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1.
The site preference of Zr atoms in Ti
3Al and the phase stability of Ti2ZrAl are examined using first-principles electronic structure total energy calculations.
Of the sixteen possible ways in which Ti, Zr and Al atoms can be arranged, in the lattice sites corresponding to D019 structure of Ti3Al, to obtain Ti2ZrAl, it is shown that Zr atoms prefer to get substituted at the Ti sites. It is further shown that among the seven crystal
structures considered, D019-like and L12-like are the competing ground-state structures of Ti2ZrAl. The above results are in agreement with the experimental results reported in the literature. Calculated values of equilibrium
lattice parameters, heat of formation and bulk modulus of Ti2ZrAl are presented. The basis for the structural stability and bonding are analysed in terms of the density of states. Between
the two possibleB2-like structures, Ti2ZrAl shows enhanced stability for the one where Zr is substituted in the Ti sublattice, which again is in agreement with the
experimental observation. 相似文献
2.
The ground state structures and properties of Si
3Hn (1 ≤n ≤ 6) clusters have been calculated using Car-Parrinello molecular dynamics with simulated annealing and steepest descent optimization
methods. We have studied cohesive energy per particle and first excited electronic level gap of the clusters as a function
of hydrogenation. Hydrogenation is done till all dangling bonds of silicon are saturated. Our results show that over coordination
of hydrogen is favoured in Si3Hn clusters and the geometry of Si3 cluster does not change due to hydrogenation. Cohesive energy per particle and first excited electronic level gap study of
the clusters show that Si3H6 cluster is most stable and Si3H3 cluster is most unstable among the clusters considered here. 相似文献
3.
Shubhra Kakani M. L. Kalra S. L. Kakani 《Journal of Superconductivity and Novel Magnetism》2008,21(5):301-311
The emergence of pressure induced superconductivity (SC) under the background of ferromagnetic state in 5f-electron based itinerant ferromagnetic superconductor UGe2 is studied in the single band model by using a mean-field approximation. The solutions to the coupled equations of superconducting
gap (Δ) and magnetization (m) are obtained using Green’s function technique and equation of motion method. It is shown that there generally exists a coexistent
(Δ≠0, m≠0) solution to the coupled equations of the order parameters in the temperature range 0<T<min (T
C,T
FM), where T
C and T
FM are respectively the superconducting and ferromagnetic transition temperatures. The study of electronic specific heat (C/T), density of states, free energy, etc. are also presented. The specific heat capacity at low temperature shows linear temperature
dependence as opposed to the activated behavior. Density of states increases as opposed to the case of a standard ferromagnetic
metal. Free energy study reveals that the superconducting ferromagnetic state has lower energy than the normal ferromagnetic
state and, therefore, realized at low enough temperature. The agreement between theory and experimental results for UGe2 is quite satisfactory.
相似文献
4.
P. P. Singh 《Bulletin of Materials Science》2003,26(1):131-135
Using density-functional-based methods, we have studied the fully-relaxed, fulltronic structure of the newly discovered superconductor,
MgB
2, and BeB2, NaB2 and AlB2. Our results, described in terms of (i) total density of states (DOS) and (ii) the partial DOS around the Fermi energy, EF, clearly show the importance of B p-electrons for superconductivity. For BeB2 and NaB2, our results indicate qualitative similarities but significant quantitative differences in their electronic structure due
to differences in the number of valence electrons and the lattice constantsa andc.
We have also studied Mg
1-xMxB2 (M = Al, Li or Zn) alloys using coherent-potential to describe disorder, Gaspari-Gyorffy approach to evaluate electron-phonon
coupling, and Allen-Dynes equation to calculate the superconducting transition temperature, Tc. We find that in Mg1-xMxB2 alloys (i) the way Tc changes depends on the location of the added/modified k-resolved states on the Fermi surface and (ii)
the variation of Tc as a function of concentration is dictated by the Bp DOS. 相似文献
5.
The reentrant magnetic phase transition in Pr
0.5Sr0.41Ca0.09MnO3 perovskite is explained using the Ising spin model on the square lattice with mixed ferromagnetic and antiferromagnetic exchange
interactions. It is shown using numerical calculations that this effect is strongly affected by the external magnetic field
and lattice disorder. 相似文献
6.
Abstract Abstract: Bulk superconductivity was observed in a new iodine-treated aggregated form of solid C60, believed to be a “polymer”, by doping with alkali metal. Evolution of superconducting (Tc = 17.2 K) and normal-state properties, has been studied as a function of doping with K. The normal-state dc susceptibility exhibited a complex behavior with the increasing K uptake, evolving from an enhanced paramagnetism at low doping level, to a weakly temperature-dependent diamagnetism at the optimum doping level for superconductivity, and ending with a strong temperature-independent diamagnetism in over-doped samples. The measured superconductivity parameters, such as Tc, London penetration depth and the Ginzburg-Landau coherence length, are compared with those of the monomer K3C60. and the main factors determining the differences between the two systems are discussed. 相似文献
7.
