Hetero-Epitaxial Growth of YBCO and MgB 2 Thin Films on Decorated Substrates with Nano-Wires and Nano-Particles

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Hetero-Epitaxial Growth of YBCO and MgB2 Thin Films on Decorated Substrates with Nano-Wires and Nano-Particles Shigetoshi Ohshima Graduate School of Science and Engineering, Yamagata University, 4-3-16, Jonan, 992-8510, Japan

ABSTRACT Superconducting NMR pickup coils have to have a low surface resistance (Rs) under a high magnetic field. One way of reducing the Rs of superconducting thin films is adding artificial pins to the films. We examined the hetero-epitaxial growth of MgB2 and YBCO thin films on decorated sapphire and MgO substrates while using nano-wires and nano-particles to add pins to the films. We used ZnO nano-wire to add artificial grain boundary pins to MgB2 films. The tilted c-plane sapphire substrate with nano-step edges was used for these substrates. The terrace width with one lattice step edge can be controlled by changing the tilt angle. AFM images showed that the tilted c-plane sapphire substrate had straight nano-step edges. ZnO nono-wires were decorated on the nano-step edges by MOCVD, and then deposited on MgB2 films by sputtering. The Rs of the films was then measured with a sapphire rod resonator. The Rs of MgB2 thin films fabricated on the tilted c-plane sapphire substrate with ZnO nano-wires was lower than that of MgB2 thin films on a conventional sapphire substrate under a high magnetic field. We used BaZrO3 (BZO) nano-particles to add the artificial pins to YBCO thin films. The BZO particles on the MgO substrates were fabricated by laser deposition. The size and density of these particles were controlled with a pulse number of laser shot. YBCO thin films were formed on decorated MgO substrate by laser deposition. The Rs of YBCO thin films on the decorated MgO substrate was lower than that of the YBCO films formed on a conventional MgO substrate under a high magnetic field. We clarified that artificial pins in MgB2 and YBCO thin films are useful for decreasing the Rs under a high magnetic field. INTRODUCTION The surface resistance of a superconducting thin film is lower than that of conventional conductors by about three orders at a microwave frequency [1-3]. Superconducting filters have therefore been used as receiving filter system in base stations for cellular telephone operations in the USA [4] and China [5]. The small surface resistance of a superconductor is applicable to many devices in various fields [6-9]. We are currently in the midst of developing a highly sensitive NMR pickup coil. The S/N of an NMR pickup coil is estimated by the following equation [10]: where, Tc is the pick-up coil temperature, Ta is the pre-amplifier temperature,K is S/N |

M 0 B1 KQ # , Tc  Ta Ts Rs  (Ta  Tc ) Rc

105

(1)

the filling factor, and Q is the quality factor of the pickup coil. Superconducting pick-up coils have been shown to be useful for increasing the S/N of NMR signals. For a high efficient coil, we need optimum superconducting thin films with a low Rs under high magnetic fields. However, there have been few reports of a superconducting thin film Rs under high magnetic fields, over