Superconductivity and electronic specific heat in V-Mo system

The parameters of superconductivity and specific heat are determined from the low temperature specific heat measurements of V_(1−x)Mo_x (0 ⩽ x ⩽ 1) solid solutions. The coefficient of electronic specific heat γ decreases with increasing Mo concentration and shows a minimum around 80 at. %Mo. Debye temperature θ_D varies slightly over the whole composition range. The superconducting transition temperature T_c also decreases with increasing Mo concentration. The variation of T_c is explained by the variation of γ and discussed in terms of the band structure.     

Study of spin-wave dynamics in Fe65Ni35 ferromagnetic via small-angle polarized-neutron scattering

The results of studying the spin dynamics of a classical Fe₆₅Ni₃₅ invar alloy are presented and analyzed. The investigations are performed via small-angle polarized-neutron scattering in the oblique geometry of a magnetic field at various temperatures (T < T _C). This approach is based on the analysis of left-right asymmetry in the magnetic scattering of polarized neutrons. The asymmetry effect arises when the magnetization direction of a sample is inclined with respect to the wave vector of the incident beam. The spin-wave scattering is concentrated within a range bounded by the cutoff angle θ_c determined by the magnetic field: θ _c ² (H) = θ ₀ ² ?(gμ_B H)θ₀/E, where \(\theta _0 = \hbar ^2 \frac{1} {{2Dm_n }}\) , H is the external magnetic field, E is the initial neutron energy, D is the spin-wave stiffness constant, and m _n is the neutron mass. The scattering is blurred by spinwave damping in the vicinity of the cutoff angle. The spin-wave stiffness constant can be obtained from a comparison of the asymmetric contribution to scattering and a model function. The temperature dependence D = D(T) is well defined by the expression D = D ₀ |τ| ^x , where \(\tau = 1 - \frac{T} {{T_C }}\) , x = 0.47 ± 0.01, D ₀ = 137 ± 3 meVŲ, and τ > 0.1 in the entire temperature range. The given method enables us to construct the temperature dependence of the spin-wave stiffness constant with a high accuracy and a small step.     

Electron-phonon coupling through the orthorhombic to rhombohedral phase transition in La2/3(Ca1−xSrx)1/3MnO3 manganites

We present a systematic study of the Raman modes of a ferromagnetic series of manganites, La_2/3(Ca_1?xSr_x)_1/3MnO₃ with 0?x?1, that presents two different regimes for the variation of the ferromagnetic order temperature, T_C, with Sr doping. This change occurs for x～0.5 when the temperature of the orthorhombic, Pbnm, to rhombohedral, R-3c, structural phase transition, T_t, coincides with T_C. We have analyzed the evolution of the frequencies and widths of the observed Raman modes as a function of Sr doping and temperature. At room temperature, features of the Pbnm structure are detected for compounds up to x=0.6, over the orthorhombic phase limit x=0.45 at 300 K. Octahedra bending modes behave as expected with doping while the tilt mode, which is related to Mn–O–Mn angles and therefore to electronic conduction mechanisms, presents different behaviors in both structures. The tilt frequency is much less sensitive to Mn–O–Mn angle in R-3c than in Pbnm structure indicating a reduction of electron-phonon coupling. Its width presents an anomalous behavior both as a function of doping and temperature with unexpectedly large width in the rhombohedral phase.     

Superconducting and magnetic properties of La3−x In Cex

Measurements of the critical temperature, T_c(x), the critical field, H_c2(x,T) and the susceptibility, χ (T), as a function de la concentration, x, in the system La₃₋x Ce_xIn, are compared with the theories of Muller-Hartmann and Zittsrtz, and Abrikosov and Gor'kov. The H_c2(x, T = 0) and χ(T) measurements indicate the appearance of short range antiferromagnetic order around x = 0.04 which is probably responsible for the anomalous behavior of T_c(x).     

McMillan-Rowell tunneling spectroscopy in the Nb-Zr system

We report the first full application of tunneling spectroscopy to a superconducting transition metal alloy: Nb_1?xZr_x at x = 0.25, corresponding to the maximum T_c in the Nb-Zr system. The spectral function α²F(ω) and related parameters, when compared to those for the Nb, confirm that the increase in T_c from 9.22 K (x = 0) to 10.8 K(x = 0.25) arises largely by softening of the effective phonon spectrum.     