B. T. Cong P. N. A. Huy P. K. Schelling J. W. Halley 《Bulletin of Materials Science》2003,26(1):155-158
We present results of numerical computation on some characteristics of BaTiO3 such as total energy, lattice constant, density
of states, band structure etc using self-consistent tight binding method. Besides strong Ti-O bond between 3
d
on titanium and 2
p
orbital on oxygen states, we also include weak hybridization between the Ba 6
s
and O 2
p
tates. The results are compared with those of other more sophisticated methods. 相似文献
8.
S. R. Murthy 《Bulletin of Materials Science》2001,24(6):611-616
Polycrystalline NiZn ferrites with different grain sizes (1.2 (Am to 10.2 (Am) were prepared by the usual ceramic method.
The magnetic properties were measured at room temperature. The ultrasonic velocity and attenuation were measured on Ni-Zn
ferrite by using the pulse transmission method at 1 MHz, in the temperature range 300–600 K. The velocity was found to be
slightly sample dependent at room temperature and decreased with increasing temperature, except near the Curie temperature,
T
c
,where a small anomaly was observed. The longitudinal attenuation (α
1
)at room temperature was found to be more sample dependent. The temperature variation of ultrasonic longitudinal attenuation
exhibited a broad maximum around 400 K and a sharp maximum just below Curie temperature (
T
c
).The above observations were carried out in the demagnetized state. The application of a 380 mT magnetic field allowed us to
reach the saturated state of the sample at all the measuring temperatures. The anomaly observed in the thermal variation of
velocities (longitudinal and transverse) and attenuation has been qualitatively explained with the help of the temperature
variation of the magneto-crystalline anisotropy constant. 相似文献
9.
The superconducting properties of iodine-intercalated high-temperature superconducting Bi2Sr2Ca2Cu3O10+x
phase (Bi-2223) were systematically studied. It was found that for samples containing a significant amount of Bi2Sr2CaCu2O8+x
, iodine intercalation results in the dramatic decrease of the inter-granular critical current density, as well as a significant
decrease of the critical temperature (T
c), the critical current density in the grains (J
cg), and of the amount of Bi-2223. For samples with a large amount of Bi-2223, T
c changes insignificantly, whereas J
cg can even increase. We argue that the different behavior of the superconducting parameters is the result of various oxygen
concentrations, and we explain the effect of iodine intercalation based on the parabolic dependence between T
c and the number of holes per CuO2 layer. The H(T) curves (determined from the peak position in the loss signal of ac susceptibility) for intercalated samples deviate significantly
from the quasi 2D-like behavior, pointing toward an enhancement of the 3D fluctuations of vortices. For the change in the
values and dimensionality of the flux pinning in the process of the intercalation, we attempted a qualitative explanation
based on the models proposed in literature. 相似文献
10.
Chao Zhang Xiao-Jia Chen Yan-Ling Li Viktor V. Struzhkin Russell J. Hemley Ho-Kwang Mao Rui-Qin Zhang Hai-Qing Lin 《Journal of Superconductivity and Novel Magnetism》2010,23(5):717-719
The electronic properties, lattice dynamics, and electron–phonon coupling of the Cmmm phase of GeH4 have been studied by first-principle calculations using density functional perturbation theory. The electronic band structure
shows the Cmmm phase metallic nature. It is found strong electron phonon interaction, and the superconducting critical temperature, predicted
by Allen–Dynes modified McMillan equation, is about 40 K at 20 GPa. 相似文献
11.
A multiband approach for hole- and electron-doped cuprate superconductors is developed. The model electron spectrum includes
nodal and antinodal defect (polaronic) subbands created by doping. Bare gaps between itinerant and defect subbands close with
extended doping. The overlap conditions determine phase diagram special points. There are analogies for both types of doping
despite the activation of different sublattices. Illustrative calculations have given self-consistent results reproducing
qualitatively doping dependences of T
c
, superconducting gaps and pseudogaps, supercarrier density and effective mass, coherence length and penetration depth, etc.
Interband pairing scenario is suggested to be an essential aspect of cuprate superconductivity.
This work was supported by the Estonian Science Foundation grant No. 7296. 相似文献
12.
