Phonon Linewidths Research Articles

Strong lattice anharmonicity exhibited by the high-energy optical phonons in thermoelectric material

Thermoelectric material SnSe has aroused world-wide interests in the past years, and its inherent strong lattice anharmonicity is regarded as a crucial factor for its outstanding thermoelectric performance. However, the understanding of lattice anharmonicity in SnSe system remains inadequate, especially regarding how phonon dynamics are affected by this behavior. In this work, we present a comprehensive study of lattice dynamics on Na0.003Sn0.997Se0.9S0.1 by means of neutron total scattering, inelastic neutron scattering, Raman spectroscopy as well as frozen-phonon calculations. Lattice anharmonicity is evidenced by pair distribution function, inelastic neutron scattering and Raman measurements. By separating the effects of thermal expansion and multi-phonon scattering, we found that the latter is very significant in high-energy optical phonon modes. The strong temperature-dependence of these phonon modes indicate the anharmonicity in this system. Moreover, our data reveals that the linewidths of high-energy optical phonons become broadened with mild doping of sulfur. Our studies suggest that the thermoelectric performance of SnSe could be further enhanced by reducing the contributions of high-energy optical phonon modes to the lattice thermal conductivity via phonon engineering.

Growth and characterization of ZnSe nanocrystals synthesized using solvothermal process

ZnSe nanocrystals with zinc blende structure are synthesized by solvothermal process at 1-h, 3-h and 5-h reaction times in a mixed solvent of hydrazine hydrate [H6N2O], ammonia [NH3] and de-ionized water. Zinc acetate [(CH3COO)2 Zn; 2H2O] and sodium selenite [Na2SeO3] are, respectively, used as precursors for zinc and selenium ions. X-ray diffraction (XRD) and scanning electron microscope (SEM) study show that the crystallinity and the chemical purity of the compound improve with increase in reaction duration. SEM images show formation of bulk rod-shaped ZnO particles in samples prepared at 1-h and 3-h reaction times, which is absent in the 5-h sample. TGA measurements also show that the ZnSe nanocrystals synthesized in this process have high purity and the nanocrystals prepared for longer duration have improved purity as well as higher thermal stability. In Raman measurements, nanocrystalline nature such as surface phonon (SP) mode and phonon line width broadening are observed. The observed variation of the relative intensities of the phonon modes follows the differences in the size of nanocrystals, in which the relative strength of the transverse optical (TO) phonon mode is found to increase with nanocrystals size and the SP phonon relative strength on the other hand decreases.

On the potential for superconductivity in ZrX (X = S and Te): a first-principles study

The phonon nature and superconductive properties of Zirconium chalcogenides (ZrS and ZrTe) in WC-type structure are evaluated leveraging density functional theory of first principles. In the WC-type structure, the phonon frequencies are positive in the Brillouin zone suggesting their stability in that region. The calculated zone center phonon dispersion curves and phonon density of states for ZrS and ZrTe are found to be in good agreement with other available theoretical results. Superconductivity in ZrS and ZrTe is explored in detail by measuring line widths of phonons, the spectral function of Eliashberg, electron-phonon coupling constant, and transition temperature. ZrS has a slightly higher Tc (3.71 K), compared to ZrTe with Tc (1.72 K).

Phonon thermodynamics and elastic behavior of GaAs at high temperatures and pressures

The phonons of wurtzite and zinc blende GaAs were calculated at simultaneously elevated temperature and pressure, and elastic constants were calculated as functions of pressure. Pressure caused instabilities of shorter-wavelength transverse acoustic modes in both wurtzite and zinc blende GaAs, causing them to fall to zero at 18 and 20 GPa, respectively. The Born stability criteria, which depend on elastic constants and only long wavelength phonons, therefore overestimated the pressure needed to induce instability at 0 K. At elevated temperatures, explicit anharmonicity pushes the onset of instability to higher pressures in both wurtzite and zinc blende GaAs. Phonon linewidth and densities of states data showed that the quasiharmonic approximation failed to account for temperature-induced phonon frequency shifts, and the quasiharmonic approximation became less reliable at elevated pressure. In general, the number of three-phonon processes increased with pressure, thereby increasing the temperature-driven broadening of phonon spectral lineshapes.

