Temperature Dependence Of Linewidth Research Articles

Temperature-dependent Raman scattering in cubic (InGa)2O3 thin films

We report measurements of Raman scattering of cubic In2O3 and (In0.83Ga0.17)2O3 films grown on sapphire substrates by pulsed laser deposition as a function of temperature (77–500 K). We analyze the temperature-dependent Raman shifts and linewidths of six Raman modes in In2O3 film and Ag(1) and Ag(2)/Tg(2) modes in (In0.83Ga0.17)2O3 film. The Raman shifts of phonon modes are found to vary linearly with temperature. The temperature coefficients for six Raman modes of In2O3 film are in the range of −0.014 and −0.006 cm−1/K, while temperature coefficients of Ag(1) and Ag(2)/Tg(2) modes in (In0.83Ga0.17)2O3 film are −0.017 and −0.024 cm−1/K, respectively. Through the aid of a model involving three- and four-phonon coupling, the effects of temperature on linewidths are clearly illustrated, which demonstrates that three-phonon process always dominates in the decay process for all the modes in both In2O3 and (In0.83Ga0.17)2O3 films. These basic properties are very important for improving the quality of In2O3 and (In0.83Ga0.17)2O3 films, which can be used as well and barrier layers in In2O3-based quantum well.

Magnetism of gold nanorods probed using electron spin resonance

Electron spin resonance (ESR) spectroscopy has been performed for gold nanorods (AuNRs) of four different sizes covered with a diamagnetic stabilizing component, cetyltrimethylammonium bromide. ESR signals were detected in AuNRs except the largest one. Two smallest AuNRs showed an abrupt change in the temperature dependence of resonance field and line width at around 60 K, indicating ferromagnetic phase transition. In medium-size AuNRs, the resonance with a large shift was observed below 100 K. The resonance field shifts at the lowest temperature exhibit systematic variation with the system size, which is explained by considering magnetic anisotropy for the ferromagnetic resonance.

Anharmonicity effects in Cu-doped ZnO nanocombs by temperature-dependent Raman scattering

Micro-Raman spectra of E2(high) phonon mode in Cu-doped ZnO nanocombs have been presented in detail with different Cu compositions under the temperature ranging from 83 to 443 K grown by a simple catalyst-free chemical vapor deposition method. The alloy disorder effect has been investigated by analyzing the asymmetric broadening of E2(high) phonon mode and Cu-induced localized vibration mode at room temperature. In addition, we resort to a theory model including the lattice thermal expansion and anharmonic phonon–phonon interaction, which can well describe the temperature dependence of Raman shift and linewidth of E2(high) phonon. In combining with the theory model, we have revealed an increasing anharmonic effect on the Raman shift and linewidth behaviors with increasing Cu composition. Furthermore, it is found that the lifetime of E2(high) phonon mode shortens with enhancing the anharmonicity.

2H nuclear magnetic resonance study of deuterated water dynamics in perfluorosulfonic acid ionomer Nafion

We have employed deuteron nuclear magnetic resonance (NMR) spectroscopy in order to study the dynamics of the deuterated water (D2O) molecules introduced into a perfluorosulfonic acid ionomer Nafion (NR-211) film. According to the 2H NMR spectral analysis, the deuterated water molecules at low temperatures occupied either relatively rigid or mobile sites up to the temperature TM=240K where all the deuterated water molecules became mobile. The temperature-dependent NMR linewidths sensitively reflected the motional narrowing of the rigid and mobile sites, and the NMR chemical shift reflected significant changes in the hydrogen bonds of the deuterated water. While a slow- to fast-limit motional transition was manifested at TM in the laboratory-frame NMR spin–lattice relaxation, the rotating-frame spin–lattice relaxation indicated no bulk liquid water state down to 200K.