A quantum-mechanical calculation of the relative stability, structural parameters, and vibrational frequencies of V2O3 molecule isomers for different spin states was carried out using the BPW91/6-311+G(d, p) method. It was shown that the isomer with the C
s
structure (nonplanar VOVO rectangle with an O atom attached to it) in the X
5
A″ electronic state possesses the maximum stability. The energy of the C
2v
symmetry structure was higher than the lowest energy by just 23 cm−1. It definitely indicated the impossibility of usage of the harmonic model in order to calculate the thermodynamic functions
of V2O3 (g). A model is proposed based on which the energy levels and vibrational sums of states for this type of motion were
calculated for the C
s
→ C
2v
→ C
s
transition coordinate. These data, as well as results obtained from quantum-mechanical calculations, were used to calculate
the thermodynamic functions of V2O3 (g) in the temperature range of T = 100–6000 K. The calculations were performed with the five excited electronic states with energies from 1000 to 9000 cm−1 taken into account. A comparison with the data calculated in the “rigid rotator-harmonic oscillator” approximation was performed. 相似文献
13.
M. Bleiweiss J. Amirzadeh M. Yin A. Lungu T. Datta 《Journal of Superconductivity and Novel Magnetism》2005,18(4):567-572
Synthesis and optical transmission of MgB2 thin films on optically transparent glass are reported. In the 400–1000 nm regime as deposited films show high metallic reflectivity
and very little transmission. After deposition, the films were annealedex situ and rendered superconducting withT
c of 38 K, approaching that of the bulk material. The reaction conditions where quite soft ∼ 10 min at 550°C. The optical absorption
coefficient,α and photon energy,E followed a Tauc-type behavior, (αE)1/2=β
T(E −E
g). The band gap (E
g) was observed to peak at 2.5 eV; but, the slope parameterβ
Tbehaved monotonically with reaction temperature. Our results indicate that an intermediate semiconducting phase is produced
before the formation of the superconducting phase; also optical measurements provide valuable information in monitoring the
synthesis of MgB2 from its metallic constituents. In addition these films have interesting optical properties that may be integrated into optoelectronics. 相似文献
14.
Dinesh Varshney A. Dube K. K. Choudhary R. K. Singh 《Bulletin of Materials Science》2005,28(2):155-171
We consider a two- peak model for the phonon density of states to investigate the nature of electron pairing mechanism for
superconducting state in fullerides. We first study the intercage interactions between the adjacent C60 cages and expansion of lattice due to the intercalation of alkali atoms based on the spring model to estimate phonon frequencies
from the dynamical matrix for the intermolecular alkali- C60 phonons. Electronic parameter as repulsive parameter and the attractive coupling strength are obtained within the random
phase approximation. Transition temperature,T
c, is obtained in a situation when the free electrons in lowest molecular orbital are coupled with alkali-C60 phonons as 5 K, which is much lower as compared to reportedT
c (≈ 20 K). The superconducting pairing is mainly driven by the high frequency intramolecular phonons and their effects enhance
it to 22 K. To illustrate the usefulness of the above approach, the carbon isotope exponent and the pressure effect are also
estimated. Temperature dependence of electrical resistivity is then analysed within the same model phonon spectrum. It is
inferred from the two- peak model for phonon density of states that high frequency intramolecular phonon modes play a major
role in pairing mechanism with possibly some contribution from alkali-C60 phonon to describe most of the superconducting and normal state properties of doped fullerides. 相似文献
15.
In order to get good quality reproducible films of Tl : HTSC system, we have studied the different annealing conditions to
finally achieve the optimized annealing condition. In the present investigation, Tl-Ca-Ba-Cu-O superconducting films have
been prepared on YSZ (100) and MgO (100) single crystal substrates via precursor route followed by thallination. The post
deposition heat treatments of the precursor films were carried out for various annealing temperatures (870°C, 890°C) and durations
(1 and 2 min). The optimized thallination procedure occurred at 870°C for 2 min into good quality films withT
c (R = 0) ∼ 103 K for YSZ andT
c (R = 0) ∼ 98 K for MgO substrates, respectively. Further we have correlated the structural/microstructural characteristics of
the films. 相似文献
16.
Harrison’s first principle pseudopotential (HFPP) technique in conjunction with BCS theory and McMillan’s formalism has been
used for the investigation of superconducting state parameters viz., Coulomb pseudopotential μ
∗, electron–phonon coupling strength λ, SC transition temperature T
C
, interaction strength N
0
V, semi band gap Δ, energy or mass renormalization parameter Z
0 and isotope effect exponent δ. The ground state properties of MgB2 have also been calculated employing full-potential linearized augmented plane wave (FLAPW) method. This enables us to estimate
the equilibrium values of bulk modulus and its pressure derivative through optimization of the crystal structure of the system.
We have also described the total density of state (DOS) and the partial DOS (PDOS) around the Fermi energy. 相似文献
17.
