Surface electronic structure of the organic superconductor κ-(ET)2Cu(NCS)2 studied via photoemission microscopy

The surface electronic structure of cleaved single crystals of the organic superconductor κ-(ET)₂Cu(NCS)₂ has been studied using photoemission microscopy. Two types of cleaved surfaces were observed, displaying different valence band photoemission spectra and different spectral behavior near the Fermi level, E_F. In particular, spectra from one surface type display relatively broad spectral features in the valence band and finite spectral intensity at E_F, while spectra from the other surface type show well-defined valence band emission features and zero photoemission intensity at E_F. We propose that the spectral differences are due to a very short electron mean free path in this material, and our results are used to explain the differences between previously published photoemission spectra from this superconductor. We also report the results of an investigation of the electronic structure of defects in this material.     

TlBaCaCuO superconducting thin films via metal-organic chemical vapour deposition (MOCVD) and thallium vapour diffusion

Summary Superconducting thin films of the TlBaCaCuO family have been prepared through a combined approach of MOCVD and thallium vapour diffusion. A three-step process has been adopted involving i) the synthesis of Ba−Ca−Cu−O films by MOCVD using ?second-generation? barium and calcium sources, ii) the hydrolysis of the fluoride phases by annealing in water vapour-saturated oxygen and iii) the annealing in the presence of a thallium source. Reproducible results, in terms of phase formation, are discussed depending on the different conditions of the preannealing process. The superconducting films consist predominantly of single-phase Tl₂Ba₂Ca₂Cu₃O _x (2223). Preferential orientation of the crystallitec-axes perpendicular to the substrate surface has been observed. The films exhibit superconducting onset temperatures of 115–120 K with zero resistance higher than 100 K. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

Transport properties of long monofilamentary Bi(2223) tapes

Summary Monofilamentary Bi(2223) tapes withJ _c(77 K, 0 tesla) up to 30 000 A/cm² have been prepared by cold rolling using the powder-in-tube method. An optimization of the precursor powders has led to a higher phase purity after the reaction heat treatment. The deformation process has been optimized in order to increase the oxide density and to reduce sausaging effects on the oxide thickness. The transport properties of these tapes have been studied in a wide range of temperature (4.2K-T _c) and magnetic fields (up to 28 tesla). The critical-current values at 77 K fields of 0.5 T and 1 T parallel to the tape surface are 10 000 A/cm² and 5400 A/cm², respectively. At 4.2 K theJ _c value decreases from 1.6·10⁵ A/cm² at 0 T to 6·10⁴ A/cm² at 15 T. At fields higher than 15 tesla a very low field dependence ofJ _c has been found, regardless of the tape orientation. Transport properties have also been studied by cutting small sections of the tape in order to investigate the local critical-current distribution. It has been found that, even in rolled tapes of good quality (J _c (77 K, 0 T)>20000 A/cm²), theJ _c distribution is homogeneous: the critical current density increases gradually from the centre of the tape to the sides, the latter exhibiting much higherJ _c (46000 A/cm²) than in the centre (18000 A/cm²). Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

亚洲托卡马克超导磁体研究新进展

简要回顾了世界托卡马克超导磁体研究的进展 ,重点介绍了全超导托卡马克装置中国的 HT- 7U、韩国的 KSTAR及日本开展的 ITER中心螺线管模型线圈研究的新进展。最后对亚洲在这一领域近几年的发展做了归纳。     

Condensation energy of the superconducting bilayer cuprates

In the present work, we report the interplay of single particle and Cooper pair tunnelings on the superconducting state of layered high-T _c cuprate superconductors. For this we have considered a model Hamiltonian incorporating the intra-planar interactions and the contributions arising due to the coupling between the planes. The interplanar interactions include the single particle tunneling as well as the Josephson tunneling of Cooper pairs between the two layers. The expression of the out-of-plane correlation parameter which describes the hopping of a particle from one layer to another layer in the superconducting state is obtained within a Bardeen-Cooper-Schriefer (BCS) formalism using the Green’s function technique. This correlation is found to be sensitive to the various parameter of the model Hamiltonian. We have calculated the out-of-plane contribution to the superconducting condensation energy. The calculated values of condensation energy are in agreement with those obtained from the specific heat and the c-axis penetration depth measurements on bilayer cuprates.     

