59 research outputs found

### Magnetic excitations of spin and orbital moments in cobalt oxide

Magnetic and phonon excitations in the antiferromagnet CoO with an unquenched
orbital angular momentum are studied by neutron scattering. Results of energy
scans in several Brillouin zones in the (HHL) plane for energy transfers up to
16 THz are presented. The measurements were performed in the antiferromagnetic
ordered state at 6 K (well below TN~290 K) as well as in the paramagnetic state
at 450 K. Several magnetic excitation modes are identified from the dependence
of their intensity on wavevector and temperature. Within a Hund's rule model
the excitations correspond to fluctuations of coupled orbital and spin degrees
of freedom whose bandwidth is controlled by interionic superexchange. The
different ordering domains give rise to several magnetic peaks at each
wavevector transfer.Comment: Accepted for publication in Canadian Journal of Physic

### Orbital and Spin Excitations in Cobalt Oxide

By means of neutron scattering we have determined new branches of magnetic
excitations in orbitally active CoO (TN=290 K) up to 15 THz and for
temperatures from 6 K to 450 K. Data were taken in the (111) direction in six
single-crystal zones. From the dependence on temperature and Q we have
identified several branches of magnetic excitation. We describe a model for the
coupled orbital and spin states of Co2+ subject to a crystal field and
tetragonal distortion.Comment: To be published in Physica B (Proceedings of SCES07 conference in
Houston

### The Ising-Kondo lattice with transverse field: an f-moment Hamiltonian for URu2Si2?

We study the phase diagram of the Ising-Kondo lattice with transverse
magnetic field as a possible model for the weak-moment heavy-fermion compound
URu2Si2, in terms of two low-lying f singlets in which the uranium moment is
coupled by on-site exchange to the conduction electron spins. In the mean-field
approximation for an extended range of parameters, we show that the conduction
electron magnetization responds logarithmically to f-moment formation, that the
ordered moment in the antiferromagnetic state is anomalously small, and that
the Neel temperature is of the order observed. The model gives a qualitatively
correct temperature-dependence, but not magnitude, of the specific heat. The
majority of the specific heat jump at the Neel temperature arises from the
formation of a spin gap in the conduction electron spectrum. We also discuss
the single-impurity version of the model and speculate on ways to increase the
specific heat coefficient. In the limits of small bandwidth and of small
Ising-Kondo coupling, we find that the model corresponds to anisotropic
Heisenberg and Hubbard models respectively.Comment: 20 pages RevTeX including 5 figures (1 in LaTeX, 4 in uuencoded EPS),
Received by Phys. Rev. B 19 April 199

### Temperature dependence of single particle excitations in a S=1 chain: exact diagonalization calculations compared to neutron scattering experiments

Exact diagonalization calculations of finite antiferromagnetic spin-1
Heisenberg chains at finite temperatures are presented and compared to a recent
inelastic neutron scattering experiment for temperatures T up to 7.5 times the
intrachain exchange constant J. The calculations show that the excitations at
the antiferromagnetic point q=1 and at q=0.5 remain resonant up to at least
T=2J, confirming the recent experimental observation of resonant
high-temperature domain wall excitations. The predicted first and second
moments are in good agreement with experiment, except at temperatures where
three-dimensional spin correlations are most important. The ratio of the
structure factors at q=1 and at q=0.5 is well predicted for the paramagnetic
infinite-temperature limit. For T=2J, however, we found that the experimentally
observed intensity is considerably less than predicted. This suggests that
domain wall excitations on different chains interact up to temperatures of the
order of the spin band width.Comment: 9 pages revtex, submitted to PR

### Evidence for decay of spin waves above the pseudogap of underdoped YBa2Cu3O6.35

The magnetic spectrum at high energies in heavily underdoped YBa2Cu3O6.35 (Tc=18 K) has been determined throughout the Brillouin zone. At low energy, the scattering forms a cone of spin excitations emanating from the antiferromagnetic (0.5,0.5) wave vector with an acoustic velocity similar to that of insulating cuprates. At high-energy transfers, below the maximum energy of 270 meV at (0.5,0), we observe zone-boundary dispersion much larger and spectral weight loss more extensive than in insulating antiferromagnets. Moreover, we report phenomena not found in insulators, an overall lowering of the zone-boundary energies and a large damping of ~100 meV of the spin excitations at high energies. The energy above which the damping occurs coincides approximately with the gap determined from transport measurements. We propose that as the energy is raised, the spin excitations encounter an extra channel of decay into particle-hole pairs of a continuum that we associate with the pseudogap

