Studies of unconventional superconductors

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- ux method. Polycrystalline samples of Lu2Fe3Si5 and Re3W were made using the arc furnace. We have also grown single crystals of ZrB12 using the optical oating zone method in a 4 mirror image furnace, and CaAlSi crystal using the Bridgman method. All the compounds have been characterized with a combination of X-ray, neutron diffraction, EDX, magnetization, resistivity or specific heat measurements. In order to investigate the pairing symmetry of the iron chalcogenide superconductors, low temperature muon spin rotation/relaxation (μSR) 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. μSR 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 μSR 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 ux line lattice in CaAlSi using small angle neutron scattering. A well defined hexagonal ux line lattice is seen just above Hc1 in an applied field of only 54 Oe. A 30° 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 μSR 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 crystallizes in a non-centrosymmetric cubic (α-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 remelting it. The zero-field μSR 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 singlegap 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 μSR and heat capacity data show that the CS material has a higher Tc and a larger superconducting gap (0) at 0 K than the NCS compound. The experimental work detailed in this thesis provides new information on the superconducting properties of FeSe0:5Te0:5, 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

The effectiveness of ultrasonogram guided intervention in chronic shoulder pain cases attending pain clinic of a government hospital of East zone, India: a prospective observational study

Background: Ultrasonogram guided intervention in chronic shoulder pain is well known for relief of pain and improvement of range of movement (ROM) of affected shoulder joint. Ultrasonogram gives accuracy helps in localized corticosteroid delivery. Corticosteroids relieve the pain by its anti-inflammatory effect. There is conflicting evidence available regarding the effectiveness of the intervention in chronic shoulder pain, questioned in many studies. We planned this study to evaluate the efficacy of ultrasonogram guided intervention in chronic shoulder pain in a tertiary teaching hospital of rural Bengal.Methods: The prospective observational study was carried out in 100 patients with chronic shoulder pain attending pain clinic who fulfilled predetermined inclusion criteria. Patients were injected with 40 mg of depomethyl prednisolone and 10 ml of 1% preservative free lignocaine via standard posterior approach. Then half an hour of standard shoulder range of motion (ROM) exercise regimen was performed under supervision. Pre injection and post injection pain level were scored by visual analogue score (VAS) shoulder exercises were taught and home-based physiotherapy was carried out by patients themselves. Patient were followed at 4, 8, and 12 weeks.Results: The mean age of patients was 53.23±5.680 years with 63 % male study subjects, After the intervention, excellent result in VAS score and improvement of ROM observed in patients. There was statistically significant difference from baseline in both pain relief and shoulder movement.Conclusions: USG guided intervention is effective in pain relief and in improvement in ROM in chronic shoulder pain in long term

Structure and superconductivity of two different phases of Re3W

Two superconducting phases of Re(3)W have been found with different physical properties. One phase crystallizes in a noncentrosymmetric cubic (alpha-Mn) structure and has a superconducting transition temperature T(c) of 7.8 K. The other phase has a hexagonal centrosymmetric structure and is superconducting with a T(c) of 9.4 K. Switching between the two phases is possible by annealing the sample or remelting it. The properties of both phases of Re(3)W have been characterized by powder neutron diffraction, magnetization, and resistivity measurements. The temperature dependences of the lower and upper critical fields have been measured for both phases. These are used to determine the penetration depths and the coherence lengths for these systems

Suppression of magnetic excitations near the surface of the topological Kondo insulator SmB6

We present a detailed investigation of the temperature and depth dependence of the magnetic properties of 3D topological Kondo insulator SmB6 , in particular near its surface. We find that local magnetic field fluctuations detected in the bulk are suppressed rapidly with decreasing depths, disappearing almost completely at the surface. We attribute the magnetic excitations to spin excitons in bulk SmB6 , which produce local magnetic fields of about ~1.8 mT fluctuating on a time scale of ~60 ns. We find that the excitonic fluctuations are suppressed when approaching the surface on a length scale of 40-90 nm, accompanied by a small enhancement in static magnetic fields. We associate this length scale to the size of the excitonic state.Comment: 5 pages, 5 figures, accepted for publication as a Rapid Communication in Phys. Rev.

Fermi-crossing Type-II Dirac fermions and topological surface states in NiTe2

Transition-metal dichalcogenides (TMDs) offer an ideal platform to experimentally realize Dirac fermions. However, typically these exotic quasiparticles are located far away from the Fermi level, limiting the contribution of Dirac-like carriers to the transport properties. Here we show that NiTe2 hosts both bulk Type-II Dirac points and topological surface states. The underlying mechanism is shared with other TMDs and based on the generic topological character of the Te p-orbital manifold. However, unique to NiTe2, a significant contribution of Ni d orbital states shifts the energy of the Type-II Dirac point close to the Fermi level. In addition, one of the topological surface states intersects the Fermi energy and exhibits a remarkably large spin splitting of 120 meV. Our results establish NiTe2 as an exciting candidate for next-generation spintronics devices

Pairing symmetry of an intermediate valence superconductor CeIr3 investigated using μSR measurements

We have investigated the bulk and microscopic properties of the rhombohedral intermediate valence superconductor CeIr3 by employing magnetization, heat capacity, and muon spin rotation and relaxation (μSR) measurements. The magnetic susceptibility indicates bulk superconductivity below T_C = 3.1 K. Heat capacity data also reveal a bulk superconducting transition at 3.1 K with a second weak anomaly near 1.6 K. Zero-field μSR data show no strong evidence of broken time-reversal symmetry but support the presence of spin fluctuations below T_C. Transverse-field μSR measurements suggest a fully gapped, isotropic, s-wave superconductivity with 2Δ(0)/k_BT_C = 3.76(3), very close to 3.53, the Bardeen-Cooper-Schrieffer gap value for weak-coupling superconductors. From the temperature variation of the magnetic penetration depth, we have also determined the London penetration depth λ_L(0) = 435(2) nm, the carrier effective-mass enhancement m^∗ = 1.69(1)m_e, and the superconducting carrier density n_s = 2.5(1) × 10^27 carriers m^−3. The fact that LaIr3, with no 4 f electrons, and CeIr3 with 4f^n electrons where n ≤ 1 (Ce ion in a valence fluctuating state), both exhibit the same s-wave gap symmetry indicates that the Ir-d band governs the physics of these two compounds near the Fermi level, which is in agreement with previous band structure calculations