39 research outputs found
Comparative experimental and Density Functional Theory (DFT) study of the physical properties of MgB2 and AlB2
In present study, we report an inter-comparison of various physical and
electronic properties of MgB2 and AlB2. Interestingly, the sign of S(T) is +ve
for MgB2 the same is -ve for AlB2. This is consistent our band structure plots.
We fitted the experimental specific heat of MgB2 to Debye Einstein model and
estimated the value of Debye temperature (theta) and Sommerfeld constant
(gamma) for electronic specific heat. Further, from gamma the electronic
density of states (DOS) at Fermi level N(EF) is calculated. From the ratio of
experimental N (EF) and the one being calculated from DFT, we obtained value of
Lembda to be 1.84, thus placing MgB2 in the strong coupling BCS category. The
electronic specific heat of MgB2 is also fitted below Tc using pi-model and
found that it is a two gap superconductor. The calculated values of two gaps
are in good agreement with earlier reports. Our results clearly demonstrate
that the superconductivity of MgB2 is due to very large phonon contribution
from its stretched lattice. The same two effects are obviously missing in AlB2
and hence it is not superconducting. DFT calculations demonstrated that for
MgB2 the majority of states come from Sigma and Pi 2p states of boron on the
other hand Sigma band at Fermi level for AlB2 is absent. This leads to a weak
electron phonon coupling and also to hole deficiency as Pi bands are known to
be of electron type and hence obviously the AlB2 is not superconducting. The
DFT calculations are consistent with the measured physical properties of the
studied borides, i.e., MgB2 and AlB2Comment: 16 pages Text + Figs: comments/suggestions welcome
([email protected])/www.freewebs.com/vpsawana
Synthesis of SmFeAsO by an Easy and Versatile Route and its Physical Property Characterization
We report synthesis, structure, electrical transport and heat capacity of
SmFeAsO. The title compound is synthesized by one-step encapsulation of
stoichiometric FeAs, Sm, and Sm2O3 in an evacuated (10-5 Torr) quartz tube by
prolong (72 hours) annealing at 1100oC. The as synthesized compound is
crystallized in tetragonal structure with P4/nmm space group having lattice
parameters a = 3.93726(33) A and c = 8.49802(07) A. The resistance (R-T)
measurements on the compound exhibited ground state spin-density-wave
(SDW)-like metallic steps below 140 K. Heat capacity CP(T) measurements on the
title compound, showed an anomaly at around 140 K, which is reminiscent of the
SDW ordering of the compound. At lower temperatures the CP(T) shows a clear
peak at around 4.5 K. At lower temperature below 20 K, Cp(T) is also measured
under an applied field of 7 Tesla. It is concluded that the CP(T) peak at 4.5 K
is due to the anti-ferromagnetic(AFM) ordering of Sm3+ spins. These results are
in confirmation with ordering of Sm in Sm2-xCexCuO4.Comment: 9 pages Text + Figs Contact Author ([email protected]
Effect of PVA doping on flux pinning in Bulk MgB2
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.Comment: 14 Pages of Text + Figs. To appear in Physica
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
Physical property characterization of Fe-tube encapsulated and vacuum annealed bulk MgB2
We report phase formation, and detailed study of magnetization and
resistivity under magnetic field of MgB2 polycrystalline bulk samples prepared
by Fe-tube encapsulated and vacuum (10-5 torr) annealed (750 0C) route.
Zero-field-cooled magnetic susceptibility (cZFC) measurements exhibited sharp
transition to superconducting state with a sizeable diamagnetic signal at 39 K
(Tc). The measured magnetization loops of the samples, despite the presence of
flux jumps, exhibited a stable current density (Jc) of around 2.4 x 105 A/cm2
in up to 2 T (Tesla) field and at temperatures (T) up to 10 K. The upper
critical field is estimated from resistivity measurements in various fields and
shows a typical value of 8 T at 21 K. Further, cFC measurements at an applied
field of 0.1 T reveal paramagnetic Meissner effect (PME) that is briefly
discussed.Comment: 13 pages text + figs. accepted: solid state commun. (2006
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]