正交Mg-Ti-O超导体的外延稳定化
Epitaxial stabilization of an orthorhombic Mg-Ti-O superconductor
摘要：
钛氧化物超导体具有许多与铜酸盐和铁磷属元素化物/硫属元素化物类似的有趣现象，为对比研究高温超导体的非常规配对机制提供了一个理想的平台。在这里，我们成功地沉积Mg-Ti-O超导薄膜MgAl $_2$ O $_4$基板上的三个主要取向通过烧蚀MgTi $_2$ O $_4$目标。特别地，令人惊讶地观察到，在（011）取向的衬底上生长了非预期结构的单晶膜，其中最高零电阻转变温度（$T_{\mathrm{c}0}$）为5.0 K。该薄膜具有高度降低的Mg/Ti比和正交晶系Ti $_9$ O $_{10}$样结构（表示为Mg：Ti $_9$ O $_{10}$），通过进一步的化学组成和结构表征证明。这样的结构在体中是不稳定的，但是由于在所形成的界面处的相对小的应变而有利于在MgAl $_2$ O $_4$的（011）表面上外延稳定。在Mg：Ti $_9$ O $_{10}$薄膜中观察到一个各向同性的上临界场（$B_{\mathrm{c}2}$），其最高可达13.7 T，突破了泡利极限，类似于其它超导钛氧化物。这种相似性表明了该族化合物超导电性的共同起源，这将为过渡金属化合物非常规超导电性的机理提供有价值的见解。
英文摘要：
The family of titanium oxide superconductors exhibits many intriguing phenomena comparable to cuprates and iron pnictides/chalcogenides, and thus provides an ideal platform to contrastively study the unconventional pairing mechanism of high-temperature superconductors. Here, we successfully deposit superconducting Mg-Ti-O films on MgAl$_2$O$_4$ substrates with three principal orientations by ablating a MgTi$_2$O$_4$ target. Particularly, it is striking to observed that a single-crystalline film of an unintended structure has been grown on the (011)-oriented substrate, with the highest zero resistance transition temperature ($T_{\mathrm{c}0}$) of 5.0 K among them. The film has a highly reduced Mg/Ti ratio and an orthorhombic Ti$_9$O$_{10}$-like structure (denoted as Mg: Ti$_9$O$_{10}$), demonstrated by further characterizations of chemical composition and structure. Such a structure is unstable in bulk but favorable to be epitaxially stabilized on the (011)-surface of MgAl$_2$O$_4$ due to a relatively small strain at the formed interface. An isotropic upper critical field ($B_{\mathrm{c}2}$) up to 13.7 T that breaks the Pauli limit is observed in the Mg: Ti$_9$O$_{10}$ film, analogous to other superconducting titanium oxides. The similarity points to a common origin for the superconductivity in the family, which will provide valuable opinions for the mechanism of unconventional superconductivity in transition metal compounds.
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