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This study investigates the structure and properties of Titanium carbide (TiC) and Tungsten carbide (WC) nanoparticles. The X‐ray diffraction patterns confirm the presence of a hexagonal structure in WC nanoparticles and cubic structure in TiC nanoparticles. Transmission electron microscopy analysis reveals the particle sizes to be within the range of 30–50 nm. The bandgap of TiC and WC nanoparticles are 3.5 and 3.6 eV, respectively based on diffuse reflectance spectroscopy (DRS) along with Kubelka–Munk. Furthermore, the optical phonons and dielectric function of WC and TiC nanoparticles are investigated using the reflectance spectra along with the Kramers–Kronig (KK) method. Furthermore, the Z‐scan technique, employing a Nd:YAG laser operating at a wavelength of 532 nm, is utilized to assess the nonlinear optical (NLO) characteristics of WC and TiC nanoparticles. The open aperture Z‐scan measurements of WC nanoparticles indicate reverse saturable absorption (RSA) behavior and a positive nonlinear refraction (NLR) coefficient. On the other hand, TiC nanoparticles show RSA behavior with a negative NLR effect. The order of NLR, nonlinear absorption coefficient, and the susceptibility of TiC and WC are 10−8 cm2 W−1, 10−4 cm W−1, and 10−6 esu, respectively. [ABSTRACT FROM AUTHOR]

Traditional optical gas sensors often require multiple components such as broadband infrared sources, detectors, and band-pass filters to detect various target gases, resulting in bulky and expensive sensor designs. A streamlined optical gas-sensing platform utilizing a narrowband thermal emitter with a spectrally selective response, capable of accommodating various target gases, has the potential to supplant current bulky designs. Through the on-chip integration of a narrowband metamaterial perfect absorber with a microelectromechanical system (MEMS) heater, a selective infrared source emitter could be designed. In this paper, a multiband metamaterial absorber with resonance modes located at different gas absorption signatures is developed for optical multi-gas-sensing applications. The proposed nanoemitter supports penta-band light absorption through the simultaneous excitation of phononic modes (within the hexagonal boron nitride (hBN) topmost layer) and plasmonic modes (with the spectrally selective underlying metal–insulator–metal (MIM) absorber stack). It achieves five near-perfect sharp absorption resonance peaks compatible with the H2S, CH4, CO2, NO, and SO2 gas absorption signatures in the mid-infrared (MIR) spectral range. This spectrally engineered multiwavelength absorption behavior is achieved by simultaneously coupling the optical phonons (OPhs) and the plasmonic modes in the vicinity of the OPh region of hBN and by exciting plasmonic modes with the help of the spacer (ZnTe: zinc telluride) and the metallic nanogratings. Finally, this low-cost and efficient penta-band absorber is combined with a MEMS-based microheater. The microheater uses a Peano-shaped configuration to provide a highly uniform surface temperature, which is crucial for accurate and reliable gas sensing. The proposed platform demonstrates excellent potential in terms of cost-effectiveness, source-free operation, and suitability for multi-gas-sensing platforms. [ABSTRACT FROM AUTHOR]

Given the practical application of sapphire as a promising transparent crystal in extreme environments, understanding the fundamental physical response of its optical properties to temperature is urgently needed. Herein, the vibrational characteristics of Raman-active modes of the strain-free sapphire and the strained sapphire substrate under the AlN epilayer were systematically investigated by Raman scattering measurements in the temperature range of 95–870 K. The observed nonlinear temperature-dependent frequency redshift for all phonon modes over the entire temperature range can be fitted with some empirical formulas. Interestingly, the frequency of these phonon modes varies linearly with temperature at high temperatures and can be explained by first-order temperature coefficients. The linear temperature coefficients of A 1g,1 modes for strain-free sapphire and strained sapphire determined at high temperatures are −0.02224 cm−1/K and −0.02189 cm−1/K, respectively. The strong temperature dependence in sapphire is expected to portray the local temperature distribution of sapphire-based devices by in situ Raman spectroscopy as a non-contact thermal probe. Furthermore, the frequency downshift and linewidth broadening observed in strain-free sapphire can be well quantified by a theoretical model involving thermal expansion and anharmonic decay. The epilayer-substrate interaction, however, needs to be additionally considered in strained sapphire. This theoretical analysis process can also be radically extended to other emerging ceramic and semiconductor materials. Our quantitative analysis may pave the way for a deeper understanding of the behavior of optical phonons in sapphire under extra perturbation effects. [ABSTRACT FROM AUTHOR]

