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My broad area of research concerns with the study of bulk and surface electronic properties of different materials where I have primarily worked on meso-scopic superconductors. Using specialized developed in-house experimental techniques like soft-point contact spectroscopy and low frequency electro-dynamic measurements, we probe the superconducting order parameter symmetry and pairing mechanisms of conventional, novel and artificially engineered superconductors and thin films.

I also work in the growth and characterization of organic electronic devices. We have successfully been able to grow, optimize and characterize organic devices for OLED and Solar Cell applications.

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SUPERCONDUCTIVITY

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1. Point contact Andreev reflection studies of a non-centro symmetric superconductor Re6Zr 

In recent times, non-centrosymmetric superconductors (NCS) have attracted considerable interest owing to the complex nature of superconductivity in these materials. In the NCS, where the crystal structure lacks a center of inversion symmetry the superconducting order parameter is characterized by a mixture of spin-singlet spin-triplet states. Despite numerous theoretical predictions, experimental evidence of spin-singlet spin-triplet mixing in NCS has been surprisingly few. A mixed singlet-triplet gap function resulting from antisymmetric spin-orbit coupling would result in two strongly anisotropic gap functions and could in principle also break time reversal symmetry (TRS). Point Contact Andreev Reflection (PCAR) spectroscopy was done on a high quality single crystal of Re6Zr which showed signatures of multiple gaps. From our experiments two possible interpretations emerged. One possibility was that one of the gaps, the bulk gap is the triplet gap which mixes with spin singlet surface gaps. This small triplet-singlet mixing results in the TRS symmetry breaking.  The other possibility is that all gaps are isotropic and the gaps originate from different Fermi sheets with the presence of strong inter-band scattering. In this model, the TRS is broken at the superconducting transition, due to the presence of non-unitary triplet pairing Thus, our results conclusively prove unconventional pairing in this NCS superconductor.

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2.Superconductivity in im-miscible Nb-Cu nanocomposite films

The main objective of the project was to investigate the role of phase fluctuations in the destruction of superconductivity in granular films with array of Josephson junctions in relation with the coupling between the grains. Nb-Cu is an immiscible binary system which is known to be quenched into a metastable alloy phase by suitable growth conditions. The evolution of superconducting properties in these 3D granular films as a function of composition was studied. Superconductivity was seen to be influenced by the coupling between the grains and the presence of phase fluctuations which affected the temperature at which the global phase coherence set in the films. Measurements were done to measure the superfluid stiffness in the films. Interestingly, signatures of the inter-granular and intra-granular coupling was visible in mutual inductance measurements in magnetic field. These were proof of the role of phase fluctuations on superconductivity in these films.

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ORGANIC ELECTRONICS

1. TADF, Exciplex Emission in Xanthone-Carbazole Derivative and Tuning its Electroluminescence with Applied Voltage

Small organic light emitters, capable of colour tuning and having broad emission spectrum is in high demand as they provide colour stability, reproducibility, simple device geometry and high efficiency. Highly efficient TADF OLED devices have been fabricated based on the donor-acceptor-donor (D-A-D) materials. The material designed and made for this study was a xanthone-carbazole (Xan-Cbz) based D-A-D material. Photophysical properties showed blue emission (470 nm) with high fluorescence quantum yield. Further, exciplex formation in blend of Xan-Cbz with NPD was observed at 525 nm. OLEDs based on Xan-Cbz were fabricated using different device geometries. OLED having devices geometry as ITO/PEDOT:PSS/NPD/Xan-Cbz/Bphen/LiF-Al showed EL with features of both neat emission (470 nm) of Xan-Cbz and its exciplex (525 nm) with NPD. Further, colour tuning was observed as a function of applied voltage. Thus, whitish emission from Xan-Cbz devices could be obtained.

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2. Nano Assembly of Dipolar Imidazoanthraquinone Derivatives Leading to Enhanced Hole Mobility

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In this work, imidazoanthraquinone derivatives which could be tailor made to give high dipole moments by changing the side electron donating/withdrawing groups were synthesized by Prof. Agarwal’s group at CEBS. Furthermore, their photo-physical and electro-chemical characterization indicated that they could be good non-fullerene acceptors with P3HT in bulk hetero-junction solar cells. We showed that the specific design of these materials gave a favourable morphology by forming one-dimensional nanostructures through H-bonding and strong p-p interaction which resulted in high charge carrier mobility (2.4 x 10-4 cm2/Vs.) important for solid state devices. Furthermore, they were successfully used as electron acceptors in bulk hetero-junction solar cells (in inverted

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