Increase In Linewidth Research Articles

Electronic structure of single-crystal solid solutions Pb1-xBaxTiO3 (0 ≤ x ≤ 1) from X-ray photoelectron spectroscopy and real-space multiple electron scattering calculations

Valence band and core level X-ray photoelectron spectra (XPS) are measured in single-crystal solid solutions Pb1-xBaxTiO3 (x = 0, 0.021, 0.146, 0.24, 0.541, 0.784, 0.906, 1). Valence band densities of states (DOS) and theoretical valence band XPS are calculated for Pb1-xBaxTiO3 (x = 0, 0.12, 0.25, 0.5, 0.875, 1) via the method of multiple electron scattering in real space. Experimental and theoretical valence band XPS are in good agreement. The increase of ionicity of the TiO bonds in Pb1-xBaxTiO3 upon the growth of relative Ba content x is predicted theoretically and shown experimentally using the energy position of the charge transfer satellites in the Ti2p XPS. The increase of Ti2p-spectrum line widths against relative Ba content x is observed.

Temperature dependence of Raman-active phonons and anharmonic interactions in layered hexagonal BN

We present a Raman scattering study of optical phonons in hexagonal BN for temperatures ranging from 80 to 600 K. The experiments were performed on high-quality, single-crystalline hexagonal BN platelets. The observed temperature dependence of the frequencies and linewidths of both Raman active ${E}_{2g}$ optical phonons is analyzed in the framework of anharmonic decay theory, and possible decay channels are discussed in the light of density-functional theory calculations. With increasing temperature, the ${E}_{2g}^{\mathrm{high}}$ mode displays strong anharmonic interactions, with a linewidth increase that indicates an important contribution of four-phonon processes and a marked frequency downshift that can be attributed to a substantial effect of the four-phonon scattering processes (quartic anharmonicity). In contrast, the ${E}_{2g}^{\mathrm{low}}$ mode displays a very narrow linewidth and weak anharmonic interactions, with a frequency downshift that is primarily accounted for by the thermal expansion of the interlayer spacing.

Change in the magnetic properties of nanoferrihydrite with an increase in the volume of nanoparticles during low-temperature annealing

The results of the investigation into the effect of low-temperature annealing of a powder of nanoparticles of bacterial ferrihydrite on its magnetic properties have been presented. It has been found that an increase in the time (up to 240 h) and temperature (in the range from 150 to 200°C) of annealing leads to a monotonic increase in the superparamagnetic blocking temperature, the coercive force, and the threshold field of the opening of the magnetic hysteresis loop (at liquid-helium temperatures), as well as to an increase in the magnetic resonance line width at low temperatures and in the magnetic susceptibility at room temperature. At the same time, according to the results of the analysis of the Mossbauer spectra, the annealing of ferrihydrite does not lead to the formation of new iron oxide phases. Most of these features are well consistent with the fact that the low-temperature annealing of ferrihydrite causes an increase in the size of nanoparticles, which is confirmed by the results of transmission electron microscopy studies.

Magnetic and absorbing properties of M-type substituted hexaferrites BaFe12–x Ga x O19 (0.1 < x < 1.2)

X-ray powder diffraction is used to determine the unit cell parameters and to refine the crystal structure of the solid solutions of M-type hexagonal barium ferrite BaFe12–xGaxO19 (x = 0.1–1.2) with isostructural diamagnetic cation Ga3+ substitution at T = 300 K. As the level of substitution increases, the unit cell parameters are shown to decrease monotonically. The temperature (300 K ≤ T ≤ 750 K, H = 8.6 kOe) and field (T = 300 K,–20 kOe ≤ H ≤ 20 kOe) dependences of the saturation magnetization of these solid solutions are studied with a vibrating-sample magnetometer. The concentration dependences of the Curie temperature TC, the specific spontaneous magnetization, and the coercive force are plotted. The magnetic parameters are found to decrease with increasing substitution. The microwave properties of the solid solutions are analyzed in an external magnetic field (0 ≤ H ≤ 4 kOe). As the cation Ga3+ concentration increases from x = 0.1 to 0.6, the natural ferromagnetic resonance (NFMR) frequency decreases; as the concentration increases further to x = 1.2, this frequency again increases. As the cation Ga3+ concentration increases, the NFMR line width increases, which indicates a widening of the frequency range where electromagnetic radiation is intensely absorbed. Here, the resonance curve peak amplitude changes insignificantly. The shift of the NFMR frequency in an applied magnetic field is more pronounced for samples with low cation Ga3+ concentrations. The role of diamagnetic substitution is revealed, and the prospects and advantages of Ga-substituted beryllium hexaferrite as the material absorbing high-frequency electromagnetic radiation are demonstrated.

