Superconductivity Lab
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Research

Our primary focus is on the study of unconventional and disordered superconductors using a variety of techniques such as, scanning tunneling spectroscopy, planar junction spectroscopy, penetration depth measurements, magnetotransport and Hall Effect measurements. The lab is equipped with various cryostats capable of measurements down to 300mK and up to a magnetic field of 12 Tesla. We currently perform scanning tunneling spectroscopy down to 2.6K in home-built scanning tunneling microscope fitted with an in-situ sputter deposition chamber for fabrication of thin films. In addition, we have various deposition facilities for the deposition of superconducting thin films.

When we are not studying superconductors, we dabble with ferromagnets and other correlated electron materials. One of our interests in recent years has been the study of spin polarization and spin fluctuations using Point Contact Andreev Reflection.

Experimental Investigations on disordered superconductors
	Understanding the role of interaction and disorder is at the heart of understanding of collective behavior of many-body quantum systems, such as High temperature superconductors, quantum Hall effect and superfluid He. In superconductors, where the order parameter is described by a complex order parameter with an amplitude and phase (Y=De^i^q), the presence of strong disorder poses an intriguing question: Is the destruction of the superconducting state always associated with the vanishing or the amplitude or could strong phase fluctuations destroy the superconducting state even when the amplitude of the superconducting pairing remains finite? The latter, if true, would give rise to novel electronic states with finite superconducting pairing amplitude but no global superconductivity. Over the past two years we have been addressing this question, by extracting all the key parameters in the superconducting state, on a set of NbN thin films with controlled amount of disorder. We use a number of experimental tools, such the low-temperature scanning tunneling microscope, a two coil mutual inductance technique for the measurement of penetration depth and conventional transport and magnetotransport at high fields and very low temperatures. (To see our publications in this area go to the publication page)
Study of gap anisotropy in quaternary borocarbides
	Anisotropic superconducting energy gaps is normally believed to be associated with the unconventional pairing mechanisms of non-phononic origin such as pairing mediated by ferromagnetic or antiferromagnetic spin fluctuations. Our interests in recent years has been on the study of a relatively less studied class of anisotropic superconductors where large gap anisotropy arises from a different origin, namely, the multiband nature of the Fermi surface. Our model system for these studies has been the quaternary borocarbide superconductor YNi2B2C which was discovered in this institute in 1994. The peculiarity of this system is the existence of very slow and very fast electrons on the Fermi surface at different k directions. Using directional Point Contact Spectroscopy we could demonstrate that the large gap anisotropy in this material arises from the difference is coupling strengths of these slow and fast electrons to the crystalline lattice. Our current emphasis in this area is on the study of impurity scattering on the multiband nature of superconductivity as well as exploration of novel systems such as Lu5Ir4Si10 where the same mechanism is likely to give rise to large gap anisotropy. (To see our publications in this area go to the publication page)
Size effect in nanostructured superconductors
Pb	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 nano-materials 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 publication page)
Spin polarization measurements using Point Contact Andreev Reflection
Temperature dependence of spin polarization and spontaneous magnetization in the low Tc ferromagnet NdNi5.	With the advent of "Spintronics" determining the degree of spin polarization in ferromagnets has gained particular significance. Point Contact Andreev reflection provides one of the simplest techniques to measure the transport spin polarization at Fermi level in ferromagnets. One of our major areas of research is to experimentally determine the spin polarization in ferromagnets that could be potentially useful as spin source in spintronic devices. An extension of this work is to investigate how the process of Andreev reflection gets modified when a superconductor is in contact with a normal metal with strong ferromagnetic spin fluctuations. (To see our publications in this area go to the publication page)