반데르발스 초전도 접합을 통한 조셉슨 커플링

Abstract

Physics has advanced as experiment and theory supported each other. Realization of monolayer graphene by mechanical exfoliation has enabled the studies of massless Dirac fermions in condensed matter systems, owing to its linear energy-momentum dispersion relation. Atomically thin two-dimensional (2D) materials are attracting attention for observing and engineering quantum effects, since quantum effects become more accessible in such a low-dimensional system due to the confinement effect. Furthermore, studies on 2D materials have acquired the following tools through the dry-transfer technique using van der Waals (vdW) force: i) encapsulation to protect intrinsic properties of 2D materials by isolating from the surrounding environment, ii) atomically clean and flat interface of vdW hetero structures, and iii) twist angle that can tune the electronic properties of that interfaces. Hence, 2D materials including graphene, with their novel properties, have been investigated extensively to date. However, there are technical difficulties studying mechanically fragile and chemically sensitive layered materials. Fabrication method compatible with sensitive layered materials is a prerequisite for widening the scope of vdW layered material research. This thesis consists of two parts. The first one focuses on realization and transport characteristics of the planar graphene Josephson coupling through vdW superconducting contacts. The second one presents the demonstration of microcleave-and-stack method, which is compatible with air-sensitive materials, and discusses twist-angle-tunable Josephson coupling of vdW-contacted high-critical-temperature superconductors, formed by microcleave-and-stack method. In the first part, two pieces of cleaved 2H-NbSe2 superconducting flakes and a monolayer graphene sheet serve as the superconducting electrodes and the normal-conducting spacer, respectively. A stack of NbSe2−graphene−h-BN heterostructure with clean and flat interfaces was prepared by a dry transfer technique. The outermost h-BN layer protected the NbSe2−graphene−NbSe2 Josephson junction from chemical contamination during the fabrication processes. The Josephson coupling was confirmed by a periodic modulation of the junction critical current I_c as a function of a perpendicular magnetic field. The temperature dependence of I_c showed long and diffusive Josephson coupling characteristics. The temperature dependence of the superconducting gap, obtained from the multiple Andreev reflection features, followed the Bardeen−Cooper−Schrieffer (BCS) prediction. Our results demonstrated the atomically clean and flat interfaces of the vdW heterostructure, and suggest their possible utilization for hybrid devices. In the second part, we discuss anisotropic superconducting order parameters in twisted Bi2Sr2CaCu2O8+x (Bi-2212) vdW junctions with an atomically clean vdW interface, achieved by microcleave-and-stack technique developed in this study. The vdW junctions with twist angles of 0° and 90° showed the maximum Josephson coupling, comparable to that of intrinsic Josephson junctions. As the twist angle approaches 45°, Josephson coupling is suppressed, and eventually disappears at 45°. The observed twist angle dependence of the Josephson coupling can be quantitatively explained by theoretical calculation with the d-wave superconducting order parameter of Bi-2212 and finite tunneling incoherence of the junction. Our results revealed the anisotropic nature of Bi-2212 and provided a novel fabrication technique for vdW-based twistronics platforms compatible with air-sensitive vdW materials.