3,096 research outputs found
Neutron Diffuse Scattering from Polar Nanoregions in the Relaxor Pb(Mg1/3Nb2/3)O3
We have studied the neutron diffuse scattering in the relaxor PMN. The
diffuse scattering appears around the Burns temperature (~620K), indicating its
origin from the polar nanoregions (PNR). While the relative diffuse intensities
are consistent with previous reports, they are entirely different from those of
the lowest-energy TO phonon. Because of that, it has been considered that this
TO mode could not be the ferroelectric soft mode. Recently, a neutron
scattering study has unambiguously shown that the TO mode does soften on
cooling. If the diffuse scattering in PMN originates from the soft mode
condensation, then the atomic displacements must satisfy the center of mass
condition. But, the atomic displacements determined from diffuse scattering
intensities do not fulfill this condition. To resolve this contradiction, we
propose a simple model in which the total atomic displacement consists of two
components: is created by the soft mode condensation, satisfying
the center of mass condition, and, represents a uniform
displacement of the PNR along their polar direction relative to the surrounding
(unpolarized) cubic matrix. Within this framework, we can successfully describe
the neutron diffuse scattering intensities observed in PMN.Comment: 7 pages, 7 figures (Revised: 11-16-2001
A Potts model for the distortion transition in LaMnO
The Jahn-Teller distortive transition of \lmo is described by a modified
3-state Potts model. The interactions between the three possible orbits depends
both on the orbits and their relative orientation on the lattice. Values of the
two exchange parameters which are chosen to give the correct low temperature
phase and the correct value for the transition temperature are shown to be
consistent with microscopy theory. The model predicts a first order transitions
and also a value for the entropy above the transition in good agreement with
experiment. The theory with the same parameters also predicts the temperature
dependence of the order parameter of orbital ordering agreeing well with
published experimental results. Finally, the type of the transition is shown to
be close to one of the most disordered phases of the generalised Potts model.
The short range order found experimentally above the transition is investigated
by this model.Comment: 16 pages, 7 figures and no tables. Re-submitted to Phys. Rev.
Quantum Impurities in the Two-Dimensional Spin One-Half Heisenberg Antiferromagnet
The study of randomness in low-dimensional quantum antiferromagnets is at the
forefront of research in the field of strongly correlated electron systems, yet
there have been relatively few experimental model systems. Complementary
neutron scattering and numerical experiments demonstrate that the spin-diluted
Heisenberg antiferromagnet La2Cu(1-z)(Zn,Mg)zO4 is an excellent model material
for square-lattice site percolation in the extreme quantum limit of spin
one-half. Measurements of the ordered moment and spin correlations provide
important quantitative information for tests of theories for this complex
quantum-impurity problem.Comment: 11 pages, 3 figures. NOTE: possible errors in PDF version of Fig. 1.
View postscript version of figure if possibl
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
Magnetism of Superconducting UPt3
The phase diagram of superconducting in pressure-temperature
plane, together with the neutron scattering data is studied within a two
component superconducting order parameter scenario. In order to give a
qualitative explanation to the experimental data a set of two linearly
independent antiferromagnetic moments which emerge appropriately at the
temperature \mbox{} and \mbox{} and
couple to superconductivity is proposed. Several constraints on the fourth
order coefficients in the Ginzburg-Landau free energy are obtained.Comment: 17 pages, figures available on request to
[email protected]
Static magnetic correlations near the insulating-superconducting phase boundary in LaSrCuO
An elastic neutron scattering study has been performed on several single
crystals of LaSrCuO for {\it x} near the lower critical
concentration {\it x} for superconductivity. % In the insulating
spin-glass phase ({\it x} = 0.04 and 0.053), the previously reported
one-dimensional spin modulation along the orthorhombic {\it b}-axis is
confirmed. % Just inside the superconducting phase ({\it x} = 0.06), however,
two pairs of incommensurate magnetic peaks are additionally observed
corresponding to the spin modulation parallel to the tetragonal axes. % These
two types of spin modulations with similar incommensurabilities coexist near
the boundary. % The peak-width along spin-modulation direction
exhibits an anomalous maximum in the superconducting phase near {\it x},
where the incommensurability monotonically increases upon doping
across the phase boundary. % These results are discussed in connection with the
doping-induced superconducting phase transition.Comment: 9pages, 9figure
The Persistence and Memory of Polar Nano-Regions in a Ferroelectric Relaxor Under an Electric Field
The response of polar nanoregions (PNR) in the relaxor compound
Pb[(ZnNb)Ti]O subject to a [111]-oriented
electric field has been studied by neutron diffuse scattering. Contrary to
classical expectations, the diffuse scattering associated with the PNR
persists, and is even partially enhanced by field cooling. The effect of the
external electric field is retained by the PNR after the field is removed. The
``memory'' of the applied field reappears even after heating the system above
, and cooling in zero field
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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