Y. Y. Xue

Superconductivity, intragrain penetration depth, and Meissner effect of RuSr 2 (Gd,Ce) 2 Cu 2 O 10+δ

The hole concentration p(δ), the transition temperature T c , the intragrain penetration depth λ, and the Meissner effect were measured for annealed RuSr 2 (Gd,Ce) 2 Cu 2 O 1 0 + Φ samples. The intragrain superconducting transition temperature T c varied from 17 to 40 K while the p changed by only 0.03 holes/CuO 2 . The intragrain superfluid density 1/λ 2 and the diamagnetic drop of the field-cooled magnetization across T c (the Meissner effect), however, increased more than 10 times. All of these findings are in disagreement with both the T c vs p and the T c vs 1/λ 2 correlations proposed for homogeneous cuprates, but are in line with a possible phase separation and the granularity associated with it.

Granularity and intragrain penetration depth in RuSr2GdCu2O8

Abstract The multi-stage transition of RuSr 2 GdCu 2 O 8 (Ru1212) ceramic samples was investigated. An intergrain phase-lock temperature of T 2 is identified around 10–30 K. Slightly above T 2 , however, an isolated Ru1212 grain still behaves as a Josephson-junction array of superconductive nano-domains, which are phase-locked below an interdomain phase-lock temperature (intragrain transition-temperature) of T 1 ≈33–40 K. The intragrain penetration depth λ 1 was subsequently deduced from the ac susceptibility χ ′ of powders with different particle-size d (0.3⩽ d ⩽10 μm in this work) between T 1 and T 2 . The sample-dependent λ 1 ≈2.5–6 μm so obtained is extremely long, and bears a systematic correlation with the T 1 observed. A phase-separation model was proposed to interpret the data.

Water content and superconductivity in Na0.3CoO2 · yH2O

Abstract We report here the correlation between the water content and superconductivity in Na 0.3 CoO 2 xa0·xa0 y H 2 O under the influences of elevated temperature and cold compression. The X-ray diffraction of the sample annealed at elevated temperatures indicates that intergrowths may exist in the compound at equilibrium when 0.6 y M s varies linearly with y , but is insensitive to the intergrowth, indicative of quasi-2D superconductivity. The T c -onset, especially, shifts only slightly with y . Our data from cold compressed samples, on the other hand, show that the water loss non-proportionally suppresses the diamagnetization, which is suggestive of weak links.

Non-invasive assessment of the heart function in unshielded clinical environment by SQUID gradiometry

Bioelectrical activity of the heart produces minute magnetic fields that may be measured above the torso with superconducting quantum interference device (SQUID) sensors. A new application has been developed which permits noninvasive assessment of the heart function in unshielded magnetically harsh clinical environment. The system incorporates a multichannel SQUID system, a supine nonmagnetic ergometer, a peripheral data interface and software tools for processing magnetocardiographic data. To demonstrate the performance of the method, biomagnetic measurements have been performed both at rest and during physical exercise. Signal processing, signal-to-noise ratio and artifact rejection procedures have been discussed.

Crypto-superconductivity in RuSr2GdCu2O8

Abstract The susceptibility χ = χ ′ + iχ ″ of RuSr 2 GdCu 2 O 8 powders at 2 K has been deduced from ac and dc magnetizations with their particle size d from 800 to 10 μm and grain size 1–5 μm. It is observed that the loss part, χ″, scales with d over a field range of 0.0001 to 100 Oe, suggesting that the supercurrent is homogeneously distributed over particles as small as 10 μm. However, the inductance part, χ′, decreases with d systematically and drastically. An effective penetration depth as large as 30–50 μm is needed, which suggests a very small super-carrier concentration. Crypto-superconductivity is proposed to accomodate the data.

Thermoelectric power and transport properties ofMg1−xAlxB2

We have measured the thermoelectric power, S, and resistivity, rho, of pure and Al-doped MgB2. S is positive and increases linearly with temperature above the superconducting transition temperature, Tc. Deviations from the linear dependence appear at higher temperature, T>T0 approx 160 K. Tc and T0 both decrease with Al doping whereas the slope of S(T) in the linear range increases with the Al content. The data are discussed in terms of doping induced changes of the Fermi surface and the density of states at the Fermi level.

The Nature of the Superconducting State in Rutheno-Cuprates

The magnetism and superconductivity (SC) of rutheno-cuprates RuSr2LCu2O8−δ (Ru1212) and RuSr2(L,Ce)2Cu2O10+δ (Ru1222) were investigated, where L = Gd or Eu. The normal state carrier concentration p, the T c, the intragrain penetration depth λ, and the diamagnetic field-cooled magnetization were measured in various annealed rutheno-cuprate samples. The p varies with annealing only slightly (from 0.09 to 0.12 holes/CuO2), but the intragrain T c by a factor of 2.3 (from 17 to 40 K). The 1/λ 2, on the other hand, was enhanced more than tenfold (from 0.3 to 6 µm−2). The data are in disagreement with both the universal T c vs. p and T c vs. 1/λ 2 proposed for bulk cuprates. These, together with the unusually large dT c/dH ≈ 100 K/T observed in both Ru1212 and Ru1222, suggest that even the intragrain SC of rutheno-cuprates is granular. A Josephson-junction-array model was then proposed to interpret the data. The memory effect observed in Ru1222 far above the main magnetic transition temperature further suggests that the root of the granularity may be a phase separation, resulting in the mesoscopic ferromagnetic and antiferromagnetic species in these rutheno-cuprates.

Thermoelectric power and transport properties of pure and Al-doped MgB2

We have measured the thermoelectric power, S, and resistivity, rho, of pure and Al-doped MgB2. S is positive and increases linearly with temperature above the superconducting transition temperature, Tc. Deviations from the linear dependence appear at higher temperature, T>T0 approx 160 K. Tc and T0 both decrease with Al doping whereas the slope of S(T) in the linear range increases with the Al content. The data are discussed in terms of doping induced changes of the Fermi surface and the density of states at the Fermi level.

Thermoelectric power and transport properties of Mg{sub 1-x}Al{sub x}B{sub 2}

We have measured the thermoelectric power, S, and resistivity, rho, of pure and Al-doped MgB2. S is positive and increases linearly with temperature above the superconducting transition temperature, Tc. Deviations from the linear dependence appear at higher temperature, T>T0 approx 160 K. Tc and T0 both decrease with Al doping whereas the slope of S(T) in the linear range increases with the Al content. The data are discussed in terms of doping induced changes of the Fermi surface and the density of states at the Fermi level.

A novel technique for identification of minor superconducting phases

Abstract A novel technique that involves isolating and characterizing particles of pulverized samples has been developed to identify minor superconducting phases. Particles as small as 10 μm can be analyzed both magnetically and structurally. Data on several multi-phase Ba–Ca–Cu–O–(C) samples demonstrate that the technique can even distinguish coexisting superconducting phases based on the difference of the transition temperature Tc. Using the technique, (Ba0.95C0.05)2CuO4−δ was identified as superconducting with a Tc up to 60 K and lattice parameters a≈b≈3.85 Axa0and c≈15.7 A.