490 research outputs found
Quantum phase transitions in three-leg spin tubes
We investigate the properties of a three-leg quantum spin tube using several
techniques such as the density matrix renormalization group method, strong
coupling approaches and the non linear sigma model. For integer spins S, the
model proves to exhibit a particularly rich phase diagram consisting of an
ensemble of 2S phase transitions. They can be accurately identified by the
behavior of a non local string order parameter associated to the breaking of a
hidden symmetry in the Hamiltonian. The nature of these transitions are further
elucidated within the different approaches. We carry a detailed DMRG analysis
in the specific cases S = 1. The numerical data confirm the existence of two
Haldane phases with broken hidden symmetry separated by a trivial singlet
state. The study of the gap and of the von Neumann entropy suggest a first
order phase transition but at the close proximity of a tricritical point
separating a gapless and a first order transition line in the phase diagram of
the quantum spin tube.Comment: 20 pages, 18 figure
Haldane Gap and Hidden Order in the S=2 Antiferromagnetic Quantum Spin Chain
We have investigated Haldane's conjecture for the S=2 isotropic
antiferromagnetic quantum spin chain with nearest-neighbor exchange J. Using a
density matrix renormalization group algorithm for chains up to L=350 spins, we
find in the thermodynamic limit a finite spin gap of Delta = 0.085(5)J and a
finite spin-spin correlation length xi = 49(1) lattice spacings. We establish
the ground state energy per bond to be E_0=-4.761248(1)J. We show that the
ground state has a hidden topological order that is revealed in a nonlocal
string correlation function. This means that the physics of the S=2 chain can
be captured by a valence-bond solid description. We also observe effective free
spin-1 states at the ends of an open S=2 chain.Comment: 6 pages, LaTeX 2.09, 3 PostScript figure
Coupled Heisenberg antiferromagnetic chains in an effective staggered field
We present a systematic study of coupled Heisenberg antiferromagnetic
chains in an effective staggered field. We investigate several effects of the
staggered field in the {\em higher} ({\em two or three}) {\em dimensional} spin
system analytically. In particular, in the case where the staggered field and
the inter-chain interaction compete with each other, we predict, using
mean-field theory, a characteristic phase transition. The spin-wave theory
predicts that the behavior of the gaps induced by the staggered field is
different between the competitive case and the non-competitive case. When the
inter-chain interactions are sufficiently weak, we can improve the mean-field
phase diagram by using chain mean-field theory and the analytical results of
field theories. The ordered phase region predicted by the chain mean-field
theory is substantially smaller than that by the mean-field theory.Comment: 13pages, 12figures, to be published in PR
Scaling behavior of the energy gap of spin-1/2 AF-Heisenberg chain in both uniform and staggered fields
We have studied the energy gap of the 1D AF-Heisenberg model in the presence
of both uniform () and staggered () magnetic fields using the exact
diagonalization technique. We have found that the opening of the gap in the
presence of a staggered field scales with , where is the
critical exponent and depends on the uniform field. With respect to the range
of the staggered magnetic field, we have identified two regimes through which
the -dependence of the real critical exponent can be numerically
calculated. Our numerical results are in good agreement with the results
obtained by theoretical approaches
Frustration of decoherence in -shaped superconducting Josephson networks
We examine the possibility that pertinent impurities in a condensed matter
system may help in designing quantum devices with enhanced coherent behaviors.
For this purpose, we analyze a field theory model describing Y- shaped
superconducting Josephson networks. We show that a new finite coupling stable
infrared fixed point emerges in its phase diagram; we then explicitly evidence
that, when engineered to operate near by this new fixed point, Y-shaped
networks support two-level quantum systems, for which the entanglement with the
environment is frustrated. We briefly address the potential relevance of this
result for engineering finite-size superconducting devices with enhanced
quantum coherence. Our approach uses boundary conformal field theory since it
naturally allows for a field-theoretical treatment of the phase slips
(instantons), describing the quantum tunneling between degenerate levels.Comment: 11 pages, 5 .eps figures; several changes in the presentation and in
the figures, upgraded reference
Dynamical Structure Factor in Cu Benzoate and other spin-1/2 antiferromagnetic chains
Recent experiments of the quasi-one-dimensional spin-1/2 antiferromagnet
Copper Benzoate established the existence of a magnetic field induced gap. The
observed neutron scattering intensity exhibits resolution limited peaks at both
the antiferromagnetic wave number and at incommensurate wave numbers related to
the applied magnetic field. We determine the ratio of spectral weights of these
peaks within the framework of a low-energy effective field theory description
of the problem.Comment: 5 pages, 3figure
An Electron Spin Resonance Selection Rule for Spin-Gapped Systems
The direct electron spin resonance (ESR) absorption between a singlet ground
state and the triplet excited states of spin gap systems is investigated. Such
an absorption, which is forbidden by the conservation of the total spin quantum
number in isotropic Hamiltonians, is allowed by the Dzyaloshinskii-Moriya
interaction. We show a selection rule in the presence of this interaction,
using the exact numerical diagonalization of the finite cluster of the
quasi-one-dimensional bond-alternating spin system. The selection rule is also
modified into a suitable form in order to interpret recent experimental results
on CuGeO and NaVO.Comment: 5 pages, Revtex, with 6 eps figures, to appear in J. Phys. Soc. Jpn.
Vol. 69 No. 11 (2000
On the ground-state properties of antiferromagnetic half-integer spin chains with long-range interactions
The Lieb-Shultz-Mattis theorem is extended to Heisenberg chains with
long-range interactions. We prove that the half-integer spin chain has no gap,
if it possesses unique ground state and the exchange decays faster than the
inverse-square of distance between spins. The results can be extended to a wide
class of one-dimensional models.Comment: 3 pages, RevTeX
Metallic Ferromagnetism in the Kondo Lattice
Metallic magnetism is both ancient and modern, occurring in such familiar
settings as the lodestone in compass needles and the hard drive in computers.
Surprisingly, a rigorous theoretical basis for metallic ferromagnetism is still
largely missing. The Stoner approach perturbatively treates Coulomb
interactions when the latter need to be large, while the Nagaoka approach
incorporates thermodynamically negligible electrons into a half-filled band.
Here, we show that the ferromagnetic order of the Kondo lattice is amenable to
an asymptotically exact analysis over a range of interaction parameters. In
this ferromagnetic phase, the conduction electrons and local moments are
strongly coupled but the Fermi surface does not enclose the latter (i.e. it is
small). Moreover, non-Fermi liquid behavior appears over a range of frequencies
and temperatures. Our results provide the basis to understand some
long-standing puzzles in the ferromagnetic heavy fermion metals, and raises the
prospect for a new class of ferromagnetic quantum phase transitions.Comment: 21 pages, 9 figures, including Supporting Informatio
- …