Takashi Manako

Orbital-State-Mediated Phase-Control of Manganites

Using the epitaxial strain, the magnetic and electronic phases can be controlled for thin films of the manganites, La1-xSrxMnO3, grown on perovskite substrates with various lattice parameters. The strain-induced orbital-ordering (disordering) via coupling

Tunneling magnetoresistance at up to 270 K in La0.8Sr0.2MnO3/SrTiO3/La0.8Sr0.2MnO3 junctions with 1.6-nm-thick barriers

Magnetic tunneling junctions are fabricated from epitaxially grown La0.8Sr0.2MnO3/SrTiO3/La0.8Sr0.2MnO3 trilayers. A large tunneling magnetoresistance of 150% is observed for a junction with a thin barrier layer (1.6 nm) under a low switching field (<10 Oe) at 5 K. A small tunneling magnetoresistance is observed even at 270 K, which is close to the ferromagnetic Curie temperature (290 K) of the La0.8Sr0.2MnO3 film. The large magnetoresistance and high operating temperature are attributed to the sufficiently thin and uniform barrier layer of SrTiO3.

Epitaxial thin films of ordered double perovskite Sr2FeMoO6

Epitaxial thin films of Sr2FeMoO6, that are ordered double perovskite with half-metallic nature, have been successfully prepared on SrTiO3 (001) and (111) substrates by pulsed laser deposition in a narrow window of the temperature and oxygen pressure. From the surface morphology analysis for the atomic scale step-and-terrace structures, the film growth is concluded to take place with the chemical formula as the growth unit when the ordering direction is normal to the surface. The films showed metallic conduction with ferromagnetic transition temperature above 400 K and intergrain tunneling type magnetoresistance even at room temperature.

Variation in Tc, structure and oxygen content in Tl2Ba2CuO6 and their correlations

Abstract The preparation condition of Tl 2 Ba 2 CuO 6 (2201 compound) and its properties including superconductivities and structural variations were studied. Preparation at relatively low heat-treatment temperatures produced the 2201 compound with an orthorhombic symmetry, while heat-treatment above 840°C produced a tetragonal symmetry. It was revealed that the appearance of superconductivity was independent of the structural symmetry, and was strongly affected by oxygen content. Decrease of oxygen content by quenching or Ar-annealing increased the superconducting transition temperatures up to about 70 K. A positive correlation between T c and the c -axis length was also found for all samples including both orthorhombic and tetragonal structures.

Rietveld analysis of Tl2Ba2Can−1CunO4+2n (n=1, 2 and 3) by powder x-ray diffraction

Abstract Crystal structures of the phases in the Tl-Ba-Ca-Cu-O system determined by powder X-ray diffraction were refined using Rietveld analysis. Three samples having the general composition Tl 2 Ba 2 Ca n −1 Cu n O 4+2 n where synthesized. These were Tl 2 Ba 2 Ca 2 Cu 3 O 10 ( n =3,2223 phase ) with a T c of 116 K, Tl 2 Ba 2 CaCu 2 O 8 ( n =2,2212 phase ) with a T c of 85 K, and Tl 2 Ba 2 CuO 6 ( n =1,2201 phase ) which did not exhibit superconductivity. The results of the Rietveld analysis revealed that as n increased the lattice parameter, a , and the interatomic distance Cu-O(2) became shorter, and the Tl-O and Ba-O layers showed a tendency to become flatter. Possible substitution of about 10% of the Ca-sites with Tl ions in the 2212 and 2223 phases was also suggested.

Correlation between the pressure-induced changes in the Hall coefficient and Tc in superconducting cuprates

Abstract The pressure dependences of the superconducting transition temperature and the Hall coefficient have been studied for YBa 2 Cu 3 O 7 , YBa 2 Cu 4 O 8 , Bi 2 Sr 2 CaCu 2 O 8 , and Tl 2 Ba 2 CuO 6+ y . The pressure coefficient of T c changes from positive to negative as one goes from the underdoped to the overdoped side. The Hall coefficient has been found to decrease by ∼ 8–10%/GPa for these cuprates. Based on these results, the correlation between the pressure dependence of T c and the carrier density dependence of T c is discussed. The present results are also compared with our earlier result on La 2− x Sr x CuO 4 .

