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.     

Brillouin zone effects on the critical temperature of superconductors and some related normal metal properties

A Fermi surface passing through a set of Bragg planes gives rise to variations of several normal metal properties, e.g. the electronic density of states N(0), the magnetic susceptibility, the Seebeck coefficient, the Debye temperature and the electron-phonon interaction strength V_ph, and consequently also to variation of the critical temperature for superconductivity, T_c.This behaviour is analysed on the basis of a nearly free electron model and a comparison is made with experimental results in various alloy systems. These systems include alloys of non-transition metals and pseudo-binary alloyed compounds with Cu₃Au- and CuAl₂-type structure.The observed variations of normal metal properties are in reasonable agreement with the theoretical estimates. The variations in T_c indicate that enhancements not only of N(0) but probably also of V_ph occur near Bragg planes.     

Electronic structure of new multiple band Pt-pnictide superconductors APt3P

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 .     

Coherent potential approximation for strong-coupling superconducting transition-metal alloys

Within the coherent potential approximation (CPA) a theory for strong-coupling superconducting transition-metal alloys is developed. The configurational dependence of the selfenergy-part is discussed and, in analogy to McMillan, an expression forT _c is derived for a singles band as well as for anΓ′₂₅-band. The electron-phonon coupling constantλ occuring in this expression depends explicitly on: the average component density of states,N _A (0) andN _B (0), the electron-phonon matrix elements of the components, the difference of the atomic energy levels,δ, and the phonon Green's function at zero frequency. In the limit of the pure metal this expression reduces to that developed by Appel and Kohn. It is shown that the change of the phonon density of states in an alloy due to the atomic mass differences — which is very complicated — does not influence the value ofλ.     

Heat capacity measurements on irradiated and non-irradiated Mo3Ge and Mo5Ge3

We have measured the heat capacity of irradiated and non-irradiated Mo₃Ge and Mo₅Ge₃ in the temperature range ≈ 1.6 to 10 K. An irradiation of 2.2 × 10¹⁹ neutrons cm^-2 results in an increase in the superconducting transition temperature T_c from < 1.6 K in the non-irradiated state to ～ 4 K after irradiation for Mo₃Ge and a corresponding change from < 1.6 K to ～ 3 K for Mo₅Ge₃. Our analysis shows that this change in T_c is not accompanied by a change in the electronic density of states (within experimental error) but rather a decrease in the Debye temperature from 392 to 322 K for Mo₃Ge and 377 to 320 K for Mo₅Ge₃.     

Anomalous behavior of superconducting transition temperature of amorphous and crystalline Nb3Ge under pressure

The superconducting transition temperature (T_c) of amorphous Nb₃Ge was studied under both hydrostatic and quasihydrostatic pressure to 3.5 and 13 GPa, respectively. Whereas hydrostatic pressure causes T_c to initially decrease, T_c is found to increase under higher quasihydrostatic pressures. T_c(p) was also studied on an A-15 crystalline Nb₃Ge sample obtained from the amorphous sample by annealing.     

Improved critical current capability of V3Ga formed in a V-6Ga/Cu-15 Ga composite

Single filament V₃Ga composite wires were prepared by imbedding cores of pure V and V-6.1 at.% Ga in a Cu-15.4 at.% Ga matrix. The parabolic growth rate of V₃Ga was enhanced by alloying the core, and the superconducting critical current density was nearly doubled. At 4.2 K J_c exceeded 10⁵ A/cm² in transverse magnetic fields up to 140 kG.     

用局域密度泛函线性丸盒轨道原子球近似超元胞法计算(Ba1-xKx)BiO3的Hopfield常数与Tc随组分x的变化

用局域密度泛函线性丸盒轨道原子球近似(LDF-LMTO-ASA)超元胞法计算(Ba_1-xK_x)BiO₃特定组分x的电子结构,其中x=0.0,0.5,1.0三种组分由扩大1倍的元胞计算;x=0.25和0.75两种组分由扩大4倍的元胞计算,所得能带参数总态密度TDOS(E_F)、分波态密度PDOS(E_F)以及自洽晶体势V^t(r),结合由实验测定的Θ_D≈210K,按McMillan强耦合公式以及Gaspari-Gyorffy近似,分别计算各原子的Hopfield常数η_t,电声子耦合常数λ以及超导转变温度T_c随组分x的变化。计算结果λ≈1而T_c最大约10K,且随x变化缓慢。与实验结果对比似乎暗示,在(Ba_1-xK_x)BiO₃中除电声子机制外,随组分变化的复杂结构相变亦将起重要的作用。 关键词：     

Superconducting phase transitions in ErcY1−cRh4B4

The superconducting transition temperature T_c1 and the reentering temperature T_c2 to the normal ferromagnetically ordered state in the system Er_cY_1?cRh₄B₄ were determined as a function of the concentration of Er. Comparing our results with the recent theory of Maekawa et al., the exchange interactions J′ among local spins and I between local spins and superconducting electrons were estimated as J′ ? 0.30 K and I²N(0) ? 0.046 K, where N(0) is the density of states. A sudden decrease of H_c2 at T_c2 was observed in a high concentration region suggesting the first order transition.     

