A型超导体的近似临界温度公式(Ⅰ)——μ*=0情形

本文用数值解方法从Eliashberg方程计算出超导临界温度T_c,并考察T_c对有效声子谱的依赖关系。在这个研究中,a²F(ω)被取为双δ函数谱,并允许其中的谱参数可以在很宽范围内改变。作者发现在λ<∧区域(即在T_c级数解的收敛圆外),T_c除了依赖λ和矩比外,还依赖T_c级数解的收敛半径倒数Λ;它们之间的关系是有规律的。在这些结果的启示下,本文在μ^*=0情形,用弥合数值解的方法得到一个适用于λ<Λ区域的T_c近似公式。接着,本文作者对吉光达和吴杭生的一篇文章进行了研究,指出:该文提出的超导体分类建议及其工作的主要结论是对的。但其中对决定A型超导体临界温度主要参量问题进行的分析,只适用于这样一些A型超导体,它们的收敛半径倒数Λ或者比λ₀小,或者虽比λ₀大、但λ又小于λ₀,其中λ₀是个依赖谱形状的参量,它的定义在正文中给出。对另一些A型超导体(λ₀<λ<Λ),决定T_c的主要参量不再是λ,而是δ=1/∧^0.5(ω_1/2/ω_log)^5.5λ^1.55。 关键词：     

非晶态超导体的2△0/(kBTc)和声子谱参量

提出了能够很好地描述非过渡金属无序和非晶态超导体的2Δ₀/(k_BT_c)与声子谱参量之间关系的一个公式:2Δ₀(k_BT_c=4.95[1-(T₀<ω>^1/2)/A(1/(λω₀)+1/(20λ<ω>)+1/(20<ω>))]。计算了大量已知声子谱的非晶和无序超导体的能隙2Δ₀对T_c的比,结果表明在百分之几的范围内与实验值符合。指出了非过渡金属和合金的非晶态超导体,既可以是一个2Δ₀/(k_BT_c)值远大于BCS理论值(3.53)的强耦合超导体,也可以是一个2Δ₀/(k_BT_c)值比BCS理论值还要小得多的弱耦合超导体。 关键词：     

关于高Tc问题的一个建议

我们建议:把超导体分成A型和B型两种。前者,λ<Λ,后者,λ>Λ。Λ的物理意义在正文中给出。本文分析表明了,有效声子谱对这两种超导体T_c的影响是不同的,因而提高它们T_c方法也是不一样的。提高A型超导体T_c的最有效方法是增大λ。提高B型超导体T_c的最有效方法是增大又λ〈ω²〉。 关键词：     

非晶态超导体转变温度Tc的经验公式

本文提出一个非晶态非过渡金属超导体的T_c经验公式,T_c=Aλ<ω>^1/2/(<ω>/ω₀+(1+λ)/20),式中A=(1/5)^(K1/2)。计算值和实验值,以及和Garland理论值的比较表明,T_c经验公式能很好地描述非晶态超导体的T_c值。 关键词：     

强磁场超导体

 超导体可分为Ⅰ类超导体和Ⅱ类超导体,前者只有一个临界磁场--热力学临界场H_c,后者有两个临界磁场--下临界场H_c1和上临界场H_c2。Ⅱ类超导体的H_c2要比Ⅰ类超导体的临界场H_c高得多,这一特性使得Ⅱ类超导体成为超导体的大规模应用的基础.Ⅱ类超导体又有理想Ⅱ类超导体和非理想Ⅱ类超导体之分,后者的主要电磁行为表现为磁化的可逆性,并且没有磁通冻结现象发生,它是均匀介质.理想Ⅱ类超导体几乎没有承载传输电流的能力,因此,它除了有科学价值外,几乎没有什么实用价值.本文将着重介绍非理想Ⅱ类超导体.科学家们已发现了数以千计的非理想Ⅱ类超导体,但是到目前为止其中只有Nb-Zr,Nb-Ti合金.     

