3,034 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
Volume contraction at the Jahn-Teller transition of LaMnO
We have studied the volume collapse of LaMnO at the Jahn- Teller (JT)
transition temperature T=750 K which has recently been found in high
temperature powder x- ray and neutron diffraction experiments. We construct a
model Hamiltonian involving the pseudospin of Mn e states, the
staggered JT distortion and the volume strain coordinate. We show that the
anharmonic coupling between these primary and secondary order parameters leads
to the first order JT phase transition associated with a comparatively large
reduction of the unit cell volume of V/V 10. We explain
the temperature dependence of JT distortions and volume strain and discuss the
volume change as function of the anharmonic coupling constant. A continuous
change to a second order transition as function of model parameters is
obtained. This behaviour is also observed under Ba doping.Comment: 5 pages, 4 figure
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
A transcriptional analysis of carotenoid, chlorophyll and plastidial isoprenoid biosynthesis genes during development and osmotic stress responses in Arabidopsis thaliana
<p>Abstract</p> <p>Background</p> <p>The carotenoids are pure isoprenoids that are essential components of the photosynthetic apparatus and are coordinately synthesized with chlorophylls in chloroplasts. However, little is known about the mechanisms that regulate carotenoid biosynthesis or the mechanisms that coordinate this synthesis with that of chlorophylls and other plastidial synthesized isoprenoid-derived compounds, including quinones, gibberellic acid and abscisic acid. Here, a comprehensive transcriptional analysis of individual carotenoid and isoprenoid-related biosynthesis pathway genes was performed in order to elucidate the role of transcriptional regulation in the coordinated synthesis of these compounds and to identify regulatory components that may mediate this process in <it>Arabidopsis thaliana</it>.</p> <p>Results</p> <p>A global microarray expression correlation analysis revealed that the phytoene synthase gene, which encodes the first dedicated and rate-limiting enzyme of carotenogenesis, is highly co-expressed with many photosynthesis-related genes including many isoprenoid-related biosynthesis pathway genes. Chemical and mutant analysis revealed that induction of the co-expressed genes following germination was dependent on gibberellic acid and brassinosteroids (BR) but was inhibited by abscisic acid (ABA). Mutant analyses further revealed that expression of many of the genes is suppressed in dark grown plants by Phytochrome Interacting transcription Factors (PIFs) and activated by photoactivated phytochromes, which in turn degrade PIFs and mediate a coordinated induction of the genes. The promoters of <it>PSY </it>and the co-expressed genes were found to contain an enrichment in putative BR-auxin response elements and G-boxes, which bind PIFs, further supporting a role for BRs and PIFs in regulating expression of the genes. In osmotically stressed root tissue, transcription of Calvin cycle, methylerythritol 4-phosphate pathway and carotenoid biosynthesis genes is induced and uncoupled from that of chlorophyll biosynthesis genes in a manner that is consistent with the increased synthesis of carotenoid precursors for ABA biosynthesis. In all tissues examined, induction of β-carotene hydroxylase transcript levels are linked to an increased demand for ABA.</p> <p>Conclusions</p> <p>This analysis provides compelling evidence to suggest that coordinated transcriptional regulation of isoprenoid-related biosynthesis pathway genes plays a major role in coordinating the synthesis of functionally related chloroplast localized isoprenoid-derived compounds.</p
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]
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