1,318 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]
Calculation of Band Edge Eigenfunctions and Eigenvalues of Periodic Potentials through the Quantum Hamilton - Jacobi Formalism
We obtain the band edge eigenfunctions and the eigenvalues of solvable
periodic potentials using the quantum Hamilton - Jacobi formalism. The
potentials studied here are the Lam{\'e} and the associated Lam{\'e} which
belong to the class of elliptic potentials. The formalism requires an
assumption about the singularity structure of the quantum momentum function
, which satisfies the Riccati type quantum Hamilton - Jacobi equation, in the complex plane. Essential
use is made of suitable conformal transformations, which leads to the
eigenvalues and the eigenfunctions corresponding to the band edges in a simple
and straightforward manner. Our study reveals interesting features about the
singularity structure of , responsible in yielding the band edge
eigenfunctions and eigenvalues.Comment: 21 pages, 5 table
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
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]
High Field Performance of Nano-Diamond Doped MgB2 Superconductor
Polycrystalline MgB2-nDx (x= 0 to 0.1) samples are synthesized by solid-state
route with ingredients of Mg, B and n-Diamond. The results from
magneto-transport and magnetization of nano-diamond doped MgB2-nDx are
reported. Superconducting transition temperature (Tc) is not affected
significantly by x up to x = 0.05 and latter decreases slightly for higher x >
0.05. R(T) vs H measurements show higher Tc values under same applied magnetic
fields for the nano-diamond added samples, resulting in higher estimated Hc2
values. From the magnetization measurements it was found that irreversibility
field value Hirr for the pristine sample is 7.5 Tesla at 4 K and the same is
increased to 13.5 Tesla for 3-wt% nD added sample at the same temperature. The
Jc(H) plots at all temperatures show that Jc value is lowest at all applied
fields for pristine MgB2 and the sample doped with 3-wt% nD gives the best Jc
values at all fields. For the pure sample the value of Jc is of the order of
105 A/cm2 at lower fields but it decreases very fast as the magnetic field is
applied and becomes negligible above 7 Tesla. The Jc is 40 times higher than
pure MgB2 at 10 K at 6 Tesla field in case of 3%nD doped sample and its value
is still of the order of 103 A/cm2 at 10 Tesla for the same sample. On the
other hand at 20K the 5%nD sample shows the best performance at higher fields.
These results are discussed in terms of extrinsic pinning due to dispersed
n-Diamond in the host MgB2 matrix along with the intrinsic pinning due to
possible substitution of C at Boron site and increased inter-band scattering
for highly doped samples resulting in extraordinary performance of the doped
system.Comment: 12 PAGES (TEXT+FIGS). ACCEPTED: J. APPL.PHYS. (MMM-2007 Proceedings
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
Colossal magnetocapacitance and scale-invariant dielectric response in phase-separated manganites
Thin films of strongly-correlated electron materials (SCEM) are often grown
epitaxially on planar substrates and typically have anisotropic properties that
are usually not captured by edge-mounted four-terminal electrical measurements,
which are primarily sensitive to in-plane conduction paths. Accordingly, the
correlated interactions in the out-of-plane (perpendicular) direction cannot be
measured but only inferred. We address this shortcoming and show here an
experimental technique in which the SCEM under study, in our case a 600
Angstrom-thick (La1-yPry)0.67Ca0.33MnO3 (LPCMO) film, serves as the base
electrode in a metal-insulator-metal (MIM) trilayer capacitor structure. This
unconventional arrangement allows for simultaneous determination of colossal
magnetoresistance (CMR) associated with dc transport parallel to the film
substrate and colossal magnetocapacitance (CMC) associated with ac transport in
the perpendicular direction. We distinguish two distinct strain-related
direction-dependent insulator-metal (IM) transitions and use Cole-Cole plots to
establish a heretofore unobserved collapse of the dielectric response onto a
universal scale-invariant power-law dependence over a large range of frequency,
temperature and magnetic field.Comment: 32 pages, 4 figures, Supplementary section included, Submitted to
Nature Physic
Magnetotransport of La0.70ca0.3-xsrxmno3 (Ag): A Potential Room Temperature Bolometer and Magnetic Sensor
Here we report the optimized magneto-transport properties of polycrystalline
La0.70Ca0.3-xSrxMnO3 and their composites with Ag. The optimization was carried
out by varying the Sr and Ag contents simultaneously to achieve large
temperature coefficient of resistance (TCR) as well as low field
magneto-resistance (MR) at room temperature. Sharpest paramagnetic
(PM)-ferromagnetic (FM) and insulator-metal (IM) transition is observed in the
vicinity of the room temperature (TC=300 K=TIM) for the composition
La0.70Ca0.20Sr00.10MnO3:Ag0.20. Partial substitution of larger Sr2+ ions at the
Ca2+ ions sites controls the magnitude of the FM and IM transition
temperatures, while the Ag induces the desired sharpness in these transitions.
For the optimized composition, maximum TCR and MR are tuned to room temperature
(300 K) with the former being as high as 9% and the later being 20 and 30
percent at 5 and 10 kOe magnetic fields respectively. Such sharp single peak
(TCR= 9 percent) at room temperature can be used for the bolometric and
infrared detector applications. The achievement of large TCR and low field MR
at T~300K in polycrystalline samples is encouraging and we believe that further
improvements can be achieved in thin films, which, by virtue of their low
conduction noise, are more suitable for device applications.Comment: 11 pages Text + Figures. Suggestions/comments welcome
([email protected]
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Laboratory Measurements of Fe XXIV Line Emission: 3→2 Transitions Near Excitation Threshold
Using the Electron Beam Ion Trap facility at Lawrence Livermore National Laboratory, we have measured relative cross sections for Fe XXIV line emission at electron energies between 0.7 and 3.0 keV. The measurements include line formation by direct electron impact excitation (DE), radiative cascades, resonant excitation (RE), and dielectronic recombination (DR) satellites with captured electrons in n≥5 levels. Good agreement with R-matrix and distorted wave calculations is found. In collisionally ionized plasmas, at temperatures near where the ion abundance peaks (kTe~1.7 keV), the RE contributions are found to be ≲5% of the line emission, while the DR satellites contribute ≲10%. While good agreement with state-of-the-art atomic physics calculations is found, there is less good agreement with existing spectral synthesis codes in common astrophysical use. For the Fe XXIV 3p3/2 → 2s1/2, 3p1/2 → 2s1/2, and 3d5/2 → 2p3/2 transitions, the synthesis code MEKAL underestimates the emissivity in coronal equilibrium by ~20% at temperatures near where the ion abundance peaks. In situations where the ionization balance is not solely determined by the electron temperature, RE and DR satellites may contribute a considerable fraction of the line emission
Enhanced Room Temperature Coefficient of Resistance and Magneto-resistance of Ag-added La0.7Ca0.3-xBaxMnO3 Composites
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([email protected]
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