PREPARATION,SUPERCONDUCTIVITY AND STRUCTURE OF THE LaBaCaCu3O7-x SUPERCONDUCTORS WITH Tc HIGHER THAN 80 K

In this paper, the preparation process of the tetragonal LaBaCaCu₃O_7-x (LBCCO) superconductor with T_c=84 K is repored. The superconductivity and structure of the LBCCO superconducting phase are investigated by means of X-ray diffraction (XRD), transmission electron microscope(TEM) and superconductivity measurements, The results indicate that the superconducting phase with T_c=82K in the LBCCO system has a tetragonal structure with a=0.3966nm, c=l.1838nm and space group P4/mmm. About 50% of the Ca and La atoms are located in 1d sites (i.e. Y site in the YBCO structure). It is confirmed that there is in fact no orthorhombic to tetragonal transition in this material. The relationship between the oxygen content and T_c is also presented.     

Superconductivity of M3C60 Within Eliashberg Theory

The important feature of the alkali-metal-doped C₆₀ is that the intramolecular high, frequency vibration modes arising from the covalent interaction and the intermolecular lowfrequency vibration modes arising from the weak Van der Wads interaction both play important roles in the properties of superconductivity. We propose a two-peak form for the phonon density of states. By solving the Eliashberg equations we get the formula for the reduced gap 2Δ₀/k_BT_c and the transition temperature T_c whidh are all in accordance with the experimental values.     

Determination of the magnetic transition temperature of dilute ferromagnetic alloys - the Pd(Gd) case

The determination of T_c in dilute alloys is examined. As an example, magnetization vs field data in a Pd_0.95Gd_0.05 alloy are presented. For this alloy, it was not possible to determine a value of T_c using Arrott-Belov-Kouvel (ABK) plots; the conditions for the applicability of ABK plots were not satisfied. Results of very-low-field ac and dc magnetization vs T, which depend mainly on the demagnetization factor, yield a more precise measure of T_c. We conclude that the latter procedures are more useful than ABK plots for determination of T_c in dilute alloys.     

NOTE ON A MCMILLAN-LIKE Tc FORMULA

In this letter, we present an analysis to understand why the McMillan-like T_c fornula: T_c=1.134ωlog(1-1.02μ^*+0.476λ)exp{-1+1.604λ/λ-μ^*(1+0.604μ)}, proposed in a previous paper by the authcrs,applies to crystalline superconductors satisfactorily but Iess satisfactorily to non-crystalline ones.     

插层FeSe高温超导体的高压研究进展

通过对FeSe进行化学插层可以将其超导转变温度（T_c）从约8 K提高到40 K以上,实现高温超导电性.最近,我们对两种插层FeSe高温超导材料（Li_0.84Fe_0.16）OHFe_0.98Se和Li_0.36（NH₃）_yFe₂Se₂开展了高压调控研究,发现压力会首先抑制高温超导相（称为SC-I相）,然后在临界压力P_c以上诱导出第二个高温超导相（称为SC-Ⅱ相）,呈现出双拱形T-P超导相图.这两个体系的P_c分别约为5和2 GPa,两个体系SC-Ⅱ相的最高T_c分别可以达到约52和55 K,比相应SC-I相的初始T_c提高了10 K.对（Li_0.84Fe_0.16）OHFe_0.98Se的正常态电输运性质分析表明,SC-I和SC-Ⅱ相的正常态分别具有费米液体和非费米液体行为,意味着这两个超导相可能存在显著差异.此外,还发现这两个体系的SC-Ⅱ相的T_c与霍尔系数倒数1/R_H（∝载流子浓度n_e）具有很好的线性依赖关系.对（Li_0.84Fe_0.16）OHFe_0.98Se的高压X射线衍射测量排除了其在10 GPa以内发生结构相变的可能,因此P_c以上SC-Ⅱ相的出现和载流子浓度的增加很可能起源于压力导致的费米面重构.     

C-15结构(Hf,Zr,Ta)V2和(Hf,Zr,Nb)V2合金超导转变的压力效应研究

本文研究了C-15结构化合物(Zr_0.5Hf_0.5-xTa_x)V₂(x≤0.2)和(Hf_0.5Zr_0.5-xNb_x)V₂(x≤0.2)的超导转变温度T_c及其压力效应?T_c/?P。报道了实验方法与结果。与(Hf_1-xTa_x)V₂和(Zr_1-xTa_x)V₂的情况不同,在(Zr_0.5Hf_0.5-xTa_x)V₂中Ta的引入使常压下的T_c下降,然而(?)T_c/(?)P却大为提高。因此高压下Zr_0.5Hf_0.45Ta_0.05V₂的T_c反而比Hf_0.5Zr_0.5V₂更高。导出了一个基于角动量分波表象能带论方法的描述压力效应的新关系式。它指出导带电子波函数中的高角动量成分变化对T_c的压力效应影响比较重要。这个公式有助于理解上述复杂的实验现象,并能合理地解释某些元素(如Cs,Ba,La等)的T_c随压力剧增的事实。 关键词：     

