5,835 research outputs found
Novel Charge Order and Superconductivity in Two-Dimensional Frustrated Lattice at Quarter Filling
Motivated by the various physical properties observed in
-(BEDT-TTF)X, we study the ground state of extended Hubbard model
on two-dimensional anisotropic triangular lattice at 1/4-filling with
variational Monte Carlo method. It is shown that the nearest-neighbor Coulomb
interaction enhances the charge fluctuation and it induces the anomalous state
such as charge-ordered metallic state and the triplet next-nearest-neighbor
-wave superconductivity. We discuss the relation to the real materials and
propose the unified view of the family of -(BEDT-TTF)X.Comment: 4 pages, 5 figure
13C NMR study of superconductivity near charge instability realized in beta"-(BEDT-TTF)4[(H3O)Ga(C2O4)3]C6H5NO2
To investigate the superconducting (SC) state near a charge instability, we
performed ^{13}C NMR experiments on the molecular superconductor
beta"-(BEDT-TTF)_{4}[(H_{3}O)Ga(C_{2}O_{4})_{3}]C_{6}H_{5}NO_{2}, which
exhibits a charge anomaly at 100 K. The Knight shift which we measured in the
SC state down to 1.5 K demonstrates that Cooper pairs are in spin-singlet
state. Measurements of the nuclear spin-lattice relaxation time reveal strong
electron-electron correlations in the normal state. The resistivity increase
observed below 10 K indicates that the enhanced fluctuation has an electric
origin. We discuss the possibility of charge-fluctuation-induced
superconductivity.Comment: 5 pages, 4 figure
The Heisenberg antiferromagnet on an anisotropic triangular lattice: linear spin-wave theory
We consider the effect of quantum spin fluctuations on the ground state
properties of the Heisenberg antiferromagnet on an anisotropic triangular
lattice using linear spin-wave theory. This model should describe the magnetic
properties of the insulating phase of the kappa-(BEDT-TTF)_2 X family of
superconducting molecular crystals. The ground state energy, the staggered
magnetization, magnon excitation spectra and spin-wave velocities are computed
as a function of the ratio between the second and first neighbours, J2/J1. We
find that near J2/J1 = 0.5, i.e., in the region where the classical spin
configuration changes from a Neel ordered phase to a spiral phase, the
staggered magnetization vanishes, suggesting the possibility of a quantum
disordered state. In this region, the quantum correction to the magnetization
is large but finite. This is in contrast to the frustrated Heisenberg model on
a square lattice, for which the quantum correction diverges logarithmically at
the transition from the Neel to the collinear phase. For large J2/J1, the model
becomes a set of chains with frustrated interchain coupling. For J2 > 4 J1, the
quantum correction to the magnetization, within LSW, becomes comparable to the
classical magnetization, suggesting the possibility of a quantum disordered
state. We show that, in this regime, quantum fluctuations are much larger than
for a set of weakly coupled chains with non-frustated interchain coupling.Comment: 10 pages, RevTeX + epsf, 5 figures Replaced with published version.
Comparison to series expansions energies include
Charge order and superconductivity in a two-dimensional triangular lattice at n=2/3
To investigate the possibility of charge order and superconductivity in a
doped two-dimensional triangular lattice, we study the extended Hubbard model
with variational Monte Carlo method. At n=2/3, a commensurate filling for a
triangular lattice, it is shown that the nearest-neighbor Coulomb interaction V
induces honeycomb-type charge order and antiferromagnetic spin order at U>10t.
We also discuss the possibility of superconductivity induced by charge
fluctuation and the relation to the superconductivity in
Na_{0.35}CoO_{2}1.3H_{2}O and theta-type organic condoctors.Comment: 4 pages, 5 figure
Ionic Hubbard model on a triangular lattice for Na_0.5CoO_2, Rb_0.5CoO_2 and K_0.5CoO_2: Mean-field slave boson theory
We introduce a strongly correlated mean-field theory of the ionic Hubbard
model on the triangular lattice with alternating stripes of site energy using
Barnes-Coleman slave bosons. We study the paramagnetic phases of this theory at
three quarters filling, where it is a model of Na_0.5CoO_2, Rb_0.5CoO_2, and
K_0.5CoO_2. This theory has two bands of fermionic quasi-particles: one of
which is filled or nearly filled and hence weakly correlated; the other is
half-filled or nearly half-filled and hence strongly correlated. Further
results depend strongly on the sign of the hopping integral, t. The light band
is always filled for t>0, but only becomes filled for |Delta/t|>=1.5 for t<0,
where Delta is the difference in the site energies of the two sublattices. A
metal--charge transfer insulator transition occurs at |Delta/t|=5.0 for t>0 and
|Delta/t|=8.0 for t<0. In the charge transfer insulator complete charge
disproportionation occurs: one sublattice is filled and the other in half
filled. We compare our results with exact diagonalisation calculations and
experiments on Na_0.5CoO_2, and discuss the relevance of our results to
Rb_0.5CoO_2 and K_0.5CoO_2. In particular we propose a resolution of seemingly
contradictory experimental results on Na_0.5CoO_2. Many experiments suggest
that there is a charge gap, yet quantum oscillations are observed suggesting
the existence of quasiparticle states at arbitrarily low excitation energies.
We argue that the heavy band is gapped while the light band, which contains
less than 1 charge carrier per 100 unit cells, remains ungapped.Comment: To appear in PRB. 19 pages, 19 figures, 6 movies. References adde
Multisite versus multiorbital Coulomb correlations studied within finite-temperature exact diagonalization dynamical mean-field theory
The influence of short-range Coulomb correlations on the Mott transition in
the single-band Hubbard model at half-filling is studied within cellular
dynamical mean field theory for square and triangular lattices.
