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In the present work, we report fabrication and current–voltage characteristics of ZnO/α-NPD:F 4 -TCNQ based inorganic–organic hybrid junction. Current–voltage characteristics of hybrid junctions show diode like behavior. The effect of organic layer doping on the diode like characteristics of the junction has been investigated. The diode parameters viz. barrier height, ideality factor and rectification ratio have been calculated from the current–voltage semi-log plots. The improvement of diode parameters i.e. increase in rectification ratio and decrease in barrier height as well as improvement in ideality factor has been observed with doping. The improvement of the diode parameters has been attributed to the modification at ZnO/α-NPD interface. The current is also found to increase with the increase in doping concentration which has been attributed to the increased conductivity of α-NPD and interface barrier lowering after doping of α-NPD with F 4 -TCNQ.

Effects of Sb doping on thermal properties like thermoelectric power (S), thermal conductivity (κ), and specific heat (Cp) of La2/3Ba1/3(Mn1−xSbx)O3 (x=0.00–0.05) perovskite manganites are reported. It has been found that S is positive at low temperature and becomes negative at higher temperatures. A pronounced peak was observed in S at the insulator–metal transition for the pristine sample La2/3Ba1/3MnO3, which gets suppressed with Sb-doping. In addition, electron to hole-like crossover temperature seen in the S(T) is also found to move towards lower temperatures with increasing Sb content. The anomalous behavior in thermal conductivity (κ) at the transition is ascribed to the local anharmonic distortions associated with small polarons. The observed reduction of the total κ(T) due to Sb-doping is discussed with various thermal scattering mechanisms. From the specific heat results, broadening of observed sharp peak in the pristine LBMO data on substitution of Sb at Mn site was recognized as the enhancement of magnetic inhomogeneity in these manganites.

The structural magnetic and magneto-transport properties of double perovskite system Ba 2− x Sr x FeMoO 6 (0⩽ x ⩽1.0) prepared in bulk polycrystalline form are reported in this paper. X-ray diffraction analysis showed that samples are single phase and the lattice constants decreases with increase in the Sr content. The degree of Fe–Mo ordering has been found decreasing in the series with an increase in the Sr content. Parent compound Ba 2 FeMoO 6 exhibits saturation magnetic moment value of 3.54 μ B /f.u. at 85 K in a magnetic field of 6000 Oe. Temperature dependence of resistivity shows metallic behavior for all the samples. The magneto-resistance (MR) of the compound with x =0.4 is higher than that of the other samples. At room temperature this system shows a saturation magnetization value of 1.73 μ B /f.u. and MR value of 7.08% (1 T). The observed variations in the structural and magnetic properties are attributed to the change of chemical pressure due to the substitution of Sr in place of Ba. The effect of antisite disorder (ASD) defects on magneto-transport properties is studied in more detail.

The variation in structural, magnetic and magneto-transport properties of the double perovskite system (Ba0.8Sr0.2)2−xNdxFeMoO6 {0.0

Double-layered manganite La1.4Ca1.6Mn2O7 has been synthesized using the solid-state reaction method. It had a metal-to-insulator transition at temperature TM1≈127 K. The temperature dependence of ac susceptibility showed a broad ferromagnetic transition. The two-dimensional (2D)-ferromagnetic ordering temperature (TC2) was observed as ≈245 K. The temperature dependence of its low-field magnetoresistance has been studied. The low-field magnetoresistance of double-layered manganite, in the temperature regions between TM1 and TC2, has been found to follow 1/T5. The observed behaviour of temperature dependence of resistivity and low-field magnetoresistance has been explained in terms of two-phase model where ferromagnetic domains exist in the matrix of paramagnetic regions in which spin-dependent tunneling of charge carriers occurs between the ferromagnetic correlated regions. Based on the two-phase model, the dimension of these ferromagnetic domains inside the paramagnetic matrix has been estimated as ∼12 A.

We have performed a systematic study on the occurrence of superconductivity in Nb1−x Mg x B2 (0.0≤x≤0.40). X-ray diffraction and magnetization measurements are carried out to determine the changes in lattice parameters, superconducting transition temperature (T c ) and critical field (H c1). The substitution of Mg at Nb site results in considerable stretching of c-parameter with only a slight change in a parameter. Rietveld analysis on X-ray diffraction patterns gives a=3.11 A and c=3.26 A for pure NbB2 while a=3.10 A and c=3.32 A for Nb0.60Mg0.40B2. This increased c-parameter introduces superconductivity in niobium diboride. Magnetization measurements though indicate the absence of superconductivity in NbB2, the same shows a clear diamagnetic signal at about 10 K for Nb0.60Mg0.40B2 sample. The magnetization M(H) plots exhibit weak superconductivity like hysteresis loops. The stretching of c-parameter from around 3.26 to 3.32 i.e. by 0.06 cannot be explained solely by substitution of Nb by Mg in the lattice. It seems that some Nb deficiencies are introduced in the Nb1−x Mg x B2 as Mg is not substituted completely at the vacant Nb sites. This could be seen from XRD results, where one can clearly notice the presence of small amount of MgO in Nb1−x Mg x B2 samples.