非平衡Fe-Pd合金的矫顽力与磁致伸缩

通过测量非平衡Fe_1-xPd_x合金的矫顽力H_c与饱和磁致伸缩系数λ_s,随合金成分的变化,发现H_c随x增加而减小,而λ_s随x增加而增大。认为晶粒尺寸减小引起的平均各向异性的降低可能是H_c减小的原因。退火温度T_a>600K时,由于具有高磁晶各向异性的有序FePd相的分离析出,矫顽力H_c随T_a升 关键词：     

钇—镧石榴石型铁氧体系统的磁性和铁磁共振的研究

本文对钇-镧石榴石型铁氧体系Y_3(1-x)La_3xFe₅O₁₂(其中x=0,0.05,0.10,O.20,0.30,0.50,0.75和1.00)的磁性和铁磁共振进行了研究。由X射线粉末照相分析和金相观察确定了石榴石转构的单相区域。测量了饱和磁矩σ_s、起始磁导率μ₀、矫顽力H_c及有效g因子g_eff和共振线宽△H(3970和9160兆赫)与La含量x的关系。由磁性和铁磁共振的测量结果表明单相区城要比由X射线分析和金相观察的结果更窄一些。讨论了不同成分的σ_s的变化,μ₀与磁化机构的关系。由g_eff与频率的关系计算出内场H_i和材料的内禀g因子。从磁的不均匀性的观点解释了实验上观测到的△H、H_i和H_c在一定成分范围内与x的指数式关系。 关键词：     

A型超导体的近似临界温度公式(Ⅰ)μ~*=0情形

本文用数值解方法从Eliashberg方程计算出超导临界温度T_c,并考察T_c对有效声子谱的依赖关系。在这个研究中,a~2F(ω)被取为双δ函数谱,并允许其中的谱参数可以在很宽范围内改变。作者发现在λ<∧区域(即在T_c级数解的收敛圆外),T_c除了依赖λ和矩比外,还依赖T_c级数解的收敛半径倒数Λ;它们之间的关系是有规律的。在这些结果的启示下,本文在μ=0情形,用弥合数值解的方法得到一个适用于λ<Λ区域的T_c近似公式。 接着,本文作者对吉光达和吴杭生的一篇文章进行了研究,指出:该文提出的超导体分类建议及其工作的主要结论是对的。但其中对决定A型超导体临界温度主要参量问题进行的分析,只适用于这样一些A型超导体,它们的收敛半径倒数Λ或者比λ_0小,或者虽比λ_0大、但λ又小于λ_0,其中λ_0是个依赖谱形状的参量,它的定义在正文中给出。对另一些A型超导体(λ_0<λ<Λ),决定T_c的主要参量不再是λ,而是δ=1/∧~(0.5)(_(1/2)/ω_(log))~(5.5)λ~(1.55)。     

三重态双极化子的超导A相与B相

本文应用对称破缺的自洽Hartree近似讨论了三重态双极化子的超导A相和B相。在弱耦合的情况下,通过自由能的计算得到B相总比A相稳定;在强耦合的情况下,加入涨落反馈项可使得A相的自由能比B相的自由能低,或者发生A相到B相的转变,这和³He超流的情形类似。本文还精确计算了超导稳定相的热力学临界场H_c随温度的变化,并与重费密子超导体UBe₁₃,UPt₃和URi₂Si₂的上临界场作了比较,发现H_c能定性解释所有这些超导体的上临界场的反常温度特性。 关键词：     

d(x2-y2)+idxy混合波正常金属/绝缘层/超导体结中的隧道谱

考虑界面粗糙散射,在Blonder-Tinkham-Klapwijk(BTK)理论框架下,通过求解Bogoliubov-de-Gennes(BdG)方程,分别计算T=0K和有限温度下,d_(x²-y²)+id_xy混合波正常金属绝缘层超导体结中的准粒子输运系数和隧道谱.研究表明:隧道谱中的电导峰的劈裂程度强烈地依赖于d_xy波分量的强度、超导体的晶轴方位和界面粗糙强度,而温度的升高能压低电导峰. 关键词： NIS结 (x²-y²)+id_xy混合波超导体')" href="#">d_(x²-y²)+id_xy混合波超导体 隧道谱     

