The upper critical field in two-band layered superconductors

The upper critical field of clean MgB₂ is investigated using the two-band layered Ginzburg--Landau (GL) theory. The calculated results are fitted to the experimental data of clean MgB₂ crystal very well in a broad temperature range. Based on the GL theory for clean superconductors, a phenomenological theory for dirty superconductor is proposed. Selecting appropriate parameters, two-band layered GL theory is successfully applied to the crystal of Mg(B_1-xC_x)₂ and the neutron irradiation samples of MgB₂.     

Upper critical field of anisotropic superconductors

The upper critical field of anisotropic superconductors described by the effective mass model is calculated in the general direction of the applied magnetic field.     

Critical magnetic field ratio of anisotropic magnetic superconductors

The upper critical field, the lower critical field and the critical magnetic field ratio of anisotropic magnetic superconductors are calculated by Ginzburg–Landau theory analytically. The effect of the Ginzburg–Landau parameter (κ₀), magnetic susceptibility (χ) and magnetic-to-anisotropic parameter ratio (θ) on the critical field ratio are considered. We find that the value of critical field ratio increases with increasing κ₀ and θ, and decreases with increasing χ. The highest and the lowest value of critical field ratio is found in the diamagnetic superconductors and the ferromagnetic superconductors, respectively.     

Investigate the effect of anisotropic order parameter on the specific heat of anisotropic two-band superconductors

The effect of anisotropic order parameter on the specific heat of anisotropic two-band superconductors in BCS weak-coupling limit is investigated. An analytical specific heat jump and the numerical specific heat are shown by using anisotropic order parameters, and the electron–phonon interaction and non-electron–phonon interaction. The two models of anisotropic order parameters are used for numerical calculation that we find little effect on the numerical results. The specific heat jump of MgB₂, Lu₂Fe₃Si₅ and Nb₃Sn superconductors can fit well with both of them. By comparing the experimental data with overall range of temperature, the best fit is Nb₃Sn, MgB₂, and Lu₂Fe₃Si₅ superconductors.     

Gapless Fermi surfaces of anisotropic multiband superconductors in a magnetic field

We propose that a new state with a fully gapless Fermi surface appears in quasi-2D multiband superconductors in magnetic field applied parallel to the plane. It is characterized by a paramagnetic moment caused by a finite density of states on the open Fermi surface. We calculate thermodynamic and magnetic properties of the gapless state for both s-wave and d-wave cases, and discuss the details of the first order metamagnetic phase transition that accompanies the appearance of the new phase in s-wave superconductors. We suggest possible experiments to detect this state both in the s-wave (2-H NbSe2) and d-wave (CeCoIn5) superconductors.     

NMR line shape in anisotropic superconductors in a tilted magnetic field

NMR line shape has been constructed for anisotropic type-II superconductors in tilted magnetic fields, with inclusion of vortex-lattice magnetic-field nonuniformities and of the skin effect near the superconductor surface. The NMR line shape parameters are shown to change considerably when the external magnetic field changes direction. This makes it possible to obtain more detailed information about the characteristics of a superconductor, in particular, its anisotropy parameter. Fiz. Tverd. Tela (St. Petersburg) 40, 993–997 (June 1998)     

Fermi surface topology and the upper critical field in two-band superconductors: application to MgB2

Recent measurements of the anisotropy of the upper critical field B(c2) on MgB2 single crystals have shown a puzzling strong temperature dependence. Here, we present a calculation of the upper critical field based on a detailed modeling of band structure calculations that takes into account both the unusual Fermi surface topology and the two gap nature of the superconducting order parameter. Our results show that the strong temperature dependence of the B(c2) anisotropy can be understood as an interplay of the dominating gap on the sigma band, which possesses a small c-axis component of the Fermi velocity, with the induced superconductivity on the pi-band possessing a large c-axis component of the Fermi velocity. We provide analytic formulas for the anisotropy ratio at T=0 and T=T(c) and quantitatively predict the distortion of the vortex lattice based on our calculations.     

