87 research outputs found
Evaluation of Born and local effective charges in unoriented materials from vibrational spectra
We present an application of the Lorentz model in which fits to vibrational
spectra or a Kramers Kronig analysis are employed along with several useful
formalisms to quantify microscopic charge in unoriented (powdered) materials.
The conditions under which these techniques can be employed are discussed, and
we analyze the vibrational response of a layered transition metal
dichalcogenide and its nanoscale analog to illustrate the utility of this
approach.Comment: 9 pages, 1 figur
Charge order, dynamics, and magneto-structural transition in multiferroic LuFeO
We investigated the series of temperature and field-driven transitions in
LuFeO by optical and M\"{o}ssbauer spectroscopies, magnetization, and
x-ray scattering in order to understand the interplay between charge,
structure, and magnetism in this multiferroic material. We demonstrate that
charge fluctuation has an onset well below the charge ordering transition,
supporting the "order by fluctuation" mechanism for the development of charge
order superstructure. Bragg splitting and large magneto optical contrast
suggest a low temperature monoclinic distortion that can be driven by both
temperature and magnetic field.Comment: 4 pages, 3 figures, PRL in prin
Magneto-optical investigation of the field-induced spin-glass insulator to ferromagnetic metallic transition of the bilayer manganite (LaPr)SrMnO
We measured the magneto-optical response of
(LaPr)SrMnO in order 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. We discuss field- and
temperature-induced trends within the framework of the Tomioka-Tokura global
electronic phase diagram picture and suggest that controlled disorder near a
phase boundary can be used to tune the magnetodielectric response. Remnants of
the spin-glass insulator to ferromagnetic metallic transition can also drive
300 K color changes in (LaPr)SrMnO.Comment: 9 pages, 8 figure
Absence of Spin Liquid Behavior in Nd3Ga5SiO14 Using Magneto-Optical Spectroscopy
We measured the low-lying crystal field levels of Nd3+ in Nd3Ga5SiO14 via magneto-optical spectroscopy and employed the extracted energies, magnetic moments, and symmetries to analyze the magnetic properties and test the spin liquid candidacy of this material. The exchange interaction is surprisingly small, a discovery that places severe constraints on models used to describe the ground state of this system. Further, it demonstrates the value of local-probe photophysical techniques for rare-earthcontaining materials where bulk property measurements can be skewed by low-lying electronic structure
Magneto-Dielectric Effect in the S = 1/2 Quasi-Two Dimensional Antiferromagnet K2V3O8
We report the optical and magneto-optical properties of K2V3O8, an S=1/2
quasi-two-dimensional Heisenberg antiferromagnet. Local spin density
approximation electronic structure calculations are used to assign the observed
excitations and analyze the field dependent features. Two large magneto-optical
effects, centered at ~1.19 and 2.5 eV, are attributed to field-induced changes
in the V 4+ d to d on-site excitations due to modification of the local crystal
field environment of the VO5 square pyramids with applied magnetic field. Taken
together, the evidence for a soft lattice, the presence of vibrational fine
structure on the sharp 1.19 eV magneto-optical feature,and the fact that these
optical excitations are due to transitions from a nearly pure spin polarized V
d state to hybridized states involving both V and O, suggest that the
magneto-dielectric effect in K2V3O8 is driven by strong lattice coupling.Comment: Zipped file containing 8 pages, 12 figures, in press PR
Lattice dynamical probe of charge order and antipolar bilayer stacking in LuFe2O4
We investigated the infrared response of LuFe2O4 through the series of charge, magnetic, and structural transitions. All vibrational modes couple strongly to the charge order, whereas the LuO zone-folding modes are also sensitive to magnetic order and structural distortion. The dramatic splitting of the LuO2 layer mode is attributed to charge-rich/poor proximity effects and its temperature dependence reveals the antipolar nature of the W layer pattern
High-pressure spectroscopic investigation of multiferroic Ni3TeO6
We combined diamond anvil cell techniques, infrared and Raman spectroscopies, and lattice dynamics calculations to explore the high pressure properties of multiferroic Ni3TeO6. Using a frequency trend analysis, we trace a subtle decrease in compressibility near 4 GPa to a minimum in the O-Ni2-O bond angle. This unique behavior emanates from the proximity of the Ni2 center in the Ni3-Ni2-Ni1-Te chain to a flexible pocket that is intrinsic to the crystal structure. At the same time, predicted trends in the superexchange pathways are consistent with greater antiferromagnetic character under compression, in line with both phase stability calculations and direct susceptibility measurements. These findings highlight opportunities for local structure control of corundumlike materials
Photoconductivity in BiFeO3 thin films
The optical properties of epitaxial BiFeO3 thin films have been characterized in the visible range. Variable temperature spectra show an absorption onset near 2.17 eV, a direct gap (2.667±0.005 eV at 300 K), and charge transfer excitations at higher energy. Additionally, we report photoconductivity in BiFeO3 films under illumination from a 100 mW/cm2 white light source. A direct correlation is observed between the magnitude of the photoconductivity and postgrowth cooling pressure. Dark conductivities increased by an order of magnitude when comparing films cooled in 760 and 0.1 Torr. Large increases in photoconductivity are observed in light
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