• Ajay

      Articles written in Pramana – Journal of Physics

    • Bilayer exchange coupling and neel temperature of YBa2Cu3O6.2

      Govind A Pratap Ajay R S Tripathi

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      The present paper attempts to study the Neel temperature of bilayer antiferromagnetic cuprate YBa2Cu3O6.2 within anisotropic Heisenberg model. The double time Green’s function formalism within random phase approximation (RPA) has been used to obtain various correlation functions. The magnetization and the Neel temperature (TN) are evaluated. It is observed that the ratio of intrabilayer to inplane exchange coupling (r=J⊥/J‖) plays an important role in the magnetic dynamics of bilayer systems. The recent experimental data of bilayer system YBa2Cu3O6.2 have been used to estimate the ratio r from the expression for Neel temperature. The estimated values of spin gap and the ratio of hopping matrix elements t⊥/t‖ are found to be in fairly good agreement with the existing experimental results.

    • Microwave absorption studies of MgB2 superconductor

      MK Bhide RM Kadam MD Sastry Ajay Singh Shashwati Sen Manmeet Kaur DK Aswal SK Gupta VC Sahni

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      Microwave absorption studies have been carried out on MgB2 superconductor using a standard X-band EPR spectrometer. The modulated low-field microwave absorption signals recorded for polycrystalline (grain size ∼ 10 µm) samples suggested the absence of weak-link character. The field dependent direct microwave absorption has been found to obey a ✓H dependence with two different slopes, which indicated a transition from strongly pinned lattice to flux flow regime.

    • Condensation energy of the superconducting bilayer cuprates

      Govind Ajay S K Joshi

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      In the present work, we report the interplay of single particle and Cooper pair tunnelings on the superconducting state of layered high-Tc cuprate superconductors. For this we have considered a model Hamiltonian incorporating the intra-planar interactions and the contributions arising due to the coupling between the planes. The interplanar interactions include the single particle tunneling as well as the Josephson tunneling of Cooper pairs between the two layers. The expression of the out-of-plane correlation parameter which describes the hopping of a particle from one layer to another layer in the superconducting state is obtained within a Bardeen-Cooper-Schriefer (BCS) formalism using the Green’s function technique. This correlation is found to be sensitive to the various parameter of the model Hamiltonian. We have calculated the out-of-plane contribution to the superconducting condensation energy. The calculated values of condensation energy are in agreement with those obtained from the specific heat and the c-axis penetration depth measurements on bilayer cuprates.

    • Preparation and characterization of MgB2 superconductor

      Shashwati Sen DK Aswal Ajay Singh TV Chandrasekhar Rao KP Muthe JC Vyas LC Gupta SK Gupta VC Sahni

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      The MgB2 superconductor, synthesized using solid-state and liquid-phase sintering methods, have been characterized for various properties. The upper critical field, irreversibility line and critical current density have been determined using magnetization data. The current-voltage characteristics recorded under an applied magnetic field revealed the existence of vortex glass transition. The surface analysis using X-ray photoelectron spectroscopy shows that MgB2 is sensitive to atmospheric degradation.

    • Effect of substrate temperature on electrical and magnetic properties of epitaxial La1−xPbxMnO3 films

      Ajay Singh DK Aswal Shashwati Sen K Shah LC Gupta SK Gupta VC Sahni

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      Epitaxial La1−xPbxMnO3 (LPMO) thin films, grown on (100) SrTiO3 substrates by laser ablation technique at different temperatures between 600 and 850°C, have been characterized for electrical and magnetic properties. The temperature dependence of resistivity showed that the metal-insulator transition temperature (TMI) decreases with increasing substrate temperature, which has been attributed to decrease in Pb content in the filsm. The YBa2Cu3Ox/La1−xMnO3 heterostructures, exhibiting both superconductivity and ferromagnetism, have been fabricated.

    • Colossal magnetoresistance in layered manganite Nd2−2xSr1+2xMn2O7 (0 ≤ x ≤ 0.5)

      K Shah D K Aswal Ajay Singh L C Gupta S K Gupta V C Sahni

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      The layered manganite Nd2−2xSr1+2xMn2O7, with x varying between 0 and 0.5, has been synthesized using solid-state reaction method. We have found that Nd2−2xSr1+2xMn2O7 do not form the single-phase layered compound with A3B2O7 structure. Instead, mixtures of various phases, such as, orthorhombic perovskite, i.e., Nd1−zSrzMnO3, layered manganite and unreacted starting compounds, have been obtained. Except for x=0.4, which is found to be an antiferromagnetic insulator, all other x values yielded metal-insulator transition and ferromagnetic ordering.

