Search Results - (Author, Cooperation:C. W. Chu)

2015-08-01

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2018-12-15

American Physical Society (APS)

1098-0121

1095-3795

Physics

Magnetism

2013-05-11

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2018-08-10

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Geosciences

Computer Science

Medicine

Natural Sciences in General

Physics

Physics

1089-7550

AIP Digital Archive

Physics

It has been demonstrated that the linear-chain charge-transfer salt, decamethylferrocenium tetracyanoethanide (DMeFc)(TCNE), is a ferromagnet with a transition temperature of ∼4.8 K. This low-temperature 3D ordering has been attributed to a strong intrachain and a weak interchain interaction. To study these interactions, we have determined the Tc up to 20 kbar by measuring the ac susceptibility χ at low frequency. Our results show that the Tc increases with pressure at a rate of ∼0.22 K/kbar, while the χ peak indicative of the ferromagnetic transition continues to decrease rapidly. A small peak was also detected above the main transition at pressures above 3 kbar. This new peak persists even after the pressure is removed. The result from dc magnetization suggests that this corresponds to a metamagnetic state. For the first time, we have observed pressure-induced phase-transition in this material.

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1089-7550

AIP Digital Archive

Physics

At the present time, there is a large amount of convergent experimental evidence pointing toward the existence of a vortex-glass state in the high temperature superconductor YBa2Cu3O7−δ in the presence of a high magnetic field, as predicted by Fisher1 and Fisher et al.2 However, the situation at low magnetic field is not so clear. The nature of the vortex state separating the Meissener state at low field and the vortex-glass state at very high field remains unsolved both theoretically and experimentally. To shed some light to this problem, we have measured the remnant magnetic moment relaxation of melt-textured bulk YBa2Cu3O7−δ samples in various magnetic fields from ∼10−4 to ∼300 s. Even at zero field, the time dependence of the relaxation follows the vortex-glass model prediction m(t)∝1/[ln(t)]1/μ but with a μ=1.4±0.2, contradicting the vortex-glass model. As the field is increased, the μ value decreases until it reaches a value of ∼0.2–0.3 above 1 T. The larger than one value of μ seems to point to a collective pinning behavior at low magnetic field.

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1089-7550

AIP Digital Archive

Physics

To make true large applications of high temperature superconductors (HTS's), one requires not just a large critical current density (Jc) and high transition temperature (Tc), but also large total current (Ic is the JcA, where A is the cross sectional area) and large total current length (Icl where l is the length) of an HTS which can be further shaped into a usable form, e.g., disk, bar, cylinder, or wire. The large anisotropy, short coherence length, and large penetration depth of HTS's pose serious challenges to efforts to enhance the Jc, Ic, or Icl of HTS's. Melt-texturing technique by slow cooling through the peritectic temperature has proven to be effective in removing large-angle grain boundaries detrimental to Jc in HTS samples of small size; but many problems remain. The short coherence length reduces the pinning potential for magnetic fluxoids and also renders ineffective the nonsuperconducting dispersion in the HTS matrix to enhance pinning through the conventional metallurgical method. Defects of atomic scale acting as pinning centers to raise Jc have been obtained through irradiation by high energy particles and in situ chemical decomposition. However, in most of these studies Jc was determined only by magnetic measurements, not by transport techniques. We have therefore carried out a systematic study on the melt-texturing of HTS's in both bulk and wire forms. We have identified major processing parameters in reducing the microcracks, increasing the dimensions of the bulk HTS, controlling the grain alignment, improving the chemical homogeneity, enhancing the processing rate, and controlling the epitaxial growth of HTS's in an Ag sheath. Effects of irradiation by various types of particles of different energies on Jc determined both by magnetic and transport techniques have also been examined. These results will be presented and discussed, following a brief review on the current status of bulk and wire processing of HTS's.

