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Title:Inelastic light scattering studies of quantum phase transitions in CuxTiSe2 and multiferroic TbMnO3
Author(s):Barath, Harini
Director of Research:Cooper, S. Lance
Doctoral Committee Chair(s):Abbamonte, Peter M.
Doctoral Committee Member(s):Fradkin, Eduardo H.; Stack, John D.
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Raman spectroscopy
Quantum Phase Transitions
Layered Dichalcogenides
Abstract:Abstract In this dissertation, inelastic (Raman) light scattering techniques are used to probe the temperature- and magnetic-field-induced phase transitions of two strongly correlated systems – the magnetoelectric multiferroic TbMnO3 and the layered dichalcogenide TiSe2 . In general, strongly correlated materials have a strong coupling between charge, spin, lattice and orbital degrees of freedom. Because of the inter- play between various competing orders, these systems have highly complex phase diagrams and exhibit interesting phenomena such as colossal magnetoresistance (CMR), high temperature superconductivity and charge/orbital ordering (COO). Magnetoelectric multiferroics are an important and interesting sub-class of strongly correlated systems. These are systems whose magnetic and electric orders are strongly coupled, thereby showing exquisite tunability of the electric polarization via applied magnetic fields, and vice-versa. One such system is the perovskite manganite, TbMnO3 , which shows magnetic-field-tuned rearrangement of the electric polarization vector in the ferroelectric phase below a critical temperature, Tc ∼ 28 K. This ferroelectric phase transition is accompanied, and in fact caused, by a magnetic phase transition from an incommensurate spiral magnetic arrangement of the Mn3+ ions to a commensurate magnetic phase as a function of applied field. We use Raman scattering to carefully probe this magnetic-field-tuned phase transition in microscopic detail. Our measurements indicate that field-induced quantum fluctuations of commensurate domains, which likely drive the field-induced polarization flop in this material, are found near the field-tuned incommensurate-commensurate phase transition. The second focus of this dissertation is the study of quantum phase transitions in TiSe2 as a function of temperature and Cu-intercalation, and the comparison of the effects of intercalation and pressure on the charge-density-wave (CDW) order in this system. All these parameters – temperature, pressure and Cu-intercalation – suppress the CDW state in TiSe2 . Our Raman measurements on Cux TiSe2 show that the x-dependent mode softening exhibits identical scaling behavior to thermal mode softening in undoped TiSe2 , suggesting that, like thermal mode softening, the x-dependent mode softening is also associated with a critical point. The softening and significant linewidth broadening of the observed CDW amplitude modes indicate strong fluctuations of the CDW. Even more interesting is the emergence of a superconducting (SC) phase – in Cux TiSe2 and pressure-tuned TiSe2 – indicating a likely coexistence of fluctuating CDW and SC phases near the quantum phase boundary.
Issue Date:2010-05-14
Rights Information:Copyright 2010 by Harini Barath. All rights reserved.
Date Available in IDEALS:2010-05-14
Date Deposited:May 2010

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