Magnetic properties and peculiarity of magnetic states in dilute antiferromagnets Mn1−x Zn x F2

The dependence of magnetic moment and susceptibility on temperature, magnetic field and frequency of some single crystals Mn_1?x Zn _x F₂ (x≈x _e=0.75—percolation limit) were experimentally investigated. Our experiments show that (Bazhan and Petrov 1984; Cowleyet al 1984; Villain 1984) in these crystals the nonequilibrium magnetic state of spinglass type with finite correlation length appears as temperature decreasesT<T in weak magnetic fields. This state is determined by fluctuation magnetic moments √nμ (wheren is the number of magnetic ions, corresponding to finite correlation length andμ the magnetic moment Mn⁺¹). In the experiments in low magnetic fields and frequencies there are no peculiarities in the magnetic susceptibility temperature dependence atT≠T _f. At temperaturesT>T _f andT<T _f magnetic susceptibility is determined by 1 $$\chi \left( {T > T_f } \right) = \frac{{N\left\langle \mu \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \frac{N}{n}\frac{{\left\langle {\sqrt n \mu } \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \chi \left( {T< T_f } \right)$$ . In strong magnetic fields and large frequencies there are peculiarities in thex(T) dependence atT=T _f. AtT<T _f and strong magnetic fieldsX(T)=x ₀ andT<T _f and at large frequenciesx(T)=x ₀+α/T. The dependences of magnetic susceptibility on the frequency are determined by the magnetic system relaxation. Calculations and comparison with experiments show that the relaxation of the investigated magnetic systems atT<T _f follows the relaxation lawM(t)=M(0) exp[?(t/τ) ^r ], suggested in Palmeret al (1984) for spin-glasses relaxation taking into account the time relaxation distributionτ ₀....τ _max in the system and its ‘hierarchically’ dynamics.     

The specific heat of Y0.7Th0.3C1.58

The specific heat of the novel high temperature superconductor Y_0.7Th_0.3C_1.58 (T_c = 17.0 K) has been measured between 4 and 22 K. Unlike the other known high temperature superconductors (T_c > 16 K) which have either an A-15 or a NaCl-type structure, this material forms in the b.c.c., Pu₂C₃-type, structure. The Debye temperature, θ_D, is 346 K and the linear term coefficient, γ, of the specific heat has the value 4.66 mJ/mole-K². Thus the electronic density of states, N(0), which is proportional to γ, is quite low. The energy gap, 2Δ/kT_c, on the other hand has an anomalously high value of 5.8. Comparisons between these parameters of Y_0.7Th_0.3C_1.58 and those for some A-15 and NaCl-type superconductors are made.     

向列相液晶电流体不稳定性——偏置电场效应

本文考虑了外电场与液晶盒玻璃片法线成倾角θ₀时,沿面排列的向列相液晶(θ_α<0,σ_α>0)的电流体不稳定性现象。在Dubois-Violette等人的一维理论的基础上,我们获得了直流、正弦波和矩形波三种电场激发下的失稳条件。重要结果包括:(1)电荷弛豫时间随θ₀单调上升。(2)对于MBBA液晶,存在一个临界角θ_c(ω)(ω是外场频率),当θ₀>θ_c(ω) 关键词：     

Superconducting transition parameters in aluminum-lithium alloys (0-10 at.% Li)

The superconducting transition temperatures T_c of face-centered cubic Al_1−x-Li_x alloys (x=0-0.10) exhibit a minimum near x=0.03 (3 at.% Li). The McMillan strong-coupling T_c equation yields a similar trend of the electron-phonon coupling constant λ. Meanwhile, the density of states at the Fermi level N(0) decreases monotonically with increasing x. It appears that T_c drops initially due to a reduced N(0), which is then overtaken by alloying-enhanced factors of phonon or electron-phonon interaction.     

c-axis penetration depth in Bi2Sr2CaCu2O8+δ single crystals measured by ac-susceptibility and cavity perturbation technique

The c-axis penetration depth Δλ_c in Bi₂Sr₂CaCu₂O_8+δ (BSCCO) single crystals as a function of temperature has been determined using two techniques, namely, measurements of the ac-susceptibility at a frequency of 100 kHz and the surface impedance at 9.4 GHz. Both techniques yield an almost linear function Δλ_c(T)∝T in the temperature range T<0.5T _c. Electrodynamic analysis of the impedance anisotropy has allowed us to estimate λ_c(0)≈50 µm in BSCCO crystals overdoped with oxygen (T _c≈84 K) and λ_c(0)≈150 µm at the optimal doping level (T _c≈90 K).     

