Angle resolved photoemission study of quantum critical solids.

Many-body systems with quantum criticality (QC) exhibit various unusual properties that can not be described by the canonical physics paradigm, i.e. the Fermi liquid (FL) theory. Research directed at understanding these systems is at the current frontier of condensed matter physics. This thesis presents experimental work on several solid materials that are proven or are promising to show QC, using angle resolved photoemission (ARPES), as part of the ongoing effort to establish electron spectroscopy as a major tool to study quantum critical phenomena. The so-called Luttinger liquid (LL) is a special kind of quantum critical system in one dimension, described by the Tomonaga-Luttinger model (TLM) for the gapless case. Li0.9Mo6O17 is a quasi-one dimensional (Q1D) metal with strong anisotropy in various properties and no evidence of gap opening. Based on previous ARPES evidence of Luttinger physics at temperatures above 250 K in this material, the present work studies temperature dependence of the spectral lineshape systematically. Our findings establish Li0.9Mo6O17 as a LL paradigm that is able to reveal new physics and demonstrate QC. A unified interpretation of the data suggests that this material actually displays a new quantum state of matter as proposed in two related theory scenarios. We also conducted a high photon energy study to confirm the bulk nature of our findings at low photon energy. NaMo6O17 and KMo6O17 are close relatives to Li0.9Mo6O17 but with quasi-two dimensional (Q2D) electronic structures. They are interesting for two reasons, the so-called hidden-1D FS and their anomalous spectroscopic lineshapes measured previously. We studied and characterized in both materials a surface charge density wave (CDW) state, whose nature was not recognized before. This study clears the way for further spectroscopic study of the bulk CDW state. Preliminary bulk measurements show non-FL (NFL) lineshape between 200 K and 16 K. Another class of quantum critical materials are from the heavy-Fermion materials, which display exceptionally large effective masses. CeCoIn 5 and UGe2 are both three dimensional (3D) metals from this category with QCP's observed in other experiments. Electronic structures of these materials are not clear yet, and having this knowledge is prerequisite to making a direct electron spectroscopic test of QC. So our task in the present work is to measure the Fermi surface (FS) topology and band structure. The results, which are the first ARPES ever achieved for both materials, serve as an experimental test for prevailing band theory calculations. These results set the stage for further studies.Ph.D.Condensed matter physicsPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126850/2/3276323.pd