Phonon Modes Research Articles

Understanding phonon transport properties on Janus XSSe (X = Hf, Pb, Pt) monolayers via density functional theory

Janus monolayers have emerged as promising candidates for thermoelectric applications due to their unique structural and electronic properties. This study investigates the phonon thermal transport of Janus XSSe (X = Hf, Pb, Pt) monolayers using density functional theory (DFT) and Boltzmann Transport Theory (BTT). Our calculations reveal exceptionally low lattice thermal conductivities of 0.80, 1.43, and 11.0 W/mK for HfSSe, PbSSe, and PtSSe, respectively, at 300 K. The transverse acoustic (TA) phonon mode dominates the thermal transport, contributing 42.49%, 46.37%, and 46.82% to the total lattice thermal conductivity in these materials. Significant anharmonicity, characterized by low phonon lifetimes and reduced group velocities, further suppresses the lattice thermal conductivity. Additionally, we report indirect bandgaps of 1.49 eV, 0.64 eV, and 2.25 eV for HfSSe, PbSSe, and PtSSe, respectively. These findings highlight the potential of Janus XSSe monolayers for thermoelectric applications and provide insights into their phonon transport mechanisms.

Effective phonon dispersion and low field transport in AlxGa1−xN alloys using supercells: An ab initio approach

To investigate the transport properties in random alloys, it is important to model the alloy disorder using supercells. Although computationally expensive, the local disorder in the system is accurately captured as translational symmetry that is imposed on the system over larger length scales. Additionally, in supercells, the error introduced by self-image interaction between the impurities is reduced. In this work, we have investigated the Effective Phonon Dispersion (EPD) and transport properties, from first principle calculations using supercells in AlxGa1−xN alloy systems. Using an in-house developed code for phonon-band unfolding, the EPD of AlGaN is obtained and the individual phonon modes are identified with good agreement with experimental values. Moreover, we report an in-house developed method to calculate low-field transport properties directly from supercells without phonon band unfolding. First, to validate our methods, we have solved the Boltzmann transport equation using Rode’s method to compare the phonon limited mobility in the 4 atom GaN primitive cell and 12 atom GaN supercell. Using the same technique, we have investigated the low field transport in random AlxGa1−xN alloy systems. The quadrupole interaction is included for transport properties of GaN and AlN to accurately capture the physics in these materials. Our calculations show that along with alloy scattering, electron–phonon scattering may also play an important role at room temperature and high-temperature device operation. This technique opens up the path for calculating phonon-limited transport properties in random alloy systems.

All-optical manipulation of bandgap dynamics via coherent phonons

The ability to actively and dynamically control electronic states at ultrafast timescales opens up a wide range of potential applications across optoelectronics, quantum computing and sensing, energy conversion and storage, etc. Yet, achieving dynamic electronic manipulation via coherent phonons has posed a considerable challenge. Here, employing time-resolved high-harmonic generation (tr-HHG) spectroscopy, we demonstrate the manipulation of bandgap dynamics in a BaF2 crystal by coherent phonons. The tr-HHG spectrum perturbed by a triply degenerate phonon mode T2g exhibits simultaneously a remarkable two-dimensional (2D) sensitivity, i.e., in both intensity and energy domains. The dynamic compression and enhancement of the harmonics in the intensity domain showed a π/2 phase shift compared to the manifestation of shifts of the harmonics in the energy domain, an astounding example of a physical phenomenon being observed simultaneously in two different perspectives. We employed a quantum model incorporating the electron–phonon coupling to complement our experimental observations, successfully reproducing the results. In addition, we demonstrated complete control over the strength and initial phase of the coherent phonon oscillations by varying the incident electric field polarizations across different crystal orientations. Our findings lay a foundation for engineering the electronic structure through coherent phonons within the terahertz frequency and picosecond to nanosecond time regimes.

Unraveling the Synergistic Role of Kinks in Zig-Zag Ag2Se Nanorod Arrays for High Room-Temperature zT and Improved Mechanical Properties: Experimental and First-Principles Studies.