Explanation of the competition between O- and E-wave induced stimulated Raman and supercontinuum in calcite under ultrafast laser excitation.

Key optical properties of calcite were measured to unravel the difference between stimulated Raman scattering (SRS) and self-phase modulation (SPM) for the supercontinuum (SC) for ordinary (O) wave and extraordinary (E) wave. These properties are group velocity dispersion, walk-off, spontaneous Raman spectra and cross section, optical 1086cm-1 phonon linewidth, nonlinear susceptibility (χ3), steady-state and transient SRS, and SC caused from SPM. These are investigated for O-waves and E-waves from a 2.7 cm thick calcite crystal. Using 390 fs pulses (∼0.8µJ pulse energy) at 517 nm, the O-wave produced a stronger sharp SRS peak at 1086cm-1 and a weaker SC spectrum in the visible range than the E-wave. The salient difference found between the O- and E-waves for SRS and SPM in calcite is attributed to the larger Raman cross section and the size of nonlinear susceptibility (χ3) for O-waves as compared to E-waves.

Isotopic Disorder: The Prevailing Mechanism in Limiting the Phonon Lifetime in Hexagonal BN.

The phonon linewidth of isotopically controlled hexagonal boron nitride (h-BN) single crystals has been determined by Raman scattering. The scattering by isotopic mass disorder induces a phonon broadening that is largest for boron 11 fractions around 0.65. Lowest-order perturbation theory does not suffice to explain the dependence of the isotopic broadening on isotopic composition. A multiple-scattering theory based on the coherent potential approximation provides a good quantitative account of the phonon shift and broadening with isotopic composition observed in the experiments. Isotopic-disorder scattering is shown to have a prominent role in limiting the optical-phonon lifetime in h-BN.

Raman and photoluminescence spectral studies in double perovskite epitaxial Nd2CoMnO6 thin films deposited by pulse laser deposition

The detailed temperature dependent optical properties in double perovskite type Nd2CoMnO6-SrTiO3 (NCMO-STO) thin films have been investigated the experimentally grown epitaxial thin films on STO substrates using pulsed laser deposition technique. At first order room temperature, a Raman spectrum of NCMO/STO thin films are measured in unpolarized geometry using 488 nm laser excitation and confirms its monoclinic bulk like crystal structure. Temperature dependent Raman spectra confirms the appearance of a sharp, intense phonon mode below magnetic ordering, signifies structural phase transition accompanied by magnetic transition. The phonon mode position and line-width are fitted by anharmonic model and found to be strongly softened phonon mode position below TC and strong spin-phonon coupling in the system. The excitation spectrum and the bandgap in NCMO film confirms the yellow emission resulting from energy transfer within the non- centrosymmetric structure.

Observation of strong higher-order lattice anharmonicity in Raman and infrared spectra

Zone-center optical phonon linewidth is a key parameter for infrared and Raman spectra as well as the Lorenz oscillator model. While three-phonon scattering was often assumed to be the leading contribution, in this work we find, surprisingly, that higher-order phonon scattering universally plays a significant or even dominant role over three-phonon scattering at room temperature, and more so at elevated temperatures, for a wide range of materials including diamond, Si, Ge, boron arsenide (BAs), cubic silicon carbide (3C-SiC), and $\alpha$-quartz. This is enabled by the large fourphonon scattering phase space of zone-center optical phonons, and distinct from heat conduction where at room temperature four-phonon scattering is still secondary to three-phonon scattering. Moreover, our results imply that five-phonon and even higher-order scattering may be significant for some large band-gap materials, e.g., BAs. Our predicted infrared optical properties through the Lorenz oscillator model, after including four-phonon scattering, show much better agreement with experimental measurements than those three-phonon based predictions. This work will raise broad interest of studying high-order scattering in various areas beyond heat conduction.