α-l-Glutamic acid under high pressure: Phase transitions studied by Raman spectroscopy

l-Glutamic acid crystallizes at ambient pressure in two different polymorphic forms, the α- and β-forms. In this work the vibrational properties of the α-form are investigated by Raman spectroscopy under high pressure. A single crystal of α-l-glutamic acid, immersed in paraffin oil, was submitted to pressures up to 7.5GPa using a diamond anvil cell. The analysis of the Raman spectra of the crystal shows that α-l-glutamic acid presents two phase transitions, one between 1.9 and 2.3GPa and another one between 3.3 and 3.7GPa, both characterized by modifications in the low-wavenumber spectral region. Additionally, an impressive increase of the band linewidth is observed for pressures between 5.6 and 6.0GPa. Such increase suggests that disorder is induced by pressure, and these changes can be directly associated with the solidification of paraffin oil. A discussion on the reversibility and possible mechanisms governing the phase transitions is also provided.

Electronic transitions of iron in almandine-composition glass to 91 GPa

Valence and spin states of Fe were investigated in a glass of almandine (Fe3Al2Si3O12) composition to 91 GPa by X-ray emission spectroscopy and energy- and time-domain synchrotron Mossbauer spectroscopy in the diamond-anvil cell. Changes in optical properties, total spin moment and Mossbauer parameters all occur predominantly between 1 bar and similar to 30 GPa. Over this pressure range, the glass changes from translucent brown to opaque and black. The total spin moment of the glass derived from X-ray emission spectroscopy decreases by similar to 20%. The complementary Mossbauer spectroscopy approaches reveal consistent changes in sites corresponding to 80-90% Fe2+ and 10-20% Fe3+. The high-spin Fe2+ doublet exhibits a continuous decrease in isomer shift and increase in line width and asymmetry. A high-spin Fe3+ doublet with quadrupole splitting of similar to 1.2 minis is replaced by a doublet with quadrupole splitting of similar to 1.9 minis, a value higher than all previous measurements of high-spin Fe3+ and consistent with low-spin Fe3+. These observations suggest that Fe3+ in the glass undergoes a continual transition from a high-spin to a low-spin state between 1 bar and similar to 30 GPa. Almandine glass is not expected to undergo any abrupt transitions in electronic state at deep mantle pressures.

Recognition of Membrane Sterols by Polyene Antifungals Amphotericin B and Natamycin, A (13)C MAS NMR Study.

The molecular action of polyene macrolides with antifungal activity, amphotericin B and natamycin, involves recognition of sterols in membranes. Physicochemical and functional studies have contributed details to understanding the interactions between amphotericin B and ergosterol and, to a lesser extent, with cholesterol. Fewer molecular details are available on interactions between natamycin with sterols. We use solid state 13C MAS NMR to characterize the impact of amphotericin B and natamycin on mixed lipid membranes of DOPC/cholesterol or DOPC/ergosterol. In cholesterol-containing membranes, amphotericin B addition resulted in marked increase in both DOPC and cholesterol 13C MAS NMR linewidth, reflecting membrane insertion and cooperative perturbation of the bilayer. By contrast, natamycin affects little either DOPC or cholesterol linewidth but attenuates cholesterol resonance intensity preferentially for sterol core with lesser impact on the chain. Ergosterol resonances, attenuated by amphotericin B, reveal specific interactions in the sterol core and chain base. Natamycin addition selectively augmented ergosterol resonances from sterol core ring one and, at the same time, from the end of the chain. This puts forward an interaction model similar to the head-to-tail model for amphotericin B/ergosterol pairing but with docking on opposite sterol faces. Low toxicity of natamycin is attributed to selective, non-cooperative sterol engagement compared to cooperative membrane perturbation by amphotericin B.

Dispersive Plasmon Damping in Single Gold Nanorods by Platinum Adsorbates.