    • Radon exhalation studies in building materials using solid-state nuclear track detectors

      Ajay Kumar Surinder Singh

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      Indoor radon has been recognized as one of the health hazards for mankind. Building materials constitute the second most important source of radon in dwellings. The common building materials used in the construction of dwellings are studied for radon exhalation rate. The ‘Can’ technique using LR-115 type-II solid-state nuclear track detector has been used for these measurements. The radon exhalation rate in these samples varies from 4.75 m Bq m−2 h−1 (0.14 m Bq kg−1 h−1) for limestone to 506.76 m Bq m−2 h−1 (15.24 m Bq kg−1 h−1) for soil.

    • Study of photon-induced L3 vacancy alignment for elements La to U

      Ajay Sharma Meenu Singh Raj Mittal

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      Alignment of photon-induced L3 vacancies is studied in rare earth and highZ elements at energies of experimental interest, near thresholds to 60 keV, under nonrelativistic dipole approximation. Numerical calculations of the matrix element are undertaken to produce theoretical data for comparison with the experimental findings. The A2 values being s>0.1 at photoelectron energies <20 keV are certainly higher than 5–8% uncertainties quoted in experimental results. Present findings are from a very basic model, hydrogen-like and can further be treated as reference to observe the impact of screening, relativistic, multipole and retardation corrections to the model

    • A high resolution X-ray crystal spectrometer to study electron and heavy-ion impact atomic collisions

      Ajay Kumar D Misra A H Kelkar U R Kadhane K V Thulasiram Lokesh C Tribedi

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      We have studied fast ion–atom and electron–atom collision processes using a reconditioned high resolution X-ray spectrometer. The X-rays, generated by the collisions, are dispersed by a curved ADP crystal (Johansson geometry) and detected by a gas proportional counter. A self-written LabVIEW based program has been used to give precise and controlled movement to the crystal and for data acquisition. The performance was tested by detecting the K𝛼 diagram and satellite lines of several elements. The K𝛼 satellite lines of Al have been studied in collision with 3–12 keV electrons and 40 MeV C4+ ions. In ion collisions as large as four L-vacancies are created simultaneously with the K-vacancy, compared to two satellites in case of the e-impact. In addition, we have measured the X-rays from H-, He- and Li-like Si ions which arise due to the electron loss/capture process in highly charged 80 MeV Si7+ ions in collision with thin carbon foil. Approximate charge state distribution has been obtained using this new technique.

    • Masses and magnetic moments of triple heavy flavour baryons in hypercentral model

      Bhavin Patel Ajay Majethiya P C Vinodkumar

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      Triple heavy flavour baryons are studied using the hypercentral description of the three-body system. The confinement potential is assumed as hypercentral Coulomb plus power potential with power index 𝑝. The ground state ($J^{P} = \dfrac{1}{2}^{+}$ and $\dfrac{3}{2}^{+}$) masses of heavy flavour baryons are computed for different power index, 𝑝 starting from 0.5 to 2.0. The predicted masses are found to attain a saturated value with respect to variation in p beyond the power index $p$ > 1.0. Using the spin-flavour structure of the constituting quarks and by defining effective mass of the confined quarks within the baryons, the magnetic moments are computed with no additional free parameters.

    • Fabrication of periodically poled lithium niobate chips for optical parametric oscillators

      Ashok Kaul Ajay Mishra

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      An electric-field poling process was established that yielded uniform periodically poled lithium niobate (PPLN) in 0.5 mm thick lithium niobate substrate. We have fabricated 50 mm long fanned as well as multigrating PPLNs having period variations from 25 𝜇m to 32 𝜇m. These PPLNs are required for quasi-phase-matched (QPM) optical parametric oscillator (OPO) applications. We have also configured a bench-top OPO set-up based on these PPLNs.

