共查询到20条相似文献,搜索用时 12 毫秒
1.
The superconductivity of Li under pressure is studied by a density functional method. Structural and elastic properties, transition temperature (Tc), density of states (DOS), are considered for the material at ambient and at higher pressures. The calculations, particularly of Tc and DOS as a function of pressure, are compared with other available results. This is in view of the wide difference between the previously predicted maximum Tc-value and those observed by both magnetic susceptibility and electrical resistivity experiments. The present calculations yield better estimates of Tc and other parameters with respect to measured values. 相似文献
2.
W. Teoh Sigurds Arajs D. Abukay E.E. Anderson 《Journal of magnetism and magnetic materials》1976,3(3):260-263
Electrical resistivity (?) of FeV alloys containing 0.5, 0.9, 2.7, and 6.1 at% V has been measured as a function of temperature (T) between 78 and 1200 K. The ? vs. T curves exhibit a change in the slope at the ferromagnetic Currie temperature (Tc). The d?/dT vs. T curves in the neibhorhood of Tc are similar to the corresponding plot for pure Fe. Our studies confirm the previously observed anomalous effect of V on Tc of Fe, i.e., that Tc increase with small additions of V to Fe. The critical index λ+ associated with the power law of d?/dT just above Tc has been determined as a function of V concentration. 相似文献
3.
In this paper, we report a model-based quantitative analysis of temperature dependent scanning tunneling spectroscopy (STS) data taken on epitaxial thin films of the hole doped manganite La0.7Ca0.3MnO3. The film, grown on lattice matched NdGaO3 substrate, has a ferromagnetic transition temperature Tc=268 K. The analysis allows us to evaluate how the tunneling curve evolves across the transition temperature. We find that there is a gap Δ in the density of states (DOS), which peaks at T≈Tc. The gap closes in the ferromagnetic state following the evolution of the magnetization. The gap closing is gradual and not sudden at T=Tc. Above Tc the gap reduces from the peak value and reaches a limiting value of ≈75 meV for T/Tc≥1.1 which is close to the value of 60 meV seen from transport experiments. 相似文献
4.
A.T. Asvini meenaatciR. Rajeswarapalanichamy K. Iyakutti 《Physica B: Condensed Matter》2011,406(17):3303-3307
First-principles calculation was performed using tight-binding LMTO method with local density approximation (LDA) and atomic sphere approximation (ASA) to understand the electronic properties of rhenium nitride. The equilibrium geometries, the electronic band structure, the total and partial DOS are obtained under various pressures and are analyzed in comparison with the available experimental data. The most stable structure of ReN is NiAs like structure. Our results indicate that ReN can be used as a super-hard conductor. We estimated the average electron-phonon coupling constant to be 1.65 and superconducting transition temperature (Tc) is 5.1 K. The Tc value increases with the increase in pressure. 相似文献
5.
S. Takács 《Solid State Communications》2003,128(12):455-459
We study some parameters of superconductors with δ-function type singularities in the electronic density of states (DOS), exhibiting (s+d)-wave symmetry of the order parameter. Starting with a pure s-wave pairing potential Vs, the critical temperature Tc at first slightly increases with increasing the d-wave interaction potential Vd, being determined by this interaction only for stronger Vd values. The ratio R=2|Δ(0)|/kBTc of the mean value of the zero temperature energy gap |Δ(0)| to Tc increases with increasing Vd, reaching a maximum which depends on the mixing interaction term. The maximum values of R are comparable with very high values obtained in some gap measurements. The jump in the specific heat at critical temperature is by a factor 2.4 higher for the extreme singularity of pure s-wave symmetry, as compared with the BCS theory with constant DOS. Such higher values of the jump are in agreement with the experimentally observed values, as well as with the calculations determined by extended saddle points in the electronic bands. By switching the d-wave channel, the value of the jump decreases. The results show the usefulness of calculations with δ-type singularities as a limiting case of very strong singularities in the DOS. 相似文献
6.