The influence of dc biasing current on temperature dependence of resistivity and low-field magnetoresistance (MR) of La0.67Ba0.33MnO3 bulk sample is reported. A prominent finding is the change in resistivity around the insulator-to-metal transition temperature (TIM) and the change in MR around the ferromagnetic transition temperature (TC). The decrease in MR around TC at higher biasing current indicates a strong interaction between carrier spin and spin of Mn ions resulting in a higher alignment of Mn ion spins. Change in resistivity around TIM is interpreted in the framework of percolative conduction model based on the mixed phase of itinerant electrons and localized magnetic polarons.

In this paper we report the effect of deposition temperature on the structural and optical properties of ZnO thin films prepared by rf magnetron sputtering. The films grown at lower deposition temperatures were in a state of large compressive stress, whereas the films grown at higher temperature (450 °C) were almost stress free. In the absorption spectra, the ZnO excitonic and the Zn surface plasmon resonance (SPR) peaks have been observed. A redshift in the optical band gap of ZnO films has also been observed with the increase in the deposition temperature. The shift in the band gap calculated from the size effect did not match with the observed shift values and the observed shift has been attributed to the compressive stress present in the films.

We have synthesized La0.7Ba0.3MnO3 (LBMO):wt%Agx and La0.7Sr0.3MnO3 (LSMO):wt%Agx composites with x=0.0, 0.1, 0.2, 0.3 and 0.4 at different sintering temperatures of 1300 and 1400 °C by solid-state reaction route. In our previous work [Awana et al. Solid State Commun. 140 (2006) 410] on La2/3Ca1/3MnO3 (LCMO):Agx (x=0.0, 0.1, 0.2, 0.3, 0.4) composites, we reported about 12% temperature coefficient of resistance (TCR) for Ag0.4 samples near metal–insulator (MI) transition, which was explained on the basis of better grains connectivity. A sharp MI transition results in high TCR. Such high-TCR material can be used as a bolometer or infrared detector at room temperature. In the present work, it is shown that the TCR is improved with Ag addition for both LBMO:wt%Agx and LSMO:wt%Agx composites. In addition to improvement in TCR, high MR is also achieved in the vicinity of room temperature.

We report synthesis, structure/micro-structure, resistivity under magnetic field [R(T)H], Raman spectra, thermoelectric power S(T), thermal conductivity K(T), and magnetization of ambient pressure argon annealed polycrystalline bulk samples of MgB2, processed under identical conditions. The compound crystallizes in hexagonal structure with space group P6/mmm. Transmission electron microscopy (TEM) reveals electron micrographs showing various types of defect features along with the presence of 3-4nm thick amorphous layers forming the grain boundaries of otherwise crystalline MgB2. Raman spectra of the compound at room temperature exhibited characteristic phonon peak at 600 cm-1. Superconductivity is observed at 37.2K by magnetic susceptibility C(T), resistivity R(T), thermoelectric power S(T), and thermal conductivity K(T) measurements. The power law fitting of R(T) give rise to Debye temperature at 1400K which is found consistent with the theoretical fitting of S(T), exhibiting ThetaD of 1410K and carrier density of 3.81x 1028/m3. Thermal conductivity K(T) shows a jump at 38K, i.e., at Tc, which was missing in some earlier reports. Critical current density (Jc) of up to 105 A/cm2 in 1-2T (Tesla) fields at temperatures (T) of up to 10K is seen from magnetization measurements. The irreversibility field, defined as the field related to merging of M(H) loops is found to be 78, 68 and 42 kOe at 4, 10 and 20K respectively. The superconducting performance parameters viz. irreversibility field (Hirr) and critical current density Jc(H) of the studied MgB2 are improved profoundly with addition of nano-SiC and nano-Diamond. The physical property parameters measured for polycrystalline MgB2 are compared with earlier reports and a consolidated insight of various physical properties is presented., 41 pages TEXT+Figs

La0.7Ba0.3MnO3 (LBMO): Ag x (x=0.0, 0.1, 0.2, 0.3, and 0.4) composites are synthesized by a solid-state reaction route, the final sintering temperatures are varied from 1300 °C (LBMO1300Ag) to 1400 °C (LBMO1400Ag), and their physical properties are compared as a function of temperature and Ag content. All samples are crystallized in single phase accompanied by some distortion in main structural phase peaks at higher angles with an increase in silver content. Though the lattice parameters (a,c) decrease, the b increases slightly with an increase in Ag content. The scanning electron micrographs (SEM) showed better grains morphology in terms of size and diffusion of grain boundaries with an increase in Ag content. In both LBMO1300Ag and LBMO1400Ag series, the metal insulator transition (T MI) and accompanied paramagnetic-ferromagnetic transition (T C) temperatures are decreased with an increase in Ag content. The sharpness of MI transition, defined by temperature coefficient of resistance (TCR), is improved for Ag added samples. At a particular content of Ag(0.3), the T MI and T C are tuned to 300 K and maximum magneto-resistance at 7 Tesla applied field (MR7 T) of up to 55% is achieved at this temperature, which is more than double to that as observed for pure samples of the both 1300 and 1400 °C series at same temperature. The MR7 T is further increased to above 60% for LBMOAg(0.4) samples, but is at 270 K. The MR7 T is measured at varying temperatures of 5, 100, 200, 300, and 400 K in varying fields from ±7 Tesla, which exhibits U and V type shapes. Summarily, the addition of Ag in LBMO improves significantly the morphology of the grains and results in better physical properties of the parent manganite system.

