尖晶石硫化物CuIr_2S_4中類Peierls相變研究

1、尖晶石硫化物CuIr_2S_4中類Peierls相變研究        在尖晶石結(jié)構(gòu)氧化物MgTi_2_4和硫化物CuIr_2S_4中,隨著溫度的降低,在一定的溫度發(fā)生金屬絕緣相變,并且伴隨有磁性、結(jié)構(gòu)等的轉(zhuǎn)變,這一相變被叫做類Peierls相變.傳統(tǒng)的Peierls相變只發(fā)生于一維金屬鏈中,這是由于特殊的能帶結(jié)構(gòu)所導致的。但是,由于特殊的晶體結(jié)構(gòu)、電荷、軌道、自旋等相互作用,在某些二維和三維結(jié)構(gòu)中也發(fā)生了類似的相變,即所謂的類Peierls相變。尖晶石結(jié)構(gòu)MgTi_2O_4和CuIr_2S_4體系就是發(fā)生類Pei

2、erls相變的典型材料.這類材料中存在著金屬絕緣轉(zhuǎn)變、電荷密度波、軌道有序、自旋二聚、磁有序等豐富的物理內(nèi)容,這些都是當前凝聚態(tài)物理學中的研究熱點。另外,對三維結(jié)構(gòu)中的類Peierls相變的研究,也進一步加深了人們對傳統(tǒng)的Peierls相變的認識和理解。在本論文中,我們對三維結(jié)構(gòu)中的類Peierls相變,以及其中的電荷有序、軌道有序、自旋二聚等進行研究。本論文分為五章.第一章綜述了類Peierls相變材料的研究歷史和進展。介紹了一維金屬鏈中的傳統(tǒng)Peierls相變,以及電荷密度波和軌道密度波概念,回顧了二維和三維結(jié)構(gòu)中的類Peierls相變的研究歷史,最后重點闡述了尖晶石結(jié)構(gòu)MgTi_2O_4

3、和CuIr_2S4材料的晶體結(jié)構(gòu)、電荷有序、軌道有序、自旋二聚。通過本章,我們將了解到類Peierls相變的基本物理性質(zhì),同時對軌道誘導Peierls態(tài)概念有所認識,為進入該研究領域作好準備.第二章中我們通過在CuIr_2S_4母體中的A位用離子半徑較大的Ag替代Cu獲得晶格壓力的釋放;并且通過A位空位來獲得晶格壓力的增強。通過對比晶格壓力釋放體系Cu_(1-x)Ag_xIr_2S_4和晶格壓力增強體系Cu_(1-y)Ir_2S_4來研究這一體系反常的壓力效應。發(fā)現(xiàn)壓力釋放抑制了相變;而壓力增強有利于相變發(fā)生。同時,我們還研究了該體系不同磁性的來源和貢獻,通過擬合,首次分開了Pauli順磁,L

4、andau抗磁,Larmor抗磁和居里順磁的不同貢獻,并且通過擬合得出了未參加自旋二聚的Ir(4+)離子的比例。第三章中我們通過直接施加靜壓力,研究了相變溫度隨壓力的變化,發(fā)現(xiàn)相變溫度隨壓力的增加而向高溫方向移動,這是由于壓力減小了晶格從而有利于相變的發(fā)生。另外,指出了CuIr_2S_4體系在低溫絕緣相的導電機制,躍遷電導項來源于Ir-Ir二聚所產(chǎn)生的能隙,而熱激活電導項來源于未二聚的Ir。第四章中我們研究了A位摻入磁性離子Fe的Cu_(1-x)Fe_xIr_2S_4體系。我們發(fā)現(xiàn)以下有趣現(xiàn)象:Fe離子在+1價態(tài)的A位表現(xiàn)為+2價,并且很少量的磁性Fe(2+)離子就可以完全抑制相變;低摻雜樣品

5、中(x=0.01和0.025),在T*=-110K左右處出現(xiàn)了一個未預料到的磁相變:隨著Fe的含量的增加,摻雜體系的磁狀態(tài)由鐵磁到順磁再到鐵磁,并且T*處相變隨著摻雜逐漸消失;高摻雜樣品中(X=0.2,0.3,0.4),T(*)=100K左右出現(xiàn)了一個新的磁相變,并且在低溫出現(xiàn)了自旋團簇玻璃轉(zhuǎn)變。這些現(xiàn)象都可以用Fe(2+)離子極化Ir(4+)離子模型得到合理的解釋。第五章中我們對B位摻雜體系CuIr_(2-x)M_xS_4(M=W/Mn)進行了研究。W在體系中以W(4+)存在,而Mn則以Mn(2+)存在。在CuIr_(2-x)W_xS_4體系中,W摻雜引入了無序,并且由于價態(tài)平衡減少了Ir(

6、4+)離子的數(shù)目,抑制了相變,同時也減弱了高溫的Pauli順磁。而在CuIr_(2-x)Mn_xS_4體系中,雖然Mn離子同樣也在體系中引入了無序,但是由于價態(tài)平衡Mn摻雜導致了Ir(4+)離子的數(shù)目的增加,這有利于相變的發(fā)生,所以在CuIr_(2-x)Mn_xS_x體系中對相變的抑制程度要比CuIr_(2-x)W_xS_4體系弱,并且Mn的磁性在體系中引入了一個順磁背景。本博士論文工作得到了國家自然科學基金(No.10334090,No.1054029)和國家重點基礎研究項目(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