Heavy fermion superconductivity

The quest for a precise identification of the symmetry of the order parameter in heavy fermion systems has really started with the discovery of the complex superconducting phase diagram in UPt₃. About 10 years latter, despite numerous experiments and theoretical efforts, this is still not achieved, and we will quickly review the present status of knowledge and the main open question. Actually, the more forsaken issue of the nature of the pairing mechanism has been recently tackled by different groups with macroscopic or microscopic measurement, and significant progress have been obtained. We will discuss the results emerging from these recent studies which all support non-phonon-mediated mechanisms.     

Unconventional superconductivity and magnetism in Sr2RuO4 and related materials

We review the normal and superconducting state properties of the unconventional triplet superconductor Sr₂RuO₄ with an emphasis on the analysis of the magnetic susceptibility and the role played by strong electronic correlations. In particular, we show that the magnetic activity arises from the itinerant electrons in the Ru d‐orbitals and a strong magnetic anisotropy occurs (χ^+‐ < χ^zz) due to spin‐orbit coupling. The latter results mainly from different values of the g‐factor for the transverse and longitudinal components of the spin susceptibility (i.e. the matrix elements differ). Most importantly, this anisotropy and the presence of incommensurate antiferromagnetic and ferromagnetic fluctuations have strong consequences for the symmetry of the superconducting order parameter. In particular, reviewing spin fluctuation‐induced Cooper‐pairing scenario in application to Sr₂RuO₄ we show how p‐wave Cooper‐pairing with line nodes between neighboring RuO₂‐planes may occur. We also discuss the open issues in Sr₂RuO₄ like the influence of magnetic and non‐magnetic impurities on the superconducting and normal state of Sr₂RuO₄. It is clear that the physics of triplet superconductivity in Sr₂RuO₄ is still far from being understood completely and remains to be analyzed more in more detail. It is of interest to apply the theory also to superconductivity in heavy‐fermion systems exhibiting spin fluctuations.     

Spontaneous internal field around the non-magnetic impurity in spin-triplet p-wave superconductivity

The electronic structure around an impurity in spin triplet p-wave superconductors is investigated by the Bogoliubov–de Gennes theory on a tight-binding model. We calculate the spontaneous current and the local density of states around the impurity and discuss the difference between -wave and -wave superconducting states.     

Anisotropy of p-wave Josephson junction

<正>The anisotropy of the dc Josephson current in the superconducting junctions of the p-wave equal spin pairing symmetry is theoretically investigated by the Furusaki-Tsukada-like formula.The current phase relations exhibit different oscillation periods and different phase shifts for the current along different directions,respectively.     

Competing spin waves and superconducting fluctuations in strongly correlated electron systems

A special diagram technique recently proposed for strongly correlated electron systems is used to study the peculiarities of a spin-density wave (SDW) in competition with superconductivity. This method allows formulation of the Dyson equations for the renormalized electron propagators of the co-existing phases of SDW antiferromagnetism and superconductivity. We find the surprising result that triplet superconductivity appears provided that we have the co-existence of singlet superconductivity and SDW antiferromagnetism. A special ansatz, which takes into account the full Green's functions as well as the dynamical structure of the correlations, is used to establish the equations determining the gap functions and order parameters.     

Temperature and magnetic field dependence of superconductivity in nanoscopic metallic grains

We study pairing correlations in ultrasmall superconductor in the nanoscopic limit by means of a toy model where electrons are confined in a single, multiply degenerate energy level. We solve the model exactly to investigate the temperature and magnetic field dependence of number parity effect (dependence of ground state energy on evenness or oddness of the number of electrons). We find a different parity effect parameter to critical temperature ratio (4 rather than 3.5) which turns out to be consistent with exact solution of the BCS gap equation for our model. This suggests the equivalence between the parity effect parameter and the superconducting gap. We also find that magnetic field is suppressed as temperature increases.     

