VIEWS: 16 PAGES: 14 POSTED ON: 11/26/2011
Superconductivity • Resistance goes to 0 below a critical temperature Tc element Tc resistivity (T=300) Ag --- .16 mOhms/m Cu -- .17 mOhms/m Ga 1.1 K 1.7 mO/m Al 1.2 .28 Sn 3.7 1.2 Res. Pb 7.2 2.2 Nb 9.2 1.3 T • many compounds (Nb-Ti, Cu-O-Y mixtures) have Tc up to 90 K. Some are ceramics at room temp P461 - Semiconductors 1 Superconductors observations • Most superconductors are poor conductors at normal temperature. Many good conductors are never superconductors • superconductivity due to interactions with the lattice • practical applications (making a magnet), often interleave S.C. with normal conductor like Cu • if S.C. (suddenly) becomes non-superconducting (quenches), normal conductor able to carry current without melting or blowing up • quenches occur at/near maximum B or E field and at maximum current for a given material. Magnets can be “trained” to obtain higher values P461 - Semiconductors 2 Superconductors observations • For different isotopes, the critical temperature depends on mass. ISOTOPE EFFECT M 0.5Tc cons tan t ( Sn115,117,119 ) K Evibrations M • again shows superconductivity due to interactions with the lattice. If M infinity, no vibrations, and Tc 0 • spike in specific heat at Tc • indicates phase transition; energy gap between conducting and superconducting phases. And what the energy difference is • plasma gas liquid solid superconductor P461 - Semiconductors 3 What causes superconductivity? • Bardeen-Cooper-Schrieffer (BCS) model • paired electrons (cooper pairs) coupled via interactions with the lattice • gives net attractive potential between two electrons • if electrons interact with each other can move from the top of the Fermi sea (where there aren’t interactions between electrons) to a slightly lower energy level • Cooper pairs are very far apart (~5,000 atoms) but can move coherently through lattice if electric field resistivity = 0 (unless kT noise overwhelms breaks lattice coupling) atoms electron electron P461 - Semiconductors 4 Conditions for superconductivity • Temperature low enough so the number of random thermal phonons is small • interactions between electrons and phonons large ( large resistivity at room T) • number of electrons at E = Fermi energy or just below be large. Phonon energy is small (vibrations) and so only electrons near EF participate in making Cooper pairs (all “action” happens at Fermi energy) • 2 electrons in Cooper pair have antiparallel spin space wave function is symmetric and so electrons are a little closer together. Still 10,000 Angstroms apart and only some wavefunctions overlap (low E large wavelength) P461 - Semiconductors 5 Conditions for superconductivity 2 • 2 electrons in pair have equal but opposite momentum. Maximizes the number of pairs as weak bonds constantly breaking and reforming. All pairs will then be in phase (other momentum are allowed but will be out of phase and also less probability to form) ip r Ppair p1 p2 0 e different times different pairs • if electric field applied, as wave functions of pairs are in phase - maximizes probability -- allows collective motion unimpeded by lattice (which is much smaller than pair size) | total |2 | 1 2 .... n |2 P461 - Semiconductors 6 Energy levels in S.C. • electrons in Cooper pair have energy as part of the Fermi sea (E1 and E2=EFD) plus from their binding energy into a Cooper pair (V12) E12 E1 E2 V12 • E1 and E2 are just above EF (where the action is). If the condition E12 2 EF is met then have transition to the lower energy superconducting state 2 EF Egap normal E12 s.c. TC Temperature • can only happen for T less than critical temperature. Lower T gives larger energy gap. At T=0 (from BCS theory) E gap 3kTC P461 - Semiconductors 7 Magnetic Properties of Materials • H = magnetic field strength from macroscopic currents • M = field due to charge movement and spin in atoms - microscopic B 0 (H M ) M cH c magnetic susceptibility can be : c (T ), c ( H ), scalar, vector • can have residual magnetism: M not equal 0 when H=0 • diamagnetic c < 0. Currents are induced which counter applied field. Usually .00001. Superconducting c = -1 (“perfect” diamagnetic) P461 - Semiconductors 8 Magnetics - Practical • in many applications one is given the magnetic properties of a material (essentially its c) and go from there to calculate B field for given geometry beamline sweeping magnet spectrometer air- gap analysis magnet D0 Iron Toroid P461 - Semiconductors 9 Paramagnetism • Atoms can have permanent magnetic moment which tend to line up with external fields • if J=0 (Helium, filled shells, molecular solids with covalent S=0 bonds…) c = 0 c 10 4 most , c 10 5 Fe • assume unfilled levels and J>0 n = # unpaired magnetic moments/volume n+ = number parallel to B n- = number antiparallel to B n = n+ + n- • moments want to be parallel as E B B (antiparallel ) B ( parallel) P461 - Semiconductors 10 Paramagnetism II • Use Boltzman distribution to get number parallel and antiparallel B / kT n Ce n n Ce B / kT n M ( n n ) • where M = net magnetic dipole moment per unit volume M e B / kT e B / kT average B / kT n e e B / kT if B kt (1 B / kT ) (1 B / kT ) 2 B (1 B / kT ) (1 B / kT ) kT • can use this to calculate susceptibility(Curie Law) B 0 H 0 M 0 H ( c small ) M n n 2 B 0 n 2 c H H kTH kT P461 - Semiconductors 11 Paramagnetism III • if electrons are in a Fermi Gas (like in a metal) then need to use Fermi-Dirac statistics 1 n C ( B E F ) / kT n e 1 1 n C ( B E F ) / kT n e 1 • reduces number of electrons which can flip, reduces induced magnetism, c smaller antiparallel B 0 kT EF parallel EF 2B turn on B field. shifts by B antiparallel states drop to lower energy parallel P461 - Semiconductors 12 Ferromagnetism • Certain materials have very large c (1000) and a non-zero B when H=0 (permanent magnet). c will go to 0 at critical temperature of about 1000 K ( non ferromagnetic) 4s2: Fe26 3d6 Co27 3d7 Ni28 3d8 6s2: Gd64 4f8 Dy66 4f10 • All have unfilled “inner” (lower n) shells. BUT lots of elements have unfilled shells. Why are a few ferromagnetic? • Single atoms. Fe,Co,Ni D subshell L=2. Use Hund’s rules maximize S (symmetric spin) spatial is antisymmetric and electrons further apart. So S=2 for the 4 unpaired electrons in Fe • Solids. Overlap between electrons bands but less overlap in “inner” shell overlapping changes spin coupling (same atom or to adjacent atom) and which S has lower energy. Adjacent atoms may prefer having spins parallel. depends on geometry internuclear separation R P461 - Semiconductors 13 Ferromagnetism II • R small. lots of overlap broad band, many possible EF energy states and magnetic effects diluted • R large. not much overlap, P A energy difference vs EF small P A • R medium. broadening of energy band similar to EF magnetic shift almost all in state vs E(unmagnetized)- Fe Co Ni E(magnetized) R Mn P461 - Semiconductors 14