Chen Si, Chen Xin, Duijnstee Elisabeth A, Sanyal Biplab, Banerjee Tamalika
Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands.
Department of Physics and Astronomy, Uppsala University, P.O. Box 516, 751 20 Uppsala, Sweden.
ACS Appl Mater Interfaces. 2020 Nov 25;12(47):52915-52921. doi: 10.1021/acsami.0c15458. Epub 2020 Nov 11.
Heterointerfaces coupling complex oxides exhibit coexisting functional properties such as magnetism, superconductivity, and ferroelectricity, often absent in their individual constituent. SrTiO (STO), a canonical band insulator, is an active constituent of such heterointerfaces. Temperature-, strain-, or mechanical stress-induced ferroelastic transition leads to the formation of narrow domains and domain walls in STO. Such ferroelastic domain walls have been studied using imaging or transport techniques and, often, the findings are influenced by the choice and interaction of the electrodes with STO. In this work, we use graphene as a unique platform to unveil the movement of oxygen vacancies and ferroelastic domain walls near the STO surface by studying the temperature and gate bias dependence of charge transport in graphene. By sweeping the back gate voltage, we observe antihysteresis in graphene typically observed in conventional ferroelectric oxides. Interestingly, we find features in antihysteresis that are related to the movement of domain walls and of oxygen vacancies in STO. We ascertain this by analyzing the time dependence of the graphene square resistance at different temperatures and gate bias. Density functional calculations estimate the surface polarization and formation energies of layer-dependent oxygen vacancies in STO. This corroborates quantitatively with the activation energies determined from the temperature dependence of the graphene square resistance. Introduction of a hexagonal boron nitride (hBN) layer, of varying thicknesses, between graphene and STO leads to a gradual disappearance of the observed features, implying the influence of the domain walls onto the potential landscape in graphene.
异质界面耦合的复合氧化物展现出诸如磁性、超导性和铁电性等共存的功能特性，这些特性在其单个组分中通常不存在。典型的能带绝缘体SrTiO（STO）是此类异质界面的活性组分。温度、应变或机械应力诱导的铁弹性转变会导致STO中形成狭窄的畴和畴壁。已经使用成像或输运技术对这种铁弹性畴壁进行了研究，并且研究结果常常受到电极与STO的选择和相互作用的影响。在这项工作中，我们使用石墨烯作为一个独特的平台，通过研究石墨烯中电荷输运对温度和栅极偏置的依赖性，来揭示STO表面附近氧空位和铁弹性畴壁的运动。通过扫描背栅电压，我们在石墨烯中观察到了通常在传统铁电氧化物中观察到的反滞后现象。有趣的是，我们在反滞后现象中发现了与STO中畴壁和氧空位运动相关的特征。我们通过分析不同温度和栅极偏置下石墨烯方块电阻的时间依赖性来确定这一点。密度泛函计算估计了STO中与层相关的氧空位的表面极化和形成能。这与由石墨烯方块电阻的温度依赖性确定的活化能在定量上相佐证。在石墨烯和STO之间引入不同厚度的六方氮化硼（hBN）层会导致观察到的特征逐渐消失，这意味着畴壁对石墨烯中势能景观的影响。