Top-Quark Condensate at Finite Temperature and Electroweak Symmetry Restoration

The gap equation at finite temperature in the top-quark condensate scheme of electroweak symmetry breaking is proven to have the identical form in both the imaginary and real time formalisms of thermal field theory. By means of the gap equation, combined with the basic relation to define the vacuum expectation value v of the effective Higgs field, we analyze the dependence on temperature T and chemical potential p of the dynamical top-quark mass as the order parameter characteristic of symmetry breaking, and obtain the p-T criticality curve for symmetry restoration. We find out that the critical temperature T_c = 2υ for μ = 0 and the critical chemical potential μ_c = 2 π υ / √3 for T = 0. When μ = 0, the top-quark mass near T_c has the leading (T_c² - T²)^1/2 behavior with an extra factor dependent on temperature T and the momentum cutoff Λ. However, it is generally argued that the symmetry restoration at T ≥ T_c is still a second-order phase transition.     

STUDY OF HALOGEN DOPING IN POLYCRYSTALLINE YBa2Cu3O7-δ

This paper presents systematically the doping effects of halogens in YBa₂Cu₃O_7-δ polycrystalline samples, with a series of concentrations of fluorine, chlorine, bromine or iodine. All samples were prepared by chemical method of citrate pyrolysis, which makes the halogen concentration in the samples more uniform. Samples doped with F have T_c as high as 94.5K, Cl, Br and I dopings also raise T_c to a certain extent. In addition, the influence of halogen doping on critical current density, morphology, constitution and so forth is discussed.     

INTERNAL FRICTION AND ULTRASONIC ATTENUATION RELATED TO CARRIERS IN HIGH Tc SUPERCONDUCTORS

For both BiSrCaCuO and TlBaCaCuO samples, internal frictions (Q^-1) in the kHz range reveal a plateau (Q^-1_p) above T_c and a rapid drop below T_c with the turning points located just at T_c for various samples with different T_c. This anomaly cannot be observed in non-superconducting samples. The ultrasonic attenuation (α) in TlBaCaCuO displays similar results to the internal friction. Moreover, it is discovered that the Q^-1_p is nearly proportional to the carrier density for Y(Pr)BaCuO and Gd(Pr)BaCuO with different Pr contents. These results show that the drop of Q^-1 and a below T_c is closely related to superconducting condensation. The Q^-1_p and α_p can be explained using coupling-model of carriers with local dynamic distortion because high T_c superconductors are strong-correlation systems. Furthermore, by taking account of the smearing of superconducting gap structure resulting from the recombination of quasi-particles and by modifying the BCS relative jump rate as S(E, E', Γ) = Re{1-Δ²/[(E-iΓ)(E'-iΓ)]}, the calculated results of internal friction and ultrasonic attenuation below T_c are in good agreement with the experimental data. The superconducting gap Δ and the damping rate Γ for both BiSrCaCuO and TlBaCaCuO have also been obtained, they are in accordance with those got by tunneling spectrum and NMR methods, etc.     

Essence of Pseudogap and Oscillation in High-Temperature Cuprate Superconductor Junction of Pseudogap State

This paper gives methods to calculate the pairing temperature T*,at which a pseudogap is opened,and the superconducting temperature Tc,at which superconductivity appears,in the high-Tc cuprates,and demonstrates directly that at Tc ＜ T ＜ T* the pseudogap is the gap of Cooper pair without long-range phase coherence,and at T ＜ Tc there is long-range phase coherence between Cooper pairs.Based on the above clear physical picture on the pseudogap state and our mechanism for the ac Josephson effect,this paper proposes that there should be a novel oscillatory current in P-I-P junction,induced by a constant bias on the junction.Here,P represents the high-Tc curates in the pseudogap state,where Cooper pairs do not have long-range phase coherence,and I represents the thin insulating barrier.This paper conjectures that there is a possible high-temperature superconductivity in the heavily underdoped high-Tc cuprates.     

Intralayer and Interlayer Couplings in the Soliton pairing Model of Superconductivity

Explicit expressions and results for T_c(n) of the layer compounds of Bi and T1 are obtained within the generalized soliton pairing framework, in which T_c, is raised by the cooperation l i of the two couplings, intralayer and interlayer. The calculated values are in good agreement I with the observed T_c's. The maximum possible T_c(∞) is predicted at≈ 200 K.     

