A type of flux instability is discussed that occurs in a
superconductor whenever flux dissipation causes a
significant destruction of the condensate and elevation of
the electronic temperature above that of the lattice. Even
when the distribution-function shape remains thermal like,
the elevated electronic temperature will cause a rise in
resistivity and hence an unstable and non-monotonic
current-voltage response. In this scenario, the vortex
expands and the quasiparticle population rises, in contrast
to the Larkin-Ovchinnikov (LO) instability where the vortex
shrinks and quasiparticles leave its vicinity. This
hot-electron-gas type of instability requires dissipation
levels that are much more intense than required for the more
subtle LO effect, and will dominate the overall non-linear
response at low temperatures, where e-e scattering is more
rapid than e-ph. The derived current-voltage response and
field dependencies of critical parameters provide good
quantitative agreement with our measurements on YBCO films.
This work was supported by the U.S. Department of Energy
through grant no. DE-FG02-99ER45763.(Related reprints and
preprints are posted at http://www.physics.sc.edu/kunchur.)
[P5.002] Interaction of vortices with acoustic waves
Boris Ivlev (Univ. of South Carolina, Columbia; Univ. of San Luis Potosi, San Luis Potosi)
An interaction of vortices in superconductors with acoustic
waves is very unusual. In a clean sample helicon waves
interact with the transverse sound resulting in the
additional sound attenuation [1]. Interaction of a vortex
lattice with ultrasound may produce the acoustic Faraday
effect and nonstationary magnetic field, acting on vortices,
may generate ultrasonic waves [2]. Vortices interact with
acoustic waves either through pinning centers or by the
mechanisms which exist even in normal metals:
electromagnetic forces or forces mediated by impurities.
When the velocity of vortices exceeds the velocity of sound,
each vortex produces the Cherenkov radiation of sound waves.
The velocity field of lattice ions is localized on the
Cherenkov cone, but the forces, acting on the lattice, are
localized on vortex positions. When the Cherenkov cone
coincides with certain directions in the moving vortex
lattice, the dissipation exhibits maxima. Since the
direction of the Cherenkov cone is determined by the vortex
velocity (electric field) one can expect a series of
resonant peaks in the current-voltage characteristic at
particular values of electric field [3]. A
radiation-friction component of the vortex dissipation also
peaks at values of vortex velocity where there are phonons
in the Brillouin zone of the crystal with matching phase and
group velocities (the van Hove condition) [4]. Thus, besides
the Crenkov peaks, also the van Hove peaks are expected in
the current-voltage characteristic of the vortex state. [1]
G. Blatter and B. Ivlev, Phys.Rev. B 52, 4588 (1995); [2] D.
Dominguez, L. Bulaevskii, B. Ivlev, M. Maley, and A. Bishop,
Phys.Rev.Lett. 74, 2579 (1995); [3] B. Ivlev, S. Mejia
Rosales, and M.Kunchur, Phys.Rev. B 60, 12419 (1999); [4] B.
Ivlev, M. Kunchur, and S. Mejia Rosales, Phys.Rev. B 64,
024508 (2001)
[P5.003] A one-dimensional chain state of vortex matter
Simon Bending (University of Bath)
The dependence of the vortex ground state on magnetic field
direction in the highly anisotropic Bi_ 2Sr_
2CaCu_ 2O_ 8+d (BSCCO) superconductor is a topic
of considerable current interest. With the magnetic field
tilted away from the high symmetry c-axis it is expected to
consist of co-existing orthogonal 6-fold Abrikosov pancake
vortex (PV) and rhombic Josephson vortex (JV) lattices.
