尖晶石硫化物CuIr_2S_4中类Peierls相变研究

1、尖晶石硫化物CuIr_2S_4中类Peierls相变研究        在尖晶石结构氧化物MgTi_2_4和硫化物CuIr_2S_4中,随着温度的降低,在一定的温度发生金属绝缘相变,并且伴随有磁性、结构等的转变,这一相变被叫做类Peierls相变.传统的Peierls相变只发生于一维金属链中,这是由于特殊的能带结构所导致的。但是,由于特殊的晶体结构、电荷、轨道、自旋等相互作用,在某些二维和三维结构中也发生了类似的相变,即所谓的类Peierls相变。尖晶石结构MgTi_2O_4和CuIr_2S_4体系就是发生类Pei

2、erls相变的典型材料.这类材料中存在着金属绝缘转变、电荷密度波、轨道有序、自旋二聚、磁有序等丰富的物理内容,这些都是当前凝聚态物理学中的研究热点。另外,对三维结构中的类Peierls相变的研究,也进一步加深了人们对传统的Peierls相变的认识和理解。在本论文中,我们对三维结构中的类Peierls相变,以及其中的电荷有序、轨道有序、自旋二聚等进行研究。本论文分为五章.第一章综述了类Peierls相变材料的研究历史和进展。介绍了一维金属链中的传统Peierls相变,以及电荷密度波和轨道密度波概念,回顾了二维和三维结构中的类Peierls相变的研究历史,最后重点阐述了尖晶石结构MgTi_2O_4

3、和CuIr_2S4材料的晶体结构、电荷有序、轨道有序、自旋二聚。通过本章,我们将了解到类Peierls相变的基本物理性质,同时对轨道诱导Peierls态概念有所认识,为进入该研究领域作好准备.第二章中我们通过在CuIr_2S_4母体中的A位用离子半径较大的Ag替代Cu获得晶格压力的释放;并且通过A位空位来获得晶格压力的增强。通过对比晶格压力释放体系Cu_(1-x)Ag_xIr_2S_4和晶格压力增强体系Cu_(1-y)Ir_2S_4来研究这一体系反常的压力效应。发现压力释放抑制了相变;而压力增强有利于相变发生。同时,我们还研究了该体系不同磁性的来源和贡献,通过拟合,首次分开了Pauli顺磁,L

4、andau抗磁,Larmor抗磁和居里顺磁的不同贡献,并且通过拟合得出了未参加自旋二聚的Ir(4+)离子的比例。第三章中我们通过直接施加静压力,研究了相变温度随压力的变化,发现相变温度随压力的增加而向高温方向移动,这是由于压力减小了晶格从而有利于相变的发生。另外,指出了CuIr_2S_4体系在低温绝缘相的导电机制,跃迁电导项来源于Ir-Ir二聚所产生的能隙,而热激活电导项来源于未二聚的Ir。第四章中我们研究了A位掺入磁性离子Fe的Cu_(1-x)Fe_xIr_2S_4体系。我们发现以下有趣现象:Fe离子在+1价态的A位表现为+2价,并且很少量的磁性Fe(2+)离子就可以完全抑制相变;低掺杂样品

5、中(x=0.01和0.025),在T*=-110K左右处出现了一个未预料到的磁相变:随着Fe的含量的增加,掺杂体系的磁状态由铁磁到顺磁再到铁磁,并且T*处相变随着掺杂逐渐消失;高掺杂样品中(X=0.2,0.3,0.4),T(*)=100K左右出现了一个新的磁相变,并且在低温出现了自旋团簇玻璃转变。这些现象都可以用Fe(2+)离子极化Ir(4+)离子模型得到合理的解释。第五章中我们对B位掺杂体系CuIr_(2-x)M_xS_4(M=W/Mn)进行了研究。W在体系中以W(4+)存在,而Mn则以Mn(2+)存在。在CuIr_(2-x)W_xS_4体系中,W掺杂引入了无序,并且由于价态平衡减少了Ir(

6、4+)离子的数目,抑制了相变,同时也减弱了高温的Pauli顺磁。而在CuIr_(2-x)Mn_xS_4体系中,虽然Mn离子同样也在体系中引入了无序,但是由于价态平衡Mn掺杂导致了Ir(4+)离子的数目的增加,这有利于相变的发生,所以在CuIr_(2-x)Mn_xS_x体系中对相变的抑制程度要比CuIr_(2-x)W_xS_4体系弱,并且Mn的磁性在体系中引入了一个顺磁背景。本博士论文工作得到了国家自然科学基金(No.10334090,No.1054029)和国家重点基础研究项目(No.2007CB925001)的支持In the spinel oxide MgTi_2O_4 and sulph