Deterministic drift instability and stochastic thermal perturbations of magnetic dissipative droplet solitons

The magnetic dissipative droplet is a strongly nonlinear wave structure that can be stabilized in a thin film ferromagnet exhibiting perpendicular magnetic anisotropy by use of spin transfer torque. These structures have been observed experimentally at room temperature, showcasing their robustness against noise. Here, we quantify the effects of thermal noise by deriving the stochastic equations of motion for a droplet based on soliton perturbation theory. First, it is found that deterministic droplets are linearly unstable at large bias currents, subject to a drift instability. When the droplet is linearly stable, our framework allows us to analytically compute the droplet's generation linewidth and center variance. Additionally, we study the influence of non-local and Oersted fields with micromagnetic simulations, providing insight into their effect on the generation linewidth. These results motivate detailed experiments on the current and temperature-dependent linewidth as well as drift instability statistics of droplets, which are important figures-of-merit in the prospect of droplet-based applications.

How two-dimensional brick layer J-aggregates differ from linear ones: Excitonic properties and line broadening mechanisms.

We study the excitonic coupling and homogeneous spectral line width of brick layer J-aggregate films. We begin by analysing the structural information revealed by the two-exciton states probed in two-dimensional spectra. Our first main result is that the relation between the excitonic couplings and the spectral shift in a two-dimensional structure is different (larger shift for the same nearest neighbour coupling) from that in a one-dimensional structure, which leads to an estimation of dipolar coupling in two-dimensional lattices. We next investigate the mechanisms of homogeneous broadening--population relaxation and pure dephasing--and evaluate their relative importance in linear and two-dimensional aggregates. Our second main result is that pure dephasing dominates the line width in two-dimensional systems up to a crossover temperature, which explains the linear temperature dependence of the homogeneous line width. This is directly related to the decreased density of states at the band edge when compared with linear aggregates, thus reducing the contribution of population relaxation to dephasing. Pump-probe experiments are suggested to directly measure the lifetime of the bright state and can therefore support the proposed model.

Anharmonicity of optic modes in monolayer MoS2

Temperature dependent frequency shift and linewidth of Γ point optic phonon modes in monolayer MoS2 are studied using the spectral energy density method. In this study, we report the temperature evolution of experimentally forbidden Raman active E″ and infra-red active A2″ modes, in-addition to the previously reported Raman active E′ and A1′ modes. All the modes are showing a redshift with an increase in temperature. We delineated the contribution of anharmonic coupling of phonon modes and thermal expansion of the lattice to the total frequency shift and linewidth, and found that a strong anharmonic coupling of phonon modes is the predominant source of the observed redshift and broadening of peaks. The temperature dependence of all modes is quantified by computing their first-order temperature co-efficients.

Existence of Large Antiferromagnetic Spin Fluctuations in Ce-Doped T′-Cuprate Superconductors

For the understanding of Ce-doped high-Tc cuprates with the so-called T′-structure, we have performed 63,65Cu and 139La NMR measurements in T′-Pr1.3−xLa0.7CexCuO4+δ single crystals with x = 0.10 and 0.15. The Neel temperature (TN) was evaluated from the temperature (T) dependence of the 139La NMR line width. For x = 0.10, TN decreased from 190 K in the as-grown crystal to 60 K in the oxygen-reduced crystal annealed at 800 °C. For x = 0.15, TN of 170 K in the as-grown crystal decreased to zero through the same heat treatment. The oxygen reduction strongly suppressed the antiferromagnetic (AF) order. 63,65Cu NMR spectra had a narrow center line and broad satellites with a small electric field gradient in the paramagnetic state. In the AF state, in contrast, 63,65Cu NMR spectra had a large electric field gradient. The T dependence of the 63Cu nuclear spin–lattice relaxation rate (1/T1) showed the existence of large spin fluctuations in the paramagnetic state. The spin fluctuations seem to play a key role in ...

