Linewidth Data Research Articles

Experimental study of the influence of variable external optical feedback on visible laser diode speckle.

This study employs an experimental setup with a variable feedback fraction (fext), external cavity length (ECL), and pump current (J) to investigate their relationships with optical linewidth and external optical feedback-induced speckle contrast (SC) reduction in a visible laser diode. In total, speckle contrast and optical linewidth data for seven feedback fractions, two pump currents, and three external cavity lengths were collected. Achieving optical linewidths up to 3.90 nm (1/e2 width) and a reduction in speckle contrast of up to 41.6%, we also revealed negative correlations between speckle contrast and optical linewidth, and speckle contrast and feedback fraction, and a positive correlation between optical linewidth and feedback fraction.

Analysis and Modeling of Line Width of Aerosol Printing based on Image Processing

This article proposes a method for calculating the line width of aerosol printing based on image processing and analyzes the line width results obtained using this method, aiming to establish an accurate mathematical model between parameters and line width. This article conducts experiments using carrier gas and sheath gas as parameters in aerosol printing and uses an industrial camera to characterize the printed lines. The line images are processed in MATLAB to obtain high-precision printed line widths. Analyzing the line width data after image processing, it was found that the line width increased with the increase of carrier gas and decreased with the increase of sheath gas. Then, using the discovered regularity, mathematical relationships between sheath gas and line width, and carrier gas and line width are established. The calculation method of print line width used in this article is highly accurate and efficient and establishes a functional relationship analysis model that can accurately describe the relationship between variables and line width, guiding printing-specific line widths.

The novel oxy-sulfide glassy ionic conductors Na4P2S7-xOx 0 ≤ x ≤ 7: Understanding the features of static and dynamic cations

We present a 23Na nuclear spin dynamics model for interpreting nuclear magnetic resonance (NMR) spin-lattice relaxation and central linewidth data in the invert glass system Na4P2S7-xOx, 0 ≤ x ≤ 7. The glassy nature of this material results in variations in local Na+ cation environments that may be described by a Gaussian distribution of activation energies. A consistent difference between the mean activation energies determined by NMR and DC conductivity measurements was observed, and interpreted using a percolation theory model. From this, the NaNa coordination number in the sodium cation sublattice was obtained. These values were consistent with jumps through tetrahedral faces of the sodium cages for the sulfur rich glasses, x < 5, consistent with proposed models of their short range order (SRO) structures. From NMR spin-echo measurements, we determined the NaNa second moment M2 resulting from the NaNa magnetic dipole interaction of nearest neighbors. Values of M2 obtained as a function of sodium number density N were in agreement with models for uniform sodium distribution, indicating that these invert glass systems form so as to maximize the average NaNa distance. A simple Coulombic attraction model between Na+ cation and X (=S−, O−) anion was applied to calculate the activation energy. In the range 1.5 ≤ x ≤ 7, an increase in activation energy with increasing oxygen content x occurred, and was consistent with the decrease in average anionic radius, and the increase in Coulombic attraction. For small oxygen additions, 0 ≤ x ≤ 1.5, the suggested minimum at low oxygen concentration seen in the activation energies obtained from DC conductivity data is not evident in the model.

Evidence of pressure-induced multiple electronic topological transitions in BiSe

BiSe, an exciting member of homologous family (Bi2)m (Bi2Se3)n [where, m = 1; n = 2], has recently gained attention in thermoelectric research due to its intrinsically ultralow lattice thermal conductivity and promising n-type thermoelectric performance near room temperature. It is experimentally observed to be a weak topological insulator and possesses a fascinating crystal structure where Bi2 bilayer is sandwiched between two Bi2Se3 quintuple layers, thus the structure resembles natural van der Waals heterostructure (Bi2Se3–Bi2–Bi2Se3). Herein, we have studied the pressure induced electronic topological transition (ETT) in BiSe through high pressure (0–10 GPa) synchrotron X-ray diffraction, Raman spectroscopy and first-principles density functional theory (DFT). We have observed clear anomalies at ∼1 and ∼2.2 GPa in the pressure dependent lattice parameters (a, c, and c/a ratio), cell volume, Raman mode shift and line width data suggesting the presence of two ETTs which is further verified by electronic structures and Fermi surface calculations under pressure. The origin of these ETTs is associated with the two different vibrational modes arising from Bi2 bilayer and Bi2Se3 quintuple layers of BiSe, respectively.

