D.P. Singh

Band structure calculation and structural stability of high pressure phases of EuSe

Ab-initio band structure calculations have been carried out to find the pressure-induced structural transitions and structural stability of the magnetic semiconductor EuSe. The first principal tight-binding linear muffin-tin orbital method (TB-LMTO) within local density approximation (LDA) has been used to study this band structure. While the magnetic phase stability is determined from the total energy calculations within the atomic-sphere approximation (ASA) for both the non-magnetic (NM) and magnetic (M) phases. Our pure theoretical calculations show that: (1) at ambient pressure, the M phase is more stable than the NM phase; and (2) it exhibits a phase transition from NaCl (B1) type to CsCl (B2) type structure at around 9.48 GPa. Although the calculated lattice parameter, transition pressure, volume of collapse is found to be less than the experimentally observed value, but this has been explained from the fact that the calculation has been carried out at 0 K while the experiments have been performed at room temperature. The bulk modulus and magnetic moments are found to be in agreement with earlier experimental result.

Direct evidence of weak-link grain boundaries in a polycrystalline MgB2 superconductor

The observation of voltage-flux oscillations due to the radio-frequency–superconducting quantum interference device (rf-SQUID) effect in polycrystalline bulk MgB2 superconductor is reported. This provides a direct evidence of the presence of Josephson junction-type weak-link grain boundaries in MgB2. Analysis of the SQUID modulation indicates that only a few grain boundaries are of weak-link type whereas others are strong links. Generation of even and odd harmonics are observed when the MgB2 sample is subjected to both ac and dc fields. This also indicates the presence of weak-link grain boundaries. Flux noise in the bulk rf-SQUID in MgB2 is studied and compared with the flux noise in bulk high-Tc rf-SQUID.

Rδ-Supercontinuous Functions

Abstract A new class of functions called ‘Rδ-supercontinuous functions’ is introduced. Their basic properties are studied and their place in the hierarchy of strong variants of continuity which already exist in the literature is elaborated. The class of Rδ-supercontinuous functions (Math. Bohem., to appear) properly contains the class of Rz-supercontinuous functions which in its turn properly contains the class of Rcl- supercontinuous functions (Demonstratio Math. 46(1) (2013), 229-244) and so includes all Rcl-supercontinuous (≡clopen continuous) functions (Applied Gen. Topol. 8(2) (2007), 293-300; Indian J. Pure Appl. Math. 14(6) (1983), 767-772) and is properly contained in the class of R-supercontinuous functions (Demonstratio Math. 43(3) (2010), 703-723).

Variable temperature electron paramagnetic resonance investigations of a kerosene-based magnetic fluid

Abstract The kerosene-based ferrofluid RP-90 has been examined by electron paramagnetic resonance (EPR) from room temperature to liquid nitrogen temperature at the X-band frequency 9.41 GHz. The variations in the g -value and ( ΔH ) with temperature indicate the presence of a magnetic phase transition from a ferromagnetic to a ‘spin glass or cluster glass’ state. This transition is associated with the freezing point of the carrier medium. No evidence of the presence of superparamagnetism was observed. The linewidth variation with temperature indicates a dominant bulk rotation relaxation mechanism ( τ B ).

Generation of even harmonics in a MgB2 bulk superconductor: an indication of the presence of weak link grain boundaries

In this paper we report the generation of higher harmonics in a bulk polycrystalline MgB2 superconductor when it is subjected to ac and dc magnetic field. The MgB2 sample shows the generation of higher odd harmonics when an ac field is applied. The application of a small dc field ( gauss) in addition to an ac field also leads to the generation of second and fourth harmonics, which is an indication of the presence of some weak link grain boundaries in a MgB2 superconductor. We have studied the variation of the amplitude of higher harmonics with temperature for different values of amplitude of ac field. The observed results are interpreted in terms of weak link grain boundaries and a critical state model.

Magnetic flux noise in MgB2 superconductor

Magnetic flux noise in MgB2 polycrystalline sample is measured using a high-TC rf-superconducting quantum interference device in the temperature range of 6–40K. A small magnetic field (∼200mG) was applied while cooling the sample. The flux noise exhibits 1∕fα type of behavior with α∼1.0–1.3 and shows enhanced noise around 24 and 37K. The flux noise seems to originate from thermally activated vortex hopping. The large magnetic noise at 24K indicates the presence of larger density of pinning sites with energies ∼0.061eV leading to enhanced magnetic fluctuations at temperatures much below TC.

