Y. Y. Hsu

Coexistence of antiferromagnetic order and superconductivity with TN higher than Tc in DyNi2B2C

Abstract A superconducting transition temperature T c of 3.8 K with an onset as high as 5.2–5.5 K was reported for quaternary borocarbide DyNi 2 B 2 C with a high antiferromagnetic Neel temperature T N around 10.1–10.8 K. No reentrant behavior was observed down to 0.3 K demonstrating a perfect coexistence between superconductivity and antiferromagnetic order. A superconducting upper critical field H c2 (0) of 3.0 kG, lower critical field H c1 (0) of 200 G and Ginzburg-Landau parameter κ of 3.7 were derived. A broad T c transition points to a possible small T c varying homogeneity range of DyNi 2 − α B 2 − β C 1 − δ in the phase diagram. The magnetic entropy S m of 10.3 J/mol K or 90% R ln 4 indicates antiferromagnetic ordering of Dy 3+ with a quadruplet ground state in the tetragonal crystal field. Two field-induced weak-ferromagnetic-like transitions with a field of around 1 T were observed at 2 K.

Co K-edge XANES and spin-state transition of RCoO3 (R=La, Eu)

Abstract Co K-edge X-ray absorption near-edge spectra at 300 K and magnetic studies for the rhombohedral LaCoO3 with low-spin state to intermediate-spin (IS) state transition T s ≃105 K and orthorhombic EuCoO 3 (T s ≃295 K ) are reported. The small pre-edge feature observed is from 1s–3d dipole transition, which is weakly allowed through the hybridization of Co 4p states with 3d states of neighboring Co atoms. For both samples in the IS state, pre-edge can be fitted using three peaks with energy separation P 2 − P 1 =2.0 eV for R=La and Eu, and P 3 − P 2 =1.5 eV for R=La and 1.3 eV for R=Eu. Since the thermally excited IS state is a mixed t2g5eg1 and d 7 L ( t 2 g 6 e g 1 or t2g5eg2), with almost filled t2g↑ states. From reported band structure calculation, the three peaks P1, P2, P3 are probably corresponding to the unfilled t2g↓, eg↑ and eg↓ states, respectively.

SUPERCONDUCTIVITY IN THE NEW QUATERNARY RUTHENIUM BOROCARBIDE Y-RU-B-C SYSTEM

Two new quaternary borocarbide phases were discovered in the Y–Ru–B–C system: the (1111) YRuBC phase of the LuNiBC-type structure with tetragonal lattice parameters a=3.625–3.630 A and c=7.775–7.874 A, and the (1221) YRu2B2C phase of the LuNi2B2C-type structure with tetragonal lattice parameters a=3.742–3.787 A and c=10.278–10.453 A. Bulk superconductivity of 9.7 K was observed from transport and magnetic measurements for the annealed YRu2B2C sample with a short Ru–Ru bond length d(Ru–Ru)=√2 a/2=2.646 A.

Wigner trajectories in resonant‐tunneling diodes under transverse magnetic field

We generalize the Wigner formalism to include effects of an in‐plane magnetic field and present theoretical study of effects of a transverse magnetic field on tunneling dynamics for resonant‐tunneling diodes. We solve transport equation for the steady‐state Wigner distribution function. Particle trajectories are determined in phase space as contour lines of the distribution function. From trajectories we estimate tunneling times. Discussions of magnetic field dependence and transverse momentum dependence of tunneling times are presented.

Correlation between oxygen content δ and metal-insulator transition in Sm1.85Ce0.15CuO4 + δ antiferromagnetic superconductors

The role of oxygen content parameter δ on the metal-insulator transition and superconductivity in oxygen-reduced Sm 1.85 Ce 0.15 CuO 4+δ (-0.06<δ<0.08) antiferromagnetic superconductors (T N (Sm)∼4.6 K) are reported. A superconductor-insulator transition was observed near δ∼ 0.05, with T c (onset) of 11 K for δ∼ 0.045, 18 K for δ∼0.04, 20 K K for δ∼0.03 and maximum T c up to 24 K for δ∼ - 0.02. The relative oxygen content 8 was determined using the Sm 2 CuO 4 as the standard and based on Ce 4+ from Ce LIII-edge X-ray absorption spectra. For tetragonal c-axis aligned Sm 1.85 Ce 0.15 CuO 3.98 powder (T c = 24K), a non-s-wave or dirty d-wave-like T 2 power law was observed for T < 0.5T c from in-plane penetration depth Δλ ab (T) = λ ab (T) - λ ab (0) data.

