Point-contact spectroscopy (PCS) is an experimental tool used for investigations of the interaction
mechanisms between electron-electron, electron-phonon, electron-magnon *etc*. in metallic systems.
The Drude model which is based on the diffusive motion of electrons describes the resistivity of
a normal metal for length scales greater than the mean free path *l*. But for length scales shorter
than *l*, the electrons move ballistically and this regime can be accessed by tunnelling into a material
from a point contact of size *a << l*. In a ballistic contact, information about the spectral weight of
interactions like electron-phonon, electron-magnon *etc*. can be obtained by measuring the
differential conductance *(dI/dV)* or its derivative w.r.t. the applied voltage V. However, new
phenomena are observed when the point-contact is established between a superconductor (S)
and N *i.e.* in a N/S junction. Below the transition temperature of the superconductor,
the resistivity drops to zero and an energy gap △ = 10^{-3} eV opens up in the density of states (DOS).
In a N/S point-contact transport measurement, three major processes can occur at the interface:

- Transmission;
- Reflection;
- Andreev reflection.

Process 1, *i.e.* transmission, occurs if the energy of the incident electron from the
N side is less than the superconducting band gap, *E > *△. For the case of *E < *△
if the dimension
of the point contact is such that *a ≥ l* process 2 is observed. However, for the case of
small enough point contact in the ballistic regime, *a ≪ l*, process 3 *i.e.* Andreev reflection
takes place at the N/S interface and the differential conductance increases by a factor
of 2 from that above the gap.

Physically, the point contact Andreev reflection (PCAR), as shown in Figure 1,
can be characterized as a process where for a spin up electron from the N side
to be injected with *E < *△ and be transmitted in the S side, it is required to form
a Cooper pair with an electron of opposite spin in the superconductor.
Consequently, a hole is retroreflected back with opposite spin and momentum
of the incident electron.

The measured *(dI/dV)* as a function of applied V is modelled according to the
Blonder-Klapwijk-Tinkham (BTK) theory and the energy gap △,
lower bounds for the BCS coherence length ΞΎ and the Fermi velocity
*v _{F}* for a superconductor.

In our group we have developed indigenously a low temperature PCAR set up
for studies of both BCS superconductors and high *T _{C}* cuprates. The tips
used are W, Au and Nb, where the superconducting Nb tip is also used
for measurement of spin polarization of magnetic materials. The tip
is moved using a programmable motor with sub-micron sensitivity.
The electrical measurements are done using state-of-the-art Keithley
electronic systems while a labview program is used to run the measuring devices.