High in-plane anisotropies arise in layered materials with large structural difference along different in-plane directions. We report an extreme case in layered TiS3, which features tightly bonded atomic chains along the b-axis direction, held together by weaker, interchain bonding along the a-axis direction. Experiments show thermal conductivity along the chain twice as high as between the chain, an in-plane anisotropy higher than any other layered materials measured to date. We found that in contrast to most other materials, optical phonons in TiS3 conduct an unusually high portion of heat (up to 66% along the b-axis direction). The large dispersiveness of optical phonons along the chains, contrasted to many fewer dispersive optical phonons perpendicular to the chains, is the primary reason for the observed high anisotropy in thermal conductivity. The finding discovers materials with unusual thermal conduction mechanism, as well as provides new material platforms for potential heat-routing or heat-managing devices.

Controlling the electrical properties of SiC requires knowledge of the nature and properties of extended defects. We have employed orthodox defect-selective etching and photo-etching methods to reveal typical and new structural defects in commercial SiC wafers. For photo-etching, the etch rate increases as the free carrier concentration decreases. The etch rate can be used to estimate the free carrier concentration with higher precision, and over a larger lateral and depth range than that accessed by Raman scattering. The logarithmic dependence of the etch rate on the free carrier concentration has been characterized. [ABSTRACT FROM AUTHOR]

This study presents an experimental implementation to determine the temperature dependence of Raman spectra of bulk 2H-MoS2 in the temperature range of 300–650 K. In addition, we have measured T AS / S according to the anti-Stokes to Stokes integrated intensity ratio ( I AS / I S ). The results indicate that Raman shift of both the vibration modes E 2 g 1 and A 1 g exhibits a linear temperature-dependent behavior in the heating and cooling processes. It is confirmed that the sample did not deteriorate by oxidization and decomposition at high temperatures. However, above 500 K, the measured phonon temperature T AS / S deviated from the heat bath temperature. It is suggested that the I AS / I S does not correspond to the equilibrium population of optical phonons, thus indicating that the measurement was not in thermal equilibrium but in a thermal steady state under laser irradiation. We conclude that this is caused by the difference in relaxation times in the thermal steady state. [ABSTRACT FROM AUTHOR]

• Acoustic phonons dominating the z-axis κ l and optical phonons contributing ∼50% to the x-axis κ l under the four-phonon picture in KCaBi. • The uncommon contribution of optical phonons in KCaBi can be used to enhance the κ l anisotropy by applying tensile strain. • The enhancement mechanism is that the strain-induced decrease of acoustic phonon contribution is more pronounced than that of optical counterpart. Acoustic phonons have long been believed to dominate the lattice thermal conductivity (κ l) and the contribution of optical phonons can be neglected in crystal structures. KCaBi, as a high-throughput screening semiconductor with ultralow κ l [J. Am. Chem. Soc. 144, 4448 (2022)], has been demonstrated that the contribution of optical phonons plays an important role in thermal transport. In this work, by solving the Boltzmann transport equation, it is found that the κ l of KCaBi is 2.2 at 300K, with acoustic phonons dominating the z-direction κ l and optical phonons contributing around 50% to the x-direction κ l under the four-phonon picture. The uncommon contribution of optical phonons also manifests the possibility of tuning the κ l anisotropy based on optical phonons. Following this line of thinking, it is found that applying tensile strain can cause a more pronounced decrease of acoustic phonon contribution than that of optical counterpart due to the highly dispersive optical branches, thus enhancing the anisotropic ratio of κ l. Moreover, the microscopic mechanism is elucidated by analyzing the phonon dispersion relation, phonon mode-wise contribution and phonon scattering rates. Our study could provide appealing alternatives for the regulation of phonon transport from the viewpoint of optical phonons. The κ l anisotropy of KCaBi can be enhanced by tensile strain based on the anomalous contribution of optical phonons. [Display omitted] [ABSTRACT FROM AUTHOR]

