446 research outputs found
Exotic phenomena in doped quantum magnets
We investigate the properties of the two-dimensional frustrated quantum
antiferromagnet on the square lattice, especially at infinitesimal doping. We
find that next nearest neighbor (N.N.) J2 and next-next N.N. J3 interactions
together destroy the antiferromagnetic long range order and stabilize a quantum
disordered valence bond crystalline plaquette phase. A static vacancy or a
dynamic hole doped into this phase liberates a spinon. From the profile of the
spinon wavefunction around the (static) vacancy we identify an intermediate
behavior between complete deconfinement (behavior seen in the kagome lattice)
and strong confinement (behavior seen in the checkerboard lattice) with the
emergence of two length scales, a spinon confinement length larger than the
magnetic correlation length. When a finite hole hopping is introduced, this
behavior translates into an extended (mobile) spinon-holon boundstate with a
very small quasiparticle weight. These features provide clear evidence for a
nearby "deconfined critical point" in a doped microscopic model. Finally, we
give arguments in favor of superconducting properties of the doped plaquette
phase.Comment: Submitted to J. of Phys. Condens. Matter (Proceedings of
International Conference "Highly Frustrated Magnets", Osaka (Japan), August
2006). 6 pages, 5 figures Display problems with Figure 2 fixe
Spin Gaps in Coupled t-J Ladders
Spin gaps in coupled - ladders are investigated by exact
diagonalization of small clusters up to 48 sites. At half-filling, the
numerical results for the triplet excitation spectrum are in very good
agreement with a second order perturbation expansion in term of small
inter-ladder and intra-ladder exchange couplings between rungs
(). The band of local triplet excitations moving
coherently along the ladder (with momenta close to ) is split by the
inter-ladder coupling. For intermediate couplings finite size scaling is used
to estimate the spin gap. In the isotropic infinite 4-chain system (two coupled
ladders) we find a spin gap of , roughly half of the single ladder
spin gap. When the system is hole doped, bonding and anti-bonding bound pairs
of holes can propagate coherently along the chains and the spin gap remains
finite.Comment: 11 pages, 5 figures, uuencoded form of postscript files of figures
and text, LPQTH-94/
Charge profile of surface doped C60
We study the charge profile of a C60-FET (field effect transistor) as used in
the experiments of Schoen, Kloc and Batlogg. Using a tight-binding model, we
calculate the charge profile treating the Coulomb interaction in a mean-field
approximation. The charge profile behaves similarly to the case of a continuous
space-charge layer, in particular it is confined to a single interface layer
for doping higher than ~0.3 electron (or hole) per C60 molecule. The morahedral
disorder of the C60 molecules smoothens the structure in the density of states.Comment: 6 pages, 9 figure
Phase diagram of the two-dimensional t--J model at low doping
The phase diagram of the planar t--J model at small hole doping is
investigated by finite size scaling of exact diagonalisation data of NXN
clusters (up to 26). Hole-droplet binding energies, compressibility and static
spin and charge correlations are calculated. Short range antiferromagnetic
correlations can produce attractive forces between holes leading to a very rich
phase diagram including a liquid of d-wave hole pairs (for ), a
liquid of hole droplets (quartets) for larger J/t ratios ()
and, at even larger coupling J/t, an instability towards phase separation.Comment: 3 pages, latex, 5 postscript figures, uuencode
Spin dynamics of the spin-Peierls compound CuGeO_3 under magnetic field
The magnetic field--driven transition in the spin-Peierls system CuGeO_3
associated with the closing of the spin gap is investigated numerically. The
field dependence of the spin dynamical structure factor (seen by inelastic
neutron scattering) and of the momentum dependent static susceptibility are
calculated. In the dimerized phase (H<H_c), we suggest that the strong field
dependence of the transverse susceptibility could be experimentally seen from
the low temperature spin-echo relaxation rate 1/T_{2G} or the second moment of
the NMR spectrum. Above H_c low energy spin excitations appear at
