A. Shibata

Signature of carrier-induced ferromagnetism in Ti(1-x)Co(x)O(2-delta): exchange interaction between high-spin Co2+ and the Ti 3d conduction band.

X-ray photoemission spectroscopy measurements were performed on thin-film samples of rutile Ti(1-x)Co(x)O(2-delta) to reveal the electronic structure. The Co 2p core-level spectra indicate that the Co ions take the high-spin Co2+ configuration, consistent with substitution on the Ti site. The high-spin state and the shift due to the exchange splitting of the conduction band suggest strong hybridization between carriers in the Ti 3d t(2g) band and the t(2g) states of the high-spin Co+2 . These observations support the argument that room temperature ferromagnetism in Ti(1-x)Co(x)O(2-delta) is intrinsic.

Growth and upper critical field of heavily overdoped Tl2Ba2CuOx single crystals

Abstract Growth technique for single crystals of Tl-based cuprate superconductors are established using a perfectly sealed gold crucible. Single crystals of Tl2Ba2CuOx can be easily obtained. Upper critical fields (Hc2) of heavily overdoped Tl2Ba2CuOx crystals (Tc=9.7, 15.7 K) have been estimated by measuring high frequency (28.5 GHz) microwave surface resistance. It is found that the temperature dependence of Hc2 is not divergent but tends to be saturated toward 0 K. This is extremely different from temperature dependence of resistive superconducting transition point of direct current resistance in magnetic fields reported before. The extrapolated upper critical fields Hc2(0) of the crystals are 0.85 T (Tc=9.7 K) and 1.9 T (Tc=15.7 K), respectively.

Quasiparticle structure in the vortex state of anisotropic s-wave superconductor YNi2B2C

Abstract We have measured the low temperature heat capacity C p in the vortex state of YNi 2 B 2 C. In contrast to conventional s-wave superconductors, C p shows a nearly H -dependence. This H -dependence persists even after the introduction of the columnar defects which change the electronic structure of the vortex core regime dramatically and strongly disturb the regular vortex lattice. These results indicate that the vortex state of YNi 2 B 2 C is fundamentally different from the conventional s-wave counterparts, in that the delocalized quasiparticle states around the vortex core play a much more important role, similar to d-wave superconductors.

Flux flow resistance and transmittivity in overdoped Tl{sub 2}Ba{sub 2}CuO{sub 6+{delta}}

We have measured the surface resistance and transmittivity of overdoped Tl2Ba2CuO6+δin magnetic field up to 7 T in order to investigate the unusual dc resistive transition. Introduction of columnar defects largely shifts the irreversibility line to higher field, showing that the resistive transition is caused by the melting of the vortex lattice. It is found that flux flow resistance of this compound is unusual as compared with that expected from the dc resistivity.

Purely viscous motion of the vortices in semiclassical d-wave superconductor

Abstract We report the free flux flow (FFF) resistivity associated with a purely viscous motion of the vortices in moderately clean d-wave superconductor Bi:2201 in the strongly overdoped regime ( T c =16 K) for a wide range of the magnetic field in the vortex state. The FFF resistivity is obtained by measuring the microwave surface impedance at different microwave frequencies. It is found that the FFF resistivity is remarkably different from that of conventional s-wave superconductors.

Unusual microwave vortex dissipation of Nd1.85Ce0.15CuO4

Abstract We have measured the microwave surface impedance Z S = R S +i X S of the electron-doped cuprate Nd 1.85 Ce 0.15 CuO 4 in magnetic field perpendicular to the superconducting layers. A broad peak was observed in the field dependence of flux flow resistivity ρ f obtained from Z S . This peak shifts to higher field with increasing microwave frequency, indicating that the peak originates not from the vortex pinning effect but from the electronic excitations. We discussed several possible origins for this peak.

Heat capacity of YNi2B2C in the vortex state: evidence of the extended s-wave state

Abstract The low temperature heat capacity C p and microwave surface impedance Z s in the vortex state of YNi 2 B 2 C have been studied. In contrast to conventional s-wave superconductors, C p shows a nearly H 1/2 -dependence. This H 1/2 -dependence is little affected by the introduction of the columnar defects. On the other hand, flux flow resistivity obtained from Z s increases linearly with H . These results indicate that in the vortex state of YNi 2 B 2 C the delocalized quasiparticle states around the vortex core play a much more important role, similar to d-wave superconductors.

Microwave surface impedance in overdoped Tl2Ba2CuO6+δ

The microwave surface impedance Zs=Rs+iXs and transmittivity TH′ of overdoped Tl2Ba2CuO6+δ have been measured in a magnetic field up to 7 T in order to clarify the unusual temperature dependence of the DC resistive-transition field H∗. From the result of TH′, columnar defects enhance H∗, indicating that H∗ is the melting point of vortices instead of the mean field Hc2. The field dependence of Rs is found to be unusual as compared with that expected from the result of the DC resistivity.

Microwave Flux Flow Resistivity of Clean Limit Superconductor YNi2B2C

We measured the microwave flux flow resistivity ρf of the clean limit s-wave superconductor YNi2B2C. We show that ρƒ increases linearly with H. This result shows that the number of the quasiparticle inside the vortex core increases in proportion to H and provides a strong evidence that the size of the vortex core does not change with H. This is inconsistent with the recent µ SR and STM results which claim that the core shrinks with H. Moreover, core shrinkage with decreasing T in constant H (Kramers-Pesch effect) was not observed down to 1.5K (~0.1Tc). The present results imply that the YNi2B2C is a s-wave superconductor with the anisotropic energy gap.

Anomalous Microwave Response and Upper Critical Field in Overdoped Tl2Ba2CuO6+δ

The transmittivity T H and microwave surface impedance Zs = Rs+iXs have been measured in order to clarify the unusual dc resistive transition of overdoped Tl2Ba2Cu2O6+δ in magnetic field. The transition field H* is enhanced by the columnar defect, indicating that H * is not the mean-field upper critical field but the depinning point of the vortices. Rs shows unusual field dependence which is quite different from that expected from the result of the dc resistivity.