Pabitra joined the Superconductivity and Magnetism Group in 2008 as an MPAGS funded PhD student working under the supervision of Dr Martin R Lees and Prof. Don McK Paul. Following his graduation Pabitra spent After a three years as an instrument scientist at the Paul Scherrer Institute in Switzerland. Pabitra is currently an Instrument Scientist in the Muon Spectroscopy Group at ISIS-RAL.
During his Ph.D. Pabitra worked on a number of unconventional superconductors including Lu2Fe3Si5, CaAlSi, ZrB12, and Re3W. He also worked on the newly discovered iron-based superconductors FeSe, FeTe0.5Se0.5, and KFe2Se2.
He grew polycrystalline and single crystal samples and characterised them using XRD and EDX, as well as transport, heat capacity and magnetometry measurements using the PPMS, VSM, and the MPMS SQUID magnetometers available at Warwick. He performed Small Angle Neutron Scattering (SANS) experiments at the ILL in France and at the PSI in Switzerland. He also worked at ISIS, the spallation source in the Rutherford Appleton Laboratory in Oxfordshire carrying out muon-spin-spectroscopy studies.
Thesis Title: Studies of Unconventional Superconductors (PDF Download: 15 MB).
In this thesis, the superconducting properties of some unconventional superconductors have been investigated using low temperature magnetic, thermal and transport measurements, small angle neutron scattering, and muon spin rotation/ relaxation techniques. The aim was to correlate the symmetry and structure of the superconducting gap with the unusual properties in these superconductors. These studies have required the preparation of high quality samples using different growth techniques. Good quality polycrystalline and single crystal samples of FeSe1-xTex and FeTe1-xSx were grown using a self-flux method. Polycrystalline samples of Lu2Fe3Si5 and Re3W were made using the arc furnace. We have also grown single crystals of ZrB12 using the optical floating zone method in a 4 mirror image furnace, and CaAlSi crystal using the Bridgman method. All the compounds have been characterised with a combination of X-ray, neutron diffraction, EDX, magnetisation, resistivity or specific heat measurements. In order to investigate the pairing symmetry of the iron chalcogenide superconductors, low temperature muon spin rotation/relaxation (muSR) measurements have been performed on FeTe0.5Se0.5. The temperature dependence of the in-plane magnetic penetration depth is found to be compatible with either a two gap s + s-wave or an anisotropic s-wave model. This result is consistent with our heat capacity data collected on the same sample. muSR results of FeTe1-xSx show an antiferromagnetic transition at low temperature and also suggest the presence of excess S in the samples. A similar magnetic transition has also been observed in the magnetization measurements. The symmetry of the superconducting gap of Lu2Fe3Si5 with Tc = 6.1 K has been investigated using low-temperature transverse-field muSR and specific heat measurements. The temperature dependence of the magnetic penetration depth, λ(T) is consistent with a two gap s+s-wave model. Low-temperature specific heat measurements on the same sample also show evidence of two distinct superconducting gaps and hence support the muon results. To resolve whether CaAlSi is a single band or multiband superconductor, we have studied the flux line lattice in CaAlSi using small angle neutron scattering. A well defined hexagonal flux line lattice is seen just above Hc1 in an applied field of only 54 Oe. A 30 degree reorientation of this vortex lattice has been observed in a very low field of 200 Oe. This reorientation transition appears to be of first-order and could be explained by non-local effects. The magnetic field dependence of the form factor is well described by a single penetration depth and a single coherence length. The penetration depth anisotropy has also been estimated with the field applied at different angles to the c-axis. The B-T phase diagram of superconducting ZrB12 has been investigated by means of muSR spectroscopy using a mosaic of single crystal. The local field distribution for different applied fields and temperatures shows evidence of the Meissner, mixed, and intermediate states in ZrB12. The intermediate state indicates that this material has some of the characteristics of a type-I superconductor, while the mixed state is typical of a type-II superconductor. Regions of coexistence have also been observed between the different states. We have not observed any distinct features of two-band or two-gap superconductivity in this material. Two different superconducting phases of Re3W have been found with different physical properties. One phase crystallises in a non-centrosymmetric cubic (alpha-Mn) structure and has a superconducting transition temperature, Tc , of 7.8 K. The other phase has a hexagonal centrosymmetric structure and is superconducting with a Tc of 9.4 K. Switching between the two phases is possible by annealing the sample or re-melting it. The zero-field muSR results indicate that time reversal symmetry is preserved for both structures of Re3W. For both phases of Re3W, the temperature dependence of the penetration depth can be explained using a single gap s-wave BCS model. Low temperature specific heat data also provide evidence for an s-wave gap-symmetry for the two phases of Re3W. Both the muSR and heat capacity data show that the CS material has a higher Tc and a larger superconducting gap Δ(0) at 0 K than the NCS compound. The experimental work detailed in this thesis provides new information on the superconducting properties of FeTe0.5Se0.5, FeTe1-xSx, Lu2Fe3Si5, CaAlSi, ZrB12, and two different superconducting phases of Re3W and contributes to our overall understanding of the physics of the different exotic superconducting features in these systems.
P. K. Biswas, M. R. Lees, G. Balakrishnan, D. Q. Liao, J. L. Gavilano, N. Egetenmeyer, C. D. Dewhurst, and D. McK. Paul, First-Order Reorientation of the Flux-Line Lattice in CaAlSi, Physical Review Letters 108, 077001 (2012).
P. K. Biswas, M. R. Lees, A. D. Hillier, and D. McK. Paul, Comparative study of the centrosymmetric and non-centrosymmetric superconducting phases of Re3W using Muon-spin-spectroscopy and heat capacity measurements, Physical Review B 85, 134505 (2012).
P. K. Biswas, M. R. Lees, A. D. Hillier, R. I. Smith, W. G. Marshall, and D. McK. Paul, Structure and superconductivity of two different phases of Re3W, Physical Review B 84, 184529 (2011).
K. Sardar, G. Catalan, J. Hong, P. K. Biswas, M. R. Lees, R. I. Walton, J. F. Scott and S. A. T, Redfern, Structural, electrical, magnetic and spectroscopic characterisation of polycrystalline Bi1-xCaxFeO3-x/2 (x≤0.1), J. Phys.: Condens. Matter 24, 045905 (2012).
P. K. Biswas, M. R. Lees, A. D. Hillier, and D. McK. Paul, Coexistence of type-I and type-II superconductivity in the single crystal of ZrB12, (in preparation).
P. K. Biswas, A. D. Hillier, G. Balakrishnan, D. M. Paul, and M.R. Lees, Structural, magnetic and transport properties of FeTe1−xSx (0.1 ≤ x ≤ 0.5), (manuscript under revision).
Training events during the thesis work:
26 November, 2008: IOP Conference on "Low Temperature Techniques" course, East Midlands Conference Centre, Nottingham, UK.
17 February, 2009: Workshop on Materials Microscopy, Warwick University, UK.
23-27 February, 2009: Neutron Training Course, ISIS, Oxford, UK.
27 May, 2009: Seminar on Mathematica 7, Warwick University, UK.
15-17 December, 2009: IOP event on Condensed Matter and Materials Physics, CMMP 2009, Warwick University, UK.
5 July, 2010: IOP Superconductivity Group Summer Science Meeting, London, UK.
29 August-3 September: SCES'11, Cambridge, UK.
ISIS Neutron and Muon Source
Science and Technology Facilities Council
Rutherford Appleton Laboratory
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