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1089-7550

AIP Digital Archive

Physics

The pressure effect on the superconducting transition temperature (dTc/dP) has been measured for Dy1−xPrxBa2Cu3O7−δ with x=0, 0.02, 0.05, 0.1, 0.15, 0.2, and 0.3. Although the Tc suppression by Pr is in a different fashion for Y1−xPrxBa2Cu3O7−δ and Dy1−xPrxBa2Cu3O7−δ, the dTc/dP is similar in these two systems, with respect to x. The measured dTc/dP is positive at small x but becomes negative for x≥0.2 which is associated with the pressure effect on the normal-state resistivity (1/R)(dR/dP). The positive (1/R)(dR/dP) at x≥0.2 indicates that the number of carriers is decreasing under pressure and it supports the picture of hole localization through the hybridization of Pr-4f and O-2p bands. The mechanical pressure has a negative effect on Tc for x≥0.2, but chemical pressure has a positive effect on Tc according to the results of chemical substitution. This suggests that the large R (=rare earth element) may possibly reduce the Pr-O distance, and increase the hybridization of Pr-4f and O-2p bands.

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1089-7550

AIP Digital Archive

Physics

The superconducting transition temperature Tc and its pressure dependence dTc/dP of HgBa2CaCu2O6+δ (Hg-1212) and HgBa2Ca2Cu3O8+δ (Hg-1223) were measured up to 17 kbar. Tc increases with pressure approximately linearly for both compounds before oxidation. However, the nonlinearity in the Tc-P correlation shows up after oxidation in both compounds. For Hg-1212, the average dTc/dP decreases as oxygen doping increases, while it increases in Hg-1223. These observations are in conflict with the modified pressure-induced charge-transfer model, but might be attributed to the possible existence of fine electronic structure. These observations suggest that a Tc much higher than 140 K might be achievable in Hg-1223 by means of higher physical or chemical pressure.

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1089-7550

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Physics

Samples of HgBa2CuO4+δ (Hg-1201) were annealed under various conditions. After carefully controlling annealing time, annealing temperature (Ta), and O2 partial pressure (P0), we were able to find the reversible annealing conditions for Hg-1201. Under 1 atm O2 at 260 °C≤Ta≤400 °C, the obtained Tc is nearly the same (∼97 K). However, it decreases quickly with Ta(approximately-greater-than)300 °C in high vacuum (P0∼10−8 atm), and reaches zero at Ta=400 °C. On the other hand, Tc decreases with the decrease of Ta in high-pressure O2 (∼500 atm) and reaches ∼20 K at about 240 °C. In the entire annealing region, the oxygen surplus varies significantly from 0.03 to 0.4, and a wide range of Tc variation (0→97 K→20 K) was obtained with anion doping alone.

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1089-7550

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Physics

HgBa2CuO4+δ (Hg1201) samples with 0.03≤δ≤0.4 have been obtained. The magnetization of the powdered Hg1201 samples was determined using a Quantum Design SQUID magnetometer. It was observed that while the magnetization of Hg1201 increased with δ in the underdoped region, the magnetization decreased with δ in the overdoped region. These results suggest an increase of ns/m* with oxidation in the underdoped region and a decrease in the overdoped region, similar to that reported in underdoped HTSs and overdoped Tl2201 and Tl1201. © 1996 American Institute of Physics.

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1089-7550

AIP Digital Archive

Physics

The microchemical composition and the electronic structure of the two superconductors HgBa2Ca2Cu3O8+δ (Hg-1223) and HgBa2Ca1Cu2O6+δ (Hg-1212) were analyzed with the technique of photoemission with high spatial resolution. This enabled primary spectral peaks to be separated from their satellites due to local charging induced by the primary photon beam. Each sample contains superconducting and impurity phases with inhomogeneous spatial distributions. The oxygenation-induced increase in the critical temperature is related to a increase in the Cu-O plane carrier concentration.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Using our recently developed pulsed critical current density (Jc) measuring system, the Jc of the high Jc melt-textured YBa2Cu3O7−δ (Y123) bulk samples has been determined. I-V curves with a voltage resolution of 0.5 μV were obtained, and transport Jc's along the a-b plane (Jc(parallel) a-b) as high as 7.2×1 04 A/cm2 were extracted. These results are comparable to the values obtained magnetically. On the other hand, transport Jc along the c axis (Jc(parallel)c) were found to be two orders of magnitude smaller, even though the magnetic Jc(parallel)c is only about five times smaller than Jc(parallel)a-b. We suggest that for the high-temperature superconducting materials which are highly anisotropic, caution should be taken when using the nontransport magnetic methods to determine Jc.