### Finite Size Scaling for Low Energy Excitations in Integer Heisenberg Spin Chains

In this paper we study the finite size scaling for low energy excitations of
$S=1$ and $S=2$ Heisenberg chains, using the density matrix renormalization
group technique. A crossover from $1/L$ behavior (with $L$ as the chain length)
for medium chain length to $1/L^2$ scaling for long chain length is found for
excitations in the continuum band as the length of the open chain increases.
Topological spin $S=1/2$ excitations are shown to give rise to the two lowest
energy states for both open and periodic $S=1$ chains. In periodic chains these
two excitations are ``confined'' next to each other, while for open chains they
are two free edge 1/2 spins. The finite size scaling of the two lowest energy
excitations of open $S=2$ chains is determined by coupling the two free edge
$S=1$ spins. The gap and correlation length for $S=2$ open Heisenberg chains
are shown to be 0.082 (in units of the exchange $J$) and 47, respectively.Comment: 4 pages (two column), PS file, to be appear as a PRB Brief Repor

### Enhancement of Anisotropy due to Fluctuations in Quasi-One-Dimensional Antiferromagnets

It is shown that the observed anisotropy of magnetization at high magnetic
fields in RbMnBr3 , a quasi-one-dimensional antiferromagnet on a distorted
stacked triangular lattice, is due to quantum and thermal fluctuations. These
fluctuations are taken into account in the framework of linear spin-wave theory
in the region of strong magnetic fields. In this region the divergent
one-dimensional integrals are cut off by magnetic field and the bare easy-plane
anisotropy. Logarithmical dependence on the cutoff leads to the "enhancement"
of the anisotropy in magnetization. Comparison between magnetization data and
our theory with parameters obtained from neutron scattering experiments has
been done.Comment: 15 pages + 5 postscript figures available upon request, RevTex

### Spin-Peierls and Antiferromagnetic Phases in Cu{1-x}Zn{x}GeO{3}: A Neutron Scattering Study

Comprehensive neutron scattering studies were carried out on a series of
high-quality single crystals of Cu_{1-x}Zn_xGeO_3. The Zn concentration, x, was
determined for each sample using Electron Probe Micro-Analysis. The measured Zn
concentrations were found to be 40-80% lower than the nominal values.
Nevertheless the measured concentrations cover a wide range which enables a
systematic study of the effects due to Zn-doping. We have confirmed the
coexistence of spin-Peierls (SP) and antiferromagnetic (AF) orderings at low
temperatures and the measured phase diagram is presented. Most surprisingly,
long-range AF ordering occurs even in the lowest available Zn concentration,
x=0.42%, which places important constraints on theoretical models of the AF-SP
coexistence. Magnetic excitations are also examined in detail. The AF
excitations are sharp at low energies and show no considerable broadening as x
increases indicating that the AF ordering remains long ranged for x up to 4.7%.
On the other hand, the SP phase exhibits increasing disorder as x increases, as
shown from the broadening of the SP excitations as well as the dimer reflection
peaks.Comment: 17 preprint style pages, 9 postscript files included. Submitted to
Phys. Rev. B. Also available from
http://insti.physics.sunysb.edu/~mmartin/pubs.htm

### Magnetic excitations in coupled Haldane spin chains near the quantum critical point

Two quasi-1-dimensional S=1 quantum antiferromagnetic materials, PbNi2V2O8
and SrNi2V2O8, are studied by inelastic neutron scattering on powder samples.
While magnetic interactions in the two systems are found to be very similar,
subtle differences in inter-chain interaction strengths and magnetic anisotropy
are detected. The latter are shown to be responsible for qualitatively
different ground state properties: magnetic long-range order in SrNi2V2O8 and
disordered ``spin liquid'' Haldane-gap state in PbNi2V2O8.Comment: 15 figures, Figs. 5,9, and 10 in color. Some figures in JPEG format.
Complete PostScript and PDF available from
http://papillon.phy.bnl.gov/publicat.ht

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