The influence of dc biasing current on temperature dependence of resistance of La 0.67 Ba 0.33 MnO 3 bulk sample is reported. A decrease in the resistance (electroresistance) on the application of higher bias current is observed. The electroresistance is maximum at metal–insulator transition temperature ( T MI ) and decreases when the temperature is either increased or decreased from T MI . A two-phase model is proposed to explain the occurrence of electroresistance. The higher bias current leads to an increase in alignment of spins and thus, in turn, leads to an increase in spin stiffness coefficient and decrease in the resistance at T MI .

The synthesis and characterization of PVA (Poly Vinyl Acetate) doped bulk MgB2 superconductor is reported here. PVA is used as a Carbon source. PVA doping effects made two distinguishable contributions: first enhancement of Jc field performance and second an increase in Hc2 value, both because of carbon incorporation into MgB2 crystal lattice. The susceptibility measurement reveals that Tc decreased from 37 to 36 K. Lattice parameter a decreased from 3.085 A to 3.081 A due to the partial substitution of Carbon at Boron site. PVA doped sample exhibited the Jc values greater than 10^5 A/cm2 at 5 & 10 K at low fields; which is almost 3 times higher than the pure one, while at high fields the Jc is increased by an order of magnitude in comparison to pure MgB2. From R(T)H measurements we found higher Tc values under magnetic field for doped sample; indicating an increase in Hc2. Also the magnetization measurements exhibited a significant enhancement in Hirr value. The improved performance of PVA doped MgB2 can be attributed to the substitution of carbon at boron site in parent MgB2 and the resulting impact on the carrier density and impurity scattering. The improved flux pinning behavior could easily be seen from reduced flux pinning force plots., 14 Pages of Text + Figs. To appear in Physica C

Temperature dependence of resistivity of double layered manganite La 2−2 x Ca 1+2 x Mn 2 O 7 (‘ x ’ = 0.1, 0.2, 0.3 and 0.4) has been investigated in the temperature range of 4.2–50 K. An upturn in the resistivity was observed for all the compositions. The increase of resistivity with the decrease in temperature was found to be due to combined effect of weak localization and electron–electron scattering. The observed minimum in the resistivity is attributed to a competition between weak localization, electron–electron scattering and electron–phonon scattering.

Bulk polycrystalline samples of nano- Mo doped MgB 2 were synthesized by Fe tube encapsulation at ambient pressure under argon annealing (850°C). Mo substitution takes place successfully at the Mg site in Mg 1-x Mo x B 2 only till x=0.2. For higher (x>0.2) Mo content the same did not enter the MgB 2 lattice but rather forms an isomorphic lattice in the host with decreased c but an increased a-parameter. The ρ(T) measurements showed superconducting transition temperature (T c ) of around 36 K for all the samples till x=0.3 and slightly decreased values of 35 and 34 K for x=0.4 and 0.5 samples, respectively. Resistivity under magnetic field [R(T)H] experiments showed distinct single peaks in dρ/dT for all applied fields up to 8 Tesla. The estimated upper critical field H c2 is 8 Tesla for pristine and x=0.2 samples at 15.6 and 19 K, respectively. Thus H c2 increases up to x=0.20 samples and decreases afterwards. Magnetic susceptibility measurements exhibited sharp transition to superconducting state with a sizeable diamagnetic signal at 38 K (T c ) in zero field-cooled measurements. Commendable current density (J c ) of up to 105 A/cm2 in 1–2 T (Tesla) fields at temperatures (T) of up to 10 K is seen from magnetization measurements invoking the Bean's critical state model for pristine samples. For higher fields above 2.5 Tesla the J c (H) characteristics of x=0.1 and 0.2 samples were found to be slightly superior to that for pristine samples with enhanced H irr (irreversibility field).