The utilization of excited charge carriers in semiconductor nanocrystals (NCs) for optoelectronic technologies has been a long-standing goal in the field of nanoscience. Experimental efforts to extend the lifetime of excited carriers have therefore been a principal focus. To understand the limits of these lifetimes, in this work, we theoretically study the time scales of pure electron relaxation in negatively charged NCs composed of two prototypical materials: CdSe and CdS. We find that hot electrons in CdSe have lifetimes that are 5 to 6 orders of magnitude longer than in CdS when the relaxation is governed only by the intrinsic properties of the materials. Although these two materials are known to have somewhat different electronic structure, we elucidate how this enormous difference in lifetimes arises from relatively small quantitative differences in electronic energy gaps and phonon frequencies, as well as the crucial role of Fröhlich-type electron-phonon couplings.

Transistors operating at high frequencies are the basic building blocks of millimeter-wave communication and sensor systems. The high charge-carrier mobility and saturation velocity in graphene can open way for ultra-fast field-effect transistors with a performance even better than what can be achieved with III-V-based semiconductors. However, the progress of high-speed graphene transistors has been hampered by fabrication issues, influence of adjacent materials, and self-heating effects. Here, we report on the improved performance of graphene field-effect transistors (GFETs) obtained by using a diamond substrate. An extrinsic maximum frequency of oscillation ${f}_{{\mathrm {max}}}$ of up to 54 GHz was obtained for a gate length of 500 nm. Furthermore, the high thermal conductivity of diamond provides an efficient heat-sink, and the relatively high optical phonon energy of diamond contributes to an increased charge-carrier saturation velocity in the graphene channel. Moreover, we show that GFETs on diamond exhibit excellent scaling behavior for different gate lengths. These results promise that the GFET-on-diamond technology has the potential of reaching sub-terahertz frequency performance. [ABSTRACT FROM AUTHOR]

X-ray studies of TlFeS2 and TlFeSe2 crystals grown by high-temperature synthesis reveal their single-phase nature and isostructurality with the space group C2/m. Raman scattering and infrared reflection of light in TlFeS2 and TlFeSe2 compounds is studied at a temperature of 300 K. Characteristic frequencies in the Raman scattering and infrared spectra of phonons in TlFeS2 and TlFeSe2 are detected. An analysis of published works on the magnetic properties of TlFeS2 and TlFeSe2 allows the conclusion that they are quasi-one-dimensional antiferromagnets each having two characteristic temperatures TN3D and , between which quasi-one-dimensional antiferromagnetic ordering in TlFeS2 and TlFeSe2 is favorable. The temperature is introduced for the first time and characterizes a highly developed short-range magnetic order, in which superparamagnetic ordering exists. [ABSTRACT FROM AUTHOR]

It has generally been believed that the lattice thermal conductivity (κ L) is mainly contributed by acoustic phonons and the contribution of optical phonons can be neglected in crystalline materials. However, in this work, we demonstrate that the contribution of optical phonons play an important role in the complex structural low- κ L oxides. The mode-wised κ L of three representative oxides, MgO, BaHfO 3 and La 2 Zr 2 O 7 , with different structural complexity was calculated using first-principles phonon Boltzmann transport equation (BTE) method. It is found that the contribution of optical phonons become significant as the structural complexity increases. Especially, for La 2 Zr 2 O 7 , the κ L is dominated by optical phonons due to the large density of states. Optical phonons exhibit amorphous-like phonon-phonon scattering characteristics that are different from those of acoustic phonons. Furthermore, calculations for materials other than the three representative oxides were conducted and the results confirmed the validity of our conclusions. [ABSTRACT FROM AUTHOR]

Recent Raman data on nanocrystallite arrays are revised within the microscopic theory for Raman peaks positions and broadening (linewidth). The theory combines the elasticity theory‐like approach for optical phonons used in order to evaluate the Raman peaks structure and the Green's function method applied for the phonon lines broadening. These theories are supported by the atomistic calculations within the dynamical matrix method for optical phonons and by the bond polarization model used to calculate the Raman intensities. The experimental data on four various nanopowders are analyzed with the use of this theory. The large width of the Raman peak in nanoparticles as compared with the corresponding peak in bulk materials and the width inverse dependence on the particle size previously observed by other researchers are explained within the framework of the theory. It is shown that the theory is capable to extract confidently from the Raman data four important microscopic characteristics of the nanopowder including the mean particle size, the variance of the particle size distribution function, the strength of intrinsic disorder in the particle, and the effective faceting number that parameterizes the particle shape. [ABSTRACT FROM AUTHOR]