Hybrid time-frequency domain dual-probe coherent anti-Stokes Raman scattering for simultaneous temperature and pressure measurements in compressible flows via spectral fitting.

We demonstrate a hybrid time-frequency spectroscopic method for simultaneous temperature/pressure measurements in nonreacting compressible flows with known gas composition. Hybrid femtosecond-picosecond, pure-rotational coherent anti-Stokes Raman scattering (CARS), with two independent, time-delayed probe pulses, is deployed for single-laser-shot measurements of temperature and pressure profiles along an ∼5-mm line. The theory of dual-probe CARS is presented, along with a discussion of the iterative fitting of experimental spectra. Temperature is obtained from spectra acquired with an early, near-collision-free probe time delay (τ 1=0p s) and pressure from spectra obtained at probe delays of τ 2=150-1000p s, where collisions significantly impact the spectral profile. Unique solutions for temperature and pressure are obtained by iteratively fitting the two spectra to account for small collisional effects observed for the near zero probe delay spectrum. A dual-probe pure-rotational CARS system, in a 1D line-imaging configuration, is developed to demonstrate effectively the simultaneous temperature and pressure profiles recorded along the axial centerline of a highly underexpanded jet. The underexpanded air jet permits evaluation of this hybrid time-frequency domain approach for temperature and pressure measurements across a wide range of low-temperature-low-pressure conditions of interest in supersonic ground-test facilities. Single-laser-shot measurement precisions in both quantities and pressure measurement accuracy are systematically evaluated in the quiet zone upstream of the Mach disk. Precise thermometry approaching 1%-2% is observed in regions of high CARS signal-to-noise ratios. Pressure measurements are optimized at probe time delays where the ratio of the late probe delay to the Raman lifetime exceeds four (τ 2/τ R>4). The impact of low-temperature Raman linewidths on CARS pressure measurements is evaluated, and comparisons of CARS pressures obtained with our recent low-temperature pure-rotational Raman linewidth data and extrapolated high-temperature Q-branch linewidths are presented. Considering all measurements with τ 2/τ R≥4.0, measured pressures were on average 7.9% of the computed isentropic values with average shot-to-shot deviations representing a combination of instrument noise and fluid fluctuations of 5.0%.

Time-domain self-broadened and air-broadened nitrogen S-branch Raman linewidths at 80-200K recorded in an underexpanded jet.

We report pure-rotational N2-N2, N2-air, and O2-air S-branch linewidths for temperatures of 80-200K by measuring the time-dependent decay of rotational Raman coherences in an isentropic free-jet expansion from a sonic nozzle. We recorded pure-rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) spectra along the axial centerline of the underexpanded jet, within the barrel shock region upstream of the Mach disk. The dephasing of the pure-rotational Raman coherence was monitored using probe-time-delay scans at different axial positions in the jet, corresponding to varying local temperatures and pressures. The local temperature was obtained by fitting CARS spectra acquired at zero probe time delay, where the impact of collisions was minimal. The measured decay of each available Raman transition was fit to a dephasing constant and corrected for the local pressure, which was obtained from the CARS-measured static temperature and thermodynamic relationships for isentropic expansion from the known stagnation state. Nitrogen self-broadened transitions decayed more rapidly than those broadened in air for all temperatures, corresponding to higher Raman linewidths. In general, the measured S-branch linewidths deviated significantly in absolute and relative magnitudes from those predicted by extrapolating the modified exponential gap model to low temperatures. The temperature dependence of the Raman linewidth for each measured rotational state of nitrogen (J ≤ 10) and oxygen (N ≤ 11) was fit to a temperature-dependent power law over the measurable temperature domain (80-200K) and extrapolated to both higher rotational states and room temperature. The measured and modeled low-temperature linewidth data provided here will aid low temperature gas-phase pressure measurements with fs/ps CARS.

Temperature dependent determination of the S-branch Raman linewidths of oxygen and carbon dioxide in an oxyfuel relevant mixture.