Structurally tuned superconductivity in heavy-fermion CeMIn5 (M=Co, Ir, Rh)

We discuss systematic trends in the high-temperature physical properties of the heavy Fermion superconductors (HFS) CeCoIn₅ (T_c=2.3 K, γ=300 mJ/molK²), CeIrIn₅ (T_c=0.4 K, γ=750 mJ/molK²), and CeRhIn₅ (P_c=16 kbar, T_c=2.1 K, γ=400 mJ/molK²) in terms of crystalline-electrical-field effects(CEF). We suggest the possibility that the interplay between the symmetry of the CEF ground-state (or low-T CEF scheme of levels) and the f–s hybridization could generate spin fluctuations relevant to the superconducting pairing mechanism in these materials. This hypothesis may provide insight into the fact that some crystal structures appear to favor superconductivity. Further, CeMIn₅ (M=Co, Ir, Rh) appear to be structural relatives of the cubic heavy Fermion superconductor CeIn₃, but with much higher T_c's. We argue that structural layering inherent in the tetragonal CeMIn₅ crystal structure determines the magnetic and electronic anisotropy responsible for the enhanced T_c's. We also describe similarities and differences between these compounds and the high-T_c cuprates.     

Unconventional superconductivity pairing induced by antiferromagnetic spin fluctuations in layered organic superconductors

The unconventional character of the superconductivity in organic compounds κ-(ET)₂X is ascribed to an antiferromagnetic spin fluctuation induced pairing. Since the band structure involves two bands (±), we assume that the large amplitude spin fluctuations arise from the band with the best nesting properties (band +), while superconductivity pairing occurs in the other band (-).We show that the nesting properties may mimic either a chemical pressure (deuterization) or a hydrostatic one. Indeed, a change of the nesting ratio t₁/t₂, according to our model, induces a modification of the Fermi surface topology. The spin fluctuations strength in the system is affected and consequently the calculated effective coupling constant of superconductivity for the even-parity singlet pairing channel. Our theory appears to be qualitatively consistent with major experimental reports.     

射频超导腔的研究新进展

 射频超导谐振腔已经大规模地应用到粒子加速器领域,其优越之处在于它可以在CW模式或长宏脉冲模式下,提供高的加速梯度。射频超导已经成为自由电子激光和能量回收直线加速器的关键技术。经过30多年的研究发展,解决了超导腔的热崩溃、场致发射等诸多关键问题,目前加速梯度已经超过40 MV/m。高加速梯度的获得是射频超导领域的前沿热点,电抛光＋低温热处理技术使射频超导腔的加速梯度提高3～4 MV/m。最新发展起来的超导腔的干式处理可以改善超导腔的表面状况,提高超导腔的Q值,抑制次级电子发射效应,有可能成为提高超导腔性能的又一有效手段。     

Spin-polarized quasiparticle transport in ferromagnet/insulator/p-wave superconductor junction

We have studied the tunneling conductance in ferromagnet/insulator/p-wave superconductor junctions, taking into account the rough interface scattering effect. We find that there exist zero-bias conductance peaks and single-minimum structure in tunneling spectroscopy. As the exchange energy increases, the Andreev reflection is always suppressed and the differential conductance decreases. The differential conductance depends on the barrier strength and the roughness at the interface. Supported by the Natural Science Foundation of Jiangsu Higher Education Institutions, China (Grant No. 06KJB140009)     

射频超导腔的研究新进展

射频超导谐振腔已经大规模地应用到粒子加速器领域,其优越之处在于它可以在CW模式或长宏脉冲模式下,提供高的加速梯度。射频超导已经成为自由电子激光和能量回收直线加速器的关键技术。经过30多年的研究发展,解决了超导腔的热崩溃、场致发射等诸多关键问题,目前加速梯度已经超过40 MV/m。高加速梯度的获得是射频超导领域的前沿热点,电抛光＋低温热处理技术使射频超导腔的加速梯度提高3～4 MV/m。最新发展起来的超导腔的干式处理可以改善超导腔的表面状况,提高超导腔的Q值,抑制次级电子发射效应,有可能成为提高超导腔性能的又一有效手段。     

Quantitative conditions for the formation of p-wave neutron halos

With the experimental binding energies and configurations, the root-mean-square radii of p-wave valence neutron distributions for nuclei up to the second p-shell have been calculated in the framework of the single-particle potential model. By analyzing the relations between the radii and the binding energies, the scaling laws of p-wave valence neutron distributions are obtained. The quantitative conditions for the formation of p-wave neutron halos are deduced from these scaling laws. Based on the investigation on the probability of finding the valence nucleon outside the range of the interaction potential, a 2p3/2 halo state in 47S is anticipated for the first time. These obtained results might provide reference for searching for p-wave neutron halos in nuclei up to the second p-shell.     