Influence of carbon concentration on structural, magnetic and electrical transport properties for antiperovskite compounds AlCxMn3

The structural, magnetic and transport properties of the antiperovskite AlC_xMn₃ (1.0≤x≤1.4) are investigated. It is found that the lattice parameter a increases monotonously with nominal carbon concentration x. The Curie temperature T_C increases with increasing x from 1.0 to 1.1 and then decreases with further increasing x. The highest T_C value is 364 K, about 70 K higher than that of stoichiometric AlCMn₃ reported previously. This may be attributed to a competition between the lattice expansion and the strong Mn 3d-C 2p hybridization. Below 100 K, the resistivity can be well described as ρ(T)=ρ₀+AT², corresponding to the electron-electron scattering. A increases with x, suggesting certain changes in the electronic structure, e.g. carrier density. Above 250 K, all ρ(T) curves depart from the linear dependence on temperature and seem to take on a tendency towards saturation.     

Ni-Zr amorphous alloys: Low temperature specific heat and superconductivity

The specific heat (C_p) of the amorphous alloys Ni_100-xZr_x for x = 75, 65, 55 and 35 was measured from 0.8K to 40K and the composition trends of the transition temperature T_c, the enhanced density of states at the Fermi level N_γ(_F) and the Debye temperatures θ_D(0), θ_D(T) established. For the three superconducting compositions (x=75, 65, 55) N_γ(E_F increases rapidly with increasing [Zr] in agreement with the trend in amorphous Cu-Zr alloys. However, for the Zr-Ni alloys the bare density of states $\text{N}{}_0\text{(E}{}_{\mathrm{F}}\text{) =}\text{N}{}_{\mathrm{\gamma }}\text{(E}{}_{\mathrm{F}}\text{)}\text{(1 + λ}{}_{\mathrm{p}}\text{)}$ increases strongly with [Zr] in contrast to the Zr-Cu alloys where it is reported to be almost constant. We conclude that for the Ni-Zr alloys the electron-ion matrix element <I²> decreases with increasing [Zr]. Other results are related to recent photoemission studies of these alloys.     

Transport properties of n-type ferromagnetic semiconductor HgCr2−xInxSe4(x = 0.100)

Measurements of the electrical conductivity, magnetoresistance, and Hall effect were performed on a n-type ferromagnetic semiconductor HgCr_2?xIn_xSe₄(x = 0.100) single crystal from 6.3 to 296 K in magnetic fields up to 1.19×l0⁶A/m. The conductivity decreases rapidly near the Curie temperatureT_c (≈120 K) as the temperature is raised. A large peak in the magnetoresistance is observed near T_c. The Hall effect measurements indicate that the temperature dependence of the conductivity and the magnetoresistance are due mostly to a change in electron mobility. The electron mobility is 1.2 × 10^?2 m²/V · s at 6.3 K, and decreases rapidly near T_c with the rise in temperature. Then it increases slowly from 5.5 × 10^?4 m²/V · s at 160 K to 7.5 × 10^?4 m²/V · s at 241 K. This temperature dependence of the electron mobility can be explained in terms of the spin-disorder scattering which takes into account the exchange interaction between charge carriers and localized magnetic moments.     

Observation of a transition from BCS to HTSC-like superconductivity in Ba1 − x KxBiO3 single crystals

The temperature dependences of the upper critical field B _c2(T) and surface impedance Z(T) = R(T) + iX(T) have been measured in Ba_{1 ? x} K_xBiO₃ single crystals with transition temperatures 6 ≤ T _c ≤ 32 K (0.6 > x > 0.4). A transition from the BCS to an unusual type of superconductivity has been revealed: B _c2(T) curves of the crystals with T _c > 20 K have positive curvature (as in some HTSCs), and those of the crystals with T _c < 15 K described by the usual Werthamer-Helfand-Hohenberg (WHH) formula. The R(T) and X(T) dependences of the crystals with T _c ≈ 32 K and T _c ≈ 11 K in the temperature range T ? T _c are linear (as in HTSCs) and exponential (BCS), respectively. The experimental results are discussed using the extended saddle point model by Abrikosov.     