Surface modifications of plasmonic nanoparticles with metal adsorbates are essential in applications such as plasmonic sensing, plasmon-enhanced photocatalysis, etc., where spectral broadening is usually observed. A single particle study is presented on plasmon damping by adsorption of platinum (Pt) clusters. Single particle dark-field spectroscopy is employed to measure exactly the same gold nanorod before and after the Pt adsorption. The Pt-induced plasmon damping in terms of linewidth increase is found dependent on the resonance wavelength of the measured nanorod, which is dispersive in nature. The measured dispersion generally matches the theoretical prediction, and it basically exhibits a gradual increase with decreasing resonance energy. This increase can be attributed to the fact that the nanorod as a better resonator is more susceptible to the Pt adsorption than the spherical particles. Moreover, simulated results based on discrete dipole approximation method further indicate that the damping is mainly contributed from the adsorbates on the ends of the nanorod and independent on the type of the metal adsorbed. Knowledge and insights gained in this study can be very important for the design and fabrication of plasmonic heterostructures as functional nanomaterials.

Anomalous Broadening in Driven Dissipative Rydberg Systems.

We observe interaction-induced broadening of the two-photon 5s-18s transition in ^{87}Rb atoms trapped in a 3D optical lattice. The measured linewidth increases by nearly 2 orders of magnitude with increasing atomic density and excitation strength, with corresponding suppression of resonant scattering and enhancement of off-resonant scattering. We attribute the increased linewidth to resonant dipole-dipole interactions of 18s atoms with blackbody induced population in nearby np states. Over a range of initial atomic densities and excitation strengths, the transition width is described by a single function of the steady-state density of Rydberg atoms, and the observed resonant excitation rate corresponds to that of a two-level system with the measured, rather than natural, linewidth. The broadening mechanism observed here is likely to have negative implications for many proposals with coherently interacting Rydberg atoms.

Effect of laser linewidth on the performance of heterodyne detection

According to the statistical theory, the influence of laser linewidth on the performance of optical heterodyne system is studied. Also, the effect of laser linewidth on the visibility of fringe pattern is discussed. The expressions of self-correlation function power-spectrum and definition of visibility of fringe patterns are obtained in this paper. Based on the analytical expressions, the numerical simulation is performed. The obtained results demonstrate that the laser linewidth influences the visibility of fringe patterns according to the result shown in Fig. 3, and that the intermediate frequency can be still detected by heterodyne detection technique as the laser linewidth increases. For different linewidths, the measurement of intermediate frequency is accurate without the influence of noise as the delay d between the received signal and locally generated signal is less than the coherence time c of laser source. If the delay d is greater than the coherence time c, the full width at half maximum of intermediate frequency in the frequency spectrum of the output signal of photodetector will broaden as the laser linewidth increases. However, for a wide linewidth, the measurement of intermediate frequency is inaccurate due to the influence of noise when the delay d is greater than the coherence time c. The wider the linewidth, the less accurate the measurement of intermediate frequency will be. In order to check the correctness of theoretical results, an experiment is carried out by using a laser with a linewidth of 1 MHz, which has an 8.1 km channel path. In our experimental set-up, a cooperative target is employed to modulate and reflect the transmitted beam. In this way, an echo signal is received. The mixing process of the received signal and local signal on the photodetector surface produces an electrical current known as the photomixing current. A spectrum analyzer is used to observe the output signal of detector. The obtained spectrum shows that intermediate frequency can be checked, which is in agreement with the theoretical result. In this work, the obtained conclusions can be directly used to choose a proper laser for optical heterodyne system. According to the target characteristics and measurement requirements, and by following the conclusions obtained in this paper, the laser linewidth can be evaluated.

Synthesis, magnetic properties and spin-wave propagation in thin Y3Fe5O12 films sputtered on GaN-based substrates

We describe the synthesis of submicron Y3Fe5O12 (YIG) films sputtered on GaN-based substrates and present the results of the investigation of ferromagnetic resonance (FMR) and spin wave propagation in YIG / GaN structures. It is found that spin waves propagated in deposited YIG films have low damping. At the frequency of 3.2 GHz the spin-wave damping parameter is less than . Sputtered YIG films have magnetic inhomogeneity, which gives the main contribution to the FMR linewidth. The FMR linewidth increase caused by the relaxation process is insignificant and is equal to low values (2.2–3.7% of the linewidth). The low relaxation of spin waves in deposited YIG films gives the opportunity to construct tuneable spin-wave band-pass filters with a high-quality factor on GaN substrates and to design radiation-tolerant monolithic microwave integrated circuits.

Calculation of the cyclotron transition line-width by confined phonon scattering using the projection-reduction method

A formula to obtain the cyclotron transition line-widths for a system of electrons interacting with confined-acoustic phonons through the deformation potential in a quantum well is derived using the projection-reduction method. The result contains the distribution functions for the electrons and phonons properly. Therefore, the phonon absorption and emission processes can be explained in an organized manner and the result can be represented diagrammatically, through which insight into the quantum dynamics of electrons in a solid can be obtained. The formula is used to calculate the cyclotron transition line-widths in silicon. It is shown that the line-width increases with increasing temperature but the well width and magnetic field dependence of the line-width are complicated. It is also shown that only a few low-energy confined modes contribute significantly to the line-widths.