    • Spectral distribution of the $2S \to 1S$ two-photon transition in atoms and few-electron ions

      Ajay Kumar S Trotsenko A V Volotka D Banaś H F Beyer H Bräuning S Fritzsche A Gumberidze S Hagmann S Hess C Kozhuharov R Reuschl U Spillmann M Trassinelli G Weber Th Stöhlker

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      The two-photon decay of the $2S$ state to the ground state in dressed atoms and oneor two-electron ions has been studied for several decades. Relativistic calculations have shown an 𝑍-dependence of the spectral shape of this two-photon transition in one- or two-electron ions. We have measured the spectral distribution of the $1s2s {}^1 S_0 \to 1_s {}^2 {}^1 S_0$ two-photon transition in He-like tin at the ESR storage ring using a new approach for such experiments. In this method, relativistic collisions of initially Li-like projectiles with a gaseous target were used to populate exclusively the first excited state, $1_s2_s$, of He-like tin, which provided a clean two-photon spectrum. The measured two-photon spectral distribution was compared with fully relativistic calculations. The obtained results show very good agreement with the calculations for He-like tin

    • Properties of light flavour baryons in hypercentral quark model

      Kaushal Thakkar Bhavin Patel Ajay Majethiya P C Vinodkumar

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      The light flavour baryons are studied within the quark model using the hypercentral description of the three-body system. The confinement potential is assumed as hypercentral Coulomb plus power potential (hCPP$_\nu$ ) with power index 𝜈. The masses and magnetic moments of light flavour baryons are computed for different power indices, 𝜈, starting from 0.5 to 1.5. The predicted masses and magnetic moments are found to attain a saturated value with respect to variation in 𝜈 beyond the power index $\nu > 1.0$. Further, we computed transition magnetic moments and radiative decay width of light flavour baryons. The results are in good agreement with the known experimental as well as other theoretical models.

    • Exact solutions of certain nonlinear chemotaxis diffusion reaction equations

      MISHRA AJAY KAUSHAL R S PRASAD AWADHESH

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      Using the auxiliary equation method, we obtain exact solutions of certain nonlinear chemotaxis diffusion reaction equations in the presence of a stimulant. In particular, we account for the nonlinearities arising not only from the density-dependent source terms contributed by the particles and the stimulant but also from the coupling term of the stimulant. In addition to this, the diffusion of the stimulant and the effect of long-range interactions are also accounted for in theconstructed coupled differential equations. The results obtained here could be useful in the studies of several biological systems and processes, e.g., in bacterial infection, chemotherapy, etc.

    • Switching from sub- to superluminal light in a Y-type atomic system using wavelength mismatching

      KAVITA YADAV AJAY WASAN

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      We study the optical properties of a four-level Y system theoretically in $^{87}\rm{Rb}$ atoms using density matrix formalism. On account of wavelength mismatching effects in the system transparency window splits and enhanced absorption regions are observed on either side of the line centre. This enhanced absorption is associated with anomalous dispersion resulting in superluminal light propagation. We also explore the dressed state analysis in the system. The effect of mismatching factor on the group index and group velocity is discussed.

    • Viscous Ricci dark energy model with matter creation: Exact solution and observational tests

      AJAY KUMAR C P SINGH

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      In this paper, the dissipative mechanism (bulk viscosity and matter creation) is introduced to describe the effects of cosmic non-perfect fluid on the Ricci dark energy (RDE) model. We consider matter creation and bulk viscosity as two independent irreversible processes. Assuming suitable forms of the bulk viscous coefficient and matter creation rate, we find the exact solution of the field equations. We carry out fitting analysis on the cosmological parameters in the model by using Type Ia supernovae data, observational Hubble data and baryon acoustic oscillation (BAO) data with cosmic microwave background. We plot the trajectory of cosmological parameters with the best-fit values of model parameters and discuss all possible (deceleration, acceleration and their transitions) evolutions of the model. The current values of deceleration parameter and equation of state parameter are found to be $q_0=-0.362$ and $\omega_{\mathrm{eff}} = -0.575$, respectively. The age of the Universe is found to be $t_0 \simeq 13.397$ Gyr, which is very close to the $\Lambda$CDM model. We further discuss the geometrical diagnostic parameters such as statefinder and $Om$ to distinguish the model with $\Lambda$CDM model. Finally, we discuss the behaviour of energy conditions for our model and find that the model satisfies the null energy condition (NEC), weak energy condition (WEC) and dominant energy condition (DEC) while it violates strong energy condition (SEC).

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