Appearance of a Maximum of Tc and a Large Negative dTc/dP in the Superconducting Tl-Ba-Ca-Cu-O Compounds under Pressure

The pressure dependence of T c for the superconducting Tl-Ba-Ca-Cu-O compounds has been measured. It is found that the pressure coefficients of T c in Tl 2 Ba 2 CaCu 2 O 8 change the sign from positive to negative with a maximum T c and that the highly hole-doped Tl 2 Ba 2 CuO 6 compounds show a large negative value of d T c /d P . These results revealed a new feature of the pressure dependence of T c in the hole-doped superconductors and can be interpreted speculatively in terms of the hole concentration in the CuO 2 planes assuming that the carrier concentration is redistributed with pressure.

Spin dynamics in heavily-doped high-Tc superconductors Tl2Ba2CuO6+y with a single CuO2 layer studied by 63Cu and 205Tl NMR

Abstract Knight shift of 63 Cu and nuclear spin lattice relaxation time T 1 of 63 Cu and 205 Tl have been measured in Tl 2 Ba 2 CuO 6+ y with T c =72 K, 40 K and 0 K. In normal state, the spin part of the 63 Cu shift is T -independent and increases with decreasing T c . 1/ T 1 of 63 Cu and 205 Tl obey a T 1 T =constant law over a wide T -range from about 150 K close to T c . Above about 150 K and near T c , (1/ T 1 T ) begins to decrease. The T -linear dependence of the resistivity observed in 72 K superconductors changes into a T 2 dependence in a non-superconducting metal in which the T 1 T =constant law holds up to around 200 K. Analysis of the anisotropy of the relaxation rate, 63 R =(1/ T 1 ) ab (1/ T 1 ) c reveals that the antiferromagnetic (AF) spin fluctuations among nearest neighbor Cu sites, which survives still in the compound with T c =72 K, becomes less distinct with decreasing T c and disappears in a non-superconducting sample. These results point out that there may be an intimate interrelation between the occurrence of superconductivity and the presence of the AF spin correlation, although the magnetic coherence length, ξ M is supposed to be as small as ξ M / a ≤∼1. Experimentally, it is shown that the T -linear dependent resistivity is relevant to the presence of the AF spin correlation. In the superconducting state, the relaxation behavior is found to possess a universal feature in most of the high- T c oxides, indicating a common relaxation mechanism to work even in heavily-doped superconductors exceeding the maximum of T c .

Superconductivity of TlBa1+xLa1−xCuO5 with 1201 structure

Abstract A new superconducting phase has been found in the Tl-Ba-La-Cu-O system. The powder X-ray diffraction and EPMA study revealed that this new phase has a chemical composition of TlBa 1+ x La 1− x CuO 5 ( x = 0-0.4) and a crystal structure identical with TlBa 2 CuO 5 (1201 phase). The lattice parameter c 0 became longer with increasing x , while a 0 exhibited no significant change. For x = 0, the temperature dependence of the resistivity was semiconductor-like. With increasing x value, the samples came to show superconductivity and finally changed to a metallic nonsuperconductor at x = 0.4. The maximum T c value was about 40 K at around x = 0.2.

T2-dependence of inverse Hall mobility observed in overdoped Tl-cuprates

Abstract Temperature dependences of the resistivity ϱ and the Hall coefficient R H for overdoped Tl 2 Ba 2 CuO 6- δ (Tl-2001) and Tl - Sr 2 CaCu 2 O 7- δ (Tl-1212) are analyzed in terms of the inverse Hall mobility μ -1 H = ϱ / R H . The temperature dependence of μ -1 H shows a clear T 2 -dependence for the entire carrier concentration range from high- T c superconductor to overdoped normal metal. This is in sharp contrast to the complicated temperature dependences of both ϱ and R H . These facts strongly suggest that the carrier concentration is variable with temperature as indicated by R H , whereas the scattering rate is always associated with a single scattering mechanism being ∽ T 2 . In that case, many “anamalous” normal state properties will disappear, and a rather simple single-band Fermi liquid picture emerges.