Superconductivity in ultrathin films II. Structure in tunneling curves

Additional structure is seen in conductance versus voltage curves when electrons are tunneled into very thin superconducting films. This peak like structure has been detected in several materials; it is observed at multiples of the energy gap above the main conductance maximum. The peak amplitude is very thickness dependent. At roughly half the transition temperature of the ultrathin film the amplitude goes to zero within a narrow temperature range. Effects of magnetic field, an overlay of a non-superconductor, and annealing are less pronounced. The most probable explanation seems to be a model where the tunneling rate is stimulated whenever the relaxation and recombination phonon frequencies equal.Supported by the Swedish Natural Science Research Council.     

高压高温处理对YNi_2B_2C晶界效应的影响

在测量经压力处理的氩弧熔炼的YNi2 B2 C的金属间化合物超导体中场冷磁化率时 ,观察到升温磁化率曲线在临界温度附近有一个负的磁化率峰。通过研究高压高温处理对粉末样品和块状样品的场冷和零场冷却磁化率曲线的影响 ,认为场冷时反常负磁化率峰是由于样品中晶界的存在 ,从而导致晶粒间的弱连接所引起的 ,而不是由磁相变或超导和磁性共存现象所引起的。并认为有可能利用高压高温技术制备这类化合物和生长单晶。     

Scanning tunneling spectroscopy in MgB2

We present scanning tunneling microscopy measurements of the surface of superconducting MgB2 with a critical temperature of 39 K. In zero magnetic field the conductance spectra can be analyzed in terms of the standard BCS theory with a smearing parameter gamma. The value of the superconducting gap is 5 meV at 4.2 K, with no experimentally significant variation across the surface of the sample. The temperature dependence of the gap follows the BCS form, fully consistent with phonon-mediated superconductivity in this novel superconductor. The application of a magnetic field induces strong pair breaking as seen in the conductance spectra in fields up to 6 T.     

Perturbation Theory in the Problem of Light Scattering by a Dielectric Body

In this letter, we describe the observation of a magnetic field dependent electronic gap, suggestive of local superconductivity, in the point-contact spectrum of microcrystalline graphite. Magnetic field dependent point-contact spectroscopy has been carried out at a temperature of 1.8 K using an etched aluminum tip. At zero field, a gap structure in the differential conductance is observed, showing a gap of Δ = 4.2 meV. On applying magnetic fields of up to 500 mT, this gap gradually closes, following the theoretical prediction by Ginzburg and Landau for a fully flux-penetrated superconductor. By applying BCS-theory, we infer a critical superconducting temperature of 14 K.     

Lee-Yang zeros of periodic and quasiperiodic anisotropic XY chains in a transverse field

The partition function zeros of the anisotropic XY chain in a complex transverse field are studied analytically and numerically. It is found that the partition function zeros of the periodic and quasiperiodic quantum Ising chain lie on the circle at zero temperature and the radius equal to the values of the critical field. For the periodic and quasiperiodic anisotropic XY chains, the closed curves are split to two or three closed curves as the anisotropic parameter gamma decreases at a given ratio of two kinds of exchange interactions. For the isotropic XX case, the partition function zeros lie on the straight segments which are parallel to the real axis and the segments move towards the real axis as the temperature goes to zero.     

Critical temperature and energy gap in superconductors with singular density of states

The influence of-functional (symmetric as well as asymmetric) singularities in density of states (DOS) on the critical temperature and zero temperature energy gap is calculated. Surprisingly, we have obtained the same function for the off-symmetry of the peak position in DOS on the corresponding critical temperature as for the temperature dependence of the energy gap in the strong-coupling limit. The enhancement of the critical temperature due to the singularity (compared with the constant DOS near the Fermi surface) is much lower for strong-coupling superconductors than in the weak-coupling limit. Hence, the singularity in DOS cannot be the exclusive reason for large values of critical temperatures in highTc superconductors.This work was supported by the grant GA SAV 188/1991.     

Critical data of niobium nitride in transverse magnetic fields

Critical fields and critical currents of a great number of niobium nitride samples with and without different amounts of zirconium have been measured. The samples for these measurements have been prepared by diffusion of nitrogen into heated niobium and niobium-zirconium wires under various conditions. For the zero upper critical field of NbN the extrapolated value (153±3) kOe has been found. The quenching curves of most of the samples show very pronounced peak effects in the high field region with a steep slope close to the upper critical field, which decreases with increasing amount of zirconium.     