We report here the electrical resistivity, ac susceptibility, low field magneto-resistance (LFMR) and structural parameters of the Pr 1-x Ba x MnO 3(x= 0.33–0.80) system. Samples with 33% and 40% Ba -content exhibit two clear transitions in their resistivity-temperature behavior while signatures of higher temperature transition are also seen in the 50% Ba sample. The higher temperature transition (insulator-metal, Tp1), like in other CMR materials, results due to competition between the double exchange and super exchange mechanisms. The lower temperature broad hump (Tp2) in resistivity reflects the strain effect at the grain boundaries due to the ionic radii mismatch between Pr +3 and Ba +2. The low field magneto-resistance (at 0.15T) also corroborates the above results as MR peaks near Tp1 and increases further at lower temperatures, reflecting the contribution from the grain boundaries. Susceptibility measurements show the Curie transition (Tc) to be near Tp1 of the ρ-T data. Results are found to be consistent with the (NdBa) MnO 3 and (LaBa) MnO 3 systems. Resistivity data on the Pr 0.67 Sr 0.33 MnO 3 system, where the lower temperature transition at Tp2 is not seen due to the smaller ionic size difference of ( Pr +3– Sr +2) compared to ( Pr +3– Ba +2), also substantiate these results. The observed resistivity upturn at low temperatures is considered in terms of the ensuing localization of carriers due to the varying degree of ionic size mismatch in these systems.

Bulk samples of double layered manganite La 1.4 Ca 1.6 Mn 2 O 7 were prepared by solid-state reaction method. Study of magnetotransport properties revealed a metal to insulator transition at 160 K, a ferromagnetic transition at 164 K and appearance of two-dimensional short range ferromagnetic correlations around 235 K. The temperature dependence of conduction noise showed three peaks at 114 K, 165 K and 235 K which are attributed to excess noise generated due to setting up of three-dimensional out-of-plane ferromagnetic transition, two-dimensional out-of-plane ferromagnetic transition temperatures and appearance of two-dimensional short range ferromagnetic correlations respectively. In the temperature region ~ 160 K, excess noise was also generated due to the metal-insulator transition. In the paramagnetic region, conduction noise was found to remain unchanged with the applied magnetic field whereas in the ferromagnetic regions, the peaks at 114 K and 235 K got disappeared. The latter is attributed to the suppression of spin fluctuations.

La0.7Ca0.2Ba0.1MnO3 (LCBMO) films were prepared by ultrasonically assisted spray pyrolysis deposition on LaAlO3 and Al2O3 single crystal substrates. Conductance–voltage ( G – V ) characteristics and conduction noise of the two films were found to be different. G – V characteristics of LCBMO film on LaAlO3 were linear indicating the spin dependent scattering across the grain boundaries whereas for the film on Al2O3, G – V characteristics at low temperature ( T 210 K ) were nonlinear indicating dominant transport mechanism as spin polarized tunnelling. Normalized conduction noise of LCBMO film on LaAlO3 was one order lower to that of film on Al2O3. These differences in conductance–voltage and conduction noise characteristics of the two films are attributed to the presence of more disordered grain boundaries in LCBMO film on Al2O3 as compared to the films on LaAlO3 substrate.

Double-layered manganite La 2−2 x Ca 1+2 x Mn 2 O 7 have been synthesized for compositions ‘ x ’=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 by solid state reaction method. From X-ray diffraction study, their crystal structures were found to be tetragonal perovskite with lattice parameters decreasing with increasing ‘ x ’. The decreasing lattice parameters affect the balance between in-plane, intra-bilayer and inter-bilayer exchange interactions, which is reflected on magnetotransport properties. The metal-to-insulator transition temperature is found to vary with composition and peaked around ‘ x ’=0.3. From ac-susceptibility study, 2D-ferromagnetic ordering was observed at higher temperatures for all compositions whereas 3D-ferromagnetic ordering was observed at quite low temperatures. In low-temperature region, decreasing susceptibility shows antiferromagnetic state for all compositions. On the basis of electrical and magnetic properties, a magnetic phase diagram is given.

Non-magnetic Zn 2+ , with varying molar concentration x (0 x 2 Cu 3 O 7− δ at Cu(2) site. The samples are treated with deionised water for 16 h. The degradation effect on both the untreated and water treated YBa 2 Cu 3− x Zn x O 7− δ samples is studied by employing temperature dependence resistance measurements, oxygen content and determination of unit cell parameters through X-ray diffraction. The results show that Zn 2+ substitution, unlike the Sr 2+ or the Al 3+ , does not control or prevent degradation of the superconducting samples rather it enhances with increasing concentration of zinc.

Double layered manganite of La 1.4 Ca 1.6 Mn 2 O 7 (DLCMO) was prepared using solid state reaction method and had a metal–insulator transition temperature ( T MI ) of 125 K. The short range 2D-feerromagnetic ordering (T C2 ) starts growing when T T C1 ) at 114 K. Low field magnetoresistance (MR) behaviour of the DLCMO was investigated and compared with an infinite layered manganite La 0.7 Ca 0.3 MnO 3 (LCMO). For DLCMO, in the temperature range between T C1 and T C2 , the MR showed a gradual increase with the magnetic field. The observed MR and R – T behaviour of double layered manganite for T C1 T T C2 has been explained in the frame work of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions] and percolative behaviour of transport in FM–PM mixture.

