Temperature Dependence Of Linewidth Research Articles

Zero-phonon line of 4T2 - 4A2 luminescence band of Cr3+ ions in Ca3Sc2Si3O12 garnet ceramics

Luminescence spectra of Cr3+ ions in Ca3Sc2Si3O12 garnet ceramics were studied in the 10–300 K temperature range. The single zero-phonon line of the Cr3+4T2 - 4A2 transition was observed below 140 K. The temperature dependence of the zero-phonon line width was measured and ascribed to direct phonon-assisted transitions between 4T2 state components.

Single-Photon Emission from Point Defects in Aluminum Nitride Films.

Quantum technologies require robust and photostable single-photon emitters. Here, room temperature operated single-photon emissions from isolated defects in aluminum nitride (AlN) films are reported. AlN films were grown on nanopatterned sapphire substrates by metal organic chemical vapor deposition. The observed emission lines range from visible to near-infrared, with highly linear polarization characteristics. The temperature-dependent line width increase shows T3 or single-exponential behavior. First-principle calculations based on density functional theory show that point defect species, such as antisite nitrogen vacancy complex (NAlVN) and divacancy (VAlVN) complexes, are considered to be an important physical origin of observed emission lines ranging from approximately 550 to 1000 nm. The results provide a new platform for on-chip quantum sources.

"Double-W-shaped" temperature dependence of emission linewidth in an InGaN/GaN multiple quantum well structure with intense phase separation

The temperature dependence of the spectra of photoluminescence (PL) from a blue InGaN/GaN multiplequantum-well (MQW) structure is investigated at lower excitation power. Two emission peaks, related to InGaN and assigned to In-rich quasi-quantum dots (QDs) and InGaN-matrix in the full PL spectrum, were observed. Upon increasing the temperature, both PL peak linewidths exhibited "double-W-shaped" (narrowing–broadening–narrowing–broadening–narrowing–broadening) temperature dependence. Combined with the observed features of the temperature dependences of the PL intensities, the temperature-dependent behaviors in this case can be interpreted as the relaxation and thermalization of carriers inside respective phase structures and the transfer of carriers between two phase structures, because of the strong phase separation and significant component fluctuation in the InGaN well layers.

Probing the direct factor for superconductivity in FeSe-Based Superconductors by Raman Scattering

The FeSe-based superconductors exhibit a wide range of critical temperature Tc under a variety of material and physical conditions, but extensive studies to date have yet to produce a consensus view on the underlying mechanism. Here we report on a systematic Raman scattering work on intercalated FeSe superconductors Lix(NH3)yFe2Se2 and (Li,Fe)OHFeSe compared to pristine FeSe. All three crystals show an anomalous power-law temperature dependence of phonon linewidths, deviating from the standard anharmonic behavior. This intriguing phenomenon is attributed to electron-phonon coupling effects enhanced by electron correlation, as evidenced by the evolution of the A1g Raman mode. Meanwhile, an analysis of the B1g mode, which probes the out-of-plane vibration of Fe, reveals a lack of influence by previously suggested structural parameters, and instead indicates a crucial role of the joint density of states in determining Tc. These findings identify carrier doping as the direct factor driving and modulating superconductivity in FeSe-based compounds.

Electron Spin Resonance Properties of CrI3 and CrCl3 Single Crystals

ABSTRACTDeveloping functional, cleavable two-dimensional materials for use in next generation devices has recently become a topic of considerable interest due to their unique properties. Of particular interest, transition metal halides CrI3 and CrCl3 have shown to be good contenders for tunable and cleavable magnetic materials due to their unique magnetic properties in the monolayer. Here, electron spin resonance spectroscopy is used to pinpoint the atomic origins and underlying mechanisms of magnetic interactions as a function of temperature (5-500 K) and microwave frequency (9.43, 120 GHz) on CrI3 and CrCl3 bulk single crystals. ESR signals from CrI3 due to Cr3+ were observed to decay at 460 K, while ESR signals from CrCl3 remain up to 500 K. In the case of CrCl3, the temperature dependences of signal behavior, line width and g-value show characteristic signatures of ferromagnetic fluctuations at around 40 K, near to the antiferromagnetic phase transition at 17 K.