Above-gap and subgap differential conductance anomaly in concentrated magnetic semiconductor Zn0.32Co0.68O1 − v/Pb superconductor hybrid junctions

The Zn_0.32Co_0.68O_1 − v/Pb hybrid junctions were prepared, where the concentrated magnetic semiconductor Zn_0.32Co_0.68O_1 − v is in the region of variable range hopping transport instead of the ballistic or diffusive transport. The high differential conductance peak at gap voltage and two above-gap peaks were observed below the superconducting critical temperature. Moreover, both the zero bias conductance peak and the finite bias conductance peak were observed below the gap voltage. All these differential conductance peaks systematically evolve and finally disappear as the temperature or the magnetic field increases. These transport phenomena were explained by phase coherent Andreev reflection in the presence of strong disorder, magnetic impurity scattering, and spin polarization.     

Nmr evidence for coexistence of superconductivity and ferromagnetic component in magnetic superconductor RuSr2YCu2O8: 99,101Ru and 63Cu NMR.

From Ru- and Cu-NMR studies, we present evidence for coexistence of superconductivity and ferromagnetism in a cuprate superconductor RuSr2YCu2O8 (RuY1212). The observation of a large enhancement of a radio-frequency field for the Ru-NMR signal at zero field reveals the existence of a ferromagnetic (FM) component in the ordered RuO2 plane below a Curie temperature of TM = 150 K. Just below the onset temperature of superconductivity T(onset)c = 45 K, a remarkable decrease of the nuclear spin-lattice relaxation rate 1/T1 was observed within the ordered RuO2 plane as well as the CuO2 plane, revealing that the superconducting gap coexists with the FM component in the RuO2 plane on a microscopic scale. In addition, from the observation of a sharp peak in 101(1/T1) at T(zero)c approximately 23 K where the resistivity becomes zero, we suggest that the motion of self-induced vortices originating from fluctuations of the FM component induces the resistivity between T(onset)c and T(zero)c in RuY1212.     

平行陶瓷棒介质阻挡放电分段模型等效参数预测

为实现对介质阻挡放电负载模型等效参数的有效预测,引入放电区域面积作为中间变量。以平行陶瓷棒为负载,通过Maxwell有限元仿真得到恒压静电场下的负载等效电容与放电区域的对应关系。结合恒压静电场下的电场分布,提出了放电区域逐步扩张下的气隙首次击穿电压、负载外加电压峰值及气隙放电维持电压的预估方法。进而,根据李萨如图形法求得各个工作点的功率。至此,建立起了放电区域同各个等效参数间的量化关系,并实现了对参数的预测。最后,对气隙间距为1,3,4 mm的工况进行了实验验证。实验结果表明:气隙放电维持电压预测值在局部变化趋势上与实测值存在一定差异;放电功率、负载外加电压峰值预测结果与实测值较为吻合。     

A microscopic theory of the lambda transition

Starting with a microscopic hamiltonian for a many-boson system with a hardcore interaction, the grand potential of the system, which contains the order-parameter of the lambda transition as one of the thermodynamical variables, is derived by making use of the finite temperature loop expansion. The divergence difficulty caused by the hardcore interaction is circumvented by the conventional field theoretic perturbational renormalization such that the chemical potential is renormalized instead of the conventional mass renormalization. The grand potential obtained consists of the superfluid part and the finite temperature elementary excitation part. The elementary excitation energy spectrum shows the Goldstone boson mode, namely, the photon, for the zero external field. A non-vanishing external field destroys such a Goldstone boson mode by causing an energy gap at zero momentum. The chemical potential and the critical temperature are also obtained for the weak coupling case. It is shown how the Bose-Einstein condensation is affected by the hardcore interaction.     

Magnetic-field-induced spin excitations and renormalized spin gap of the underdoped La1.895Sr0.105CuO4 superconductor

High-resolution neutron inelastic scattering experiments in applied magnetic fields have been performed on La1.895Sr0.105CuO4 (LSCO). In zero field, the temperature dependence of the low-energy peak intensity at the incommensurate momentum transfer QIC=(0.5,0.5+/-delta,0),(0.5+/-delta,0.5,0) exhibits an anomaly at the superconducting Tc which broadens and shifts to lower temperature upon the application of a magnetic field along the c axis. A field-induced enhancement of the spectral weight is observed, but only at finite energy transfers and in an intermediate temperature range. These observations establish the opening of a strongly downward renormalized spin gap in the underdoped regime of LSCO. This behavior contrasts with the observed doping dependence of most electronic energy features.     

Signatures of superfluidity for Feshbach-resonant Fermi gases

We consider atomic Fermi gases where Feshbach resonances can be used to study the whole BCS-Bose-Einstein condensate crossover. We show how a probing field transferring atoms out of the superfluid can be used to detect the onset of the superfluid transition in the high-T(c) and BCS regimes. The number of transferred atoms, as a function of the energy given by the probing field, peaks at the gap energy. The shape of the peak is asymmetric due to the single particle excitation gap. Since the excitation gap also includes a pseudogap contribution, the asymmetry alone is not a signature of superfluidity. The incoherent nature of the noncondensed pairs leads to broadening of the peak. The broadening decays below the critical temperature, causing a drastic increase in the asymmetry. This provides a signature of the transition.     