incommensurate wave vectors where the longitudinal susceptibility chi_{zz}(q)
peaks.Comment: 4 pages, LaTeX, postscript figures include
Low-frequency current fluctuations in doped ladders
Charge current static and dynamical correlations are computed by exact
diagonalisation methods on a 2-leg t-t'-J ladder which exhibits a sharp
transition between a Luther-Emery (LE) phase of hole pairs and a phase with
deconfined holes. In the LE phase, we find short-range low-energy
incommensurate current fluctuations which are intrinsically connected to the
internal charge dynamics within one hole pair. On the contrary, when holes
unbind, the maximum of the current susceptibility moves abruptly to the
commensurate wavevector and strongly increases for decreasing doping
suggesting an instability towards a staggered flux state at sufficiently small
doping.Comment: 4 pages, 5 figure
Superconducting fluctuations in the Luther-Emery liquid
The single-particle superconducting Green's functions of a Luther-Emery
liquid is computed by bosonization techniques. Using a formulation introduced
by Poilblanc and Scalapino [Phys. Rev. B v. 66, art. 052513 (2002)], an
asymptotic expression of the superconducting gap is deduced in the long
wavelength and small frequency limit. Due to superconducting phase
fluctuations, the gap exhibits as a function of size L a (1/L)^{1/2K_\rho}
power-law decay as well as an interesting singularity at the spectral gap
energy. Similarities and differences with the 2-leg t-J ladder are outlined.Comment: RevTeX 4, 3 pages, 2 EPS figure
Checkerboard order in the t--J model on the square lattice
We propose that the inhomogeneous patterns seen by STM in some underdoped
superconducting cuprates could be related to a bond-order-wave instability of
the staggered flux state, one of the most studied "normal" state proposed to
compete with the d-wave RVB superconductor. A checkerboard pattern is obtained
by a Gutzwiller renormalized mean-field theory of the t-J model for doping
around 1/8. It is found that the charge modulation is always an order of
magnitude smaller than the bond-order modulations. This is confirmed by an
exact optimization of the wavefunction by a variational Monte Carlo scheme. The
numerical estimates of the order parameters are however found to be strongly
reduced w.r.t their mean-field values
Charge density correlations in t-J ladders investigated by the CORE method
Using 4-site plaquette or rung basis decomposition, the CORE method is
applied to 2-leg and 4-leg t-J ladders and cylinders. Resulting range-2
effective hamiltonians are studied numerically on periodic rings taking full
advantage of the translation symmetry as well as the drastic reduction of the
Hilbert space. We investigate the role of magnetic and fermionic degrees of
freedom to obtain the most reliable representation of the underlying model.
Spin gaps, pair binding energies and charge correlations are computed and
compared to available ED and DMRG data for the full Hamiltonian. Strong
evidences for short-range diagonal stripe correlations are found in periodic
4-leg t-J ladders.Comment: Computation of Luttinger liquid parameters (charge velocity and
charge correlation exponent) adde
Binding of holes and pair spectral function in the t-J model
Clusters of the two-dimensionnal t--J model with 2 holes and up to 26 sites
are diagonalized using a Lanczos algorithm. The behaviour of the binding energy
with system size suggests the existence of a finite critical value of J above
which binding occurs in the bulk. Only the d-wave pair field operator acting on
the Heisenberg GS has a finite overlap with the 2 hole ground state for all the
clusters considered. The related spectral function associated with the
propagation of a d-wave (spin singlet) pair of holes in the antiferromagnetic
background is calculated. The quasiparticle peak at the bottom of the spectrum
as well as some structure appearing above the peak survive with increasing
cluster size. Although no simple scaling law was found for the quasiparticle
weight the data strongly suggest that this weight is finite in the bulk limit
and is roughly proportional to the antiferromagnetic coupling J (for J<1).Comment: Report LPQTH-93/01, 18 pages (REVTEX), 8 postscript files include
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