5,798 research outputs found
Spiral Spin Order and Transport Anisotropy in Underdoped Cuprates
We discuss the spiral spin density wave model and its application to explain
properties of underdoped LaSrCuO. We argue that the spiral
picture is theoretically well justified in the context of the extended
model, and then show that it can explain a number of observed features, such as
the location and symmetry of the incommensurate peaks in elastic neutron
scattering, as well as the in-plane resistivity anisotropy. A consistent
description of the low doping region (below 10% or so) emerges from the spiral
formulation, in which the holes show no tendency towards any type of charge
order and the physics is purely spin driven.Comment: 6 pages, 3 figures; Proceedings of the International Workshop on
Effective Models for Low-Dimensional Strongly Correlated Systems, September
2005, Peyresq, Franc
Theory of incommensurate magnetic correlations across the insulator-superconductor transition in underdoped La_{2-x}Sr_xCuO_4
The main feature in the elastic neutron scattering of La_{2-x}Sr_xCuO_4 is
the existence of incommensurate peaks with positions that jump from 45 to 0
degrees at 5% doping. We show that the spiral state of the t-t'-t''-J model
with realistic parameters describes this data perfectly. We explain why in the
insulator the peak is at 45 degrees while it switches to 0 degrees precisely at
the insulator-metal transition. The calculated positions of the peaks are in
agreement with the data in both phases.Comment: 5 pages, 1 figure; minor change
Thermodynamics of a gas of deconfined bosonic spinons in two dimensions
We consider the quantum phase transition between a Neel antiferromagnet and a
valence-bond solid (VBS) in a two-dimensional system of S=1/2 spins. Assuming
that the excitations of the critical ground state are linearly dispersing
deconfined spinons obeying Bose statistics, we derive expressions for the
specific heat and the magnetic susceptibility at low temperature T. Comparing
with quantum Monte Carlo results for the J-Q model, which is a candidate for a
deconfined Neel-VBS transition, we find excellent agreement, including a
previously noted logarithmic correction in the susceptibility. In our
treatment, this is a direct consequence of a confinement length scale Lambda
which is proportional to the correlation length xi raised to a non-trivial
power; Lambda ~ xi^(1+a) ~1/T^(1+a), with a>0 (with a approximately 0.2 in the
model).Comment: 4+ pages, 3 figures. v2: cosmetic changes onl
Low-energy singlet and triplet excitations in the spin-liquid phase of the two-dimensional J1-J2 model
We analyze the stability of the spontaneously dimerized spin-liquid phase of
the frustrated Heisenberg antiferromagnet - the J1-J2 model. The lowest triplet
excitation, corresponding to breaking of a singlet bond, is found to be stable
in the region 0.38 < J2/J1 < 0.62. In addition we find a stable low-energy
collective singlet mode, which is closely related to the spontaneous violation
of the discrete symmetry. Both modes are gapped in the quantum disordered phase
and become gapless at the transition point to the Neel ordered phase
(J2/J1=0.38). The spontaneous dimerization vanishes at the transition and we
argue that the disappearance of dimer order is related to the vanishing of the
singlet gap. We also present exact diagonalization data on a small (4x4)
cluster which indeed show a structure of the spectrum, consistent with that of
a system with a four-fold degenerate (spontaneously dimerized) ground state.Comment: 4 pages, 4 figures, small changes, published versio
Theory of Anisotropic Hopping Transport due to Spiral Correlations in the Spin-Glass Phase of Underdoped Cuprates
We study the in-plane resistivity anisotropy in the spin-glass phase of the
high- cuprates, on the basis of holes moving in a spiral spin
background. This picture follows from analysis of the extended model with
Coulomb impurities. In the variable-range hopping regime the resistivity
anisotropy is found to have a maximum value of around 90%, and it decreases
with temperature, in excellent agreement with experiments in
LaSrCuO. In our approach the transport anisotropy is due to the
non-collinearity of the spiral spin state, rather than an intrinsic tendency of
the charges to self-organize.Comment: 5 pages, 4 figures; expanded versio
Single hole dynamics in dimerized spin liquids
The dynamics of a single hole in quantum antiferromagnets is influenced by
magnetic fluctuations. In the present work we consider two situations. The
first one corresponds to a single hole in the two leg t-J spin ladder. In this
case the wave function renormalization is relatively small and the
quasiparticle residue of the S=1/2 state remains close to unity. However at
large t/J there are higher spin (S=3/2,5/2,..) bound states of the hole with
the magnetic excitations, and therefore there is a crossover from
quasiparticles with S=1/2 to quasiparticles with higher spin.
The second situation corresponds to a single hole in two coupled
antiferromagnetic planes very close to the point of antiferromagnetic
instability. In this case the hole wave function renormalization is very strong
and the quasiparticle residue vanishes at the point of instability.Comment: 12 pages, 3 figure
Low-lying excitations and magnetization process of coupled tetrahedral systems
We investigate low-lying singlet and triplet excitations and the
magnetization process of quasi-1D spin systems composed of tetrahedral spin
clusters. For a class of such models, we found various exact low-lying
excitations; some of them are responsible for the first-order transition
between two different ground states formed by local singlets. Moreover, we find
that there are two different kinds of magnetization plateaus which are
separated by a first-order transition.Comment: To appear in Phys.Rev.B (Issue 01 August 2002). A short comment is
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