5 research outputs found
Andreev Reflection in Heavy-Fermion Superconductors and Order Parameter Symmetry in CeCoIn_5
Differential conductance spectra are obtained from nanoscale junctions on the
heavy-fermion superconductor CeCoIn along three major crystallographic
orientations. Consistency and reproducibility of characteristic features among
the junctions ensure their spectroscopic nature. All junctions show a similar
conductance asymmetry and Andreev reflection-like conductance with reduced
signal (~ 10%-13%), both commonly observed in heavy-fermion superconductor
junctions. Analysis using the extended Blonder-Tinkham-Klapwijk model indicates
that our data provide the first spectroscopic evidence for
symmetry. To quantify our conductance spectra, we propose a model by
considering the general phenomenology in heavy fermions, the two-fluid
behavior, and an energy-dependent density of states. Our model fits to the
experimental data remarkably well and should invigorate further investigations.Comment: 4 pages, 4 figures; Phys. Rev. Lett., published versio
Andreev reflection and order parameter symmetry in heavy-fermion superconductors: the case of CeCoIn
We review the current status of Andreev reflection spectroscopy on the heavy
fermions, mostly focusing on the case of CeCoIn, a heavy-fermion
superconductor with a critical temperature of 2.3 K. This is a well-established
technique to investigate superconducting order parameters via measurements of
the differential conductance from nanoscale metallic junctions. Andreev
reflection is clearly observed in CeCoIn as in other heavy-fermion
superconductors. The measured Andreev signal is highly reduced to the order of
maximum 13% compared to the theoretically predicted value (100%).
Analysis of the conductance spectra using the extended BTK model provides a
qualitative measure for the superconducting order parameter symmetry, which is
determined to be -wave in CeCoIn. A phenomenological model is
proposed employing a Fano interference effect between two conductance channels
in order to explain both the conductance asymmetry and the reduced Andreev
signal. This model appears plausible not only because it provides good fits to
the data but also because it is highly likely that the electrical conduction
occurs via two channels, one into the heavy electron liquid and the other into
the conduction electron continuum. Further experimental and theoretical
investigations will shed new light on the mechanism of how the coherent
heavy-electron liquid emerges out of the Kondo lattice, a prototypical strongly
correlated electron system. Unresolved issues and future directions are also
discussed.Comment: Topical Review published in JPCM (see below), 28 pages, 9 figure
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