64 research outputs found
Magnon Bose condensation in symmetry breaking magnetic field
Magnon Bose condensation (BC)in the symmetry breaking magnetic field is a
result of unusual form of the Zeeman energy, which has terms linear in the
spin-wave operators and terms mixing excitations differ in the Wave-vector of
the magnetic structure. The following examples are considered: simple
easy-plane tetragonal antiferromagnets (AF), frustrated AF family
where etc. and cubic magnets with the Dzyaloshinskii-Moriya
interaction ( etc.). In all cases the BC becomes important when the
magnetic field becomes comparable with the spin-wave gap. The theory is
illustrated by existing experimental results.Comment: Submitted to J. of Phys. Condens. Matter (Proceedings of
International Conference "Highly Frustrated Magnets", Osaka (Japan), August
2006). 8 pages, 5 figure
X-ray and neutron diffraction studies of coupled structural phase transitions in DyBaCoO
A structural transition at K from the to phase
is found to coincide with an anomaly of resistivity. Another structural phase
transition doubling the lattice parameter , which has been postulated
earlier to accompany a low-temperature magnetic transition in
TbBaCoO, is observed in a single crystal DbBaCoO by
means of the X-ray and neutron diffraction. The low temperature phase does not
belong to the space group that has been chosen earlier as the highest
subgroup of the . The transition is of the first order with the
temperature hysteresis, between and K, which
probably explains anomalous magnetic properties in this temperature range.Comment: 6 pages, 4 figure
X-ray Diffraction Study of Superstructure in GdBaCo2O5.5
A single crystal of GdBaCo2O5.47(2) has been studied by means of X-ray
diffraction. Appearance of superstructure reflections at T = 341.5(7) K gives
an evidence of continuous transition to the phase with unit cell doubled along
the shortest edge a1. Critical exponent for the order parameter is found to be
beta=0.33(1). The superstructure reflections are about 2-4 orders of magnitude
weaker than the basic ones. Their systematic extinction indicates the crystal
symmetry change from Pmmm to Pmma. The integrated intensities allow to
calculate displacements of atoms from the positions in the high-temperature
phase. The cobalt-ligand distances in the ordered phase are discussed in terms
of the spin-state/orbital ordering of Co3+ ions.Comment: 4 page
Quantum Phase Transition in Pr2CuO4 to Collinear Spin State in Inclined Magnetic Field: A Neutron Diffraction Observation
In the external field slightly inclined to the - or y-axis of the
frustrated tetragonal atiferromagnet Pr2CuO4, a transition is discovered from
the phase with orthogonal antiferromagnetic spin subsystems along [1,0,0] and
[0,1,0] to the phase with the collinear spins. This phase is shown to be due to
the pseudodipolar interaction, and transforms into the spin-flop phase S perp H
asymptotically at very high field. The discovered phase transition holds at T=0
and is a quantum one, with the transition field being the critical point and
the angle between two subsystems being the order parameter
Polarized beam operation of the Hybrid Spectrometer at the pulsed Spallation Neutron Source
The concept of a neutron Hybrid Spectrometer (HYSPEC) combines the
time-of-flight spectroscopy with the focusing Bragg optics and incorporates a
polarized beam option. Here we describe the polarization analysis scheme
proposed for HYSPEC and quantify its performance via the Monte-Carlo
simulations. We find that the broadband supermirror-bender transmission
polarizers provide reasonably good polarization analysis capability within
about 8-10 meV energy window for scattered neutron energies in the thermal
range up to about 25 meV.Comment: Preprint, to appear in Physica B. 10 pages, 4 figure
Temperature excitation of the Co3+ triplet spin-state in LaCoO3 determined by polarized neutron diffraction
The magnetic moment induced by an external magnetic field of 5.8 T on the
Co3+ ions in LaCoO3 has been measured by polarized neutron diffraction.
