t-J Ladder: A Stable Finite Temperature Density Matrix Renormalization Group Calculation

Accurate numerical simulations of a doped t-J model on a two-leg ladder are presented for the particle number, chemical potential, magnetic susceptibility and entropy in the limit of large exchange coupling on the rung using a finite temperature density matrix renormalization group (TDMRG) method. This required an improved algorithm to achieve numerical stability down to low temperatures. The thermal dissociation of hole pairs and of the rung singlets are separately observed and the evolution of the hole pair binding energy and magnon spin gap with hole doping is determined.

cond-mat↗

arXiv1998

Truncation of a 2-dimensional Fermi surface due to quasiparticle gap formation at the saddle points

We study a two-dimensional Fermi liquid with a Fermi surface containing the saddle points $(π,0)$ and $(0,π)$. Including Cooper and Peierls channel contributions leads to a one-loop renormalization group flow to strong coupling for short range repulsive interactions. In a certain parameter range the characteristics of the fixed point, opening of a spin and charge gap and dominant pairing correlations are similar to those of a 2-leg ladder at half-filling. An increase of the electron density we argue leads to a truncation of the Fermi surface with only 4 disconnected arcs remaining.

cond-mat.str-el↗

arXiv1998

Theory for Metal Hydrides with Switchable Optical Properties

Recently it has been discovered that lanthanum, yttrium, and other metal hydride films show dramatic changes in the optical properties at the metal-insulator transition. Such changes on a high energy scale suggest the electronic structure is best described by a local model based on negatively charged hydrogen (H$^-$) ions. We develop a many-body theory for the strong correlation in a H$^-$ ion lattice. The metal hydride is described by a large $U$-limit of an Anderson lattice model. We use lanthanum hydride as a prototype of these compounds, and find LaH$_3$ is an insulator with a substantial gap consistent with experiments. It may be viewed either as a Kondo insulator or a band insulator due to strong electron correlation. A H vacancy state in LaH$_3$ is found to be highly localized due to the strong bonding between the electron orbitals of hydrogen and metal atoms. Unlike the impurity states in the usual semiconductors, there is only weak internal optical transitions within the vacancy. The metal-insulator transition takes place in a band of these vacancy states.

cond-mat.str-el↗

arXiv1998

Instability of a Landau Fermi liquid as the Mott insulator is approached

We examine a two-dimensional Fermi liquid with a Fermi surface which touches the Umklapp surface first at the 4 points $(\pm π/2, \pm π/2)$ as the electron density is increased. Umklapp processes at the 4 patches near $(\pm π/2, \pmπ/2)$ lead the renormalization group equations to scale to strong coupling resembling the behavior of a 2-leg ladder at half-filling. The incompressible character of the fixed point causes a breakdown of Landau theory at these patches. A further increase in density spreads the incompressible regions so that the open Fermi surface shrinks to 4 disconnected segments. This non-Landau state, in which parts of the Fermi surface are truncated to form an insulating spin liquid, has many features in common with phenomenological models recently proposed for the cuprate superconductors.

cond-mat.str-el↗

arXiv1998

Generalization of the Luttinger Theorem for Fermionic Ladder Systems

We apply a generalized version of the Lieb-Schultz-Mattis Theorem to fermionic ladder systems to show the existence of a low-lying excited state (except for some special fillings). This can be regarded as a non-perturbative proof for the conservation under interaction of the sum of the Fermi wave vectors of the individual channels, corresponding to a generalized version of the Luttinger Theorem to fermionic ladder systems. We conclude by noticing that the Lieb-Schultz-Mattis Theorem is not applicable in this form to show the existence of low-lying excitations in the limit that the number of legs goes to infinity, e.g. in the limit of a 2D plane.

cond-mat.str-el↗