Phonon Decay Research Articles

Temperature dependence of Raman spectra and first principles study of NiMn2O4 magnetic spinel oxide thin films. Application in efficient photocatalytic removal of RhB and MB dyes.

Magnetic spinel oxide NiMn2O4 thin films were grown on glass substrates using the spray pyrolysis deposition technique at 350 °C. First, structural optimization, electronic structure and vibrational properties were obtained for NiMn2O4 by density functional theory (DFT) with an all electron basis set and the hybrid functional B3LYP implemented in the CRYSTAL14 code. Our DFT results show good agreement with previous experimental and theoretical data. Second, Raman spectra of as-prepared thin films have been measured over the temperature range from 100 K to 620 K. We report on the temperature dependence of their dominant first-order scattering F2g and A1g phonons modes. The phonon dynamic behavior is well described by a model taking into account explicit lattice anharmonicity associated to both cubic and quartic phonon decay processes and thermal expansion related quasiharmonicity. Finally, the photocatalytic activities of the obtained thin films were evaluated through the photocatalytic degradation of Rhodamine B (RhB) and methylene blue (MB) solutions under solar light irradiation as a function of time. All the results indicate that NiMn2O4 thin film can be used as a potential solar-light-driven photocatalyst.

Phononic heat transport in molecular junctions: Quantum effects and vibrational mismatch.

Problems of heat transport are ubiquitous to various technologies such as power generation, cooling, electronics, and thermoelectrics. In this paper, we advocate for the application of the quantum self-consistent reservoir method, which is based on the generalized quantum Langevin equation, to study phononic thermal conduction in molecular junctions. The method emulates phonon-phonon scattering processes while taking into account quantum effects and far-from-equilibrium (large temperature difference) conditions. We test the applicability of the method by simulating the thermal conductance of molecular junctions with one-dimensional molecules sandwiched between solid surfaces. Our results satisfy the expected behavior of the thermal conductance in anharmonic chains as a function of length, phonon scattering rate, and temperature, thus validating the computational scheme. Moreover, we examine the effects of vibrational mismatch between the solids' phonon spectra on the heat transfer characteristics in molecular junctions. Here, we reveal the dual role of vibrational anharmonicity: It raises the resistance of the junction due to multiple scattering processes, yet it promotes energy transport across a vibrational mismatch by enabling phonon recombination and decay processes.

Raman scattering studies on very thin layers of gallium sulfide (GaS) as a function of sample thickness and temperature

Gallium sulfide is a semiconducting material with a layered structure and a characteristic low interlayer interaction. Because of weak van der Waals forces, GaS crystals are relatively easy to exfoliate to very thin layers. In this work nanometric-GaS layers were obtained by a micro-mechanical exfoliation process and were transferred to Si/SiO2 substrate. The thickness of these layers was estimated from AFM measurements. Raman spectra were collected for different layer thicknesses ranging from one layer to bulk crystal. An analytical function fitted to experimental data is proposed to determine layer thickness from Raman measurements. For the first time, the Raman position and the FWHM of the main Raman peaks were measured on very thin GaS layers as a function of temperature in the range from 80 to 470 K. The first order temperature coefficients of the A1g Raman peaks were determined. Phonon decay due to anharmonic processes at temperatures above 300 K in layers of thickness below 4 nm was observed. Contribution of optical phonon scattering processes to thermal properties of very thin GaS layers is discussed.

Phonon anharmonicity in single-crystalline SnSe

Understanding the temperature-dependent behavior of phonons that affects the thermal transport properties is crucial for developing efficient thermoelectric materials. Previously, SnSe was reported to exhibit a record high thermoelectric performance above 900 K, which was debated in subsequent reports. Nonetheless, there is a growing consensus that anharmonicity is one of the contributing factors which yields ultralow thermal conductivity in SnSe. While there is experimental and theoretical evidence for anharmonicity in SnSe, its effect on the entire phonon band structure, which is responsible for the thermal transport, is investigated here. We performed a combined temperature-dependent polarized Raman spectroscopic and heat-capacity study on fully dense single-crystalline SnSe, which revealed that the anharmonicity is driven by soft optical modes in the $b\text{\ensuremath{-}}c$ plane. These modes exhibited strong high-temperature broadening, indicating ultrashort phonon lifetimes and high scattering rates. Analysis of the Raman peak frequencies and linewidths revealed phonon decay to be dominated by a three-phonon scattering process. Consistent with this observation, the analysis of our temperature-dependent heat-capacity data also revealed the presence of strong anharmonicity in SnSe. The anharmonic coefficients ${\ensuremath{\alpha}}_{R}$ and ${\ensuremath{\alpha}}_{C}$ calculated from the Raman and heat-capacity measurements, respectively, are in excellent agreement with each other. This study provides a deeper understanding of the role of phonon-phonon scattering and anharmonicity leading to outstanding thermal transport properties of SnSe.

