Transcript Slide 1
ORGANIC SUPERCONDUCTORS by Eugenio Gutierrez Joe Nowell Physics 335 Evolution of Organic SC 1908- Kamerlingh liquifies helium 1911- low temp. resistivity of mercury 1981- Bechgard superconducts first organic material 1964- Bill Little predicts High critical temperature 2004- 400 organic conductors have been synthesized, over 50 of which are superconducting Principles of Superconductors •Abnormally high conductivity •Zero resistivity at low temperatures current flows for long periods of time without dissipation. •Perfect Diamagnet dB/dt=0 induced magnetic field cancels out any applied field. •Superconducting state can be destroyed by a strong enough magnetic field •Magnetic flux is ejected in the presence of a weak magnetic field (Meissner Effect- zero magnetic field of a SC) Meissner Effect • Zero resistance would imply that if you tried to magnetize a superconductor, current loops would be generated to exactly cancel the imposed field (Lenz’s law). • If there were no change in the applied magnetic field, there would be no generated voltage (Faraday’s law) to drive currents, even in a perfect conductor. Faraday’s law summarizes how voltage can be generated. BCS Theory-Cooper Pairs • The effective net attraction between the normally repulsive electrons produces a pair binding energy on the order of milli-electron volts, enough to keep them paired at very low temperatures. Cooper Pair Attraction • A passing electron attracts the lattice, causing a slight ripple toward its path. • Another electron passing in the opposite direction is attracted to that displacement. Critical Temperatures Metals: • Aluminum 1.2K • Lead 7.2K • YBaCu oxide 92K • TlBaCu oxide 125K Organics: • K3C60 18K • C60 w/I Cl >60K • SWCNT 15K Charge Transfer Salts •Consists of segregated stacks of donor and acceptor molecules •Donor molecules have π orbital delocalization •Need unpaired electrons •Electronegativity is important in acceptors (ClO4, PF6) •Both donor and acceptor are planar Donor molecule example SC fullerenes • Bucky balls • Carbon Nanotubes Fullerenes • Usually doped with metals (K,Rb) Organic SC properties • Tc pressure dependence • Magnetic field induced superconductivity • Isotope effect Organic SC advantages • Generally lighter in weight than other superconductors • Increased ability to fine tune electrical properties Future studies • Increase Tc • More research with large magnetic fields • Use quantum mechanics to take superconductivity beyond BCS theory For More Information • http://hyperphysics.phyastr.gsu.edu/hbase/solids/coop.html • http://superconductors.org/ • http://physics.clarku.edu/superconductor/s uperconductor.html • http://arxiv.org/PS_cache/condmat/pdf/0302/0302157.pdf • Organic Superconductors by Ishiguro, Yamaji and Saito