88 research outputs found
Simulations of cubic-tetragonal ferroelastics
We study domain patterns in cubic-tetragonal ferroelastics by solving
numerically equations of motion derived from a Landau model of the phase
transition, including dissipative stresses. Our system sizes, of up to 256^3
points, are large enough to reveal many structures observed experimentally.
Most patterns found at late stages in the relaxation are multiply banded; all
three tetragonal variants appear, but inequivalently. Two of the variants form
broad primary bands; the third intrudes into the others to form narrow
secondary bands with the hosts. On colliding with walls between the primary
variants, the third either terminates or forms a chevron. The multipy banded
patterns, with the two domain sizes, the chevrons and the terminations, are
seen in the microscopy of zirconia and other cubic-tetragonal ferroelastics. We
examine also transient structures obtained much earlier in the relaxation;
these show the above features and others also observed in experiment.Comment: 7 pages, 6 colour figures not embedded in text. Major revisions in
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Theory of Metal-Insulator Transition in PrRu4P12 and PrFe4P12
All symmetry allowed couplings between the 4f^2-electron ground state doublet
of trivalent praseodymium in PrRu4P12 and PrFe4P12 and displacements of the
phosphorus, iron or ruthenium ions are considered. Two types of displacements
can change the crystal lattice from body-centred cubic to simple orthorhombic
or to simple cubic. The first type lowers the point group symmetry from
tetrahedral to orthorhombic, while the second type leaves it unchanged, with
corresponding space group reductions Im3 --> Pmmm and Im3 --> Pm3 respectively.
In former case, the lower point-group symmetry splits the degeneracy of the
4f^2 doublet into states with opposite quadrupole moment, which then leads to
anti-quadrupolar ordering, as in PrFe4P12. Either kind of displacement may
conspire with nesting of the Fermi surface to cause the metal-insulator or
partial metal-insulator transition observed in PrFe4P12 and PrRu4P12. We
investigate this scenario using band-structure calculations, and it is found
that displacements of the phosphorus ions in PrRu4P12 (with space group
reduction Im3 --> Pm3) open a gap everywhere on the Fermi surface.Comment: 6 page
Impurity induced density of states and residual transport in nonunitary superconductors
We obtain general expressions for the residual density of states, electrical
conductivity and thermal conductivity for non-unitary superconductors due to
impurity scattering. We apply the results to the so-called `B phase' of
PrOs4Sb12, which we describe using a non-unitary gap function derived from
symmetry considerations. The conductivity tensor has inequivalent diagonal
components due to off-axis nodal positions which may be detectable in
experiments.Comment: 8 pages, 1 figur
Corrigendum to “Femur associated with the Deep Skull from the West Mouth of the Niah Caves (Sarawak, Malaysia)” [Journal of Human Evolution 127 (2019) 133–148](S0047248418301520)(10.1016/j.jhevol.2018.12.008)
In the original analyses of the Deep Skull femoral midshaft section, anteroposterior (AP) and mediolateral (ML) orientations were transposed, that is, AP and ML second moments of area, as well as linear external breadths, were reversed. Cross-sectional properties were also calculated using an approximate method employing radiographic breadths, while a more accurate method using actual periosteal and endosteal contours is possible given that a complete section is available (MomentMacro; https://fae.johnshopkins.edu/chris-ruff/). When this method is applied and orientations are corrected, the following properties are obtained. C.B. Ruff has been added to the author list of this article as he carried out the analyses that resulted in the new cross-sectional properties. [Table presented] The authors apologize for the original error
Order parameter in superconductors with non-degenerate bands
In noncentrosymmetric metals, the spin degeneracy of the electronic bands is
lifted by spin-orbit coupling. We consider general symmetry properties of the
pairing function Delta(k) in noncentrosymmetric superconductors with spin-orbit
coupling (NSC), including CePt3Si, UIr and Cd2Re2O7. We find that Delta(k) =
chi(k) t(k), where chi(k) is an even function which transforms according to the
irreducible representations of the crystallographic point group and t(k) is a
model dependent phase factor. We consider tunnelling between a NSC and a
conventional superconductor. It is found that, in terms of thermodynamical
properties as well as the Josephson effect, the state of NSC resembles a
singlet superconducting state with gap function chi(k).Comment: 8 pages, references updated. Accepted to PR
Superconducting states in the tetrahedral compound PrOs4Sb12
