高压诱导新型拓扑半金属的晶体结构及费米面演化

Topological semimetal could exhibit spectacular macroscopic quantum phenomena like extremely large magnetoresistance and ultrahigh mobility of carriers. Pressure, as a viable thermodynamic parameter, without introducing composition disorder, increasing the orbital wavefunction overlap between neighbouring sites in a crystal, in turn increasing the ratio of kinetic (inter-site charge hopping) to potential (on-site Coulomb repulsion) energy, is a "clean" and continuously controllable tuning parameter in discovering novel superconductor. Our preliminary results have demonstrated that pressure could induce superconductivity in topological semimetal WTe2 and MoP. In this project, utilizing pressure as the tuning knob in combination with high magnetic field and ultralow temperature, we plan to explore pressure-induced superconductivity in novel topological semimetals via high pressure electrical transport measurement, determine the crystal structure of superconducting phase via synchrotron x-ray diffraction measurement, investigate the pressure-induced evolution of Fermi surface topology via quantum oscillation measurement, determine the upper critical field of superconducting phase via electrical transport measurement under external magnetic fields, construct the temperature-pressure-magnetic field phase diagram of novel topological semimetals, uncover the microscopic mechanism underlying the emergence of pressure-induced superconductivity, discuss the superconducting pairing mechanism, and provide valuable clues for the microscopic mechanism of topological superconductivity and high-Tc superconductivity.

拓扑半金属可展示巨磁阻效应和超高载流子迁移率等宏观量子现象。压强，作为一个重要的热力学参数，在不引入组分无序的前提下，通过体积塌缩提高晶体中近邻位置电子轨道波函数之间的交叠，转而增加电荷跃迁动能对电荷库仑排斥势能之比，是发掘新超导体的一种连续可控的“洁净”物理手段。我们的前期工作表明：压强可以诱导拓扑半金属WTe2和MoP发生超导转变。本项目拟采取压强为调控手段，结合强磁场和极低温两种极端条件，通过电输运性能测量探索新型拓扑半金属的高压超导相，通过同步辐射x射线衍射测量研究晶体结构随压力的演化并确定超导相的晶体结构，通过量子震荡测量研究费米面拓扑随压力的演化，通过外磁场下电输运性能测量确定超导相的临界磁场，构建温度―压强―磁场（T≥50 mK，P≤200 GPa，H≤40 T）三维相图，揭示“压强诱导的超导电性”发生的微观机理，探讨超导配对机制，为拓扑超导及高温超导机理提供有价值的线索。

拓扑量子材料具有拓扑保护的非平庸电子能带，其宏观物理性能起源于维度、量子限域、量子相干、量子涨落、电子轨道波函数拓扑、电子关联、自旋-轨道耦合及对称性等因素之间的复杂相互作用。开发拓扑与宏观量子现象超导电性共存的本征拓扑超导体是凝聚态物理领域一个重要的研究课题。高压诱导拓扑量子材料包括拓扑绝缘体和拓扑半金属的超导电性提供了一种探索拓扑超导体的有效手段。压强，作为一个重要的热力学参数，在不引入组分无序的前提下，通过体积塌缩提高晶体中近邻位置电子轨道波函数之间的交叠，转而增加电荷跃迁动能对电荷库仑排斥势能之比，是发掘新超导体的一种连续可控的“洁净”物理手段。.通过高压低温电输运测量结合同步辐射x射线衍射对第二类外尔拓扑半金属WP2研究表明：纵向磁阻随外加压力升高逐渐被抑制，在40 GPa左右，超导电性出现，临界温度约为3.5 K，同时伴随发生单胞体积的突然塌缩和反常的晶格软化。继续加压，纵向磁阻逐渐被强化，同时超导电性也逐渐增强。卸压过程中，超导电性可以保持到20 GPa左右。晶体结构在50 GPa范围内保持稳定，无结构相变发生。综合分析认为：超导电性的出现是由于在40 GPa左右发生了等结构电子拓扑转变，即利夫希茨转变。.通过高压低温电输运测量结合同步辐射x射线衍射对反铁磁拓扑绝缘体EuSn2As2研究表明：在15 GPa左右，超导电性出现，临界温度约为4 K，同时伴随发生从准二维的菱方结构到三维的单斜结构的相变。.通过上述对外尔拓扑半金属和反铁磁拓扑绝缘体的高压研究，为探索新型拓扑超导体提供了有价值的线索。