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Languages : en
Pages : 28
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Department of Energy EPSCoR The University of Mississippi Award: DE-FG02-04ER46121 Resonant Ultrasound Spectroscopy Studies of Complex Transition Metal Oxides The central thrust of this DOE funded research program has been to apply resonant ultrasound spectroscopy (RUS), an elegant and efficient method for determining the elastic stiffness constants of a crystal, to the complex and poorly understood class of materials known as transition metal oxides (TMOs). Perhaps the most interesting and challenging feature of TMOs is their strongly correlated behavior in which spin, lattice, and charge degrees of freedom are strongly coupled. Elastic constants are a measure of the interatomic potentials in a crystal and are thus sensitive probes into the atomic environment. This sensitivity makes RUS an ideal tool to study the coupling of phase transition order parameters to lattice strains. The most significant result of the project has been the construction of a high temperature RUS apparatus capable of making elastic constant measurements at temperatures as high as 1000 degrees Celsius. We have designed and built novel acoustic transducers which can operate as high as 600 degrees Celsius based on lithium niobate piezoelectric elements. For measurement between 600 to 1000 C, a buffer rod system is used in which the samples under test and transducers are separated by a rod with low acoustic attenuation. The high temperature RUS system has been used to study the charge order (CO) transition in transition metal oxides for which we have discovered a new transition occurring about 35 C below the CO transition. While the CO transition exhibits a linear coupling between the strain and order parameter, this new precursor transition shows a different coupling indicating a fundamentally different mechanism. We have also begun a study, in collaboration with the Jet Propulsion Laboratory, to study novel thermoelectric materials at elevated temperatures. These materials include silicon germanium with various doping and Zintl phase materials. Such materials show promise for increased figures of merit, vital to making thermolectrics competitive with traditional power generation mechanisms.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 28
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Book Description
Department of Energy EPSCoR The University of Mississippi Award: DE-FG02-04ER46121 Resonant Ultrasound Spectroscopy Studies of Complex Transition Metal Oxides The central thrust of this DOE funded research program has been to apply resonant ultrasound spectroscopy (RUS), an elegant and efficient method for determining the elastic stiffness constants of a crystal, to the complex and poorly understood class of materials known as transition metal oxides (TMOs). Perhaps the most interesting and challenging feature of TMOs is their strongly correlated behavior in which spin, lattice, and charge degrees of freedom are strongly coupled. Elastic constants are a measure of the interatomic potentials in a crystal and are thus sensitive probes into the atomic environment. This sensitivity makes RUS an ideal tool to study the coupling of phase transition order parameters to lattice strains. The most significant result of the project has been the construction of a high temperature RUS apparatus capable of making elastic constant measurements at temperatures as high as 1000 degrees Celsius. We have designed and built novel acoustic transducers which can operate as high as 600 degrees Celsius based on lithium niobate piezoelectric elements. For measurement between 600 to 1000 C, a buffer rod system is used in which the samples under test and transducers are separated by a rod with low acoustic attenuation. The high temperature RUS system has been used to study the charge order (CO) transition in transition metal oxides for which we have discovered a new transition occurring about 35 C below the CO transition. While the CO transition exhibits a linear coupling between the strain and order parameter, this new precursor transition shows a different coupling indicating a fundamentally different mechanism. We have also begun a study, in collaboration with the Jet Propulsion Laboratory, to study novel thermoelectric materials at elevated temperatures. These materials include silicon germanium with various doping and Zintl phase materials. Such materials show promise for increased figures of merit, vital to making thermolectrics competitive with traditional power generation mechanisms.