Terahertz probe of photoexcited carrier dynamics in the Dirac semimetal Cd3As2

The relaxation dynamics of photoexcited quasiparticles of three-dimensional (3D) Dirac semimetals are vital towards their application in high performance electronic and optoelectronic devices. In this work, the relaxation dynamics of photoexcited carriers of 3D Dirac semimetal Cd3As2 are investigated by transient terahertz spectroscopy. The visible pump-THz probe spectroscopy measurement shows clear biexponential decays with two characteristic time constants. According to the pump-power and temperature dependence, these two characteristic time constants are attributed to the electron phonon coupling (1-4 ps) and anharmonic decay of hot coupled phonons to electronic uncoupled phonons (2-9 ps), respectively. An anomalous electron-optical phonon coupling reduction and a bottleneck slowing of hot optical phonons relaxation are observed with higher excitation intensities similar to that in graphene. On the other hand, the electron-optical phonon coupling can be enhanced due to the phonon frequency broadening and softening at elevated lattice temperature. Furthermore, the transient THz spectrum response is strongly modified by the phonon assisted intraband absorption of hot carriers from a pure electronic Drude model, which is evidenced by a characteristic THz absorption dip in the transient THz absorption spectrum. This absorption dip is pinned by the discrete optical phonon energy that assists the intraband transition enabled by photoexcitation of hot carriers.

Effect of quantum confinement on lifetime of anharmonic decay of optical phonons in semiconductor nanostructures

The anharmonic decay of phonons underlies many important effects in semiconductors, e.g. hotspot formation, phonon bottleneck and thermal resistivity. In this article, we evaluate the effect of quantum confinement on the anharmonic decay transition probability in a cubic isotropic material. This article focuses on the anharmonic decay of longitudinal optical phonons, generated from hot electrons, are directly related to formation of hotspots in the active region of semiconductor devices. The confinement effect has been realized in double interface heterostructure quantum well (DHSQW) (e.g. AlAs/GaAs/AlAs) and free-standing quantum well (FSQW) (e.g. GaAs) structures as the confined phonon modes have different properties inside the structures. The longitudinal–optical phonon decay rate is reduced for the case of a DHSQW compared to bulk material and for a FSQW the decay rate has a strong dependence on wavevector value of the three phonons involved.

Effect of optical phonons on the charge qubit dynamics in a semiconductor microcavity

We consider the possibility of using optical vibrational degrees of freedom as an additional resource for controlling the evolution of a charge qubit based on a semiconductor single-electron double quantum dot. The effect of electron – photon – phonon processes on the accuracy of single-qubit operations is investigated. It is shown that, depending on the phonon decay rate and detuning of the electronic transition frequency from the optical phonon frequency, these processes can both facilitate and prevent coherent electron transfer between quantum dots.

Quenching of Infrared-Active Optical Phonons in Nanolayers of Crystalline Materials by Graphene Surface Plasmons

Optical phonons are fundamental excitations in many solid-state materials and have crucial influences on numerous material properties. Therefore, achieving extrinsic control of optical phonon properties, such as the phonon frequency, lifetime and population, may lead to new ways of tailoring various material properties relevant to key technological applications. Here, we experimentally demonstrate that infrared-active optical phonons in thin (tens of nm) layers of crystalline materials such as III–V semiconductors can be significantly quenched by a monolayer graphene. The optical phonon quenching effect is attributed to the ultrafast decay of optical phonons into resonant graphene surface plasmons at a rate which is significantly higher than the intrinsic decay rate of optical phonons due to lattice anharmonicity. Our results point to a new approach to engineering optical phonon properties and potentially other related material properties. Such an approach can be applied to a wide range of materials with ...