Furthermore it has been shown recently that small
displacements of PVs driven by the underlying JV
supercurrents can lead to an attractive interaction between
these two ‘crossing’ lattices and can give rise to a very
rich variety of composite lattice structures. We report here
the use of high resolution scanning Hall probe microscopy
(SHPM) to directly probe the static and dynamic properties
of these structures in BSCCO single crystals under
independently applied H_ c and H_ // fields. At very
low c-axis fields we observe a novel 1D vortex chain state
where all pancake vortex stacks become trapped on underlying
stacks of Josephson vortices. The remarkable dynamic
properties of this system of interacting orthogonal vortex
lattices will be described. In particular it will be shown
how one sub-lattice can be used to manipulate the other with
the exciting potential for building novel flux logic and
flux amplifier devices. In addition the existence of 1D
vortex chains explains many of the features observed in the
magnetisation of HTS under strongly tilted magnetic fields.
The dependence of vortex structures on in-plane field is in
good quantitative agreement with theoretical predictions,
yielding an almost temperature-independent anisotropy
parameter of g=640±25 in the range 77-85K. We directly
confirm that the PV/JV attraction arises from small PV
displacements in the presence of JV supercurrents. In
addition we demonstrate how the presence of quenched
disorder leads to indirect JV pinning via interactions with
weakly pinned PV stacks, and develop a theoretical model for
this phenomenon by analogy with the problem of vortex
deformation near planar defects. Finally we will show how
fragmentation of both PV and JV stacks can occur when stacks
of JVs ‘decorated’ with PVs are forced abruptly through a
region of disorder which is inhomogeneous along the c-axis.
[P5.004] Crossing vortex lattice melting transition in BSCCO single crystals
Kazuo Kadowaki (Institute of Materials Science, University of Tsukuba)
We have studied the vortex lattice melting transition and established the vortex matter phase diagram for whole angle region, in particular, within a few degrees from the ab-plane in detail, by means of both resistivity measurement in Corbino geometry and local ac-magnetic response measurement using miniature coils in high quality single crystal BSCCO[1,2]. The Corbino resistivity measurement is a unique technique and essential to avoid the surface pinning (and the edge barrier) effects, while it is impossible to study the vortex solid phases because the resistivity is simply zero. On the other hand, the ac-magnetic response measurement can give an access to the vortex solid state. The first order melting trasition can clearly be observed in both measurements except about \pm0.3 degree from the ab-plane. In this extremely narrow angle region, the first order nature of the phase transition changes the character to the second order one. We found another anomaly in temperature dependence of the Corbino resistivity in a magnetic field. The resistivity at a higher field changes the slope well above the line to the solid phase, and the corresponding anomaly sharply goes beyond our accessible field window of 7T as temperature is decreased. Both transition lines merge to one at the triple point at about 2.5 kOe and changes to the first order nature in a temperature region just a few derees below T_c. Appearance of two phases at low temperatures can reasonably be interpreted by the three-dimensional long range order of vortex alignment and the Kosterlitz-Thouless type of transition, although the estimated triple point is quite inconsistent, yielding at 1.7 T with \gamma=170[3]. The revival of the first order nature of the transition occuring very close to T_c also agrees well with the result of the recent theoretical simulation study.
[1]. J. Mirkovic, et al., PRL86 (2001) 886. [2]. J.
Mircovic, et al., PRB66 (2002) 132505. [3].X. Hu and M.
Tachiki, cond-mat/0211620
[P5.005] Do Superconductors Have Zero Resistance in a Magnetic Field?
C. J. Lobb (University of Maryland, Department of Physics, Center for Superconductivity Research, College Park, MD 20742-4111)
DC voltage versus current measurements of superconductors in a magnetic field are widely interpreted to imply that a phase transition occurs into a state of zero resistance. We show that the widely-used scaling function approach has a problem: Good data collapse occurs for a wide range of critical exponents and temperatures [1]. This strongly suggests that agreement with scaling alone does not prove the existence of the phase transition. We discuss a criterion to determine if the scaling analysis is valid, and find that all of the data in the literature that we have analyzed fail to meet this criterion. Our data on YBCO films, and other data that we have analyzed, are more consistent with the occurrence of small but non-zero resistance at low temperature.
[1] D. R. Strachan, M. C. Sullivan, P. Fournier, S. P. Pai, T. Venkatesan, and C. J. Lobb, Phys. Rev. Lett. 87, 067007 (2001)