Thermodynamics of SU(3) lattice gauge theory

The pressure and the energy density of the SU(3) gauge theory are calculated on lattices with temporal extent N_τ = 4, 6 and 8 and spatial extent N_σ = 16 and 32. The results are then extrapolated to the continuum limit. In the investigated temperature range up to five times T_c we observe a 15% deviation from the ideal gas limit. We also present new results for the critical temperature on lattices with temporal extent N_τ = 8 and 12. At the corresponding critical couplings the string tension is calculated on 32⁴ lattices to fix the temperature scale. An extrapolation to the continuum limit yields T_c/√σ = 0.629(3). We furthermore present results on the electric and magnetic condensates as well as the temperature dependence of the spatial string tension. These observables suggest that the temperature dependent running coupling remains large even at T ≅ 5T_c. For the spatial string tension we find √σ_s/T=0.566(13)g²2(T) with g² (5T_c) ≅ 1.5.     

A—15型化合物的超导性经验规律

本文给出一张既能考虑A-15结构型变,又能描述A-15型化合物T_c(超导临界温度)变化规律的图。从而预言:所述A-15系列中大概不会存在T_c很高(>25K)的化合物;铌系列中Nb₃Ge的T_c可能是最高的;钒系列中T_c可望还能提高(>17.1K),V₃Ge可望提高到12K以上。检查Nb₃Si,得不到Geller那样的乐观估计,并认为:以新工艺得到的A-15相Nb₃Si,是以牺牲正当化学配比或改变正常原子体积为代价而获得的非理想结构。从T_c与半径比关系及理想A-15结构对半径比的要求来看,笔者认为文献上关于Nb₃Si在有较高T_c(>23K)的种种预言,都是缺乏充分依据而不足置信的。 关键词：     

Comparison of amorphous melt-spun and vapor-quenched Zr-Rh alloys

Amorphous Zr-Rh alloys produced by melt spinning have a higher superconducting transition temperature T_c than materials generated by r.f. cosputtering techniques. However, the temperature dependence of the upper critical field H_c²(T is consistent with the GLAG theory for both materials. The T_c differences, as well as variations in the electronic density of states, can be partially understood in terms of slight differences in the average density, perhaps due to the inclusion of Ar gas during sputtering.     

Some parameters of superconductors with an extreme singularity in the density of state: (s+d) model

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 V_s, the critical temperature T_c at first slightly increases with increasing the d-wave interaction potential V_d, being determined by this interaction only for stronger V_d values. The ratio R=2|Δ(0)|/k_BT_c of the mean value of the zero temperature energy gap |Δ(0)| to T_c increases with increasing V_d, 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.     

The superconducting transition temperature of bulk samples of the niobium-germanium A15-phase after implantation of Ge,Si, O and Ar ions

Bulk material of Nb₃ (Ge_0.8Nb_0.2) with A15 structure and a superconducting transition temperature T_c of 6.5 K has been implanted with Ge, Si, Ar and O ions and subsequently annealed at high temperatures. After annealing between 700 and 750°C the Ge implanted samples showed a strong increase in T_c up to 16.2 K. With Si ions only a T_c of 13 K was obtained, with Ar and O ions T_c remained below 9 K. From X-ray measurements carried out on high T_c Ge implanted samples it could be concluded that the implanted surface layer grows up to a high degree epitaxially on the single crystallites of the bulk material. The lattice constant $\text{a}$₀ of the implanted film was reduced by 0.02 Å with respect to the bulk material. This reduction in $\text{a}$₀ is stronger than expected from the transition temperature of the implanted surface layer.     

