196 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
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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]
Analysis of transient interactions between a PWR nuclear power plant and a faulted electricity grid
This paper presents the transient stability analysis of a pressurised water-type nuclear power plant following faults and disturbances affecting the electricity grid to which it is connected. The modelled nuclear plant consists of various integrated subsystems, such as core neutronics and thermal-hydraulics, piping and plenum, pressuriser, steam generator, turbine, governor, and dynamics shaft, in addition to the turbine-speed controller. The nonlinear nuclear power plant model is linearised at the operating point to acquire a linear model for controller design. The turbine-speed control loop effectively enacts a closed-loop implementation of the nuclear power plant connected to the electric grid. The various transient stability enhancement components such as the power system stabiliser, static var compensator, and static synchronous compensator are employed to test performance during severe contingencies. The interaction between the nuclear power plant, electric grid, and protection system is studied under various scenarios such as single-phase fault, three-phase fault, and permanent load loss. The performance of the nonlinear plant is further observed during load-following operation. The dynamic behaviour of the overall system is analysed using simulations in the MATLAB/Simulink/Simscape environment
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
Anomalous heat capacity and X-ray photoelectron spectroscopy of Superconducting FeSe1/2Te1/2
The bulk polycrystalline sample FeSe1/2Te1/2 is synthesized by solid state
reaction route in an evacuated sealed quartz tube at 750 oC. The presence of
superconductivity is confirmed through magnetization/thermoelectric/resistivity
studies. It is found that the superconducting transition temperature (Tc) is
around 12 K. Heat capacity (Cp) of superconducting FeSe1-xTex exhibited a hump
near Tc, instead of well defined Lambda transition. X-ray Photo electron
spectroscopy (XPS) studies revealed well defined positions for divalent Fe, Se
and Te but with sufficient hybridization of Fe (2p) and Se/Te (3d) core levels.
In particular divalent Fe is shifted to higher BE (binding energy) and Se and
Te to lower. The situation is similar to that as observed earlier for famous Cu
based HTSc (High Tc superconductors), where Cu (3d) orbital hybridizes with O
(2p). We also found the satellite peak of Fe at 712.00 eV, which is attributed
to charge carrier localization induced by Fe at 2c site.Comment: 12 pages text + Figs contact Author-VPS Awana
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