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.