Admixture of an s-Wave Component to the d-Wave Gap Symmetry in High-Temperature Superconductors

Neutron crystal-field spectroscopy experiments in the Y- and La-type high-temperature superconductors HoBa₂Cu₃O_6.56, HoBa₂Cu₄O₈, and La_1.81Sr_0.15Ho_0.04- CuO₄ are reviewed. By this bulk-sensitive technique, information on the gap function is obtained from the relaxation behavior of crystal-field transitions associated with the Ho³⁺ ions which sit as local probes close to the superconducting copper-oxide planes. The relaxation data exhibit a peculiar change from a convex to a concave shape between the superconducting transition temperature T _c and the pseudogap temperature T ^* which can only be modeled satisfactorily if the gap function of predominantly d-wave symmetry includes an s-wave component of the order of 20–25%, independent of the doping level. Moreover, our results are compatible with an unusual temperature dependence of the gap function in the pseudogap region (T _c≤T≤T ^*), i.e., a break up of the Fermi surface into disconnected arcs.     

Critical Current Density Through Grain Boundaries in High-Temperature Superconductors

In this paper, a theoretical study is proposed based on the assumption that the vortices on low-angle grain boundaries (GBs) in high-temperature superconductor (HTS) are mixed Abrikosov-Josephson (AJ) vortices. The critical current density through GB is obtained on the basis of the Bean critical model and the assumption that the periods of AJ vortices coincide with the ones of Abrikosov (A) vortices. The model also enables us to calculate J _c of HTS with an inclined GB. In addition, the effect of strain on critical current density is also taken into account in this model by considering the strain dependence of deparing current density within GB. There is a good agreement of our results with the classical power-law expression. The model proposed in this work can be used for simultaneous studies of the effects of misorientation angles, GB-inclined angles, and applied fields on the critical current density of polycrystalline HTS.     

Josephson Current Through Planar Pb–Cu–Pb Nanobridges

We report on the fabrication and characterization of coplanar Pb–Cu–Pb nanobridges. In such superconductor (S) – normal metal (N) – superconductor junctions the Josephson coupling is mediated via the proximity effect at the S–N interface. For a junction in the dirty limit (ℓ≪ ξ _N where ℓ is the mean free path and ξ _N is the coherence length in N) the Josephson current I _c is proportional to L/ξ _N ·exp(−L/ξ _N ). The relation defines an upper limit for the length L of the normal-metal bridge in order to observe Josephson coupling. A Josephson current of up to 750 μA at 1.5 K was observed in junctions with L/ξ _N = 0.82 which is only 1/8 of the theoretically expected value. The reduction might originate from oxide layers at the normal metal – superconductor interfaces.     

Temperature Dependence of the Josephson Current in the d-Wave Superconductor Junction Based on the 2-D Extended Hubbard Model

We present a theoretical study of the Josephson current in the d_x ^{2-y 2}-wave superconductor junction. We calculate the current-phase relation and the temperature dependence of the Josephson current for the (100) oriented and the (110) oriented junction using the two-dimensional (2-D) extended Hubbard model. We obtain the anomalous temperature dependence of the current in the (110) oriented junction which has been reported within the quasiclassical theory.     

Variation of Vortex Charges in Hole Over-Doped High-Temperature Superconductors with Competing Anti-Ferromagnetic and d-Wave Superconducting Order

Vortex charges in copper oxide high-temperature superconductor (HTS) with hole over-doping are investigated by Bogoliubov-de Gennes (BdG) equations based on a model Hamiltonian with competing anti-ferromagnetic (AF) and d-wave superconducting order (DSC) with variation of temperature T. For the DSC, the spin density wave (SDW) and the charge density wave (CDW), the transition from stripe to checkerboard pattern and to 2D-structure may occur with variation of temperature T. Especially, the vortex core will be negatively charged for hole over-doped HTS at the lower temperature where the AF and DSC orders coexist, whereas positive core charges are expected for relatively large temperature (still below Superconducting critical temperature T _c ) where AF order vanishes.     

Chaos Synchronization Between Josephson Junctions Coupled with Time Delays

We investigate chaos synchronization between Josephson junctions coupled unidirectionally with time-delay. We demonstrate the possibility of high-quality synchronization with numerical simulations of such systems. The results are of certain importance for obtaining the high-power system of Josephson junctions, which is promising for practical applications.     

Transmission of Sound Through the Interface of Phase-Separated 3He-4He Liquid Mixtures Kinetic Growth Coefficient of the Interface

We report the results of the first systematic study of the dynamical properties of the interface between the phase-separated ³ He- ⁴ He liquid mixtures. With the acousic sound wave of 10MHz, we determined the kinetic growth coefficient of the interface in the temperature range from 150mK down to 7mK. Below about 70mK, it starts increasing with the decrease of temperature and shows an approximate temperature dependence of 1/T ² . The magnitude and the temperature dependence cannot be explained with the existing theory of the hydrodynamic regime.     

Josephson junction with an amorphous silicon barrier

A Josephson junction Nb/Si/Nb with a 10 nm thick amorphous silicon barrier is studied. The upper electrode contains a 2 nm thick sublayer of amorphous phase adjacent to the barrier, as revealed by cross-sectional TEM. Thus, the junction can be considered as a S-I-N-S system with the N layer represented by amorphous niobium. Peculiarities in the I–V and Δ(T) dependencies are observed and explained as a consequence of a proximity effect present in the upper electrode.     

Nonlinear Response of High- T c Superconductors with Small Current Density

By using the Anderson pinning model and considering the reversal transition probability of the magnetic flux bundle, we use the consistent theory to derive the three relaxation phenomena with small current density consisting of the relaxation of the fixed external field, the scanning rate $\dot H$ of the external field, and the ac response with driving frequency ν of the external field, respectively. We also explain the corresponding physical phenomena.     