Effect of Ar heat treatment on oxygenated R1−xCaxBa2Cu3O7−δ (R = Y, Sm) superconductors

The influence of heat treatment, at 450°C in Ar environment, on oxygenated (6.72 < 7 − δ < 6.93) samples of R1−xCaxBa2Cu3O7−δ (i.e. R(Ca)-123, with R = Y, Sm) with 0 < x < 0.3 has been investigated. Measurements of oxygen loss, normal state resistivity behaviour ρ(T), superconducting critical temperature Tc and transition widths reveal that Ar treatment effects both the series differently. The Sm(Ca)-123 samples, for all values of x, show a considerable loss of oxygen (Δδ(x) = 0.4 to 0.6) and decrease in Tc (ΔTc(x) = 35 to 55 K). Whereas, surprisingly, the Y(Ca)-123 samples show a much smaller loss of oxygen (Δδ(x) = 0 to 0.3), along with a decrease in Tc for x < 0.2 and an increase in it for x > 0.2. In particular, the x = 0.3 sample shows a negligible change in oxygen content accompanied by an increase of 20 K in Tc. These and host of other observations suggest that the presence of Ca may influence the oxygen loss mechanism in these series.

110K phase and oxygen balance in BSCCO system

Abstract Bi2Sr2Ca2Cu3Oy (2223) specimens added with 5 wt% and 7.5 wt% of PbO have been prepared. 110K phase is formed in specimens with 5 wt% PbO addition when annealed in vacuum (10−1 torr) at 500°C for short period (2–3 hr). On the other hand, the formation of 110K phase needs oxygen annealing (500°C × 2 hr) for specimens with 7.5 wt% PbO addition. These results indicate that the annealing condition (vacuum or oxygen) is closely related to the Pb contents - both balancing the oxygen stoichiometry and the hole concentration to the optimum level for the 110K phase. The presence of the residual 80K phase indicates that the addition of PbO, to the master (2223) specimen already containing 80K phase, requires a long sintering time as the 110K phase grows around 80K phase through a slow kinetic diffusion.

Effect of substrate on magneto-transport properties of polycrystalline manganite films

Polycrystalline films of La0.7Ca0.2Ba0.1MnO3 (LCBMO) were deposited on LaAlO3, SrTiO3 (STO), Al2O3 and YSZ single crystal substrates by spray pyrolysis technique employing an ultrasonic nebulizer. Deposition and annealing conditions for all four films were kept identical. All films have been found to be single phase and have orthorhombic unit cells with only slightly different lattice parameters. The magneto-transport properties (magnetoresistance (MR), TC and TIM) of these polycrystalline LCBMO films are found to depend strongly on the substrate. The lattice mismatch between the film and substrate induces strain of various degrees. The strain is believed to get relaxed and accommodated. This produces disorders dominantly through the increase in the grain boundary density and hence decrease in grain size. The largest strain leads to the creation of highest degree of disorder. The observed MR for the case of YSZ/LCBMO is found to be significantly higher than other case of STO/LCBMO. As for example, for YSZ/LCBMO where the lattice mismatch is −24.280%, MR is ~7%, on the other hand for STO/LCBMO where the lattice mismatch is −0.333%, MR is ~2.3%. The TC and TIM for the film with higher disorder (YSZ/LCBMO) is significantly reduced as compared to the case of the film having lower disorder (STO/LCBMO). The TC for YSZ/LCBMO (higher disorder) is 243 K and for STO/LCBMO (lower disorder), it is 285 K.

Synthesis and characterization of ferroelectric bismuth titanate (Bi4Ti3O12)

The Lead free, Bi4Ti3O12 has been synthesized by sol-gel method. The synthesized samples have been subjected to the structural, morphological, P-E hysteresis loop measurements. The X-Ray diffraction (XRD) studies have confirmed the pure phase Bismuth Titanate Bi4Ti3O12 with orthorhombic symmetry. The Scanning electron microscope (SEM) studies have shown the good surface morphology. The hysteresis measurement of BIT is also done to confirm the ferroelectric property of the material. The values of remnant polarization (Pr) and coercive field (Ec) is found to be dependent on applied electric field.