A superconducting state is formed when
electrons in a metal form pairs and condense into a single
macroscopic quantum state. Several peculiar properties can arise
from the formation of the superconducting state such as
electrical conduction without resistance, perfect diamagnetism,
and the quantization of magnetic flux into elementary fluxons
(also known as flux vortices). The properties of perfect
conduction and diamagnetism hold only for low currents and
fields, and at higher current densities a superconductor becomes
resistive and can dissipate energy intensely.
A large current drastically modifies the
superconducting state and destroys it altogether when the
current's kinetic energy overcomes the condensation energy of
superconductive pairing---the so-called pair-breaking effect. At
intermediate current densities, below the onset of pair-breaking,
one expects to see the free flow of fluxons. Technical obstacles
resulting from the high dissipation levels (exceeding 1010
W/cm3!) have hindered experimental investigation of
these effects in the past. In our work special measurement
techniques were developed to successfully demonstrate the
dynamics of free fluxons for the first time in any
superconductor, and provide the first evidence of current induced
pair-breaking in a high-temperature superconductor. In addition
to its fundamental significance, this work has important
implications for the technology and practical applications of
superconductors, including the extension of their potential
usefulness to current densities higher than had been previously
recognized.
Laboratory facilities presently include a
16-Tesla large-bore superconducting magnet with a
variable-temperature environment and a comprehensive complement
of measurement electronics. In order to access the
high-current/high-dissipation regime, a custom-built pulsed
current apparatus is used that has submicrosecond rise times and
precise control over waveform shape. The samples we study are
superconducting films. Although most of them are made by
collaborators at other universities and national labs, we have
several vacuum deposition chambers and furnaces for thin-film
preparation. For sample analysis and characterization we have
STEM, TEM, and SEM electron microscopes, scanning probe
microscopes, and x-ray diffractometers. The films are patterned
into narrow structures using optical-projection and electron-beam
lithographies. The data-acquisition and computer control of the
experiment is performed on a state-of-the-art Labwindows Visual-C
platform.
The research program is designed to give the
student a well-rounded and diversified experience in
interdisciplinary areas---helpful for success in today's
professional scientific enivironment. The training emphasizes
gaining a clear understanding of research problems and tackling
them independently, while developing the requisite skills along
the way, including effective technical cummunication.
Selected Publications
"Observation of free flux flow at high
dissipation levels in YBa2Cu3O7
epitaxial films", M. N. Kunchur, D. K. Christen, and J. M.
Phillips, Phys. Rev. Lett 70, 998 (1993).
"Pair-breaking effect of high current
densities on the superconducting transition on YBa2Cu3O7 ",
M. N.Kunchur, D. K. Christen, C. E. Klabunde, and J. M. Phillips,
Phys. Rev. Lett. 72, 752 (1994).
"Hall effect in YBa2Cu3O7
in the limit of free flux flow", M. N. Kunchur, D. K.
Christen, C. E. Klabunde, and J. M. Phillips, Phys. Rev. Lett. 72,
2259 (1994).
"Absence of Superconductivity in Metallic Granular Aluminum",
M. Kunchur, P. Lindenfeld, W. L. Mclean, and J. S. Brooks, Phys. Rev. Lett.
59, 1232 (1987).
Critical fields and critical currents of
superconducting discs in transverse magnetic fields, M.N.
Kunchur and S.J. Poon, Phys. Rev. B 43, 2916 (1991).