The temperature dependence of the ${\rm O}_2$ and ${\rm CO}_2$ S-branch linewidths in a 30/70% ${\rm O}_2 - {\rm CO}_2$ mixture between 295 K and 1900 K has been studied by a picosecond time-resolved pure rotational coherent anti-Stokes Raman scattering (RCARS) approach. The S-branch Raman linewidths are required for diagnostics of thermodynamic properties in oxyfuel combustion processes by RCARS, where this mixture is of special interest, because it is regularly used to replace air when transiting from air-fed to oxyfuel combustion. The obtained linewidths for oxygen and carbon dioxide show a strong deviation from pure self-broadened linewidths and previously used Q-branch linewidths, respectively. A discussion on the expected impact on RCARS thermometry and concentration evaluations as well as a description of specific properties of oxygen and carbon dioxide and their effect on the dephasing behavior of the Raman coherences and, thereby the Raman linewidths, is included, along tabulated linewidths data of both molecules.

Improvement of the coherent model function for S-branch Raman linewidth determination in oxygen.

Determination of S-branch Raman linewidths of oxygen from picosecond time-domain pure rotational coherent anti-Stokes Raman spectroscopy (RCARS) measurements requires consideration of coherence beating. We present an optimization of the established model for fitting the coherence decay in oxygen, which leads to an improvement in Raman linewidth data quality, especially for the challenging small signal intensity and decay constant regime, enabling the application for low oxygen concentrations. Two modifications to the fitting procedure are discussed, which aim at reliably fitting the second coherence beat properly. These are evaluated statistically against simulated decay traces, and weighing the data by the inverse of the data magnitude gives the best agreement. The presented temperature dependent ${{\rm O}_2} {-} {{\rm O}_2}$ S-branch Raman linewidth from the modified model shows an improved data quality over the original model function for all studied temperatures. ${{\rm O}_2} {-} {{\rm N}_2}$ linewidths of oxygen in air for the temperature range from 295 K to 1900 K demonstrate applicability to small concentrations. Use of the determined RCARS ${{\rm O}_2} {-} {{\rm O}_2}$ S-branch linewidth instead of regularly used Q-branch derived linewidths leads to a lowering in evaluated RCARS temperature by about 21 K, thereby, a much better agreement with thermocouple measurements.

Calculation of the damping constant and the relaxation time of the LA mode in the incommensurate phase of quartz

The damping constant (linewidth) of the LA mode is calculated as a function of temperature for the incommensurate (INC) phase of quartz by using the models of the pseudospin-phonon (PS) coupled and the energy fluctuation (EF). For this calculation, the observed linewidth () data are used at and from the literature. Close to the incommensurate phase between the and phases of quartz, the observed and the frequency shifts () are also analyzed by the power-law formula for both pressures ( and ). By using the and the inverse relaxation time () is predicted as a function of temperature ( and ) in the incommensurate phase of quartz and the values of the activation energy () are deduced. Our calculated from both models (PS and EF) explain adequately the observed behavior of the linewidth of the LA mode in the incommensurate phase of quartz. Also, our predicted can be compared with the measurements in the INC phase of this molecular crystal.

Role of Ce concentration on the structural and magnetic properties of functional magnetic oxide particles.

Functional magnetic oxide particles offer exceptional GHz frequency capabilities, which can significantly enhance the utility of communication and signal processing devices. In the present work, we have investigated the structural and magnetic properties of rational multifunctional oxide Y2.9−xCexBi0.1Fe5O12 particles – a full series with x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1 via a conventional solid-state route. The X-ray diffraction pattern validated the Ia3̄d cubic garnet phase in all samples. From Rietveld refinement, it is observed that the ceric oxide (CeO2) impurity increases with an increase of Ce concentration, evincing a partial substitution of cerium (Ce) element into the garnet structure. The magnetic oxide particles with Ce concentration x = 0.4 showed a better crystallite size, dodecahedral site occupancy and solubility of cerium in the garnet phase. The morphological visualization of random shaped grains in the micrometer range was performed using the scanning electron microscopy (SEM) technique. The static magnetic properties showed that the saturation magnetization (Ms) decreases up to 43% and coercivity increases up to 59% with the increase of Ce concentration. The dynamic investigation on these oxide particles exhibits various intriguing and novel properties. Various intrinsic material parameters such as saturation magnetization (Ms), gyromagnetic ratio (γ), Gilbert damping constant (α) and extrinsic contribution (ΔHo) to linewidth were determined from the fitting of resonance field (Hr) and field linewidth (ΔHr) data. We ascertained that the damping constant increases with the increase of Ce concentration, which can be explained in terms of two magnon scattering and local defects caused by CeO2 inhomogeneity. The proposed doped garnets can be a potential candidate for high frequency microwave applications and spin-transfer-torque devices.

