2H结构过渡族金属二硫属化物电子结构的高分辨角分辨光电子能谱研究
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摘要
过渡族金属二硫属化物(Transition metal dichalcogenides,TMD's)作为人类最早发现的一类二维电荷密度波体系,因其不同构份所呈现的丰富多彩的电子基态物理相图,一直引起凝聚态物理学界持续而广泛的兴趣。近年来,随着高温超导体研究的深入,人们又逐渐发现具有简单2H构型的TMD's系统具备和高温超导体类似的准二维电子结构特点和多自由度间相互竞争的低温基态,以及各向异性的物理性质,是研究复杂的高温超导体的一个十分合适的中间阶梯,有着十分重要的科研价值。而角分辨光电子能谱作为目前唯一可以同时测量固体中费米能级附近价电子的能量、运动方向和散射性质的实验工具,可以有效地用来研究各种此类材料的电子结构、相变及其中的各种有序现象。本文主要就是围绕着2H结构过渡族金属二硫属化物在低温下所呈现的如超导、电荷密度波有序及二者共存等丰富基态产生的微观机理,利用高分辨角分辨光电子能谱谱仪技术对其展开了一系列研究,并得到了如下一些结果:
1.针对2H-Na_xTaS_2体系中的CDW转变提出了“费米域”驱动的新机理。利用角分辨光电子能谱技术发现了当2H-Na_xTaS_2体系中CDW有序态形成时,占布里渊区很大比例的“费米域”范围内的谱线都发生了费米能量附近谱重被部分压制,从而形成特殊CDW能隙的结果;同时通过联合态密度计算证明正是这些“费米域”内的电子态的共同作用导致了2H-Na_xTaS_2体系的CDW转变。以往一些长期困绕2H-Na_xTaS_2体系化合物CDW转变微观机理研究的难点,例如电荷密度波转变时的调制波矢和费米面大小不匹配,发生电荷密度波转变时费米面附近测量不到能隙打开等较反常的现象,都得到了比较好的解释。不同于其它基于简单金属能带理论的CDW转变机理,这个机理植根于体系电子结构中很强的电声子相互作用。
2.发现了长期隐藏着的引起2H-NbSe_2中CDW转变的nesting条件,从而解决了一个长期以来困扰理解2H-NbSe_2中CDW机理的难题。利用角分辨光电子能谱技术,通过在整个布里渊区上对2H-NbSe_2进入CDW态后所形成的能隙寻找,发现了真正和其CDW转变相关的电子态所在动量区域,而这些区域之间的距离恰好和之前发现的CDW波矢相一致。此外,利用联合态密度的计算方法,我们还进一步证明了这些动量区域中的电子态在2H-NbSe_2的CDW有序态形成中所起的重要作用。
3.对一系列不同Cu插层掺杂浓度的2H-NbS_2单晶样品进行了角分辨光电子能谱测量研究。首次得到了2H-Cu_xNbS_2体系中电子结构随Cu掺杂量改变的具体演化结果;通过对沿不同动量方向所测得的低能能带色散关系进行分析,发现2H-Cu_xNbS_2中的电声子相互作用存在较明显的各向异性,同时其相应的电声子耦合常数也较其同构的化合物2H-NbSe_2要弱,这样就解释了为什么2H-NbS_2及其微量Cu插层化合物具有较低的超导转变温度Tc;此外,结合相关的声子色散曲线计算结果和联合态密度计算分析,还解释了2H-Cu_xNbS_2体系中电荷密度波转变缺失的原因。
The transition metal dichalcogenides(TMD's),which are one class of the first discovered 2D charge density wave(CDW) compounds,have received continuous and considerable interest because of their relatively simple crystalline structures but complex electronic ground states.Their particular features,such as the layered crystal structures and the competition and/or coexistence between different electronic ground states,are especially intriguing in view of the anomalous properties of another class of layered materials,the high-Tc cuprate superconductors. The angle resolved photoemission spectroscopy(ARPES),which is the sole tool to simultaneously detect the electron's energy,moving direction and scattering property near Fermi energy in solids,is appropriate for investigating the novel electronic structures,phase transitions,various orderings etc in such layered materials.In this dissertation,we report some progresses on studying the electronic structures of the TMD's by means of ARPES.We would like to reveal the microscopic mechanism for the formation of their different electronic ground states.The corresponding results are listed as follows.
1.We report a new CDW mechanism discovered in a 2H-structured transition metal dichalcogenide 2H-Na_xTaS_2,where the two essential ingredients of the CDW are realized in very anomalous ways due to the strong-coupling nature of the electronic structure.Namely,the CDW gap is only partially open,and charge density wave vector match is fulfilled through participation of states of the large Fermi patches.In this scheme,some long-standing difficulties in understanding the CDW of 2H-Na_xTaS_2,such as no gap opening near the Fermi surface and no match of the CDW wavevector with the Fermi surfaces,are sovled successfully. Unlike other CDW mechanisms based on band structure effects,it is rooted in the strongly coupling nature of its electronic structure,which provides phase space needed for CDW fluctuations.
2.2H-NbSe_2 is a prototype CDW material.However,the nesting required by the conventional CDW mechanism,i.e.separation of certain Fermi surface sections should match the ordering wavevector,is mysteriously missing here.With ARPES, we show that the long-lost nesting condition is fullfiled not just between the six saddle points,but the major contributions come from a large honeycomb region away from the Fermi surface.Although the spectral suppression in these regions is weak,it clearly tracks CDW formation,and the summed suppression overweights those around the saddle points.The estimated charge susceptibility further confirms that the CDW instability in 2H-NbSe_2 are mostly caused by electrons in these regions.
