Experimental study of yttrium barium copper oxide superconducting tape’s critical current under twisting moment

Critical current (I _c) characteristics of 2G YBCO superconducting tape under the influence of twisting moment was experimentally investigated at varying current ramp rates in the self-field. Under a uniform twist, the degradation in the current-carrying capacity of YBCO tape up to 30% was observed at 77 K. The degradation is largely attributed to the shear stress and torsional shear strain resulting from the twisting. The superconductor to resistive transition index, n, is also found to behave in an identical manner with increase in the twisting. Finite element analysis (FEA) of the tape in the experimental configuration with twisting moment being applied on to it has been carried out in COMSOL. The torsional strain calculated analytically as per the experimental configuration matches closely with that of FEA results, which shows that the critical current degradation is a function of strain.     

Bianchi .Type-I Massive String Magnetized Barotropic Perfect Fluid Cosmological Model in General Relativity

Bianchi type-I massive string cosmological model with magnetic field of barotropic perfect fluid distribution through the techniques used by Latelier and Stachel is investigated. To obtain the deterministic model of the universe, it is assumed that the universe is filled with barotropic perfect fluid distribution. The magnetic field is due to electric current produced Mong the x-axis with infinite electrical conduictivity. The behaviour of the model in the presence and absence of magnetic field together with other physical aspects is further discussed.     

Higher Dimensional Strange Quark Matter Coupled to the String Cloud with Electromagnetic Field Admitting One Parameter Group of Conformal Motion

We solve Einstein＇s field equations in higher-dimensional spherically symmetric spacetime with strange quark matter attached to the string cloud, assuming one parameter group of con formal motions. The solutions match with the higher-dimensional Reissner-Nordstroem metric on the boundary at r=ro. The features of the solutions are also discussed in the framework of higher-dimensional spacetime.     

Stability analysis and quasinormal modes of Reissner–Nordstrøm space-time via Lyapunov exponent

We explicitly derive the proper-time (τ) principal Lyapunov exponent (λ_p) and coordinate-time (t) principal Lyapunov exponent (λ_c) for Reissner–Nordstrøm (RN) black hole (BH). We also compute their ratio. For RN space-time, it is shown that the ratio is \(({\lambda _{p}}/{\lambda _{c}})={r_{0}}/{\sqrt {{r_{0}^{2}}-3Mr_{0}+2Q^{2}}}\) for time-like circular geodesics and for Schwarzschild BH, it is \(({\lambda _{p}}/{\lambda _{c}})={\sqrt {r_{0}}}/{\sqrt {r_{0}-3M}}\). We further show that their ratio λ_p/λ_c may vary from orbit to orbit. For instance, for Schwarzschild BH at the innermost stable circular orbit (ISCO), the ratio is \(({\lambda _{p}}/{\lambda _{c}})|_{r_{\text {ISCO}}=6M}=\sqrt {2}\) and at marginally bound circular orbit (MBCO) the ratio is calculated to be \(({\lambda _{p}}/{\lambda _{c}})|_{r_{\mathrm {m}\mathrm {b}}=4M}=2\). Similarly, for extremal RN BH, the ratio at ISCO is \(({\lambda _{p}}/{\lambda _{c}})|_{r_{\text {ISCO}}=4M}={2\sqrt {2}}/{\sqrt {3}}\). We also further analyse the geodesic stability via this exponent. By evaluating the Lyapunov exponent, it is shown that in the eikonal limit, the real and imaginary parts of the quasinormal modes of RN BH is given by the frequency and instability time-scale of the unstable null circular geodesics.     

Temperature profile evolution in quenching high-T c superconducting composite tape

Irreversible normal zones leading to quench is an important aspect of high-temperature superconductors (HTS) in all practical applications. As a consequence of quench, transport current gets diverted to the matrix stabilizer material of the high-T _c composite and causes Joule heating till the original conditions are restored. The nature of growth of the resistive zone in the superconductor greatly influences the temperature evolution of the quenched zone. In this investigation, a complete mathematical analysis of the temperature profile evolution following a quench in a HTS has been carried out. Such prediction in temperature profile would aid the design of HTS tape-based practical applications in limiting the thermal stress-induced damages in off-normal scenarios.     

Spectral narrowing of coherent population trapping resonance in laser-cooled and room-temperature atomic gas

We have investigated coherent population trapping (CPT) in laser-cooled as well as room-temperature (with and without buffer gas) rubidium atoms. The characteristic broad signal profile emerging from the two-photon Raman resonance for room-temperature atomic vapour is consistent with the theoretical calculation incorporating associated thermal averaging. The spectral width of the dark resonance obtained with cold atoms is found to be broadened, compared to room-temperature vapour cell, due to the feeble role played by thermal averaging, although the cold atomic sample significantly overcomes the limitation of the transit time broadening. An alternative way to improve transit time is to use a buffer gas, with which we demonstrate that the coherent population trapping signal width is reduced to < 540 Hz.     

Bianchi Type-Ⅲ String Cosmological Models with Time Dependent Bulk Viscosity

Bianchi type-Ⅲ string cosmological models with bulk viscous fluid for massive string are investigated. To obtain the determinate model of the universe, we assume that the coeffcient of bulk viscosity ξ is inversely proportional to the expansion θ in the model and expansion θ in the model is proportional to the shear g. This leads to B =lC^n, where l and n are constants. Behaviour of the model in the presence and absence of bulk viscosity is discussed. The physical implications of the models are also discussed in detail.     

Magnetic field modulation spectroscopy of rubidium atoms

The magnetically modulated saturation absorption profile is studied for a wide range of external DC magnetic field. The salient features of Doppler-free signal generated by laser frequency modulation and atomic energy level modulation are compared. The DC offset of the signal profile is found to be unstable as the external DC magnetic field is changed. The technical difficulty of tuning laser frequency under locked condition over a large frequency span is discussed along with possible solutions.