새소식

Department of Physics & Astronomy

[이진호 교수 연구실] Nano letters 논문 게재

2019-03-03l 조회수 2105

Cooper Pair Density of Bi2Sr2CaCu2O8+x in Atomic Scale at 4.2 K /

고온초전도체 연구에 있어서 주사터널 현미경은 필수적인 역할을 수행했으나 금속 팁을 이용하기에 준입자(quasiparticle)만을 측정한다는 한계가 있었다. 이를 극복하기 위해 Josephson effect 이용하여 Cooper pair density 직접 측정하는 새로운 STM 기법(SJTM) 개발되었고, 가치가 많은 보고들을 통해 입증되었다. 하지만 보고된 실험들은 대부분 mK 단위의 극저온에서만 이뤄졌다. 연구에서는 기존의 실험 온도에 40 높은 온도인 4.2K에서 구리 화합물 고온초전도체 Bi2Sr2CaCu2O8+x 대한 SJTM연구를 진행하였다. Ambegaokar-Baratoff 이론 등을 통하여 실제로 Josephson effect 측정하고 있음을 확인했으며, 나아가 Ambegaokar-Baratoff 이론을 통해 계산한 에너지 (gap) 공간 상의 분포로 기존에 널리 사용된 방식을 통해 계산한 에너지 갭의 공간 상의 분포가 음의 상관 관계(anticorrelation) 가짐을 보였다.

In pursuit of the elusive mechanism of high-TC superconductors (HTSC), spectroscopic imaging scanning tunneling microscopy (SI-STM) is an indispensable tool for surveying local properties of HTSC. Since a conventional STM utilizes metal tips, which allow the examination of only quasiparticles and not superconducting (SC) pairs, Josephson tunneling using STM has been demonstrated by many authors in the past. An atomically resolved scanning Josephson tunneling microscopy (SJTM), however, was realized only recently on Bi2Sr2CaCu2O8+x (Bi-2212) below 50 mK and on the Pb(110) surface at 20 mK. Here we report the atomically resolved SJTM on Bi2Sr2CaCu2O8+x at 4.2 K using Bi-2212 tips created in situ. The I-V characteristics show clear zero bias conductance peaks following Ambegaokar-Baratoff (AB) theory. A gap map was produced for the first time using an atomically resolved Josephson critical current map IC(r) and AB theory. Surprisingly, we found that this new gap map is anticorrelated to the gap map produced by a conventional method relying on the coherence peaks. Quasiparticle resonance due to a single isolated zinc atom impurity was also observed by SJTM, indicating that atomically resolved SJTM was achieved at 4.2 K. Our result provides a starting point for realizing SJTM at even higher temperatures, rendering possible investigation of the existence of SC pairs in HTSC above the TC.

Authors: Sang Hyun Joo, Jae-Joon Kim, Jung Hoon Yoo, Min Seok Park, Kyung Seok Lee, Jinho Lee

Nano Letters 게재

DOI: 10.1021/acs.nanolett.8b04415
https://pubs.acs.org/doi/10.1021/acs.nanolett.8b04415