The high-frequency performance of top-gated graphene field-effect transistors (GFETs) depends to a large extent on the saturation velocity of the charge carriers, a velocity limited by inelastic scattering by surface optical phonons from the dielectrics surrounding the channel. In this work, we show that, by simply changing the graphene channel surrounding dielectric with a material having higher optical phonon energy, one could improve the transit frequency and maximum frequency of oscillation of GFETs. We fabricated GFETs on conventional SiO2/Si substrates by adding a thin Al2O3 interfacial buffer layer on top of SiO2/Si substrates, a material with about 30% higher optical phonon energy than that of SiO2, and compared performance with that of GFETs fabricated without adding the interfacial layer. From S-parameter measurements, a transit frequency and a maximum frequency of oscillation of 43 and 46 GHz, respectively, were obtained for GFETs on Al2O3 with 0.5- μm gate length. These values are approximately 30% higher than those for state-of-the-art GFETs of the same gate length on SiO2. For relating the improvement of GFET high-frequency performance to improvements in the charge carrier saturation velocity, we used standard methods to extract the charge carrier velocity from the channel transit time. A comparison between two sets of GFETs with and without the interfacial Al2O3 layer showed that the charge carrier saturation velocity had increased from 1.5·107 to 2·107 cm/s. [ABSTRACT FROM AUTHOR]

The transport property of six types of individual carbon nanotubes (CNT) with specified chiral indices at high temperature is investigated on a home‐made apparatus. It is found that the resistivity of all individual CNTs increases with temperature monotonically, whereas the mobility decreases with temperature and follows different laws in different temperature ranges. The carrier concentration increases exponentially with −1/T. The mobility of CNT deviates from traditional trends with temperature, T−β, at high temperature (1070–1728 K). This deviation is attributed to scattering with optical phonons at high temperature. The data show that the electron‐optical phonon interaction becomes dominant in the temperature range of 1070–1362 K for all measured CNTs. Finally, result within a polaron model is discussed. [ABSTRACT FROM AUTHOR]

The effect of point-like lattice impurities on nanoparticle Raman spectra is studied using both numerical and analytical methods. Particular cases of replacement atoms of various masses, vacancies, and disorder in interatomic bonds are considered. It is shown that the disorder leads not only to the broadening of optical phonon lines but also to the shift of the corresponding Raman peak. The latter can be either positive (i.e., blueshift) or negative (redshift) depending on the type of impurities. Thus there is an additional contribution to the well-known redshift that occurs due to the size-quantization (confinement) effect. Considering nanometer-sized diamond particles as a representative example, we show that the broadening and the shift are, as a rule, of the same order of magnitude. The results are discussed in the framework of the self-consistent T-matrix approach. It is argued that both effects should be considered for accurate treatment of experimental Raman spectra. Simple recipes to do so are formulated for several important cases including NV centers in nanodiamonds. [Display omitted] [ABSTRACT FROM AUTHOR]

Biological membranes exhibit a great deal of compositional and phase heterogeneity due to hundreds of chemically distinct components. As a result, phase separation processes in cell membranes are extremely difficult to study, especially at the molecular level. It is currently believed that the lateral membrane heterogeneity and the formation of domains, or rafts, are driven by lipid-lipid and lipid-protein interactions. Nevertheless, the underlying mechanisms regulating membrane heterogeneity remain poorly understood. In the present work, we combine inelastic X-ray scattering with molecular dynamics simulations to provide direct evidence for the existence of strongly coupled transient lipid pairs. These lipid pairs manifest themselves experimentally through optical vibrational (a.k.a. phononic) modes observed in binary (1,2-dipalmitoyl-sn-glycero- 3-phosphocholine [DPPC]-cholesterol) and ternary (DPPC-1, 2-dioleoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoylglycero- 3-phosphocholine [DOPC/POPC]-cholesterol) systems. The existence of a phononic gap in these vibrational modes is a direct result of the finite size of patches formed by these lipid pairs. The observation of lipid pairs provides a spatial (subnanometer) and temporal (subnanosecond) window into the lipid-lipid interactions in complex mixtures of saturated/unsaturated lipids and cholesterol. Our findings represent a step toward understanding the lateral organization and dynamics of membrane domains using a well-validated probe with a high spatial and temporal resolution. [ABSTRACT FROM AUTHOR]