Temperature dependence of conduction noise and low field magnetoresistance of layered manganite La 1.4 Ca 1.6 Mn 2 O 7 (DLCMO) are reported and compared with the infinite layered manganite La 0.7 Ca 0.3 MnO 3 (LCMO). The double layered manganite was prepared using standard solid state reaction method and had a metal–insulator transition temperature ( T M–I ) of 155 K. The temperature dependence of susceptibility showed evolution of ferromagnetic ordering at 168 K. The observed voltage noise spectral density ( S V ) shows 1/ f α type of behaviour at all temperatures from 77 K to 300 K. In the ferromagnetic region ( T S V / V 2 shows two peaks at 164 K and 114 K. The observed two peaks in normalised conduction noise of DLCMO is attributed to the excess noise generated due to setting up of short range 2D-ferromagnetic ordering and long range 3D-ferromagnetic ordering at two different temperatures T C2 and T C1 . In temperature range between T C1 and T C2 , the magnetoresistance (MR) showed a gradual increase with the magnetic field. The observed MR has been explained in the framework of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions].

A single artificial grain boundary in La 0.67 Ba 0.33 MnO 3 (LBMO) thin film has been prepared by depositing the film on a bicrystal substrate using laser ablation technique. We investigated the magnetic field dependence of magnetoresistance and conductance–voltage characteristics of the grain boundary at 77 K. A decrease of nonlinearity of current–voltage characteristics was observed upon application of magnetic field. The results are explained by assuming the presence of two different types of parallel conducting channels (metallic and highly resistive) across the grain boundary. The analysis of the results reveals that the application of magnetic field suppresses magnetic disorders at the grain boundary region and increases metallic conduction channels across the grain boundary. The temperature dependence of the conduction noise of the bicrystal grain boundary was measured at 0 and 1.5 kG magnetic field and compared with a microbridge on the LBMO film having no grain boundary. The presence of the grain boundary was found to enhance noise by one order of magnitude. The noise of a bicrystal grain boundary showed a decrease in the presence of 1.5 kG magnetic field for T

Modified Ambegaokar–Halperin model has been applied to the temperature dependent resistivity (R–T) curves of polycrystalline Ba2YCu3O7−δ superconductor. The curves, drawn using the modified AH model, fit well with the experimental R–T data points for different magnetic fields ranging from 0 to ∼0.5 T. Each curve is found to possess two distinct regions, which suggests the dissipation phenomena in Ba2YCu3O7−δ superconductor is caused by two mechanisms, viz. the order parameter fluctuations and the vortex-dynamics. A new dissipation transition temperature TBP has been observed, across which one dissipation mechanism possibly changes over to another. On either side of TBP there exists a small region where both the mechanisms seem to operate.

The magnetoresistance curves of polycrystalline Ba 2 YCu 3 O 7-δ samples for H=0, 1050 and 4880 G are analyzed using the modified Ambegaokar–Halperin model. The analysis shows the occurrence of a crossover in the R–T curves at a field dependent temperature T BP , revealing that the dissipation in Ba 2 YCu 3 O 7-δ samples is possibly caused by two different mechanisms: vortex-dynamics in the low temperature region (below T BP ) and the order parameter fluctuations in the high temperature region (above T BP ).

Magneto-resistance studies in a high-Tc granular superconductor Bi1.7Pb0.3Sr2Ca2Cu3Ox in an applied magnetic field ranging from 0 to 0.6 T in the low current limits (1 mA) have been carried out to understand the dissipation mechanism in high-Tc superconducting materials. The Kosterlitz–Thouless (KT) as well as the Ambegaoker–Halperin (AH) widely employed models are invoked to interpret the results. The KT theory is found to account for dissipation in the temperature range TKT

Diamagnetic Al 3+ ions have been substituted in YBa 2 Cu 3 O 7− δ sites. Comparative stability studies on 8 h and 16 h deionized water-treated samples (0 x x =0.2 is more resistant to water corrosion as compared to other compositions with x =0, 0.05, 0.1 and 0.3.

The substitution of Al at Cu(1) sites in YBa 2 Cu 3-x Al x O 7-δ (0 < x < 0.3) was carried out. The samples were treated with deionized water of conductivity 9.3 x 10 -6 Ω -1 cm -1 . The effect of water was examined by studying the changes produced in T c (K), oxygen content and lattice parameters. The investigations indicated that the sample with composition x = 0.2, i.e. YBa 2 Cu 2.8 Al 0.2 O 6.934 , was more water resistant as compared to samples with compositions x = 0, 0.05, 0.1 and 0.3.

