1,732 research outputs found
Double Exchange Ferromagnetism in the Peierls Insulator State
We study the effects of opening of the band gap on the double exchange
ferromagnetism. Applying the density-matrix renormalization group method and an
analytical expansion from the dimer limit to the one-dimensional double
exchange model, we demonstrate for a relevant region of the exchange coupling
that, in the weak dimerization regime, the Peierls gap opens in the fully
spin-polarized conduction band without affecting its ferromagnetism, whereas in
the strong dimerization regime, the ferromagnetism is destroyed and the Mott
gap opens instead, leading the system to the antiferromagnetic quasi-long-range
order. An insulator version of the double exchange ferromagnetism is thus
established.Comment: 5 pages, 3 figures, Phys. Rev. Lett., in pres
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
Phase diagram of the one-dimensional Hubbard model with next-nearest-neighbor hopping
We study the one-dimensional Hubbard model with nearest-neighbor and
next-nearest-neighbor hopping integrals by using the density-matrix
renormalization group (DMRG) method and Hartree-Fock approximation. Based on
the calculated results for the spin gap, total-spin quantum number, and
Tomonaga-Luttinger-liquid parameter, we determine the ground-state phase
diagram of the model in the entire filling and wide parameter region. We show
that, in contrast to the weak-coupling regime where a spin-gapped liquid phase
is predicted in the region with four Fermi points, the spin gap vanishes in a
substantial region in the strong-coupling regime. It is remarkable that a large
variety of phases, such as the paramagnetic metallic phase, spin-gapped liquid
phase, singlet and triplet superconducting phases, and fully polarized
ferromagnetic phase, appear in such a simple model in the strong-coupling
regime.Comment: 11 pages, 8 figure
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]
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