Mg2Si1−xSnx solid solutions are prepared in the composition range from x = 0 to 1 by mechanical alloying using high‐energy ball milling followed by direct‐current sintering at 600–800 °C. X‐ray diffraction analysis confirms that the samples obtained are uniform solid solutions. Raman spectroscopy shows two first‐order phonon Raman bands and a distribution of Raman signals due to second‐order phonon Raman scattering. The first‐order Raman bands are assigned to the nondegenerate Raman‐forbidden F1u (LO) mode and the triply degenerate Raman‐allowed F2g mode. Both modes exhibit a linear shift of the phonon frequency with a composition showing a clear one‐mode behavior for this solid solution. The findings are discussed in the context of existing theories for the occurrence of the one‐mode or two‐mode behavior of phonons in solid solutions. [ABSTRACT FROM AUTHOR]

A theory of interband magnetooptical light absorption in a nano-dimensional quantum well in a strong magnetic field conditioned by interaction of electrons with optical phonons was developed. We investigated a behavior of magnetooptical peaks corresponded to transitions of electrons from Landau levels with quantum numbers n ≥ 3 in the region of magnetic fields that satisfy the condition of equality of the electron cyclotron frequency in the conduction band to the frequency of longitudinaloptical phonon. For n = 3 in a point of resonance there is an intersection of four terms of the electron-phonon system (electron on Landau level n = 3, electron on the level n = 2 and one optical phonon, electron on the level n = 1 and two phonons, and electron on the level n = 0 and three phonons), which are considered to be a function of the cyclotron frequency of electron. Interaction with phonons removes a degeneracy of the terms, and leads to appearance of four non-intersected branches of electron-phonon spectrum. Accordingly, a magnetooptical peak splits into four peaks. Equations to describe a frequency dependence of a position and intensity of peaks on the magnetic field are presented. [ABSTRACT FROM AUTHOR]

A thorough analysis of the first-order vibrational spectrum of LuVO4 is presented by using polarized micro-Raman spectroscopy with special focus on the phonon modes with the weakest intensity and occasional controversial assignment. Group-theory analysis is carried out to demonstrate the determination of numbers and symmetries of the Raman active modes. Crystal- and correlation-field splitting effects in the vibrational spectrum of LuVO4 are discussed. Under conditions adjusted to minimize the birefringence effects we recorded, in each main scattering configuration, a series of Raman spectra in different sample orientations achieved by rotating the sample around the incident laser beam. The dependence of the Raman intensity on the rotational angle allowed us to identify the correct symmetry of the phonons with exceptionally weak scattering cross-section. A complete assignment of all twelve first-order Raman active phonons of LuVO4 is thus obtained.

Surface ligand modification of colloidal semiconductor nanocrystals has been widely used as a means of controlling photoexcited-state generation, relaxation, and coupling to the environment. While progress has been made in understanding how surface ligand modification affects the behavior of electronic states, less is known about the influence of surface ligand modification on phonon behavior, which impacts relaxation dynamics and transport phenomena. In this work, we compare the dynamics of optical and acoustic phonons in CdTe quantum dots (QDs), CdTe/CdSe core/shell QDs capped with octadecylphosphonic acid ligands, and CdTe QDs capped with Se2- to ascertain how ligand exchange from native aliphatic ligands to single-atom Se2- ligands affects phonon behavior. We use transient absorption spectroscopy and observe modulations in the kinetics of excited-state decay due to QD lattice vibrations from both optical and acoustic phonons, which we describe using the damped oscillator model. The longitudinal optical phonons have similar frequencies and damping behavior in all three samples. In contrast, the longitudinal acoustic phonon mode in the Se2--capped CdTe QDs is severely damped, much more so than in CdTe and CdTe/CdSe QDs capped with the native aliphatic ligands. We attribute these differences in the acoustic phonon behavior to the differences in how the QD dissipates vibrational energy to its surroundings as a function of ligand identity. Our results indicate that these inorganic surface-capping ligands enhance not only the electronic but also the mechanical coupling of nanocrystals with their environment. [ABSTRACT FROM AUTHOR]

