2,990 research outputs found
Unconventional metallic conduction in two-dimensional Hubbard-Wigner lattices
The interplay between long-range and local Coulomb repulsion in strongly
interacting electron systems is explored through a two-dimensional
Hubbard-Wigner model. An unconventional metallic state is found in which
collective low-energy excitations characteristic of the Wigner crystal induce a
flow of electrical current despite the absence of one-electron spectral weight
at the Fermi surface. Photoemission experiments on certain quarter-filled
layered molecular crystals should observe a gap in the excitation spectrum
whereas optical spectroscopy should find a finite Drude weight indicating
metallic behavior.Comment: 10 pages, accepted for publication in PR
Charge ordering in \theta-(BEDT-TTF)2RbZn(SCN)4: Cooperative effects of electron correlations and lattice distortions
Combined effects of electron correlations and lattice distortions are
investigated on the charge ordering in \theta-(BEDT-TTF)2RbZn(SCN)4
theoretically in a two-dimensional 3/4-filled extended Hubbard model with
electron-lattice couplings. It is known that this material undergoes a phase
transition from a high-symmetry metallic state to a low-symmetry insulating
state with a horizontal-stripe charge order (CO) by lowering temperature. By
means of the exact-diagonalization method, we show that electron-phonon
interactions are crucial to stabilize the horizontal-stripe CO and to realize
the low-symmetry crystal structure.Comment: 7 peges, 7 figures, accepted for publication in Phys. Rev.
Point-contact tunneling involving low-dimensional spin-triplet superconductors
We modify and extend previous microscopic calculations of tunneling in
superconducting junctions based on a non-equilibrium Green function formalism
to include the case of spin-triplet pairing. We show that distinctive features
are present in the I-V characteristics of different kinds of junctions, in
particular when the effects of magnetic fields are taken into account, that
permit to identify the type of pairing. We discuss the relevance of these
results in the context of quasi one-dimensional organic superconductors like
(TMTSF)_2(PF_6) and layered compounds like Sr_2RuO_4.Comment: 4 pages, 1 figur
Phase diagram of the one-dimensional half-filled extended Hubbard model
We study the ground state of the one-dimensional half-filled Hubbard model
with on-site (nearest-neighbor) repulsive interaction () and
nearest-neighbor hopping . In order to obtain an accurate phase diagram, we
consider various physical quantities such as the charge gap, spin gap,
Luttinger-liquid exponents, and bond-order-wave (BOW) order parameter using the
density-matrix renormalization group technique. We confirm that the BOW phase
appears in a substantial region between the charge-density-wave (CDW) and
spin-density-wave phases. Each phase boundary is determined by multiple means
and it allows us to do a cross-check to demonstrate the validity of our
estimations. Thus, our results agree quantitatively with the renormalization
group results in the weak-coupling regime (), with the
perturbation results in the strong-coupling regime (), and with
the quantum Monte Carlo results in the intermediate-coupling regime. We also
find that the BOW-CDW transition changes from continuous to first order at the
tricritical point and the BOW
phase vanishes at the critical end point .Comment: 4 pages, 5 figure
Quantum critical behavior of electrons at the edge of charge order
We consider quantum critical points (QCP) in which quantum fluctuations
associated with charge rather than magnetic order induce unconventional
metallic properties. Based on finite-T calculations on a two-dimensional
extended Hubbard model we show how the coherence scale T* characteristic of
Fermi liquid behavior of the homogeneous metal vanishes at the onset of charge
order. A strong effective mass enhancement reminiscent of heavy fermion
behavior indicates the possible destruction of quasiparticles at the QCP.
Experimental probes on quarter-filled layered organic materials are proposed
for unveiling the behavior of electrons across the quantum critical region.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Let
Density Induced Quantum Phase Transitions in Triplet Superconductors
We consider the possibility of quantum phase transitions in the ground state
of triplet superconductors where particle density is the tunning parameter. For
definiteness, we focus on the case of one band quasi-one-dimensional triplet
superconductors but many of our conclusions regarding the nature of the
transition are quite general. Within the functional integral formulation, we
calculate the electronic compressibility and superfluid density tensor as a
function of the particle density for various triplet order parameter symmetries
and find that these quantities are non-analytic when a critical value of the
particle density is reached.Comment: 4 pages, 3 figure
Electronic and magnetic properties of the ionic Hubbard model on the striped triangular lattice at 3/4 filling
We report a detailed study of a model Hamiltonian which exhibits a rich
interplay of geometrical spin frustration, strong electronic correlations, and
charge ordering. The character of the insulating phase depends on the magnitude
of Delta/|t| and on the sign of t. We find a Mott insulator for Delta >> U >>
|t|; a charge transfer insulator for U >> \Delta >> |t|; and a correlated
covalent insulator for U >> \Delta ~ |t|. The charge transfer insulating state
is investigated using a strong coupling expansion. The frustration of the
triangular lattice can lead to antiferromagnetism or ferromagnetism depending
on the sign of the hopping matrix element, t. We identify the "ring" exchange
process around a triangular plaquette which determines the sign of the magnetic
interactions. Exact diagonalization calculations are performed on the model for
a wide range of parameters and compared to the strong coupling expansion. The
regime U >> \Delta ~ |t| and t<0 is relevant to Na05CoO2. The calculated
optical conductivity and the spectral density are discussed in the light of
recent experiments on Na05CoO2.Comment: 15 pages, 15 figure
Phase diagram of the one dimensional Hubbard-Holstein Model at 1/2 and 1/4 filling
The Hubbard-Holstein model is one of the simplest to incorporate both
electron-electron and electron-phonon interactions. In one dimension at half
filling the Holstein electron-phonon coupling promotes onsite pairs of
electrons and a Peierls charge density wave while the Hubbard onsite Coulomb
repulsion U promotes antiferromagnetic correlations and a Mott insulating
state. Recent numerical studies have found a possible third intermediate phase
between Peierls and Mott states. From direct calculations of charge and spin
susceptibilities, we show that (i) As the electron-phonon coupling is
increased, first a spin gap opens, followed by the Peierls transition. Between
these two transitions the metallic intermediate phase has a spin gap, no charge
gap, and properties similar to the negative-U Hubbard model. (ii) The
transitions between Mott/intermediate and intermediate/Peierls states are of
the Kosterlitz-Thouless form. (iii) For larger U the two transitions merge at a
tritical point into a single first order Mott/Peierls transition. In addition
we show that an intermediate phase also occurs in the quarter-filled model.Comment: 10 pages, 10 eps figure
Magnetic Determination of under Accurate Alignment in (TMTSF)ClO
Cantilever magnetometry has been used to measure the upper critical magnetic
field of the quasi-one dimensional molecular organic superconductor
(TMTSF)ClO. From simultaneous resistivity and torque magnetization
experiments conducted under precise field alignment, at low
temperature is shown to reach 5T, nearly twice the Pauli paramagnetic limit
imposed on spin singlet superconductors. These results constitute the first
thermodynamic evidence for a large in this system and provide support
for spin triplet pairing in this unconventional superconductorComment: Submitted July 1, 2003, Accepted December 9, 2003, Physical Review
Letter
- …