Flexible thermoelectric materials are usually fabricated by incorporating conducting or organic polymers; however, it remains a formidable task to achieve high thermoelectric properties comparable to those of their inorganic counterparts. Here, we present a high zT value of 1.29 ± 0.31 at room temperature in the hierarchical zig-zag Ag2Se nanorod arrays fabricated using the glancing angle deposition (GLAD) technique followed by a facile selenization process. The high zT value at 300 K is ascribed to the ultrahigh power factor of 3101 ± 252 μW/m-K2 and the reduced thermal conductivity of 0.72 ± 0.01 W/mK. Based on ab initio computational and experimental evidence, we reveal that kinked Ag2Se nanorod arrays consisting of rough interfaces modulate the lattice thermal conductivity up to 48.5% at room temperature. The modulation results from interchanging of phonon modes at kink points and enhanced scattering from a large number of rough interfaces. Further, benefiting from kinked hierarchy, a notable improvement in the mechanical performance is observed for zig-zag Ag2Se nanorods which is confirmed by nanoindentation measurements. The synergic improvement in thermoelectric and mechanical performance not only unravels a paradigm to harness thermoelectric heat but also offers deeper insights into tuning the mechanical properties of inorganic thermoelectric materials.

Optoacoustic Entanglement in a Continuous Brillouin-Active Solid State System

Entanglement in hybrid quantum systems comprised of fundamentally different degrees of freedom, such as light and mechanics, is of interest for a wide range of applications in quantum technologies. Here, we propose to engineer bipartite entanglement between traveling acoustic phonons in a Brillouin active solid state system and the accompanying light wave. The effect is achieved by applying optical pump pulses to state-of-the-art waveguides, exciting a Brillouin Stokes process. This pulsed approach, in a system operating in a regime orthogonal to standard optomechanical setups, allows for the generation of entangled photon-phonon pairs, resilient to thermal fluctuations. We propose an experimental platform where readout of the optoacoustics entanglement is done by the simultaneous detection of Stokes and anti-Stokes photons in a two-pump configuration. The proposed mechanism presents an important feature in that it does not require initial preparation of the quantum ground state of the phonon mode. Published by the American Physical Society 2024

Superconductivity and strain-enhanced phase stability of Janus tungsten chalcogenide hydride monolayers

Janus transition metal-dichalcogenide materials have attracted a great deal of attention due to their remarkable physical properties arising from the two-dimensional geometry and the breakdown of the out-of-plane symmetry. Using first-principles density functional theory, we investigated the phase stability, strain-enhanced phase stability, and superconductivity of Janus WSeH and WSH. In addition, we investigated the contribution of the phonon linewidths from the phonon energy spectrum responsible for the superconductivity and the electron-phonon coupling as a function of phonon wave vectors and modes. Previous work has examined hexagonal 2H and tetragonal 1T structures, but we found that neither is a ground-state structure. The metastable 2H phase of WSeH is dynamically stable with Tc≈11.60K, similar to WSH. Compressive biaxial strain—the two-dimensional equivalent of pressure—can stabilize the 1T structures of WSeH and WSH with Tc≈9.23K and 10.52 K, respectively. Published by the American Physical Society 2024

Chemical Pressure-Induced Unconventional Band Convergence Leads to High Thermoelectric Performance in SnTe.

Band convergence is considered a net benefit to thermoelectric performance as it decouples the density of states effective mass ( ) and carrier mobility (µ) by increasing valley degeneracy. Unlike conventional methods that typically prioritize at the expense of µ, this study theoretically demonstrates an unconventional band convergence strategy to enhance both and µ in SnTe under pressure. Density functional theory calculations reveal that increasing pressure from 0 to 5 GPa moves the Σ-band of SnTe upward, reducing the energy offset between L- and Σ-band from 0.35 to 0.2 eV while preserving the light band feature of the L-band. Consequently, a high power factor (PF) of 119.2 µW cm-1 K-2 at 300 K is achieved for p-type SnTe under 5 GPa. Chemical pressure also induces conduction band convergence, significantly enhancing the PF of n-type SnTe. Additionally, the interplay between pressure-induced phonon modes leads to a moderate increase in lattice thermal conductivity of SnTe below 3 GPa, which combined with the significantly enhanced PF, contributes to a large enhancement in ZT. Consequently, predicted ZT values of 2.12 at 650 K and 2.55 at 850 K are obtained for p- and n-type SnTe, respectively, showcasing substantial performance enhancements.

