2,497 research outputs found
Density matrix renormalisation group for a quantum spin chain at non-zero temperature
We apply a recent adaptation of White's density matrix renormalisation group
(DMRG) method to a simple quantum spin model, the dimerised chain, in
order to assess the applicabilty of the DMRG to quantum systems at non-zero
temperature. We find that very reasonable results can be obtained for the
thermodynamic functions down to low temperatures using a very small basis set.
Low temperature results are found to be most accurate in the case when there is
a substantial energy gap.Comment: 6 pages, Standard Latex File + 7 PostScript figures available on
reques
Density matrix renormalisation group study of the correlation function of the bilinear-biquadratic spin-1 chain
Using the recently developed density matrix renormalization group approach,
we study the correlation function of the spin-1 chain with quadratic and
biquadratic interactions. This allows us to define and calculate the
periodicity of the ground state which differs markedly from that in the
classical analogue. Combining our results with other studies, we predict three
phases in the region where the quadratic and biquadratic terms are both
positive.Comment: 13 pages, Standard Latex File + 5 PostScript figures in separate (New
version with SUBSTANTIAL REVISIONS to appear in J Phys A
Dynamical effects of the nanometer-sized polarized domains in Pb(Zn1/3Nb2/3)O3
Recent neutron scattering measurements performed on the relaxor ferroelectric
Pb[(Zn1/3Nb2/3)0.92Ti0.08]O3 (PZN-8%PT) in its cubic phase at 500 K, have
revealed an anomalous ridge of inelastic scattering centered ~0.2 A-1 from the
zone center (Gehring et al., Phys. Rev. Lett. 84, 5216 (2000)). This ridge of
scattering resembles a waterfall when plotted as a phonon dispersion diagram,
and extends vertically from the transverse acoustic (TA) branch near 4 meV to
the transverse optic (TO) branch near 9 meV. No zone center optic mode was
found. We report new results from an extensive neutron scattering study of pure
PZN that exhibits the same waterfall feature. We are able to model the dynamics
of the waterfall using a simple coupled-mode model that assumes a strongly
q-dependent optic mode linewidth Gamma1(q) that increases sharply near 0.2 A-1
as one approaches the zone center. This model was motivated by the results of
Burns and Dacol in 1983, who observed the formation of a randomly-oriented
local polarization in PZN at temperatures far above its ferroelectric phase
transition temperature. The dramatic increase in Gamma1 is believed to occur
when the wavelength of the optic mode becomes comparable to the size of the
small polarized micro-regions (PMR) associated with this randomly-oriented
local polarization, with the consequence that longer wavelength optic modes
cannot propagate and become overdamped. Below Tc=410 K, the intensity of the
waterfall diminishes. At lowest temperatures ~30 K the waterfall is absent, and
we observe the recovery of a zone center transverse optic mode near 10.5 meV.Comment: 8 pages, 9 figures (one color). Submitted to Physical Review
Anomalous transverse acoustic phonon broadening in the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O_3
The intrinsic linewidth of the transverse acoustic (TA) phonon
observed in the relaxor ferroelectric compound
Pb(MgNbTiO (PMN-20%PT) begins to broaden
with decreasing temperature around 650 K, nearly 300 K above the ferroelectric
transition temperature ( K). We speculate that this anomalous
behavior is directly related to the condensation of polarized, nanometer-sized,
regions at the Burns temperature . We also observe the ``waterfall''
anomaly previously seen in pure PMN, in which the transverse optic (TO) branch
appears to drop precipitously into the TA branch at a finite momentum transfer
\AA. The waterfall feature is seen even at
temperatures above . This latter result suggests that the PNR exist as
dynamic entities above .Comment: 6 pages, 4 figure
Elastic anomaly of heavy fermion systems in a crystalline field
An elastic anomaly, observed in the heavy fermi liquid state of Ce alloys
(for example, CeCu and CeTe), is analyzed by using the infinite-
Anderson lattice model. The four atomic energy levels are assumed for
f-electrons. Two of them are mutually degenerate. A small crystalline splitting
is assumed between two energy levels. The fourfold degenerate
conduction bands are also considered in the model. We solve the model using the
mean field approximation to slave bosons, changing the Fermi energy in order to
keep the total electron number constant. The nonzero value of the mean field of
the slave bosons persists over the temperatures much higher than the Kondo
temperature. This is the effect of the constant electron number. Next, the
linear susceptibility with respect to is calculated in order to obtain
the renomalized elastic constant. The resulting temperature dependence of the
constant shows the downward dip. We point out the relation of our finding with
the experimental data.Comment: submitted to J. Phys.: Condens. Matter, please request figure copies
to [email protected]
Interface driven magnetoelectric effects in granular CrO2
Antiferromagnetic and magnetoelectric Cr2O3-surfaces strongly affect the
electronic properties in half metallic CrO2. We show the presence of a Cr2O3
surface layer on CrO3 grains by high-resolution transmission electron
microscopy. The effect of these surface layers is demonstrated by measurements
of the temperature variation of the magnetoelectric susceptibility. A major
observation is a sign change at about 100 K followed by a monotonic rise as a
function of temperature. These electric field induced moments in CrO3 are
correlated with the magnetoelectric susceptibility of pure Cr2O3. This study
indicates that it is important to take into account the magnetoelectric
character of thin surface layers of Cr2O3 in granular CrO2 for better
understanding the transport mechanism in this system. The observation of a
finite magnetoelectric susceptibility near room temperature may find utility in
device applications.Comment: Figure 1 with strongly reduced resolutio
Vibrations of the cubane molecule: inelastic neutron scattering study and theory
Cataloged from PDF version of article.Cubane (C8H8) is an immensely strained molecule whose C-C-C bond angle is 90 degrees rather than 109.5 degrees as expected for sp(3) bonding of carbon. We have measured the intramolecular vibrational spectrum of cubane using inelastic neutron scattering. The neutron data are used to test the transferability of various phenomenological potentials and tight-binding models to this highly strained molecule. Unlike these models, first-principles calculations of the INS spectrum (both energy and intensity) agree well with the experimental data. (C) 1999 Published by Elsevier Science B.V. All rights reserved
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