567 research outputs found
Cooperative dynamics in doped manganite films: phonon anomalies in the ferromagnetic state
We present optical measurements of phononic excitations in
LaCaMnO (LCMO) and LaSrMnO (LSMO)
thin films covering the full temperature range from the metallic ferromagnetic
to the insulating paramagnetic phase. All eight phonons expected for the
Rc symmetry in LSMO and 17 out of the expected 25 phonons for the Pnma
symmetry in LCMO have been determined. Close to the
ferromagnetic-to-paramagnetic transition both compounds reveal an anomalous
behavior but with different characteristics. Anomalies in the phononic spectra
are a manifestation of the coupling of lattice degrees of freedom (DOF) to
electronic DOF. Specifically, the low-frequency external group proves to be an
indicator for lattice modifications induced by electronic correlations. The
enhanced electron-phonon coupling in LCMO is responsible for Fano-like
interference effects of distinct phonon modes with electronic continuum
excitations: we observe asymmetric phonon line shapes, mode splitting and
spectral weight transfer between modes.Comment: 10 pages, 10 figure
Large and Small Polaron Excitations in La2/3(Sr/Ca)1/3MnO3 Films
We present detailed optical measurements of the mid-infrared (MIR)
excitations in thin films of La2/3Sr1/3MnO3 (LSMO) and La2/3Ca1/3MnO3 (LCMO)
across the magnetic transition. The shape of the excitation at about 0.2 eV in
both samples is analyzed in terms of polaron models. We propose to identify the
MIR resonance in LSMO as the excitation of large polarons and that in LCMO as a
small polaron excitation. A scaling behavior for the low-energy side of the
polaronic MIR resonance in LSMO is established
Phonon-Metamorphosis in Ferromagnetic Manganite Films: Probing the Evolution of an Inhomogeneous State
The analysis of phonon anomalies provides valuable information about the
cooperative dynamics of lattice, spin and charge degrees of freedom.
Significant is the anomalous temperature dependence of the external modes
observed in LaSrMnO (LSMO) films. The two external modes
merge close to the ferromagnetic to paramagnetic transition at and,
moreover, two new modes evolve in this temperature range with strong resonances
at slightly higher frequencies. We propose that this observed phonon
metamorphosis probes the inhomogeneous Jahn-Teller distortion, manifest on the
temperature scale . The analysis is based on the first observation of all
eight phonon modes in the metallic phase of LSMO and on susceptibility
measurements which identify a Griffiths-like phase.Comment: 4 pages, 4 figure
How a realistic magnetosphere alters the polarizations of surface, fast magnetosonic, and Alfvén waves
System-scale magnetohydrodynamic (MHD) waves within Earth's magnetosphere are often understood theoretically using box models. While these have been highly instructive in understanding many fundamental features of the various wave modes present, they neglect the complexities of geospace such as the inhomogeneities and curvilinear geometries present. Here, we show global MHD simulations of resonant waves impulsively excited by a solar wind pressure pulse. Although many aspects of the surface, fast magnetosonic (cavity/waveguide), and Alfvén modes present agree with the box and axially symmetric dipole models, we find some predictions for large-scale waves are significantly altered in a realistic magnetosphere. The radial ordering of fast mode turning points and Alfvén resonant locations may be reversed even with monotonic wave speeds. Additional nodes along field lines that are not present in the displacement/velocity occur in both the perpendicular and compressional components of the magnetic field. Close to the magnetopause, the perpendicular oscillations of the magnetic field have the opposite handedness to the velocity. Finally, widely used detection techniques for standing waves, both across and along the field, can fail to identify their presence. We explain how all these features arise from the MHD equations when accounting for a non-uniform background field and propose modified methods that might be applied to spacecraft observations
Solution Equilibrium Studies of Anticancer Ruthenium(II)-η6-p-cymene Complexes of Pyridinecarboxylic Acids
Stoichiometry and stability of antitumor ruthenium(II)-η6-p-cymene complexes of picolinic acid and its 6-methyl and 6-carboxylic acid derivatives were determined by pH-potentiometry, 1H NMR spectroscopy and UVâVis spectrophotometry in aqueous solution in the presence or absence of coordinating chloride ions. The picolinates form exclusively mono-ligand complexes in which they can coordinate via the bidentate (O,N) mode and a chloride or a water molecule is found at the third binding site of the ruthenium(II)-η6-p-cymene moiety depending on the conditions. [Ru(η6-p-cymene)(L)(H2O/Cl)] species are predominant at physiological pH in all studied cases. Hydrolysis of the aqua complex or the chlorido/hydroxido co-ligand exchange results in the formation of the mixed-hydroxido species [Ru(η6-p-cymene)(L)(OH)] in the basic pH range. There is no indication for the decomposition of the mono-ligand complexes during 24 h in the ruthenium(II)-η6-p-cymene-picolinic acid system between pH 3 and 11; however, a slight dissociation with a low reaction rate was found in the other two systems leading to the appearance of the dinuclear trihydroxido-bridged species [Ru2(η6-p-cymene)2(OH)3]+ and free ligands at pH > 10. The replacement of the chlorido by an aqua ligand in [Ru(η6-p-cymene)(L)Cl] was also monitored and equilibrium constants for the exchange process were determined
Apparent giant dielectric constants, dielectric relaxation, and ac-conductivity of hexagonal perovskites La1.2Sr2.7BO7.33 (B = Ru, Ir)
We present a thorough dielectric investigation of the hexagonal perovskites
La1.2Sr2.7IrO7.33 and La1.2Sr2.7RuO7.33 in a broad frequency and temperature