K.H.J. Buschow 《Solid State Communications》1976,19(5):421-423
X-ray diffraction studies of the hydrogen absorption in several YFe and CeFe intermetallic compounds showed that no structural changes occur upon hydrogen absorption in Y6Fe23, YFe3, YFe2. The lattice constants of the hydrides were found to be appreciably larger than those of the pure intermetallic compounds. The magnetic properties of the hydrides were determined and compared with the original compounds. In all cases the magnetic moment per Fe atom proved to be much larger in the hydride phases. Hydrogen absorption can lead to a decrease as well as to an increase of the magnetic ordering temperature (Tc). These changes in Tc could adequately be explained in terms of the observed increases in lattice constant and the data available for the pressure derivative of Tc of these compounds. 相似文献
7.
Alternating-current electrical resistance measurements between 17 Hz and 100 kHz were made on high purity Fe, Co, and Ni wires in the vicinity of their Curie temperatures (T c). The electrical resistance was independent of frequency for temperatures (T) aboveT c. As the temperature was lowered, however, there was an abrupt jump in the electrical resistance atT c followed by a gradual decrease toward its dc value. The magnitude of the electrical resistance jump atT c increased as the square root of the frequency. The enhancement of the electrical resistance forT≦T c is produced by an abrupt decrease of the skin depth atT c which, in turn, is due to the sudden increase in the initial magnetic permeability atT c. Measurements of the ac electrical resistance in the vicinity of the Curie temperature of certain ferromagnetic metals can be utilized to 1) accurately determine the Curie temperature using frequencies as low as 17 Hz, and 2) quantitatively determine the initial magnetic permeability as a function of temperature and heat treatment. 相似文献
8.
F.A. Khwaja 《Solid State Communications》1981,38(7):615-617
The calculations for the temperature dependence of the first shell short- range order (SRO) parameter for Ni3Fe using the cubic approximation of Tahir Kheli, and the concentration dependence of order-disorder temperature Tc for NiFe and NiPt systems using the linear approximation, have been carried out in the framework of pseudopotential theory. It is shown that the cubic approximation yields a good agreement between the theoretical prediction of the α1 and the experimental data. Results for the concentration dependence of the Tc show that improvements in the statistical pseudopotential approach are essential to achieve a good agreement with experiment. 相似文献
9.
With the advent of Fe–As based superconductivity it has become important to study how superconductivity manifests itself in details of 57Fe Mössbauer spectroscopy of conventional, Fe-bearing superconductors. To this end, the iron-based superconductor Lu2Fe3Si5 has been studied by 57Fe Mössbauer spectroscopy over the temperature range from 4.4 K to room temperature with particular attention to the region close to the superconducting transition temperature (Tc=6.1 K). Consistent with the two crystallographic sites for Fe in this structure, the observed spectra appear to have a pattern consisting of two doublets over the whole temperature range. The value of Debye temperature was estimated from temperature dependence of the isomer shift and the total spectral area and compared with the specific heat capacity data. Neither abnormal behavior of the hyperfine parameters at or near Tc, nor phonon softening were observed. 相似文献
10.
William Sacks Alain Mauger Yves Noat 《The European Physical Journal B - Condensed Matter and Complex Systems》2016,89(8):183
Key properties of the cuprates, such as the pseudogap observed above the criticaltemperature Tc, remain highlydebated. Given their importance, we recently proposed a novel mechanism based on theBose-like condensation of mutually interacting Cooper pairs [W. Sacks, A. Mauger, Y. Noat,Supercond. Sci. Technol. 28, 105014 (2015)]. In this work, we calculate thetemperature dependent DOS using this model for different doping levels from underdoped tooverdoped. In all situations, due to the presence of excited pairs, a pseudogap is foundabove Tc while the normal DOSis recovered at T?, the pair formation temperature. Asimilar behavior is found as a function of magnetic field, crossing a vortex, where apseudogap exists in the vortex core. We show that the precise DOS shape depends oncombined pair (boson) and quasiparticle (fermion) excitations, allowing for a deeperunderstanding of the SC to the PG transition. 相似文献
11.
