9 research outputs found
Charge-density waves in one-dimensional Hubbard superlattices
We study the formation of charge density waves (CDW's) in one-dimensional
Hubbard superlattices, modeled by a repeated pattern of repulsive (U>0) and
free (U=0) sites. By means of Lanczos diagonalizations for the ground state, we
calculate the charge structure factor. Our results show that while the
superlattice structure affects the modulation of the charge density waves, the
periodicity can still be predicted through an effective density. We also show
that, for a fixed repulsive layer thickness, the periodicity of the CDW is an
oscillatory function of the free layer thickness.Comment: 4 pages, 4 figure
Charge-density waves in the Hubbard chain: evidence for 4k_F instability
Charge density waves in the Hubbard chain are studied by means of
finite-temperature Quantum Monte Carlo simulations and Lanczos diagonalizations
for the ground state. We present results both for the charge susceptibilities
and for the charge structure factor at densities \rho=1/6 and 1/3; for \rho=1/2
(quarter filled) we only present results for the charge structure factor. The
data are consistent with a 4k_F instability dominating over the 2k_F one, at
least for sufficiently large values of the Coulomb repulsion, U. This can only
be reconciled with the Luttinger liquid analyses if the amplitude of the 2k_F
contribution vanishes above some U^*(\rho).Comment: RevTeX, 4 two-column pages with 7 colour figures embedded in tex
Crossover behavior of the J1-J2 model in a staggered magnetic field
The ground states of the Heisenberg chain with the
nearest-neighbor and the next-nearest-neighbor antiferromagnetic couplings are
numerically investigated in a staggered magnetic field. While the staggered
magnetic field may induce the N\'eel-type excitation gap, and it is
characterized by the Gaussian fixed point in the spin-fluid region, the
crossover to the behavior controlled by the Ising fixed point is expected to be
observed for the spontaneously dimerized state at finite field. Treating a
low-lying excitation gap by the phenomenological renormalization-group method,
we numerically determine the massless flow connecting the Gaussian and Ising
fixed points. Further, to check the criticalities, we perform the
finite-size-scaling analysis of the excitation gap.Comment: 4 pages, 3 figure
Finite-size investigation of scaling corrections in the square-lattice three-state Potts antiferromagnet square-lattice three-state Potts antiferromagnet
We investigate the finite-temperature corrections to scaling in the
three-state square-lattice Potts antiferromagnet, close to the critical point
at T=0. Numerical diagonalization of the transfer matrix on semi-infinite
strips of width sites, , yields finite-size estimates of
the corresponding scaled gaps, which are extrapolated to . Owing to
the characteristics of the quantities under study, we argue that the natural
variable to consider is x\eta_3=2.00(1)\eta_{{\bf P}_{\rm
stagg}}=3$, corresponding to the staggered polarization.Comment: RevTex4, 5 pages, 2 .eps figures include
Quantum-fluctuation-induced collisions and subsequent excitation gap of an elastic string between walls
An elastic string embedded between rigid walls is simulated by means of the
density-matrix renormalization group. The string collides against the walls
owing to the quantum-mechanical zero-point fluctuations. Such ``quantum
entropic'' interaction has come under thorough theoretical investigation in the
context of the stripe phase observed experimentally in doped cuprates. We found
that the excitation gap opens in the form of exponential singularity DeltaE ~
exp(-Ad^sigma) (d: wall spacing) with the exponent sigma =0.6(3), which is
substantially smaller than the meanfield value sigma=2. That is, the excitation
gap is much larger than that anticipated from meanfield, suggesting that the
string is subjected to robust pinning potential due to the quantum collisions.
This feature supports Zaanen's ``order out of disorder'' mechanism which would
be responsible to the stabilization of the stripe phase
Magnetization plateau in the spin ladder with the four-spin exchange
The magnetization process of the =1/2 antiferromagnetic spin ladder with
the four-spin cyclic exchange interaction at T=0 is studied by the exact
diagonalization of finite clusters and size scaling analyses. It is found that
a magnetization plateau appears at half the saturation value if the ratio of
the four- and two-spin exchange coupling constants is larger than the
critical value 0.04. The phase transition with respect to
at is revealed to be the Kosterlitz-Thouless-type.Comment: 4 pages, Revtex, with 5 eps figure
Search for Kosterlitz-Thouless transition in a triangular Ising antiferromagnet with further-neighbour ferromagnetic interactions
We investigate an antiferromagnetic triangular Ising model with anisotropic
ferromagnetic interactions between next-nearest neighbours, originally proposed
by Kitatani and Oguchi (J. Phys. Soc. Japan {\bf 57}, 1344 (1988)). The phase
diagram as a function of temperature and the ratio between first- and second-
neighbour interaction strengths is thoroughly examined. We search for a
Kosterlitz-Thouless transition to a state with algebraic decay of correlations,
calculating the correlation lengths on strips of width up to 15 sites by
transfer-matrix methods. Phenomenological renormalization, conformal invariance
arguments, the Roomany-Wyld approximation and a direct analysis of the scaled
mass gaps are used. Our results provide limited evidence that a
Kosterlitz-Thouless phase is present. Alternative scenarios are discussed.Comment: 10 pages, RevTeX 3; 11 Postscript figures (uuencoded); to appear in
Phys. Rev. E (1995
Z_3 Quantum Criticality in a spin-1/2 chain model
The stability of the magnetization plateau phase of the XXZ spin-1/2
Heisenberg chain with competing interactions is investigated upon switching on
a staggered transverse magnetic field. Within a bosonization approach, it is
shown that the low-energy properties of the model are described by an effective
two-dimensional XY model in a three-fold symmetry-breaking field. A phase
transition in the three-state Potts universality class is expected separating
the plateau phase to a phase where the spins are polarized along the
staggered magnetic field. The Z critical properties of the transition are
determined within the bosonization approach.Comment: 5 pages, revised versio