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Authors
I.K. Yanson
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The point-contact spectroscopy, in contrast to the tunneling spectrocopy, considers small electrical contacts with direct conductivity. In the normal state, it enables one to measure the spectral function of electron-boson interaction. In the superconducting state, new features appear connected with the energy dependence of excess current. The non-linearities of the current-voltage characteristic are due to: i) The inelastic scattering of electron quasiparticles in the contact region; ii) The energy dependence of the superconducting energy gap, and iii) The non-equilibrium superconducting effects. These effects are discussed from the experimental point of view
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We introduce a two-channel tunneling model to generalize the widely used BTK theory of point-contact conductance between a normal metal contact and superconductor. Tunneling of electrons can occur via localized surface states or directly, resulting in a Fano resonance in the differential conductance $G=dI/dV$. We present an analysis of $G$ within the two-channel model when applied to soft point-contacts between normal metallic silver particles and prototypical heavy-fermion superconductors CeCoIn$_5$ and CeRhIn$_5$ at high pressures. In the normal state the Fano line shape of the measured $G$ is well described by a model with two tunneling channels and a large temperature-independent background conductance. In the superconducting state a strongly suppressed Andreev reflection signal is explained by the presence of the background conductance. We report Andreev signal in CeCoIn$_5$ consistent with standard $d_{x^2-y^2}$-wave pairing, assuming an equal mixture of tunneling into [100] and [110] crystallographic interfaces. Whereas in CeRhIn$_5$ at 1.8 and 2.0 GPa the signal is described by a $d_{x^2-y^2}$-wave gap with reduced nodal region, i.e., increased slope of the gap opening on the Fermi surface. A possibility is that the shape of the high-pressure Andreev signal is affected by the proximity of a line of quantum critical points that extends from 1.75 to 2.3 GPa, which is not accounted for in our description of the heavy-fermion superconductor.
The newly discovered oxypnictide family of superconductors show very high critical temperatures of up to 55K. Whilst there is growing evidence that suggests a nodal order parameter, point contact Andreev reflection spectroscopy can provide crucial information such as the gap value and possibly the number of energy gaps involved. For the oxygen deficient NdFeAsO0.85 with a Tc of 45.5K, we show that there is clearly a gap value at 4.2K that is of the order of 7meV, consistent with previous studies on oxypnictides with lower Tc. Additionally, taking the spectra as a function of gold tip contact pressure reveals important changes in the spectra which may be indicative of more complex physics underlying this structure.
An overview of the recent efforts in point-contact (PC) spectroscopy of the nickel borocarbide superconductors RNi2B2C in the normal and superconducting (SC) state is given. The results of measurements of the PC electron- boson(phonon) interaction spectral function are presented. Phonon maxima and crystalline-electric-field (CEF) excitations are observed in the PC spectra of compounds with R=Dy, Ho, Er and Tm, while for R=Y a dominant phonon maximum around 12 meV is characteristic. Additionally, non-phonon and non-CEF maxima are observed near 3 meV in R=Ho and near 6 meV in R=Dy. Directional PC study of the SC gap gives evidence for the multi-band nature of superconductivity in R=Y, Lu. At low temperature the SC gap in R=Ho exhibits a standard single-band BCS-like dependence, which vanishes above T_c^*= 5.6K< T_c=8.5K, where a specifc magnetic ordering starts to play a role. For R=Tm (T_c=10.5 K) a decrease of the SC gap is observed below 5 K.
In the hole-doped $d_{x^{2}-y^{2}}$-wave cuprate superconductor, due to the midgap surface state (MSS), a zero bias conductance peak (ZBCP) is widely observed in [110] interface point contact spectroscopy (PCS). However, ZBCP of this geometry is rarely observed in the electron-doped cuprates, even though their pairing symmetry is still likely the $d_{x^{2}-y^{2}}$-wave. We argue that this is due to the coexistence of antiferromagnetic (AF) and the superconducting (SC) orders. Generalizing the Blonder-Tinkham-Klapwijk (BTK) formula to include an AF coupling, it is shown explicitly that the MSS is destroyed by the AF order. The calculated PCS is in good agreement with the experiments.
Point-contact Andreev reflection spectroscopy (PCAR) has proven to be one of the most powerful tools in the investigation of superconductors, where it provides information on the order parameter (OP), a fundamental property of the superconducting state. In the past 20 years, successive improvements of the models used to analyze the spectra have continuously extended its capabilities, making it suited to study new superconductors with exotic properties such as anisotropic, nodal and multiple OPs. In Fe-based superconductors, the complex compound- and doping-dependent Fermi surface and the predicted sensitivity of the OP to fine structural details present unprecedent challenges for this technique. Nevertheless, we show here that PCAR measurements in Fe-based superconductors carried out so far have already greatly contributed to our understanding of these materials, despite some apparent inconsistencies that can be overcome if a homogeneous treatment of the data is used. We also demonstrate that, if properly extended theoretical models for Andreev reflection are used, directional PCAR spectroscopy can provide detailed information not only on the amplitude and symmetry of the OPs, but also on the nature of the pairing boson, and even give some hints about the shape of the Fermi surface.
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