7、ide CuIr_2S_4,a metal-insulator transition, accompanied with the transition of magnetism and structure etc.,happens when cooling the temperature.This phase transition is called the Peierles-like phase transition.In fact,the traditional Peierls phase transition only occurs to the one-dimensional meta

8、llic chains due to the special energy band structure.But a Peierls-like phase transition can also happen to some two- or three-dimensional materials due to the complex interaction between the lattice,charge,orbital,spin.The spinel structure MgTi_2O_4 and Culr_2S_4 are the typical materials with the

9、Peierls-like phase transition.There are abundant physical mechanisms in this kind of materials,such as the metal-insulator transition,the charge density wave,the orbital ordering,spin-dimerization,the magnetic ordering,etc.These mechanisms are hot topics of the present physical research.Besides,the

10、investigation of the Peierls-like phase transition is help for the further understanding of the classic Peierls phase transition.In this dissertation,the author devoted his effort to the study of the Peierls-like phase transition,together with the charge ordering,the orbital ordering and the spin-di

11、merization.The whole dissertation consists of five chapters.In the first chapter,we give a brief overview of the progresses of the Peierls-like phase transition.Firstly,we introduce the conception of the classic Peierls phase transition in the one-dimensional metallic chains,and the charge ordering

12、wave and orbital ordering wave.Secondly,we elucidate the research of the Peierls-like phase transition in the two-dimensional and three-dimensional system.Finally,we lay emphasis on the lattice structure,charge ordering,orbital ordering,spin-dimerization in the spinel structure MgTi_2O_4 and CuIr_2S

13、_4.We can achieve the basic physical concepts about the Peierls-like phase transition and the orbital-induced Peierls state in this chapter.This part is helping to build up background knowledge for the research.In the second chapter,we obtain the lattice pressure released system through the substitu

14、tion of Ag with big ionic radius for Cu in the A site,while achieve the lattice pressure enhanced system through the vacancies in the A site.We investigate the abnormal pressure effect by the comparison of the lattice pressure released system Cu_(1-x)Ag_xIr_2S_4 and the lattice pressure enhanced sys

15、tem Cu_(1-y)Ir_2S_4.It is found that the Peierls-like phase transition is suppressed by the releasing of lattice pressure while it is strengthened by the enhancement of the lattice pressure.Meanwhile,for the first time,the contributions of the Pauli paramagentism,Landau diamagnetism,Larmor diamagnet

16、ism and Curie paramagnetism are distinguished by fitting the magnetization, and we obtained the proportion of Ir ions which do not participate the spin-dimerization.In the third chapter,we study the pressure effect by means of applying direct hydrostatic pressures.It is found that the phase transiti

17、on temperature moves to higher temperature with the increasing of the hydrostatic pressure.This is because that the application of hydrostatic pressure decreases the lattice,which favors the phase transition.In addition,we give an explanation for the origin of the different conductive mechanisms in

18、the insulating phase,i.e.,the variable range hopping originate from the energy gap produced by the dimerized Ir-Ir chains,while the thermal activation originate from the non-dimerized Ir.In the fourth chapter,the doping of magnetic Fe in the A site Cu_(1-x)Fe_xIr_2S_4 system is discussed.Many intere

19、sting phenomena are found in this system:the Fe ionexists in the A site with the valence of +1 as +2,and the phase transition is completely suppressed by gentle introduction of Fe;a unexpected magnetic phase transition appears at T*=110 K in the low doping system(x=0.01 and 0.025);the magnetic state

20、 of this system evolve from ferromagnetic to paramagnetic,and back to ferromagnetic state with the increase of Fe,while the magnetic transition at T* disappears gradually with the increase of Fe;a new magnetic transition presents at T(*)= 100 K in the heavy doping system(x=0.2,0.3,0.4),and a spin-cl

21、uster transition appears at low temperatures.All these phenomenia can explain by the cluster model based on Fe polaring Ir.In the fifth chapter,the substitution in the B site CuIr_(2-x)M_xS_4(M=W/Mn) system is investigated.W acts as W(4+) in the CuIr_(2-x)W_xS_4 system,while Mn exists as Mn(2+)in the CuIr_(2-x)Mn_xS_4 system.In the CuIr_(2-x)W_xS_4 system,the phase transition is suppressed due to the random and the reduction of Ir(4+) caused by the doping of