Anharmonic phonon decay in polycrystalline CdTe thin film

The anharmonic decay of both the longitudinal optical phonon (LO) and its overtone (2LO) was found to decay asymmetrically into a transverse optical (TO) and a transverse acoustic (TA) phonon, both of which are at the L point along the Γ-L direction of the Brillouin zone. For the LO and its overtone 2LO, both the Raman shift and Raman linewidth were decreased/increased almost linearly with the temperature in the range of 78–523 K. This temperature-dependent phonon decay characteristics were induced by LO anharmonic decay to the TA phonon with an energy of only ∼29 cm−1. A TA phonon mode with such low energy is readily excited, and its phonon density is almost linearly increased with increased temperature. Strong multi-phonon scatterings, which involved the LO, the surface optical mode, and the TO, were funded to contribute to the anharmonic decay of the 1LO, especially at temperature higher than room temperature.

Coupling of phonons with orbital dynamics and magnetism in CuSb2O6

Strongly interacting phonons and orbital excitations are observed in the same energy range for CuSb$_2$O$_6$, unlocking a so-far unexplored type of electron-phonon interaction. An orbital wave at $\sim 550$ cm$^{-1}$ softens on warming and strongly interferes with a phonon at $\sim 500$ cm$^{-1}$, giving rise to a merged excitation of mixed character. An electronic continuum grows on warming to the orbital ordering temperature $T_{OO}$=400 K, generating an important phonon decay channel. This direct and simultaneous observation of orbital and vibrational excitations reveals details of their combined dynamics. In addition, phonon frequency anomalies due to magnetic correlations are observed below $\sim 150$ K, much above the three-dimensional magnetic ordering temperature $T_N^{3D}=8.5$ K, confirming one-dimensional magnetic correlations along Cu-O-O-Cu linear chains in the paramagnetic state.

Isotopic effects on phonon anharmonicity in layered van der Waals crystals: Isotopically pure hexagonal boron nitride

Hexagonal boron nitride (h-BN) is a layered crystal that is attracting a great deal of attention as a promising material for nanophotonic applications. The strong optical anisotropy of this crystal is key to exploit polaritonic modes for manipulating light-matter interactions in 2D materials. h-BN has also great potential for solid-state neutron detection and neutron imaging devices, given the exceptionally high thermal neutron capture cross section of the boron-10 isotope. A good knowledge of phonons in layered crystals is essential for harnessing long-lived phonon-polariton modes for nanophotonic applications and may prove valuable for developing solid-state 10BN neutron detectors with improved device architectures and higher detection efficiencies. Although phonons in graphene and isoelectronic materials with a similar hexagonal layer structure have been studied, the effect of isotopic substitution on the phonons of such lamellar compounds has not been addressed yet. Here we present a Raman scattering study of the in-plane high-energy Raman active mode on isotopically enriched single-crystal h-BN. Phonon frequency and lifetime are measured in the 80–600-K temperature range for 10B-enriched, 11B-enriched, and natural composition high quality crystals. Their temperature dependence is explained in the light of perturbation theory calculations of the phonon self-energy. The effects of crystal anisotropy, isotopic disorder, and anharmonic phonon-decay channels are investigated in detail. The isotopic-induced changes in the phonon density of states are shown to enhance three-phonon anharmonic decay channels in 10B-enriched crystals, opening the possibility of isotope tuning of the anharmonic phonon decay processes.

Mutual interactions of phonons, rotons, and gravity

We introduce an effective point-particle action for generic particles living in a zero-temperature superfluid. This action describes the motion of the particles in the medium at equilibrium as well as their couplings to sound waves and generic fluid flows. While we place the emphasis on elementary excitations such as phonons and rotons, our formalism applies also to macroscopic objects such as vortex rings and rigid bodies interacting with long-wavelength fluid modes. Within our approach, we reproduce phonon decay and phonon-phonon scattering as predicted using a purely field-theoretic description of phonons. We also correct classic results by Landau and Khalatnikov on roton-phonon scattering. Finally, we discuss how phonons and rotons couple to gravity, and show that the former tend to float while the latter tend to sink but with rather peculiar trajectories. Our formalism can be easily extended to include (general) relativistic effects and couplings to additional matter fields. As such, it can be relevant in contexts as diverse as neutron star physics and light dark matter detection.