《Solid State Communications》1986,59(3):159-161
A spin fluctuation theory for itinerant electrons that includes short-range magnetic order (SRMO) is used to calculate the Curie-temperature (Tc and the temperature dependence of the magnetization and the susceptibility of bulk Fe. When spin correlations are included the Curie-temperature is reduced by 9% to Tc = 2000 K. The calculated temperature-dependence of the magnetization and the magnetic susceptibility are in excellent agreement with experimental results. 相似文献
12.
Debye-Waller factors f were determined over the temperature range 4.2 to 298°K in Fe(bipy)2(NCS)2 from 57Fe Mössbauer spectra (bipy = 2,2'-bipyridyl). Whereas magnetic data and other physical properties show an abrupt change at the temperature of a cooperative 5T2 ? 1A1 transition, Tc = 214°K, a continuous change of -ln f is found. The significance of this result is discussed. 相似文献
13.
P. V. Leksin A. A. Kamashev N. N. Garif’yanov I. A. Garifullin Ya. V. Fominov J. Schumann C. Hess V. Kataev B. Büchner 《JETP Letters》2013,97(8):478-482
The spin valve effect for the superconducting current based on the superconductor/ferromagnet proximity effect has been studied for a CoO x /Fe1/Cu/Fe2/Cu/Pb multilayer. The magnitude of the effect ΔT c = T c AP ? T c P , where T c P and T c AP are the superconducting transition temperatures for the parallel (P) and antiparallel (AP) orientation of magnetizations, respectively, has been measured for different thicknesses of the Fe1 layer d Fe1. The obtained dependence of the effect on d Fe1 reveals that ΔT c can be increased in comparison with the case of a half-infinite Fe1 layer considered by the previous theory. A maximum of the spin valve effect occurs at d Fe1 ~ d Fe2. At the optimal value of d Fe1 almost full switching from the normal to the superconducting state when changing the mutual orientation of magnetizations of the iron layers Fe1 and Fe2 from P to AP is demonstrated. 相似文献
14.
《Journal of magnetism and magnetic materials》1987,65(1):63-70
We report magnetic and transport measurements in the metallic pseudobinary compound LaNi5-xFex, which show a Fe spin freezing. The magnetic phase diagram in the x-T plane is derived in the range 0 ⩽ x 1.2, 20 K ⩽ T ⩽ 300 K. Above a critical Fe concentration xc ≈ 0.3, the system undergoes a superparamagnetic to long range ferromagnetic ordering at a finite temperature T. At lower concentrations, no long range spin ordering is observed, which suggests that x is the percolation threshold. Instead, a transition to a phase characterized by strong irreversibilities is observed at temperature Tf(x). Very strong ferromagnetic coupling interactions between Fe atoms are observed at low concentrations (x < xc), which contrast with small values of T observed for x ∼ xc. The results, together with the nature of the spin freezing are discussed in relation to the 2d character of the magnetic lattice. 相似文献
15.
G. Shirane 《Journal of magnetism and magnetic materials》1984,45(1):33-37
A review is presented of our current neutron scattering experiments on Fe and Ni above the Curie temperature. Our experimental results show that the picture of propagating spin waves above Tc, widely accepted since 1973, is incorrect. In addition, we will demonstrate that over wide ranges of ω, q and temperature, both Fe and Ni, as well as Pd2MnSn, follow a simple paramagnetic scattering function of the spin diffusion type. 相似文献
16.
F.L.A. Machado L.L.L. Sousa F.A.O. Cabral A.R. Rodrigues R.C. Santana 《Journal of Physics and Chemistry of Solids》2010,71(5):862-787
The influence of highly diluted impurities (Cu, Mn, Fe, Ni) on the temperature (T) dependence of the specific heat (cp) of l-arginine phosphate monohydrate (LAP) was investigated in the temperature range 1.8-300 K. The doped samples yielded values for cp in excess to those obtained for a pure LAP sample. The melting temperatures (Tm≈420 K) obtained by differential scanning calorimetry for pure and doped LAP samples were found not to be significantly affected by the impurities. The T-dependence of cp was fully accounted for by taking into consideration the Debye contribution, an Einstein term and a contribution due to both Frenkel and Schottky defects. The model fit all cp versus T data using a single value for both the Debye (θD=160 K) and the Einstein (TE=376.8 K) temperatures, and for the energy (εd=157.9 meV) required to create the defects. 相似文献
17.
