1. How would you determine the variation of critical field from resistivity
measurements ?

One first measures the resistivity variation with temperature
in zero magnetic field. Such a measurement is repeated for various
applied magnetic fields. The superconducting transition temperature
is noted for each of these fields. One then has a set of magnetic
field and critical temperature values.

2. How can the superconducting condensation energy be estimated from
resistivity measurements ?

Measurements are done as in question 1 above. The zero temperature
critical field (the minimum field needed to reduce the
to zero) can be estimated. Thus
can be
estimated.

3. If one measures the resistivity of a sample and finds that it becomes
zero below a certain temperature can one conclude that the sample
exhibits superconductivity ?

Not quite! In case of a polycrystalline sample, the presence
of a minority phase which is superconducting can give rise to such
behaviour. For making definitive conclusions, one has to back this
up with other measurements.

4. The Meissner fraction for a sample is found to be 50%. To what
susceptibility (in dimensionless units) does this correspond ?

-1/(8)

5. The Meissner fraction is found to be less than 100%. What could
be the reasons for this ?

One possibility is that the sample contains multiple phases
of which only some are superconducting. Another possibility is that
it is a Type-II superconductor and exhibits flux pinning. Yet another
possibility is that the penetration depth of the superconductor is
comparable to the particle size.

6. For a superconductor, why is the heat capacity expected to decreases
exponentially with temperature below
?

This is due to the gap in the excitation spectrum that opens
up below
.

7. In case of nodes in the gap (existence of points in -space
where the gap becomes zero), what is the effect on the temperature
dependence of the heat capacity ?

Below
, the variation will no longer be exponential
but will acquire a power-law behaviour.

8. What are the advantages of local probe techniques such as NMR and
muSR over measurement of bulk properties such as resistivity and heat
capacity?

While bulk quantities such as resistivity and magnetic susceptibility
constitute quick and inexpensive methods for showing the existence
of superconductivity in a sample, local probe techniques such as muSR
and NMR enable one to examine the nature of the superconducting state.
For instance, the variation of the Knight shift and spin-lattice relaxation
rate with temperature as also the muon depolarisation rate with temperature
can throw light on the symmetry of the order parameter.

9. What type of information is obtained from photoemission studies ?

Photoemission studies can determine the band structure of
the material. There are characteristic features associated with the
peak in the density of states near the Fermi level which can be measured
by photoemission. In case angle resolved measurements are done on
single crystals information about the anisotropy can also be obtained.
A possibly negative point about photoemission is that it is sensitive
to only the surface and not the bulk.