Electron paramagnetic resonance and 1H nuclear magnetic resonance study of Y-doping effect on the hydrogen shallow donors in ZnO nanoparticles

Hydrogen shallow donors in sol-gel-derived pristine and rare-earth Y-doped ZnO nanoparticles have been investigated by electron paramagnetic resonance (EPR) and high-resolution 1H nuclear magnetic resonance (NMR). It is shown by EPR measurements that the energy level of the hydrogen shallow donors in the Y-doped ZnO is much deeper (E ∼ 174 meV) than in the pristine ZnO (E ∼ 75 meV). The temperature-dependent 1H NMR chemical shift and linewidth measurements of the pristine and the Y-doped ZnO systems indicated that Y-doping effectively modifies the lattice environment and hinders the hydrogen motions in the ZnO nanoparticles.

Pure rotational coherent anti-Stokes Raman spectroscopy of ethylene, experiments and modelling

Rotational coherent anti-Stokes Raman spectroscopy (CARS) is an established diagnostic technique for temperature and concentration measurements in combustion-related experiments. CARS has mainly been applied on di- and tri-atomics with linear molecular symmetry, for which good agreement is achieved between experimental and theoretical spectra. Hydrocarbons have so far been less attractive for rotational CARS studies, partly because of their complex molecular symmetries making it challenging to obtain molecular parameters necessary for a successful modeling. Nevertheless, in this work we have studied the asymmetric top molecule ethylene (C2H4) using rotational CARS through experiments and theoretical modelling. Experimental spectra have been recorded at temperatures between 293 K and 804 K, and theoretical spectra have been calculated with a novel method for which the crucial temperature-dependent linewidth parameters were determined by comparison of experimental and theoretically calculated spectra. Line-mixing effects and temperature dependence of the isolated linewidths were studied using a semi-classical approach considering an exponential gap law and a scaling law. Subsequently, theoretically calculated spectra were used to evaluate the temperatures of the experimental spectra for validation of the method. The evaluated temperatures were in good agreement with thermocouple temperatures with differences up to ±7 K. Hence, this work not only investigates the potential of using rotational CARS on ethylene for diagnostic purposes, but also expands the applicability of the technique to a whole new class of molecules with asymmetric-top molecular symmetry.

Decay mechanism of optical phonons in γ-CuI

Temperature-dependent frequencies and linewidths of transverse-optical (TO) and longitudinal-optical (LO) in γ–CuI were fitted according to the anharmonic phonon coupling theory. The results indicate that the asymmetric three-phonon and symmetric four-phonon processes are both responsible for the decay of LO and TO phonons in γ–CuI. Additionally, the possible decay channels are also suggested as: TO (135 cm-1) →LA (∼100 cm-1) +TA (35 cm-1) at L point and three TA (45cm-1) at X point; LO (152 cm-1) → LA (∼102 cm-1) +TA (∼50 cm-1) or three TA (50cm-1) at K point in Brillouin zone. These results are meaningful for the future applications of γ–CuI in optoelectronic and electronic fields.

Longer Cations Increase Energetic Disorder in Excitonic 2D Hybrid Perovskites.

We synthesize and characterize derivatives of the two-dimensional hybrid perovskite (2DHP) phenethylammonium lead iodide ((PEA)2PbI4) in which the para H on the cation is replaced with F, Cl, CH3, or Br. These substitutions increase the length of the cation but leave the cross-sectional area unchanged, resulting in structurally similar PbI42- frameworks with increasing interlayer spacing. Longer cations result in broader, blue-shifted excitonic absorption spectra with reduced or eliminated structure, indicating greater energetic disorder. Photoluminescence spectra are largely invariant and insensitive to cation length, suggesting polaron formation stabilizes a structural and electronic minimum. Temperature-dependent line width analysis reveals excitons couple to a vibration on the organic framework that is weakly sensitive to these cation substitutions, and Raman spectra and electronic structure calculations support the presence of such a cationic mode. Despite carriers being confined to the inorganic framework, the length of the organic cation alters the optical and electronic properties of 2DHPs.

