43,682 research outputs found
Hydrogen adsorption and phase transitions in fullerite
Hydrogen desorption and adsorption properties of the fullerene materials C60, C70, and fullerite (a mixture of C60 and C70) were measured volumetrically using a Sievert's apparatus. Over several cycles of isotherm measurements at 77 K, the hydrogen storage capacities of one of the fullerite samples increased from an initial value of 0.4 wt % for the first cycle to a capacity of 4.4 wt % for the fourth cycle. Correspondingly, the surface area of this sample increased from 0.9 to 11 m^2/g, and there were changes in its x-ray powder diffraction pattern. In comparison, two other fullerite samples, prepared by a different procedure showed no such behavior. Pure C60 and pure C70 were also cycled and exhibited small and constant capacities of 0.7 and 0.33 wt %, respectively, as a function of number of cycles. The enhanced storage capacity of fullerite material is tentatively attributed to the presence of C60 oxide
A statistical model approximation for perovskite solid-solutions: a Raman study of lead-zirconate-titanate single crystal
Lead titanate (PbTiO3) is a classical example of a ferroelectric perovskite
oxide illustrating a displacive phase transition accompanied by a softening of
a symmetry-breaking mode. The underlying assumption justifying the soft-mode
theory is that the crystal is macroscopically sufficiently uniform so that a
meaningful free energy function can be formed. In contrast to PbTiO3,
experimental studies show that the phase transition behaviour of
lead-zirconate-titanate solid solution (PZT) is far more subtle. Most of the
studies on the PZT system have been dedicated to ceramic or powder samples, in
which case an unambiguous soft-mode study is not possible, as modes with
different symmetries appear together. Our Raman scattering study on
titanium-rich PZT single crystal shows that the phase transitions in PZT cannot
be described by a simple soft-mode theory. In strong contrast to PbTiO3,
splitting of transverse E-symmetry modes reveals that there are different
locally-ordered regions. The role of crystal defects, random distribution of Ti
and Zr at the B-cation site and Pb ions shifted away from their ideal
positions, dictates the phase transition mechanism. A statistical model
explaining the observed peak splitting and phase transformation to a complex
state with spatially varying local order in the vicinity of the morphotropic
phase boundary is given.Comment: Article contains four black-and-white figures, one colour figure and
one Table. Symmetry analysis and details of the model are given in Appendices
I and II, respectivel
Topological Quantum Phase Transition in Synthetic Non-Abelian Gauge Potential
The method of synthetic gauge potentials opens up a new avenue for our
understanding and discovering novel quantum states of matter. We investigate
the topological quantum phase transition of Fermi gases trapped in a honeycomb
lattice in the presence of a synthetic non- Abelian gauge potential. We develop
a systematic fermionic effective field theory to describe a topological quantum
phase transition tuned by the non-Abelian gauge potential and ex- plore its
various important experimental consequences. Numerical calculations on lattice
scales are performed to compare with the results achieved by the fermionic
effective field theory. Several possible experimental detection methods of
topological quantum phase tran- sition are proposed. In contrast to condensed
matter experiments where only gauge invariant quantities can be measured, both
gauge invariant and non-gauge invariant quantities can be measured by
experimentally generating various non-Abelian gauges corresponding to the same
set of Wilson loops
A Fast Modal Space Transform for Robust Nonrigid Shape Retrieval
Nonrigid or deformable 3D objects are common in many application domains. Retrieval of such objects in large databases based on shape similarity is still a challenging problem. In this paper, we take advantages of functional operators as characterizations of shape deformation, and further propose a framework to design novel shape signatures for encoding nonrigid geometries. Our approach constructs a context-aware integral kernel operator on a manifold, then applies modal analysis to map this operator into a low-frequency functional representation, called fast functional transform, and finally computes its spectrum as the shape signature. In a nutshell, our method is fast, isometry-invariant, discriminative, smooth and numerically stable with respect to multiple types of perturbations. Experimental results demonstrate that our new shape signature for nonrigid objects can outperform all methods participating in the nonrigid track of the SHREC’11 contest. It is also the second best performing method in the real human model track of SHREC’14.postprin
Magnetic Interaction in the Geometrically Frustrated Triangular Lattice Antiferromagnet
The spin wave excitations of the geometrically frustrated triangular lattice
antiferromagnet (TLA) have been measured using high resolution
inelastic neutron scattering. Antiferromagnetic interactions up to third
nearest neighbors in the ab plane (J_1, J_2, J_3, with
and ), as well as out-of-plane coupling (J_z, with
) are required to describe the spin wave dispersion
relations, indicating a three dimensional character of the magnetic
interactions. Two energy dips in the spin wave dispersion occur at the
incommensurate wavevectors associated with multiferroic phase, and can be
interpreted as dynamic precursors to the magnetoelectric behavior in this
system.Comment: 4 pages, 4 figures, published in Phys. Rev. Let
The complex multiferroic phase diagram of MnCoWO
The complete magnetic and multiferroic phase diagram of
MnCoWO single crystals is investigated by means of magnetic,
heat capacity, and polarization experiments. We show that the ferroelectric
polarization in the multiferroic state abruptly changes
its direction twice upon increasing Co content, x. At x=0.075,
rotates from the axis into the plane and at
x=0.15 it flips back to the axis. The origin of the multiple
polarization flops is identified as an effect of the Co anisotropy on the
orientation and shape of the spin helix leading to thermodynamic instabilities
caused by the decrease of the magnitude of the polarization in the
corresponding phases. A qualitative description of the ferroelectric
polarization is derived by taking into account the intrachain (axis) as
well as the interchain (axis) exchange pathways connecting the magnetic
ions. In a narrow Co concentration range (0.1x0.15), an
intermediate phase, sandwiched between the collinear high-temperature and the
helical low-temperature phases, is discovered. The new phase exhibits a
collinear and commensurate spin modulation similar to the low-temperature
magnetic structure of MnWO.Comment: 18 pages, 6 figure
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