K. P. Singh V. P. S. Awana S. Balamurugan M. Shahabuddin H. K. Singh M. Husain H. Kishan E. R. Bauminger I. Felner 《Journal of Superconductivity and Novel Magnetism》2008,21(1):39-44
Nano particle of Fe3O4 (nFe3O4) up to 6 at% were doped in the superconducting MgB2 samples. Despite the strong ferromagnetic nature of Fe3O4, both the ac susceptibility and the resistivity measurements show that up to 4 at% of Fe3O4, T
c
=38 K is not changed, whereas for 6% T
c
decreases by 6 K. This indicates that a low concentration of Fe does not substitute either the Mg or B sites and probably
occupies the intergrain spaces. For 0.5% doped Fe3O4, an increase in J
c
with respect to the pure MgB2 samples is observed in the lower field and temperature regions (H<2 T and 20 K) indicating an enhanced flux pinning and the magnetic activation, i.e., the interaction between the magnetic
dipole of Fe ion and the vortices is weak in comparison to the effective pinning potential. Whereas, at H>2 T, J
c
of the doped samples is always less than that of MgB2, and the activation is dominant in comparison with the effective pinning potential provided by the doping. Flux jumps are
observed in lower T and H regions for the samples doped up to 1% nFe3O4 only. Magnetization plots of higher Fe content samples exhibited clear paramagnetic background. Mossbauer measurements for
the higher (4, 6 at%) nFe3O4 doped MgB2 samples show that at RT, the hyperfine field for both samples is ∼100 kOe and ∼120 kOe at 90 K. This means that the nFe3O4 particles decompose and form possibly an intermetallic Fe-B phase in the matrix. 相似文献
18.
Toshikazu Nakamura Tomoko Nobutoki Masao Miyamoto Yuichi Tsubokura Ryota Tsuchiya Toshihiro Takahashi Kazushi Kanoda Gunzi Saito 《Journal of Superconductivity》1994,7(3):671-674
The electronic structure of the title superconductors has been investigated by electrical resistivity, complex susceptibility, and electron paramagnetic resonance (EPR) measurements. The superconducting properties (pressure dependence ofT
c
, magnetic penetration depth, upper critical field, and so on) of these three salts are similar to each other, while transport properties in the normal state have shown a large variety in the temperature dependence. In order to clarify the electronic structure in the normal state, the EPR parameters, the spin susceptibility (
spin), and the linewidth (H
pp), are compared. An anomalous temperature dependence of theg-value has been observed below 150 K in the Cu(NCS)2 and Cu(CN)[N(CN)2] salts. 相似文献
19.
A. V. Semeno A. A. Pronin N. E. Sluchanko N. A. Samarin A. V. Kuznetsov S. V. Demishev 《Journal of Superconductivity and Novel Magnetism》2007,20(2):109-112
Electron spin resonance of a single crystal of CuGeO3 doped with 2% of Co has been studied at f = 99 GHz in temperature range 1.8–50 K. Contributions to ESR absorption from Cu2+ chains and from Co2+ ions were derived. It is found that functions obtained for ESR integrated intensities: Curie-Weiss for Cu2+ (χCu ∼ C
Cu/(T + Θ), with Θ = 92 K) and Curie for Co2+ (χCo ∼ C
Co/T) are well consistent with temperature dependence of static magnetic susceptibility. Strong dependence of ESR absorption on
polarization of oscillating magnetic field was discovered for Co2+ contribution. Polarization effect was studied for magnetic field applied along a, b and c directions. Values of g-factors of resonance lines are presented. 相似文献
20.
D. C. Kim A. N. Baranov J. S. Kim H. R. Kang B. J. Kim Y. C. Kim J. S. Pshirkov E. V. Antipov Y. W. Park 《Journal of Superconductivity》2002,15(5):331-334
Ba1 – x
K
x
BiO3 (BKBO) samples with 0.35 < x < 1 were synthesized by the high pressure and high temperature technique. XRD analysis showed that the BKBO samples were single phase for the whole range of the potassium doping concentration. The change of superconducting transition temperature, T
c, as well as lattice parameters have been investigated upon doping concentration. As the K doping concentration (x) increases from x = 0.37, T
c decreases from 30.4 K to almost zero at x = 0.74. However, in some BKBO samples without including any barium in the starting composition (x = 1), which is denoted as KBO samples, superconductivity is observed with T
c as high as 9 K with partial substitutions of Bi at the K site. Depending on the synthesis condition of the KBO samples, T
c and lattice parameters were different from sample to sample. Compared with other superconducting bismuthates, the evolution of T
c by potassium doping in the cubic BKBO system is discussed in terms of its electronic band structure. 相似文献