Conductive polycrystalline diamond probes for local anodic oxidation lithography

This is the first report characterizing local anodic oxidation (LAO) lithography performed using conductive monolithic polycrystalline diamond (MD) and conductive polycrystalline diamond-coated (DC) tips and comparing it to the diamond-like carbon-coated and metal-coated silicon tips. The range and the rate of increase in the lithographic linewidth and height with tip bias (dw/dV and dh/dV) differed based on the tip material. The DC tips resulted in wider and taller lines and a higher dw/dV and dh/dV compared to metal-coated tips with a similar force constant (kAvg). The metal-coated and the DC tips with comparable kAvg showed comparable threshold voltages, whereas the MD tips with similar kAvg showed a higher threshold voltage. The MD tips exhibited less than half the height and nearly half the dw/dV and dh/dV obtained with the metal-coated tips with similar kAvg, thus also resulting in a smaller width at −10 V. The linewidths were found to be proportional to the inverse of the log of write speed ( for all the tips; however, the proportionality constant varied with tip material; the DC tips had larger values, and the MD and the metal-coated tips had comparable values. When varying the speed, the height was found to be a sigmoidal function of width, with the MD probes achieving lower height compared to the metal-coated and the DC tips with comparable kAvg. This study expands the application of monolithic conductive polycrystalline diamond (PCD) probes with outstanding wear resistance to fine LAO lithography.

Determination of phonon decay rate in p-type silicon under Fano resonance by measurement of coherent phonons

We determine phonon decay rate by measuring the temperature dependence of coherent phonons in p-type Si under Fano resonance, where there is interference between the continuum and discrete states. As the temperature decreases, the decay rate of coherent phonons decreases, whereas that evaluated from the Raman linewidth increases. The former follows the anharmonic decay model, whereas the latter does not. The different temperature dependences of the phonon decay rate of the two methods originate from the way that the continuum state, which originates from the Fano resonance, modifies the time- and frequency-domain spectra. The observation of coherent phonons is useful for evaluating the phonon decay rate free from the interaction with the continuum state and clarifies that the anharmonic decay is dominant in p-type Si even under Fano resonance.

Optical signatures of spin-dependent coupling in semimagnetic quantum dot molecules

We present photoluminescence studies of CdTe and ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ quantum dots grown in two adjacent layers. We show that when the dots are 8 nm apart, their magneto-optical properties---Zeeman shifts and transition linewidths---are analogous to those of individual CdTe or ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ dots. When the dots are grown closer, at a distance of 4 nm, it becomes possible to tune the electron states to resonance and obtain a formation of a molecular state hybridized over the two dots. As a result of the resonant enhancement of the electron--Mn ion exchange interaction, spectroscopic signatures specific to spin-dependent interdot coupling appear. Namely, an anomalous increase of the Zeeman shift and a resonant increase in the transition linewidth are observed. A simple model calculation allows us to quantitatively reproduce the experimental results.

Structural characterization of selective area growth GaN nanowires by non-destructive optical and electrical techniques

The growth selectivity and structural quality of GaN nanowires obtained by plasma-assisted molecular beam epitaxy on pre-patterned GaN(0001) templates are investigated by means of non-destructive techniques. Optimum control over the nanowire arrangement and size requires a pitch between the mask apertures below twice the diffusion length of Ga atoms. Lower pitches, however, seem to slightly diminish the structural quality of the material, as revealed by the increase of the Raman peak linewidths. The photoluminescence spectra of the nanowires show a considerable presence of basal plane stacking faults, whose density increases for decreasing nanowire diameter. The capabilities of Kelvin probe force microscopy for imaging these kind of defects are also demonstrated.

Cavity enhanced nonlinear optics for few photon optical bistability.

Weak material nonlinearity at optical frequencies poses a serious hurdle to realizing optical bistability at low optical powers, which is a critical component for digital optical computing. In this paper, we explore the cavity enhancement of the second-order optical nonlinearity in order to determine the feasibility of few photon optical bistability. Starting from a quantum optical formalism of a doubly resonant cavity (required to meet the condition of phase matching), we derive a dynamic classical model of a cavity that is bistable at the fundamental mode. We analyze the optical energy and the switching speed as a function of the cavity quality factors and mode volumes and identify the regime where only ten's of photons are required to perform the switching. An unusual trend in the switching speed is also observed, where the speed monotonically decreases as the cavity linewidth increases. This is ascribed to the increase in the switching gain with increasing cavity linewidth.

