Browsing by Author "Knigavko, A"

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90 degrees reorientation in the vortex lattice of borocarbide superconductors
(2000-12-01)
Knigavko, A
;
Rosenstein, B
;
交大名義發表
;
電子物理學系
;
National Chiao Tung University
We explain 90 degrees reorientation in the vortex lattice of borocarbide superconductors on the basis of a phenomenological extension of the nonlocal London model that takes full account of the symmetry of the system. Microscopic mechanisms that could generate the correction terms are analyzed. We show that for any dispersion relation and longitudinal phonon-electron pairing mechanism the relevant quantity is strongly suppressed. Possible phenomenological interaction terms providing the effect are studied.
Anisotropic peak effect due to structural phase transition in the vortex lattice
(2000-05-01)
Rosenstein, B
;
Knigavko, A
;
電子物理學系
;
Department of Electrophysics
The recently observed new peak effect in YBCO is explained by softening of the vortex lattice (VL) due to a structural phase transition in the VL. At this transition, square lattice transforms into a distorted hexagonal one. While conventional peak effect is associated with softening of shear modes at melting, in this case the relevant mode is the point. The squash mode is highly anisotropic and we point out some peculiar effects associated with this feature. (C) 2000 Published by Elsevier Science B.V. All rights reserved.
Anisotropic peak effect due to structural phase transition in the vortex lattice
(1999-07-26)
Rosenstein, B
;
Knigavko, A
;
電子物理學系
;
Department of Electrophysics
It is shown that the recently observed new peak effect in YBCO could be explained by the softening of the vortex lattice due to a structural phase transition in the vortex lattice. At this transition square lattice transforms into a distorted hexagonal one. While conventional peak effect is associated with the softening of shear modes (elastic modulus c(66) vanishes) at melting, in this case the relevant mode is "squash" mode(c(11) + c(22) - 2c(12) vanishes).
Ferromagnetic behavior of a triplet superconductor
(1999-04-15)
Knigavko, A
;
Rosenstein, B
;
Chen, YF
;
Huang, HL
;
Lin, MT
Properties of a type II triplet superconductor with equal spin pairing are investigated using the phenomenological Ginzburg-Landau approach. It is shown that there exist two kinds of fluxons: vortices and magnetic skyrmions. Vector nature of the order parameter allows a direct coupling of the effective spin of the condensate to a magnetic field. This coupling, which is reminiscent to Zeeman interaction in the form, significantly modifies the structure of an isolated vortex, as compared to the usual Abrikosov vortex, and changes the energetics of a vortex lattice. If the coupling is sufficiently strong, then magnetization of the triplet superconductor becomes positive at high magnetic fields. Formation of spontaneous vortex state is also possible. (C) 1999 American Institute of Physics. [S0021-8979(99)55408-X].
Magnetic Skyrmion lattices in heavy fermion superconductor UPt3
(1999-02-08)
Knigavko, A
;
Rosenstein, B
;
電子物理學系
;
Department of Electrophysics
Topological analysis of nearly SO(3)(spin) symmetric Ginzburg-Landau theory, proposed for UPt3 by Machida ct al. shows that there exists a new class of solutions carrying two units of magnetic flux-the magnetic Skyrmion. These solutions do not have singular corelike Abrikosov vortices and at low magnetic fields they become lighter fur strongly type-II superconductors. Magnetic Skyrmions repel each other as 1/r at distances much larger than the magnetic penetration depth lambda, forming a relatively robust triangular lattice. The magnetic induction near H-c1 is found to increase as (H - H-c1)(2). This behavior agrees well with experiments. [S0031-9007(98)08317-3].
Magnetic skyrmions and their lattices in triplet superconductors
(1999-07-01)
Knigavko, A
;
Rosenstein, B
;
Chen, YF
;
交大名義發表
;
電子物理學系
A complete topological classification of solutions in SO(3) symmetric Ginzburg-Landau free energy has been performed and a new class of solutions in weak external magnetic field carrying two units of magnetic flux has been identified. These solutions, magnetic skyrmions, do not have a singular core as Abrikosov vortices do and at low magnetic field they become lighter for strongly type II superconductors. As a consequence, the lower critical magnetic field H-c1 is reduced by a factor of In kappa. Magnetic skyrmions repel each other as 1/r at distances much larger than magnetic penetration depth lambda forming a relatively robust triangular lattice. Magnetization near H-c1 increases gradually as (H-Hcl)(2). This behavior agrees very well with experiments on heavy fermion superconductor UPt3. Newly discovered Ru based compounds Sr2RuO4 and Sr2YRu1-xCuxO6 are other possible candidates to possess skyrmion lattices. Deviations from exact SO(3) symmetry are also studied, [S0163-1829(99)06425-5].
Nature of the 45 degrees vortex lattice reorientation in tetragonal superconductors
(2000-07-01)
Knigavko, A
;
Kogan, VG
;
Rosenstein, B
;
Yang, TJ
;
電子物理學系
The transformation of the vortex lattice in a tetragonal superconductor which consists of the 45 degrees reorientation relative to the crystal axes is studied using the nonlocal London model. It is shown that the reorientation occurs as two successive second order (continuous) phase transitions. The transition magnetic fields are calculated for a range of parameters relevant for borocarbide superconductors in which the reorientation has been observed.
Spontaneous vortex state and ferromagnetic behavior of type-II p-wave superconductors
(1998-10-01)
Knigavko, A
;
Rosenstein, B
;
電子物理學系
;
Department of Electrophysics
The mixed phase in type-II superconductors with equal spin p-wave pairing is considered using the Ginzburg-Landau approach. Due to direct spin coupling of the condensate to magnetic field the mixed state acquires ferromagnetic properties. For sufficiently large Zeeman coupling a spontaneous vortex phase appears at H=0 and exists and for an arbitrarily large magnetic field. The Meissner phase therefore completely disappears. Vortices become thinner when H grows. There exists a value of Zeeman coupling above which, in the presence of external magnetic field, a mixed phase might occur even for temperatures above T-c. The structure of the vortex core is markedly different from the usual one. [S0163-1829(98)06137-2].
Spontaneous vortex state and ferromagnetic behavior of type-II p-wave superconductors
(1998-10-01)
Knigavko, A
;
Rosenstein, B
;
電子物理學系
;
Department of Electrophysics
The mixed phase in type-II superconductors with equal spin p-wave pairing is considered using the Ginzburg-Landau approach. Due to direct spin coupling of the condensate to magnetic field the mixed state acquires ferromagnetic properties. For sufficiently large Zeeman coupling a spontaneous vortex phase appears at H=0 and exists and for an arbitrarily large magnetic field. The Meissner phase therefore completely disappears. Vortices become thinner when H grows. There exists a value of Zeeman coupling above which, in the presence of external magnetic field, a mixed phase might occur even for temperatures above T-c. The structure of the vortex core is markedly different from the usual one. [S0163-1829(98)06137-2].