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superconductors the superconducting gap exists over the entire Fermi
surface, and the electrons pair in a zero angular momentum, spin-singlet
state which is produced by the electron-phonon interaction. A non-
vanishing gap gives rise to exponential temperature dependences for all
measurements involving the thermal excitation of quasiparticles across the
gap: specific heat, magnetic-field penetration depth (X) and nuclear spin
lattice relaxation rate (1/T1), as examples. However, measurements of these
quantities in heavy fermion superconductors all exhibit power-law
temperature dependences, usually considered as evidence for nodes in the
energy gap. Table I gives the expected power-laws for two different kinds
of nodal structures on a spherical Fermi surface, lines and points.
TABLE I
Polar Axial
Cp T.2 T3
lTI T3 T5
1/X211 T3 T2
1/a21 T T4
Xspin T2 T3
Table 1. Temperature dependence of specific heat (Cp), spin lattice relaxation rate
(lIT1), magnetic field penetration depth (for vector potential parallel (X1 1) and
perpendicular (X1) to tie order-parameter symminetry axis) and spin susceptibility (Xs pin)
The polar and axial gap structures correspond to odd-parity states with an equatorial line
of nodes and two point nodes, respectively An isotropic gap produces morc neatly
exponential temperature dependencies
Power -law temperatures dependencies do not provide definitive evidence fi an
uincionventional gap strticture, however 121 (A conventional gap-less supei conductor can
also exhibit power -law behavior, fli example ) More decisive conclusions can only be
dr awn fiomlr tests of the symmetiy-hi caking natore of the or der paitmeter A(k) In
cinventional supet conductors A(k) obeys the symmetry of the lamiltonian, which
includes totittiotnal, reflection (parity) and time-r ever sal syninetry An unconventional
super)conductor has it wei symmuetr y in at least one ofthese resp:"cts .xpei iimcntal tests2
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Heffner, R. H. Muon spin relaxation studies of heavy fermion superconductors, article, June 1, 1993; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1384878/m1/4/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.