理想第二类超导薄膜的纵场临界电流

本文分析了一个在纵向磁场H_e中,载流的理想第二类超导薄膜(ξ(T)《d≤λ(T))的混合态结构。指出,只有当H_e>(H_c1)(d)时,超导膜的混合态才具有无阻负载电流的能力。本文得到的纵场临界电流曲线具有复杂的结构,并且一般均呈现峰值效应。Heaton及Rose-Innes测得的Nb₅₅Ta₄₅合金的I_c-H_e曲线和d>5λ时的理论曲线相比较,在主要特征上是相同的。 关键词：     

On the isotope effect of Tc in squaric acid (H2C4O4)

The phase transition mechanism in squaric acid is reviewed from the viewpoint of the pressure dependence of the O—H—O bond length (2R). The two-dimensional pressure effect on T_c is extracted from the hydrostatic pressure behaviour by taking account of the uniaxial stress applied perpendicular to the layer planes. The result shows that if 2R(H₂SQ) is stretched to 2R(D₂SQ) the phase transition temperature 7^H _c of H₂SQ coincides with T^D _c of D₂SQ. This result supports the suggestion that the phase transition mechanism is of the order-disorder type.     

Upper critical field <Emphasis Type="Italic">H</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis>2</Subscript> in anisotropic superconductors with varying charge carrier density: Application of the theory to doped MgB<Subscript>2</Subscript>

A theory describing the magnetic properties of a two-band superconductor with a varying charge carrier density is constructed. The upper critical field H _c2^(ab) parallel to the ab plane and field H _c2^(c) parallel to the c axis are determined in the entire temperature range 0 < T < T _c . A considerable increase in upper critical field H _c2^(ab) as compared to H _c2^(c) because of strong anisotropy of the system is detected. Anisotropy of coefficient γ _H = H _c2^(ab) / H _c2^(c) is obtained as a function of temperature for pure MgB₂ and as a function of the chemical potential in the case when Mg and B atoms are replaced with other chemical elements. A correlation between the variation in the superconducting transition temperature upon an increase in the chemical potential and critical magnetic fields H _c2^(ab) and H _c2^(c) is observed. The effect of doping on magnetic anisotropy is also determined.     

200 MeV Ag ion irradiation created columnar defects and enhanced critical current density of La-2125 type superconducting thin films

We have deposited c-axis oriented thin films of La_1.5Dy_0.5CaBa₂Cu₅O_z (La-2125) tetragonal superconductor on LaAlO₃(001) substrates by pulsed laser deposition. These films were irradiated with 200 MeV Ag⁺¹⁵ ions. Atomic force microscopy and elastic recoil detection analysis indicate that the irradiation has created columnar defects through the entire thickness (2000 Å) of these films. With ion irradiation up to 1×10¹¹ ions/cm², the critical current density (J_c(H)) enhances by fivefold, which is attributed to the augmented flux pinning by the columnar defects. A further increase in irradiation to 1×10¹² ions/cm² causes reduction in J_c(H) due to distorted morphology of the film. Our work shows that the enhancement in J_c(H) of the irradiated La-2125 film is comparable to that in irradiated RE-123 (RE = rare earth ion). Also, as the La-2125 type films have greater chemical stability than RE-123, La-2125 type superconductors are potential candidates for applications. It is interesting to note that there are partial flux jumps observed to occur symmetrically in the magnetic hysteresis of irradiated La-2125 thin films with enhanced J_c(H).     

Ll2超导体的Tc表达式

通过对L1₂超导体的研究,我们得出了这类超导体临界温度的表达式T_c=(15.9T_BV(B)G_A)/(M^1/2V(L1₂)G_B)。其中T_B是B组元的超导转变温度,V(B)是B组元的原子体积,V(L1₂)为L1₂化合物的平均原子体积,M是L1₂化合物的平均原子量,G_A,G_B分别为A组元、B组元的Gordy电负性值。 关键词：     

Thermal,magnetic and structural aspects of transitions in Mn0.63Cr0.37As. Thermodynamic properties from 10 to 350 K

Subambient crystallographic and magnetic structures and heat capacity on Mn_0.63Cr_0.37As are measured below 350 K. A complex first order magnetic (helimagnetic H_a^- to H_c-type) and structural (MnP- to MnP-type) transition, accompanied by a discontinuous unit cell volume change, and a second order magnetic [heli-(H_c) to paramagnetic] transition are observed at 165 and 219 K, respectively. The thermodynamic functions are derived and the thermodynamic characteristics of the transitions are discussed. The enthalpy and entropy of the combined MnP,H_a^- to MnP,H_c^- and MnP,H_c ⇌ MnP,P-type transitions are assessed to be 124.8R K and 0.749R, respectively. Two different models for the deconvolution of the magnetic heat-capacity envelope into contributions from the MnP,H_a^- to MnP,H_c^- and the MnP,H_c ⇌ MnP,P-type transitions are considered.     