Evidence of surface spin-glass behavior in NiFe2O4 nanoparticles determined using magnetic resonance technique

Abstract Nanosized nickel ferrite (NiFe2O4) is successfully synthesized by the sol-gel method using citric acid (C6H8O7) as fuel. The X-ray diffraction pattern of the as-synthesized sample shows formation of NiFe2O4 nanoparticles (NPs) as the main phase, with mean crystalline size ∼50 nm. While transmission electron microscopy (TEM) reveals nearly spherical morphology with mean particle diameter of 41 ± 5 nm, high-resolution TEM shows inter-planar distance (2.55 A) corresponding to the (1 1 3) plane of the spinel structure. Additionally, the Raman spectrum exhibits five Raman active modes (two A1g at 575 cm−1 and 710 cm−1; two Eg at 345 cm−1 and 670 cm−1; and one T2g at 497 cm−1) typical of NiFe2O4. X-band magnetic resonance (MR) data reveal a single broad resonance line in the whole temperature range (3.8 K ≤ T ≤ 300 K), with g-value decreasing monotonically from 3.10 ± 0.01 at 3.8 K to 2.77 ± 0.01 at 300 K. The temperature dependence of both resonance field and resonance linewidth show a remarkable change in the range of 60–70 K, herein credited to surface spin-glass behavior. The model picture used to explain the MR data assumes NPs with a core-shell structure. Below about 60–70 K the shell’s spin system progressively reveals a paramagnetic to spin-glass-like transition upon cooling, with a freezing temperature estimated at 2.1 ± 0.1 K. However, above about 60–70 K spins in the paramagnetic shell align along the NP’s core dipolar field, resulting in an effective enlarged nanoparticle size.

Some radiation effects in II-VI quantum dots

The effect of electron irradiation (E=0.6-1.8 MeV) on the optical characteristics (photoluminescence, PL) of CdTe/ZnTe structures with quantum dots, QDs, was investigated in the temperature range from 4.2 to 250 K. The data on the influence of irradiation on the temperature dependence of PL intensity, energy position and PL line width, W, from QDs were obtained. The narrowing of PL band and the blue shift of the QD peak position are explained by quenching of the low energy component connected with larger QDs. A slight decrease in PL intensity for both QDs and the buffer ZnTe layer as well as a sharp drop in PL intensity for 1 ML CdTe quantum wells, QWs, and fragments accompanied by a change in the activation energy of the PL quenching is explained by the radiation defects localized near the QD interface. The obtained results show that under e-beam irradiation QDs are more stable than QWs, which is in agreement with our previous investigations.

Spin Dynamics of Two-Dimensional Triangular-Lattice Antiferromagnet 3R-AgFeO2

Spin dynamics of quasi two-dimensional triangular-lattice antiferromagnet delafossite 3R-AgFeO2 have been investigated by electron spin resonance. The divergence of the temperature-dependent linewidth has been analyzed in the frame of two possible scenarios: (1) critical broadening due to slowing down of spins on approaching the Neel temperature TN from above and (2) in terms of a Berezinski–Kosterlitz–Thouless formalism on triangular lattice due to magnetic Z2-vortex–antivortex pairing. It has been found that the latter model gives better description of the experimental data indicating traces of Z2-vortices formation.

