Download Organic Superconductors

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