Tunneling Conductance in Normal Metal/Insulator/Triplet Superconductor Junction

Tunneling conductance in normal metal/insulator/triplet superconductor junctions is studied theoretically as a function of the bias voltage at zero temperature and finite temperature. The results show there are zero-bias conductance peak, zero-bias conductance dip and double-minimum structures in the spectra for p-wave superconductor junctions. The existence of such structures in the conductance spectrum can be taken as evidence that the pairing symmetry of Sr2RuO4 is p-wave symmetry.     

Tunneling Conductance in Normal Metal/Insulator/Triplet Superconductor Junction

Tunneling conductance in normal metal/insulator/triplet superconductor junctions is studied theoretically as a function of the bias voltage at zero temperature and finite temperature. The results show there are zero-bias conductance peak, zero-bias conductance dip and double-minimum structures in the spectra for p-wave superconductor junctions. The existence of such structures in the conductance spectrum can be taken as evidence that the pairing symmetry of Sr2RuO4 is p-wave symmetry.     

Quantitative conditions for the formation of p-wave neutron halos

With the experimental binding energies and configurations, the root-mean-square radii of p-wave valence neutron distributions for nuclei up to the second p-shell have been calculated in the framework of the single-particle potential model. By analyzing the relations between the radii and the binding energies, the scaling laws of p-wave valence neutron distributions are obtained. The quantitative conditions for the formation of p-wave neutron halos are deduced from these scaling laws. Based on the investigation on the probability of finding the valence nucleon outside the range of the interaction potential, a 2p_3/2 halo state in ⁴⁷S is anticipated for the first time. These obtained results might provide reference for searching for p-wave neutron halos in nuclei up to the second p-shell.     

具有p波超流的一维非公度晶格中迁移率边研究

研究了具有p波超流的一维非公度晶格中迁移率边的性质. 发现适当的p波超流可以增加体系中的迁移率边的数目, 并且通过多分形分析确定了迁移率边所在的位置.     

Factors affecting the superconductivity in the process of depositing Nd1.85Ce0.15CuO4-δ by the pulsed electron deposition technique

On SrTiO3 single crystal substrate, by using the pulsed electron deposition technique, the high-quality electron doped Nd1.85Ce0.15CuO4-δsuperconducting film was successfully fabricated. After careful study on the R-T curves of the obtained samples deposited with different substrate temperatures, thicknesses, annealing methods and pulse frequencies, the effects of them on the superconductivity of the films were found, and the reasons were also analyzed. Additionally, by using the same model of the pulsed laser deposition technique, the relation between the target-to-substrate distance and the deposition pressure was drawn out as a quantitative one.     

Pressure induced superconductivity in Bismuth

Abstract

In this paper, we report the high pressure band structure and superconductivity calculated for Bi at 9 GPa. The band structure calculation was performed using the LMTO method. The theoretically calculated value of superconducting transition temperature is found to be in good agreement with the experimentally observed value. It is concluded that the same mechanism of s → electron transfer under pressure, which has been used by us for explaining the pressure induced superconductivity in many solids, is found to be responsible for promoting T, in Bi also.     

Non‐interacting single‐impurity Anderson model: solution without using the equation‐of‐motion method

Ground‐state properties of the non‐interacting symmetric single‐impurity Anderson model (SIAM) are derived from the corresponding eigenenergy equation. Explicit formulae are given for the ground‐state energy, the hybridization, and the momentum distribution that are essential quantities for variational approaches to the interacting model. Various spectral functions, e.g., the total density of states, the phase shift function, and the impurity spectral function, are shown to agree with those obtained from the equation‐of‐motion method (see supplementary material). For a constant hybridization strength and a semi‐elliptic host density of states it is seen that the impurity spectral function builds up weight at the band edges.

     

正常金属/自旋三重态p波超导体结隧道谱的奇异性

利用Blonder，Tinkham和Klapwijk理论计算了正常金属/绝缘层/正常金属/自旋三重态的p波超导体结的隧道谱和平均电流.计算结果表明：在自旋三重态p波超导结的隧道谱中存在零偏压电导峰、零偏压电导凹陷和双凹陷结构，并有微分电导随偏压震荡的现象出现，在I-V曲线上出现电流台阶.这些结果在理论上支持Sr₂RuO₄的超导态是自旋三重态p波超导态. 关键词： 自旋三重态超导体 p波超导体 隧道谱