FeSi/Si成份调制膜的二维磁性和死层效应

成功地制成了FeSi/Si非晶态成分调制膜,固定Si层厚度,系统改变FeSi层厚度d_m发现,随d_m下降,饱和磁化强度M_s单调减小,这主要是死层效应的贡献,OK下死层厚度约为5.8?,随d_m减小,调制膜逐渐由三维磁性转变为二维磁性,表现为出现低磁滞和无磁滞效应,线性的M_s(T)-T关系和Curie温度T_c的单调下降,T_c满足如下关系:△T_{c关键词：}     

INFLUENCE OF γ-RADIATION ON THE DIELECTRIC CHARACTERISTICS IN Rb2ZnCl4 SINGLE CRYSTALS AT INCOMMENSURATE-COMMENSURATE PHASE TRANSITION

In this paper the influence of γ-radiation on the dielectric constants of Rb₂ZnCl₄ crystal at incommensurate-commensurate phase transition (hereafter abbreviated as INC-C transition) are studied. The thermal hysteresis occurs upon both cooling and heating runs, irrespective of whether the samples have been treated with γ-radiation or not. For the γ-irradiated sample, its transition point, T_c, between the INC and C phases is not changed, but the peak value of the dielectric constant at T_c increases abruptly, compared with that before γ-irradiation, When this sample is annealed at 40℃, the peak value restores to the incipient value for the sample free from γ-irradiation. The origin of the phenomenon of the thermal hysteresis of the dielectric constant may be due to the pinning effect of dejects or impurities in the samples.     

a—Fe—(Co,Cr)—Zr系列合金的穆斯堡尔谱研究

本文研究了a-Fe_90-xCo_xZr₁₀(x=4,10,20,…,70)和Fe_90-yCr_yZr₁₀(y=0,4,7,16,20)系列合金的M?ssbaner谱。实验结果表明a-Fe_90-yCo_yZr₁₀和富Fe区(x<30)的a-Fe_90-xCo_xZr_{10 关键词：}     

Multiple Exchange-Mediated Interaction and Superconductivity

It is shown that in a strongly correlated system there exists multiple exchange-mediated interaction (MEIW). From the numerical calculations of T_c, (T_c vs. x and y for La_2-xSr_xCuO₄ and Y₁Ba₂Cu₃O_7-y), the small isotope effect, the large gap and some explanations for the observed phenomena, this paper affirms that the high-T, superconductivity is caused by the MEIW, the phonon-mediated interaction and the tunneling coupling. The heavy-fermion superconductivity is mainly caused by the MEMI, and, in the meantime, the phonon-mediated interaction h h also some contributions. The extremely large gap below T_N in AF Chevrel phase, some AF materials come from the MEMI. The organic superconductivity might be caused mainly by the MEMI.     

Magnetic Excitations and the Curie Temperature of Decamethylchromocenium Tetracyanoethylenide

We describe the molecular ferromagnet decamethylchromocenium tetracyanoethanide [C_rC_p2^*] [TCNE] by a quasi-one-dimensional alternating-spin model with ferromagnetic exchange interaction. The spin-wave theory is used to discuss the magnetic excitation of the molecular ferromagnet. We obtain the magnetizations corresponding to different external magnetic fields without interchain exchange interaction which is consistent with the experimental results. The formula of the Curie temperature T_c is obtained by using the thermodynamic Green's function method. It is found to be related to the spin of donors, to the intrachain exchange interaction and to the spatial anisotropic parameter. The result obtained by this T_c formula is consistent with the experimental result T_c = 3.6 K for [C_rC_p2^*][TCNE].     

Lifshitz时空s波超导模型的关联长度和穿透深度

在D=d+2维各向异性的Lifshitz黑洞时空背景中, 在探子极限下, 用解析方法研究了临界温度附近引力系统的微扰, 计算出超导的关联长度ξα(1/T_c)(1-(T/T_c)^-1/2, 这与平均场论的结果一致. 进一步, 考虑在该系统中加一个均匀外磁场, 计算出穿透深度λα(T_c-T)^-1/2, 该结果与Ginzburg-Landau理论相符.     

Magnetic impurities in amorphous superconductors:TKondo>Tc

Measurements of upper critical field as a function of temperature and Fe-concentration are reported for the amorphous superconductors ((Mo_.6Ru_.4)_.80G_.20)_1−cFe_c, where G is a glass-former. The linear shapes of the H_c2 vs. T curves near T_c agree well with predictions made by Matsura, Ichinose, and Nagaoka, for T_Kondo > T_c.     

On the Liguid-Gas Phase Transition and the Instability of Hot Nuclei

Based on a simple semi-empirical energy density, the free energy density for a hot nucleon system is derived, from which the equations of state P(T, p) and chemical potentials μ_q(T, p) for both finite nucleus and infinite nuclear matter are acquired. The liquid-gas phase transition and the instability of hot nuclei are analysed with these ingredients within the framework of thermodynamic's. Both critical temperature T_c for infinite nuclear matter and limiting temperature T_lim, for finite nuclei are predicted and compared with previous calculations.     

On the Free Carrier-Negative U Center Interacting Model for the High-Tc Oxide Superconductor

Using the free carrier-negative U center interacting model, we have investigated the superconducting transitionr,temperature T_c, the specific heat jump at T_c, the coherence length, the penetration depth, the thermodynamic critical field and the isotope effect for the high-T_c oxide superconductor. The theoretical values of these physical quantities are consistent with the experimental values. Further, the residual electronic specific heat in the superconducting state is explained as the result of the inherent instability in this superconductive mechanism.