碱（土）金属氢化物及稀土金属氢化物压力诱发的金属化及超导电性研究

"Metallic hydrogen" could exhibit a wealth of physical phenomena, for example, high-temperature superconductivity,superconducting and superfluid ground state. However,high density "metallic hydrogen" is exceedingly difficult to be experimentally realized due to the extremely high pressure required. In the case of hydrogen-dense metal hydride,it is highly expected to be metallized and even become superconductive at pressure far below that necessitated by pure hydrogen due to the "chemical precompression" effect imposed on hydrogen atom by metal atom. As a result,they are promising candidates to realize "metallic hydrogen" indirectly. We propose to start with pure metal and hydrogen as reaction agents to synthesize metal hydride with high stoichiometric amount of hydrogen.Pressure-induced metallization and superconductivity would be explored by virtue of electrical transportation and magnetic susceptibility measurement at low temperature。In combination with Raman scattering and synchrotron X-ray diffraction,the correlation between insulator-metal transition and superconductivity transition and structural transition would be established. Through the replacement of H2 with D2, the isotope effect of hydrogen would be unravelled.Tuning the isotope effect via applied magnetic field,the pressure-magnetic field-temperature phase diagram of metal hydrides under investigation would be constructed,and the mechanism of potential superconductivity would also be unravelled which is highly expected to open new pathway toward "metallic hydrogen" and shed light on the superconducting mechanism of cuprate and iron-based superconductors.

"金属氢"蕴含着丰富的物理现象，比如高温超导及超导超流量子基态。然而，由于需要极高的静态压力，高密度"金属氢"的实验研究是极其困难的。对富氢金属氢化物体系而言，由于金属原子对氢原子的"化学预压效应"，有望在远低于纯氢金属化所需的压力条件下呈现金属态和超导电性，因而是间接实现"金属氢"的理想载体。本项目拟采用纯金属和流体H2为原材料，在金刚石对顶砧中合成高化学计量比氢含量的金属氢化物。通过高压低温电输运和磁化率测量探索压力诱发的金属化及超导电性。结合拉曼散射和同步辐射X射线衍射，建立绝缘体-金属转变及超导转变与晶体结构转变之间的关联性。用同位素D2取代H2，研究氢的同位素效应。通过外磁场调控凸显氢的同位素效应，建立所研究金属氢化物的压力-磁场-温度相图，揭示其可能的超导物理机制，为实现"金属氢"开辟新的途径，为阐释铜氧化物和铁基超导体的超导物理机制提供有价值的线索。

随着石墨烯研究如火如荼的开展，单层过渡金属二硫属化物由于其比石墨烯更加丰富奇特的物理性质成为了凝聚态物理领域的一个研究热点。过渡金属二硫属化物MX2（M为过渡金属Ti, Nb, Ta, Mo, W; X为硫族元素S, Se, Te）具有和石墨类似的层状结构，根据单胞中X-M-X三明治单层的数目及MX6配位多面体的不同，可以分为1T，1T′，Td，2H等多形体。其中2H结构的第五族过渡金属二硫属化物NbS2，NbSe2, TaS2, TaSe2在常压条件下即呈现超导电性，除了NbS2，电荷密度波和超导共存竞争。另外，通过化学插层和施加压力抑制电荷密度波转变，可以诱导1T结构的TiSe2和TaS2呈现超导电性。同样，实验表明施加压力可以诱导常压无电荷密度波转变的Td-WTe2呈现超导电性。2H结构的第六族过渡金属二硫属化物MoS2常压无电荷密度波转变，实验上已经证实通过化学插层和施加静电场偏压可以诱导超导电性出现，但迄今还没有压力诱导超导电性发生的实验证据。为此，我们开展了这方面的研究工作。通过超高压电输运测量，我们发现2H-MoS2在90 GPa左右开始呈现超导电性，超导转变临界温度Tc约为3 K，随着压力升高，Tc急剧提升，并在120 GPa左右达到11 K 左右，然后在130 GPa到220 GPa压力范围内，Tc保持在12 K左右不变。高压同步辐射X射线衍射测量表明2H-MoS2在155 GPa压力范围内无结构相变发生。密度泛函计算表明超导的出现可能与电子结构中一个新的空穴型费米口袋的出现有关。我们的工作首次在常压无电荷密度波转变的2H结构过渡金属二硫属化物中通过压力调控观察到超导电性出现。