26 research outputs found
Charge Excitations in Doped Mott Insulator in One Dimension
The doped Mott insulator in one dimension has been studied based on the phase
Hamiltonian with the Umklapp scattering process, in which the charge degree of
freedom is described by the quantum sine-Gordon model. The well-known
equivalence between the quantum sine-Gordon model and the massive Thirring
model for the spinless fermion makes it clear that the Mott-Hubbard gap
originates from the Umklapp scattering process as was indicated by Emery and
Giamarchi. Compressibility, density-density correlation function, frequency
dependence of optical conductivity and Drude weight have been calculated in the
presence of the impurity scattering treated in the self-consistent Born
approximation. It is seen that there exists a crossover behavior in the
spectral weight of charge excitations: the acoustic mode is dominant in small
wave number region while the optical excitations across the Mott-Hubbard gap
lie in large wave number region and that this crossover wave number is reduced
as the Mott transition is approached.Comment: We revised our previous manuscript. 17 pages and 11 figures, to be
published in Journal of the Physical Society of Japan Vol.65 No.1
Field-induced XY behavior in the S=1/2 antiferromagnet on the square lattice
Making use of the quantum Monte Carlo method based on the worm algorithm, we
study the thermodynamic behavior of the S=1/2 isotropic Heisenberg
antiferromagnet on the square lattice in a uniform magnetic field varying from
very small values up to the saturation value. The field is found to induce a
Berezinskii-Kosterlitz-Thouless transition at a finite temperature, above which
a genuine XY behavior in an extended temperature range is observed. The phase
diagram of the system is drawn, and the thermodynamic behavior of the specific
heat and of the uniform and staggered magnetization is discussed in sight of an
experimental investigation of the field-induced XY behavior.Comment: 4 pages, 4 figure
Dynamical spin correlations in Heisenberg ladder under magnetic field and correlation functions in SO(5) ladder
The zero-temperature dynamical spin-spin correlation functions are calculated
for the spin-1/2 two-leg Heisenberg ladder in a magnetic field above the lower
critical field Hc1. The dynamical structure factors are calculated which
exhibit both massless and massive excitations. These modes appear in different
sectors characterized by the parity in the rung direction and by the momentum
in the direction of the chains. The structure factors have power-law
singularities at the lower edges of their support. The results are also
applicable to spin-1 Heisenberg chain. The implications are briefly discussed
for various correlation functions and the pi-resonance in the SO(5) symmetric
ladder model.Comment: 15 pages, 6 figures, added references; final version to appear in
Phys. Rev.
Theory of adsorbate induced surface reconstruction on W(100)
We report results of a theoretical study on an adsorbate induced surface
reconstruction. Hydrogen adsorption on a W(100) surface causes a switching
transition in the symmetry of the displacements of the W atoms within the
ordered c(2x2) phase. This transition is modeled by an effective Hamiltonian,
where the hydrogen degrees of freedom are integrated out. Based on extensive
Monte Carlo renormalisation group calculations we show that the switching
transition is of second order at high temperatures and of first order at low
temperatures. This behavior is qualitatively explained in terms of an XY model
where there is an interplay between four and eight fold anisotropy fields. We
also compare the calculated phase diagrams with a simple mean field theory.Comment: CSC Preprint, 31 pages (plain TeX file, no figures