30 research outputs found
Enhanced Critical parameters of nano-Carbon doped MgB2 Superconductor
The high field magnetization and magneto transport measurements are carried
out to determine the critical superconducting parameters of MgB2-xCx system.
The synthesized samples are pure phase and the lattice parameters evaluation is
carried out using the Rietveld refinement. The R-T(H) measurements are done up
to a field of 140 kOe. The upper critical field values, Hc2 are obtained from
this data based upon the criterion of 90% of normal resistivity i.e. Hc2=H at
which Rho=90%Rho; where RhoN is the normal resistivity i.e., resistivity at
about 40 K in our case. The Werthamer-Helfand-Hohenberg (WHH) prediction of
Hc(0) underestimates the critical field value even below than the field up to
which measurement is carried out. After this the model, the Ginzburg Landau
theory (GL equation) is applied to the R-T(H) data which not only calculates
the Hc2(0) value but also determines the dependence of Hc2 on temperature in
the low temperature high field region. The estimated Hc(0)=157.2 kOe for pure
MgB2 is profoundly enhanced to 297.5 kOe for the x=0.15 sample in MgB2-xCx
series. Magnetization measurements are done up to 120 kOe at different
temperatures and the other parameters like irreversibility field, Hirr and
critical current density Jc(H) are also calculated. The nano carbon doping
results in substantial enhancement of critical parameters like Hc2, Hirr and
Jc(H) in comparison to the pure MgB2 sample.Comment: 25 pages with 9 Figs:
comments/suggestions([email protected]
Superconductivity of non- stoichiometric intermetallic compound NbB2
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.Comment: 14 pages TEXT+Figs; comments/suggestions
([email protected]). ACCEPTED: Solid State Communications (2008
Superconductivity of various borides: The role of stretched c-parameter
The superconductivity of MgB2, AlB2, NbB2+x, and TaB2+x is intercompared. The stretched c-lattice parameter (c = 3.52 Å) of MgB2 in comparison to NbB2.4 (c = 3.32 Å) and AlB2 (c = 3.25 Å) decides empirically the population of their π and σ bands and as a result their transition temperature Tc values, respectively, at 39 and 9.5 K for the first two and no superconductivity for the later. The nonstoichiometry induces an increase in c parameter with Boron excess both in NbB2+x and TaB2+x. Magnetization (M-T) and resistivity measurements (ρ-T) in case of niobium boride samples show the absence of superconductivity in stoichiometric NbB2 sample (c = 3.26 Å) while a clear diamagnetic signal and a ρ = 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
Anomalous thermoelectric power of Mg1-xAlxB2 system with x = 0.0 to 1.0
Thermoelectric power, S(T) of the Mg1-xAlxB2 system has been measured for x =
0.0, 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0. XRD, resistivity and magnetization
measurements are also presented. It has been found that the thermoelectric
power is positive for x = 0.4 and is negative for x = 0.6 over the entire
temperature range studied up to 300 K. The thermoelectric power of x = 0.4
samples vanishes discontinuously below a certain temperature, implying
existence of superconductivity. In general, the magnitude of the thermoelectric
power increases with temperature up to a certain temperature, and then it
starts to decrease towards zero base line. In order to explain the observed
behavior of the thermoelectric power, we have used a model in which both
diffusion and phonon drag processes are combined by using a phenomenological
interpolation between the low and high temperature behaviors of the
thermoelectric power. The considered model provides an excellent fit to the
observed data. It is further found that Al doping enhances the Debye
temperature.Comment: 19 pages Text + Figs.
suggestions/comments([email protected]
Superconductivity in SmFe1-xCoxAsO (x = 0.0 to 0.30)
We report synthesis, structural details and magnetization of SmFe1-xCoxAsO
with x ranging from 0.0 to 0.30. It is found that Co substitutes fully at Fe
site in SmFeAsO in an iso-structural lattice with slightly compressed cell. The
parent compound exhibited known spin density wave (SDW) character below at
around 140 K. Successive doping of Co at Fe site suppressed the SDW transition
for x = 0.05 and later induced superconductivity for x = 0.10, 0.15 and 0.20
respectively at 14, 15.5 and 9K. The lower critical field as seen from
magnetization measurements is below 200Oe. The appearance of bulk
superconductivity is established by wide open isothermal magnetization M(H)
loops. Superconductivity is not observed for higher content of Co i.e. x =
0.30. Clearly the Co substitution at Fe site in SmFe1-xCoxAsO diminishes the Fe
SDW character, introduces bulk superconductivity for x between 0.10 and 0.20
and finally becomes non-superconducting for x above 0.20. The Fe2+ site Co3+
substitution injects mobile electrons to the system and superconductivity
appears, however direct substitution introduces simultaneous disorder in
superconducting FeAs layer and thus superconductivity disappears for higher
content of Co.Comment: 14 Pages Text + Figs comments ([email protected]
Synthesis and Physical Properties of FeSe1/2Te1/2 Superconductor
One of the most important properties of very recently reported FeSe based
superconductors is the robustness of their superconductivity under applied
magnetic field. The synthesis and control of superconductivity in FeSe based
compounds is rather a difficult task. Synthesis and physical property
characterization for optimized superconductivity of FeSe1/2Te1/2 at 13 K is
reported here. The compound crystallized in a tetragonal structure with lattice
parameters a = 3.8008(10) and c = 6.0187 (15) A. Magnetization measurements
indicated bulk superconductivity with lower critical field (Hc1) of around 180
Oe. By applying Ginzburg Landau (GL) theory, the Hc2(0) value is estimated to
be = 1840 kOe for the 90% of resistive transition. A heat capacity measurement
revealed bulk superconductivity by a hump at Tc near 13 K, and an expected
decrease was observed under an applied magnetic field.Comment: 13 pages text + Figs: commenta ([email protected]
Role of Carbon in Enhancing the Performance of MgB2 superconductor
The enhancement of the critical current density (Jc(H)) of carbon and
nano-SiC doped MgB2 is presented and compared. The upper critical field (Hc2)
being determined from resistivity under magnetic field experiments is though
improved for both C substitution and nano-SiC addition the same is more
pronounced for the former. In MgB2-xCx carbon is substituted for boron that
induces disorder in the boron network and acts as internal pinning centres. The
optimal Jc(H) values are obtained for x = 0.1 sample . In case of nano-SiC
doped in MgB2, the Jc(H) improves more profoundly and two simultaneous
mechanisms seems responsible to this enhancement. Highly reactive nano-SiC
releases free carbon atom, which gets easily incorporated into the MgB2 lattice
to act as intrinsic pinning centres. Further enhancement is observed for higher
nano-SiC concentrations, where the un-reacted components serve as additional
extrinsic pinning centres.Comment: 17 Pages text + Fig