Impact of Temperature and Substrate Type on the Optical and Structural Properties of AlN Epilayers: A Cross-Sectional Analysis Using Advanced Characterization Techniques

AlN, with its ultra-wide bandgap, is highly attractive for modern applications in deep ultraviolet light-emitting diodes and electronic devices. In this study, the surface and cross-sectional properties of AlN films grown on flat and nano-patterned sapphire substrates are characterized by a variety of techniques, including photoluminescence spectroscopy, high-resolution X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and Raman spectroscopy. The results indicate that different sapphire substrates have minimal impact on the photoluminescence spectrum of the epitaxial films. As the temperature increased, the radius of curvature of the AlN films increased, while the warpage decreased. The AlN films grown on nano-patterned substrates exhibited superior quality with less surface oxidation. During the growth of AlN thin films on different types of substrates, slight shifts in the energy bands occurred due to differences in the introduction of carbon-related impurities and intrinsic defects. The Raman shift and full width at half maximum (FWHM) of the E2(low), A1(TO), E2(high), E1(TO), and E1(LO) phonon modes for the cross-sectional AlN films varied with the depth and temperature. The stress state within the film was precisely determined with specific depths and temperatures. The FWHM of the E2(high) phonon mode suggests that the films grown on nano-patterned substrates exhibited better crystalline quality.

Deep Ultraviolet Excitation Photoluminescence Characteristics and Correlative Investigation of Al-Rich AlGaN Films on Sapphire.

AlGaN is attractive for fabricating deep ultraviolet (DUV) optoelectronic and electronic devices of light-emitting diodes (LEDs), photodetectors, high-electron-mobility field-effect transistors (HEMTs), etc. We investigated the quality and optical properties of AlxGa1-xN films with high Al fractions (60-87%) grown on sapphire substrates, including AlN nucleation and buffer layers, by metal-organic chemical vapor deposition (MOCVD). They were initially investigated by high-resolution X-ray diffraction (HR-XRD) and Raman scattering (RS). A set of formulas was deduced to precisely determine x(Al) from HR-XRD data. Screw dislocation densities in AlGaN and AlN layers were deduced. DUV (266 nm) excitation RS clearly exhibits AlGaN Raman features far superior to visible RS. The simulation on the AlGaN longitudinal optical (LO) phonon modes determined the carrier concentrations in the AlGaN layers. The spatial correlation model (SCM) analyses on E2(high) modes examined the AlGaN and AlN layer properties. These high-x(Al) AlxGa1-xN films possess large energy gaps Eg in the range of 5.0-5.6 eV and are excited by a DUV 213 nm (5.8 eV) laser for room temperature (RT) photoluminescence (PL) and temperature-dependent photoluminescence (TDPL) studies. The obtained RTPL bands were deconvoluted with two Gaussian bands, indicating cross-bandgap emission, phonon replicas, and variation with x(Al). TDPL spectra at 20-300 K of Al0.87Ga0.13N exhibit the T-dependences of the band-edge luminescence near 5.6 eV and the phonon replicas. According to the Arrhenius fitting diagram of the TDPL spectra, the activation energy (19.6 meV) associated with the luminescence process is acquired. In addition, the combined PL and time-resolved photoluminescence (TRPL) spectroscopic system with DUV 213 nm pulse excitation was applied to measure a typical AlGaN multiple-quantum well (MQW). The RT TRPL decay spectra were obtained at four wavelengths and fitted by two exponentials with fast and slow decay times of ~0.2 ns and 1-2 ns, respectively. Comprehensive studies on these Al-rich AlGaN epi-films and a typical AlGaN MQW are achieved with unique and significant results, which are useful to researchers in the field.