Temperature dependent behavior of localized and delocalized electrons in nitrogen-doped 6H SiC crystals as studied by electron spin resonance

We have studied the temperature behavior of the electron spin resonance (ESR) spectra of nitrogen (N) donors in n-type 6H SiC crystals grown by Lely and sublimation sandwich methods (SSM) with donor concentration of 1017 cm−3 at T = 60–150 K. A broad signal in the ESR spectrum was observed at T ≥ 80 K with Lorentzian lineshape and g|| = 2.0043(3), g⊥ = 2.0030(3), which was previously assigned in the literature to the N donors in the 1s(E) excited state. Based on the analysis of the ESR lineshape, linewidth and g-tensor we attribute this signal to the conduction electrons (CE). The emergence of the CE ESR signal at T > 80 K was explained by the ionization of electrons from the 1s(A1) ground and 1s(E) excited states of N donors to the conduction band while the observed reduction of the hyperfine (hf) splitting for the Nk1,k2 donors with the temperature increase is attributed to the motional narrowing effect of the hf splitting. The temperature dependence of CE ESR linewidth is described by an exponential law (Orbach process) with the activation energy corresponding to the energy separation between 1s(A1) and 1s(E) energy levels for N residing at quasi-cubic sites (Nk1,k2). The theoretical analysis of the temperature dependence of microwave conductivity measured by the contact-free method shows that due to the different position of the Fermi level in two samples the ionization of free electrons occurs from the energy levels of Nk1,k2 donors in Lely grown samples and from the energy level of Nh residing at hexagonal position in 6H SiC grown by SSM.

Effect of strong phonon–phonon coupling on the temperature dependent structural stability and frequency shift of 2D hexagonal boron nitride

The temperature dependent structural stability, frequency shift and linewidth of 2D hexagonal boron nitride (h-BN) are studied using a combination of lattice dynamics (LD) and molecular dynamics (MD) simulations. The in-plane lattice parameter shows a negative thermal expansion in the whole computed temperature range (0-2000 K). When the in-plane lattice parameter falls below the equilibrium value, the quasi-harmonic bending (ZA) mode frequency becomes imaginary along the Γ-M direction in the Brillouin zone, leading to a structural instability of the 2D sheet. The ZA mode is seen to be stabilized in the dispersion obtained from MD simulations, due to the automatic incorporation of higher order phonon scattering processes in MD, which are absent in a quasi-harmonic dispersion. The mode resolved phonon spectra computed with a quasi-harmonic method predict a blueshift of the longitudinal and transverse (LO/TO) optic mode frequencies with an increase in temperature. On the other hand, both canonical (NVT) and isobaric-isothermal (NPT) ensembles predict a redshift with an increase in temperature, which is more prominent in the NVT ensemble. The strong phonon-phonon coupling dominates over the thermal contraction effect and leads to a redshift in LO/TO mode frequency in the NPT ensemble simulations. The out-of-plane (ZO) optic mode quasi-harmonic frequencies are redshifted due to a membrane effect. The phonon-phonon coupling effects in the NVT and NPT ensemble simulations lead to a further reduction in the ZO mode frequencies. The linewidth of the LO/TO and ZO mode frequencies increases in a monotonic fashion. The temperature dependence of acoustic modes is also analyzed. The quasi-harmonic calculations predict a redshift of ZA mode, and at the same time the TA (transverse acoustic) and LA (longitudinal acoustic) mode frequencies are blueshifted. The strong phonon-phonon coupling in MD simulations causes a redshift of the LA and TA mode frequencies, while the ZA mode frequencies are blueshifted with an increase in temperature.

Temperature dependence of Raman scattering in β-(AlGa)2O3 thin films

We report a detailed investigation on temperature-dependent Raman scattering of β-(AlGa)2O3 thin films with different Al content (0-0.72) under the temperature range of 77-300 K. The temperature-dependent Raman shifts and linewidths of the phonon modes were obtained by employing Lorentz fitting. The linewidths broadening of phonon modes with the temperature can be well explained by a model involving the effects of thermal expansion, lattice-mismatch-induced strain, and decay of optical phonon into two and three phonons. It is clearly demonstrated dependence of the linewidths and decay process on the Al content in β-(AlGa)2O3 thin films, which can provide an experimental basis for realization of (AlGa)2O3-based optoelectronic device applications.

