Ken Horiuchi

Effect of oxidation gases on the formation of Bi2Sr2CuO6 thin films

Three kinds of oxidation gases, O2, N2O, and NO2, have been used during the deposition of Bi2Sr2CuO6 thin films, and the effect of the gases on the film formation has been compared. Ambient N2O with substrate irradiation by ArF excimer laser is effective for lowering the substrate temperature to form an as‐grown film. The as‐grown film formed in N2O, however, has a higher resistivity due to the lower oxygen concentration in the film. The mixed gas of N2O and O2 gives the best result. NO2 is very effective for film formation under low‐pressure conditions, and makes it possible to form a crystallized as‐grown film at such a low pressure of 1×10−4 Torr even in a conventional laser ablation experiment.

Bi2Sr2Ca1Cu2O8/Bi2Sr2Cu1O6 Superconducting Superlattices: Modulation of the Number of CuO2 Plane between Bi2O2 Layers

Superlattices with a combination of a superconductor and semiconductor have been formed by the periodic stacking of Bi2Sr2CaCu2O8 and Bi2Sr2CuO6 layers. The observed X-ray diffraction patterns of the superlattices are consistent with those calculated from the ideal superstructures. The resistivity-temperature curve of the superlattices containing three unit layers (45 A) of Bi2Sr2CaCu2O8 exhibits the same superconducting behavior as that of a Bi2Sr2CaCu2O8 film, indicating the Bi-Sr-Ca-Cu-O system has a strong two-dimensional nature.

Superconductivity of strained YBa2Cu3O7/(La1-xSrx)2CuO4 superlattices

Abstract We have formed three kinds of YBa2Cu3O7-based superconducting superlattices with different carrier concentration and different strain. These superlattices are YBa2Cu3O7 ( a = 3.88 A , b = 3.83 A )/ PrBa2Cu3O7 ( a = 3.92 A , b = 3.86 A ), YBa2Cu3O7/ La1.85Sr0.15CuO4 ( a = b = 3.78 A ) and YBa2Cu3O7/La2CuO4 ( a = 3.82 A , b = 3.79 A ). The lengths of the a- and b-axes in the YBCO layer can be controlled by changing the neighboring layers. When the thickness of the YBCO layer decreases, the YBCO/ PBCO superlattice shows large Tc deterioration, while the YBCO/LSCOor LCO superlattices with compressed ab-plane show smaller Tc deterioration for thickness larger than 36 A (3 unit cells). This tendency is independent of carrier concentration of the neighboring layer. When the thickness is below 36 A, YBCO/LSCO shows higher Tc than YBCO/LCO, although the strain is the same for both superlattices. These behaviors suggest that the in-plane stress along the ab-axis is important to control the Tc value, and the hole filling effect is also important for ultra-thin films.

(La2−xSrxCuO4)m(Ca1−zSrzCuO2)n and (Ba2CuO3+y)m(Ca1−zSrzCuO2)n superlattices prepared by laser ablation

Abstract The preparation of new materials with superlattice, consisting of La 2− x Sr x CuO 4 or Ba 2 CuO 3+ y and the infinite layer (Ca 1− z Sr z CuO 2 ) was studied to obtain CuO 2 multilayered new copper-oxides. From the X-ray diffraction patterns and transmission electron microscopy (TEM) images on the ( La 2−x Sr x CuO 4 ) m ( CaCuO 2 ) n superlattice, it was found that one, two or three unit cells of the infinite layer can be inserted between the La 2− x Sr x CuO 4 blocking layers. A slight bend was observed at around 105 K in the temperature-dependence curve of the resistivity ( ϱ - T curve) of the superlattice consisting of nine unit cells of La 1.6 Sr 0.4 CuO 4 and two unit cells of CaCuO 2 . On the other hand, a bend was observed in the ϱ - T curve at about 150 K on the thin films being stacked up with ten unit cells of Ba 2 CuO 3+ y and three unit cells of CaCuO 2 alternately.

High Tc superconducting artificial lattice as a new material

Abstract High Tc superconducting artificial lattices have been constructed by a layer-by-layer laser molecular beam epitaxy method with atomic layer and sub-unit cell accuracy, and the basic structural factors in the high Tc cuprates have been controlled artificially. In the Bi 2 Sr 2 Ca n−1 Cu n O 2n+4 artificial lattices, the numbers of CuO 2 planes are changed from one to even eight in the unit formula. The compression and expansion of the CuO 2 plane are demonstrated in the strained superlattices of YBa 2 Cu 3 O 7 /(La,Sr) 2 CuO 4 and (La,Sr) 2 CuO 4 /Sm 2 CuO 4 . In these superlattices, the change of the lattice constant along a,b axes is proved to be one of the important controlling factors in the high Tc cuprates. This technique has been also applied to obtain reliable tunneling spectra of Au/Bi 2 Sr 2 CiO 6 /Bi 2 Sr 2 CaCu 2 O 8 , and reproducible BCS-like spectra have been obtained in this system. The results indicate that the high Tc cuprate is essentially the BCS superconductor.

Crystal structures and superconductivities of (La,Sr)2CuO4/Sm2CuO4 and YBa2Cu3O7/(La,Sr)2CuO4

Abstract Superconducting strained superlattices have been formed by epitaxial stacking along c-axes of two kinds of unit layers, such as (La,Sr) 2 CuO 4 /Sm 2 CuO 4 and YBa 2 Cu 3 O 7 /(La,Sr) 2 CuO 4 . In the case of (La,Sr) 2 CuO 4 /Sm 2 CuO 4 superlattice, the Tc becomes lower and ab-plane are expanded with decreasing the number of stacking layers. For YBa 2 Cu 3 O 7 /(La,Sr) 2 CuO 4 , on the other hand, the Tc and is 72K even in the range of 24A (2 unit layers) thickness of YBCO layer with the compression of the ab-plane. The correlation between the Tc and the in-plane strain along ab-axes observed here suggests that the pressure effect to change the CuO bond length along the CuO 2 plane is important for the Tc value.

Superconducting artificial lattices grown by laser molecular beam epitaxy

The laser molecular beam epitaxial technique for forming superconducting thin films and artificial lattices is presented. Tunnel junctions, superlattices and tailored thin films have been formed by the layer-by-layer method using this technique. In the junction of Au/Bi2Sr2CuO6/Bi2Sr2CaCu2O8, reproducible tunnel spectra have been obtained. Strain effects are observed to induce Tc changes in YBa2Cu3O7 La2CuO4 and La2CuO4 superlattices and this effect is discussed on the basis of the pressure effect of high Tc cuprates. The growth mechanism of the Bi2Sr2Can−1CunO2n+4 (n = 1−8) films in the order of unit cell, subunit cell and atomic layer has been revealed.

Layered Growth Of HTSC Thin Films Using Pulsed Laser Deposition

I. Introduction: Characteristic Features Of Laser Ablation Method For HTSC Thin Films Among the various methods for the HTSC film formation, the laser ablation method has following advantages. First, the film formation can be performed under a high oxygen or N20 pressure, such as 10-1 torr.1 This condition is not easily satisfied by other techniques such as MBE and evaporation. Second, there is small deviation of the composition of the films from the target composition used for the film formation. This is advantageous for the films with multi-component systems, such as Y-Ba-CL-O2 or Bi-Sr-Ca-Cu-0.3) Third, photochemically excited species produced by ablation can contribute to the low temperature film formation.4) Furthermore, the laser beam can be intentionally used for the substrate excitation to lower the substrate temperature and to increase the crystallinity of the films.