Single-shot fs/ns rotational CARS for temporally and spectrally resolved gas-phase diagnostics

We report thermometry and new Raman linewidth data using a novel method for time-domain rotational coherent anti-Stokes Raman spectroscopy (RCARS) on single-shot basis. The coherences are generated by two fs pulses and probed by a ns pulse. The resulting signal is detected using a combined spectrograph/streak camera setup with high spectral and temporal resolution. Rotational CARS spectrograms of nitrogen (N2), ethylene (C2H4), ethane (C2H6), and argon (Ar) are demonstrated at ambient condition. Self-broadened N2–N2 S-branch Raman linewidths were measured on single-shot basis and show excellent agreement with literature values. Furthermore, Raman linewidths of N2 S-branch perturbed by C2H4 and C2H6 were measured for the first time. The time-resolved approach was also utilized to separate complex spectra from a mixture of species through their difference in coherence lifetimes. Since the Raman coherences decay much faster for both ethane and ethylene than for nitrogen, pure nitrogen spectra could be obtained by analyzing only the temporal tail of the signal. Moreover, a model for time-resolved RCARS of N2 was developed, which employs the single-shot measured Raman linewidths to calculate the corresponding spectro-temporally resolved spectrograms at different temperatures. The model was used to evaluate the temperature from single-shot data of an N2/C2H6 mixture, showing very high accuracy. The demonstrated method constitutes an essential improvement for single-shot thermometry as it does not require pre-knowledge about collider species concentrations and their impact on linewidths.

Thickness dependent magneto-static and magneto-dynamic properties of CoFeB thin films

The authors report thickness dependent magneto-static and magneto-dynamic properties of amorphous Co40Fe40B20 films with thickness, t = 10–200 nm deposited on thermally oxidized Si substrates using magnetron sputtering technique at ambient temperature. Magnetic hysteresis loops of films with t ≤ 20 nm transformed from rectangular or flat type shape to transcritical nature for t ≥ 50 nm. This is attributed to a change in the magnetic domain structure from in-plane magnetization with uniaxial anisotropy in thinner films to stripe domains with effective magnetic anisotropy in thicker films. Magnetization dynamics in the films has been investigated using ferromagnetic resonance spectra. Out-of-plane angular dependent resonance field (Hr) and linewidth (ΔH) data have been interpreted using the Smit–Beljers formula. It is found that ΔH contains large extrinsic contribution with the major one being two-magnon scattering. An inverse scaling of Gilbert damping constant (α) with the film thickness over the entire thickness range of 10–200 nm is observed with the lowest α value of 0.004. The results reveal a cost effective strategy for developing films for spintronics based microwave devices and spin-transfer-torque based magnetic devices.

Direct measurement of CO2S-branch Raman linewidths broadened by O2, Ar, and C2H4.

Direct measurements of CO2S-branch Raman coherence-decay lifetimes resulting from CO2-O2, CO2-Ar, and CO2-C2H4 collisions by employing time-resolved picosecond rotational coherent anti-Stokes Raman scattering (CARS) spectroscopy are reported. Based on the measured Raman coherence dephasing rates, the corresponding Raman linewidths of various J levels are obtained. Our measured linewidth data will aid CO2 CARS fitting, which is applied to the high-accuracy extraction of temperature and species concentration information from CARS spectra.

Determination of N2-N2 and N2-O2 S-branch Raman linewidths using time-resolved picosecond pure rotational coherent anti-Stokes Raman scattering.

N2-N2 and N2-O2 S-branch Raman linewidths have been determined using picosecond dual-broadband pure rotational coherent anti-Stokes Raman scattering (CARS). Time-resolved rotational CARS measurements were performed in gas-phase N2 and air for temperatures up to 1900 K in order to determine the time constants of the coherence decay to subsequently calculate the S-branch Raman linewidths. Coherence decay time traces and the resulting S-branch Raman linewidths are presented for N2-N2 and N2-O2 collisions. Therewith, we reduce the gap of widely missing S-branch linewidth data in the temperature regime of many combustion processes. Further, we demonstrate that the standard monoexponential fitting of the coherence decay, as it is commonly done for nitrogen, is not applicable to oxygen.