Time dependencies of interband photoluminescence are studied in InGaAsSb/AlGaAsSb quantum well structures with various barrier materials and quantum well widths. The experimentally determined dependencies of photoluminescence intensity on the optical pumping level are compared with calculated dependencies of photoluminescence intensity on the nonequilibrium carrier concentration. Time of charge carrier trapping into quantum wells and the energy relaxation time related to the polar optical phonon emission were obtained from analysis of the photoluminescence dynamics at different optical pumping levels. The recombination rates with respect to nonradiative Shockley–Read–Hall and Auger recombination were determined. It is shown that, at certain parameter sets, resonant Auger recombination can exist in InGaAsSb/AlGaAsSb quantum well structures, which should results in the decrease of the concentration of carriers taking part in the radiative recombination. [ABSTRACT FROM AUTHOR]

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The investigation of phononic collective excitations in soft matter systems at the molecular scale has always been challenging due to limitations of experimental techniques in resolving low-energy modes. Recent advances in inelastic X-ray scattering (IXS) enabled the study of such systems with unprecedented spectral contrast at meV excitation energies. In particular, it has become possible to shed light on the low-energy collective motions in materials whose morphology and phase behavior can easily be manipulated, such as mesogenic systems. The understanding of collective mode behavior with a Q-dependence is the key to implement heat management based on the control of a sample structure. The latter has great potential for a large number of energy-inspired innovations. As a first step toward this goal, we carried out high contrast IXS measurements on a liquid crystal sample, D7AOB, which exhibits solid-like dynamic features, such as the coexistence of longitudinal and transverse phononic modes. For the first time, we found that these terahertz phononic excitations persist in the crystal, smectic A, and isotropic phases. Furthermore, the intermediate smectic A phase is shown to support a van der Waals-mediated nonhydrodynamic mode with an optical-like phononic behavior. The tunability of the collective excitations at nanometer-terahertz scales via selection of the sample mesogenic phase represents a new opportunity to manipulate optomechanical properties of soft metamaterials. [ABSTRACT FROM AUTHOR]

We investigate the long-wavelength dispersion of longitudinal and transverse optical phonon modes in polar two-dimensional materials, multilayers, and their heterostructures. Using analytical models and density-functional perturbation theory in a two-dimensional framework, we show that at variance with the three-dimensional case these modes are degenerate at the zone center but the macroscopic electric field associated with the longitudinal-optical modes gives rise to a finite slope at the zone center in their corresponding phonon dispersions. This slope increases linearly with the number of layers and it is determined solely by the Born effective charges of the material and the dielectric properties of the surrounding media. Screening from the environment can greatly reduce the slope splitting between the longitudinal and transverse optical modes and can be seen in the experimentally relevant case of boron nitride-graphene heterostructures. As the phonon momentum increases, the intrinsic screening properties of the two-dimensional material dictate the transition to a momentum-independent splitting similar to that of three-dimensional materials. These considerations are essential to understand electrical transport and optical coupling in two-dimensional systems. [ABSTRACT FROM AUTHOR]

This paper shows a systematic experimental and theoretical study on the temperature dependence of the infrared optical properties of pyrolytic boron nitride (pBN), from 390 to 1050 °C for wavelengths between 4 and 16 μm. The temperature dependence of these properties has never been analyzed before. The measured emissivity spectra were fitted to a dielectric function model and an effective medium approximation. The phonon frequencies and dielectric constants agreed well with room temperature experimental values from the literature, as well as with ab initio and first principles calculations. In addition, the phonon frequencies of the perpendicular mode and the dielectric constants of the parallel mode showed an appreciable parabolic temperature dependence, which justifies the interest of more theoretical efforts in order to explain this behavior. Finally, the results of this work demonstrate that thermal emission spectroscopy allows obtaining the values of the optical and dielectric parameters of impure ceramic materials in a simple manner as a function of temperature. [ABSTRACT FROM AUTHOR]

