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Published on October 16, 2007

Author: Arkwright26

Source: authorstream.com

Why Make Holes in Superconductors? :  Why Make Holes in Superconductors? Saturday Morning Physics December 6, 2003 Dr. Sa-Lin Cheng Bernstein Type I Superconductors:  Type I Superconductors Two Conditions: Zero resistivity Meissner effect Superconducting state: T < Tc H < Hc Theoretical Understanding:  Theoretical Understanding 1930s – quantum model could not explain superconductivity 1950 – phenomenological theory by Vitaly Ginzburg and Lev Landau Ginzburg-Landau equation Describe superconductivity near Tc 2003 – Ginzburg wins Nobel Prize in physics (Nobel Lectures will be held on December 8) Vitaly L. Ginzburg::  Vitaly L. Ginzburg: The New York Times, October 8, 2003 "They have been nominating me for about 30 years, so in that sense it didn't come out of the blue. But I thought, `Well, they're not giving it to me, I guess that's it.' After all, there are a lot of contenders. So, you know, I had long ago forgotten to think about this." BCS Theory:  BCS Theory BCS = John Bardeen, Leon Cooper, and Robert Schrieffer 1957 – explain why superconductivity occurs Cooper pairs = Pairing of electrons 1972 – Nobel Prize in physics http://superconductors.org Critical Fields in Type II:  Critical Fields in Type II Lower critical field Upper critical field vortices Incomplete Meissner Effect Mixed State in Type II :  Mixed State in Type II Hc1< H < Hc2 Also called “vortex state” Magnetic field can penetrate but not completely Flux lines D. J. Bishop et al., © Scientific American, 48 (Feb. 1993) Abrikosov Vortex Lattice:  Abrikosov Vortex Lattice 1953 – Abrikosov Vortex Lattice Based on Ginzburg – Landau equation Flux lines repel each other Triangular vortex lattice 2003 – Nobel Prize in physics A. A. Abrikosov © AT&T, 1995 Nobel Prize Winning:  Nobel Prize Winning Abrikosov Lattice Images:  Abrikosov Lattice Images First image of Vortex lattice, 1967 Bitter Decoration Pb-4at%In rod, 1.1K, 195G U. Essmann and H. Trauble © Physics Letters 24A, 526 (1967) Vortices in MgB2, 2002 Scanning Tunnel Spectroscopy MgB2 crystal, 2K, 2000G M. R. Eskildsen et al. © Phys. Rev. Lett. 89, 187003 (2002) What is a vortex?:  What is a vortex? Photo courtesy of the National Severe Storms Laboratory Vortex (flux line) in superconductor:  Vortex (flux line) in superconductor Has a core, circled by supercurrents Inside the core: normal electrons Outside the core: superconducting electrons (Cooper pairs) Examples of Type II:  Examples of Type II Mostly compounds Record holder: Tc =138 K High Hc2: Hc2 > 1000 000 G (YBCO) See the Light . . .:  See the Light . . . More useful higher Tc higher Hc2 Easily cooled: He (4.22 K): $5/liter N (77.36 K): 10¢/liter whole milk: 66¢/liter Columbia Pictures The Fifth Element The “but”. . .:  The “but”. . . Dilemma of Type II superconductors Solutions? Magnetic Force on Current:  Magnetic Force on Current Thumb = current Fingers = magnetic field Palm = magnetic force Lorentz force © John Wiley & Sons, Inc. Superconducting Wires:  Superconducting Wires Top View Magnetic field: out of page D. J. Bishop et al., © Scientific American, 48 (Feb. 1993) Resistance in Superconductors:  Resistance in Superconductors Lorentz force pushes vortices (Flux motion) Dissipation of energy Resistance Increase of temperature Quench!!! Critical Surface Phase Diagram:  Critical Surface Phase Diagram Superconducting state: T < Tc H < Hc J < Jc Dilemma:  Dilemma For practical applications, high Tc high Hc high Jc Dilemma: highest Jc is at Tc=0 and Hc=0 Solution:  Solution Prevent flux from moving Tarp (or pin) flux Flux Pinning:  Flux Pinning Defects in crystalline structure Impurities Grain boundaries Artificial pinning centers: Holes (antidots) Magnetic dots Arrays of dots Single defect in a YBa2Cu3O7 film (magneto-optical imaging) http://www.fys.uio.no/super/mo/ Energy Surface:  Energy Surface Potential energy drops discontinuously when the vortex enters the defect zone Experiment Evidence:  Experiment Evidence Rectangular array of antidots http://www.fys.kuleuven.ac.be/vsm/nsm/research/lieve2.html Interstitial Vortices:  Reside in the area between pinning sites Trapped by other vortices (due to mutual repulsion) More mobile Energy Dissipation!! Interstitial Vortices http://www.physics.colostate.edu/groups/fieldgroup/Holes.html Solution:  Solution Large defects Solution2:  Solution2 Magnetic pinning centers Summary:  Summary Type II superconductor: Incomplete Meissner effect in vortex state (Hc1 < H < Hc2) Superconducting state: T < Tc H < Hc2 J < Jc Flux consists of whirlpools supercurrent called vortex Abrikosov vortex lattice = triangular array Interstitial vortices are mobile  energy lost Effective pinning centers: Larger size Magnetic dots Summary

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