Phase diagram of the electron-doped La2-xCexCuO4 cuprate superconductor from Andreev bound states at grain boundary junctions

We use quasiparticle tunneling across La2-xCexCuO4 grain boundary junctions to probe the superconducting state and its disappearance with increasing temperature and magnetic field. A zero bias conductance peak due to zero energy surface Andreev bound states is a clear signature of the phase coherence of the superconducting state. Hence, such a peak must disappear at or below the upper critical field Bc2(T). For La2-xCexCuO4 this approach sets a lower bound for Bc2(0) approximately 25 T which is substantially higher than reported previously. The method of probing the superconducting state via Andreev bound states should also be applicable to other cuprate superconductors.     

Quantum fluctuations in low-dimensional easy-plane spin models

A self-consistent harmonic approximation is used to treat the low temperature limit of the one and two dimensional S=1 easy plane magnets. For the one dimensional (1D) model, we calculate the gap caused by the presence of an external magnetic field applied in the easy-plane. The quantum phase transition of the one-dimensional model at T=0 is also studied. For the two-dimensional case, the Kosterlitz-Thouless (KT) transition temperature as a function of a single-ion anisotropy term is calculated. The line ends at a quantum critical point, where the KT temperature goes to zero.     

Relationship between the critical field and energy gap of a superconductor

The theory of Lewis concerning the relationship between the critical field and the exponent of the superconducting electronic specific heat is found to be verified over an extended region of this exponent in the light of new data. If the identification of this exponent with the zero temperature energy gap is valid, then the theory of Toxen, for large energy gaps, is found to be incorrect.     

Dynamics of the anisotropic two-dimensional XY model

In this paper we study the dynamics of the two-dimensional XY model with single-ion anisotropy, and spin S = 1, in the large D phase, and low temperatures, using the bond operator formalism. The in-plane structure factor is a delta function. The out of plane shows a three peak structure, which merges in a single peak at the Brillouin zone boundary. We analyze also spin currents generated by a magnetic field gradient. The spin conductivity is calculated, at finite temperature, using the Kubo formula. The model shows unconventional ballistic spin transport at finite temperature. The computed spin conductivity exhibits a nonzero Drude weight at finite temperature. For ω< 2m, where m is the energy gap, the spin conductivity is described solely by the Drude weight. There is a regular contribution to the spin conductivity for ω> 2m, which persist in the zero temperature limit. The conductivity at the critical point, and for small frequencies, is (gμ_B)²/ħ times a universal scaling function of ħω/k_B T.     

Spin reorientation and magnon-phonon hybridization in a ferromagnet

A model is presented for the magnetic excitations and magnon-phonon coupling in a localised moment ferromagnet in which spins can reorientate by application of a magnetic field. The model is suitable for those materials which possess a spin wave gap at zero wave vector and therefore the magnon and acoustic phonon branches can intersect. A magnon-phonon coupling linear in both spin and phonon operators is employed which has proved successful for the ferrous salts. The main effect of the applied field is to modify the spin wave gap, and to introduce a critical value for the coupling constant which enables the system to remain stable as the gap goes to zero. Furthermore the wave vector of the anticrossing point decreases as the spin wave gap increases and therefore the value of the sound velocity determined by high resolution inelastic neutron experiments is dependent on the gap.     

横向随机晶场Ising模型的相图和磁化行为研究

在有效场理论和切断近似框架内，选择自旋S=1的二维方格子，研究横向随机晶场Ising模型的相图和磁化行为，重点是横向随机晶场浓度和晶场比率对相图和磁化的影响.给出了i>T-D_x空间的相图和m-T空间的磁化图.在晶场稀疏情况下，负晶场方向存在临界温度的峰值，正方向可出现重入现象.晶场比率取+0.5和-0.5时，磁有序相范围缩小，特别是晶场比率取-0.5时，随晶场浓度的降低，临界温度峰值从横向晶场负方向渡越到正方向.固定某一负晶场值，不同晶场比率的磁化行为有明显差异.同时与纵向稀疏晶场Ising模型结果进行有意义的比较. 关键词： 横向随机晶场Ising模型 相图 磁化行为     

Tunneling through narrow-gap semiconductor Sb2Te3 barrier

Tunnel experiments have been performed on Au/Sb₂Te₃/Al tunnel junctions to study elastic interelectrode tunneling through the small energy gap of a narrow-gap semiconductor. Tunnel conductance exhibited narrow width conductance peak at zero bias voltage. This behaviour is in accordance with the result of the theoretically calculated tunnel conductance, in which the nonparabolic dispersion relation within the energy gap of the narrow-gap semiconductor used as a tunnel barrier in a metal/narrow-gap semiconductor/metal tunnel structure is included. And some interesting structures are also observed in the conductance curves.