K. Vinod

Monte Carlo simulations using the Metropolis algorithm were carried out for the Heisenberg model, the two-state Ising model, and the 2 S + 1 state Ising model, focusing on ferromagnetic systems. The relationship between magnetic exchange interaction strength and the ferromagnetic transition temperature ( T C ) was evaluated and compared across the three models within the range of 1 / 2 ⩽ S ⩽ 10 . Additionally, the magnetocaloric properties, such as the isothermal magnetic entropy change ( Δ S mag ) and the relative cooling power, were assessed for two well-known ferromagnetic materials: Gd and EuS. Among the three models examined, the 2 S + 1 state Ising model demonstrated the best quantitative agreement with experimental results, whereas the Heisenberg model showed poor agreement.

Knowledge of electronic band structure and its role on the electronic transport as well as optical properties of semiconducting as well as insulating materials are important to characterize/evaluate their potential for practical applications. In this work, the crystallographic, morphological, electronic transport, optical and electronic band structure properties of three Lanthanum (La) based double perovskite oxides, La₂ZnMnO₆, La₂NiTiO₆ and La₂NiRuO₆ were evaluated and compared. Polycrystalline samples were synthesized by following the solid-state reaction method. Analysis of powder x-ray diffraction (XRD) data indicates monoclinic (space group: P2₁/n) crystal structure. The unit cell size of these three systems correlates with their constituent transition metal ionic sizes. Analysis of Raman spectra and Fourier transform infrared (FTIR) spectra indicates that the octahedral vibrational modes correlate with the ionic mass of the constituent transition metals for these three systems. The temperature depended resistivity data (ρ(T)) indicate semiconducting behaviour for all the three systems. Among the samples, the La₂NiRuO₆ system shows lowest ρ(T) and the La₂NiTiO₆ system shows the highest ρ(T) values. The thermal activation energy is extracted from the analysis of ρ(T) data, following the thermally activated transport model. The estimated activation energy values are 134(8) meV for La₂NiRuO₆, 203(1) meV for La₂ZnMnO₆ and 280(5) meV for La₂NiTiO₆, respectively. Also, it is observed that the normalized resistivity and activation energy of the three systems follow a direct/simple scaling trend. Additionally, diffuse reflectance spectroscopy (DRS) measurements were carried out on these systems. The DRS results further supports the observed trend in ρ(T) data among the three systems. The experimental observations is further supported by the density functional theory calculations.

First order ferromagnetic to paramagnetic phase transition is studied using the Bean-Rodbell model within the mean-field framework. The presence of sample inhomogeneities is modeled by considering distributions of the Bean-Rodbell parameter. The corresponding Arrott plots with and without distributions of the Bean-Rodbell parameter are generated and compared. Results indicate that in the presence of inhomogeneities, ferromagnetic to paramagnetic phase transition is broadened. The initial negative slopes in the Arrott plots due to first order phase transition changes to positive slopes for temperatures lying in the broadened tail region of the ferromagnetic to paramagnetic phase transitions. However, the initial negative slopes in the paramagnetic regime (for temperatures above the broadened tail region) are preserved even in the presence of sample inhomogeneities. Also, the effect of inhomogeneities on the magnetic entropy change near the first order transition is studied.

Superconductor-topological insulator (TI) heterostructures are interesting due to induced p-wave superconductivity on the TI surface states near the interface. Transport studies carried out in this work on the TI surface of superconductor-TI (NbN-Bi_1.95Sb_0.05Se₃) heterostructures have revealed anomalous resistance upturn and downturn at millimetre length scales away from the interface. Magnetotransport measurements have indicated that the anomaly is caused due to the superconducting transition of the NbN layer. The strength of the jump in resistance has been found to be strongest at the edges and the sign of the jump found to be opposite along opposite edges. Such resistance upturns and downturns have been previously reported in literature, wherein the upturns have been attributed to the antagonistic ground states of p-wave Cooper pairing and the spin-polarized TI states, and the downturns attributed to induced long-range proximity effects. However, the possibility of long-range superconducting proximity effect has been ruled out in this study through the observation of similar anomaly in NbN-Au and NbN-Al heterostructures. The present study demonstrates that the anomalies in resistance occur due to current redistribution (CRD) effects at the superconducting transition due to the geometry of the heterostructure. Results obtained from finite element analysis using COMSOL software have validated the proposed CRD model of long-range resistance anomalies in superconductor-TI and superconductor-metal heterostructures.

A comparative study was carried out using magnetization measurements to understand the diamagnetic response of the superconducting thin films of Bi _1.75 Pb _0.25 Sr ₂ Ca ₂ Cu ₃ O _{10± δ} grown on two different substrates. The film grown on SrTiO ₃ substrate was found to exhibit enhanced superconducting properties than the film grown on LaAlO ₃ substrate. The strengthening of the intergrain coupling by the growth of the larger grains with better grain connectivity and enhanced pinning mechanism yielded a critical current density as high as ∼4.8 × 10 ⁵ Acm ⁻² in the film grown on SrTiO ₃ substrate. An enhancement in the value of the critical current density with the decrease in the dimensionality of the sample on account of a change in the microstructure was observed. As revealed from the AC susceptibility measurements, the metallic nature of the weak links conveys that the weak links are essentially the coexisting secondary phases in these films. Furthermore, the paramagnetic Meissner effect was observed in these films, which can be explained within the realm of the flux compression models.

The superconducting properties of DC magnetron sputtered Nb, NbN, and MoN films deposited on Sb-doped bismuth selenide (BSS) topological insulator films have been studied. In this two-fold study, firstly, thick superconducting films of ∼100 nm have been studied to find the modifications in the superconducting properties of the films caused by the surface morphology of pulsed laser deposition-grown BSS substrates. A drastic suppression in the critical current density has been observed in the bilayers, which has been ascribed to the strong influence of the substrate on the morphology of the superconducting film and the consequent enhancement in the density of superconducting weak links. The temperature dependence of the critical magnetic field has been carefully analyzed within the purview of the Werthamer–Hohenberg–Helfand theory, taking into account the effects of spin paramagnetism and spin–orbit scattering. In the second part of the study, ultra-thin bilayers of NbN-BSS, with thickness of the order of the coherence length (∼5 nm), have been grown to study the superconducting properties within the proximity regime. A ∼3.5 K suppression in T _c is seen in the NbN(5 nm)-BSS bilayer, due to a combination of interface roughness and superconducting proximity, while a suppression of 0.6 K has been observed in the BSS(6 nm)-NbN(5 nm) bilayer, which is surmised to be solely due to the superconducting proximity effect.

Low temperature magnetization studies on polycrystalline EuSe have been performed. The system shows anti-ferromagnetic magnetic ordering with Neel temperature TN ~ 4.6 K and exhibits a field induced meta-magnetic transition, for temperatures below the TN. Arrot plot analysis indicates first order nature for both these magnetic phase transitions. Using the isothermal magnetization measurements, the isothermal magnetic entropy change (−∆SM) is evaluated. The magnetization measurements were done for two different measurement protocols to check the effect of thermo-magnetic history/hysteresis on the magneto caloric properties. Results indicate negligible effect of thermo-magnetic history/hysteresis on magnetic and magneto-caloric properties of the EuSe system.