Subject Number :S-19-MS-1063
Support Type : Common use
Proposal Title (English) : Magnetic structure of semiconductor/superconductor nanocomposites
Username (English) : T. Uchino
Affiliation (English) : Kobe University

1. Summary
We explore the electrical and magnetic properties of a fractal assembly of Josephson junctions with transparent interfaces. For this purpose, we employ an Mg/MgO/MgB2 nanocomposite with ~16 vol. % of MgB2 nanograins which are distributed in a fractal manner in the normal matrix. Irrespective of the low volume fraction of MgB2 nanograins, the nanocomposite behaves as a bulk-like superconductor, i.e., zero resistivity, perfect diamagnetism, and strong vortex pinning. Thus, a global Josephson phase coherence is achieved in the nanocomposite. The lower and higher critical fields of the Josephson network are exceptionally high as compared to those reported previously for granular superconductors. This will give an example of robust macroscopic superconducting coherence derived from long-range proximity coupling among fractally distributed superconducting nanograins through quantum interference of Andreev quasiparticles. Transverse-field muon spin rotation measurements reveal that the mean internal field in the superconducting mixed state increases with decreasing temperature below the superconducting transition temperature, opposite to the diamagnetic response observed in magnetization measurements. This unusual behavior implies a highly disordered and fluctuating nature of the Josephson vortices in the present superconducting nanocomposite.

2. Experimental
For the magnetoresistivity measurements, a square cuboid sample with a size of 2×2×10 mm3 was used. The magnetic field was provided by superconducting magnets in a commercial superconducting quantum interference device (SQUID) magnetometer (MPMS-XL, Quantum Design, San Diego, USA).

3. Results and Discussion
Figure 1 presents temperature-dependent electrical resistivity ρ(T) data taken at a variety of applied fields H ranging from 0 to 70 kOe. in zero applied field, ρ(T)exhibits Tc onset at 38.5 K and becomes virtually zero at temperatures below 33.4 K, implying the establishment of the intergrain Josephson network of MgB2 nanograins. Figure 1(a) further reveals that there is a large magnetoresistance ∆ρ/ρ0, where ρ0 is the zero-field resistivity in the normal state. The isothermal ∆ρ/ρ0 data shown in Fig. 1(b) vary almost linearly with H, in contrast to the case of MgB2.

4. Others
We are grateful to M. Fujiwara, and S. Iki for their strong support in the electrical conductivity and magnetoresistivity measurements.

5. Publication/Presentation
(1) T. Uchino, N. Teramachi, R. Matsuzaki, E. Tsushima, S. Fujii, Y. Seto, K. Takahashi, T. Mori, Y. Adachi, Y. Nagashima, Y. Sakaguchi, K. Ohishi, A. Koda, T. Sakurai, and H. Ohta, Phys. Rev. B 101, 035146 (2020).

6. Patent
N/A