A Zero-loop Sagnac Interferometer for Probing Spontaneous Time Reversal Symmetry Breaking in Topological Superconductors

该提案是为了购买建立零环路Sagnac干涉仪所需的设备，用于探测克尔旋转，具有亚µrad的灵敏度和µm的空间分辨率。近年来二维材料领域最大的突破无疑是在扭曲双层石墨烯中发现了相关相和超导性。此外，去年刚刚报道了单层高温超导体的成功分离。受到这些发展的启发，我们的理论学家同事Marcel Franz提出了一个新理论，预测第一个高tc拓扑超导体可以从扭曲的d波超导体双层中出现。该理论预测，当两层单层d波超导体以45度的扭转角堆叠时，两层之间的超导序参量会产生1 /2的相位差。结果，超导总序参量获得手性对称，高tc超导体成为具有拓扑间隙的高tc拓扑超导体。

This proposal is for the purchase of equipment required to build a zero-loop Sagnac interferometer for probing Kerr rotation with a sub-µrad sensitivity and a µm spatial resolution. The biggest breakthrough in two-dimensional materials recently is inarguably the discovery of correlated phases and superconductivity in twisted bilayer graphene. In addition, the successful isolation of monolayer high-temperature (Tc) superconductors was just reported last year. Inspired by these developments, our theorist colleague, Marcel Franz, proposed a new theory predicting that the first high-Tc topological superconductor can emerge out of a twisted bilayer of d-wave superconductors. The theory predicts that when two monolayers of d-wave superconductors are stacked with a forty-five degree twist angle, a /2 phase difference arises in the superconducting order parameter between the two layers. As a result, the total superconducting order parameter acquires chiral symmetry and the high-Tc superconductor becomes a high-Tc topological superconductor with a topological gap. Here we propose to experimentally test this theory by fabricating twisted bilayers of a high-Tc superconductor, Bi2Sr2CaCu2O8+, and probing their topological superconducting phase with light. According to the theory, the time reversal symmetry will spontaneously break below the transition temperature, which will give rise to a Polar Kerr Effect. Since the topological gap is much smaller than the probe light's photon energy, the Kerr rotation angle is expected to fall below µrad. Therefore we plan to build a state-of-the-art Sagnac interferometer with an extreme sensitivity for Kerr rotation (tens of nrads). This instrument will allow us to demonstrate the first high-Tc topological superconductor, as well as resolve some open fundamental questions in the field. Our new approach to realizing topological superconductivity at high temperature might enable new opportunities for topological quantum computing in the future. In collaboration with colleagues, we will also apply the proposed equipment to probing the marginal time reversal symmetry breaking in other low-dimensional systems, such as the twisted bilayer graphene (Folk group), the MBE-grown monolayer ferromagnet, Fe3GeTe2 (Zou group), the surface state in HfNiSn along the (111) direction (Aronson group), high-entropy magnetic alloys (Hallas group), and transition-metal dichalcogenide heterostructures, which is of my own group's interest. The experience of working with the state-of-the-art Sagnac interferometer will provide our HQPs with extremely valuable skills which will help their careers both within and beyond academia.