Improving Performance in HPC Centers by Modeling Users Through an Evolutionary Virtual Interface

An agent-based profiling approach is presented in this paper in the form of a virtual interface that models user behavior and satisfaction with the objective of improving the performance of high-performance computing (HPC) centers. The interface's function is to translate user requests and satisfaction criteria into what is really necessary, thus permitting the optimization of the HPC center's scheduling, taking into account the predicted user satisfaction together with the objectives of the management of the center in terms of resource usage and cost. The system is built using an evolutionary agent-based profiling architecture where agents are evolved in real time to adapt to the different users. These agents cooperate with the scheduling mechanism, providing resource usage estimations for the different tasks, as well as predicting the effect of possible strategies on user satisfaction. This paper focuses on the user behavior modeling component, although the global architecture is also presented. Some experiments are carried out where the proposed architecture interacts with a real job-management system (JMS) (Sun Grid Engine). They clearly show that modeling the user and taking user satisfaction into account helps to improve system performance.      

Nonlinear Response of High- Tc Superconductors with Small Current Density

By using the Anderson pinning model and considering the reversal transition probability of the magnetic flux bundle, we use the consistent theory to derive the three relaxation phenomena with small current density consisting of the relaxation of the fixed external field, the scanning rate of the external field, and the ac response with driving frequency of the external field, respectively. We also explain the corresponding physical phenomena.     

Detecting Current Noise with a Josephson Junction in the Macroscopic Quantum Tunneling Regime

We discuss the use of a hysteretic Josephson junction to detect current fluctuations with frequencies below the plasma frequency of the junction. These adiabatic fluctuations are probed by switching measurements observing the noise-affected average rate of macroscopic quantum tunneling of the detector junction out of its zero-voltage state. In a proposed experimental scheme, frequencies of the noise are limited by an on-chip filtering circuit. The third cumulant of current fluctuations at the detector is related to an asymmetry of the switching rates.      

Elastic Wave Scattering by an Interface Crack Between a Piezoelectric Layer and an Elastic Substrate

This paper analyzes the consequences on fracture of the combined effects of triaxial stress states generated by notches of very different geometries and microstructural evolution produced by a heavy cold drawing when eutectoid high-strength prestressing steels are manufactured. The anisotropic fracture behaviour of these materials with high level of strain hardening is rationalized on the basis of the markedly oriented pearlitic microstructure of the drawn steels which influences the operative micromechanism of fracture in this case.     

Axisymmetric Flow in an Oil Reservoir of Finite Depth Caused by a Point sink above an Oil-Water Interface

The flow of a stratified fluid (e.g., oil/water) withdrawn from a vertically confined porous medium through a point sink is considered. The withdrawal tends to cause the oil-water interface to move upwards. So long as the interface is below the well, the less dense fluid (oil) is pumped into the well without the denser fluid (water) until a critical flow rate is reached. The flow is considered to be axisymmetric, and involves a nonlinear boundary condition along the free surface. A boundary-integral equation method (BIEM) is used to find the interface position for different pumping rates. For small flow rates, a small-parameter expansion is derived and the results are compared with numerical solutions to the problem. There exists a critical withdrawal rate beneath which the water does not break through into the sink, this rate depending on the sink location and bottom geometry.     

Microwave-Induced Constant Voltage Steps at Series Arrays of Josephson Tunnel Junctions with Near-Zero Current Bias

Microwave-induced, constant-voltage steps in the current-voltage characteristics of Josephson tunnel junctions are used as a reference voltage in cryogenic voltage standards. This relatively low reference voltage of a few millivolts could be increased by using series arrays of tunnel junctions instead of a single junction. Series arrays consisting of 2, 6, 14, and 54 tunnel junctions were tested at a frequency of 70 GHz. The use of this relatively high frequency and of lead-alloy base electrodes allows zero crossing steps of up to 34 mV to be observed for an array with only 54 junctions when irradiated by an RF power of 32 mW. The measurements were performed at 4.2 K.     

Significant Correlations Between Levitation-Suspension Forces and Critical Current Densities in Bulk YBCO/Ag Composite Superconductors Fabricated by Infiltration and Growth Processing Technique

We report on significant correlations between the levitation-suspension forces and critical current densities in bulk YBCO/Ag composite superconductors fabricated by infiltration and growth processing (IGP) technique. Based on our studies, we have found that there is a strong correlation between various parameters obtained from levitation and suspension force curves and the magnetic J _cs calculated from M-H loops using the standard Bean’s model-type approaches. Significantly, the levitation force gap and the critical current density of the bulk sample are in direct correlation. We also find that it is possible to qualitatively predict the J _cs of samples from the levitation and suspension force curves by using this cost-effective measurement technique as a preliminary tool. These correlations provide a novel approach to rapid selection and characterization of bulk superconducting samples for their employment in practical applications.     

1V脉冲驱动型交流量子电压源研究

脉冲驱动型交流量子电压标准ACJVS通过高速脉冲序列驱动约瑟夫森结阵芯片的方式实现宽频带交流量子电压的合成,相比于可编程型交流量子电压标准PJVS,具有免台阶切换、频谱纯净、频带宽等优点。搭建的系统主要包括8位高速脉冲码型发生器、微波放大器、直流阻断、约瑟夫森结阵芯片等。通过驱动包含4个子阵列,每个子阵列含12810个约瑟夫森结的结阵芯片,并结合4通道联合低频补偿的方式,成功产生了1V有效值的脉冲驱动型交流量子电压,为进一步建立交流量子电压基准打下了坚实的基础。最后,展望了脉冲驱动型交流量子电压在量子阻抗桥、交流量子功率源、交流量子功率表方面的应用价值。