1,108 research outputs found
Thermodynamic properties of the coupled dimer system Cu(CHN)Cl
We re-examine the thermodynamic properties of the coupled dimer system
Cu(CHN)Cl under magnetic field in the light of
recent NMR experiments [Cl\'emancey {\it et al.}, Phys. Rev. Lett. {\bf 97},
167204 (2006)] suggesting the existence of a finite Dzyaloshinskii-Moriya
interaction. We show that including such a spin anisotropy greatly improves the
fit of the magnetization curve and gives the correct trend of the insofar
unexplained anomalous behavior of the specific heat in magnetic field at low
temperature.Comment: published version with minor change
Contractor-Renormalization approach to frustrated magnets in magnetic field
We propose to use the Contractor Renormalization (CORE) technique in order to
derive effective models for quantum magnets in a magnetic field. CORE is a
powerful non-perturbative technique that can reduce the complexity of a given
microscopic model by focusing on the low-energy part. We provide a detailed
analysis of frustrated spin ladders which have been widely studied in the past:
in particular, we discuss how to choose the building block and emphasize the
use of their reduced density matrix. With a good choice of basis, CORE is able
to reproduce the existence or not of magnetization plateaux in the whole phase
diagram contrary to usual perturbation theory. We also address the issue of
plateau formation in two-dimensional bilayers and point out the analogy between
non-frustrated strongly anisotropic models and frustrated SU(2) ones.Comment: 13 pages, 20 figures; published version with minor change
Effective Theory of Magnetization Plateaux in the Shastry-Sutherland Lattice
We use the non-perturbative Contractor-Renormalization method (CORE) in order
to derive an effective model for triplet excitations on the Shastry-Sutherland
lattice. For strong enough magnetic fields, various magnetization plateaux are
observed, e.g. at 1/8, 1/4, 1/3 of the saturation, as found experimentally in a
related compound. Moreover, other stable plateaux are found at 1/9, 1/6 or 2/9.
We give a critical review of previous works and try to resolve some apparent
inconsistencies between various theoretical approaches.Comment: published version with minor change
Quantum phase transitions in multileg spin ladders with ring exchange
Four-spin exchange interaction has been raising intriguing questions
regarding the exotic phase transitions it induces in two-dimensional quantum
spin systems. In this context, we investigate the effects of a cyclic four-spin
exchange in the quasi-1D limit by considering a general N-leg spin ladder. We
show by means of a low-energy approach that, depending on its sign, this ring
exchange interaction can engender either a staggered or a uniform dimerization
from the conventional phases of spin ladders. The resulting quantum phase
transition is found to be described by the SU(2)_N conformal field theory. This
result, as well as the fractional value of the central charge at the
transition, is further confirmed by a large-scale numerical study performed by
means of Exact Diagonalization and Density Matrix Renormalization Group
approaches for N \le 4
Finite Size Scaling of the Spin Stiffness of the Antiferromagnetic S=1/2 XXZ chain
We study the finite size scaling of the spin stiffness for the
one-dimensional s=1/2 quantum antiferromagnet as a function of the anisotropy
parameter Delta.Previous Bethe ansatz results allow a determination of the
stiffness in the thermodynamic limit. The Bethe ansatz equations for finite
systems are solvable even in the presence of twisted boundary conditions, a
fact we exploit to determine the stiffness exactly for finite systems allowing
for a complete determination of the finite size corrections. Relating the
stiffness to thermodynamic quantities we calculate the temperature dependence
of the susceptibility and its finite size corrections at T=0. A Luttinger
liquid approach is used to study the finite size corrections using
renormalization group techniques and the results are compared to the
numerically exact results obtained using the Bethe ansatz equations. Both
irrelevant and marginally irrelevant cases are considered
Dynamical dimer correlations at bipartite and non-bipartite Rokhsar-Kivelson points
We determine the dynamical dimer correlation functions of quantum dimer
models at the Rokhsar-Kivelson point on the bipartite square and cubic lattices
and the non-bipartite triangular lattice. Based on an algorithmic idea by
Henley, we simulate a stochastic process of classical dimer configurations in
continuous time and perform a stochastic analytical continuation to obtain the
dynamical correlations in momentum space and the frequency domain. This
approach allows us to observe directly the dispersion relations and the
evolution of the spectral intensity within the Brillouin zone beyond the
single-mode approximation. On the square lattice, we confirm analytical
predictions related to soft modes close to the wavevectors (pi,pi) and (pi,0)
and further reveal the existence of shadow bands close to the wavevector (0,0).
On the cubic lattice the spectrum is also gapless but here only a single soft
mode at (pi,pi,pi) is found, as predicted by the single mode approximation. The
soft mode has a quadratic dispersion at very long wavelength, but crosses over
to a linear behavior very rapidly. We believe this to be the remnant of the
linearly dispersing "photon" of the Coulomb phase. Finally the triangular
lattice is in a fully gapped liquid phase where the bottom of the dimer
spectrum exhibits a rich structure. At the M point the gap is minimal and the
spectral response is dominated by a sharp quasiparticle peak. On the other
hand, at the X point the spectral function is much broader. We sketch a
possible explanation based on the crossing of the coherent dimer excitations
into the two-vison continuum.Comment: 16 pages, 7 figures, published versio
Breathers and Raman scattering in a two-leg ladder with staggered Dzialoshinskii-Moriya interaction
Recent experiments have revealed the role of staggered Dzialoshinskii-Moriya
interaction in the magnetized phase of an antiferromagnetic spin 1/2 two-leg
ladder compound under a uniform magnetic field. We derive a low energy
effective field theory describing a magnetized two-leg ladder with a weak
staggered Dzialoshinskii-Moriya interaction. This theory predicts the
persistence of the spin gap in the magnetized phase, in contrast to standard
two-leg ladders, and the presence of bound states in the excitation spectrum.
Such bound states are observable in Raman scattering measurements. These
results are then extended to intermediate Dzialoshinskii-Moriya interaction
using Exact Diagonalizations.Comment: RevTeX 4, 14 pages, 11 EPS figure
Combined analytical and numerical approach to magnetization plateaux in one-dimensional spin tube antiferromagnets
In this paper, we investigate the properties of frustrated three-leg spin
tubes under a magnetic field. We concentrate on two kind of geometries for
these tubes, one of which is relevant for the compound
. We combine an analytical path integral
approach with a strong coupling approach, as well as large-scale Density Matrix
Renormalization Groups (DMRG) simulations, to identify the presence of plateaux
in the magnetization curve as a function of the value of spin . We also
investigate the issue of gapless non-magnetic excitations on some plateaux,
dubbed chirality degrees of freedom for both tubes.Comment: 17 page
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