2,002 research outputs found
Charge Ordering and Spin gap in NaV_2O_5
A possible ground state of NaV_2O_5 is proposed based on the Hartree
approximation for both on-site and intersite Coulomb interactions. The results
indicate that the intersite Coulomb interaction induces a zigzag type of charge
disproportionation (i.e. charge ordering) along the ladders of V-ions resulting
in the localized spins between neighboring ladders to form a spin gap. This new
state, which is different from the spin-Peierls state so far believed, seems to
be consistent with the existing experimental results.Comment: 3 pages, 4 figures, submitted to J. Phys. Soc. Jp
A Model Study of the Low-Energy Charge Dynamics of NaV_2O_5
An exact-diagonalization technique on small clusters is used to calculate the
dynamical density correlation functions of the dimerized t-J chain and coupled
anisotropic t-J ladders (trellis lattice) at quarter filling, i.e., the systems
regarded as a network of pairs (dimers or rungs) of sites coupled weakly via
the hopping and exchange interactions. We thereby demonstrate that the
intersite Coulomb repulsions between the pairs induce a low-energy collective
mode in the charge excitations of the systems where the internal charge degrees
of freedom of the pairs play an essential role. Implications to the electronic
states of NaV_2O_5, i.e., fluctuations of the valence state of V ions and phase
transition as a charge ordering, are discussed.Comment: 4 pages, 4 gif figures. Hardcopies of figures (or the entire
manuscript) can be obtained by e-mail request to [email protected]
Optical conductivity in the CuO double chains of PrBa_2Cu_4O_8: Consequences of charge fluctuation
We calculate the optical conductivity of the CuO double chains of
PrBaCuO by the mean-field approximation for the coupled two-chain
Hubbard model around quarter filling. We show that the 40 meV peak
structure, spectral shape, and small Drude weight observed in experiment are
reproduced well by the present calculation provided that the stripe-type charge
ordering presents. We argue that the observed anomalous optical response may be
due to the presence of stripe-type fluctuations of charge carriers in the CuO
double chains; the fast time scale of the optical measurement should enable one
to detect slowly fluctuating order parameters as virtually a long-range order.Comment: 7 pages, 5 eps figure
Optical Conductivity of the Trellis-Lattice t-J Model: Charge Fluctuations in NaV_2O_5
Optical conductivity of the trellis lattice t-J model at quarter filling is
calculated by an exact-diagonalization technique on small clusters, whereby the
valence state of V ions of NaV_2O_5 is considered. We show that the
experimental features at \sim 1 eV, including peak positions, presence of
shoulders, and anisotropic spectral weight, can be reproduced in reasonable
range of parameter values, only by assuming that the system is in the charge
disproportionated ground state. Possible reconciliation with experimental data
suggesting the presence of uniform ladders at T>T_c is discussed.Comment: 4 pages, 4 gif figures. Minor revisions have been made. Hardcopies of
figures (or the entire manuscript) can be obtained by e-mail request to
[email protected]
Basic properties of three-leg Heisenberg tube
We study three-leg antiferromagnetic Heisenberg model with the periodic
boundary conditions in the rung direction. Since the rungs form regular
triangles, spin frustration is induced. We use the density-matrix
renormalization group method to investigate the ground state. We find that the
spin excitations are always gapped to remove the spin frustration as long as
the rung coupling is nonzero. We also visibly confirm spin-Peierls dimerization
order in the leg direction. Both the spin gap and the dimerization order are
basically enhanced as the rung coupling increases.Comment: 4 pages, 2 figure
Coexistence of distinct charge fluctuations in -(BEDT-TTF)X
Using the Lanczos exact-diagonalization and density-matrix renormalization
group methods, we study the extended Hubbard model at quarter filling defined
on the anisotropic triangular lattice. We focus on charge ordering (CO)
phenomena induced by onsite and intersite Coulomb interactions. We determine
the ground-state phase diagram including three CO phases, i.e., diagonal,
vertical, and three-fold CO phases, based on the calculated results of the hole
density and double occupancy. We also calculate the dynamical density-density
correlation functions and find possible coexistence of the diagonal and
three-fold charge fluctuations in a certain parameter region where the onsite
and intersite interactions compete. Furthermore, the characteristic features of
the optical conductivity for each CO phase are discussed.Comment: 9 pages, 7 figure
Parallelization Strategies for Density Matrix Renormalization Group Algorithms on Shared-Memory Systems
Shared-memory parallelization (SMP) strategies for density matrix
renormalization group (DMRG) algorithms enable the treatment of complex systems
in solid state physics. We present two different approaches by which
parallelization of the standard DMRG algorithm can be accomplished in an
efficient way. The methods are illustrated with DMRG calculations of the
two-dimensional Hubbard model and the one-dimensional Holstein-Hubbard model on
contemporary SMP architectures. The parallelized code shows good scalability up
to at least eight processors and allows us to solve problems which exceed the
capability of sequential DMRG calculations.Comment: 18 pages, 9 figure
NaV_2O_5 as an Anisotropic t-J Ladder at Quarter Filling
Based on recent experimental evidences that the electronic charge degrees of
freedom plays an essential role in the spin-Peierls--like phase transition of
NaVO, we first make the mapping of low-energy electronic states of the
model for NaVO to the quarter-filled ladder with
anisotropic parameter values between legs and rungs, and then show that this
anisotropic ladder is in the Mott insulating state, of which
lowest-energy states can be modeled by the one-dimensional Heisenberg
antiferromagnet with the effective exchange interaction whose value
is consistent with experimental estimates. We furthermore examine the coupling
between the ladders as the trellis lattice model and show that the
nearest-neighbor Coulomb repulsion on the zigzag-chain bonds can lead to the
instability in the charge degrees of freedom of the ladders.Comment: 4 pages, 5 gif figures. Fig.3 corrected. Hardcopies of figures (or
the entire manuscript) can be obtained by e-mail request to
[email protected]
Disorder and superconductivity in doped semiconductor nanotubes
Finite-size systems of the one-dimensional attractive Hubbard model with random potential are studied as an effective model for doped semiconductor nanotubes. We calculate the binding energy of Cooper pairs and pair correlation function by the density-matrix renormalization group method. We show that, when the scattering potential is strong, there appears the ground state where Cooper pairs are formed but are localized spatially, with a decay length of pair correlation smaller than the system size. Experimental relevance is discussed. © 2009 IOP Publishing Ltd.JSPS Research Fellowship for Young ScientistsMinistry of Education, Science, Sports and Culture of Japa
Correlation function for the one-dimensional extended Hubbard model at quarter filling
We examine the density-density correlation function in the Tomonaga-Luttinger
liquid state for the one-dimensional extended Hubbard model with the on-site
Coulomb repulsion and the intersite repulsion at quarter filling. By
taking into account the effect of the marginally irrelevant umklapp scattering
operator by utilizing the renormalization-group technique based on the
bosonization method, we obtain the generalized analytical form of the
correlation function. We show that, in the proximity to the gapped
charge-ordered phase, the correlation function exhibits anomalous crossover
between the pure power-law behavior and the power-law behavior with logarithmic
corrections, depending on the length scale. Such a crossover is also confirmed
by the highly-accurate numerical density-matrix renormalization group method.Comment: 6 pages, 3 figure
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