Measurements were made at 100 K, near to the spin-state transition, at 1.5 K
and at 285 K. The data give evidence for a transition from the ground singlet
state (S = 0) to the excited triplet intermediate spin state (S = 1). The
energy of K estimated for the high-spin (S = 2) state, is too high to
contribute significantly even at T = 285 K. There is evidence for a small
negative covalent spin density associated with the oxygen ligands amounting to
about one fifth of the aligned Co3+ moment at all three temperatures.Comment: 10 pages, 3 figures, 2 tables, 25 reference
Anomalous spin density distribution on oxygen and Ru in CaSrRuO: A polarised neutron diffraction study
By means of polarized neutron diffraction in a magnetic field of 7.0 T at 1.6
K an anomalously large magnetization density is observed on the in-plane oxygen
in CaSrRuO. Field-induced moments of different ions are
determined by refinement on the flipping ratios, yielding =
0.346(11) , = 0.076(6) and = 0.009(6)
. The moment on the oxygen arises from the strong hybridization
between the Ru-4d and O-2p orbitals. %The maximum entropy method is used for
the %reconstruction of the magnetization density and reveals a strongly
anisotropic The maximum entropy magnetization density reconstruction reveals a
strongly anisotropic density at the Ru site, consistent with the distribution
of the {\it xy} ( band) {\it d}-orbitals.Comment: 4 pages 3 figure
Local distortion in LaCoO3 and PrCoO3: EXAFS, XRD and XANES studies
Room temperature Co K-edge extended x-ray absorption fine structure (EXAFS),
x-ray absorption near edge structure (XANES) including pre-edge and x-ray
diffraction (XRD) studies are carried out on LaCoO3 and PrCoO3. The Co-O,
Co-La/Pr and Co-Co bond lengths are obtained from EXAFS analysis and compared
with those obtained from XRD. The EXAFS analysis of data indicates that CoO6
octahedron is distorted in both LaCoO3 and PrCoO3. Such distortion is expected
in orthorhombic PrCoO3 but not in rhombohedral LaCoO3. This distortion in CoO6
octahedron is attributed to Jahn-Teller active Co3+ ion in intermediate spin
state in these compounds. Higher shell studies reveal that Debye-Waller (DW)
factors of Co-Pr and Co-Co bonds in PrCoO3 are more in comparison to Co-La and
Co-Co bonds in LaCoO3 indicating that these bonds are structurally more
disordered in PrCoO3. The comparison of Co-Co bond lengths and corresponding DW
factors indicates that the structural disorder plays an important role in
deciding the insulating properties of these compounds. XANES studies have shown
changes in the intensities and positions of different near edge features.Comment: 22 pages, 8 figures, 2 tables. To appear in J. Phys.: Condens. Matte
Spin fluctuations in the stacked-triangular antiferromagnet YMnO3
The spectrum of spin fluctuations in the stacked-triangular antiferromagnet
YMnO3 was studied above the Neel temperature using both unpolarized and
polarized inelastic neutron scattering. We find an in-plane and an out-of-plane
excitation. The in-plane mode has two components just above TN, a
resolution-limited central peak and a Debye-like contribution. The
quasi-elastic fluctuations have a line-width that increases with q like Dq^z
and the dynamical exponent z=2.3. The out-of-plane fluctuations have a gap at
the magnetic zone center and do not show any appreciable q-dependence at small
wave-vectors.Comment: JETP LETTERS, in pres
Nonequilibrium relaxation study of the anisotropic antiferromagnetic Heisenberg model on the triangular lattice
Effect of exchange anisotropy on the relaxation time of spin and vector
chirality is studied for the antiferromagnetic classical Heisenberg model on
the triangular lattice by using the nonequilibrium relaxation Monte Carlo
method. We identify the Berezinskii-Kosterlitz-Thouless (BKT) transition and
the chiral transition in a wide range of the anisotropy, even for very small
anisotropy of 0.01%. As the anisotropy decreases, both the critical
temperatures steeply decrease, while the BKT critical region becomes
divergently wide. We elucidate a sharp "V shape" of the phase diagram around
the isotropic Heisenberg point, which suggests that the isotropic case is
exceptionally singular and the associated Z vortex transition will be isolated
from the BKT and chiral transitions. We discuss the relevance of our results to
peculiar behavior of the spin relaxation time observed experimentally in
triangular antiferromagnets.Comment: 5 pages, 4 figures, accepted for publication in J. Phys. Soc. Jp
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