Exciton-magnon interactions in NicMg1−c O single crystals

The effect of chemical composition and temperature on exciton-magnon interactions in Ni_cMg_1?c O single crystals was studied using optical absorption spectra in the region of the magnetic-dipole ³ A _2g (G)→³ T _2g (F) and electric-dipole ³ A _2g (F)→¹ E _g (D) transitions. The two zero-phonon lines at ～7800 and ～7840 cm^?1 on the low-energy side of the magnetic-dipole transition band were assigned to a pure exciton transition and an exciton-one-magnon transition at the center of the Brillouin zone. The intensity of the exciton-one-magnon peak decreases rapidly with increasing magnesium ion concentration and/or temperature, to vanish altogether at T=6 K for c<0.95 and at T=130 K for c≥0.99. Thus, the contribution of long-wavelength magnons in optical absorption spectra of Ni_cMg_1?c O becomes negligible at temperatures substantially lower than the Néel point T _N (the antiferromagnetic ordering temperature). This observation can be explained as being due to a substantial decrease in the characteristic spin-spin interaction length with increasing concentration of the diamagnetic magnesium impurity ions (static disorder) and/or with increasing amplitude of thermal atomic vibrations (dynamic disorder). At the same time, the peak at ～14500 cm^?1, which lies in the electric-dipole transition region and corresponds to excitation of an exciton and two magnons at the Brillouin zone edge, remains visible up to the Néel temperature. This is accounted for by the short-wavelength magnons being sensitive to short-range magnetic order, which persists up to T _N .     

Role of magnetism in the formation of a short-range order in an Fe-Ga alloy

The formation of a short-range order in an Fe-Ga bcc alloy has been studied by Monte Carlo simulation with the use of effective interaction potentials calculated within the density functional theory for the ferromagnetic and paramagnetic states. It has been found that the pronounced short-range order of the D0₃ type is formed at Ga concentrations close to the boundary of the two-phase region at T < T _c, whereas no short-range order is observed at T < T _c. The results obtained are in agreement with the experimental X-ray diffraction analysis. The relation of the features of the short-range order in the Fe-Ga alloy to the magnetostriction value has been discussed.     

Variation of electronic density in the superconducting phase transition in Nb<Subscript>3</Subscript>Al

The variation of electronic density in the superconducting phase transition in the classical superconductor Nb₃Al with critical temperature T_c=18.6 K was studied using ⁷³Ge emission Mössbauer spectroscopy. A comparison of the results obtained and the data available for the ⁶⁷Zn isotope in the lattices of high-temperature superconductors revealed a correlation between the electronic density variation at the Mössbauer probe nuclei sites and the value of T_c. This correlation is assumed to be related to the dependence of the electronic density variation on the standard correlation length.     

Slow transient response of Ge:Ga far-infrared photoconductors for space applications

Two slow transient times in the response of Ge:Ga photoconductors to photon influx are found to fit well to τ_a=τ_o(N_d/p₁) and τ_m=τ_o(S₁/p₁), derived by analyzing a two-region model, where τ_o is hole lifetime, N_d is the donor concentration, and p₁ and S₁ are concentrations of hole and space charge, respectively in the region near the metal–p⁺ ohmic contact. Both τ_a and τ_m are consistent with the response times observed in Ge:Ga photoconductors with Ga concentrations of 2×10¹⁴ cm⁻³ and 1×10¹⁴ cm⁻³ as well as in a stressed Ge:Ga photoconductor under low background photon influx at low temperatures: Those characteristic times are mainly determined by the compensation and carrier density in the crystal. We discuss the relation between the characteristic times proposed in this study and those theoretically derived by Westervelt and Teitsworth [R.M. Westervelt, S.W. Teitsworth, J. Appl. Phys. 57 (1985) 5457–5469], by Fouks [B.I. Fouks, Proc. ESA Symp. Photon Detectors for Space Instrumentation (ESA SP-356), 1992, pp. 167–174], and by Haegel et al. [N.M. Haegel, C.A. Latasa, A.M. White, Appl. Phys. A 56 (1993) 15–21; N.M. Haegel, C.R. Brennan, A.M. White, J. Appl. Phys. 80 (1996) 1510–1514]. As the compensation of Ge:Ga photoconductors decreases, responsivity to step change in photon influx and the ratio of slow response in the total response increases. These results also indicate that the slow response characteristics depend mainly on crystal qualities such as compensation if the metal–p⁺ contact is good enough.     

Superconductivity of quench-condensed beryllium and beryllium-germanium films

Tunneling experiments on Be films condensed onto helium cooled substrates show that these films are homogeneously disordered with a uniform transition temperatureT _c when Ge is codeposited, whereas thick films of pure Be do not grow homogeneously. For films of Be+10 at.% Ge a ratio 2Δ/k _B T _c =3.7 is found. Phonon induced structure in the tunneling density of states is not observed. The metastable phase obtained by quench-condensation is considered to be a disordered high-temperature phase of Be which transforms to the room temperature phase at about 60 K.