Obata Reiji, Kosugi Mioko, Kikkawa Takashi, Kuroyama Kazuyuki, Yokouchi Tomoyuki, Shiomi Yuki, Maruyama Shigeo, Hirakawa Kazuhiko, Saitoh Eiji, Haruyama Junji
Faculty of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa, 252-5258, Japan.
Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan.
Adv Mater. 2024 May;36(19):e2311339. doi: 10.1002/adma.202311339. Epub 2024 Feb 13.
SrTiO (STO) substrate, a perovskite oxide material known for its high dielectric constant (ɛ), facilitates the observation of various (high-temperature) quantum phenomena. A quantum Hall topological insulating (QHTI) state, comprising two copies of QH states with antiparallel two ferromagnetic edge-spin overlap protected by the U(1) axial rotation symmetry of spin polarization, has recently been achieved in low magnetic field (B) even as high as ≈100 K in a monolayer graphene/thin hexagonal boron nitride (hBN) spacer placed on an STO substrate, thanks to the high ɛ of STO. Despite the use of the heavy STO substrate, however, proximity-induced quantum spin Hall (QSH) states in 2D TI phases, featuring a topologically protected helical edge spin phase within time-reversal-symmetry, is not confirmed. Here, with the use of a monolayer hBN spacer, it is revealed the coexistence of QSH (at B = 0T) and QHTI (at B ≠ 0) states in the same single graphene sample placed on an STO, with a crossover regime between the two at low B. It is also classified that the different symmetries of the two nontrivial helical edge spin phases in the two states lead to different interaction with electron-puddle quantum dots, caused by a local surface pocket of the STO, in the crossover regime, resulting in a spin dephasing only for the QHTI state. The results obtained using STO substrates open the doors to investigations of novel QH spin states with different symmetries and their correlations with quantum phenomena. This exploration holds value for potential applications in spintronic devices.
钛酸锶（STO）衬底是一种以高介电常数（ɛ）闻名的钙钛矿氧化物材料，有助于观察各种（高温）量子现象。最近，在置于STO衬底上的单层石墨烯/薄六方氮化硼（hBN）间隔层中，即使在低磁场（B）下高达约100 K时，也实现了一种量子霍尔拓扑绝缘（QHTI）态，该态由两个具有反平行两个铁磁边缘自旋重叠的量子霍尔（QH）态副本组成，受自旋极化的U(1)轴向旋转对称性保护，这得益于STO的高ɛ。然而，尽管使用了较重的STO衬底，但二维拓扑绝缘体（TI）相中的近邻诱导量子自旋霍尔（QSH）态（其在时间反演对称性内具有拓扑保护的螺旋边缘自旋相）尚未得到证实。在此，通过使用单层hBN间隔层，发现在置于STO上的同一单石墨烯样品中存在QSH（在B = 0T时）和QHTI（在B ≠ 0时）态的共存，且在低B时有两者之间的交叉区域。还分类表明，这两种态中两个非平凡螺旋边缘自旋相的不同对称性导致在交叉区域与由STO的局部表面口袋引起的电子 puddle量子点有不同的相互作用，从而仅导致QHTI态的自旋退相。使用STO衬底获得的结果为研究具有不同对称性的新型量子霍尔自旋态及其与量子现象的相关性打开了大门。这种探索对于自旋电子器件的潜在应用具有价值。
