Study of Charge Density Waves in Rare-Earth Transition-Metal Silicides Dy5Ir4Si10 and Lu2Ir3Si5 by Transmission Electron Microscopy

• Main Authors: Ming-Hao Lee, 李明浩
• Other Authors: Cheng-Hsuan Chen
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/91892987854737052654

博士 === 國立臺灣大學 === 物理研究所 === 101 === The charge-density wave (CDW) is a ground state with broken translational symmetry of metals, brought by electron-phonon interactions. It was recognized that highly anisotropic band structures are important in leading to these ground state. And the ground states are coherent superposition of electron-hole pairs, and result in a periodic space variation of charge density as name. The CDW ground state is also accompanied by the opening of energy gap at the Fermi surface. Since the energy gap forms within the former conduction band, a 1D metal would be expected to become insulating in CDW state. Rare-earth transition-metal ternary silicides with three-dimensional crystallographic structures, such as the R5T4Si10 and R2T3Si5 types, have been shown to exhibit CDW phase transitions with remarkable anomalies observable in the thermal and electrical transport measurements. In this thesis, I describe the experimental results of probing CDW formation in two different compounds of 3-D materials: Dy5Ir4Si10 and Lu2Ir3Si5. The CDW study in Dy5Ir4Si10 The tetragonal rare-earth transition-metal silicide system, R5T4Si10, where R is Dy, Ho, Er, Tm, and Lu, and T = Ir and Rh, with seemingly three-dimensional crystallographic structure, has been shown to exhibit fascinating charge density wave (CDW) phase transitions, a phenomenon found largely in low-dimensional systems. In this study we report the investigations of CDW in Dy5Ir4Si10 at different temperatures using transmission electron microscopy (TEM) techniques including electron diffraction and dark-field imaging. Superlattice diffraction spots along c-axis were observed in the electron diffraction pattern when the sample was cooled below the CDW transition temperature (TCDW ~ 200K), indicating the presence of incommensurate CDW state with the modulation wave vector of . CDW become commensurate with further cooling below ~ 160K. Configurations of CDW dislocations convincingly show that the CDW phase transition is accompanied by a concomitant cell-doubling crystallographic structural phase transition. Furthermore, symmetry breakdown along c-axis observed by convergent beam electron diffraction (CBED) gives rise to two different type of CDW domains. Detailed characteristics of this unusual behavior will also be discussed. The CDW study in Lu2Ir3Si5 We report the investigation of charge density wave (CDW) in Lu2Ir3Si5 by electron diffraction and dark-field imaging using superlattice diffraction spots. The CDW state is confirmed by the presence of superlattice reflections. Most interestingly, the CDW state at low temperatures is found to be electronically phase-separated with the coexistence of CDW domains and low-temperature normal phase domains. Upon change of temperatures, unlike other typical incommensurate CDW systems in which commensurability varies with temperatures, we find that commensurability remains unchanged in the present case and the predominant change is in the redistribution of the area ratio of the two coexisted phases, which is clearly revealed in the dark-field images obtained from the CDW superlattice reflections. The electronic phase separation in the CDW state of Lu2Ir3Si5 is unprecedented in CDW systems, and its temperature dependence is also anomalous.