309 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
A comparison of the ground magnetic responses during the 2013 and 2015 St. Patrick's Day geomagnetic storms
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
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
Associating ground magnetometer observations with current or voltage generators
A circuit analogy for magnetosphere‐ionosphere current systems has two extremes for drivers of ionospheric currents: ionospheric electric fields/voltages constant while current/conductivity vary—the “voltage generator”—and current constant while electric field/conductivity vary—the “current generator.” Statistical studies of ground magnetometer observations associated with dayside Transient High Latitude Current Systems (THLCS) driven by similar mechanisms find contradictory results using this paradigm: some studies associate THLCS with voltage generators, others with current generators. We argue that most of this contradiction arises from two assumptions used to interpret ground magnetometer observations: (1) measurements made at fixed position relative to the THLCS field‐aligned current and (2) negligible auroral precipitation contributions to ionospheric conductivity. We use observations and simulations to illustrate how these two assumptions substantially alter expectations for magnetic perturbations associated with either a current or a voltage generator. Our results demonstrate that before interpreting ground magnetometer observations of THLCS in the context of current/voltage generators, the location of a ground magnetometer station relative to the THLCS field‐aligned current and the location of any auroral zone conductivity enhancements need to be taken into account.Key PointsConductivity and location assumptions used to interpret ground magnetic perturbations yield conflicting resultsHigh‐latitude currents associated with voltage generators may instead be associated with current generators, and vice versaWithout better constraints on conductivity/station location relative to currents, conflicts will not be resolvedPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138366/1/jgra53632.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138366/2/jgra53632_am.pd
The global structure and time evolution of dayside magnetopause surface eigenmodes
Theoretical work and recent observations suggest that the dayside magnetopause may support its own eigenmode, consisting of propagating surface waves which reflect at the northern and southern ionospheres. These magnetopause surface eigenmodes (MSEs) are a potential source of magnetospheric ultralow‐frequency (ULF) waves with frequencies less than 2 mHz. Here we use the Space Weather Modeling Framework to study the magnetospheric response to impulsive solar wind dynamic pressure increases. Waves with 1.8 mHz frequency are excited whose global properties are largely consistent with theoretical predictions for MSE and cannot be explained by other known ULF wave modes. These simulation results lead to two key findings: (1) MSE can be sustained in realistic magnetic field geometries with nonzero flow shear and finite current layer thickness at the magnetopause and (2) MSE can seed the growth of tailward propagating surface waves via the Kelvin‐Helmholtz instability.Key PointsDayside ULF response to pulse consistent with magnetopause surface eigenmodeMagnetopause surface eigenmodes are a potential source of ULF waves below 2 mHzMagnetopause surface eigenmodes seed tailward propagating surface wave growthPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111803/1/grl52799.pd
Characterization of the binding sites of the anticancer ruthenium(III) complexes KP1019 and KP1339 on human serum albumin via competition studies
Indazolium trans-[tetrachloridobis(1H-indazole)ruthenate(III)] (KP1019) and its Na+ analogue (KP1339) are two of the most prominent non-platinum antitumor metal complexes currently undergoing clinical trials. After intravenous administration, they are known to bind to human serum albumin (HSA) in a noncovalent manner. To elucidate their HSA binding sites, displacement reactions with the established site markers warfarin and dansylglycine as well as bilirubin were monitored by spectrofluorimetry, ultrafiltration-UV-vis spectrophotometry, and/or capillary zone electrophoresis. Conditional stability constants for the binding of KP1019 and KP1339 to sites I and II of HSA were determined, indicating that both Ru(III) compounds bind to both sites with moderately strong affinity (log K (1)' = 5.3-5.8). No preference for either binding site was found, and similar results were obtained for both metal complexes, demonstrating low influence of the counter ion on the binding event
Incorporation of β‐alanine in Cu(II) ATCUN peptide complexes increases ROS levels, DNA cleavage and antiproliferative activity
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