We find possible superconducting states for tetrahedral (Th) symmetry
crystals with strong spin-orbit coupling using Landau theory. Additional
symmetry breaking within the superconducting state is considered. We discuss
nodes of the gap functions for the different states, secondary superconducting
order parameters and coupling to the elastic strain. By comparing our results
to experiments, we find that superconductivity in PrOs4Sb12 is best described
by the three-dimensional representations of point group Th.Comment: 9 pages, 2 figures. Expanded version submitted to Physical Review
Quantum Oscillations of Electrons and of Composite Fermions in Two Dimensions: Beyond the Luttinger Expansion
Quantum oscillation phenomena, in conventional 2-dimensional electron systems
and in the fractional quantum Hall effect, are usually treated in the
Lifshitz-Kosevich formalism. This is justified in three dimensions by
Luttinger's expansion, in the parameter . We show that in two
dimensions this expansion breaks down, and derive a new expression, exact in
the limit where rainbow graphs dominate the self-energy. Application of our
results to the fractional quantum Hall effect near half-filling shows very
strong deviations from Lifshitz-Kosevich behaviour. We expect that such
deviations will be important in any strongly-interacting 2-dimensional
electronic system.Comment: 4 pages, 3 figures, LaTe
Metallic "Ferroelectricity" in the Pyrochlore Cd2Re2O7
A class of materials known as ``ferroelectric metals'' was discussed
theoretically by Anderson and Blount in 1965 [Phys. Rev. Lett. 14, 217 (1965)],
but to date no examples of this class have been reported. Here we present
measurements of the elastic moduli of Cd2Re2O7 through the 200 K
cubic-to-tetragonal phase transition. A Landau analysis of the moduli reveals
that the transition is consistent with Cd2Re2O7 being classified as a
``ferroelectric metal'' in the weaker sense described by Anderson and Blount
(loss of a center of symmetry). First-principles calculations of the lattice
instabilities indicate that the dominant lattice instability corresponds to a
two-fold degenerate mode with Eu symmetry, and that motions of the O ions
forming the O octahedra dominate the energetics of the transition.Comment: 4 pages, 2 figure
Twin wall of cubic-tetragonal ferroelastics
We derive solutions for the twin wall linking two tetragonal variants of the
cubic-tetragonal ferroelastic transformation, including for the first time the
dilatational and shear energies and strains. Our solutions satisfy the
compatibility relations exactly and are obtained at all temperatures. They
require four non-vanishing strains except at the Barsch-Krumhansl temperature
TBK (where only the two deviatoric strains are needed). Between the critical
temperature and TBK, material in the wall region is dilated, while below TBK it
is compressed. In agreement with experiment and more general theory, the twin
wall lies in a cubic 110-type plane. We obtain the wall energy numerically as a
function of temperature and we derive a simple estimate which agrees well with
these values.Comment: 4 pages (revtex), 3 figure
Statics and dynamics of domain patterns in hexagonal-orthorhombic ferroelastics
We study the statics and the dynamics of domain patterns in proper
hexagonal-orthorhombic ferroelastics; these patterns are of particular interest
because they provide a rare physical realization of disclinations in crystals.
Both our static and dynamical theories are based entirely on classical,
nonlinear elasticity theory; we use the minimal theory consistent with
stability, symmetry and ability to explain qualitatively the observed patterns.
After scaling, the only parameters of the static theory are a temperature
variable and a stiffness variable. For moderate to large stiffness, our static
results show nested stars, unnested stars, fans and other nodes, triangular and
trapezoidal regions of trapped hexagonal phase, etc observed in electron
microscopy of Ta4N and Mg-Cd alloys, and also in lead orthovanadate (which is
trigonal-monoclinic); we even find imperfections in some nodes, like those
observed. For small stiffness, we find patterns like those observed in the
mineral Mg-cordierite. Our dynamical studies of growth and relaxation show the
formation of these static patterns, and also transitory structures such as
12-armed bursts, streamers and striations which are also seen experimentally.
The major aspects of the growth-relaxation process are quite unlike those in
systems with conventional order parameters, for it is inherently nonlocal; for
example, the changes from one snapshot to the next are not predictable by
inspection.Comment: 9 pages, 3 figures (1 b&w, 2 colour); animations may be viewed at
http://huron.physics.utoronto.ca/~curnoe/sim.htm
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