New research from a team of scientists co-led by Dr. Runze Yu from HPSTAR observed pressure-induced superconductivity in the nonsymmorphic topological insulator KHgAs. This study published in the journal of NPG Asia Materials also showed the possible coexistence of superconductivity and topologically nontrivial features protected by nonsymmorphic symmetries and could potentially provide a new platform to investigate th interplay between topological properties and superconductivity.
The investigation of topological materials, including topological insulators (TIs), topological semimetals, and topological superconductors, has attracted tremendous interest over the past few decades. The topological configuration of the electronic states of the TI system has become increasingly rich by the consideration of additional symmetries. Among them, crystal symmetries have played an important role in topological classification, such as crystal point group symmetries, leading to the concept of topological crystalline insulators. Recently, nonsymmorphic symmetries were predicted to be able to protect a novel surface fermion with dispersion shapes similar to an hourglass, and KHgX (X = As, Sb, Bi) was proposed as a material class to achieve this new hourglass shaped dispersion. Different from the previously discovered TIs protected by either time-reversal or mirror crystal symmorphic symmetry, the band topology of KHgX relied on nonsymmorphic symmetries. Usually, topological materials show superconductivity under pressure or by carrier doping, which is considered a possible way to achieve topological superconductivity.
In this study, Yu and colleagues carried out high-pressure electrical measurements, high-pressure X-ray diffractions, and high-pressure structure predictions on KHgAs. They observed a pressure-induced semiconductor-metal transition between ~16 and 20 GPa, followed by the appearance of superconductivity with a Tc of ~3.5 K at approximately 21 GPa. The superconducting transition temperature was enhanced to a maximum of ~6.6 K at 32 GPa and then slowly decreased until 55 GPa. Furthermore, three high pressure phases were observed within the studied pressure range. They thus established the phase diagram of KHgAs. This study can be used to facilitate further research on superconductivity and topologically nontrivial features protected by nonsymmorphic symmetries.
Caption: The phase diagram of KHgAs as a function of pressure up to 55 GPa.
在过去的几十年中拓扑材料的研究，包括拓扑绝缘体（TI）、拓扑半金属和拓扑超导体，引起了人们的极大兴趣。最近，理论预测非点式对称操作可以保护具有类似沙漏的色散形状的新型表面费米子，KHgX（X=As，Sb，Bi）也被提出可以作为材料类来实现这种新的沙漏形色散。与先前发现的受时间反演或镜像晶体对称性保护的TI不同，KHgX的能带拓扑依赖于非对称性。通常拓扑材料在压力下或通过载流子掺杂表现出超导性，这被认为是实现拓扑超导性的一种可能方式。北京高压科学研究中心于润泽团队与中科院物理所靳常青团队等国内外团队合作，生长了高质量的具有沙漏型色散的拓扑材料KHgAs单晶，并系统研究了KHgAs在高压下的电输运和结构演化行为。他们发现KHgAs在~16和20 GPa之间发生了压力诱导的半导体-金属转变，随后在约21 GPa处出现了Tc为~3.5K的超导电性。超导转变温度在32GPa时达到最高约6.6K，随后缓慢降低。另外，在55 GPa压力范围内，该团队观察到KHgX的三次晶体结构转变，通过高压结构预测结合衍射数据成功的解析出了KHgAs的高压晶体结构，并基于上述结果建立了KHgAs的相图。这项研究将促进人们对超导和受非点式对称操作保护的拓扑非平庸特性的进一步研究。相关结果近期以“Pressure-induced superconductivity in the nonsymmorphic topological insulator KHgAs”为题发表在NPG Asia Materials上。