Rotational States of Methyl Radical Monitored by EPR Line Shape of Matrix-Isolated CH $$_{3}$$ 3 in CO $$_{2}$$ 2 and N $$_{2}$$ 2 O Solids

Methyl radicals, CH\(_{3}\), were trapped in matrices of solid CO\(_{2}\) and N\(_{2}\)O by condensing the methyl along with the matrix constituents from the gas phase. The obtained EPR spectra were analyzed using EasySpin and/ or SimFonia simulation software and ab-initio computed quantum-chemistry parameters. We focus on two distinguished features of the spectra: the temperature-dependent linewidth anisotropy and the origin of the weak satellite doublets. The low-temperature spectral analysis of the satellites was based on the coupled representation of the three proton spins of CH\(_{3}\) and a recently obtained parameterization of the spin Hamiltonian based on one nuclear quartet and two doublets within the D\(_{3}\) group. These features were closely correlated to the state of rotation of the trapped radical.

Evidence of superconductivity-induced phonon spectra renormalization in alkali-doped iron selenides

Polarized Raman scattering spectra of superconducting KxFe2−ySe2 and non-superconducting K0.8Fe1.8Co0.2Se2 single crystals were measured in the temperature range from 10 K up to 300 K. Two Raman active modes from the I4/mmm phase and seven from the I4/m phase are observed in the frequency range from 150 to 325 cm−1 in both compounds, suggesting that the K0.8Fe1.8Co0.2Se2 single crystal also has a two-phase nature. The temperature dependence of the Raman mode energy is analyzed in terms of lattice thermal expansion and phonon–phonon interaction. The temperature dependence of the Raman mode linewidth is dominated by temperature-induced anharmonic effects. It is shown that the change in Raman mode energy with temperature is dominantly driven by thermal expansion of the crystal lattice. An abrupt change of the A1g mode energy near was observed in KxFe2−ySe2, whereas it is absent in non-superconducting K0.8Fe1.8Co0.2Se2. Phonon energy hardening at low temperatures in the superconducting sample is a consequence of superconductivity-induced redistribution of the electronic states below the critical temperature.

Comparative Raman spectroscopic study of phase stability and anharmonic effects in AZr2(PO4)3 (A = K, Rb and Cs)

AZr2(PO4)3 (A=Na, K, Rb, Cs) are a set of framework structured compounds that exhibit tunable ultralow thermal expansion over the wide temperature range of 293–1273K. We report a systematic Raman spectroscopic investigation on AZr2(PO4)3 (A=K, Rb and Cs) compounds as a function of temperature in the range 80–860K and pressures of up to 32GPa. To get insight into the thermal expansion property, phonon anharmonicity has been investigated by studying the temperature and pressure dependence of Raman peak shifts and line widths and computed bulk modulus. We have compared the phase transition and amorphization pressures of the various members of AZr2(PO4)3 to account for the stability of the ambient rhombohedral phase. We find that unlike most of the anomalous thermal expansion materials, in AZr2(PO4)3 (A=K, Rb and Cs), the phonons that are anharmonic with temperature do not necessarily exhibit anharmonicity with pressure.

Temperature dependence of free carriers and optical phonons inLiInSe2in the terahertz frequency regime

We investigate free carriers dynamics and anharmonic decay of optical phonons in ${\mathrm{LiInSe}}_{2}$ by using low temperature THz time-domain spectroscopy. The quantitatively determined refractive index and absorption coefficient are significantly influenced by temperature. By fitting the evaluated dielectric functions with a Drude-Lorentz model, the temperature dependencies of the plasmon frequency, scattering rates of free carriers, activation energies of carriers, and frequencies and linewidths of optical phonons are investigated. The contributions of three- and four-phonon processes to the temperature dependent optical phonon frequency and linewidth variations are quantitatively determined.

