287 research outputs found
Jordan-Wigner Approach to Dynamic Correlations in 2D Spin-1/2 Models
We discuss the dynamic properties of the square-lattice spin-1/2 XY model
obtained using the two-dimensional Jordan-Wigner fermionization approach. We
argue the relevancy of the fermionic picture for interpreting the neutron
scattering measurements in the two-dimensional frustrated quantum magnet
Cs_2CuCl_4.Comment: Presented at 12-th Czech and Slovak Conference on Magnetism,
Ko\v{s}ice, 12-15 July 200
Spin wave theory for antiferromagnetic XXZ spin model on a triangle lattice in the presence of an external magnetic field
Spin wave theory is applied to a quantum antiferromagnetic XXZ model on a
triangle lattice in the presence of an in-plane magnetic field. The effect of
the field is found to enhance the quantum fluctuation and to reduce the
sublattice magnetization at the intermediate field strength in the anisotropic
case. The possible implication to the field driven quantum phase transition
from a spin solid to a spin liquid is discussed.Comment: 5 pages,4 figure
Spin gaps and magnetic structure of NaxCoO2
We present two experiments that provide information on spin anisotropy and
the magnetic structure of NaxCoO2. First, we report low-energy neutron
inelastic scattering measurements of the zone-center magnetic excitations in
the magnetically ordered phase of Na0.75CoO2. The energy spectra suggest the
existence of two gaps, and are very well fitted by a spin-wave model with both
in-plane and out-of-plane anisotropy terms. The gap energies decrease with
increasing temperature and both gaps are found to have closed when the
temperature exceeds the magnetic ordering temperature T_m~22 K. Secondly, we
present neutron diffraction studies of Na0.85CoO2 with a magnetic field applied
approximately parallel to the c axis. For fields in excess of ~8T a magnetic
Bragg peak was observed at the (0,0,3) position in reciprocal space. We
interpret this as a spin-flop transition of the A-type antiferromagnetic
structure, and we show that the spin-flop field is consistent with the size of
the anisotropy gap.Comment: 9 pages, 7 figure
Direct observation of charge order in triangular metallic AgNiO2 by single-crystal resonant X-ray scattering
We report resonant X-ray scattering measurements on the orbitally-degenerate
triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous
transition to a triple-cell superstructure at temperatures below 365 K. We
observe a strong resonant enhancement of the supercell reflections through the
Ni K-edge. The empirically extracted K-edge shift between the
crystallographically-distinct Ni sites of 2.5(3) eV is much larger than the
value expected from the shift in final states, and implies a core-level shift
of ~1 eV, thus providing direct evidence for the onset of spontaneous honeycomb
charge order in the triangular Ni layers. We also provide band-structure
calculations that explain quantitatively the observed edge shifts in terms of
changes in the Ni electronic energy levels due to charge order and
hybridization with the surrounding oxygens.Comment: 5 pages, 4 figure
Ground states of a frustrated spin-1/2 antifferomagnet: Cs_2CuCl_4 in a magnetic field
We present detailed calculations of the magnetic ground state properties of
CsCuCl in an applied magnetic field, and compare our results with
recent experiments. The material is described by a spin Hamiltonian, determined
with precision in high field measurements, in which the main interaction is
antiferromagnetic Heisenberg exchange between neighboring spins on an
anisotropic triangular lattice. An additional, weak Dzyaloshinkii-Moriya
interaction introduces easy-plane anisotropy, so that behavior is different for
transverse and longitudinal field directions. We determine the phase diagram as
a function of field strength for both field directions at zero temperature,
using a classical approximation as a first step. Building on this, we calculate
the effect of quantum fluctuations on the ordering wavevector and components of
the ordered moments, using both linear spinwave theory and a mapping to a Bose
gas which gives exact results when the magnetization is almost saturated. Many
aspects of the experimental data are well accounted for by this approach.Comment: 13 Pages, 9 Figure
Where is the spectral weight in magnetic neutron scattering in the cuprates?
We present estimates in the Hubbard and Heisenberg models for the spectral
weight in magnetic neutron scattering experiments on the cuprates. With the aid
of spin-wave theory and the time dependent Gutzwiller approximation we discuss
how the spectral weight is distributed among the different channels and between
high and low energies. In addition to the well known total moment sum rule we
discuss sum rules for each component of the dynamical structure factor tensor
which are peculiar for spin 1/2 systems. The various factors that reduce the
spectral weight at the relevant energies are singled out and analyzed like:
shielding factors, weight at electronic energies, multimagnon process etc.
Although about 10% ~ 15% of the naively expected weight is detected in
experiments after consideration of these factors the missing weight is within
the experimental uncertainties. A large fraction of the spectral weight is hard
to detect with present experimental conditions.Comment: 16 pages, 13 figures, submitted to PR
Bose-Einstein Condensation of Magnons in Cs2CuCl4
We report on results of specific heat measurements on single crystals of the
frustrated quasi-2D spin-1/2 antiferromagnet Cs_2CuCl_4 (T_N=0.595 K) in
external magnetic fields B30 mK. Decreasing B from
high fields leads to the closure of the field-induced gap in the magnon
spectrum at a critical field B_c = 8.51 T and a magnetic phase transition is
clearly seen below B_c. In the vicinity to B_c, the phase transition boundary
is well described by the power-law T_c(B)\propto (B_c-B)^{1/\phi} with the
measured critical exponent \phi\simeq 1.5. These findings are interpreted as a
Bose-Einstein condensation of magnons.Comment: 5 pages, 4 figures, experiment and theor
Excitation spectra of the spin-1/2 triangular-lattice Heisenberg antiferromagnet
We use series expansion methods to calculate the dispersion relation of the
one-magnon excitations for the spin-1/2 triangular-lattice nearest-neighbor
Heisenberg antiferromagnet above a three-sublattice ordered ground state.
Several striking features are observed compared to the classical (large-S)
spin-wave spectra. Whereas at low energies the dispersion is only weakly
renormalized by quantum fluctuations, significant anomalies are observed at
high energies. In particular, we find roton-like minima at special wave-vectors
and strong downward renormalization in large parts of the Brillouin zone,
leading to very flat or dispersionless modes. We present detailed comparison of
our calculated excitation energies in the Brillouin zone with the spin-wave
dispersion to order 1/S calculated recently by Starykh, Chubukov, and Abanov
[cond-mat/0608002]. We find many common features but also some quantitative and
qualitative differences. We show that at temperatures as low as 0.1J the
thermally excited rotons make a significant contribution to the entropy.
Consequently, unlike for the square lattice model, a non-linear sigma model
description of the finite-temperature properties is only applicable at
extremely low temperatures.Comment: 14 pages, 11 figure
On the weak confinement of kinks in the one-dimensional quantum ferromagnet CoNb2O6
In a recent paper Coldea et al (2010 Science {\bf 327} 177) report
observation of the weak confinement of kinks in the Ising spin chain
ferromagnet CoNb2O6 at low temperatures. To interpret the entire spectra of
magnetic excitations measured via neutron scattering, they introduce a
phenomenological model, which takes into account only the two-kink
configurations of the spin chain. We present the exact solution of this model.
The explicit expressions for the two-kink bound-state energy spectra and for
the relative intensities of neutron scattering on these magnetic modes are
obtained in terms of the Bessel function.Comment: 18 pages, 9 figures; v2: figures 1,3,4 replaced, few misprints
correcte
- …