3.For the first time,we report the detailed evolution of the electronic structures of 2H-Cu_xNbS_2.The extracted self-energy information on these compounds implies rather weak and anisotropic electron-phonon couplings therein,which naturally answer for the anomalous properties of their superconductivity.On the other hand,we compared the estimated charge susceptibility for two different TMD's with and without CDW,2H-Na_(0.05)TaS_2 and 2H-Cu_(0.09)NbS_2 respectively.While there is a clear maximum at the CDW wavevector for 2H-Na_(0.05)TaS_2,the result for 2H-Cu_(0.09)NbS_2 does not show such behavior,which is consistent with the absence of charge density wave in the system.
1.针对2H-Na_xTaS_2体系中的CDW转变提出了“费米域”驱动的新机理。利用角分辨光电子能谱技术发现了当2H-Na_xTaS_2体系中CDW有序态形成时,占布里渊区很大比例的“费米域”范围内的谱线都发生了费米能量附近谱重被部分压制,从而形成特殊CDW能隙的结果;同时通过联合态密度计算证明正是这些“费米域”内的电子态的共同作用导致了2H-Na_xTaS_2体系的CDW转变。以往一些长期困绕2H-Na_xTaS_2体系化合物CDW转变微观机理研究的难点,例如电荷密度波转变时的调制波矢和费米面大小不匹配,发生电荷密度波转变时费米面附近测量不到能隙打开等较反常的现象,都得到了比较好的解释。不同于其它基于简单金属能带理论的CDW转变机理,这个机理植根于体系电子结构中很强的电声子相互作用。
2.发现了长期隐藏着的引起2H-NbSe_2中CDW转变的nesting条件,从而解决了一个长期以来困扰理解2H-NbSe_2中CDW机理的难题。利用角分辨光电子能谱技术,通过在整个布里渊区上对2H-NbSe_2进入CDW态后所形成的能隙寻找,发现了真正和其CDW转变相关的电子态所在动量区域,而这些区域之间的距离恰好和之前发现的CDW波矢相一致。此外,利用联合态密度的计算方法,我们还进一步证明了这些动量区域中的电子态在2H-NbSe_2的CDW有序态形成中所起的重要作用。
3.对一系列不同Cu插层掺杂浓度的2H-NbS_2单晶样品进行了角分辨光电子能谱测量研究。首次得到了2H-Cu_xNbS_2体系中电子结构随Cu掺杂量改变的具体演化结果;通过对沿不同动量方向所测得的低能能带色散关系进行分析,发现2H-Cu_xNbS_2中的电声子相互作用存在较明显的各向异性,同时其相应的电声子耦合常数也较其同构的化合物2H-NbSe_2要弱,这样就解释了为什么2H-NbS_2及其微量Cu插层化合物具有较低的超导转变温度Tc;此外,结合相关的声子色散曲线计算结果和联合态密度计算分析,还解释了2H-Cu_xNbS_2体系中电荷密度波转变缺失的原因。
The transition metal dichalcogenides(TMD's),which are one class of the first discovered 2D charge density wave(CDW) compounds,have received continuous and considerable interest because of their relatively simple crystalline structures but complex electronic ground states.Their particular features,such as the layered crystal structures and the competition and/or coexistence between different electronic ground states,are especially intriguing in view of the anomalous properties of another class of layered materials,the high-Tc cuprate superconductors. The angle resolved photoemission spectroscopy(ARPES),which is the sole tool to simultaneously detect the electron's energy,moving direction and scattering property near Fermi energy in solids,is appropriate for investigating the novel electronic structures,phase transitions,various orderings etc in such layered materials.In this dissertation,we report some progresses on studying the electronic structures of the TMD's by means of ARPES.We would like to reveal the microscopic mechanism for the formation of their different electronic ground states.The corresponding results are listed as follows.
1.We report a new CDW mechanism discovered in a 2H-structured transition metal dichalcogenide 2H-Na_xTaS_2,where the two essential ingredients of the CDW are realized in very anomalous ways due to the strong-coupling nature of the electronic structure.Namely,the CDW gap is only partially open,and charge density wave vector match is fulfilled through participation of states of the large Fermi patches.In this scheme,some long-standing difficulties in understanding the CDW of 2H-Na_xTaS_2,such as no gap opening near the Fermi surface and no match of the CDW wavevector with the Fermi surfaces,are sovled successfully. Unlike other CDW mechanisms based on band structure effects,it is rooted in the strongly coupling nature of its electronic structure,which provides phase space needed for CDW fluctuations.
2.2H-NbSe_2 is a prototype CDW material.However,the nesting required by the conventional CDW mechanism,i.e.separation of certain Fermi surface sections should match the ordering wavevector,is mysteriously missing here.With ARPES, we show that the long-lost nesting condition is fullfiled not just between the six saddle points,but the major contributions come from a large honeycomb region away from the Fermi surface.Although the spectral suppression in these regions is weak,it clearly tracks CDW formation,and the summed suppression overweights those around the saddle points.The estimated charge susceptibility further confirms that the CDW instability in 2H-NbSe_2 are mostly caused by electrons in these regions.
3.For the first time,we report the detailed evolution of the electronic structures of 2H-Cu_xNbS_2.The extracted self-energy information on these compounds implies rather weak and anisotropic electron-phonon couplings therein,which naturally answer for the anomalous properties of their superconductivity.On the other hand,we compared the estimated charge susceptibility for two different TMD's with and without CDW,2H-Na_(0.05)TaS_2 and 2H-Cu_(0.09)NbS_2 respectively.While there is a clear maximum at the CDW wavevector for 2H-Na_(0.05)TaS_2,the result for 2H-Cu_(0.09)NbS_2 does not show such behavior,which is consistent with the absence of charge density wave in the system.
引文
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