Mossbauer studies were performed in 57Fe impurities (0.3% at.) in solid methane, in order to investigate the properties of the order-disorder phase transition taking place at Tc=20.4K. From the temperature dependence of the relative values of the recoil-free-fraction - f - it was found that near Tc a change occurs in the slope of f(T2) without discontinuity in the value of f. A consistent model is proposed by which immediately below Tc both phases, I and II coexist with a gradual evolution of the ordered state (II) on the account of the high temperature plastic state (I). The Debye temperature of phase II (θII = 101±7K) increases with respect to that of phase I (θI = 72±3K), indicating a hardening in the phonon spectrum and precluding the onset of rotational modes of the disordered molecules. 相似文献
18.
We report LDA calculated band structure, densities of states and Fermi surfaces for recently discovered Pt-pnictide superconductors APt3P (A = Ca, Sr, La), confirming their multiple band nature. Electronic structure is essentially three dimensional, in contrast to Fe pnictides and chalcogenides. LDA calculated Sommerfeld coefficient agrees rather well with experimental data, leaving little space for very strong coupling super-conductivity, suggested by experimental data on specific heat of SrPt3P. Elementary estimates show, that the values of critical temperature can be explained by rather weak or moderately strong coupling, while the decrease in superconducting transition temperature T c from Sr to La compound can be explained by corresponding decrease in total density of states at the Fermi level N(E F). The shape of the density of states near the Fermi level suggests that in SrPt3P electron doping (such as replacement Sr by La) decreases N(E F) and T c , while hole doping (e.g., partial replacement of Sr with K, Rb or Cs, if possible) would increase N(E F) and possibly T c . 相似文献
19.
The a, b, c, and β crystallographic parameters of the (CH3)2NH2Al(SO4)2 · 6H2O crystal (DMAAS) have been measured by x-ray diffraction in the 90–300-K temperature range. The thermal expansion coefficients along the principal crystallographic axes αa, αb, and αc have been determined. It was shown that, as the temperature is increased, the parameter α decreases and b increases, whereas c decreases for T<T c (where T c is the transition temperature) and increases for T>T c, so that one observes a minimum in the c=f(T) curve in the region of the phase transition (PT) temperature T c ~ 152 K. The thermal expansion coefficients αa, αb, and αc vary in a complicated manner with increasing temperature, more specifically, αa and αc assume negative values at low temperatures, and the αa=f(T), αb=f(T), and αc=f(T) curves exhibit anomalies at the PT point. The crystal has been found to be substantially anisotropic in thermal expansion. 相似文献
20.
Systematic ab initio LDA calculations were performed for all the typical representatives of recently discovered class of iron-based high-temperature superconductors: REOFe(As,P) (RE = La, Ce, Nd, Sm, Tb), Ba2Fe2As, (Sr,Ca)FFeAs, Sr4Sc2O6Fe2P2, LiFeAs and Fe(Se,Te). Non-monotonic behavior of total density of states at the Fermi level is observed as a function of anion height relative to Fe layer with maximum at about Δz a ~ 1.37 Å, attributed to changing Fe-As (P, Se, Te) hybridization. This leads to a similar dependence of superconducting transition temperature T c as observed in the experiments. The fit of this dependence to elementary BCS theory produces semiquantitative agreement with experimental data for T c for the whole class of iron-based superconductors. The similar fit to Allen-Dynes formula underestimates T c in the vicinity of the maximum, signifying the possible importance of non-phonon pairing in this region. These results unambiguously demonstrate that the main effect of T c variation between different types of iron-based superconductors is due to the corresponding variation of the density of states at the Fermi level. 相似文献