Author: Yanbing Luan
Publisher:
ISBN:
Category :
Languages : en
Pages : 143
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The elastic properties of novel transition metal oxides have been investigated, using a powerful technique known as Resonant Ultrasound Spectroscopy (RUS). Two sets of transition metal oxides have been studied. One is the ruthenate Ca2-[subscript x]Sr[subscript x]RuO4 series with a layered perovskite structure, a Mott transition system that connects the Mott insulator Ca2RuO4 with the unconventional superconductor Sr2RuO4. The other set contains geometrically frustrated materials, including vanadium spinels AV2O4 (A = Zn, Mn and Fe) and titanate pyrochlores A2Ti2O-- (A= Y, Tb, Yb, Ho and Dy). The elastic response of five Ca2-[subscript x]Sr[subscript x]RuO4 single crystals (x = 2.0, 1.9, 0.5, 0.3 and 0.2) has been measured. For 2.0 [greater than or equal to] x [greater than or equal to] 0.5, a dramatic softening over a wide temperature range is observed upon cooling, caused by the rotational instability of RuO6 octahedra (for x = 2.0 and 1.9) or the static rotation of the octahedra (for x = 0.5). For the Ca-rich samples (x = 0.3 and 0.2), the softening occurs in a very narrow temperature range, corresponding to the structural phase transition from high-temperature-tetragonal to low-temperature-orthorhombic symmetry. Elastic softening in ZnV2O4 is observed near the cubic-to-tetragonal structural phase transition at 50 K. The elastic response of MnV2O4 is quite unusual, displaying a softening over a wide temperature range with decreasing temperature. Upon cooling, C' of FeV2O4 becomes so soft that it drops to almost zero around 140 K, where the cubic-to-tetragonal structural transition occurs. For Y2Ti2O--, all three elastic constants show normal "Varshni" behavior. For spin liquid Tb2Ti2O--, all three elastic constants show a pronounced softening below 50 K, indicative of a possible Jahn-Teller, cubic-to-tetragonal transition at very low temperatures. It is also found that the application of a magnetic field suppresses the elastic softening in this compound. Another spin liquid Yb2Ti2O-- shows no elastic softening. The elastic moduli of the spin-ice compounds, Ho2Ti2O-- and Dy2Ti2O--, show a broad "dip" around 100 K, which is believed to be caused by the strong crystal field effect in those two compounds.
Author: Мирослав Драмићанин
Publisher: Institut za nuklearne nauke VINČA
ISBN: 8673061342
Category :
Languages : en
Pages : 327
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Author: Guangyan Li
Publisher:
ISBN:
Category :
Languages : en
Pages : 416
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The goal of this dissertation is to elastically characterize thermoelectrics and other novel materials using resonant ultrasound spectroscopy (RUS) at elevated temperatures. A "direct-contact" transducer system with test sample in direct contact with the piezoelectric elements was developed. This new transducer system with improved signal-to-noise ratio (SNR) and the conventional buffer-rod system were used for elasticity measurements and phase transition studies. The temperature dependent elastic moduli of thermoelectric materials of four nanostructured polycrystalline silicon-germanium (SiGe) samples were obtained up to 950°C. Abnormal elastic behavior (stiffening) in the temperature range of 350-550°C was observed in the two n-doped SiGe samples, which is associated with dopant (phosphorus) precipitation. The elastic moduli of the complex Zintl phase Yb 14 MnSb 11 were also measured up to 600°C. Using the quick "mode-tracking" method, the various temperature-induced phase transitions in Yb 14 MnSb 11, transition metal oxide LuFe 2 O 4 and bulk metallic glass Zr 50 Cu 31 Pd 9 Al 10 were investigated. In addition to the high-temperature RUS measurements, the continuum elastic model in RUS was also applied to numerically study several lower normal modes of vibration in carbon nanotubes (CNTs).
Author: Jinbo Cao
Publisher:
ISBN:
Category : Metallic oxides
Languages : en
Pages : 170
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In this dissertation, I present spectroscopic studies of several model electronic and magnetic materials. Compounds of interest include VO[subscript lowercase chi] nanoscrolls, VOHPO4·[1/2]H2O, and (La[subscript 0.4]Pr[subscript 0.6])[subscript 1.2]Sr[subscript 1.8]Mn2O--. These materials are attractive systems for the investigation of optical gap tuning, lattice and charge dynamics, spin-lattice-charge coupling, and hydrogen bonding effects. I measured the optical properties of VO[subscript lowercase chi] nanoscrolls and the ion-exchanged derivatives to investigate the lattice and charge degrees of freedom. Selected V-O-V stretching modes sharpen and redshift with increasing amine size, which are microscopic manifestations of strain. We observed bound carrier localization in the metal exchanged nanoscrolls, indicating they are weakly metallic in their bulk form. I also investigated the variable temperature vibrational properties of single crystals of the S = 1/2 Heisenberg antiferromagnet VOHPO4·[subscript 1/2]H2O. In order to explain the activation and polarization dependence of the singlet-to- triplet gap in the far-infrared response, we invoke a dynamic Dzyaloshinskii-Moriya mechanism and we identify the low-energy phonons that likely facilitate this coupling. Vibrational mode splitting of VOHPO4·[1/2]H2O also points toward a weak local symmetry breaking near 180 K, and the low-temperature redshift of V-O and H-O related modes demonstrates enhanced low-temperature hydrogen bonding. Finally, I measured the magneto-optical response of (La[subscript 0.4]Pr[subscript 0.6])[subscript 1.2]Sr[subscript 1.8]Mn2O-- to investigate the microscopic aspects of the magnetic field driven spin-glass insulator to ferromagnetic metal transition. Application of a magnetic field recovers the ferromagnetic state with an overall redshift of the electronic structure, growth of the bound carrier localization associated with ferromagnetic domains, development of a pseudogap, and softening of the Mn-O stretching and bending modes that indicate a structural change. By exploiting the electronic mechanisms, we can induce large high energy magnetodielectric contrast in (La[subscript 0.4]Pr[subscript 0.6])[subscript 1.2]Sr[subscript 1.8]Mn2O--. The dielectric contrast is over 100% near 0.8 eV at 4.2 K. Remnants of the transition also drive the high energy magnetodielectric effect at room temperature.