Probing Phonon Dynamics in Individual Single-Walled Carbon Nanotubes.

Interactions between elementary excitations, such as carriers, phonons, and plasmons, are critical for understanding the optical and electronic properties of materials. The significance of these interactions is more prominent in low-dimensional materials and can dominate their physical properties due to the enhanced interactions between these excitations. One-dimensional single-walled carbon nanotubes provide an ideal system for studying such interactions due to their perfect physical structures and rich electronic properties. Here we investigated G-mode phonon dynamics in individual suspended chirality-resolved single-walled carbon nanotubes by time-resolved anti-Stokes Raman spectroscopy. The improved technique allowed us to probe the intrinsic phonon information on a single-tube level and exclude the influences of tube-tube and tube-substrate interactions. We found that the G-mode phonon lifetime ranges from 0.75-2.25 ps and critically depends on whether the tube is metallic or semiconducting. In comparison with the phonon lifetimes in graphene and graphite, we revealed structure-dependent carrier-phonon and phonon-phonon interactions in nanotubes. Our results provide new information for optimizing the design of nanotube electronic/optoelectronic devices by better understanding and utilizing their phonon decay channels.

Relationship between the Fano asymmetry parameter and time-domain spectra studied by time-resolved measurement of carriers and phonons in p -type Si

We performed time-resolved reflectivity measurements in $p$-type Si to study the Fano asymmetry parameter as a function of time-domain parameters. We observed a fast decay of intervalence band transitions, and slow decay of optical phonons, which respectively correspond to the continuum and discrete states. The experimental results revealed that the initial phase of the discrete state and the ratio of the relative transition amplitude between the discrete and continuum states affect the Fano asymmetry parameter. We derived an equation for the Fano asymmetry parameter as a function of the time-domain parameters that explained the experimental results. The present study indicates how the Fano asymmetry parameter is controlled by the time-domain parameters.

Nonequilibrium dynamics of the phonon gas in ultrafast-excited antimony

The ultrafast relaxation dynamics of a nonequilibrium phonon gas towards thermal equilibrium involves many-body collisions that cannot be properly described by perturbative approaches. Here, we develop a nonperturbative method to elucidate the microscopic mechanisms underlying the decay of laser-excited coherent phonons in the presence of electron-hole pairs, which so far are not fully understood. Our theory relies on ab initio molecular dynamics simulations on laser-excited potential-energy surfaces. Those simulations are compared with runs in which the laser-excited coherent phonon is artificially deoccupied. We apply this method to antimony and show that the decay of the ${A}_{1g}$ phonon mode at low laser fluences can be accounted mainly to three-body down-conversion processes of an ${A}_{1g}$ phonon into acoustic phonons. For higher excitation strengths, however, we see a crossover to a four-phonon process, in which two ${A}_{1g}$ phonons decay into two optical phonons.

Hot carrier cooling mechanisms in halide perovskites

Halide perovskites exhibit unique slow hot-carrier cooling properties capable of unlocking disruptive perovskite photon–electron conversion technologies (e.g., high-efficiency hot-carrier photovoltaics, photo-catalysis, and photodetectors). Presently, the origins and mechanisms of this retardation remain highly contentious (e.g., large polarons, hot-phonon bottleneck, acoustical–optical phonon upconversion etc.). Here, we investigate the fluence-dependent hot-carrier dynamics in methylammonium lead triiodide using transient absorption spectroscopy, and correlate with theoretical modeling and first-principles calculations. At moderate carrier concentrations (around 1018 cm−3), carrier cooling is mediated by polar Fröhlich electron–phonon interactions through zone-center delayed longitudinal optical phonon emissions (i.e., with phonon lifetime τLO around 0.6 ± 0.1 ps) induced by the hot-phonon bottleneck. The hot-phonon effect arises from the suppression of the Klemens relaxation pathway essential for longitudinal optical phonon decay. At high carrier concentrations (around 1019 cm−3), Auger heating further reduces the cooling rates. Our study unravels the intricate interplay between the hot-phonon bottleneck and Auger heating effects on carrier cooling, which will resolve the existing controversy.