ESR Study of the Nature of Melanin from Micellar Fungi Nadsoniella Nigra

Melanin-containing powders and their aqueous solutions have been studied by electron spin resonance (ESR). Lorentzian lineshape, temperature dependence of magnetic susceptibility and linewidth resemble those for conducting polymers in which polyconjugated chemical links between their paramagnetic centers play an important role. Aqueous solution of one powder has been studied by the spin-probe method. It has been found that the addition of probes Fe(CN) 6 3− does not affect substantially both the width and the amplitude of melanin ESR lines probably due to strong electrostatic repulsion between the probes and negatively charged surrounding of melanin. The addition of probes Co2+ decreases the amplitude of melanin ESR line and does not affect its width. This may be explained by adsorption of the probes on negatively charged sites in the vicinity of melanin paramagnetic centers.

Nonmagnetic impurity effect on magnetic correlation in the triangular-lattice antiferromagnet Ba3Mn1−xZnxSb2O9(0 < x < 0.16)

The nonmagnetic impurity effects on the triangular-lattice antiferromagnet (TLAF) Ba3Mn1−xZnxSb2O9 () by Zn2+ () dilutions are investigated based on magnetic susceptibility, specific heat and electron spin resonance (ESR) measurements. The ac susceptibility and specific heat result reveal that the two-dimensional (2D) antiferromagnetic (AFM) ground state is stable rather than forming a spin-glass (SG) phase for Zn2+ substitution up to , where the AFM transition temperature (TN) shifts slowly from 10.2 K () to 8.2 K (). For all samples, the divergence of the temperature-dependent linewidth of ESR can be well described in terms of the Berezinskii-Kosterlitz-Thouless (BKT) scenario. The evolution of ESR spectra suggests that the nonmagnetic impurity does not induce magnetic defects decoupled with AFM matrix but rather change the bulk AFM properties.

Temperature dependence of optical centers in transmission electron microscope irradiated natural IIa diamond

Diamond, a wide band gap semiconductor material, has been attracting interest in several fields from electrics and optics to biomedicine and quantum computing due to its outstanding properties. These properties of diamond are related to its unique lattice and optically active defect centers. In this paper, we present a detailed study of the temperature-dependent linewidth and energy shift for two zero-phonon lines at 503.5 nm (3H center) and 741 nm (GR1 center). We also discuss the uses and properties of electron-photon coupling and zero-phonon lines. The results show that with the increasing temperature, both centers red-shifted and broadened, and the intrinsic defects of the GR1 center were mainly controlled by the homogeneous broadening mechanism, while the inhomogeneous broadening mechanism of the 3H center was probably caused by local stress.

Effect of disordered structure and crystal defects on heat transfer behavior in Er:Yb: YCa4O(BO3)3 crystal

The anomalous glasslike behavior of heat transfer in Er:Yb: YCa4O(BO3)3 crystal was explored by means of temperature dependence Raman scattering spectra and synchrotron radiation X-ray topography. Crystal samples were prepared by the Czochralski technique, and the corresponding thermal properties, including thermal expansion, specific heat and thermal conductivity were investigated. The temperature-dependent Raman scattering results indicate the frequencies of Raman peaks as a function of crystal temperature followed a linear dependency on crystal temperature with different slopes. Nevertheless, the temperature-dependent linewidths of different Raman peaks were inconsistent and complicated. The X-ray topography firstly revealed a core situated in the center of crystal. Surrounding the core, a number of other defects like striations and dislocations were observed. The existence of such defects can lead to phonon scattering, which will introduce complicated influence into heat transfer process. As a result, the anomalous behavior of heat transfer can be influenced by disordered structure and crystal defects.

Giant Electron-Phonon Coupling and Deep Conduction Band Resonance in Metal Halide Double Perovskite.

