84 research outputs found
Quasiclassical calculation of spontaneous current in restricted geometries
Calculation of current and order parameter distribution in inhomogeneous
superconductors is often based on a self-consistent solution of Eilenberger
equations for quasiclassical Green's functions. Compared to the original Gorkov
equations, the problem is much simplified due to the fact that the values of
Green's functions at a given point are connected to the bulk ones at infinity
(boundary values) by ``dragging'' along the classical trajectories of
quasiparticles. In finite size systems, where classical trajectories undergo
multiple reflections from surfaces and interfaces, the usefulness of the
approach is no longer obvious, since there is no simple criterion to determine
what boundary value a trajectory corresponds to, and whether it reaches
infinity at all. Here, we demonstrate the modification of the approach based on
the Schophol-Maki transformation, which provides the basis for stable numerical
calculations in 2D. We apply it to two examples: generation of spontaneous
currents and magnetic moments in isolated islands of d-wave superconductor with
subdominant order-parameters s and d_{xy}, and in a grain boundary junction
between two arbitrarily oriented d-wave superconductors. Both examples are
relevant to the discussion of time-reversal symmetry breaking in unconventional
superconductors, as well as for application in quantum computing.Comment: 6 pages, Submitted for publication in the proceedings of MS+S2002
conference, Japa
Breaking of the overall permutation symmetry in nonlinear optical susceptibilities of one-dimensional periodic dimerized Huckel model
Based on infinite one-dimensional single-electron periodic models of
trans-polyacetylene, we show analytically that the overall permutation symmetry
of nonlinear optical susceptibilities is, albeit preserved in the molecular
systems with only bound states, no longer generally held for the periodic
systems. The overall permutation symmetry breakdown provides a fairly natural
explanation to the widely observed large deviations of Kleinman symmetry for
periodic systems in off-resonant regions. Physical conditions to experimentally
test the overall permutation symmetry break are discussed.Comment: 7 pages, 1 figur
Phase transformation mechanism between γ- and θ-alumina
gamma-alumina transforms to theta-alumina and finally to alpha-alumina in the sequence of thermal dehydration of boehmite. We report a detailed theoretical investigation of the gamma- to theta-alumina transformation based on first-principles density-functional calculations. Although the unit cells of cubic gamma-alumina and monoclinic theta-alumina look quite different, we have identified cells for both the polytypes (with the composition Al16O24) that look very similar and can be continuously transformed one to another. The transformation may be described by a set of aluminum atom migrations between different interstitials while the oxygen atoms remain fixed. Total-energy calculations along the paths of the atomic migrations have been used to map out possible transformation pathways. The calculated conversion rate accurately predicts the experimentally measured transformation temperature. The deduced orientation relationships between the gamma- and theta-alumina forms also agree with experimental observations. The formation of several different interfaces observed in domain boundaries of theta-alumina may correspond to different migration paths of the aluminum atoms in neighboring domains during the gamma- to theta-alumina phase transition
Atomic scale mechanism of the transformation of γ-alumina to θ-alumina
gamma-alumina is known to transform to theta-alumina and finally to alpha-alumina upon thermal treatment with a catastrophic loss of porosity and catalytic activity. First-principles calculations were performed to investigate the atomic scale mechanism of the gamma- to theta-alumina transformation. The transformation pathways between the two different forms have been mapped out and identified as a sequence of Al cation migrations. Different possible Al migration paths may be responsible for the experimentally observed formation of domains and twins in theta-alumina. The estimated temperature dependence of the conversion rate is in excellent agreement with the experimental transformation temperature
Second harmonic generation in SiC polytypes
LMTO calculations are presented for the frequency dependent second harmonic
generation (SHG) in the polytypes 2H, 4H, 6H, 15R and 3C of SiC. All
independent tensor components are calculated. The spectral features and the
ratios of the 333 to 311 tensorial components are studied as a function of the
degree of hexagonality. The relationship to the linear optical response and the
underlying band structure are investigated. SHG is suggested to be a sensitive
tool for investigating the near band edge interband excitations.Comment: 12 pages, 10 figure
Transport, optical and electronic properties of the half metal CrO2
The electronic structure of CrO_2 is critically discussed in terms of the
relation of existing experimental data and well converged LSDA and GGA
calculations of the electronic structure and transport properties of this half
metal magnet, with a particular emphasis on optical properties. We find only
moderate manifestations of many body effects. Renormalization of the density of
states is not large and is in the typical for transition metals range. We find
substantial deviations from Drude behavior in the far-infrared optical
conductivity. These appear because of the unusually low energy of interband
optical transitions. The calculated mass renormalization is found to be rather
sensitive to the exchange-correlation functional used and varies from 10%
(LSDA) to 90% (GGA), using the latest specific heat data. We also find that
dressing of the electrons by spin fluctuations, because of their high energy,
renormalizes the interband optical transition at as high as 4 eV by about 20%.
Although we find no clear indications of strong correlations of the Hubbard
type, strong electron-magnon scattering related to the half metallic band
structure is present and this leads to a nontrivial temperature dependence of
the resistivity and some renormalization of the electron spectra.Comment: 9 Revtex 2 column pages, including 8 postscript figures. Two more
figures are included in the submission that are not embedded in the paper,
representing DOS and bandstructure of the paramagnetic CrO
Electronic Raman scattering in YBCO and other superconducting cuprates
Superconductivity induced structures in the electronic Raman spectra of
high-Tc superconductors are computed using the results of ab initio LDA-LMTO
three-dimensional band structure calculations via numerical integrations of the
mass fluctuations, either in the whole 3D Brillouin zone or limiting the
integrations to the Fermi surface. The results of both calculations are rather
similar, the Brillouin zone integration yielding additional weak structures
related to the extended van Hove singularities. Similar calculations have been
performed for the normal state of these high-Tc cuprates. Polarization
configurations have been investigated and the results have been compared to
experimental spectra. The assumption of a simple d_(x^2-y^2)-like gap function
allows us to explain a number of experimental features but is hard to reconcile
with the relative positions of the A1g and B1g peaks.Comment: 14 pages, LaTeX (RevTeX), 5 PostScript figures, uses multicol.sty,
submitted to PR
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