Magneto-transport measurements have been carried out to investigate the effect of 250 MeV Ag+17 ion irradiation on the flux pinning properties in YBa2Cu3O7-δ polycrystalline films. The ohmic resistivity measurements of the films above the Bose-glass temperature, TBG, have been carried out as a function of temperature, magnitude and orientation of the magnetic field with respect to the columnar defects. The experimental results have been analysed on the basis of Bose-glass theory. The angular dependence of resistance shows a dip and the Bose-glass temperature is found to be a maximum when the magnetic field is aligned with the columnar pins. Angular critical scaling laws have been used to determine the static ν' and dynamic z' exponents. In addition, the prediction of universal behaviour of Bose-glass theory below the critical angle has been observed.

Angle dependent magnetoresistance of the 250 MeV Ag+17 ion irradiated YBa2Cu3O7−δ melt-textured sample and polycrystalline films have been carried out to study the pinning behaviour in the presence of columnar defects. Bose glass scaling laws, both to the melt-textured sample and polycrystalline film have been applied uniquely to the angle dependent magnetoresistance to determine the critical static (v′), dynamic (z′) exponents and the critical angle (θcri). The values were found to be v′∼1.0 and z′∼4.0, respectively for both irradiated melt-textured sample and polycrystalline film. The scaling laws are found to hold good below the critical angle which is found to be different in these samples. The Bose glass temperature ‘TBG’ is found to be maximum along the columnar defects. Besides, the vortex binding energy per unit length has been computed.

Columnar defects along c- axis as well as in crossed geometry in YBa 2 Cu 3 O 7− δ melt-textured samples have been created by 250 MeV Ag +17 ion irradiation. Angle-dependent magnetoresistance measurements have been carried out to study the flux pinning behaviour of the samples in the presence of columnar defects and to examine the Bose-glass theory. The Bose-glass temperature ‘ T BG ’ has been found to be maximum along the direction of columnar defects. In addition, angular critical scaling laws have been applied to determine the critical exponents. The value of the critical angle, θ cri , is found to be ∼30° and c- axis and for the crossed linear defect geometry, respectively.

Dissipation in high-T c superconducting Bi 1.7 Pb 0.3 Sr 2 Ca 2 Cu 3 O x samples in different magnetic fields has been investigated in terms of the modified Ambegaokar–Halperin theory. Our observed dissipation behaviour in the transition region is well described on this model by employing two exponents instead of one. Curiously, each resistance–temperature curve seems to possess mainly two different regions, which, we believe, can be accounted for in terms of order parameter fluctuations and vortex dynamics. A new dissipation mechanism transition temperature T BP has been observed, across which one dissipation mechanism possibly changes over to another mechanism.

High-temperature superconducting YBa2Cu3O7−δ melt-textured samples have been irradiated with 250 MeV Ag+17 ion irradiation to create columnar defects at an angle ±45° from c-axis. Subsequently, angle-dependent magnetoresistance measurements have been carried out at different angles, viz. θ=0,±15°,30° and ±45°, respectively. The Bose-glass theory is examined by the application of critical scaling laws.

Single crystals of the common polytype 2H of lead iodide were grown by zone melting and subjected to argon ion laser irradiation. Both time and intensity of radiation were systematically varied to assess their effect on the magnitude of band gap of the crystals. The band gap was determined by UV spectrophotometry. When the beam intensity was varied, the band gap value steadily decreased from 2.53 to 2.02 eV, but the value drastically decreased from 2.53 to 1.04 eV when the irradiation time was varied. The results are explained in terms of changes in polycrystallinity, surface roughness, amount of absorption, and anisotropic nature of thermal conductivity of the material.

Stability of YBa2−xSrxCu3O7−δ (0

Vortex dynamics of YBa 2 Cu 3 O 7−δ films with controlled columnar defects (CD) were investigated using angle resolved resistivity measurements. The Bose Glass Temperature T BG is found to decrease with increasing angle between the applied field and CDs. The Critical exponent ν ( z − 2), where ν is the static exponent and z is the dynamic exponent, were found to be angle independent.

Crystals of CdI 2 and PbI 2 were grown by zone melting. They were subjected to argon ion laser irradiation. Both time and intensity of radiation were systematically varied. The crystals of CdI 2 were of type 4H while those of PbI 2 were of type 2H. Their energy gaps were determined by UV spectrophotometry. The CdI 2 crystals showed a linear variation of energy gap with intensity of radiation (0 to 125 mW), with the energy gap value decreasing from 3.28 to 3.11 eV, when the exposure time was kept fixed (7 min). When the time of exposure was varied, with the intensity of radiation fixed at 50 mW, the energy gap initially decreased from 3.28 to 3.07 eV and then gradually rose to be restored to the original value, thus showing a nonlinear variation. For the PbI 2 crystals, the energy gap steadily decreased from 2.53 to 2.02 eV, when the intensity of exposure was varied. However, the value drastically decreased from 2.53 to 1.04 eV for the case of variation of exposure time. The results have been explained in terms of changes in phonon frequency, amount of absorption and anisotropy of thermal conductivity of the material.