Temperature dependent phonon frequency shift and structural stability of free-standing graphene: a spectral energy density analysis

A spectral energy density based formalism is implemented to probe the temperature dependent frequency shift, linewidth, structural stability and coupling of normal modes of vibrations of free-standing graphene using a combination of lattice dynamics and molecular dynamics (MD). The in-plane lattice parameter shows a thermal contraction upto 1300 K and it expands thereafter. Frequency of the bending mode (ZA) becomes imaginary in the quasi-harmonic dispersion at higher temperatures, suggestive of a structural instability. However, the frequency of the ZA mode becomes real in the dispersion obtained from MD. Dynamical stability to the structure is restored by strong anharmonic coupling of phonon modes which is automatically incorporated in the MD simulations, whereas it is ignored in the quasi-harmonic dispersion. The mode resolved phonon spectra at Γ point show a blue-shift of degenerate longitudinal and transverse (LO/TO) optic modes. The blue-shift observed in canonical (NVT) and isobaric–isothermal (NPT) ensembles are more prominent than the shift predicted by quasi-harmonic approximation (QHA) due to the additional contribution from phonon–phonon coupling. The out-of-plane optic (ZO) mode frequencies are red-shifted in the QHA due to membrane-effect, whereas MD simulations show that the strong phonon–phonon coupling dominates the membrane effect leading to a blue-shift. The linewidth of LO/TO and ZO modes increases non-monotonically with temperature. The anharmonic coupling of normal modes at high symmetry points in the Brillouin zone is also discussed.

Magnetoabsorption of elliptically polarized electromagnetic radiation by graphene: The relaxation-time approximation and Monte Carlo method

It has been shown that the linewidth of cyclotron absorption in band-gap graphene is nonzero even in the absence of electron scattering. The functional temperature dependence of the cyclotron absorption linewidth, which is applicable to band-gap graphene in the absence of collisions, has been analytically determined. The power of the elliptically polarized electromagnetic wave absorbed by graphene in the presence of a dc magnetic field has been numerically calculated. The Monte Carlo numerical experiment has confirmed the analytical calculations based on the Boltzmann equation.

Temperature dependence of sapphire fiber Raman scattering

Anti-Stokes Raman scattering in sapphire fiber has been observed for the first time. Temperature dependence of Raman peaks' intensity, frequency shift, and linewidth were also measured. Three anti-Stokes Raman peaks were observed at temperatures higher than 300°C in a 0.72-m-long sapphire fiber excited by a second-harmonic Nd YAG laser. The intensity of anti-Stokes peaks are comparable to that of Stokes peaks when the temperature increases to 1033°C. We foresee the combination of sapphire fiber Stokes and anti-Stokes measurement in use as a mechanism for ultrahigh temperature sensing.

Temperature Dependence of Raman Scattering in 4H-SiC Films under Different Growth Conditions**Supported by the National Natural Science Foundation of China under Grant Nos 61176085, 11474365 and 61377055, the Department of Education of Guangdong Province under Grant No gjhz1103, and the Open-Project Program of the State Key laboratory of Opto-Electronic Material and Technologies of Sun Yatsen University.

The microRaman scattering of 4H-SiC films, fabricated by low pressure chemical vapor deposition under different growth conditions, is investigated at temperatures ranging from 80K to 550K. The effects of growth conditions on E2(TO), E1(TO) and A1(LO) phonon mode frequencies are negligible. The temperature dependences of phonon linewidth and lifetime of E2(TO) modes are analyzed in terms of an anharmonic damping effect induced by thermal and growth conditions. The results show that the lifetime of E2(TO) mode increases when the quality of the sample improves. Unlike other phone modes, Raman shift of A1 (longitudinal optical plasma coupling (LOPC)) mode does not decrease monotonously when the temperature increases, but tends to blueshift at low temperatures and to redshift at relatively high temperatures. Theoretical analyses are given for the abnormal phenomena of A1(LOPC) mode in 4H-SiC.

Ytterbium ESR in a lattice with weak coupling: the case of YbPt2Sn

We observed a well-defined Yb3+ electron spin resonance (ESR) line in YbPt2Sn. The single crystal ESR data show strongly anisotropic g factors, being largest in the basal plane, in agreement with magnetization data. This proves the intrinsic nature of the ESR signal. Analysis of these results indicates the crystal electric field ground state to be the Γ7 doublet. Although the Kondo and intersite interactions are at least one order of magnitude weaker than in the Kondo lattices YbRh2Si2 and YbIr2Si2, the temperature dependence of linewidth and g factors show qualitative similarities.