The room-temperature charge carrier mobility and excitation-emission properties of metal halide perovskites are governed by their electronic band structures and intrinsic lattice phonon scattering mechanisms. Establishing how charge carriers interact within this scenario will have far-reaching consequences for developing high-efficiency materials for optoelectronic applications. Herein we evaluate the charge carrier scattering properties and conduction band environment of the double perovskite Cs2AgBiBr6 via a combinatorial approach; single crystal X-ray diffraction, optical excitation and temperature-dependent emission spectroscopy, resonant and nonresonant Raman scattering, further supported by first-principles calculations. We identify deep conduction band energy levels and that scattering from longitudinal optical phonons- via the Fröhlich interaction-dominates electron scattering at room temperature, manifesting within the nominally nonresonant Raman spectrum as multiphonon processes up to the fourth order. A Fröhlich coupling constant nearing 230 meV is inferred from a temperature-dependent emission line width analysis and is found to be extremely large compared to popular lead halide perovskites (between 40 and 60 meV), highlighting the fundamentally different nature of the two "single" and "double" perovskite materials branches.

Temperature-dependent Raman linewidths in transition-metal dichalcogenides

Using a semi-ab-initio theoretical method we examine the temperature-dependent linewidth of the Raman modes in bulk and monolayer transition-metal dichalcogenides (TMDs) ${\mathrm{MoS}}_{2}, {\mathrm{WS}}_{2}$, and ${\mathrm{MoTe}}_{2}$. It is found that different Raman modes show different linewidths and different temperature dependencies with respect to each other for a given sample and across different TMDs. We explain these characteristics as arising from a combination of phonon density of states, Raman mode frequency, and the relative contributions of temperature-independent mass-defect scattering and temperature-dependent intrinsic anharmonic interactions. Reported measurements for samples prepared under experimental conditions have been explained by adding frequency-dependent inhomogeneity-related background contribution to our theoretical results for the pure and homogeneous samples.

Temperature-dependent photoluminescence of Mn doped CsPbCl3 perovskite nanocrystals in mesoporous silica

Highly luminescent Mn doped perovskite nanocrystals (NCs) are promising in solid state lighting. The thermal stability of Mn2+ luminescence in Mn: CsPbCl3 NC/mesoporous silica (MPS) composites was studied at temperature ranging from 80 K to 450 K by steady-state and time-resolved photoluminescence (PL) spectroscopy. The Mn: CsPbCl3 NCs with various Mn doping concentrations were synthesized by changing the Mn/Pb molar ratios at 190 °C. It was found that the Mn doped NC films showed a more significant decrease in PL intensities and lifetimes at high temperature above 320 K than the Mn doped NC/MPS films, indicating that the Mn2+ luminescence in doped NC/MPS films exhibited better thermal stability than that of NC films. Further the broadening and shifts of Mn2+ emission bands in doped NC and NC/MPS films were observed at elevated temperature. The temperature-dependent linewidths of Mn2+ emission bands were discussed by electron-phonon coupling.

Dynamic investigations of liquid crystalline elastomers and their constituents by 2H NMR spectroscopy

ABSTRACTSeveral side-chain liquid single crystalline elastomers (LSCEs) were investigated by means of 2H NMR spectroscopy focusing on the temperature dependence of the spectral line width and spectral shape, as well as the T-dependence of spin–lattice and spin–spin relaxation times (T1 and T2). All measurements were performed at the Larmor frequency of 76.753 MHz for 2H. The LSCEs were prepared in the form of monodomain films by using either 2H-labelled monomers (or co-monomers), 2H-labelled cross-linkers or 2H-labelled molecular probes free to diffuse in the LSCE matrix. In all cases, deuterons are on the phenyl moiety. The main features of the measured 2H NMR properties (static and dynamic ones) in these different cases were discussed and compared with those of low-molecular-weight mesogens (i.e. the same molecules used as monomers or co-monomers in the side-chain LSCEs) in the nematic and smectic A phases. Among all investigated LSCE samples, those 2H-labelled on the cross-linker sites present distinct properties either on the 2H NMR spectral line width and spectral shape in the paranematic and in the nematic phases, which can be interpreted in terms of director reorientational dynamics in the intermediate–slow motional regime, taking into account the inherent local director misalignment of LSCEs.