Upper critical fields of superconducting Gd and Tm doped LaSn3: Effects of crystalline electric fields

The temperature dependence of the upper critical fields, H_c²(T), are presented for (La_1-xGd_x)Sn₃ and (La_1-xTm_x)Sn₃. For samples with nearly the same T_c, H_c²(T) of the Tm-doped LaSn₃ samples are always larger than those for the Gd-doped samples. The results are interpreted in terms of crystalline electric field splitting of magnetic levels of the Tm³⁺. Pure LaSn₃ is found to be a Type I superconductor.     

The effect of hydrogen on the superconducting and structural properties of b.c.c. Nb-Ru alloys

The superconducting transition temperature (T_c) has been measured before and after the introduction of hydrogen into Nb_(1?x)Ru_x$\text{(0.20?x?0.33)}$. In all cases, the presence of appreciable amounts of this interstitial component led to a sharp increase in the T_c. All the evidence suggests that conversion of the host metal lattice to f.c.c. is necessary for the appearance of the elevated T_c.     

Spin freezing properties in LaNi5-xFex

We report magnetic and transport measurements in the metallic pseudobinary compound LaNi_5-xFe_x, 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 x_c ≈ 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 T_f(x). Very strong ferromagnetic coupling interactions between Fe atoms are observed at low concentrations (x < x_c), which contrast with small values of T observed for x ∼ x_c. The results, together with the nature of the spin freezing are discussed in relation to the 2d character of the magnetic lattice.     

Effect of renormalization of magnetic fluctuations on the kinetic coefficients of high-T c superconductors

Temperature dependences of resistivity, ρ(T), and Hall coefficient, R _H (T), in a 2D doped antiferromagnet are studied for various forms of the dynamic spin susceptibility X(q, θ) (in the mean-field approximation, taking into account attenuation and renormalization of the magnetic excitation spectrum θ_q, and for so-called strongly overdamped magnons). Doped CuO₂ planes in cuprates are considered in the one-band model of the Kondo lattice. Charge carrier scattering anisotropy, which strongly depends on temperature, is taken into account using the density matrix formalism and seven-moment approximation for the nonequilibrium distribution function. It is shown that the behavior of ρ(T) and R _H (T) is completely determined by the renormalization θ_q → $\omega _q \to \tilde \omega _q $ of the spin wave spectrum (the renormalization is essentially controlled by the fulfillment of the sum rule for X(q, θ) and by the strong temperature dependence of the gap δ(T). The resultant ρ(T) and R _H (T) dependences match the experimental data for optimally doped high-T _c superconductors.     

Specific heat of A-15 Nb3Al0.83Ge0.21

The specific heat of A-15 Nb₃Al_.83Ge_.21 with a transition temperature, T_c, of 20.0 K has been measured from 6 to 25 K. Values of the parameters derived from the data are γ=35.0±2 mJ/mole-K²; the Debye temperature, θ_D=278±5 K; the energy gap, △, normalized as 2△/kT_c, is 4.9±.3, and the thermodynamic critical field, H_c(0), is 4700±300 Gauss.     

Preparation and magnetic properties of LaMnO3 + δ (0 ≤ δ ≤ 0.154)

The structural, electrical, and magnetic properties of ceramic perovskite manganites LaMnO_{3 + δ} (δ = 0–0.154) are investigated. It is found that, in a weak magnetic field (B = 2 G), the LaMnO_{3 + δ} manganite with δ = 0.065 at temperatures below the Curie temperature T _C of the paramagnet-ferromagnet phase transition has a mixed (spin glass + ferromagnet) phase. In LaMnO_{3 + δ} manganites with the parameter δ = 0.100–0.154, this phase transforms into a frustrated ferromagnetic phase. A similar transformation was observed previously in La_1?x Ca_xMnO₃ compounds at calcium contents in the range 0 ≤ x ≤ 0.3. This similarity is explained by the fact that, in both materials, the Mn⁴⁺ concentration and, accordingly, the hole concentration c change equally in the concentration range from ～0.13 to 0.34 with an increase in x or δ. However, the magnetic irreversibility, the concentration dependences of the Curie temperature T _C(c) and the magnetic susceptibility X(c), and the critical behavior of the temperature dependence of the susceptibility X(T) in the vicinity of the Curie temperature T _C differ substantially for these two materials. The observed differences are associated with the distortion of the cubic perovskite structure, the decrease in the degree of lattice disorder, and a more uniform distribution of holes in the LaMnO_{3 + δ} manganites as compared to the La_{1 ? x} Ca_xMnO₃ compounds.