588 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
Quantum phase transition in the Plaquette lattice with anisotropic spin exchange
I study the influence of anisotropic spin exchange on a quantum phase
transition in the Plaquette lattice driven by the purely quantum effect of
singlet formation. I study the influence of i) a Dzyaloshinskii-Moriya exchange
and ii) four spin exchange on the transition point by evaluating spin--spin
correlations and the spin gap with exact diagonalization. The results point to
a stabilization of the Neel-like long range order when the
Dzyaloshinskii-Moriya exchange is added, whereas the four-spin exchange might
stabilize the singlet order as well as the Neel-like order depending on its
strength.Comment: LaTeX article with 4 pages and 3 figures, prepared with material for
the ICM 200
Gender discourse, awareness, and alternative responses for men in everyday living
In this paper, the authors use examples from their experiences to explore the nuances and complexities of contemporary gender practices. They draw on discourse and positioning theories to identify the ways in which culturally dominant, and difficult to notice, gender constructions help shape everyday experiences. In addition, the authors share their view that there are benefits in developing skills in noticing contemporary practices made available by dominant gender constructions. Such noticing expands possibilities for ways of responding and relating that might produce outcomes for men and women that fit with their hopes for living
The Quasi-1D S=1/2 Antiferromagnet Cs2CuCl4 in a Magnetic Field
Magnetic excitations of the quasi-1D S=1/2 Heisenberg antiferromagnet (HAF)
Cs2CuCl4 have been measured as a function of magnetic field using neutron
scattering. For T<0.62 K and B=0 T the weak inter-chain coupling produces 3D
incommensurate ordering. Fields greater than Bc =1.66 T, but less than the
field (~8 T) required to fully align the spins, are observed to decouple the
chains, and the system enters a disordered intermediate-field phase (IFP). The
IFP excitations are in agreement with the predictions of Muller et al. for the
1D S=1/2 HAF, and Talstra and Haldane for the related 1/r^2 chain (the
Haldane-Shastry model). This behaviour is inconsistent with linear spin-wave
theory.Comment: 10 pages, 4 encapsulated postscript figures, LaTeX, to be published
in PRL, e-mail comments to [email protected]
Fermi-surface topology and the effects of intrinsic disorder in a class of charge-transfer salts containing magnetic ions: β" — (BEDT — TTF)₄ [(H₃O)M(C₂O₄)₃]Υ (M = Ga, Cr, Fr; Υ = C₅H₅N)
We report high-field magnetotransport measurements on β" — (BEDT — TTF)₄ [(H₃O)M(C₂O₄)₃]Υ, where M =Ga, Cr and Fe and Υ = C₅H₅N. We observe similar Shubnikov–de Haas oscillations in all compounds, attributable to four quasi-two-dimensional Fermi-surface pockets, the largest of which corresponds to a cross-sectional area ≈ 8.5% of the Brillouin zone. The cross-sectional areas of the pockets are in agreement with the expectations for a compensated semimetal, and the corresponding effective masses are ∼mₑ, rather small compared to those of other BEDT-TTF salts. Apart from the case of the smallest Fermi-surface pocket, varying the M ion seems to have little effect on the overall Fermi-surface topology or on the effective masses. Despite the fact that all samples show quantum oscillations at low temperatures, indicative of Fermi liquid behavior, the sample and temperature dependence of the interlayer resistivity suggest that these systems are intrinsically inhomogeneous. It is thought that intrinsic tendency to disorder in the anions and/or the ethylene groups of the BEDT-TTF molecules leads to the coexistence of insulating and metallic states at low temperatures. A notional phase diagram is given for the general family of β" — (BEDT — TTF)₄ [(H₃O)M(C₂O₄)₃]Υ salts
Dimensionality-driven spin-flop transition in quasi-one-dimensional PrBa2Cu4O8
In the quasi-one-dimensional cuprate PrBaCuO, the Pr cations
order antiferromagnetically at 17 K in zero field. Through a combination of
magnetic susceptibility, torque magnetometry, specific heat and interchain
transport measurements, the anisotropic temperature-magnetic field phase
diagram associated with this ordering has been mapped out. A low-temperature
spin-flop transition in the Pr sub-lattice is found to occur at the same
magnetic field strength and orientation as a dimensional crossover in the
ground state of the metallic CuO chains. This coincidence suggests that the
spin reorientation is driven by a change in the anisotropic
Rudermann-Kittel-Kasuya-Yosida (RKKY) interaction induced by a corresponding
change in effective dimensionality of the conduction electrons.Comment: 8 pages, 8 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
Theoretical Analysis of the Reduction of Neel Temperature in La(CuZn(or Mg)O
Using Tyablikov's decoupling approximation, we calculate the initial
suppression rate of the Neel temperature, , in a quasi two-dimensional diluted Heisenberg antiferromagnet with
nonmagnetic impurities of concentration . In order to explain an
experimental fact that of the Zn-substitution is different
from of the Mg-substitution, we propose a model in which
impurity substitution reduces the intra-plane exchange couplings surrounding
impurities, as well as dilution of spin systems. The decrease of 12% in
exchange coupling constants by Zn substitution and decrease of 6% by Mg
substitution explain those two experimental results, when an appropriate value
of the interplane coupling is used.Comment: 2 pages, 3 figure
Quasiparticle Breakdown and Spin Hamiltonian of the Frustrated Quantum Pyrochlore YbTiO in Magnetic Field
The frustrated pyrochlore magnet YbTiO has the remarkable
property that it orders magnetically, but has no propagating magnons over wide
regions of the Brillouin zone. Here we use inelastic neutron scattering to
follow how the spectrum evolves in cubic-axis magnetic fields. At high fields
we observe in addition to dispersive magnons also a two-magnon continuum, which
grows in intensity upon reducing the field and overlaps with the one-magnon
states at intermediate fields leading to strong renormalization of the
dispersion relations, and magnon decays. Using heat capacity measurements we
find that the low and high field regions are smoothly connected with no sharp
phase transition, with the spin gap increasing monotonically in field. Through
fits to an extensive data set we re-evaluate the spin Hamiltonian finding
dominant quantum exchange terms, which we propose are responsible for the
anomalously strong fluctuations and quasiparticle breakdown effects observed at
low fields.Comment: 5 pages main text + 19 pages supplemental materia
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