Universal behavior of the upper critical field in iron-based superconductors

The newly discovered iron-based high temperature superconductors have demonstrated rich physical properties. Here we give a brief review on the recent studies of the upper critical field and its anisotropy in a few typical series of the iron-based superconductors (FeSCs). In spite of their characters of a layered crystal structure, all the FeSCs possess an extremely large upper critical field and a weak anisotropy of superconductivity, being unique among the layered superconductors. These particular properties indicate potential applications of the FeSCs in the future. Based on the experimental facts of the FeSCs, we will discuss the possible mechanisms of pair breaking in high magnetic fields and its restrictions on the theoretical analysis of the superconducting pairing mechanisms.     

Pressure dependence of high-Tc superconductors

Abstract

We recently developed a method to derive the charge carrier concentration 6 (holes per planar copper atom) under pressure from the measured T_c and H_c2. The different behaviour of YBa₂Cu₄O₈, CaLaBaCu₃)₇ and other compounds can be understood from the difference in ?δ/?p using a model where T_c is quadratically dependent upon δ and where δ is linearly dependent upon pressure, as observed experimentally.     

Strong Pauli paramagnetic effect in the upper critical field of KCa2Fe4As4F2

Recently, 12442 system of Fe-based superconductors has attracted considerable attention owing to its unique double-Fe As-layer structure. A steep increase in the in-plane upper critical field with cooling has been observed near the superconducting transition temperature, Tc, in KCa2Fe4As4F2 single crystals. Herein, we report a high-field investigation on upper critical field of this material over a wide temperature range, and both out-of-plane(H∥c, Hc2c) and in-plane(H∥ab, Hc2ab ) directions have been measured.A sublinear temperature-dependent behavior is observed for the out-of-plane Hc2c , whereas strong convex curvature with cooling is observed for the in-plane Hc2ab . Such behaviors could not be described by the conventional Werthamer-Helfand-Hohenberg(WHH) model. The data analysis based on the WHH model by considering the spin aspects reveals a large Maki parameter α=9,indicating that the in-plane upper critical field is affected by a very strong Pauli paramagnetic effect.     

射频超导谐振腔中的电磁场研究

由高纯铌制成的射频超导谐振腔在高场时会出现Q值下降现象，这是高场下超导腔表面局部磁场增强所致. 为研究高场下超导腔表面的电磁场特性，用单cell超导腔设计了腔表面峰值磁场的研究实验. 通过对单cell超导腔表面峰值磁场的测量，可以确定超导腔焊缝及iris附近表面是否存在缺陷，以便进一步进行腔的处理来改进腔的性能.     

Critical Behaviors and Finite-Size Scaling of Principal Fluctuation Modes in Complex Systems

Complex systems consisting of N agents can be investigated from the aspect of principal fluctuation modes of agents. From the correlations between agents, an N×N correlation matrix C can be obtained. The principal fluctuation modes are defined by the eigenvectors of C. Near the critical point of a complex system, we anticipate that the principal fluctuation modes have the critical behaviors similar to that of the susceptibity. With the Ising model on a two-dimensional square lattice as an example, the critical behaviors of principal fluctuation modes have been studied. The eigenvalues of the first 9 principal fluctuation modes have been invesitigated. Our Monte Carlo data demonstrate that these eigenvalues of the system with size L and the reduced temperature t follow a finite-size scaling form λ_n(L, t)=L^γ/ν f_n(tL^1/ν), where γ is critical exponent of susceptibility and ν is the critical exponent of the correlation length. Using eigenvalues λ₁, λ₂ and λ₆, we get the finite-size scaling form of the second moment correlation length ξ(L, t)=Lξ(tL^1/ν). It is shown that the second moment correlation length in the two-dimensional square lattice is anisotropic.     

Magnetic field dependence ofT c and temperature dependence ofH c2 in layered superconductors with open normal state Fermi surface

A theoretical framework for treating the effects of magnetic fieldH on the pairing theory of superconductivity is considered, where the field is taken in an arbitrary direction with respect to crystal axes. This is applicable to closed, as well as open normal state Fermi surface (FS), including simple layered metals. The orbital effects of the magnetic field are treated semiclassically while retaining the full anisotropic paramagnetic contribution. Explicit calculations are presented in the limits |H| → |H _c2(T)|,T ∼ 0 andT →T _c(|H|), |H| ∼ 0. Effects of weak nonmagnetic impurity scattering, without vertex corrections, have also been taken into account in a phenomenological way. The final results for the case of open FS and layered materials are found to differ considerably from those of the closed FS. For example, an important parameter,h(T=0)=|H_c2(0)|/[-Tδ|H _c2 T|δT]_T{s0} for the case of a FS open ink _z-direction with thek _z-bandwidth, 4t ₃, very small compared to the Fermi energy,E _F, is close to 0.5906, compared to 0.7273 for the closed FS, in the clean limit. Analytical results are given for the magnetic field dependence ofT _c and the temperature dependence of H _c2 for a model of layered superconductors with widely open FS. For a set of band structure parameters for YBa₂Cu₃O₇ used elsewhere, we find reasonable values for the upper critical fieldH _c2(0), the slope (dH _c2/dT)_{T c0}, anisotropic coherence lengths ζ_i(T=0),i=x, y, z, and (dT _c/d|H|)_{|H| → 0}.     

