1,208 research outputs found
Spin-1/2 Triangular Lattice with Orbital Degeneracy in a Metallic Oxide Ag2NiO2
A novel metallic and magnetic transition metal oxide Ag2NiO2 is studied by
means of resistivity, magnetic susceptibility, specific heat and X-ray
diffraction. The crystal structure is characterized by alternating stacking of
a Ni3+O2 layer and a (Ag2)+ layer, the former realizing a spin-1/2 triangular
lattice with eg orbital degeneracy and the latter providing itinerant
electrons. It is found that the NiO2 layer exhibits orbital ordering at Ts =
260 K and antiferromagnetic spin ordering at TN = 56 K. Moreover, a moderately
large mass enhancement is found for the itinerant electrons, suggesting a
significant contribution from the nearly localized Ni 3d state to the Ag 5s
state that forms a broad band.Comment: 9 pages, 5 figures, to be published in Rapid Communications, Phys.
Rev.
Tuning the spin dynamics of kagome systems
Despite the conceptional importance of realizing spin liquids in solid states
only few compounds are known. On the other side the effect of lattice
distortions and anisotropies on the magnetic exchange topology and the
fluctuation spectrum are an interesting problem. We compare the excitation
spectra of the two s=1/2 kagome lattice compounds volborthite and vesignieite
using Raman scattering. We demonstrate that even small modifications of the
crystal structure may have a huge effect on the phonon spectrum and low
temperature properties.Comment: 3 pages, 2 figure
The second phase transition in the pyrochlore oxide Cd2Re2O7
Evidence for another phase transition at 120 K in the metallic pyrochlore
oxide Cd2Re2O7, following the structural transition at 200 K and followed by
the superconducting transition at 1.0 K, is given through resistivity,
magnetoresistance, specific heat, and X-ray diffraction measurements. The
results indicate unique successive structural and electronic transitions
occurring in the pyrochlore compound, revealing an interesting interplay
between the crystal and electronic structures on the itinerant electron system
in the pyrochlore lattice
Structural Order Parameter in the Pyrochlore Superconductor Cd2Re2O7
It is shown that both structural phase transitions in Cd2Re2O7, which occur
at T_{s1}=200 K and T_{s2}=120 K, are due to an instability of the Re
tetrahedral network with respect to the same doubly degenerate long-wavelength
phonon mode. The primary structural order parameter transforms according to the
irreducible representation E_u of the point group O_h. We argue that the
transition at T_{s1} may be of second order, in accordance with experimental
data. We obtain the phase diagram in the space of phenomenological parameters
and propose a thermodynamic path that Cd2Re2O7 follows upon cooling. Couplings
of the itinerant electronic system and localized spin states in pyrochlores and
spinels to atomic displacements are discussed.Comment: 5 pages. Submitted to J. Phys. Soc. Jpn. Best quality figures are
available at http://www.physics.mun.ca/~isergien/pubs.htm
Kagom\'{e} ice state in the dipolar spin ice Dy_{2}Ti_{2}O_{7}
We have investigated the kagom\'{e} ice behavior of the dipolar spin-ice
compound Dy_{2}Ti_{2}O_{7} in magnetic field along a [111] direction using
neutron scattering and Monte Carlo simulations. The spin correlations show that
the kagom\'{e} ice behavior predicted for the nearest-neighbor (NN) interacting
model, where the field induces dimensional reduction and spins are frustrated
in each two-dimensional kagom\'{e} lattice, occurs in the dipole interacting
system. The spins freeze at low temperatures within the macroscopically
degenerate ground states of the NN model.Comment: 5 pages, 3 figures, submitted to PR
A Possible Phase Transition in beta-pyrochlore Compounds
We investigate a lattice of interacting anharmonic oscillators by using a
mean field theory and exact diagonalization. We construct an effective
five-state hopping model with intersite repulsions as a model for
beta-pyrochlore AOs_2O_6(A=K, Rb or Cs). We obtain the first order phase
transition line from large to small oscillation amplitude phases as temperature
decreases. We also discuss the possibility of a phase with local electric
polarizations. Our theory can explain the origin of the mysterious first order
transition in KOs_2O_6.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp
Heterogeneous spin state in the field-induced phase of volborthite as seen via 51V nuclear magnetic resonance
We report results of 51V NMR in the field-induced phase of volborthite
Cu3V2O7(OH)dot2H2O, a spin-1/2 antiferromagnet on a distorted kagome lattice.
In magnetic fields above 4.5 T, two types of V sites with different spin-echo
decay rates are observed. The hyperfine field at the fast decaying sites has a
distribution, while it is more homogeneous at the slowly decaying sites. Our
results indicate a heterogeneous state consisting of two spatially alternating
Cu spin systems, one of which exhibits anomalous spin fluctuations contrasting
with the other showing a conventional static order.Comment: 5 pages, 4 figure
Vesignieite BaCu3V2O8(OH)2 as a Candidate Spin-1/2 Kagome Antiferromagnet
A polycrystalline sample of vesignieite BaCu3V2O8(OH)2 comprising a nearly
ideal kagome lattice composed of Cu2+ ions carrying spin 1/2 has been
synthesized and studied by magnetization and heat capacity measurements.
Magnetic susceptibility shows a neither long range order, a spin glass
transition nor a spin gap down to 2 K, in spite of a moderately strong
antiferromagnetic interaction of J/kB = 53 K between nearest-neighbor spins. A
broad peak observed at a temperature corresponding to 0.4J in intrinsic
magnetic susceptibility indicates a marked development of the short-range
order. The ground state of vesignieite is probably a gapless spin liquid or is
accompanied by a very small gap less than J/30.Comment: 4 pages, 5 figure
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