Research Areas
The research interest in our group is
broadly of the study of superconductors using tunneling and ballistic electron
transport spectroscopy. We study unconventional and disordered superconductors
using a combination of Point Contact Andreev reflection spectroscopy, planar
tunnel junctions and scanning tunneling microscopy. Our current activities are
on studying the evolution of superconducting properties with disorder in
strongly homogeneously disordered epitaxial NbN thin films using a combination
of transport and spectroscopic techniques.
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Studies
on disordered superconductors |
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The effect of disorder on
superconductivity which has been a topic of considerable interest for several
decades has been enjoying a serious revival, which is primarily motivated
around the phenomenon of the loss of bulk superconductivity not coincident
with the loss of pairing. While this is natural in granular superconductors where bulk
superconductivity can be destroyed through phase fluctuations between grains,
while the pairing amplitude inside individual grains remains finite, the same
is not expected in a homogeneously disordered system. However, it has been
suggested in recent years that in the presence of strong disorder where the
mean-free path is of the order of the inverse Fermi wave vector the
superconductor can spontaneously break up into domains, giving rise to a
scenario similar to granular systems. Superconductivity can get destroyed due
to phase fluctuations between these domains, giving rise to novel metallic
and insulating phases with finite pairing amplitude but no global superconductivity. Over the past couple of years
we have been exploring the evolution of superconductivity with disorder in
epitaxial NbN thin films where the disorder can be controlled by tuning the
growth conditions. We observe that as the disorder is increased the
superconductor normal metal transition is no longer associated with a
vanishing of the superconducting energy gap, but rather a large broadening of
the density of states. Our studies strongly suggest the existence of
pseudogap state above Tc with finite amplitude of the
superconducting order parameter but no global phase coherence. Students
and post-docs: Madhavi Chand, Anand
Kamlapure, Mintu Mondal, Archana Mishra and S. P. Chockalingam References: Tunneling studies in a homogeneously disordered s-wave
superconductor: NbN S. P. Chockalingam, Madhavi Chand, Anand Kamlapure, John Jesudasan,
Archana Mishra, Vikram Tripathi and Pratap Raychaudhuri Phys. Rev. B 79, 094509 (2009). Superconducting properties and Hall effect in epitaxial
NbN thin films S. P. Chockalingam, Madhavi Chand, John Jesudasan, Vikram
Tripathi and Pratap Raychaudhuri Phys. Rev. B 77, 214503 (2008) |
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Study
of gap anisotropy in multiband superconductors |
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Temperature dependence of the
large and small gap in YNi2B2C measured through
directional Point Contact Spectroscopy. The presence of fast and slow
electrons give rise to two apparent Tcs, with the smaller gap closing at a temperature smaller than the bulk
transition temperature. |
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 electron 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. Students and Post docs: Sourin
Mukhopadhyay, Goutam Sheet, Deepshikha Jaiswal-Nagar References: Magnetic-field dependence of
superconducting energy gaps in YNi2B2C: Evidence of
multiband superconductivity Evidence of gap anisotropy in
superconducting YNi2B2C using directional point contact spectroscopy |
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Size effect in nanostructured superconductors |
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Bright field and dark field
transmission electron picture of the nanostructured Nb films with grain size
of 60nm (upper) and 5 nm (lower). |
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. Students and Post docs: Sangita Bose, S P Chockalingam,
Charu Galande References: Mechanism of the size dependence of superconductivity in
nanocrystalline Nb Size induced metal–insulator transition in nanostructured niobium thin
films: intra-granular and inter-granular contributions Upper critical field
in nanostructured Nb: Competing effects of the reduction in density of states
and the mean free path Submitted: cond-mat/0609202 |
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Variation of the superconducting
energy gap with Tc for nanostructured Nb film with different
particle sizes. The linear variation shows that Nb remains in the
intermediate coupling limit down to the smallest particle size. |
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Al/Al2O3/nanostructured Pb
junctions fabricated through thermal evaporation and sputtering to measure
the variation of superconducting energy gap with particle size. |
Temperature variation of the
superconducting energy gap for Pb with varying particle size. |
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Spin polarization measurements using Point Contact Andreev Reflection |
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Temperature variation of the spontaneous magnetization and transport spin polarization 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. Students and Post docs: Sourin Mukhopadhyay, Goutam Sheet References: Temperature dependence of transport spin polarization in NdNi5 measured using Point Contact Andreev reflection Sourin Mukhopadhyay, Pratap Raychaudhuri, Devang A. Joshi, C. V. Tomy Submitted: cond-mat/0608214 CeMnNi4: A soft ferromagnet with a high degree of transport spin polarization Surjeet Singh, Goutam Sheet, Pratap Raychaudhuri and Sudesh Kumar Dhar Appl.
Phys. Lett. 88, 022506 (2006).
High spin polarization in the ferromagnetic filled skutterudites KFe4Sb12 and NaFe4Sb12 Goutam Sheet, H. Rosner, S. Wirth, A. Leithe-Jasper, W. Schnelle, U. Burkhardt, J. A. Mydosh, P. Raychaudhuri, and Yu. Grin Phys.
Rev. B 72, 180407 (2005). Transport spin polarization in
SrRuO3 measured through point-contact
Andreev reflection
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