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1089-7550

AIP Digital Archive

Physics

The magnetic properties and depth profiles of Co-modified γ-Fe2O3 films have been studied. The Hc of these films varies between 0.3 and 2.5 kOe nonmonotonically with Co concentration and depends only slightly on temperature. The non-diffusion-like Co-rich layer that appears underneath the surface of the Fe-oxide films displaying an enhanced Hc has been positively demonstrated to be responsible for the Hc enhancement.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Magnetic field trapping and shielding by several unirradiated single-grain YBa2Cu3O7−δ disks have been investigated between 4.2 and 71 K up to 20 T. Flux avalanches due to thermal instabilities limited the trapping field to ∼5 T below 45 K and often caused microcracks in the disks. However, a shielding field greater than 19 T was detected at 4.2 K in the same sample assembly even after it was damaged by flux avalanches. This violates the expected symmetry between shielding and trapping fields. The shielding field of (approximately-greater-than)19 T exceeds the total quench field of 10 T predicted by the Swartz–Bean model. The observations suggest that thermomechanical properties play an important role in field shielding and trapping and mechanical stress may be effectively used to enhance the field trapped by disks of high temperature superconductors.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Superconducting preferred-oriented films of HgBa2CaCu2O6+δ of a thickness of ∼1 μm have been successfully synthesized. The process involves reacting rf-magnetron sputtered films of Ba2CaCu2Ox in a controlled Hg atmosphere at ∼800 °C for ∼5 h, followed by oxygenation. The films so obtained show a relatively sharp superconducting transition up to ∼120 K, as determined magnetically, which is similar to that of bulk Hg-1212.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Results of our study on high magnetic field trapping in unirradiated, high quality monolithic single-grain YBa2Cu3O7−δ disks are reported. A record high 4 T trapped field at the surface of the unirradiated disks is observed. However, below 11 K, large flux avalanches caused by thermal instability severely limit the remnant trapped field. Therefore, flux avalanche, rather than Jc×d, dictates the maximum trapped field at low temperatures. To overcome this problem, a strong high temperature superconductor trapped field magnet is proposed. A novel application of the avalanche effect is also mentioned.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report results on the temperature dependence of the trap field BT and the thermal instability (flux jump) field Hfj in single-grain YBa2Cu3O7−δ disks. Above 45 K, BT is limited by the critical current density Jc and BT=BT0(1−T/Tc)q with q≈1.67. At 4.2 K, the first flux jump field Hfj of ∼3 T is about an order of magnitude larger than theoretical prediction based on the adiabatic thermal instability model. We also find that the occurrence of large flux avalanches probably causes severe microcracking damages to the superconducting disks.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Millimeter-sized single crystals of C60 were grown by sublimation of C60 powder in a vacuum for 6–24 h. The crystals had excellent facets, were free of C70 or solvent, and showed face-centered cubic symmetry with a very small mosaic spread down to 0.01°.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

High quality c-axis oriented HgBa2Ca2Cu3O8 thin films have been fabricated with the stable Re-doped Ba2Ca2Cu3Ox precursor powder by pulsed laser deposition followed by postannealing without any special handling. As-grown films on (100) SrTiO3 exhibit a zero-resistance transition (Tc,zero) at ∼131 K with a narrow transition width ΔT∼1.5 K after oxygen annealing at 340 °C for 12 h. The critical current densities are observed 1.1×107 A/cm2 at 10 K and 1.2×105 A/cm2 at 120 K in zero field. The x-ray diffraction pattern indicates highly c-axis oriented thin films normal to the substrate plane containing a minor HgBa2CaCu2O6 phase. The scanning electron microscopy analysis shows that the surface morphology of the film is well connected platelike crystals but reveals Ba–Cu–Ox impurities are uniformly scattered across the film surface. © 1998 American Institute of Physics.

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