Our Research Interests and Key Results
-Synthesis of New Superconducting Materials
"At the extream forefront of research in superconductivity is the empirical search for new materials." One of the targets of our research is the synthesis of new high-temperature superconductors by Molecular beam epitaxy (MBE), which is a potential synthetic technique leading to a breakthrough in the search for new materials. In recent years, we have discovered new superconducting Ba2CuO4-d (Tc~90K), Sr2CuO4-d (Tc~75K) and PbSr2CuO5+d (Tc~40K), which cannot be obtained by conventional solid-state reaction. Moreover, we have succeeded in synthesizing (La, Ce) 2CuO4 (Tc~30K) and (Sr, La)CuO2 (Tc~39K), high-quality samples of which are difficult to obtain by bulk synthesis.
Recently, we also presented that T and T'-La214 films can be selectivly grown by MBE with an appropriate choice of growth temperature and substrate material. Regarding non-cuprate superconductors, we have succeeded in preparing as-grown superconducting MgB2 films with Tc of 36 K.
These works demonstrate that MBE is capable of opening up a novel synthetic route for new high-temperature superconductors.
-HTSC Thin Films under Epitaxial StrainBy utilizing lattice-mismatch strain generated at an interface between a film and a substrate (epitaxial strain), we firstly obtained HTSC thin films with Tcvalues higher than those for bulk samples under the ambient pressure; for the compressed films of La2-xSrxCuO4 and La2-xBaxCuO4 on LaSrAlO4 substrates, Tc reached 44 K and 47K, respectively. Moreover, the Tc-x phase diagrams for the films did not show a local minimum at x~0.125, the so-called "1/8 anormaly." The compressive strain expands the c-axis via the Poisson effect and seems to suppress formation of the low-temperature tetragonal phase. We supeculate that the both effects result an increase in the bond length between Cu and the apical oxygen, which is responsible for the enhancement of Tc. We also found that the lower residual resistivity gives a higher Tc. Based on the experimental results, we suggest that reduction of spin fluctuation is effective to increase Tc in the HTSCs.
The minimum number of CuO2 plane prerequisite for superconductivity is one of the most important issues in the study of the thin film growth of HTSCs. We have succeeded in growing superconducting (001) La1.85Sr0.15CuO4 films with thickness less than 5 unit cells on LaSrAlO4 substrates without any buffer or cap layers. 4-unit-cell-thick films show bulk-like transport properties and Tc(end), whereas Tc disappears in films with thickness less than 2 unit cells, suggesting that the minimum number of CuO2 planes necessary for superconductivity is less than or equal to four.
Application -Large-Area High-Tc Wafers by MBE Double-sided large-area high-Tc wafers for microwave applications have been grown so far by coevaporation with a special substrate heater design, PLD with substrate scanning, sputtering, and MOCVD. Although these techniques have produced fairly high-quality films, we believe that there is a need for further improvement of the quality, uniformity, and reproducibility to meet the reliability requirements for real commercialization. For this purpose, we have been growing double-sided large-area (so far up to 2 inches) films by UHV-MBE. The resultant c-axis oriented Dy-123 films on MgO substrates show Tc(end) > 90 K, r(300 K) < 300 mWcm. The full-width at half-maximum of the (005) XRD peak is 0.3 - 0.5 degree. AFM images show that grains of films are square with a dimension of 200-300 nm, indicating that the microstructure is quite different from that typically observed for PLD or sputtered films.