The recovery of superconductivity in water degraded YBa 2 Cu 3 O 7− δ samples has been studied by subjecting them to heat treatments. The possible recovery of superconductivity has been estimated through structural, electrical, magnetic and morphological investigations. The results reveal the transformation of secondary phases formed during degradation into YBCO phase. The presence of some additional phases of YBCO, possibly arising from deficiency of the constituent elements resulting from degradation/lack of proper reaction between the secondary phases, is also observed. The heat treatment carried out on a sample after pulverising it led to full recovery of superconductivity in it. A possible mechanism for the observed recovery of superconductivity has been suggested.

The partial substitution of Sr at Ba sites in YBa 2-x Sr x Cu 3 O 7-δ (0 < x < 0.6) has been carried out. The samples are treated with deionized water for 16 h. The influence of water on the corrosion resistance of YBa 2-x Sr x Cu 3 O 7-δ is examined by the changes in T c , lattice parameters, orthorhombicity and oxygen contents. The results indicate that the corrosion resistance of YBa 1.8 Sr 0.2 Cu 3 O 6.882 against treatment with water is enhanced.

Highly purified single crystals of cadmium iodide obtained through repeated zone refining have been subjected to laser beam exposure (Argon ion laser), both for various time durations and to various beam intensities, and then subjected to band gap determination by UV spectroscopy. The band gap has been found to decrease gradually with increase in the laser beam intensity, whereas its variation with increase in time of exposure shows an unusual behaviour such that it initially falls sharply followed by a gradual rise back to its original value. The results have been analysed and interpreted in terms of indirect band gap character of the material and an unusual variation in phonon frequency. The interpretation is well supported by X-ray diffraction and scanning electron microscopy (SEM) studies.

We report here the temperature dependence of the out-of-phase susceptibility of 60 Co γ-irradiated granular YBa 2 Cu 3 O 7−δ samples under the zero field cooling (ZFC) and field cooling (FC) configurations. DC field of different magnitudes was superimposed on a fixed 1 Oe ac field of frequency 111.1 Hz. A modified form of the Anderson-Kim model has been invoked to compute the critical current density ( J c ) and the pinning force density ( f p ). J c has been seen to be a non-linear function of the increasing radiation dosage. Both J c and f p are found to be higher for the FC case than for the ZFC case. Attempts have been made to account for the experimental results in terms of the formation of point defects like vacancies, interstitials and their recombination during irradiation.

Samples of the high temperature superconductor, YBCO, were encapsulated with thermoset silicone and epoxy resins by a dip coating method, to control environmental degradation of YBCO. The change in the encapsulating films in relation to the superconductor under repeated cryo-thermal cycles was examined by scanning electron microscopy (SEM). The SEM studies reveal that the cryo-thermal cycles cause progressive crazing (plastic deformation) of the encapsulating resins. The overall change in the behaviour of the resin is found to be somewhat different in the two cases. The process of films rupturing and detachment, is induced earlier in the epoxy film than in the silicone film.

The stability of the high-temperature superconductor YBa 2 Cu 3 O 7- δ on treatment with water has been examined and quantitatively determined by XRD, AC magnetic susceptibility measurements and TGA, and by calculating the lattice parameters through Cohen's method. The kinetic parameters, viz. activation energy and order of reaction, have been determined from the TGA data. Progressively longer treatment results in the degradation of the material, as evidenced by corresponding changes in the values of lattice parameters, changes in orthorhombicity, and a continuous increase in the value of the activation energy. The reaction leading to the decomposition of the material is found to be of first order.

In this paper we report an enhanced temperature coefficient of resistance (TCR) close to room temperature in La0.7Ca0.3-xBaxMnO3 + Agy (x = 0.10, 0.15 and y = 0.0 to 0.40) (LCBMO+Ag) composite manganites. The observed enhancement of TCR is attributed to the grain growth and opening of new conducting channels in the composites. Ag addition has also been found to enhance intra-granular magneto-resistance. Inter-granular MR, however, is seen to decrease with Ag addition. The enhanced TCR and MR at / near room temperature open up the possibility of the use of such materials as infrared bolometric and magnetic field sensors respectively., Comment: 22 pages of Text + Figs:comments/suggestions(awana@mail.nplindia.ernet.in)

The superconductivity of MgB2, AlB2, NbB2+x and TaB2+x is inter-compared. The stretched c-lattice parameter (c = 3.52 A) of MgB2 in comparison to NbB2.4 (c = 3.32 A) and AlB2 (c = 3.25 A) decides empirically the population of their p and s bands and as a result their transition temperature, Tc values respectively at 39K and 9.5K for the first two and no superconductivity for the later. Besides the electron doping from substitution of Mg+2 by Al+3, the stretched c-parameter also affects the Boron plane constructed hole type Sigma-band population and the contribution from Mg or Al plane electron type Pi band. This turns the electron type (mainly Pi-band conduction) non-superconducting AlB2 to hole type (mainly s-band conduction) MgB2 superconductor (39 K) as indicated by the thermoelectric power study. Keeping this strategy in mind that stretching of c-parameter enhances superconductivity, the NbB2+x and TaB2+x samples are studied for existence of superconductivity. The non-stoichiometry induces an increase in c parameter with Boron excess in both borides. Magnetization (M-T) and Resistivity measurements (R-T) in case of niobium boride samples show the absence of superconductivity in stoichiometric NbB2 sample (c = 3.26 A) while a clear diamagnetic signal and a R = 0 transition for Boron excess NbB2+x samples. On the other hand, superconductivity is not achieved in TaB2+x case. The probable reason behind is the comparatively lesser or insufficient stretching of c-parameter., 14 pages Text+Figs contact (awana@mail.nplindia.ernet.in). ACCEPTED: J. APPL.PHYS. (MMM-2008 Proceedings)