Experimental and DFT generated Raman study of two bent-core monomeric liquid crystalline compounds

ABSTRACTThe temperature dependence Raman spectra of two liquid crystalline compounds defined by the chemical formula of 3,5-difluoro-4ʹ-(4-pentylcyclohexyl)-(1,1ʹ-biphenyl)-4-carbonitrile and 3,4,5-trifluoro-4ʹ-(4-pentylcyclohexyl)-1,1ʹ-biphenyl is being first reported in this study. These compounds are bent-core monomers and their bent nature has been verified by the Density Functional Theory (DFT). The temperature-dependent Raman spectroscopy has been widely used in understanding the effects of temperature-based phase transitions on the molecular vibrations. The same spectroscopic technique; helps to understand various phase transitions temperature in the liquid crystalline compounds (LC) and also their molecular arrangements during the phase transitions. This study has successfully revealed the nature of intermolecular interactions between the investigated compounds during the phase transitions and the correlation between the observed Raman spectra and the measurement temperature. The contributions of different types of chemical bonds in the investigated LC compounds to their recorded Raman spectra have also been discussed in detail. In predicting the observed Raman spectra, the theoretical Raman spectra obtained from the DFT calculation was used as a reliable tool. In the light of the calculated data, the peak position, line width, and integral intensity data for each band in the observed Raman spectra were reported.

Magnetic and crystallographic properties of rare-earth substituted yttrium-iron garnet

Yttrium Iron Garnet (YIG) is a versatile material for microwave device applications since last five decades. The goal of this investigation is to synthesize rare-earth doped YIG to be applicable for broad-band microwave and logic devices. YIG with lanthanide substitution of x = 0.15 atoms per formula unit(Y3-xRxFe5O12) was synthesized using conventional solid-state method (R = La, Nd, Ce). X-ray diffraction pattern confirms the cubic Ia3¯d symmetry in all the doped samples with significant change in lattice parameters, bond angles and cell volume (Nd-YIG having the highest). Microstructural analysis using high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) captures the well-defined d-spacings and grain boundaries, respectively. Raman spectroscopy confirms the significant distortion in FeO4 tetrahedron. X-ray photoelectron spectroscopy (XPS) verified the effect of doping of lanthanides (Rare earth: La, Nd, Ce) ions as well as the occurrence of Fe2+ and Fe3+ ion concentrations to support the magnetization increase (highest for Nd-YIG). The exchange interaction between Fe2+ and Fe3+ ions increases with rare-earth doping. Hence, saturation magnetization increases from 24.42 emu/gm for YIG sample to 26.23 emu/gm for Nd doped YIG sample.Room temperature microwave characteristics of lanthanide ions (La, Nd, Ce) doped YIG have been investigated using field sweep ferromagnetic resonance (FMR) technique in a broad frequency range (4–20 GHz) using the fabricated microstrip transmission line device. From the measured FMR fields and linewidths data dynamic parameters were derived. It is observed that rare-earth ions enhance the Gilbert damping parameter (α) and reduce the relaxation time in rare-earth doped YIG. α increased from 2.45 × 10−3 for un-doped YIG to 3.8 × 10−3 for Nd doped YIG samples. From the observed results, it may be concluded that the lanthanide doped-YIG can be a good candidate for broad-band-width microwave filters for higher-band 5G technology. The relaxation time reduced from 38.45 ns for YIG to 22.3 ns for Nd doped YIG which can be useful for designing high speed memory devices.