Phonon linewidths in InAs/AlSb superlattices derived from first-principles—application towards quantum well hot carrier solar cells

Hot electrons generated by the absorption of high-energy photons typically thermalize by dissipating energy through phonon mediated relaxation pathways. Existence of non-equilibrium optical phonon populations (hot phonons) can suppress thermalization of electrons by facilitating a stable hot carrier population. In this work, we derive optical phonon scattering rates in InAs/AlSb superlattices using first-principles calculations and compare them with bulk InAs. Computations are performed for the two shortest period superlattices with equal widths of InAs and AlSb. While in pure InAs, the highest optical phonon scattering rates reach 5 cm−1; in the superlattice, the peak scattering rates of optical phonons with frequencies above the energy gap, are much smaller, reaching ∼1 cm−1. These lower scattering rates in the superlattice have important implications for design of high efficiency hot carrier solar cells and explain some of the robust hot carriers observed recently in InAs/AlAsSb quantum wells.

Raman scattering measurements of phonon anharmonicity in the delafossite CuGa1‐xCrxO2 (0 ≤ x ≤ 1) films

AbstractRaman spectra of the delafossite CuGa1‐xCrxO2 (0 ≤ x ≤ 1) solid solution films have been measured in the temperature range from 80 to 823 K. Raman phonon modes are observed at 224 (Ag), 328 (A2u), 396 (Eg), and 757 (A1g) cm−1 for the CuGaO2 film, and phonon modes of the CuCrO2 film are around 211 (Ag), 324 (A2u), 458 (Eg), and 712 (A1g) cm−1, respectively. The A1g phonon mode has a distinct redshift with increasing Cr composition, which indicates that the interatomic potential becomes weaker between Cu and O atoms. Except Raman modes at the Brillouin zone center, the additional phonon modes in the spectral range of 500−670 cm−1 were observed due to Raman selection rules being relaxed by the crystal defects. With increasing temperature, the A1g phonon mode undergoes a larger phonon softening but a smaller line broadening relative to that of the Eg phonon mode, which indicated the anisotropic behavior for the delafossite CuGa1‐xCrxO2 films. The nonlinear temperature dependence of the phonon frequency and linewidth is an indication of nonharmonic lattice dynamics. The Raman shift and the linewidth of the Eg and A1g optical modes as a function of temperature were studied as an experimental evidence of the combined effect of lattice expansion and anharmonic phonon–phonon interaction. And the three‐phonon anharmonic coupling plays the dominant role in the anharmonicity behavior. The phonon anharmonicity behavior of the CuGaO2 and CuCrO2 films is slightly different. The larger Γ0 is intrinsic for the A1g mode of the CuGa0.8Cr0.2O2 film due to the broadening by the crystalline defects, which perturb the translation symmetry of the harmonic crystal, making the momentum conservation rule for phonon scattering partially relaxed.

Broken adiabaticity induced by Lifshitz transition in MoS2 and WS2 single layers

The breakdown of the adiabatic Born-Oppenheimer approximation is striking dynamical phenomenon, however, it occurs only in a handful of layered materials. Here, I show that adiabaticity breaks down in doped single-layer transition metal dichalcogenides in a quite intriguing manner. Namely, significant nonadiabatic coupling, which acts on frequencies of the Raman-active modes, is prompted by a Lifshitz transition due to depopulation and population of multiple valence and conduction valleys, respectively. The outset of the latter event is shown to be dictated by the interplay of highly non-local electron-electron interaction and spin-orbit coupling. In addition, intense electron-hole pair scatterings due to electron-phonon coupling are inducing phonon linewidth modifications as a function of doping. Comprehending these intricate dynamical effects turns out to be a key for mastering characterization of electron doping in two-dimensional nano-devices by means of Raman spectroscopy.

Temperature Dependence of Raman and Photoluminescence Spectra of Ternary Alloyed CdSe0.3Te0.7 Quantum Dots

The Raman and photoluminescence spectra of ternary alloyed CdSe0.3Te0.7 quantum dots (QDs) have been studied at temperatures between 84 K and 293 K. The average diameter of QDs is about 5.1 nm. The temperature dependence of the longitudinal optical (LO) phonon frequencies and the phonon band width was analyzed and the anharmonic constants relating to various high-order phonon processes was determined. While the three-phonon processes plays a dominant role in the temperature-dependent shift of the LO-phonon frequencies, the four-phonon processes contribute to the increase of the phonon linewidth with increasing temperature. The photoluminescence (PL) spectra were used to study the temperature dependence of the bandgap, the linewidth and the integrated PL intensity. At low temperatures below about 120 K, the PL linewidth decreases and the PL intensity increases as temperature increases. This temperature behavior can be ascribed to the exciton fine structure.