range, supplemented by additional infrared measurements. The occurrence of
giant dielectric constants up to 10^5 is revealed to be due to electrode
polarization. Aside of dc and ac conductivity contributions, we detect two
intrinsic relaxation processes that can be ascribed to ionic hopping between
different off-center positions. In both materials we find evidence for charge
transport via hopping of localized charge carriers. In the infrared region,
three phonon bands are detected, followed by several electronic excitations. In
addition, these materials provide further examples for the occurrence of a
superlinear power law in the broadband ac conductivity, which recently was
proposed to be a universal feature of all disordered matter.Comment: 8 pages, 7 figure
The effect of magnetopause motion on fast mode resonance
The Earth's magnetosphere supports several types of ultralow frequency (ULF) waves. These include fast mode resonance (FMR): cavity modes, waveguide modes, and tunneling modes/virtual resonance. The magnetopause, often treated as the outer boundary for cavity/waveguide modes in the dayside magnetosphere, is not stationary. A rapidly changing outer boundary conditionâe.g., due to rapid magnetopause motionâis not favorable for FMR generation and may explain the sparseness of FMR observations in the outer magnetosphere. We examine how magnetopause motion affects the dayside magnetosphere's ability to sustain FMR with idealized Space Weather Modeling Framework (SWMF) simulations using the BATSâRâUS global magnetohydrodynamic (MHD) code coupled with the Ridley Ionosphere Model (RIM). We present observations of FMR in BATSâRâUS, reproducing results from other global MHD codes. We further show that FMR is present for a wide range of solar wind conditions, even during periods with large and rapid magnetopause displacements. We compare our simulation results to FMR observations in the dayside magnetosphere, finding that FMR occurrence does not depend on solar wind dynamic pressure, which can be used as a proxy for dynamic pressure fluctuations and magnetopause perturbations. Our results demonstrate that other explanations besides a nonstationary magnetopauseâsuch as the inability to detect FMR in the presence of other ULF wave modes with large amplitudesâare required to explain the rarity of FMR observations in the outer magnetosphere. Key Points Typical magnetopause motion does not affect fast mode resonance occurrence Magnetopause motion cannot explain why FMR is rarely observed Selection criteria and nonâFMR wave activity affect FMR occurrence ratePeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109612/1/2014JA020401readme.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/109612/2/Auxiliary_Material_fs01.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/109612/3/Auxiliary_Material_fs02.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/109612/4/jgra51354.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/109612/5/Auxiliary_Material_fs03.pd
How a realistic magnetosphere alters the polarizations of surface, fast magnetosonic, and Alfvén waves
Funding: MOA holds a UKRI (STFC / EPSRC) Stephen Hawking Fellowship EP/T01735X/1. DJS was supported by STFC grant ST/S000364/1. MDH was supported by NASA grant 80NSSC19K0127. A.N.W. was partially funded by STFC grant ST/N000609/1.System-scale magnetohydrodynamic (MHD) waves within Earth?s magnetosphere are often understood theoretically using box models. While these have been highly instructive in understanding many fundamental features of the various wave modes present, they neglect the complexities of geospace such as the inhomogeneities and curvilinear geometries present. Here we show global MHD simulations of resonant waves impulsively-excited by a solar wind pressure pulse. Although many aspects of the surface, fast magnetosonic (cavity/waveguide), and Alfvén modes present agree with the box and axially symmetric dipole models, we find some predictions for large-scale waves are significantly altered in a realistic magnetosphere. The radial ordering of fast mode turning points and Alfvén resonant locations may be reversed even with monotonic wave speeds. Additional nodes along field lines that are not present in the displacement/velocity occur in both the perpendicular and compressional components of the magnetic field. Close to the magnetopause the perpendicular oscillations of the magnetic field have the opposite handedness to the velocity. Finally, widely-used detection techniques for standing waves, both across and along the field, can fail to identify their presence. We explain how all these features arise from the MHD equations when accounting for a non-uniform background field and propose modified methods which might be applied to spacecraft observations.Publisher PDFPeer reviewe
Polaronic excitations in CMR manganite films
In the colossal magnetoresistance manganites polarons have been proposed as
the charge carrier state which localizes across the metal-insulator transition.
The character of the polarons is still under debate. We present an assessment
of measurements which identify polarons in the metallic state of
La{2/3}Sr{1/3}MnO{3} (LSMO) and La{2/3}Ca{1/3}MnO{3} (LCMO) thin films. We
focus on optical spectroscopy in these films which displays a pronounced
resonance in the mid-infrared. The temperature dependent resonance has been
previously assigned to polaron excitations. These polaronic resonances are
qualitatively distinct in LSMO and LCMO and we discuss large and small polaron
scenarios which have been proposed so far. There is evidence for a large
polaron excitation in LSMO and small polarons in LCMO. These scenarios are
examined with respect to further experimental probes, specifically charge
carrier mobility (Hall-effect measurements) and high-temperature
dc-resistivity.Comment: 16 pages, 10 figure
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