Masatsune Kato

Metal-insulator transition and spin gap in the spin-12 ladder system Sr14−xAxCu24O41 (A Ba and Ca)

Abstract We have investigated the transport and magnetic properties of the spin - 1 2 ladder system with two legs Sr 14− x A x Cu 24 O 41 (A  Ba and Ca). For x = 0, both electrical resistivity and thermoelectric power exhibit semiconductive behavior. It becomes more insulating with the increase in x (Ba), while it becomes more conductive with the increase in x (Ca). A metal-insulator transition is found at x (Ca) = 6.0–8.4 from the thermoelectric power measurements. The magnetic susceptibility exhibits a broad peak around 80 K for x = 0. Subtracting the Curie component at low temperatures, the remainder of the susceptibility χ s ( T ) decreases toward zero with decreasing temperature below ∼80 K, which implies the presence of a spin gap. The temperature at which the broad peak shows the maximum changes little through the partial substitution of Ba and Ca for Sr. However, the value of χ s ( T ) decreases with the increase in x (Ba), while it tends to increase with the increase in x (Ca). These properties are discussed in terms of the redistribution of holes between the two different sites of Cu. It appears that the observed spin gap behavior is due to Cu 2+ spins in the CuO 2 chain rather than in the Cu 2 O 3 plane.

Growth, Annealing Effects on Superconducting and Magnetic Properties, and Anisotropy of FeSe1-xTex (0.5≤x≤1) Single Crystals

Single crystals of FeSe 1- x Te x (0.5≤ x ≤1) have been grown by the Bridgman method. After annealing them at 400 °C for 100 h in vacuum, single crystals of x =0.5–0.9 have exhibited bulk superconductivity. Anisotropic properties of the electrical resistivity and upper critical field, H c2 , have been investigated for the single-crystal FeSe 1- x Te x with x =0.6. It has been found that the in-plane resistivity, ρ a b , shows a metallic temperature-dependence, while the out-of-plane resistivity, ρ c , shows a broad maximum around 100 K. The resistivity ratio, ρ c /ρ a b , is 44 and 70 at 290 K and just above the superconducting transition temperature, T c , respectively. The anisotropic parameter, γ≡ H c2 || / H c2 ⊥ (superscripts ∥ and ⊥ indicate field directions parallel and perpendicular to the a b -plane, respectively.), is estimated as 2.7 just below T c .

Superconductivity in YBa2Cu3-yNiyO7-δ

Superconductivity in alloyed compounds YBa2Cu3-yNiyO7-δ has been studied in order to gain direct insights on the roles different Cu sites play in electrical conduction. Electron probe microanalysis as well as X-ray powder diffraction analysis shows that Ni is actually substituted for Cu in the oxygen-deficient perovskite compound for y at least as much as 0.27. Electrical resistivity measurements revealed that the substituted Ni does not substantially destroy the superconductivity.

Superconductivity in Impurity-Induced Tetragonal YBa2(Cu1-xFex)3O7-δ

Lattice deformation and superconductivity in YBa2(Cu1-xFex)3O7-δ are studied in detail. An orthorhombic to tetragonal phase change is induced at xc≈0.02. A phase diagram in the temperature-concentration plane clarifies two regions of the tetragonal phase: one is, as known previously, induced mainly by the release of oxygen, and the other by the direct action of impurities. Bulk nature of superconductivity in the novel impurity-induced tetragonal phase is confirmed by magnetic susceptibility measurements.

Anomalous x dependence of Tc and possibility of low-temperature structural phase transition in La2−xSrxCu0.99M0.01O4 (M Ni, Zn, Ga)

Abstract Effects of the 1% substitution of Ni, Zn and Ga for Cu on the x dependence of T c have been investigated in La 2−x Sr x CuO 4 . The T c increases by the Cu-site substitution in all compositions of x. The local minimum of T c as a function of x, appearing slightly near x = 0.115 for La 2−x Sr x CuO 4 , becomes conspicuous with decreasing T c . However, structural phase transition from the orthorhombic mid-temperature phase to the tetragonal low-temperature phase has not been detected from powder X-ray diffraction measurements at low temperatures down to 12 K. The local minimum of T c is attributable to the fluctuation of the undiscovered structural phase transition, which will occur at a low temperature below 12 K as long as samples are in the normal-conducting state. In the 1% Ga-substituted system for Cu, the local minimum clearly shifts from x = 0.115 to 0.125. This suggests that the undiscovered low-temperature structural phase transition is due to the optimum hole concentration of 0.115 per Cu atom, analogously to La 2−x Ba x CuO 4 .