Investigation of the spectral characteristics of silicon-vacancy centers in ultrananocrystalline diamond nanostructures and single crystalline diamond

Silicon-vacancy (SiV) centers were produced in single crystalline diamond (SCD) and ultrananocrystalline diamond (UNCD) nanostructures via Si ion implantation or in situ Si doping. SiV-embedded UNCD (SiV-UNCD) was fabricated by both top-down and bottom-up methods. The spectral properties of the SiV centers, including the zero phonon line (ZPL) width and decay time, were investigated in the SCD and UNCD nanostructures. All the SiV-UNCD nanostructures showed bright emission regardless of the preparation method. However, the decay time of the SiV centers was affected by the synthesis procedure. A SiV decay time of τ ∼ 0.19 ns was observed for UNCD nanostructures formed by in situ doping, whereas the SiV decay time was ∼0.43 ns for SiV-UNCD clusters prepared by Si ion implantation into UNCD deposited on Ti/sapphire substrates. The ultrasonication of UNCD clusters on Ti/sapphire pyramids produced bright SiV-UNCD nanoclusters with sizes of ∼50 nm, a ZPL width of 13.5 nm, and a decay time of 0.35 ns, suggesting promising potential in bioimaging applications. SiV-containing SCD (type Ia or type IIa) showed enhanced SiV spectral properties with a ZPL width of 6.08 nm and longer decay time of 1.3 ns.

单原子层薄膜热传导性质的晶格动力学研究(I)——声子线宽和热传导系数公式

单原子层薄膜热传导性质的晶格动力学研究(I)——声子线宽和热传导系数公式

Uncovering electron-phonon scattering and phonon dynamics in type-I Weyl semimetals

Weyl semimetals are 3D phases of matter with topologically protected states that have remarkable macroscopic transport behaviors. As phonon dynamics and electron-phonon scattering play a critical role in the electrical and thermal transport, we pursue a fundamental understanding of the origin of these effects in type-I Weyl semimetals NbAs and TaAs. In the temperature-dependent Raman spectra of NbAs, we reveal a previously unreported Fano lineshape, a signature stemming from the electron-phonon interaction. Additionally, the temperature dependence of the $A_{1}$ phonon linewidths in both NbAs and TaAs strongly deviate from the standard model of anharmonic decay. To capture the mechanisms responsible for the observed Fano asymmetry and the atypical phonon linewidth, we present first principles calculations of the phonon self-energy correction due to the electron-phonon interaction. Finally, we investigate the relationship between Fano lineshape, electron-phonon coupling, and locations of the Weyl points in these materials. Through this study of the phonon dynamics and electron-phonon interaction in these Weyl semimetals, we consider specific microscopic pathways which contribute to the nature of their macroscopic transport.

Temperature dependent X‐ray diffraction and Raman spectroscopy studies of polycrystalline YCrO3 ceramics across the TC ~ 460 K

AbstractIn this paper, we report the X‐ray diffraction studies over the temperature range of 300–900 K and subsequent Rietveld refinement of the diffraction patterns shows that there is no evidence of any structural phase transition in YCrO3 (YCO) across the paraelectric to ferroelectric phase transitions (TC ~ 460 K), and the material retains the orthorhombic structure with Pnma space group. However, a detailed analysis reveals local distortions in the CrO6 octahedra. To probe the local structural distortion, we have carried out temperature‐dependent, unpolarized Raman measurements, from 300 to 600 K. YCrO3 shows a strong anomalous deviation in both phonon wavenumbers and line widths around TC. YCrO3 is reported to possess as an incipient ferroelectric material to show that there are two competing phenomenon—onset of ferroelectricity due to rotation of CrO6 octahedra and displacement of Y atom leading to suppression of ferroelectricity. This competition reveals that although the octahedral rotations favor a lower symmetry state, the Y atom displacement opposes it leaving YCO to exhibit only an incipient ferroelectric state. These results while being in agreement with the earlier theoretical predictions can also help suggest a pathway to a more stable ferroelectric state in these oxides by using a larger cationic substitution at the Y site.