Magnetotransport properties of untwinned YBa2Cu3Oy single crystals: novel 60-K-phase anomalies in the charge transport

We present the result of our accurate measurements of the a- and b-axis resistivity (ρ_a and ρ_b), magnetoconductivity Δσ/σ, Hall coefficient R_H, and the a-axis thermopower S_a in untwinned YBa₂Cu₃O_y single crystals in a wide range of doping (6.45≤y≤7.0). The systematics of our data reveals a number of novel 60-K-phase anomalies in the charge transport: (i) temperature dependences of ρ_a show anomalous overlap below ∼130 K for 6.65≤y≤6.80, (ii) Hall mobility μ_H shows an enhancement near y?6.65, which is reflected in an anomalous y dependence of σ_xy, (iii) with decreasing temperature R_H shows a marked drop upon approaching T_c only in samples with 6.70≤y≤6.85, (iv) superconducting fluctuation magnetoconductivity is anomalously enhanced near y?6.7, and (v) H_c2 is anomalously reduced near y?6.70. We discuss that the fluctuating charge stripes might be responsible for these anomalies in the charge transport.     

Excess resistance in the superconducting transition of a mesoscopic Al disk

We report resistance measurements on a mesoscopic Al disk whose size is comparable to the superconducting coherence length. As the magnetic field increases, resistance peaks successively appear and some of the peak resistances are larger than the normal state value R_N. These peaks are ascribed to the transitions between different vortex states in the superconducting Al disk. The experimental results suggest that some anomalous energy dissipation is caused by the dynamics of the vortices in the confined geometry.     

Quenched! The ISABELLE Saga, II

This is the second part of a two-part article about ISABELLE, a colliding-beam accelerator conceived in 1971, officially approved in 1978, partially constructed, and terminated in 1983. I cover here the period from ISABELLE’s groundbreaking in 1978 to its termination in 1983. I treat the problems with ISABELLE’s superconducting magnets, the steps by which the problems became clear within the laboratory and to the outside, the initial failure of the laboratory administration to take forceful steps, conflicts with Fermilab, the alternate magnet projects, the repeated missing of openings that eventually used up the window of opportunity. I cover Robert Palmer’s successful creation of an alternate magnet that used Brookhaven’s existing tooling and techniques and provided (too late) a technical solution to the project’s problems, and the name change to the Colliding Beam Accelerator (CBA). I also discuss the failure of the machine to hold the physics community’s interest, and the project’s termination.     

First on-line test of the LINAC superbuncher at nuclear science centre

An on-line test of the LINAC superbuncher at Nuclear Science Centre has been successfully performed. DC O⁷⁺ beam of nominal energy 92 MeV was accelerated through the superbuncher resonator, operating at a field of 4.54 MV/m. The total energy gain of the beam was measured to be 4.5 MeV. For the pulsed beam test a phase locked bunched beam of O⁷⁺ of nominal energy 92 MeV, FWHM 1.3 ns from the pre-tandem multiharmonic buncher was injected into the superbuncher. By properly adjusting the phase and amplitude of the resonator, the best FWHM of the bunched beam was measured to be 185 ps near the entrance of the first LINAC module. Fully depleted cooled surface barrier detector was used for measuring the time width. In a separate experiment the intrinsic time resolution of the same detector was measured to be 134 ps. Consequently the intrinsic time width of the bunched beam, after correcting for the detector resolution, would be 127 ps. Details of the experiment and results are presented.     

没有薛定谔猫态

新近有个超导量子干涉器件的实验,使澄清量子力学基本解释有了可能.分析此实验和相关实验之后,作出断言,抽象量子态没有信息,且波函数是个尚待观察者要去做的实验的几率幅.故用常用术语将量子态或波函数过分具体化,恐怕要招致误解.因而从本质上讲,薛定谔猫态是不存在的.