Ce_mM_nIn_{3m+2n}(M=Rh,Pd,Pt等）及相关重费米子材料中维度对量子临界行为和超导态的影响

Heavy-fermion materials can be tuned to a quantum critical point by different tuning parameters such as pressure and have displayed rich and exotic quantum phenomena like superconductivity. Recently, a unified global phase diagram in the Kondo lattice system has been proposed, which expands into a new dimension by tuning the magnetic frustration or dimensionality of the electronic structure rather than the conventionalway of pressure, doping and magnetic field tuning the effective Kondo interaction. In the global phase diagram, different quantum critical behaviors can be accessed with different routes. However, it is still lacking and thus desirable to have systematical experimental investigations on therelationship between electronic dimensionalityand the quantum criticality. The series of Ce_mM_nIn_{3m+2n}heavy-fermion compoundswith alternating CeIn3 and MIn2 layers have been synthesized such as CeIn3, Ce2MIn8, CeMIn5 and CePt2In7, where the separation of CeIn3 layers among these compounds increases monotonically with increased n/m and it tunesthe electronic structure from 3D into more 2D-like. Among them, Ce2MIn8 (M=Rh, Pd, Pt) and CePt2In7 will be our proposed focus in this project as the representatives with quasi-3D and quasi-2D electronic structures, respectively.Meanwhile, superconductivity emerging around the quantum critical point is generally believed to be closely related to the dimensionality of spin fluctuations. However, the general trend of increasing superconducting temperature with reduced dimensionality doesn't hold in CePt2In7, indicating a complicated relationship between them. Different experimental methods under pressure, such as electrical resistivity, heat capacity, ac susceptibility and soft point-contact spectroscopy,are planned to characterize their quantum critical behaviors and related superconductivity. In all,Ce_mM_nIn_{3m+2n}compounds serve as an ideal collection to systematically explore quantum criticality and superconductivity and deepen our understanding of their complicated dependence on electronic dimensionality.

重费米子化合物在压力等参量的调控下表现出丰富的宏观量子现象，是研究非常规超导和量子相变的理想体系。最近理论上提出近藤晶格体系的一个全局相图（Global Phase Diagram）：通过压力、掺杂或磁场等参量调控体系的有效近藤作用，同时改变体系的磁阻挫或电子结构的维度，系统可能表现出不同类型的量子临界点。该理论较成功地解释了少数重费米子化合物的量子临界行为，但量子相变对维度的依赖尚缺乏系统研究。重费米子系列化合物Ce_mM_nIn_3m+2n可通过改变n/m的比值实现对电子结构维度的调控。在本项目中，我们选取压力作为主要调控参量，以该系列中Ce2MIn8(M=Rh,Pd,Pt)（准三维）和CePt2In7（准二维）作为主要研究对象，通过测量压力下的电阻、比热、软点接触隧道谱等物理量及其随磁场转角的变化来研究压力诱导的量子临界行为和超导序参量，旨在研究电子结构维度对量子相变和超导态的影响。

重费米子体系是研究量子临界和非常规超导现象的重要体系之一，维度、掺杂和压力的混合调控对其影响是目前研究的热点。我们利用电阻和比热等手段重点研究了CemMnIn3m+2n中"115"临界超导体CeCoIn5和CeIrIn5的掺杂和压力的电子相图，实验结果表明掺杂诱导的长程反铁磁序被压力抑制，但没有表现出量子临界行为，压力诱导的超导态与纯净样品对应压力下的超导行为类似。该行为符合“自旋液滴”模型：CeCoIn5和CeIrIn5常压下位于反铁磁量子临界点附近，剧烈的量子涨落在杂质附近形成反铁磁性的"自旋液滴",进而形成长程序。压力调控使体系远离临界点，量子涨落减弱，自旋液滴尺寸减小，彼此退相干。该研究以重费米子体系为例证明量子临界点附近量子涨落对掺杂效应的放大。另外，CeIrIn5的系列掺杂研究表明超导也处于反铁磁量子临界附近，因此反铁磁涨落诱导超导态可能是115体系超导的统一图像。该类研究指出强关联电子体系的普遍规律，值得更加深入的探索。.我们还通过压力手段研究了低载流子浓度的近藤晶格CeNi2As2，超导体EuBiS2F和BaPt2As2等体系的压力-温度相图，关注其中的量子临界和超导等行为。另外，我们的点接触谱测量发现可能的拓扑超导体系β-PdBi2仅仅是经典的BCS超导，没有所谓的马约罗拉零能模。这些结果加深了对相关强关联电子体系中量子临界和超导行为的理解，拓宽了以后的研究范围。