Nanostructured Superconductors

The evolution of superconductivity as the size of the superconductor is reduced towards atomic length scales has been an active field of research for many decades. The reduction in size of the superconductor has two effects: (i) The quasi-continuous energy levels in the conduction band become discrete (with the spacing between energy levels known as the Kubo gap), and (ii) An effective softening of the phonon spectrum happens due to large fraction of atoms being on the surface with lower coordination number than the bulk atoms. The formation of the Kubo gap causes an effective decrease in the density of states at Fermi level and an associated decrease in the superconducting transition temperature. The softening of the phonon spectrum causes an increase in the electron-phonon coupling strength. In weak coupling superconductors this effect is believed to give rise to an increase in Tc. In strong coupling superconductor this increase is often offset by the increase zero energy cut off of the phonon spectrum giving rise to an effective decrease in Tc. However, despite decades of intensive research the relative strength of these two effects have still remained unclear.

Our system of choice to study nanoscale superconductors is in the form of nanostructured thin films grown by our collaborators in the Nanomaterials Group, through r.f. magnetron sputtering. Using this technique, the size of the superconducting grains can be controlled from a few nanometers to few tens of nanometers. Through a detailed measurement of the superconducting energy gap and Tc in nanostructured Nb and Pb films we could establish a sceme to discriminate between the two dominant mechanisms governing Tc at nanometer length scales. Our results showed that in Nb the Tc gradually decreases below 20nm due to the reduction in the density of states at Fermi level arising from quantum size effects. In Pb, this is offset by the increase in surface phonon softening keeping the Tc almost constant down to 10nm.

(To see our publications in this area go to the publications page)

Some more details can be found here:

Disordered Superconductors

Quaternary Borocarbides

Spin Polarization Measurements using PCAR