Exchange-coupled hard-soft ferrites; A new microwave material

During the past decade, hexaferrite has become an important candidate for a variety of microwave and millimeter-wave devices due to their large uniaxial magneto-crystalline anisotropy and high saturation magnetization. The goal of this investigation is to synthesize BaFe12O19/Y3Fe5O12 (Barium hexaferrite/Yttrium Iron Garnet - BaM/YIG) hard-soft ferrite nanocomposites (NCs) as an advanced microwave material. A detailed microstructure and magnetic properties of these nano-composites were investigated using various techniques; viz. X-ray diffractometry (XRD), vibrating sample magnetometry (VSM), Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).With recent advances in nanostructure synthesis techniques, we have realized a thin nano-composite layer on top of coplanar waveguide (CPW) planar transmission lines to design microwave notch-filters. Using a vector network analyzer we have checked the device operation from X-to Ka-band through ferromagnetic resonance (FMR) spectroscopy. The FMR linewidth (device bandwidth) is observed to be tunable with (BaM)1-x (YIG)x nanocomposite from 2 kOe (900 MHz) for pure BaM (x = 0) to 0.2 kOe (90 MHz) for pure YIG (x = 1). Various mathematical models were employed to fit the experimentally observed FMR linewidth (ΔH) data to yield intrinsic and extrinsic damping parameters as a function of ‘x’ value in (BaM)1-x (YIG)x nanocomposite.

H + , O 2+ , O 3+ and high resolution PIXE spectra of Yb 2 O 3

Abstract The number of X-ray spectrometry systems having energy resolution of the order of 10 eV, or less, has increasing recently, included already energy dispersive systems (EDS). Access to previous unseen spectra details and enhanced information including speciation, becomes more common and available. Analysis of high resolution EDS PIXE spectra is, nevertheless a complex task due to the need to carefully account for contributions from minor and satellite transitions. In this work, a pure Yb 2 O 3 sample was irradiated at the HRHE-PIXE setup of C2TN, and simultaneous CdTe and X-ray Microcalorimeter Spectrometer (XMS) spectra were collected. The L-shell spectrum of Yb emitted during irradiations using H + , O 2 + and O 3 + ions in the energy range from 1.0 to 6.5 MeV was studied. Measured L X-ray spectra were analysed taking into account the effects of the multiple ionization in the L and M shells. All spectra were analysed using the DT2 code, which allows to include in the fitting model diagram lines as well as multi-ionization satellites and any other contributions. In this communication we present the results and discuss details and problems related to the transition energies, intensity, line width data, and multiple ionization satellites.

Constraining the Single-degenerate Channel of Type Ia Supernovae with Stable Iron-group Elements in SNR 3C 397

Abstract Recent Suzaku X-ray spectra of supernova remnant (SNR) 3C 397 indicate enhanced stable iron group element abundances of Ni, Mn, Cr, and Fe. Seeking to address key questions about the progenitor and explosion mechanism of 3C 397, we compute nucleosynthetic yields from a suite of multidimensional hydrodynamics models in the near-Chandrasekhar-mass, single-degenerate paradigm for Type Ia supernovae (SNe Ia). Varying the progenitor white dwarf (WD) internal structure, composition, ignition, and explosion mechanism, we find that the best match to the observed iron peak elements of 3C 397 are dense (central density ≥6 × 109 g cm−3), low-carbon WDs that undergo a weak, centrally ignited deflagration, followed by a subsequent detonation. The amount of 56Ni produced is consistent with a normal or bright normal SNe Ia. A pure deflagration of a centrally ignited, low central density (≃2 × 109 g cm−3) progenitor WD, frequently considered in the literature, is also found to produce good agreement with 3C 397 nucleosynthetic yields, but leads to a subluminous SN Ia event, in conflict with X-ray line width data. Additionally, in contrast to prior work that suggested a large supersolar metallicity for the WD progenitor for SNR 3C 397, we find satisfactory agreement for solar- and subsolar-metallicity progenitors. We discuss a range of implications our results have for the single-degenerate channel.

Broadband ferromagnetic resonance characterization of anisotropies and relaxation in exchange-biased IrMn/CoFe bilayers

The magnetization dynamics of exchange-biased IrMn/CoFe bilayers have been investigated using broadband and in-plane angle-dependent ferromagnetic resonance spectroscopy. The interface energy of the exchange bias effect in these bilayers exceeds values previously reported for metallic antiferromagnets. A strong perpendicular magnetic anisotropy and a small in-plane uniaxial anisotropy are also observed in these films. The magnetization relaxation of the bilayers has a strong unidirectional contribution, which is in part caused by two-magnon scattering. However, a detailed analysis of in-plane angle-- and thickness-dependent linewidth data strongly suggests the presence of a previously undescribed unidirectional relaxation mechanism.