6 research outputs found
Nodal Structure of Unconventional Superconductors Probed by the Angle Resolved Thermal Transport Measurements
Over the past two decades, unconventional superconductivity with gap symmetry
other than s-wave has been found in several classes of materials, including
heavy fermion (HF), high-T_c, and organic superconductors. Unconventional
superconductivity is characterized by anisotropic superconducting gap
functions, which may have zeros (nodes) along certain directions in the
Brillouin zone. The nodal structure is closely related to the pairing
interaction, and it is widely believed that the presence of nodes is a
signature of magnetic or some other exotic, rather than conventional
phonon-mediated, pairing mechanism. Therefore experimental determination of the
gap function is of fundamental importance. However, the detailed gap structure,
especially the direction of the nodes, is an unresolved issue in most
unconventional superconductors. Recently it has been demonstrated that the
thermal conductivity and specific heat measurements under magnetic field
rotated relative to the crystal axes are a powerful method for determining the
shape of the gap and the nodal directions in the bulk. Here we review the
theoretical underpinnings of the method and the results for the nodal structure
of several unconventional superconductors, including borocarbide YNiBC,
heavy fermions UPdAl, CeCoIn, and PrOsSb, organic
superconductor, -(BEDT-TTF)Cu(NCS), and ruthenate
SrRuO, determined by angular variation of the thermal conductivity and
heat capacity.Comment: topical review, 55 pages, 35 figures. Figure quality has been reduced
for submission to cond-mat, higher quality figures available from the authors
or from the publishe
Strong-coupling theory of magnetic-exciton-mediated superconductivity in UPd<sub>2</sub>Al<sub>3</sub>
There is compelling evidence from inelastic-neutron-scattering and tunneling
experiments that the heavy-fermion superconductor UPdAl can be
understood as a dual system consisting of magnetic excitons, arising from
crystal-field-split U levels, coupled to delocalised f-electrons. We
have computed the superconducting transition temperature and the mass
renormalisation arising from a dual model with maximal spin anisotropy using a
strong-coupling approach. We find an instability to two possible
opposite-spin-pairing states with even- or odd-parity gap functions. Each has a
line node perpendicular to the c-direction, in agreement with NMR
relaxation-rate, specific-heat and thermal-conductivity measurements. In
addition, both have total spin component =0, compatible with the
observation of a pronounced Knight shift and Pauli limiting. For
parameter values appropriate to UPdAl, we determine the dependence of
the superconducting transition temperature on a phenomenological coupling
constant and we investigate the associated mass enhancement and its
anisotropy.Comment: 11 pages, 2 figures, 1 tabl