Trimerization-induced zone-folded phonons in 1T′-TaTe2

The trimerized state is an intriguing structure in the superlattice with three unit cells distorted periodically. Here, we report on two kinds of featured phonon modes induced by the trimerization transition in 1T′-TaTe2. One is the almost temperature-independent zone-folded mode, stemming from the collective excitation due to the superlattice formation and the other is the normal mode with the step-like temperature-dependent intensity and frequency, which is the characteristic of the first-order transition of the trimerization. Furthermore, the trimerization can be totally suppressed when Mo-dopant content is less than 6 at. %. Our work not only unveils the featured phonon modes in trimerized system but also provides a pathway to identify the multimerization transition in solids by Raman spectroscopy.

Measurement of temporal evolution of antiferromagnetic domain change in nickel oxide driven by 2TO phonon mode excitation via pump-probe experiment

The temporal evolution of antiferromagnetic domain pattern changes under resonant excitation of the second-order transverse optical phonon mode in nickel oxide was investigated using mid-infrared free electron laser (MIR-FEL) pulses and visible nanosecond probe laser pulses at room temperature. We visualized the domain patterns through magneto-optical polarized microscopy in transmission and observed their temporal variation after the phonon excitation via MIR-FEL. We evaluated the differences throughout the timeline using the structural similarity index measure technique from domain images with and without MIR-FEL irradiation. We found that the MIR-FEL irradiation induces significant structural changes in the domain patterns. The maximum difference was observed at the timing of the MIR-FEL irradiation and exponentially recovered with the time constant of 1.4 ± 0.2 ms. This rather long-time constant could be owing to the spin relaxation, whilst further investigation is needed to confirm the underlying mechanism.

Novel Mo: CoFe2O4 nanoparticles combustion synthesis for opto-magneto-electrochemical applications: A systematic analysis

In current work, various concentrations (0.0 wt%, 0.10 wt%, 0.25 wt%, 0.50 %,0.75 wt%, and 1 wt%) of Molybdenum (Mo) - doped cobalt ferrite (CFO) nanoparticles (Mo:CFO NPs) were synthesized using the flash combustion approach. The structural analysis of the prepared Mo:CFO was examined by the XRD patterns, and the obtained crystallite size 48.64, 46.72, 22.81, 21.05, 18.03, and 19.31 nm for 0.0 wt%, 0.10 wt%, 0.25 wt%, 0.50 wt%, 0.75 wt%, and 1 wt% Mo:CFO NPs, respectively. The presence of stoichiometry and homogeneity of the prepared Mo:CFO NPs was confirmed by the EDX analysis. The five phonon modes of the prepared Mo:CFO NPs were recorded by FT-Raman spectra, and the phonon modes were observed around 220, 312, 479, 624, and 685 cm−1 that corresponded o T2g(2), Eg, T2g(1), A1g(2), and A1g(1) symmetries, respectively. The grain sizes of the pure CFO and Mo:CFO NPs were evaluated using the images of scanning electron microscopy (SEM) and obtained in the range of 39–61 nm, respectively. The presence of valence states Co (2p), Fe (2p), O (1 s), and Mo (3d) in the prepared 1 wt% Mo:CFO NPs were examined XPS spectra. The particle sizes ~26.4 nm and ~ 16.7 nm were obtained for pure CFO and 1 wt% Mo:CFO NPs using lognormal function fitting. The emission peaks at 445 ± 3, 521 ± 3, and 620 ± 2 nm in the PL spectra were observed by PL spectroscopy. The decrease in saturation magnetization Ms. (70.80–66.54 emu/g) and reduced remanent magnetization Mr. (24.22–18.64 emu/g) of prepared Mo: CFO NPs was observed in the MH analysis by SQUID analysis. The electrochemical study of Mo: CFO NPs (0.0 %, 0.25 %, 0.50 %, and 1.0 %) was done in a three-electrode assembly cell. The capacitance of values 650.0 Fg−1, 800.0 Fg-1, and 810.0 Fg−1 for pure CFO, 0.25 % Mo: CFO, and 0.50 % Mo: CFO were recorded in electrochemical analysis. The highest capacitance of 840.0 Fg−1 was observed for the electrode with 1.0 % Mo: CFO NPs. It was analyzed that the increase in CFO electrodes enhances their performance, and therefore, it can be utilized for multifunctional devices.