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：     

An evidence for a possible stable dibaryon

A search for stableH-dibaryon ofI=0,Y=0,J ^P =0⁺ was undertaken in negative pion-, neutron-,¹²C-nucleus- and proton-propane interactions at 4.0, 7.0, 50.4 and 10 GeV/c, respectively, using 55 cm and 2 m propane bubble chambers. This technique permite one to examine the whole range of possibleH-mass value. In the range ofM _H >2M _Λ=2231.2 MeV/c² an enhancement ofM _ΛΛ=(2365.3±9.6) MeV/c², Γ _ΛΛ =(47.2±15.1) MeV/c² with production cross section σ=(24.2±7.0) μb per carbon nucleus was found. The mass rangeM _Σ?p<M _H <2M _Λ was explored via the search for a weak decay modeH→Σ^?+p. AV ⁰-particle was found pointing to a two-prong interaction and satisfying the kinematics of the above two-body decay mode with a high confidence level. The possibility of imitation of the observed event by background processes was investigated for a series of two-step strong reactions and weak decays. We succeeded to show the significance of theH-production hypothesis on a deuteron-mass nuclear fluctuon via thepd→HpK ⁺ K ⁰ reaction. The mass of the event treated as aH→Σ^?+p weak decay appeared to be (2173.94±1.32) MeV/c², its life-time being 0.668·10^?10 s. Both are in agreement with predictions of the MIT Bag Model for the so-calledH-dihyperon. The formally determined production cross section of theH in proton-propane interactions at 10 GeV/c is estimated to be 40 nb. Perhaps, this event can be considered as a first evidence for a six-quark bound state-H-dihyperon. An independent way for the search for theH-particle, based on a specific conversion processHN→YYN, peculiar to its intranuclear strong interactions was proposed and tried. For this purpose all events with two Λ-hyperons and identified protons were tested for the hypothesesH _p →ΛΛp at three fixed masses:M _H =1880.00, 2173.94 MeV/c² and the ΛΛ invariant mass for the event in question. The kinematics of this reaction fits successfully only one event and only atM _H =2173.94 MeV/c². It seems worthy of note that the final state ΛΛ invariant massM _ΛΛ=2344 MeV/c² is rather close to the observed peak at 2365 MeV/c². It is shown that the intranuclear conversion processes Ξ^? p→ΛΛ and Ξ⁰ n→ΛΛ fail to imitate the observed event. The ΛΛp invariant mass of this event equal to 3363 MeV/c² is out of a 3568 MeV/c² peak region of the ΛΛp invariant mass spectrum observed earlier. TheH conversion cross section exceeds the production one. Thus, we consider this event as providing a new evidence for the stableH-dibaryon ofM _H =(2173.94±1.32) MeV/c².     

Bestimmung der Atomspinkorrelationszeiten der Fe3+-Ionen in FeNH4(SO4)2·12 H2O mit Hilfe des Mössbauereffektes

The Mössbauer effect studies of⁵⁷Fe in the paramagnetic salt FeNH₄(SO₄)₂·12 H₂O reported byObenshain et al. were extended to higher values of the applied external magnetic fieldH _a. The results are discussed in terms of the theoretical calculations given byWegener. The data can be fitted using a maximum internal fieldH _max=(598±10) kOe and two atomic spin correlation times τ_c=2,4 · 10^?9 sec andτ′ _c=2,1 · 10^?9 sec.τ _c andτ′ _c are independent ofH _a and the temperatureT. It is assumed in the analysis that the distribution w(H) of the internal fields can be described by a continuous function of the Boltzmann factor type with two parameters. These two parameters are determined by the normalization and the macroscopic magnetization.