A pseudopotential approach has been employed so as to investigate the pressure dependence of refractive index, dielectric constants and optical phonon frequencies for InAs. At zero pressure our findings are compared to those reported in the literature and found to be generally in reasonably good agreement. The pressure dependence of all features of interest showed a monotonic behavior. The linear pressure coefficient of the refractive index and dielectric constants are calculated and reported. The transverse and longitudinal optical phonon frequencies are found to shift towards high frequencies when pressure is applied. [ABSTRACT FROM AUTHOR]

Micro-photoluminescence measurements have been performed on single cubic-shaped CsPbCl 3 nanocrystals (NCs) with side lengths of nearly twice the Bohr diameter, i.e. in the intermediate confinement regime. We observed two peaks, linearly polarized and spectrally separated by 1–3 meV associated to the bright states of the exciton fine structure. We took advantage of these emission polarization properties to track the energy and the linewidth broadening of individual peaks with increasing temperature. Two regimes exist in the broadening of emission peaks, at low and high temperature, dominated respectively by acoustic or longitudinal optical phonons couplings. We deduce the respective strengths of the interactions. We also measured micro-Raman spectra of an ensemble of CsPbCl 3 NCs. The energy of the mean optical phonon responsible for the high temperature broadening of the emission peaks is in good agreement with the highlighted longitudinal phonon modes observed in the Raman spectra. Finally, we compare our results to previous experimental works done on other lead halide perovskites emitting at lower energy. • Excitonic emission in a single CsPbCl3 perovskite nanocrystal is explored. • Temperature broadening of excitonic linewidths mainly due to LO phonons. • Agreement for phonon energies from Raman and results of the homogeneous broadening. [ABSTRACT FROM AUTHOR]

• The MAPbBr 3 SC possess a significant third-order nonlinear response. • The optical parameters i.e. n(λ) and ε(λ) were evaluated via Kramers-Kronig method. • The LO and TO phonon modes are 678, and 578 cm−1, respectively. The optical and structural aspects of methylamine lead bromide (MAPbBr 3) single crystal (SC) are investigated. Based on Kramers–Kronig (KK) equations, the optical and dielectric parameters are investigated. The optical phonons Longitudinal optical (LO = 678 cm−1) and Transversal optical (TO = 578 cm−1) phonon modes are associated with the optical interaction with lattices in the visible region for MAPbBr 3 SC. The Z-scan method is used to investigate the third-order nonlinear coefficients of SC hybrid organic–inorganic lead halide perovskites, MAPbBr 3 with the 2.19 eV optical band gap energy. According to the nonlinear optical (NLO) properties, MAPbBr 3 has a high third-order NLO response. The two-photon absorption (2PA) coefficient is found to be 3 × 10-4 cm W−1 using the transmission open aperture Z-scan technique. The nonlinear refraction (NLR) is obtained in the order of 10-9 cm2 W−1 utilizing a close aperture Z-scan, which revealed self-defocusing phenomena. This research shows that MAPbBr 3 is a practical and low-priced nonlinear material for ultrafast photonics applications. [ABSTRACT FROM AUTHOR]

• InAs nanocrystals are formed faster in the SiO 2 films covered with Si 3 N 4 layers. • InAs nanocrystals ion-beam synthesized in the SiO 2 films are not stressed. • A room-temperature visible luminescence was obtained from the InAs nanocrystals. The Raman spectroscopy and room-temperature photoluminescence were used to study the properties of the In + and As + ion-implanted SiO 2 films as a function of annealing temperature T a at 700–1100 °C. No features of the InAs phase were obtained in the Raman spectra of the native SiO 2 films irrespective of T a. In the ion-implanted SiO 2 films encapsulated with Si 3 N 4 , three Raman peaks at 202 cm−1, 256 cm−1 and 232 cm−1 corresponding to transverse and longitudinal optical phonons in black As and to longitudinal optical phonons in InAs, respectively, were detected. The SiO 2 encapsulation effect on the enhanced InAs phase formation is discussed. A photoluminescence of around 556 nm was observed from the encapsulated ion-implanted SiO 2 films and reached its intensity maximum after the annealing at 1000 °C. [ABSTRACT FROM AUTHOR]

