A superconductor is a material which, when cooled below a certain
temperature, loses all resistance to electricity. This means that
superconducting materials can carry large electrical currents without any energy
loss- but there are limits to how much current can flow before superconductivity
is destroyed. The current at which superconductivity breaks down is called the
critical current Jc. The value of the maximum possible, or critical current density
Jc in superconducting materials is determined by the balance between Lorentz and
pinning forces acting on flux lines (FL). Lorentz forces, which act on the
current flowing in a vortex, drive FL into motion, which dissipates energy and
destroys the perfect conductivity of the superconducting state (Fig.1).
· Fig.1. The value of the critical current
in superconducting materials is determined by the balance of Lorenz forces and
pinning forces acting on the flux lines. Lorenz forces which are proportional to
the current flow tend to drive the flux lines into motion, which dissipates
energy and destroys zero resistance
Isolated defects act as local zones of reduced superconducting
condensation energy. These sites attract ("pin") FL and oppose their motion and
thereby increase Jc. The insertion of many kinds of artificial pinning centers
(APC) into practical superconducting materials has been proposed to increase
their current carrying capacity, ranging from non-superconducting inclusions
(second phases) to proton, neutron or heavy-ion damage tracks inserted via
irradiation in accelerator beams. In all of these methods, the APC are randomly
distributed over the superconducting material, causing them to operate well
below their maximum efficiency. This drawback can be avoided by using special
techniques to create a periodic lattice of nano-holes (see Fig.2) so that each
pin interacts strongly with at least one, or even a few FL. The introduction of
periodic arrays of APC into superconducting samples has been shown to increase
the crical current densities by more than 2 orders of magnitude and give rise
to novel FL behavior that is not observed in the presence of pinning due to
random growth defects or radiation damage.
· Fig.2. SEM image of nanostructured
superconducting device. Nano-patterning leads to the effective pinning of
flux lines and the critical current enhancement by more than 100 times in
comparison with un-patterned superconductor.
· After V.Metlushko et al. Phys.Rev.B 59,
603 (1999)
From technological point of view, it has become possible to create superconductor quantum devices using nanotechnology,
which was previously thought to be practically impossible, and the connection with superconducting quantum computers makes this an important
and innovative even further. (V.V. Metlushko, M. Baert, R.Jonckheere, V.V. Moshchalkov, and Y. Bruynseraede, Solid State Communications 91 (1994)
331-335, M. Baert, V.V. Metlushko, R.Jonckheere, V.V. Moshchalkov, and Y.Bruynseraede, Physical Review Letters, 74 (1995)
3269- ISI Citation Classic Award, June 21, 2000 (159 citations)).
This discovery is now a foundation of Joint European Science Foundation (ESF) Vortex Programme and JSPS Core-to-Core Integrated Action Initiative "Nanoscience and Engineering in Superconductivity" in Japan. Prof. Metlushko was Invited Speaker at all major ESF-JSP Conferences: Combined ESF Vortex and ESF PiShift Workshop, Bad Münstereifel, Germany, 2004, Joint meeting of JSPS Core-to-Core Integrated Action Initiative "Nanoscience and Engineering in Superconductivity" (CTC-NES) and The 4th International Symposium on Intrinsic Josephson Effect and Plasma Oscillations in High-Tc Superconductors (PLASMA 2004), Tsukuba, Japan, November 26-28, 2004, Joint JSPS and ESF Conference on Vortex Matter in Nanostructured Superconductors, (VORTEX IV), Crete, Greece, September 3-9, 2005, the Nanoscale Superconductivity and Magnetism - NSM2006, Satellite M2S-HTSC-VIII Conference, Vaalbeek, Belgium, July 6-8, 2006 .
This work was done in
collaboration with: · G.Crabtree, U.Welp, V.Vlasko-Vlasov,
Materials Science Division, Argonne National Laboratory
· B.Ilic, Cornell University
· S.R.J.Brueck, University of New Mexico
· A. Zhukov, University of Southampton, UK
· L. DeLong, University of Kentucky
· S.Field, Colorado State University
· M.Roseman, P. Grütter, McGill
University, Canada
· V.V. Moshchalkov,Y. Bruynseraede, K.U.
Leuven, Belgium