Evidence of d-wave behavior in Pr1.85Ce0.15CuO3.97 aligned superconductors

Abstract The c -axis and in-plane alignment of 23 K Pr 1.85 Ce 0.15 CuO 3.97 superconducting powder were achieved with powder/epoxy cylindrical holder perpendicular and parallel to the alignment field. The temperature dependencies of basal-plane penetration depth λ ab ( T ) and c -axis Josephson penetration depth λ c ( T ) were derived using zero-field-cooled diamagnetic susceptibility χ d ( T ) in a low applied field ( B a H c1 ) parallel or perpendicular to the aligned powder where small Pr Curie–Weiss paramagnetic susceptibility χ p ( T ) was subtracted. Up to 0.5 T c , Δλ ab ( T )= λ ab ( T )− λ ab (0) as well as Josephson Δλ c ( T ) can be well fitted with a d-wave-like power law c 2 T 2 + c 4 T 4 , where T 2 -term is dominated below 0.3 T c . The present result agrees with recent phase-sensitive experiment, which indicates d-wave nature for electron-doped R 1.85 Ce 0.15 CuO 4− δ (R=Pr, Nd) cuprates.

Magnetic and transport properties of PrBa{sub 2}Cu{sub 4}O{sub 8-{delta}} prepared at 1 atm oxygen pressure

Measurements of physical properties were performed on the PrBa2Cu4O8-δ sample prepared by the sol-gel method and sintered in I atm oxygen pressure. Instead of exhibiting superconductivity as for R Y or Gd in the RBa2Cu4O8-δ compound. This compound is unstable, insulating but metallic-like at low temperatures. No superconductivity was observed down to 2 K. Since PrBa2Cu4O8-δ can be re-written as Cu2Ba2PrCu2O8-δ which is a compound of two CuO2-layers and two CuO-chains, the possibility of anomalous Pr antiferromagnetic ordering around 18 K observed from preliminary magnetic data and the effect of oxygen stoichiotmetry parameter δ on the metal-insulator transition are explored.

Superconductivity up to 10 K in the Sr(Pd1−xPtx)2Ge2 layer system

Metastable filamentary superconductivity with Tc = 10.2 K was observed in SrPt2Ge2. This is the highest Tc reported outside the iron-based 122-layer compounds. The stable phase for this compound is a non-superconducting monoclinic LaPt2Ge2-type structure with lattice parameters a = 0.4401(4) nm, b = 0.4435(4) nm, c = 0.9764(10) nm, β = 90.5°–90°. Weak diamagnetic shielding signal indicates that superconductivity is not from the major monoclinic phase, but from the minor BaFe2As2-type body-center-tetragonal (122-bct) phase with a=0.44 nm, c=0.98 nm,β=90°. Investigation on the pseudoternary system Sr(Pd1−xPtx)2Ge2 suggests that the stable bct to monoclinic structure phase transition occurs near x = 0.15. For x 0.15 with metastable filamentary bct phase, Tc increases from 7.5 K for x = 0.5 to 9.2 K for x = 0.7 and reaches a Tc maximum of 10.2 K for SrPt2Ge2. High Tc is due to a strong quasi-2D Pt-Ge-Pt 5dxz,yz-...

Mn K-edge XANES of hexagonal manganites RMnO3 (R = Sc, Y)

Abstract Mn K-edge X-ray absorption near-edge spectra (XANES) and magnetic studies for ferroelectromagnetic compounds RMnO3 (R=Sc,Y) are reported. ScMnO3 shows stronger d–p hybridization with a triangular antiferromagnetic (AF) order of T N =133 K and a spin reorientation T sr =23 K , as compared to T N =74 K for YMnO3. The identical main threshold edge energy of 6545 eV indicates Mn3+ character. Small pre-edge features with energy separation P2–P1 of 2.27 eV for ScMnO3 and 2.72 eV for YMnO3 is from the 1s–3d dipole transition, which is weakly allowed through the hybridization of Mn 4p with 3d states of neighboring Mn atoms. The larger P1 peak is corresponding to transition into empty e↓′ and e↓″ minority states, and the smaller P2 peak is into leftover e↓′ states.

Metal-Insulator Transition and Cu K-Edge XANES Studies for Pr1.85Ce0.15CuO4+δ High-Tc Superconductors

AbstractMetal-insulator transition near oxygen content parameter δ∼−0.018 was observed for electron-doped Pr1.85Ce0.15CuO4+δ (−0.003≤δ≤0.03) cuprates. Cu K-edge X-ray absorption near-edge structure (XANES) studies with nearly identical threshold edge energy E0 of 8980.8 eV indicate a Cu formal valence smaller than 2 for all samples, which is consistent with the estimated Cu valence of ∼1.84 for 20.5 K superconductor Pr1.85Ce0.15CuO3.997 and ∼1.91 for Pr1.85Ce0.15CuO4.03 insulator. The XANES spectrum reflects the Cu 3dn character where low energy peak A1 reflects the 3d10 configuration of Cu(I) oxidation state and A2 peak reflects the 3d10