Extremely space and time restricted thermal transport in the high temperature Cmcm phase of thermoelectric SnSe

Phonon dispersions and linewidths of layered thermoelectric SnSe in the high temperature Cmcm phase have been mapped by inelastic neutron scattering measurements. Downturns in the phonon dispersion reveal soft phonon energies at larger wave vectors for phonon modes propagating in the crystallographic basal a-c plane. The downturns can be described by a q2-dependency, indicative of the appearance of strong electron-phonon scattering in the Cmcm phase. A q3-dependency is also needed for a satisfactory description of the dispersions, revealing the existence of a huge lattice anharmonicity. Group velocities of the transverse acoustic phonons are reduced by as much as 37% through the structural phase transition at 798 K. The scattering of phonons is so strong that they propagation only over a few unit cells in length, with the lifetime as short as ~0.3 ps. The very short phonon lifetimes and very limited phonon propagation length, together with negative phonon group velocity at large wavevectors restricts heat transport at high temperatures. Our results reveal the origin of the extremely low thermal conductivity of SnSe in the Cmcm phase.

Topological phase transition between distinct Weyl semimetal states in MoTe2

We present experimental evidence of an intriguing phase transition between distinct topological states in the type-II Weyl semimetal MoTe2. We observe anomalies in the Raman phonon frequencies and linewidths as well as electronic quasielastic peaks around 70 K, which, together with structural, thermodynamic measurements, and electron-phonon coupling calculations, demonstrate a temperature-induced transition between two topological phases previously identified by contrasting spectroscopic measurements. An analysis of experimental data suggests electron-phonon coupling as the main driving mechanism for the change of key topological characters in the electronic structure of MoTe2.We also find the phase transition to be sensitive to sample conditions distinguished by synthesis methods. These discoveries of temperature and material condition-dependent topological phase evolutions and transitions in MoTe2 advance the fundamental understanding of the underlying physics and enable an effective approach to tuning Weyl semimetal states for technological applications.

Crystalline phase transitions and vibrational spectra of silicon up to multiterapascal pressures

A composite high-pressure phase diagram for silicon has been predicted for up to 4 TPa. This diagram has been built using a combination of evolutionary algorithm-based structure searches, electronic density functional theory, lattice dynamics of perfect crystals, and anharmonic corrections for the solid state in conjunction with molecular dynamics for evaluating the melt curve. The anharmonic corrections to free energy, arising from both finite-temperature multiphonon interactions and temperature dependence of the lattice axial ratios, play a critical role in properly identifying the solid-solid transition boundaries of the orthorhombic structures. A double hexagonal close-packed structure has been found to be thermodynamically and dynamically stable, sandwiched between the experimentally observed base-centered orthorhombic and hexagonal close-packed structures. In addition, beyond 2.8 TPa, there exists a sequence of face-centered cubic-to-body-centered cubic-to-simple cubic transitions that are accompanied by the localization of electrons in the interstitial spaces between the ions. Supplementing the structural calculations, second- and third-order interatomic force constants were evaluated to compute the phonon vibration modes and linewidths, respectively. This allowed an elaborate analysis of the Raman and infrared spectra for all of the structures of silicon identified so far.

Body-centered-cubic structure and weak anharmonic phonon scattering in tungsten

It was recently found that the anharmonic phonon–phonon scattering in tungsten is extremely weak at high frequencies, leading to a predominance of electron–phonon scattering and consequently anomalous phonon transport behaviors. In this work, we calculate the phonon linewidths of W along high-symmetry directions from first principles. We find that the weak phonon–phonon scattering can be traced back to two factors. The first is the triple degeneracy of the phonon branches at the P and H points, a universal property of elemental body-centered-cubic (bcc) structures. The second is a relatively isotropic character of the phonon dispersions. When both are met, phonon–phonon scattering rates must vanish at the P and H points. The weak phonon–phonon scattering feature is also applicable to Mo and Cr. However, in other elemental bcc substances like Na, the isotropy condition is violated due to the unusually soft character of the lower transverse acoustic phonon branch along the Γ-N direction, opening emission channels and leading to much stronger phonon–phonon scattering. We also look into the distributions of electron mean-free paths (MFPs) at room temperature in tungsten, which can help engineer the resistivity of nanostructured W for applications such as interconnects.