NMR Study on Spin Gap in Spin Ladder Sr14Cu24O41

We have investigated spin dynamics of both the ladder and the chain sites in Sr 14 Cu 24 O 41 by Cu-NMR and Cu-NQR. From the NMR shift, K spin , and nuclear spin-lattice relaxation time, T 1 , the energy gaps in the spin excitation spectrum are confirmed to be Δ=140 K for the chain site and Δ=470 K for the ladder site for Sr 14 Cu 24 O 41 . Doping effects on the spin gaps and electric field gradient at the Cu sites were also investigated by the substituting of Ca, Y and La for Sr. The spin gap of the ladder site decreases with increasing x for Sr 14- x Ca x Cu 24 O 41 and seems to collapse at around x =13 for Ca substitution.

New Intercalation Superconductor Lix(C6H16N2)yFe2−zSe2 with a Very Large Interlayer-Spacing and Tc = 38 K

A new iron-based superconductor, Lix(C6H16N2)yFe2−zSe2, with Tc = 38 K has successfully been synthesized via the intercalation of lithium and hexamethylenediamine into FeSe. The superconducting transition has been confirmed not only by magnetic susceptibility measurements but also by electrical resistivity ones. The interlayer spacing, namely, the distance between neighboring Fe layers, d, is 16.225(5) A, which is the largest among those of FeSe-based intercalation compounds. It has been found that the dependence of Tc on d in FeSe-based intercalation superconductors appears domic.

Dependence of Tc on the excess oxygen content δ in Bi2Sr2CaCu2O8+δ annealed under high pressures of oxygen

Abstract The dependence of the superconducting transition temperature T c on the excess oxygen content δ has been investigated for Bi 2 >Sr 2 >CaCu 2 >O 8+δ annealed under high pressures of oxygen. With increasing δ, T c decreases for δ≲0.3 and then becomes constant for δ≳0.3. It has been found by chemical titration that this strange δ dependence of T c is related to the δ dependence of the average valency of Cu ions. That is, the average valency increases with increasing δ for δ≲0.3 and then is saturated for δ≳0.3. For δ≳0.3, the Curie-Weiss component in the DC magnetic susceptability has been found to increase with increasing δ. Therefore, the saturation is ascribed to the fact that most of the excess oxygen atoms exceeding δ≈0.3 are introduced in the form of O 2 instead of O 2- .

Spin gap of the one-dimensional CuO2 chains in Sr14−yYyCu24O41−δ

Abstract We have measured the magnetic susceptibility χ of Sr 14− y Y y Cu 24 O 41−δ consisting of two-dimensional Cu 2 O 3 planes, namely, so-called spin- 1 2 ladders with two legs and also of one-dimensional CuO 2 chains. The susceptibility χ chain ( T ) due to the CuO 2 chains, obtained by subtracting the susceptibility χ ladder ( T ) due to the Cu 2 O 3 planes from the experimental data of χ ( T ), is described by the Curie-Weiss law at T > T max ∼ 80 K, where χ exhibits the maximum for y = 0. With increasing y , χ chain becomes large owing to the increase of Cu 2+ spins introduced into the CuO 2 chains. Moreover, the antiferromagnetic interaction between Cu 2+ spins in the CuO 2 chains becomes weak with the increase in y , namely, with the increase of Cu 2+ spins introduced into the CuO 2 chains, leading to suppression of the spin gap of the CuO 2 chains. As the result, the broad peak of χ, observed around 80 K for y = 0, is found to disappear for y ≧ 1.0 .

Rapid Preparation of YBa 2Cu 3O7-x with T c ∼ 90 K Using a Domestic Microwave Oven

High-Tc superconducting samples of YBa2Cu3O7-x with T c ~90 K have been prepared in a short time of 25 min, using a domestic microwave oven operated at 2.45 GHz and 200 W, without any post-heat treatment. Post-heat treatment is not necessary due to improvement of the sample environment during the microwave process. That is, a pellet of mixed powders of raw materials is surrounded by mixed powders of raw materials and subsequently wrapped in glass wool, in order to suppress rapid dissipation of heat from the surface of the pellet. This leads to successful preparation of homogeneous and oxygenated samples of YBa2Cu3O7-x.