London penetration depth and thermal fluctuations in the sulphur hydride 203 K superconductor

Recently, compressed H₂S has been shown to become superconducting at 203 K under a pressure of 155 GPa. One might expect fluctuations to dominate at such temperatures. Using the magnetisation critical current, we determine the ground‐state London penetration depth, λ₀=189 nm, and the superconducting energy gap, Δ₀=27.8 meV, and find these parameters are similar to those of cuprate superconductors. We also determine the fluctuation temperature scale, K, which shows that, unlike the cuprates, of the hydride is not limited by fluctuations. This is due to its three dimensionality and suggests the search for better superconductors should refocus on three‐dimensional systems where the inevitable thermal fluctuations are less likely to reduce the observed .

     

Direct observation of nonlocal effects in a superconductor

We have used the technique of low energy muon spin rotation to measure the local magnetic field profile B(z) beneath the surface of a lead film maintained in the Meissner state (z depth from the surface, z less, similar 200 nm). The data unambiguously show that B(z) clearly deviates from an exponential law and represent the first direct, model independent proof for a nonlocal response in a superconductor.     

Effective "penetration depth" in the vortex state of a d-wave superconductor

The temperature and the field dependence of the effective magnetic penetration depth (lambdaeff) in the vortex state of a d-wave superconductor, as measured by muon spin rotation (muSR) experiments, is calculated using a nonlocal London model. We show that at temperatures below [EQUATION: SEE TEXT], the linear T dependence of lambda-2eff crosses over to a T3 dependence. This could provide an explanation for the low temperature flattening of the lambda-2eff curve observed in a recent muSR experiment.     

Direct observation of rectified motion of vortices in a niobium superconductor

Using Lorentz microscopy to directly image vortices, we investigate vortex motion control and rectification in a niobium superconductor. We directly observe a net motion of vortices along microfabricated channels with a spatially asymmetric potential, even though the vortices were driven by an oscillatory field. By observing the individual motion of vortices, we clarify elementary processes involved in this rectification. To further demonstrate the ability to control the motion of vortices, we created a tiny vortex "racetrack" to monitor the motion of vortices in a closed circuit channel.     

London penetration depth and coherence peak in ammonia-intercalated fulleride superconductors

The compound (NH₃)_xNaRb₂C₆₀ (x ～ 1.6) has been studied by¹H nuclear magnetic resonance. The proton line shows no shift at any temperature, while lineshape analysis of the low-temperature (30 K) spectra reveals the presence of a Knight shift interaction of pure anisotropic nature, whose amplitude has been estimated. Relaxation measurements show a single-exponential recovery of the magnetization at all investigated temperatures and fields. A Korringa behavior is observed below 65 K and a clear Hebel-Slichter coherence peak is detected just below T_c. The London penetration depth can be estimated from the line broadening observed below T_c, which yields λ ≈ 825 nm.     

Magnetothermal instabilities in an organic superconductor

We have studied the occurrence of magnetothermal instabilities in a single crystal during field sweep magnetization experiments, equivalent to short time relaxation studies. We find instability behaviour in good agreement with a recent model by Mints, for a non-linear E(J) characteristic. In particular, we find that a decrease of the dynamic relaxation rate, characterizing the effective activation energy, precedes the unstable regime. We point out formal analogies between such instabilities and the general predictions for flux avalanches. Received: 11 December 1997 / Revised: 15 January 1998 / Accepted: 31 March 1998     

Measurements of the London penetration depth in Bi-based high-T c compounds

The muon spin relaxation was measured in the high-T _c superconductors Bi₂Sr_1.3Ca_0.7 CuO_6.15, Bi₂Sr₂CaCu₂O₈ and Bi₂Pb _x Sr₂Ca₂Cu₃O₁₀. We present our method to determine the London penetration depth perpendicular toc-axis λ_⊥ (0) taking also contributions from the anisotropic magnetization and nuclear dipoles to the muon frequency spectrum into account.     

Direct observation of the flux-line vortex glass phase in a type II superconductor

The order of the vortex state in La1.9Sr0.1CuO4 is probed using muon-spin rotation and small-angle neutron scattering. A transition from a Bragg glass to a vortex glass is observed, where the latter is composed of disordered vortex lines. In the vicinity of the transition the microscopic behavior reflects a delicate interplay of thermally induced and pinning-induced disorder.     

STM spectroscopy of an organic superconductor

Electron tunneling spectroscopy of the organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂using low temperature scanning tunneling microscope (STM) is reported. The tunneling differential conductance in the superconducting phase was obtained in theb–cplane of a single crystal, by varying the tip position on the sample surface. The differential conductance is reduced near zero bias voltage and enhanced at the gap edge, associated with the superconducting gap structure below[formula] K. The gap width differs slightly from sample to sample, while the overall functional shape of the conductance is sample-independent. The tunneling conductance is reduced to almost zero near zero bias voltage, while it is finite inside the gap edge. The curve obtained cannot be fit to the BCS density of states withs-wave pairing symmetry, even if the life-time broadening of one-electron levels is taken into account. Finite conductance inside the gap edge suggests anisotropy of the gap. However, the conductance curve obtained is not explained by a simpled-wave symmetry for Δ(k). The reduced conductance near zero bias voltage suggests a finite gap. An anisotropic model with a finite gap, in which Δ(k) varies depending on the direction ink-space, is examined. The tunneling conductance in the low-energy region is almost fit by the model with Δ_min = 2 meV and Δ_max = 6 meV. The finite conductance is explained by introducing a small effect of life time broadening. We conclude that the gap is anisotropic and is finite (at least Δ_min = 2 meV) on the entire Fermi surface.     

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.