We report the synthesis, magnetic susceptibility and crystal structure analysis for NbB2+x (x = 0.0 to 1.0) samples. The study facilitates in finding a correlation among the lattice parameters, chemical composition and the superconducting transition temperature Tc. Rietveld analysis is done on the X- ray diffraction patterns of all synthesized samples to determine the lattice parameters. The a parameter decreases slightly and has a random variation with increasing x, while c parameter increases from 3.26 for pure NbB2 to 3.32 for x=0.4 i.e. NbB2.4. With higher Boron content (x>0.4) the c parameter decreases slightly. The stretching of lattice in c direction induces superconductivity in the non- stoichiometric niobium boride. Pure NbB2 is non-superconductor while the other NbB2+x (x>0.0) samples show diamagnetic signal in the temperature range 8.9-11K. Magnetization measurements (M-H) at a fixed temperature of 5K are also carried out in both increasing and decreasing directions of field. The estimated lower and upper critical fields (Hc1 & Hc2) as viewed from M-H plots are around 590 and 2000Oe respectively for NbB2.6 samples. In our case, superconductivity is achieved in NbB2 by varying the Nb/B ratios, rather than changing the processing conditions as reported by others., 14 pages TEXT+Figs; comments/suggestions (awana@mail.nplindia.ernet.in). ACCEPTED: Solid State Communications (2008)

We report the synthesis and variation of superconductivity parameters such as transition temperature Tc, upper critical field Hc, critical current density Jc, irreversibility field Hirr and flux pinning parameter (Fp) for the MgB2-xCx system with nano-Carbon doping up to x=0.20. Carbon substitutes successfully on boron site and results in significant enhancement of Hirr and Jc(H). Resistivity measurements reveal a continuous decrease in Tc under zero applied field, while the same improves remarkably at higher fields with an increase in nano-C content for MgB2-xCx system. The irreversibility field value (Hirr) is 7.6 & 6.6 Tesla at 5 and 10K respectively for the pristine sample, which is enhanced to 13.4 and 11.0 Tesla for x = .08 sample at same temperatures. Compared to undoped sample, critical current density (Jc) for the x=0.08 nano-Carbon doped sample is increased by a factor of 24 at 10K at 6 Tesla field., 14 pages Text+Figs; Comments/suggestions welcome (awana@mail.nplindia.ernet.in)

We study the effect of nano(n)-SiC addition on the crystal structure, critical temperature (Tc), critical current density (Jc) and flux pinning in MgB2 superconductor. X-ray diffraction patterns show that all the samples have MgB2 as the main phase with very small amount of MgO, further with n-SiC addition the presence of Mg2Si is also noted and confirmed by SEM & EDS. The Tc value for the pure MgB2 is 18.9K under 8 Tesla applied field, while is 20.8K for the 10-wt % n-SiC doped sample under the same field. This points towards the increment in upper-critical field value with n-SiC addition. The irreversibility field (Hirr) for the 5% n-SiC added sample reached 11.3, 10 and 5.8 Tesla, compared to 7.5, 6.5, and 4.2 Tesla for the pure MgB2 at 5, 10 and 20K respectively. The critical current density (Jc) for the 5-wt % n-SiC added sample is increased by a factor of 35 at 10K and 6.5 Tesla field and by a factor 20 at 20K and 4.2 Tesla field. These results are understood on the basis of superconducting condensate (sigma band) disorder and ensuing intrinsic pining due to B site C substitution clubbed with further external pinning due to available n-SiC/Mg2Si pins in the composite system., 19 pages of Text + Figs. comments/suggestions are welcome (awana@mail.nplindia.ernet.in)

In this paper we report the results of specific heat measurements carried out on single phase samples of Bi-based 2-1-2-2 and Tl-based 2-2-2-3 compounds. The data are compared with those obtained on Y-based 1-2-3 system. The samples have been simultaneously characterized for superconductivity using AC susceptibility and four-probe resistivity techniques and for their structural aspects using XRD. All samples studied unambiguously reveal the characteristic specific heat jump at T c . The data are used for estimation of various electronic parameters which are compared for the three systems. The parameter Δ C /(γƒ T c ) is found close to the BCS weak-coupling value.