两带超导体LaNiC_2上临界磁场的理论分析

非中心对称超导体LaNiC_2是传统BCS超导体还是能隙存在节点又或是两带超导体,目前仍然存在争议.基于此,文章用两带Ginzburg-Landau理论分析了超导体LaNiC_2的上临界磁场随温度的变化关系,计算结果与实验结果在整个温度区间内符合得很好,说明LaNiC_2是两带超导体,和陈健等人的观点一致.文章还分析了两个不同能带对上临界磁场的影响,发现相对较小的相干长度对LaNiC_2的上临界磁场影响较大.     

Measurement and understanding of magnetization in AC and DC fields and the determination of intragrainH c1 in highT c RBa2Cu3O7 superconductors

We present DC and low frequency AC magnetization measurements on various RBa₂Cu₃O₇ superconductors. We identify features intrinsic to these compounds, and establish the features originating from intergranular links in sintered pellets. The isothermal magnetization curves, and the temperature dependence of magnetization in field-cooled and zero field-cooled states are shown to be consistent with the calculations done following a recent extension of Bean’s model. Low field anomalies predicted within this model are observed, and yieldH _c1 values of a few Oe. These values are shown to be consistent with the temperature variation of magnetization. A comparison is made with the other existing data and it is demonstrated that earlier quoted values ofH _c1 are gross overestimates.     

Field and temperature dependence of intergranular hysteresis in YBa2Cu3O7

We have studied the variation of low field magnetization hysteresis in YBa₂Cu₃O₇ as a function of the maximum magnetic field applied during a hysteresis cycle (1 G<H _max<7.3 G) and also as a function of temperature (77 K<T<95 K). The remnant magnetization is studied as a function ofH _max andT and the measured dependences are explained using the extended critical state model. The potential of this technique as a contactless method of probing the temperature dependence ofJ _c is discussed.     

The Effect of Interface Potential Barriers on Boundary Conditions in the Theory of Dirty Inhomogeneous Superconducting Systems Near Transition Temperature

A theoretical treatment of the superconductor-normal metal contact with interface potential barrier is presented in the dirty limiting case. Our result for the kernel of the linearized self-consistency condition allows near transition temperature the determination of boundary conditions and the behaviour of Usadel's function at interfaces. This represents a microscopic derivation of the so-called “extended” de Gennes boundary condition. With the help of a variational procedure an expression is derived for the Ginsburg-Landau extrapolation length of dirty SN systems with interface potential barrier.     

矩形截面超导线圈的径高比对最大磁场点位置和幅值的影响分析

为了充分利用超导线材的载流能力,需要精确计算超导线圈产生的最大磁场值,还需要明确最大磁场值所处的具体位置.其中,最大磁场点的位置主要由线圈的形状(径高比)决定.本文基于单积分法并通过MATLAB编程,将矩形截面线圈的径高比α和β参数化,计算分析线圈内壁边上和端面边上各点磁场的变化趋势.同时,利用电磁场有限元软件ANSYS,对矩形截面线圈的空间磁场进行仿真分析,得到线圈的内壁磁场系数、端面磁场系数和最大磁场系数随α和β的变化规律;进而,计算并寻找到了线圈截面上最大磁场点的位置和幅值.综合分析表明,线圈内壁边上的最大磁场点并不是始终位于内壁中点B_c处,而是可能偏离端点B_e一小段距离的某点(a₁,b-δ)处;线圈端面边上的最大磁场点一定不位于端点B_e处,而是偏离端点B_e一小段距离的某点(a₁+δ,b)处.本文给出了线圈截面上最大磁场系数K_mc对应于线圈径高比(α,β)变化的等高曲线,矩形截面超导线圈最大磁场值可以通过计算内壁中点的磁场值与最...     

