M. Hirai

Preparation of polycrystals with various Tc and single crystal growth of Ba2Ca3Cu4O8(O1-yFy)2 under high pressure

Abstract Polycrystalline samples of Ba 2 Ca 3 Cu 4 O 8 (O 1− y F y ) 2 (0234F) with a variety of T c (55–105 K) were synthesized under high pressure. The dynamic range of the T c 50 K in an underdoped region, has been found to be remarkably wide. We also found that plate-like single crystals of 0234F (∼1 mm 2 in plane area and 30 μm in thickness) grow when samples with a nominal composition of y ∼1.0 are heated at high temperature (∼1150 °C). Cleavage was observed for the crystals. The 0234F will be a convenient material for investigations of physics of the multi-layered superconductors and/or the underdoped state that has a key for understanding high- T c superconductivity.

Crystal growth of Ba2Can−1CunO2n(O,F)2 (n = 3 and 4) multi-layered superconductors under high pressure

Single crystals of multi-layered superconductors of Ba2Can−1CunO2n(O,F)2 (F-02(n − 1)n, n = 3 and 4) have been grown under a pressure of 4.5 GPa. Crystals were obtained by cooling samples with nominal compositions of Ba2Can−1CunO2nF2 (n = 2, 2.5, 3 and 4) across the melting point. The crystals are thin plate-like shapes with typical sizes of 500 µm in plane length and 30 µm in thickness. The crystals are found to show cleavage, which will be convenient for surface sensitive measurements. The F-0223 and F-0234 crystals showed Tc of 90 K and 70 K, respectively after oxygen annealing. The temperature dependence of resistivity showed pronounced underdoped behaviour. Composition analysis showed that a ratio of Ba:Ca:Cu is close to the stoichiometric composition one. We think that the F-system is a rare multi-layered cuprate that can largely grow under high pressure and its crystals are quite valuable to study both high-Tc superconductivity and the multi-layered system.

Zn and Ni Doping Effect on Anomalous Suppression of Tc in an Over Doped Region of TlBa2Ca2Cu3O9−δ

Variation of Tc in the over doped region have been investigated for Zn and Ni doped samples of TlBa2Ca2Cu3O9−δ to obtain a clue to understand the anomalous suppression of Tc. We expected that if the anomaly in Tl-1223 originates from the stripe order like La-214 system, the suppression of Tc is enhanced by the doping. The results showed that the anomaly disappeared by both of the doping contrary to the expectation so that the possibility of the stripe order was ruled out. Two possible explanations were proposed based on a self-doping mechanism and inhomogeneous charge distribution to crystallography different CuO2 layers.

Electron-doped superconductivity induced by oxygen vacancies in as-grown Sr0.6Ca0.4CuO2?? infinite-layer films

Thin films of the infinite-layer compound Sr0.6Ca0.4CuO2?? have been prepared by off-axis rf magnetron sputtering on SrTiO3 (001). For an optimum level of oxygen vacancy superconductivity was observed (Tc onset = 42 K and Tc(? = 0) = 11 K). Structural and transport data suggesting n-type superconductivity are presented and discussed. Infinite-layer thin films of Sr0.6Ca0.4CuO2??, without trivalent cation doping and showing n-type superconductivity, are reported for the first time.

Preparation of CuBa2Ca3Cu4Oy superlattice thin film by self-assembling epitaxy method

Abstract We report on the epitaxial growth of 1:2:0:1 type CuBa2CuOy layer, an infinite layer CaCuO2 and their superlattice (1×3) by self-assembling epitaxy method, involving rf magnetron sputtering, on (1xa00xa00) SrTiO3 substrate. Excellent (0xa00xa01) films of both the CuBa2CuOy and CaCuO2 with c∼8.4 and 3.2 A, respectively, were obtained. A superlattice with a period of about 18.5 A which corresponds to the formula CuBa2Ca3Cu4Oy was prepared by growing alternatively the 1:2:0:1 and CaCuO2 at about 500°C in a gas mixture 10–20 mTorr of Ar+N2O. The as-grown superlattice film showed a superconducting diamagnetic signal at about 20 K.

Dynamics of multiple order parameters in the multi-band superconductor studied by Raman spectroscopy

Abstract We controlled the dynamics of the superconducting order parameter keeping the same T c on Cu x Ba 2 Ca 3 Cu 4 O y . There are two pair-breaking bands corresponding to two superconducting gaps in Raman spectrum of the as-synthesized sample. One of them smeared out by a simple annealing.

Annealing study of superconducting properties in a Cu-1223 superconductor using O2-HIP apparatus

Abstract A (Cu,C)Ba 2 Ca 2 Cu 3 O x superconductor [(Cu,C)-1223)] is synthesized under GPa order pressure condition. Due to the high oxidizing condition, an as-prepared sample becomes overdoped with T c at about 70 K. The T c rises up to 120 K (an optimal doped state) by reduction of oxygen. Details of oxygen behavior in reduction annealing has not been investigated yet. In this paper, we clarified the relationship between oxygen contents and T c . We also investigate control of reversible oxygen contents by annealing under both 1 atm and high pressure using an O 2 -HIP apparatus in a (Cu,C)-1223 superconductor and annealing effects in a (Cu,C)Ba 2 Ca 3 Cu 4 O x superconductor [(Cu,C)-1234].

Anomalous suppression of Tc in an overdoped region of TlBa2Ca2Cu3O9−δ

Abstract Anomalous suppression of T c has been found in an overdoped region of TlBa 2 Ca 2 Cu 3 O 9− δ superconductors. A heavily overdoped sample ( T c ≃100 K) was synthesized under high pressure. The T c was measured with decreasing the oxygen content until the sample becomes optimally doped state with a T c ≃131 K. A dip of T c was observed before the T c reaches to an optimal value i.e. in an overdoped region. This behavior is quite strange because T c is known to change monotonically in an overdoped region. Some possible explanations for this anomalous behavior will be presented.

Heavy-ion irradiation dependence of the superconducting properties of (Cu,C)Ba2Ca3Cu4O10.5−δ

Abstract To enhance or improve critical currents density ( J c ) and irreversibility field ( H irr ) of (Cu,C)Ba 2 Ca 3 Cu 4 O 10.5− δ ((Cu,C)-1234), pinning centers were introduced by heavy-ion irradiation. The polycrystalline samples were irradiated with Au 15+ ions (240 MeV energy) at various fluence of 1×10 11 , 2.5×10 11 and 5×10 11 ions/cm 2 . J c and H irr were determined for the irradiated samples as well as unirradiated samples. J c (77 K, 1 T) increase from 3.9×10 4 A/cm 2 to 4.1×10 6 A/cm 2 for at fluence of 1×10 11 ions/cm 2 of heavy-ion irradiated sample and decreases with further increase of fluence. These results indicate the possibility of further enhancement of J c and of achieving a very high H irr of (Cu,C)-1234 below a fluence of 1×10 11 ions/cm 2 .

Fish-Tail Effect and Irreversibility Field of (Cu, C)Ba2Ca3Cu4Ox-(LiF)y Superconductor

Addition of (LiF)