Upconversion of phonon modes into microwave photons in a lithium niobate bulk acoustic wave resonator coupled to a microwave cavity

Upconversion of phonon modes into microwave photons in a lithium niobate bulk acoustic wave resonator coupled to a microwave cavity

Phonons reveal coupled cholesterol-lipid dynamics in ternary membranes

Experimental studies of collective dynamics in lipid bilayers have been challenging due to the energy resolution required to observe these low-energy phonon-like modes. However, inelastic X-ray scattering (IXS) measurements — a technique for probing vibrations in soft and biological materials — are now possible with sub-meV resolution, permitting direct observation of low energy, phonon-like modes in lipid membranes. Here, IXS measurements with sub-meV energy resolution reveal a low-energy optic-like phonon mode at roughly 3 meV in the liquid-ordered (Lo) and liquid-disordered (Ld) phases of a ternary lipid mixture. This mode is only observed experimentally at momentum transfers greater than 5 nm−1 in the (Lo) system. A similar gapped mode is also observed in all-atom molecular dynamics (MD) simulations of the same mixture, indicating that the simulations accurately represnt the fast, collective dynamics in the (Lo) phase. It’s optical nature and the Q range of the gap together suggest that the observed mode is due to the coupled motion of cholesterol-lipid pairs, separated by several hydrocarbon chains within the membrane plane. Analysis of the simulations provide molecular insight into the origin of the mode in transient, nanoscale substructures of hexagonally packed hydrocarbon chains. This nanoscale hexagonal packing was previously reported based on molecular dynamics simulations and later by NMR measurements. Here, however, the integration of IXS and MD simulations identifies a new signature of the L° substructure in the collective lipid dynamics, thanks to the recent confluence of IXS sensitivity and MD simulation capabilities.

Phonon modes and electron-phonon coupling at the FeSe/SrTiO3 interface.

The remarkable increase in superconducting transition temperature (Tc) observed at the interface of one-unit-cell FeSe films on SrTiO3 substrates (1 uc FeSe/STO)1 has attracted considerable research into the interface effects2-6. Although this high Tc is thought to be associated with electron-phonon coupling (EPC)2, the microscopic coupling mechanism and its role in the superconductivity remain elusive. Here we use momentum-selective high-resolution electron energy loss spectroscopy to atomically resolve the phonons at the FeSe/STO interface. We uncover new optical phonon modes, coupling strongly with electrons, in the energy range of 75-99 meV. These modes are characterized by out-of-plane vibrations of oxygen atoms in the interfacial double-TiOx layer and the apical oxygens in STO. Our results also demonstrate that the EPC strength and superconducting gap of 1 uc FeSe/STO are closely related to the interlayer spacing between FeSe and the TiOx terminated STO. These findings shed light on the microscopic origin of the interfacial EPC and provide insights into achieving large and consistent Tc enhancement in FeSe/STO and potentially other superconducting systems.

Superorders and terahertz acoustic modes in multiferroic BiFeO3/LaFeO3 superlattices

Superlattices are materials created by the alternating growth of two chemically different materials. The direct consequence of creating a superlattice is the folding of the Brillouin zone, which gives rise to additional electronic bands and phonon modes. This phenomenon has been successfully exploited to achieve new transport and optical properties in semiconductor superlattices. Here, we show that multiferroic BiFeO3/LaFeO3 superlattices exhibit several structural orders parallel and perpendicular to the growth direction, not existing in individual bulk materials. Using transmission electron microscopy, x-ray diffraction, and first-principles calculations, we reveal in particular a new long-range order of tilted FeO6 octahedra, with a period along the growth direction about twice that of the chemical supercell, i.e., a superorder. The effect of this new structural order on the phonon dynamics is studied with ultrafast optical pump-probe experiments. While a folded-mode at 1.2 THz is attributed solely to the chemical modulation of the superlattice, the existence of another 0.7 THz mode seems to be explained only by a double Brillouin zone folding in agreement with the structural out-of-plane superorder. Our work shows that multiferroic BiFeO3/LaFeO3 superlattices can be used to tune the spectrum of coherent THz phonons, and potentially that of magnons or electromagnons.

