738 research outputs found
Hopping and clustering of oxygen vacancies in SrTiO3 by anelastic relaxation
The complex elastic compliance s11(w,T) of SrTiO3-d has been measured as a
function of the O deficiency d < 0.01. The two main relaxation peaks in the
absorption are identified with hopping of isolated O vacancies over a barrier
of 0.60 eV and reorientation of pairs of vacancies involving a barrier of 1 eV.
The pair binding energy is ~0.2 eV and indications for additional clustering,
possibly into chains, is found already at d ~0.004. The anistropic component of
the elastic dipole of an O vacancy is Deltalambda = 0.026.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Non-linear Elastic Response in Solid Helium: critical velocity or strain
Torsional oscillator experiments show evidence of mass decoupling in solid
4He. This decoupling is amplitude dependent, suggesting a critical velocity for
supersolidity. We observe similar behavior in the elastic shear modulus. By
measuring the shear modulus over a wide frequency range, we can distinguish
between an amplitude dependence which depends on velocity and one which depends
on some other parameter like displacement. In contrast to the torsional
oscillator behavior, the modulus depends on the magnitude of stress, not
velocity. We interpret our results in terms of the motion of dislocations which
are weakly pinned by 3He impurities but which break away when large stresses
are applied
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Phase transitions and phase diagram of the ferroelectric perovskite NBT-BT by anelastic and dielectric measurements
The complex elastic compliance and dielectric susceptibility of
(Na_{0.5}Bi_{0.5})_{1-x}Ba_{x}TiO_{3} (NBT-BT) have been measured in the
composition range between pure NBT and the morphotropic phase boundary
included, 0 <= x <= 0.08. The compliance of NBT presents sharp peaks at the
rhombohedral/tetragonal and tetragonal/cubic transitions, allowing the
determination of the tetragonal region of the phase diagram, up to now
impossible due to the strong lattice disorder and small distortions and
polarizations involved. In spite of ample evidence of disorder and structural
heterogeneity, the R-T transition remains sharp up to x = 0.06, whereas the T-C
transition merges into the diffuse and relaxor-like transition associated with
broad maxima of the dielectric and elastic susceptibilities. An attempt is made
at relating the different features in the anelastic and dielectric curves to
different modes of octahedral rotations and polar cation shifts. The
possibility is also considered that the cation displacements locally have
monoclinic symmetry, as for PZT near the morphotropic phase boundary.Comment: 11 pages, 9 figures, submitted to Phys. Rev.
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