Finite-temperature exact diagonalization is used to investigate correlations
within two-, three-, and four-site clusters. Transforming the non-local
self-energy from a site basis to a molecular orbital basis, we focus on the
inter-orbital charge transfer between these cluster molecular orbitals in the
vicinity of the Mott transition. In all cases studied, the charge transfer is
found to be small, indicating weak Coulomb induced orbital polarization despite
sizable level splitting between orbitals. These results demonstrate that all
cluster molecular orbitals take part in the Mott transition and that the
insulating gap opens simultaneously across the entire Fermi surface. Thus, at
half-filling we do not find orbital-selective Mott transitions, nor a
combination of band filling and Mott transition in different orbitals.
Nevertheless, the approach towards the transition differs greatly between
cluster orbitals, giving rise to a pronounced momentum variation along the
Fermi surface, in agreement with previous works. The near absence of Coulomb
induced orbital polarization in these clusters differs qualitatively from
single-site multi-orbital studies of several transition metal oxides, where the
Mott phase exhibits nearly complete orbital polarization as a result of a
correlation driven enhancement of the crystal field splitting. The strong
single-particle coupling among cluster orbitals in the single-band case is
identified as the source of this difference.Comment: 12 pages, 10 figure
Feynman scaling violation on baryon spectra in pp collisions at LHC and cosmic ray energies
A significant asymmetry in baryon/antibaryon yields in the central region of
high energy collisions is observed when the initial state has non-zero baryon
charge. This asymmetry is connected with the possibility of baryon charge
diffusion in rapidity space. Such a diffusion should decrease the baryon charge
in the fragmentation region and translate into the corresponding decrease of
the multiplicity of leading baryons. As a result, a new mechanism for Feynman
scaling violation in the fragmentation region is obtained. Another numerically
more significant reason for the Feynman scaling violation comes from the fact
that the average number of cutted Pomerons increases with initial energy. We
present the quantitative predictions of the Quark-Gluon String Model (QGSM) for
the Feynman scaling violation at LHC energies and at even higher energies that
can be important for cosmic ray physics.Comment: 21 pages, 11 figures, and 1 table. arXiv admin note: substantial text
overlap with arXiv:1107.1615, arXiv:1007.320
Antiferromagnetic Heisenberg model on anisotropic triangular lattice in the presence of magnetic field
We use Schwinger boson mean field theory to study the antiferromagnetic
spin-1/2 Heisenberg model on an anisotropic triangular lattice in the presence
of a uniform external magnetic field. We calculate the field dependence of the
spin incommensurability in the ordered spin spiral phase, and compare the
results to the recent experiments in CsCuCl by Coldea et al. (Phys.
Rev. Lett. 86, 1335 (2001)).Comment: 4 pages with 4 figures include
Charge Fluctuations in Geometrically Frustrated Charge Ordering System
Effects of geometrical frustration in low-dimensional charge ordering systems
are theoretically studied, mainly focusing on dynamical properties. We treat
extended Hubbard models at quarter-filling, where the frustration arises from
competing charge ordered patterns favored by different intersite Coulomb
interactions, which are effective models for various charge transfer-type
molecular conductors and transition metal oxides. Two different lattice
structures are considered: (a) one-dimensional chain with intersite Coulomb
interaction of nearest neighbor V_1 and that of next-nearest neighbor V_2, and
(b) two-dimensional square lattice with V_1 along the squares and V_2 along one
of the diagonals. From previous studies, charge ordered insulating states are
known to be unstable in the frustrated region, i.e., V_1 \simeq 2V_2 for case
(a) and V_1 \simeq V_2 for case (b), resulting in a robust metallic phase even
when the interaction strenghs are strong. By applying the Lanczos exact
diagonalization to finite-size clusters, we have found that fluctuations of
different charge order patterns exist in the frustration-induced metallic
phase, showing up as characteristic low energy modes in dynamical correlation
functions. Comparison of such features between the two models are discussed,
whose difference will be ascribed to the dimensionality effect. We also point
out incommensurate correlation in the charge sector due to the frustration,
found in one-dimensional clusters.Comment: 8 pages, 9 figure
Electronic correlation in the infrared optical properties of the quasi two dimensional -type BEDT-TTF dimer system
The polarized optical reflectance spectra of the quasi two dimensional
organic correlated electron system -(BEDT-TTF)Cu[N(CN)],
Br and Cl are measured in the infrared region. The former shows the
superconductivity at 11.6 K and the latter does the
antiferromagnetic insulator transition at 28 K. Both the
specific molecular vibration mode of the BEDT-TTF molecule and
the optical conductivity hump in the mid-infrared region change correlatively
at 38 K of -(BEDT-TTF)Cu[N(CN)]Br, although
no indication of but the insulating behaviour below 50-60 K are found in -(BEDT-TTF)Cu[N(CN)]Cl. The
results suggest that the electron-molecular vibration coupling on the
mode becomes weak due to the enhancement of the itinerant
nature of the carriers on the dimer of the BEDT-TTF molecules below ,
while it does strong below because of the localized carriers on
the dimer. These changes are in agreement with the reduction and the
enhancement of the mid-infrared conductivity hump below and , respectively, which originates from the transitions between the upper
and lower Mott-Hubbard bands. The present observations demonstrate that two
different metallic states of -(BEDT-TTF)Cu[N(CN)]Br are
regarded as {\it a correlated good metal} below including the
superconducting state and {\it a half filling bad metal} above . In
contrast the insulating state of -(BEDT-TTF)Cu[N(CN)]Cl
below is the Mott insulator.Comment: 8 pages, 7 figure
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