层状二硒化物体系的电声耦合效应研究

Layered diselenides within low temperatures undergo several collective quantum phase transitions like charge density wave and superconducting. The underlying mechanisms have been deeply explored ever since their discoveries. With the latest development of high resolution X-ray scattering techniques and growing technology of quasi-two-dimensional materials, the newest experimental results show that the main factor responsible for the formation of charge density wave in diselenide materials is the electron-phonon coupling effect, in contradiction with the original perspective that the pure electronic state effect like Fermi surface nesting is enough to lead to formation of charge density wave. Based on these fresh knowledge, here we propose to apply systematic theoretical researches on the electron-phonon coupling effects in various diselenide materials, aiming to i) reveal the underlying physical mechanisms for different quantum phase transitions in layered diselenides and present a physical description for their competition-cooperation relationships; ii) explain the specific influences of layer stacking, pressure and interlayer doping on the electron-phonon coupling effects of diselenides, as well as the resulting quantum controlling of collective quantum phases like charge density wave and superconducting; and iii) predicting the quantum properties of monolayer diselenide films coming from the electron-phonon coupling effects. Through the running of our proposal, we intend to make out innovative work on the electron-phonon coupling effects of diselenide materials, and promote as far as we can the research progress of our country in related fields of condensed matter physics.

层状二硒化物体系在低温下会出现电荷密度波、超导等多体集体相的转变，关于其形成机理一直处于深入的探索之中。随着高能量分辨X 光散射技术及准二维材料制备技术的发展，最新的实验结果表明，二硒化物体系中电荷密度波形成的主要原因是电声耦合作用，而非早先认为的单纯电子态效应（费米面嵌套）。基于此，本项目拟针对二硒化物体系的电声耦合效应展开系统的理论模拟研究，致力于揭示二硒化物体系中电荷密度波、超导等集体相的微观形成机理，理解这些集体相之间的竞争-协作关系；阐明层间堆积方式、压力及层间掺杂对二硒化物电声耦合作用的影响，及由此产生的电荷密度波、超导等集体相的量子调控行为；预言单层二硒化物超薄膜的电声耦合效应会导致何种量子现象。通过本项目的实施，我们希望对二硒化物的电子态及电声耦合效应做出创新性的工作，促进我国在凝聚态物理相关领域的研究进展。

通过对NbSe2和TaS2等二硒化物的声子色散曲线与电子结构的系统研究，发现存在电荷密度波转变的体系，不论是金属型还是半导体型电子能级结构，其声子色散曲线中都会出现对应的虚频，而通过对应的电子能级展宽，均能有效抑制这个虚频，我们的这部分研究结果表明对于二硒化物，电荷密度波的出现与声子色散曲线紧密相关；通过对TiSe2电荷密度波转变温度的模拟计算，我们发现电子和空穴掺杂均会降低TiSe2的电荷密度波转变温度，而且当电子掺杂浓度大于1.47×10-3 Å-3时，或空穴掺杂浓度大于2.94×10-3 Å-3时，TiSe2的电荷密度波转变会完全被抑制；此外，我们发现拉应变与压应变对于TiSe2材料电声耦合效应的调制作用不一样，拉伸应变会增大TiSe2的电荷密度波转变温度，而压应变降低小TiSe2的电荷密度波转变温度。在压应力作用下，当外加晶格应变大于8%时，电荷密度波转变被完全抑制；最后，我们预言单层TaS2和TaSe2在低温下存在电荷密度波转变。在本项目的资助下，项目组成员共在国际知名期刊上合作发表了十余篇学术文章。