Who
Anand Bhattacharya
Abstract
The unique electronic structure found at interfaces between materials can allow novel quantum states to emerge, that are very distinct from those found in the bulk. While of great fundamental interest, these interfacial states may also have potential for applications due to their inherent tunability. Thus, it is of some interest to look for interfacial systems that may have collective properties like magnetism or superconductivity, that may also be gated using electric fields for novel functionalities. In this talk I will discuss our recent discovery of superconductivity in electron gases formed at interfaces between (111) orientedKTaO3 and insulating over layers of either EuO or LaAlO3. KTaO3is an incipient ferroelectric, a cousin of the more widely studied SrTiO3,but with much higher spin-orbit coupling arising from the 5d states in Ta. In fact, interfacial superconductivity was discovered in the LaAlO3/SrTiO3system more than a decade ago, with a transition temperature Tc~ 200 mK. In the KTaO3 (111) interfacial superconductor, the value Tc can be as high as 2.2 K, about one order of magnitude higher than in the LaAlO3/SrTiO3 system. Critical field and current-voltage measurements indicate that the superconductivity is two-dimensional. Using transmission electron microscopy and resonant x-ray scattering, we establish the presence of substitutional defects and vacancies at the KTO interface that can act as donors of electrons, and may lead to the formation of an interfacial electron gas. Furthermore, we have uncovered a number of unusual and intriguing properties of the superconducting state at the KTO (111) interface. Firstly, the superconductivity is orientation selective - similar electron gases at (001)KTaO3 interfaces remain normal down to 25 mK. Secondly, in higher mobility EuO/KTaO3 (111) samples, we observe a large in-plane anisotropy in transport properties at low temperatures prior to onset of superconductivity, where the nominally six-fold symmetry of the KTO (111)surface is spontaneously broken. Measurements of magnetoresistance suggest that the transport anisotropy is a due to the emergence of a nematic phase of ‘stripe’ like superconductivity, which is nearly homogeneous in one direction, but strongly modulated in the other. Being that EuO is magnetic, the interaction between magnetism and superconductivity may play an important role in the emergence of this nematicity. These findings have provoked a number of theoretical ideas and experimental developments that I will briefly review, and also point towards possible future directions in this area of research. Reference: “Two-dimensional superconductivity and anisotropic transport at KTaO3 (111)interfaces”, Changjiang Liu, Xi Yan, Dafei Jin, Yang Ma, Haw-Wen Hsiao, YulinLin, Terence M Bretz-Sullivan, Xianjing Zhou, John Pearson, Brandon Fisher, JSamuel Jiang, Wei Han, Jian-Min Zuo, Jianguo Wen, Dillon D Fong, Jirong Sun,Hua Zhou, Anand Bhattacharya, Science 10.1126/science.aba5511 (2021)(see arXiv:2004.07416 (2020) for related paper).
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