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Title:Exploring the pressure- and magnetic-field-induced phases of orbital-ordering KCuF3 and single molecule magnet Mn12-acetate: Crystal growth and Raman scattering studies
Author(s):Yuan, Shi
Director of Research:Cooper, S. Lance
Doctoral Committee Chair(s):Abbamonte, Peter M.
Doctoral Committee Member(s):Cooper, S. Lance; Phillips, Philip W.; Peng, Jen-Chieh
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Strongly correlated material
Crystal growth and characterization
Inelastic light (Raman) scattering
Abstract:In this dissertation, inelastic light (Raman) scattering techniques are used to probe the novel phases and properties of the orbital ordering material KCuF3 and the single molecule magnet Mn12-acetate as functions of temperature, applied pressure, and applied magnetic field. Single crystal samples of KCuF3, Mn12-acetate, 1T-TiSe2, and Bi2Se3 are also grown as part of this thesis work. Our experiments on KCuF3 provide evidence that the Kugel and Khomskii description of orbital ordering does not, by itself, explain orbital ordering behavior in this material, as is commonly believed. This evidence includes the observation of temperature-dependent phonon mode softening below the putative orbital-ordering transition—suggesting that a lattice instability persists below this transition—and a previously unidentified tetragonal-to-orthorhombic structural transition near TS=50 K at temperatures slightly higher than the onset of Néel order, TN=40 K. A model based on these observations, developed by Siddhartha Lal and Paul Goldbart, suggests that the orbital ordering transition in KCuF3 takes place in two steps: a high temperature transition (T>800 K) removes some of the d-electron level degeneracy, but leaves a pair of nearly degenerate hybrid states whose degeneracy is split by the structural transition near TS=50 K. Our pressure-dependent, low-temperature Raman measurements also show that applied pressure above P^*~ 7 kbar suppresses the low temperature structural phase transition temperature to zero temperature in KCuF3, resulting in the development of a quasielastic fluctuational response near T~ 0 K. This pressure-induced fluctuational response—which we associate with slow fluctuations of the CuF6 octahedral orientation—is temperature independent and exhibits a characteristic fluctuation rate that is much larger than the temperature, consistent with quantum fluctuations of the CuF6 octahedra. We show that a model of pseudospin-phonon coupling—where the pseudospin represents distinct orientations of the CuF6 octahedra—provides a qualitative description of both the temperature-dependent “soft mode” behavior exhibited by F ion vibrations and the pressure-dependent evolution of fluctuational (quasielastic) scattering in KCuF3. In Mn12-acetate, our temperature-dependent Raman measurements reveal anomalous phonon behavior near 200 K, indicating a structural transition to a lower crystal symmetry below 200 K. These results support the inclusion of a second-order rhombic term E(S_x^2-S_y^2 ) in the spin Hamiltonian, which would be consistent with phonon-assisted tunneling in Mn12-acetate. The phonon modes in the range 100 cm-1 to 550 cm-1—which involve Mn ion vibrations—are consistent with previous theoretical and infrared studies and make an important contribution to the fourth-order anisotropy energy. Our field-dependent measurements at T ~ 3 K show that a magnetic field oriented perpendicular to the Mn12 magnetization direction (the “hard plane” direction) does not affect the phonon vibrational energies. However, when the magnetic field is oriented along the easy-axis direction, there is a clear phonon mode splitting at 540 cm-1, indicating a strong spin-phonon coupling associated with this phonon mode in Mn12 acetate. An applied field oriented in the longitudinal direction induces a small tilt of the anisotropy axis responsible for the odd resonance tunneling. The molecular relaxation rate—inferred from the linewidth of the Raman-active phonon—increases significantly with increasing magnetic field at T=3 K, indicating an important spin relaxation contribution to the quantum tunneling magnetization at low temperatures. Pressure-dependent Raman results at T=3 K reveal a pressure-induced structural phase transition near 15 kbar associated with the Mn3+ center; this transition is believed to reflect a saturation of the pressure-induced Jahn-Teller distortion. Financial support for this research was provided in part by the Department of Energy under Grant No. DE-FG02-07ER46453.
Issue Date:2014-05-30
Rights Information:Copyright 2014 Shi Yuan
Date Available in IDEALS:2014-05-30
Date Deposited:2014-05

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