Ultralow thermal conductivity in Si–Ge nanograin mixtures: A cost-effective granular material for thermoelectric applications

This paper presents a theoretical study of the thermal conductivity of Si–Ge nanograin mixtures using a multiscale computational methodology based on solving the Boltzmann transport equation for phonons with first-principles techniques. A size-dependent correction factor is developed to account for the spatial dependence of the phonon distribution function on nanograin size, with parameters derived from the phonon properties of infinite Si and Ge crystals. This approach makes it possible to accurately calculate the thermal conductivity within a single nanograin, using force constants obtained from first-principles calculations. Thermal energy transport by phonons across grain boundaries is modeled by accounting for phonon transmission by two-phonon processes, weighting specular, and diffuse transmission for each phonon mode as a function of the root-mean-square roughness of the boundary relative to the phonon wavelength. The boundary thermal conductance model, previously validated against experimental data, is implemented using first-principles techniques. This approach excludes specular transmission for phonon modes with specific symmetries while ensuring conservation of the total number of modes in each symmetry class. The study examines the influence of grain size, nanograin mixture composition, temperature, and boundary asperities on the thermal conductivity of nanograin mixtures.

Localized interfacial phonon modes at the electronic axion domain wall

Localized interfacial phonon modes at the electronic axion domain wall

Crystalline electric field excitations and their nonlinear splitting under magnetic fields in YbOCl

Recently reported van der Waals layered honeycomb rare-earth chalcohalides REChX (RE = rare earth, Ch = chalcogen, and X = halogen) are considered to be promising Kitaev spin liquid (KSL) candidates. The high-quality single crystals of YbOCl, a representative member of the family with an effective spin of 1/2, are available now. The crystalline electric field (CEF) excitations in a rare-earth spin system are fundamentally important for understanding both finite-temperature and ground-state magnetism but remain unexplored in YbOCl so far. In this paper, we conduct a comprehensive Raman scattering study to unambiguously identify the CEF excitations in YbOCl and determine the CEF parameters and wave functions. Our Raman experiments further reveal the anomalous nonlinear CEF splitting under magnetic fields. We have grown single crystals of YbOCl, the nonmagnetic LuOCl, and the diluted magnetic Lu0.86Yb0.14OCl to make a completely comparative investigation. Polarized Raman spectra on the samples at 1.8 K allow us to clearly assign all the Raman-active phonon modes and explicitly identify the CEF excitations in YbOCl. The CEF excitations are further examined using temperature-dependent Raman measurements and careful symmetry analysis based on Raman tensors related to CEF excitations. By applying the CEF Hamiltonian to the experimentally determined CEF excitations, we extract the CEF parameters and eventually determine the CEF wave functions. The study experimentally pins down the CEF excitations in the Kitaev compound YbOCl and sets a foundation for understanding its finite-temperature magnetism and exploring the possible nontrivial spin ground state. Published by the American Physical Society 2024

Influence of Zinc Ion Concentration on the Structural, Surface Morphology and Optical Properties of Zinc Selenide Thin Films

ZnSe thin films were deposited on nonconducting glass substrates using different Zn[Formula: see text] ion concentrations. The films were deposited at 80[Formula: see text]C for 2.0[Formula: see text]h via photo-assisted chemical bath technique and annealed for 2.0[Formula: see text]h at 250[Formula: see text]C. X-ray diffraction revealed a hexagonal structure with preferred orientation along the (002) plane and the average crystallite size decreased from 10.5[Formula: see text]nm to 6.8[Formula: see text]nm with increased Zn[Formula: see text] ions. Raman spectra were used to confirm ZnSe phonon modes whose intensity increased with Zn[Formula: see text] ion concentrations although with fluctuation. Optical analysis showed higher absorbance and low transmittance in the visible region than near infrared making the thin films good materials for selective absorber surfaces. The band gap increased from 2.52[Formula: see text]eV to 2.78[Formula: see text]eV as the Zn[Formula: see text] ion concentration varied from 0.05% to 0.25%. The presence of the desired elements was confirmed by the EDS. Photoluminescence studies revealed three emission peaks which were all ascribed to defect state levels in ZnSe and all the samples emitted in reddish color according to CIE color chromaticity analysis. The selective absorption, wide band gap and broad emission properties suggest that the material is promising for optoelectronic applications.