465 research outputs found
Strain-induced interface reconstruction in epitaxial heterostructures
We investigate in the framework of Landau theory the distortion of the strain
fields at the interface of two dissimilar ferroelastic oxides that undergo a
structural cubic-to-tetragonal phase transition. Simple analytical solutions
are derived for the dilatational and the order parameter strains that are
globally valid over the whole of the heterostructure. The solutions reveal that
the dilatational strain exhibits compression close to the interface which may
in turn affect the electronic properties in that region.Comment: 7 pages, 5 figures, to be published in Physical Review
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Piezoelectricity and electrostriction in ferroelastic materials with polar twin boundaries and domain junctions
Weak piezoelectricity, compared with electrostriction, occurs in twinned ferroelastic materials even when the uniform bulk material is centro-symmetric. In a simple computer simulation, polarity is exclusively generated by the flexoelectric effect. Simple twinned structures (parallel twin walls) are electrostrictive and show no piezoelectricity. Complex twinned structures break inversion symmetry by the simultaneous appearance of junctions, kinks, needle domains, etc. Such structures show weak piezoelectricity (d ∼ 10−4 pm/V) under periodic boundary conditions together with significant electrostriction. The macroscopic piezoelectric response is stronger (d ∼ 10−3 pm/V) under free boundary conditions due to the effect of relaxing surfaces.EPSR
Effect of internal friction on transformation twin dynamics in SrxBa1-xSnO3 perovskite
The dynamics of transformation twins in SrxBa1-xSnO3 (x=0.6,0.8) perovskite
has been studied by dynamical mechanical analysis in three-point bend geometry.
This material undergoes phase transitions from orthorhombic to tetragonal and
cubic structures on heating. The mechanical loss signatures of the
transformation twins include relaxation and frequency-independent peaks in the
orthorhombic and tetragonal phases, with no observed energy dissipation in the
cubic phase. The macroscopic shape, orientation and relative displacements of
twin walls have been calculated from bending and anisotropy energies. The
mechanical loss angle and distribution of relaxation time are discussed in term
of bending modes of domain walls.Comment: 20 pages, 4 figure
Tin telluride: a weakly co-elastic metal
We report resonant ultrasound spectroscopy (RUS),
dilatometry/magnetostriction, magnetotransport, magnetization, specific heat,
and Sn M\"ossbauer spectroscopy measurements on SnTe and
SnCrTe. Hall measurements at K indicate that our
Bridgman-grown single crystals have a -type carrier concentration of cm and that our Cr-doped crystals have an -type
concentration of cm. Although our SnTe crystals are
diamagnetic over the temperature range , the Cr-doped crystals are room temperature ferromagnets with a Curie
temperature of 294 K. For each sample type, three-terminal capacitive
dilatometry measurements detect a subtle 0.5 micron distortion at K. Whereas our RUS measurements on SnTe show elastic hardening near the
structural transition, pointing to co-elastic behavior, similar measurements on
SnCrTe show a pronounced softening, pointing to
ferroelastic behavior. Effective Debye temperature, , values of SnTe
obtained from Sn M\"ossbauer studies show a hardening of phonons in the
range 60--115K ( = 162K) as compared with the 100--300K range
( = 150K). In addition, a precursor softening extending over
approximately 100 K anticipates this collapse at the critical temperature, and
quantitative analysis over three decades of its reduced modulus finds with , a value
indicating a three-dimensional softening of phonon branches at a temperature
K, considerably below . We suggest that the differences in
these two types of elastic behaviors lie in the absence of elastic domain wall
motion in the one case and their nucleation in the other
Optimal interlayer hopping and high temperature Bose–Einstein condensation of local pairs in quasi 2D superconductors
Both FeSe and cuprate superconductors are quasi 2D materials with high transition temperatures and local fermion pairs. Motivated by such systems, we investigate real space pairing of fermions in an anisotropic lattice model with intersite attraction, V, and strong local Coulomb repulsion, U, leading to a determination of the optimal conditions for superconductivity from Bose–Einstein condensation. Our aim is to gain insight as to why high temperature superconductors tend to be quasi 2D. We make both analytically and numerically exact solutions for two body local pairing applicable to intermediate and strong V. We find that the Bose–Einstein condensation temperature of such local pairs pairs is maximal when hopping between layers is intermediate relative to in-plane hopping, indicating that the quasi 2D nature of unconventional superconductors has an important contribution to their high transition temperatures
High-pressure synthesis, crystal and electronic structures of a new scandium tungstate, Sc0.67WO4
Negative thermal expansion (NTE) materials possess a low-density, open
structure which can respond to high pressure conditions, leading to new
compounds and/or different physical properties. Here we report that one such
NTE material -- white, insulating, orthorhombic Sc2W3O12 -- transforms into a
black compound when treated at 4 GPa and 1400 oC. The high pressure phase,
Sc0.67WO4, crystallizes in a defect-rich wolframite-type structure, a dense,
monoclinic structure (space group P2/c) containing 1-D chains of edge-sharing
WO6 octahedra. The chemical bonding of Sc0.67WO4 vis-a-vis the ambient pressure
Sc2W3O12 phase can be understood on the basis of the Sc defect structure.
Magnetic susceptibility, resistivity, thermoelectric power and IR spectroscopic
measurements reveal that Sc0.67WO4 is a paramagnet whose conductivity is that
of a metal in the presence of weak localization and electron-electron
interactions. Oxygen vacancies are suggested as a potential mechanism for
generating the carriers in this defective wolframite material.Comment: 29 pages total, 1 table, 7 figure
Antiferromagnetic correlations in Fe-Cu granular alloys: the role of the surface structure
Fe precipitates in a Cufcc matrix, prepared using the Bridgeman method and with an average composition of Cu97Fe3, displayed the coexistence of ferromagnetism ~FM!, spin glass-like ~SGL! behavior and antiferromagnetic ~AFM! correlations. The two former contributions may be attributed, respectively, to the segregation of FM, a-Febcc precipitates and to the few Fe spins distributed in the matrix. The annealing procedures increased the FM contribution and, as particle growth and phase segregation took place, the SGL behavior progressively disappeared. Results from high resolution transmission electron microscopy ~HRTEM!, x-ray photoelectron spectroscopy ~XPS!, and electron energy-loss spectroscopy ~EELS! suggest that the AFM correlations are due to the a-Fe particles that show a surface layer of a few nanometers in thickness, of either FeO and/or g-Fefcc . XPS and EELS measurements confirm the presence of FeO; however, the latter is only tentatively suggested by the HRTEM analysis of the particle/matrix interfaces
Optical conductivity of polaronic charge carriers
The optical conductivity of charge carriers coupled to quantum phonons is
studied in the framework of the one-dimensional spinless Holstein model. For
one electron, variational diagonalisation yields exact results in the
thermodynamic limit, whereas at finite carrier density analytical
approximations based on previous work on single-particle spectral functions are
obtained. Particular emphasis is put on deviations from weak-coupling,
small-polaron or one-electron theories occurring at intermediate coupling
and/or finite carrier density. The analytical results are in surprisingly good
agreement with exact data, and exhibit the characteristic polaronic excitations
observed in experiments on manganites.Comment: 23 pages, 11 figure
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