过渡金属化合物的电子结构研究与调制

【作者】 赵嘉峰；

【导师】 封东来；

【作者基本信息】 复旦大学 ， 凝聚态物理， 2009， 博士

【摘要】 过渡金属包括元素周期表中从3B到12B共10族,这一类元素的共同特点就是具有未填满的d轨道,因此在形成化合物时表现出与其他元素的明显不同,在光,电,磁等方面表现出许多新奇的性质。而且这些奇异性质对温度,掺杂,压力等可调参数非常敏感,从而形成了许多丰富多彩的相图:例如过渡金属二硫族化合物中典型的电荷密度波与超导共存的行为;锰氧化合物中的金属.绝缘体相变相图;以及新型铁基超导体中的自旋密度波与超导共存的相图。这些相图的背后蕴藏了丰富的物理信息并且可能具有广阔的实用前景,因此成为了目前凝聚态物理研究的热点。然而d电子的奇异特性使传统的能带理论在研究过渡金属化合物时已经失效,实验测量成为了理解所有这些奇异性质最直接也是最有效的手段,针对这类体系的实验手段例如扫描隧道显微镜（STM）以及角分辨光电子能谱（ARPES）等在近十年得到的巨大的发展。现代的角分辨光电子能谱技术已经不局限于测量固体中的带隙,通过对数据的进一步分析我们还可以同时得到固体中价电子的能量、运动方向和散射性质,从而对固体内部的能带,轨道等给出一个全面的理解。本论文的前半部分主要介绍了利用光屯子能谱对经典的过渡金属二硫族化合物1T-Cu_xTiSe₂以及新型的铁基超导体BaFe_2-xCo_xAs₂的测量,并得到如下结果:1、针对具有广泛争议的1T-TiSe₂的2×2×2的电荷密度波机理给出了合理的解释并且合理的理解了掺杂后样品的相图:利用角分辨光电子能谱对这类样品的电子结构随掺杂的变化进行了系统的研究;观察到化学势随着掺杂的抬升符合刚带模型,同时电荷密度波引起的价带折叠受到了掺杂的显著压制。通过对费米能量处的Ti 3d能带谱权重的分析得到3d能带得到了来自插层铜原子的填充,这为超导的产生提供了必要的条件。综合我们的分析,我们提出1T-TiSe2中的电荷密度波行为起源于电子-空穴能带之间的相互作用,并且通过影响价带节省系统能量。随着掺杂提升化学势,一方面电荷-空穴的相互作用被显著削弱,压制电荷密度波:另一方面费米面的态密度得到增加,产生超导。我们的实验利用掺杂后的样品,针对以往争论的根源,从能带和态密度两个角度做出了合理的解释,解决了一个长期的争论。2、研究了新型超导体BaFe2-xCoxAs2,得到了这类样品的能带结构以及随掺杂的变化,观察到了电子掺杂掺杂对M点和Γ点不同的影响。针对以上样品的研究一方面让我们惊叹光电子能谱这一实验手段的强大,另一方面也让我们感叹由于它的表面敏感,目前的研究领域还局限于能解理的块材样品。许多同样有意思的样品例如La1-xSrxMnO3等,由于无法解理只能放弃。为了扩大我们的研究领域同时也是对进一步推广光电子能谱进行了以下的尝试:1、氧化物分子束外延技术与角分辨光电子能谱仪有机的结合起来,自主设计了与光电子能谱仪的设计和搭建工作。完成了低温和常温蒸发源的设计和制作:设计并且完成了臭氧提纯系统。目前位于复旦大学的氧化物分子束外延系统的搭建已经进入尾声,处于调试阶段。2、受ICTP资助在意大利交流期间.参与了LSMO体系的分子束外延生长,La_1-xSr_xMnO₃由于其独特的磁电性质,是下一代磁存储材料的热门侯选。借助MBE的手段,我们可以生长出比PLD等常规手段更高质量的单晶,同时由于采用了Layer by layer的生长模式,我们可以对最上层的界面做人工的优化,其对居里温度的影响大大的超出了我们的预期。更多还原

【Abstract】 Transitional Metal Elements includes ten groups of elements ranges fromⅢB toⅡB in the period table.This large group of material has unfilled d orbital and thus could form many novel compounds which has unique behavior in electronic,optic and magnetic properties.Moreover all these unique properties could be finely tuned by changing temperature,doping and pressure which in turn gives a very colorful phase diagram.For example,the coexistence of Superconductivity（SC） and Charge Ordering（CDW） in Transitional Metal Dichalcogenides（TMDs）;the Metal-Insulator Transition （MIT） in Mangnite Compound;and the competition of Spin-Density-Wave （SDW） and Superconductivity（SC） in the newly discovered Iron Pnictides Superconductors.All these diagrams include rich physics information inside and could have potential in future applications and this make them hot topics in condensed matter physics.For the complexity of d-orbital,classical band structure theories are no longer valid in studying the transitional metal compound which makes experimental method the most direct and efficient way in understanding the unique properties of these novel materials.Advanced experimental techniques such as STM（Scanning Tunneling Microscopy） and ARPES （Angle Resolved Photoemission Spectroscopy） have been greatly developed during the last two decades.With modern ARPES we could not only obtain the band structure of solids but also the Fermi velocities and scattering properties et al.,which help to give a detail understanding of the orbital and band information.In the first half of this thesisⅠpresent the ARPES measurements on classical TMDs material 1T-Cu_xTiSe₂ and the new superconductor BaFe_2-xCo_xAs₂ with the following conclusions:1.The electronic structure of a new charge-density-wave system or superconductor,1T-CuxTiSe2,has been studied by photoemission spectroscopy.A correlated semiconductor band structure is revealed for the undoped case,which resolves a long-standing controversy in the system. With Cu doping,the charge density wave is suppressed by the raising of the chemical potential,while the superconductivity is enhanced by the enhancement of the density of states,and possibly suppressed at higher doping by the strong scattering.2.Systematic study of the electronic structure of new type electron dopped material BaFe_2-xCo_xAs₂ is finished and the evolution of the band structure with doping the observed.We note the different doping effect to the electronic structure at M and G.While being inspired by the powerful method,we feel somehow regret for the oversensitivity to surface which limits this method to only a few cleavable layered structure samples.Other interesting sample such as La1-xSrxMnO3 for the difficulty to obtaining a high quality surface is beyond our research area. To further develop the state of art photoemission technique,we make the following attempts to extend our research area:1.We make seamless combination of the ARPES and OxMBE（Oxide Molecular Beam Epitaxy） which could provide high quality single crystal for insitu measurements.I designed and built the MBE system and managed to construct our own evaporation source;I designed and finished the ozone distilling system.Now the Oxide MBE system in Fudan University is being debugged and would be ready to use in a few weeks.2.With the finical support,I participated in the epitaxial growth of La1-xSrxMnO3 films which has unique magnetic and electronic properties and is the key candidate for the next generation information storage material.With the ozone assistant MBE technique,we could grow better quality films than PLD ever did before and through the artificial engineering of the top-most layer we could increase the Curie temperature much higher than we have expected.更多还原