Parametric magnon–phonon instability in the structure of YIG films

The effects of parametric magnon–phonon instability and the self-modulation of magnetostatic and fast magnetoelastic waves are revealed. It is found that instabilities are caused by the decay of the initial waves stimulated by high-Q acoustic resonances. Decays of the first and the second order (three- and four-magnon–phonon decays) are observed. Their characteristics and existing conditions are determined. Magnetostatic wave decays have both upper and lower thresholds. It is shown that magnetoelastic waves become stable above the upper thresholds. The existence of the upper threshold is due to several competing decay scenarios.

Nanosecond long excited state lifetimes observed in hafnium nitride

Increasing photovoltaic conversion efficiency by overcoming the fundamental loss mechanisms due to hot carrier thermalisation remains one of the challenges in solar energy research. A large proportion of dynamic energy is lost due to thermalisation where hot carriers lose their energy to lattice vibrations within picoseconds. Extending hot carrier lifetimes will potentially improve the efficiency of photovoltaic devices. In this study, the bulk hafnium nitride film is proposed as a potential hot carrier absorber from its phononic properties. Hafnium nitride thin films were deposited by sputtering on quartz substrates. The films are polycrystalline as determined from X-ray diffraction and from X-ray photoelectron spectroscopy the chemical composition of the films are estimated to be HfN0.9. The absorption coefficient of the film exhibits strong amplitude below 500nm and free electron absorption at longer wavelengths. Transient absorption spectroscopy was used to investigate carrier dynamics in the visible and near-infrared spectral regions. For the first time we present evidence of up to nanosecond long lifetimes of photo-excited carriers observed in hafnium nitride. We propose that the long lived excited states are due to restricted phonon decay pathways in hafnium nitride. The long lived excited states in the bulk hafnium nitride film provide a simple route to devices designed to utilise hot carriers efficiently.

Suppression of supercollision carrier cooling in high mobility graphene on SiC( 0001¯ )

Graphene, a two-dimensional monatomic layer of crystal carbon, has recently emerged as a potential material for next-generation optoelectronic devices owing to unique properties arising from its massless Dirac fermions. However, the intrinsic carrier dynamics of graphene has remained a mystery even for the simple case of carrier cooling after the photoexcitation. This is because the observed temporal variations of the nonequilibrium carriers in graphene have been thoroughly described in terms of a defect-induced extrinsic effect known as ``supercollision'' (SC). The SC process is based on defect-mediated electron-acoustic phonon scattering and theoretically has been predicted to reduce with the increase of mobility of a material. Here, the authors have prepared extremely high mobility graphene and traced the dynamics of photoexcited carriers in the Dirac bands directly by time- and angle-resolved photoemission spectroscopy. They successfully observed suppression of SC and extracted the intrinsic dynamical properties of graphene, such as anharmonic decay of the optical phonons and the bottleneck relaxation at the Dirac point. Breaking the limit of SC, their research also has technological significance in developing graphene-based optoelectronic devices.

Scattering of carriers by coupled plasmon-phonon modes in bulk polar semiconductors and polar semiconductor heterostructures

We present a general treatment of carrier scattering by coupled phonon-plasmon collective modes in polar semiconductors, taking anharmonic phonon decay into account and self-consistently calculating carrier momentum relaxation rates and carrier mobility in a parabolic band model. We iteratively solve the weak-field Boltzmann equations for carriers and collective modes and obtain their nonequilibrium distribution functions. Both the scattering rates and the anharmonic decay of the coupled modes are expressed through the total dielectric function of the semiconductor, consisting of a damped lattice dielectric function, and a temperature dependent random phase approximation dielectric function for the carrier plasma. We show that the decay of the coupled modes has a significant effect on the contribution to the mobility limited by carrier-coupled mode scattering. We also propose a scalar quantity, the phonon dissipation weight factor, with which this effect can be estimated from an analytic expression. We apply this treatment to dynamically screened electron-longitudinal optical phonon scattering in bulk polar semiconductors, and to dynamically screened remote phonon scattering in polar heterostructures where monolayers of ${\mathrm{MoS}}_{2}$ are sandwiched between various polar dielectrics. We find that a dynamic treatment of the remote phonon scattering yields mobilities up to 75% higher than a static screening approximation does for structures which consist of a monolayer of ${\mathrm{MoS}}_{2}$ between hafnia and silica. Moreover, we show that accounting for the nonzero thickness of the ${\mathrm{MoS}}_{2}$ interface layer has an important effect on the calculated mobility in the same structure.