The present report is concerned with the dynamical behavior of -electronic valley states, under the interaction with transverse zone-boundary optical phonons, in graphene. It is assumed that the phonons are thermal and obey the Bose-Einstein distribution, while the -electrons are initially prepared in an experimentally realizable particular valley state. In our study, we take the view that such a mixture is completely described by a time-dependent density operator which is then determined, to the second-order of perturbation, from the governing Schrödinger equation. Employing the density operator so calculated, an analytical expression for the valley polarization, as a function of time, phonon frequency and temperature, is obtained. The results, accompanying with illustrative figures, reveal that the -electrons, through the elastic exchange of energy with phonons, change the valley states periodically with characteristics that strongly depend upon the temperature. It is in particular shown that as the temperature is raised, the time-averaged valley polarization approaches zero, as expected. Our calculations also show that the amplitude of valley oscillations is solely determined by the temperature and phonon frequency: an increase in the temperature enlarges the amplitudes in contrast to the phonon frequency which does the reverse. Along these lines, moreover, we demonstrate that the frequency of valley oscillations is determined by the electronic momentum deviation from the valley states, along with the phonon frequency. [ABSTRACT FROM AUTHOR]

New calculations of both the equation of state and the frequency of the Raman-active optical phonon in phases I and II of solid hydrogen at pressures larger than about 20 GPa show that no semiempirical potential so far suggested, even containing three-body components, is able to reproduce the experimental findings. The result poses the question of whether, in the solid phases I and II, an expansion of the hydrogen intermolecular potential in an increasing number of many-body contributions is really possible. [ABSTRACT FROM AUTHOR]

Exploring supersolidity in naturally occurring and artificially designed systems has been and will continue to be an area of immense interest. Here we study the cooperation/competition of the superfluid and charge-density-wave (CDW) orders in a two-dimensional Bose–Holstein (BH) model where hard-core-bosons (HCBs) are coupled locally to optical phonons. In the parameter regimes of strong HCB–phonon coupling and nonadiabaticity, we find a novel mechanism for lattice-supersolidity (namely, sizeable same-sublattice tunneling in presence of large nearest-neighbor repulsion) in the system. The ground state phase diagram is obtained using Quantum Monte Carlo simulation involving stochastic-series-expansion technique. At densities not far from half filling and in the parameter regime where the double-hopping terms are non-negligible (negligible) compared to the nearest-neighbor hopping, we get checkerboard-supersolidity (phase separation) with CDW being characterized by ordering wavevector Q → = ( π , π ) . [ABSTRACT FROM AUTHOR]

In this work, the analysis of both experimental and theoretical angular dispersion of the Raman frequencies of the optical polar phonons in LiNbO3 (LN) single crystal has been determined with the emphasis of the determination without ambiguity of a new and complete set of the normal mode assignments that clarify long-standing debates. The coherency of such couple assignments is assessed by the computed oscillator strengths that are revealed to be in satisfactory agreement with others on the one hand and by the comparison with the oblique frequencies measured from the compilation of the Raman spectra acquired under different geometrical configurations in the 90° scheme on the other hand. Finally, the directional behavior of the quasinormal vectors has been established and discussed with regard to the symmetry mixing of modes or the mode with oblique polarization. [ABSTRACT FROM AUTHOR]

An optical-phonon-limited velocity model has been employed to investigate high-field transport in a selection of layered 2-D materials for both, low-power logic switches with scaled supply voltages, and high-power, high-frequency transistors. Drain currents, effective electron velocities, and intrinsic cutoff frequencies as a function of carrier density have been predicted, thus providing a benchmark for the optical-phonon-limited high-field performance limits of these materials. The optical-phonon-limited carrier velocities for a selection of multi-layers of transition metal dichalcogenides and black phosphorus are found to be modest compared to their n-channel silicon counterparts, questioning the utility of biasing these devices in the source-injection dominated regime. h-BN, at the other end of the spectrum, is shown to be a very promising material for high-frequency, high-power devices, subject to the experimental realization of high carrier densities, primarily due to its large optical-phonon energy. Experimentally extracted saturation velocities from few-layer MoS2 devices show reasonable qualitative and quantitative agreement with the predicted values. The temperature dependence of the measured v{\mathrm{sat}} is discussed and compared with the theoretically predicted dependence over a range of temperatures. [ABSTRACT FROM AUTHOR]