Author: Joshua J. Turner
Publisher:
ISBN: 9780549567615
Category : Grenz rays
Languages : en
Pages : 414
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Author: Joseph Robert Shannon
Publisher:
ISBN:
Category :
Languages : en
Pages : 228
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Author: Tsezar F. Seman
Publisher:
ISBN:
Category :
Languages : en
Pages : 148
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The following studies are presented: theory of K-edge resonant inelastic x-ray scattering and its application for La0.5Sr1.5MnO4, effects of rare earth ion size on the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites, and symmetry-mode-based classical and quantum mechanical formalism of lattice dynamics. The formula based on tight-binding approach for the calculation of K-edge resonant inelastic x-ray scattering (RIXS) spectrum for transition metal oxides is presented first, by extending the previous existing result to include explicit momentum dependence and a basis with multiple core-hole sites. This formula is applied to layered charge, orbital and spin ordered manganites, La0.5Sr1.5MnO4, and good agreement with experimental data was obtained, in particular, with regard to the large variation of the intensity with momentum. As a consequence, it is established that the electron screening in La0.5Sr1.5MnO4 is highly localized around the core hole site and demonstrates the potential of K-edge RIXS, as a probe for the screening dynamics in materials. Theoretical study is then introduced on the relation between the size of the rare earth ions, often known as chemical pressure, and the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites. Using a Keating model expressed in terms of atomic scale symmetry modes, it is shown that there exists a coupling between the uniform shear distortion and the staggered buckling distortion within the Jahn-Teller energy term. It is found that this coupling provides a mechanism by which the coherent Jahn-Teller distortion is more stabilized by smaller rare earth ions. Further analysis shows the appearance of the uniform shear distortion below the Jahn-Teller ordering temperature; the Jahn-Teller ordering temperature is estimated and its variation between NdMnO3 and LaMnO3, and the relations between distortions are obtained. A good agreement is found between theoretical results and the experimental data. Finally, the classical and quantum mechanical descriptions of lattice dynamics are presented, from the atomic to the continuum scale, using atomic scale symmetry modes and their constraint equations. This approach is demonstrated for a one- dimensional chain and a two-dimensional square lattice on a monatomic basis. For the classical description, it is found that rigid modes, in addition to the distortional modes found before, are necessary to describe the kinetic energy. The long wavelength limit of the kinetic energy terms expressed in terms of atomic scale modes is shown to be consistent with the continuum theory, and leading order corrections are obtained. For the quantum mechanical description, conjugate momenta for the atomic scale symmetry modes are presented. In direct space, graphical rules for their commutation relations are obtained. Commutation relations in the reciprocal space are also calculated. As an example, phonon modes are analyzed in terms of symmetry modes. The approach presented here based on atomic scale symmetry modes could be useful for the study of complex emerging materials, in which competing structural phases and non-linearity of the lattice energy play an important role.
Author: Md. Motin Seikh
Publisher: LAP Lambert Academic Publishing
ISBN: 9783838342177
Category :
Languages : en
Pages : 292
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The coupled dynamics of charge, spin, orbital and lattice degrees of freedom, related to the emergence of complex phases in transition metal oxides has been a long standing research topic among condensed matter community. This monograph largely emphasis on the experimental investigations of several oxide systems by employing a number of techniques. The synthetic methods include conventional solid state route, single crystal growth using floating zone melting technique and thin films. The various aspects of investigations include electronic and magnetic properties, charge-ordering and electronic phase segregation in rare earth manganites, spin- state transitions in rare earth cobaltites and insulator-metal transitions in magnetite (Fe3O4) and karelianite (V2O3). Besides the measurements of magnetoelectric properties, several spectroscopic techniques are employed to study the various problems. The LS IS HS spin-state transition in cobaltite is explored using infrared and Raman spectroscopy. Brillouin scattering has been successfully utilized to emphasize role of low energy excitations in the strongly correlated electron systems
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Publisher:
ISBN:
Category :
Languages : de
Pages : 249
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