Anatomy of point-contact Andreev reflection spectroscopy from the experimental point of view (review)

We review application of point-contact Andreev-reflection spectroscopy to study elemental superconductors, where theoretical conditions for the smallness of the point-contact size with respect to the characteristic lengths in the superconductor can be satisfied. We discuss existing theoretical models and identify new issues that have to be solved, especially when applying this method to investigate more complex superconductors. We will also demonstrate that some aspects of point-contact Andreev-reflection spectroscopy still need to be addressed even when investigating ordinary metals.Comment: 20 pages, 18 figs. V2: Ref.60 and footnote 3 are added, a number of minor fixe

Investigation of the superconducting energy gap in the compound LuNi2_{2}B2_{2}C by the method of point contact spectroscopy: two-gap approximation

It is shown that the two-gap approximation is applicable for describing the $dV/dI(V)$ spectra of LuNi$_{2}$B$_{2}$C-Ag point contacts in a wide interval of temperatures. The values and the temperature dependences of the large and the small gaps in the $ab$ plane and in the $c$ direction were estimated using the generalized BTK model and the equations of Beloborodko. In the BCS extrapolation the critical temperature of the small gap is 10 $K$ in the $ab$ plane and 14.5 $K$ in the $c$ direction. The absolute values of the gaps are $\Delta_0^{ab}=2.16$ $meV$ and $\Delta_0^c=1.94$ $meV$. For the large gaps the critical temperature coincides with the bulk $T_c$, $T_c^{bulk}=16.8$ $K$, and their absolute values are very close, being about 3 $meV$ in both orientations. In the $c$ direction the contributions to the conductivity from the small and the large gaps remain practically identical up to $10 \div 11$ $K$. In the $ab$ plane the contribution from the small gap is much smaller and decreases rapidly as a temperature rises.Comment: 10 pages, 10 figures, submitted to Fiz. Nizk. Temp. (Low Temp. Phys.

The Superconducting Gap Behavior in the Antiferromagnetic Nickel-Borocarbide Compounds RNi2B2C (R=Dy, Ho, Er, Tm) Studied by Point-Contacts Spectroscopy

An general survey of the superconducting (SC) gap study in the title compounds by point-contact (PC) spectroscopy is presented. The SC gap was determined from dV/dI of PCs employing the well-known theory of conductivity for normal metal-superconductor PCs accounting Andreev reflection. The theory was modified by including pair-breaking effects considering the presence of magnetic rare-earth ions. A possible multiband structure of these compounds was also taken into account. The PC study of the gap in the Er-compound (TN=6K<Tc=11K) gives =evidence for the presence of two SC gaps. Additionally, a distinct decrease of both gaps is revealed for R = Er in the antiferromagnetic (AF) state. For R = Tm (TN=1.5K<Tc=10.5K) a decrease of the SC gap is observed below 4-5K, while for R = Dy (TN=10.5K>Tc=6.5K) the SC gap has a BCS-like dependence in the AF state. The SC gap for R = Ho (TN=5.2K<Tc=8.5K) exhibits below T*=5.6K a single-band BCS-like dependence vanishing above T*, where a specific magnetic order occurs. The difference in the SC gap behavior in the title compounds is attributed to different AF ordering.Comment: 4 pages, 4 figs., submitted to LT25; V2. missed author is adde