Structural, optical and magnetic properties of γ-irradiated SiO2 xerogel doped Fe2O3

The paper deals with the structure, morphology and magnetic properties of two different iron concentrations (20 and 33mol.%) of Fe2O3−SiO2 nanocomposites, prepared by sol–gel technique and exposed to different gamma-irradiation doses (0, 30 and 60kGy). The nanocomposites were investigated through XRD, TEM, SEM, FTIR and EPR measurements. Superparamagnetic iron (III) oxide nanoparticles with a narrow size distribution, dispersed over the amorphous silica matrix, are assumed to be present in all the samples before and after irradiation. Before irradiation, a lot of γ-Fe2O3 crystalline ferromagnetic nanoparticles are assumed to be formed particularly for sample containing 33mol.% Fe2O3, while exposing the samples to irradiation results in the transformation of γ- to α-Fe2O3. Iron concentrations and/or irradiation of the samples are assumed to cause changes in the bond angles and/or bond lengths of the structural silicate units within network, as well, the increase of more defect centers induced by irradiation as evident through the variations of the IR bands intensity. The EPR results show both intensity and line width increase with increasing Fe2O3 concentration. The EPR signals for the samples consist of a well defined symmetrical broad signal at g≈2.0 ascribed to antiferromagnetic interactions between the Fe2+ and Fe3+ clusters. Condensed clusters of Fe3+ ions are observed to give rise to a resonance line at g∼2 whose position and width do not depend on the Fe2O3 concentrations. The EPR signal intensity is observed to be significantly decreased in the sample 33mol.% or stabilized in the sample 20mol.% by γ-irradiation. This reflects simultaneous spin transformation from the high-spin state of Fe III to the low-spin state of Fe II. As a final point, an effort has been given to found the possibility to use one of these studied nanocomposite materials as candidate for radiation shielding purposes.

Spectroscopic studies for the changes of a Cr(II) compound in solution triggered by the deprotonation of an aqua ligand

Recently, we reported dioxygen binding and its activation by the Cr(II) compound [Cr(H2O)(LISQ)2] (1) [(LISQ)1− is o-iminothionebenzosemiquinonate(1–) π-radical]. In this work, we report the existence of ferromagnetism in its solid form. The plot of (χm)−1 versus T from 250 to 20 K reveals a Curie–Weiss constant of θ ~26 K suggesting a ferromagnetic coupling. A theoretical simulation on the susceptibility curve and the fitting of the χm versus T yielded g = 1.83 ± 0.002, J = 33.4 ± 0.001 cm−1, and θ = 25.2 ± 0.02 K with J >> D. Increase in EPR line width on cooling helped us calculate the Cr(II)–Cr(II) distance in the proposed dimer. Also we find that deprotonation of the aqua ligand in 1 is essential for dioxygen binding and its activation. Reaction with dioxygen is completely inhibited when there is no deprotonation. Reaction of 1 with dioxygen in pyridine (py) resulted in isolation of the O=Cr(IV) compound [O=Cr(LAP)2(py)] (2) [(LAP)1– is o-aminothiophenolate anion] and no monomeric O=Cr(V) has been detected by EPR. 1H NMR study of 1 in d6-DMSO with time shows increase in intensity of the –NH2 peak at 5.46 ppm, suggesting gradual reduction of the –NH groups in (LISQ)1– ligands.

Ultrafast electron dynamics in epitaxial graphene investigated with time- and angle-resolved photoemission spectroscopy

In order to exploit the intriguing optical properties of graphene it is essential to gain a better understanding of the light-matter interaction in the material on ultrashort timescales. Exciting the Dirac fermions with intense ultrafast laser pulses triggers a series of processes involving interactions between electrons, phonons and impurities. Here we study these interactions in epitaxial graphene supported on silicon carbide (semiconducting) and iridium (metallic) substrates using ultrafast time- and angle-resolved photoemission spectroscopy (TR-ARPES) based on high harmonic generation. For the semiconducting substrate we reveal a complex hot carrier dynamics that manifests itself in an elevated electronic temperature and an increase in linewidth of the π band. By analyzing these effects we are able to disentangle electron relaxation channels in graphene. On the metal substrate this hot carrier dynamics is found to be severely perturbed by the presence of the metal, and we find that the electronic system is much harder to heat up than on the semiconductor due to screening of the laser field by the metal.