179 research outputs found
Transverse instability due to the space charge during the electron-cooling bunching of ion beams
A beam-tracking simulation has been done for study of the electron- cooling bunching of ion beams around a few hundred MeV/u. In the simulation, the field due to the space charge of a beam in a round vacuum chamber is calculated from the charge distribution of macro- particles and the image particles due to the macro-particles. The simulation results show that the bunched beams meet a transverse instability more easily than coasting beams. (2 refs)
Ginzburg Landau theory for d-wave pairing and fourfold symmetric vortex core structure
The Ginzburg Landau theory for d_{x^2-y^2}-wave superconductors is
constructed, by starting from the Gor'kov equation with including correction
terms up to the next order of ln(T_c/T). Some of the non-local correction terms
are found to break the cylindrical symmetry and lead to the fourfold symmetric
core structure, reflecting the internal degree of freedom in the pair
potential. Using this extended Ginzburg Landau theory, we investigate the
fourfold symmetric structure of the pair potential, current and magnetic field
around an isolated single vortex, and clarify concretely how the vortex core
structure deviates from the cylindrical symmetry in the d_{x^2-y^2}-wave
superconductors.Comment: 12 pages including 8 eps figs, LaTeX with jpsj.sty & epsfi
Vortex lattice structure in a d_{x^2-y^2}-wave superconductor
The vortex lattice structure in a d_{x^2-y^2}-wave superconductor is
investigated near the upper critical magnetic field in the framework of the
Ginzburg Landau theory extended by including the correction terms such as the
higher order derivatives derived from the Gor'kov equation. On lowering
temperature, the unit cell shape of the vortex lattice gradually varies from a
regular triangular lattice to a square lattice through the shape of an
isosceles triangle. As for the orientation of the vortex lattice, the base of
an isosceles triangle is along the a axis or the b axis of the crystal. The
fourfold symmetric structure around a vortex core is also studied in the vortex
lattice case. It is noted that these characteristic features appear even in the
case the induced s-wave order parameter is absent around the vortex of the
d_{x^2-y^2}-wave superconductivity. We also investigate the effect of the
induced s-wave order parameter. It enhances (suppresses) these characteristic
features of the d_{x^2-y^2}-wave superconductor when the s-wave component of
the interaction is attractive (repulsive).Comment: 20 pages, RevTex, 9 figures in 3 PS-files and 5 GIF-file
Reentrant vortex lattice transformation in four-fold symmetric superconductors
The physics behind the rhombicsquarerhombic flux line lattice
transformation in increasing fields is clarified on the basis of Eilenberger
theory. We demonstrate that this reentrance observed in LuNiBC is due
to intrinsic competition between superconducting gap and Fermi surface
anisotropies. The calculations reproduce not only it but also predict yet not
found lock-in transition to a square lattice with different orientation in
higher field. In view of physical origin given, this sequence of transitions is
rather generic to occur in four-fold symmetric superconductors.Comment: 5 pages, 4 figures,submitted to Phys. Rev. Let
Anisotropy of the upper critical field in MgB2: the two-gap Ginzburg-Landau theory
The upper critical field in MgB2 is investigated in the framework of the
two-gap Ginzburg-Landau theory. A variational solution of linearized
Ginzburg-Landau equations agrees well with the Landau level expansion and
demonstrates that spatial distributions of the gap functions are different in
the two bands and change with temperature. The temperature variation of the
ratio of two gaps is responsible for the upward temperature dependence of
in-plane Hc2 as well as for the deviation of its out-of-plane behavior from the
standard angular dependence. The hexagonal in-plane modulations of Hc2 can
change sign with decreasing temperature.Comment: 6 pages, 6 figures, accepted in the European Physical Journal
The Phase Transition to a Square Vortex Lattice in Type-II Superconductors with Fourfold Anisotropy
We investigate the stability of the square vortex lattice which has been
recently observed in experiments on the borocarbide family of superconductors.
Taking into account the tetragonal symmetry of these systems, we add fourfold
symmetric fourth-derivative terms to the Ginzburg-Landau(GL) free energy. At
these terms may be treated perturbatively to lowest order to locate
the transition from a distorted hexagonal to a square vortex lattice. We also
solve for this phase boundary numerically in the strongly type-II limit,
finding large corrections to the lowest-order perturbative results. We
calculate the relative fourfold anisotropy for field in the plane
to be 4.5% at the temperature, , where the transition occurs at
for field along the axis. This is to be compared to the 3.6%
obtained in the perturbative calculation. Furthermore, we find that the phase
boundary in the phase diagram has positive slope near .Comment: 15 pages including 2 figures, LaTe
Theory of equilibrium flux lattices in unconventional superconductors
We investigate equilibrium flux lattice structures in superconductors with
unconventional order parameters, such as high- cuprates, using a
generalized London model with non-local electrodynamics derived from a simple
microscopic model. We find a rich phase diagram containing triangular, centered
rectangular and square lattices with various orientations to the ionic lattice,
as a function of magnetic field and temperature.Comment: 4 pages, RevTeX, 2 PostScript figures included [The version to appear
in PRL. Stylistic changes in the text, a brief discussion of new results
involving the non-linear Meissner effect in the vortex lattice added.
Vortex lattice structures of SrRuO
The vortex lattice structures of SrRuO for the odd parity
representations of the superconducting state are examined for the magnetic
field along the crystallographic directions. Particular emphasis is placed upon
the two dimensional representation which is believed to be relevant to this
material. It is shown that when the zero-field state breaks time reversal
symmetry, there must exist two superconducting transitions when there is a
finite field along a high symmetry direction in the basal plane. Also it is
shown that a square vortex lattice is expected when the field is along the
-axis. The orientation of the square lattice with respect to the underlying
ionic lattice yields information as to which Ru 4d orbitals are relevant to the
superconducting state.Comment: 5 pages, 2 figure
Ginzburg-Landau theory of vortices in a multi-gap superconductor
The Ginzburg-Landau functional for a two-gap superconductor is derived within
the weak-coupling BCS model. The two-gap Ginzburg-Landau theory is, then,
applied to investigate various magnetic properties of MgB2 including an upturn
temperature dependence of the transverse upper critical field and a core
structure of an isolated vortex. Orientation of vortex lattice relative to
crystallographic axes is studied for magnetic fields parallel to the c-axis. A
peculiar 30-degree rotation of the vortex lattice with increasing strength of
an applied field observed by neutron scattering is attributed to the multi-gap
nature of superconductivity in MgB2.Comment: 11 page
Vortex structure in -wave superconductors
Vortex structure of pure -wave superconductors is
microscopically analyzed in the framework of the quasi-classical Eilenberger
equations. Selfconsistent solution for the -wave pair potential is obtained
for the first time in the case of an isolated vortex. The vortex core
structure, i.e., the pair potential, the supercurrent and the magnetic field,
is found to be fourfold symmetric even in the case that the mixing of -wave
component is absent. The detailed temperature dependences of these quantities
are calculated. The fourfold symmetry becomes clear when temperature is
decreased. The local density of states is calculated for the selfconsistently
obtained pair potential. From the results, we discuss the flow trajectory of
the quasiparticles around a vortex, which is characteristic in the
-wave superconductors. The experimental relevance of our results
to high temperature superconductors is also given.Comment: 22 pages, RevTex, 23 figures available upon reques
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