超导材料NbS2上临界磁场的理论分析

根据两带Ginzburg-Landau 理论计算了层状超导材料NbS₂的上临界磁场, 以及上临界磁场各向异性参数随温度的变化情况, 并将NbS₂与MgB₂, NbSe₂上临界磁场的各向异性进行比较. 所得计算结果与已有实验数据符合得很好, 充分说明了NbS₂的超导电性具有两能隙特征. NbS₂上临界磁场各向异性参数在5.0 K附近逐渐变小, 这与MgB₂和NbSe₂有相似之处. 但NbS₂的上临界磁场各向异性参数大约为7.3, 明显大于MgB₂和NbSe₂. 计算结果还表明, NbS₂较大能隙所对应能带的有效质量比约为54, 另一能带的有效质量基本为各向同性.     

Critical current densities of high pressure oxygen sputtered thin films of YBa2Cu3O7−x by non-resonant rf absorption method

The critical current densities (J _c) have been measured at 77K in high pressure oxygen sputtered thin films of YBa₂Cu₃O_7−x superconductor using the non-resonant rf absorption technique. High values ofJ _c (∼ 10⁵ A/cm²) are observed in these relatively large area (∼ 1·2 cm²) films.     

Effect of Sb substitution on the superconductivity of Bi2(Ca,Sr) n+1Cu n O4n+2 (n=1, 2 and 3) systems

Effect of incorporation of Sb in place of Bi in the bismuth cuprate superconductors has been examined. The nominal compositions studied are MCa_1·5Sr_1·5Cu₂O_8+δ and MCa₂Sr₂Cu₃O_10+δ, whereM=Bi_2−x Sb _x or Bi_1·5−x Sb _x Pb_0·5. Different preparative routes such as the ceramic method, the matrix route as well as the melt route were employed to prepare the materials. No indication of either Sb entering the lattice or enhancement ofT _c is noted from resistivity, magnetic susceptibility and microwave absorption measurements. Communication No. 162 from Materials Research Centre.     

Superconducting state of the organic conductor (TMTSF)2ClO4

(TMTSF)2ClO4 is a quasi-one-dimensional organic conductor and superconductor with Tc=1.4 K, and one of at least two Bechgaard salts observed to have upper critical fields far exceeding the paramagnetic limit. Nevertheless, the 77Se NMR Knight shift at low fields reveals a decrease in spin susceptibility chi(s) consistent with singlet spin pairing. The field dependence of the spin-lattice relaxation rate at 100 mK exhibits a sharp crossover (or phase transition) at a field Hs approximately 15 kOe, to a regime where chi(s) is close to the normal state value, even though Hc2> Hs.     

Why pinning by surface irregularities can explain the peak effect in transport properties and neutron diffraction results in NbSe2 and Bi- 2212 crystals?

The existence of a peak effect in transport properties (a maximum of the critical current as function of magnetic field) is a well-known but still intriguing feature of Type II superconductors such as NbSe₂ and Bi-2212. Using a model of pinning by surface irregularities in anisotropic superconductors, we have developed a calculation of the critical current which allows estimating quantitatively the critical current in both the high critical current phase and the low critical current phase. The only adjustable parameter of this model is the angle of the vortices at the surface. The agreement between the measurements and the model is really very impressive. In this framework, the anomalous dynamical properties close to the peak effect is due to coexistence of two different vortex states with different critical currents. Recent neutron diffraction data in NbSe₂ crystals in the presence of transport current support this point of view.     

Anisotropy of Critical Fields in MgB2： Two-Band Ginzburg-Landau Theory for Layered Superconductors

The temperature dependence of the anisotropy parameter of upper critical field γHc2(T)=H_{c2}^||(T)/H_{c2}^{\bot}(T) and London penetration depth γ_λ(T)=λ_{\perp} (T)/λ_{\bot} (T) are calculated using two-band Ginzburg-Landau theory for layered superconductors. It is shown that, with decreasing temperature the anisotropy parameter γ_{H_{c2}}(T) is increased, while theLondon penetration depth anisotropy γλ(T) reveals n opposite behavior.Results of our calculations are in agreement with experimental datafor single crystal MgB₂ nd with other calculations. Results of an analysis of magnetic field H_c1 in a single vortex between superconducting layers are also presented.     

Anisotropy of the upper critical field in c-axis oriented MgB2 thin films

C-axis oriented MgB_2 thin films were synthesized on single-crystal MgO (111) substrates using a chemical vapour deposition technique. The as-formed films revealed a sharp superconducting transition temperature of 38K with the transition width 0.2K. The temperature dependence of the upper critical magnetic field H_{c2}(T) in the films was determined via resistivity for magnetic field H parallel and perpendicular to the c axis of the films. Using the Werthamer－Helfand－Hohenberg formula, we obtained the anisotropy ratio of the upper critical field γ=1.2.