二带超导体上临界磁场的各向异性

采用二带Ginziburg-Landau(G-L)理论模型计算了各向异性超导体的上临界磁场,发现上临界磁场Hc2的解析式中包含序参量的各向异性和有效质量的各向异性;上临界磁场ab-面内分量Hc|2|ab、c-轴方向分量Hc|2|ab与实验数据符合;上临界磁场比率在实验许可范围之内。     

The Ginzburg-Landau expansion and the slope of the upper critical field in superconductors with anisotropic normal-impurity scattering

We carry out the Ginzburg-Landau expansion for superconductors with anisotropic s and d pairing in the presence of anisotropic normal-impurity scattering, which enhances the stability of d pairing with respect to disordering. We find that the slope of the curve of the upper critical field, |dH _c2/dT|_{T c}, in superconductors with d pairing behaves nonlinearly as disorder grows: at low scattering anisotropy the slope rapidly decreases with increasing impurity concentration, then gradually but nonlinearly increases with concentration, reaches its maximum, and then rapidly decreases, vanishing at the critical impurity concentration. In superconductors with anisotropic s pairing, |dH _c2/dT|_{T c} always increases with impurity concentration, finally reaching the familiar asymptotic value characteristic of the isotropic case, irrespective of whether there is anisotropic impurity scattering. Zh. éksp. Teor. Fiz. 112, 2124–2133 (December 1997)     

Theory of the upper critical field in layered superconductors with magnetic intercalates

We propose a model for the layered superconductors with electronically isolated magnetic intercalates in which the electrons propagate freely in the dirty superconducting layers with arbitrarily strong spin-orbit scattering, and spin-flip scatter off the magnetic ions during interlayer tunneling. We calculate the upper critical field H_c2 including demagnetization effects. In a parallel field below the dimensional-crossover temperature T¹, a new type of spin ordering is predicted.     

The theory of ultrasonic attenuation in two-band superconductors

The ultrasonic attenuation coefficient for longitudinal waves is formulated for two-band superconductors. The resulting expression differs from that previously obtained by Fal'ko.     

Time-reversal symmetry-breaking in two-band superconductors

Competition between an interband Josephson interaction and a biquadratic interband interaction can break time-reversal symmetry when the interband interactions are very weak compared with the intraband interaction. We demonstrate this using the phenomenological Ginzburg–Landau free energy, taking into account terms essential for this phenomenon. This time-reversal symmetry-breaking state peels away a sheet of an interband phase difference soliton wall. This sheet is the domain wall between two different chiral states. The peeling reduces the formation energy of the domain wall. The domain wall can simultaneously erase the Meissner effect and the specific heat jump when the entropy invokes many domain walls, and its life time is prolonged by a pinning owing to an inhomogeneity such as surface roughness.     

The transition temperature and critical magnetic field of the localized superconductive states for space-inhomogeneous superconductors

A theory of the proximity effects in the inhomogeneous dirty superconductors has been developed. The critical magnetic field as a function of temperature and of the length of the inhomogeneity has been investigated. The angular dependence has been obtained.     

Extended Ginzburg-Landau formalism for two-band superconductors

Recent observation of unusual vortex patterns in MgB(2) single crystals raised speculations about possible "type-1.5" superconductivity in two-band materials, mixing the properties of both type-I and type-II superconductors. However, the strict application of the standard two-band Ginzburg-Landau (GL) theory results in simply proportional order parameters of the two bands-and does not support the "type-1.5" behavior. Here we derive the extended GL formalism (accounting all terms of the next order over the small τ=1-T/T(c) parameter) for a two-band clean s-wave superconductor and show that the two condensates generally have different spatial scales, with the difference disappearing only in the limit T→T(c). The extended version of the two-band GL formalism improves the validity of GL theory below T(c) and suggests revisiting the earlier calculations based on the standard model.     

用两带Ginzburg-Landau理论分析两带超导体Lu2Fe3Si5的表面临界磁场

用两带Ginzburg-Landau(GL)理论分析了Lu₂Fe₃Si₅的表面临界磁场,当超导体的表面与任一主晶面重合,且外磁场平行于超导体的表面时,比值H_c3/H_c2(H_C2是Lu₂Fe₃Si₅的上临界磁场)强烈依赖于温度. 当超导体的表面是bc平面,且 关键词： 两带超导体 GL理论 2Fe₃Si₅')" href="#">Lu₂Fe₃Si₅ 表面临界磁场