Longitudinal Acoustic Phonons Research Articles

On thermal stability of Fröhlich polarons in bulk and nano-structured polar solids II: Diabatic ionization and depolarization

In Paper I of this bipartite article, the two-particle vibrational DOS functions for spatially confined longitudinal acoustic and longitudinal optic phonons are implemented at evaluation on Fröhlich Polarons (FP) decay probability via the “Klemens channel” and lifetime for FP states confined within InAs quantum dots. Here, an alternative model based on the idea of diabatic thermally activated excitation of the FP charge carrier and consecutive lattice “depolarization” is used for evaluations on the spectral dependence of the phononic and polaronic lifetime(s). Key parameters of the “depolarization” phenomenon are evaluated quantitatively using the “entirely quantum” generalized “displaced harmonic oscillator” framework, which is closely related in spirit to the so-called “Feynman polaron” approximation.

On thermal stability of Fröhlich polarons in bulk and nano-structured polar solids I: Klemens channel formalism and two-phonon DOS

The “Klemens channel” formalism is commonly used in interpretations of experimental results on the lifetime of longitudinal optic (LO) phonons and Fröhlich polarons (FPs) in polar bulk, polycrystalline, and nanostructured solids. Here, in Paper I of this bipartite article, the two-particle vibrational DOS function(s) for spatially confined longitudinal acoustic and LO phonons is implemented at evaluation on FP decay probability via the “Klemens channel” and its lifetime for the case of FPs confined within InAs quantum dots. This approach yields a convincing elucidation of the earlier experimental data. An alternative model based on the idea of diabatic thermally activated excitation of FP charge carriers and consecutive lattice “depolarization” will be discussed in detail and implemented at simulations on FP lifetime in Paper II [V. Ligatchev, “On thermal stability of Fröhlich polarons in bulk and nano-structured polar solids II: Diabatic ionization and depolarization,” AIP Adv. 14, (2024)].

Ab initio calculation of nonequilibrium quasiparticle-phonon dynamics in superconductors

Phonon-induced Cooper pair breaking, inciting nonequilibrium quasiparticle (QP) bursts, is known to deteriorate the performance of superconducting devices. However, a detailed understanding of QP-phonon dynamics is lacking due to the absence of a well-established theoretical framework. This paper presents a fully ab initio scheme of calculating nonequilibrium, polarization-dependent QP-phonon dynamics in superconductors. The authors find that with only an 8% deviation from the equilibrium phonon Bose–Einstein distribution, the resulting nonequilibrium QP population is 83 times larger than the equilibrium Fermi–Dirac distribution, and the longitudinal acoustic (LA) phonon polarization is most responsible for QP generation. The authors demonstrated that the qubit transition rate in Josephson junction-based transmon qubits is increased by orders of magnitude when taking these nonequilibrium distributions into account, compared to equilibrium distributions. This framework allows an in-depth exploration of nonequilibrium QP-phonon dynamics in various Josephson-junction-based superconducting devices. It paves the way for formulating advanced phonon shielding strategies to target the LA polarization, potentially leading to enhanced device performance, such as increased coherence time of transmon qubits or reduced thermal noise in cryogenics.

On the importance of Akhiezer damping to thermal conductivity in silicon at elevated temperatures above 300 K

Recently, Chiloyan et al. [Appl. Phys. Lett. 116, 163102 (2020)] have reported that phonon transport could exceed bulk heat conduction if low-frequency phonons with long mean free path (MFP) remain in the nonthermal regime in silicon. To gain a better understanding of their findings, we investigated the effects of temperature-induced anharmonicity on both Landau–Rumer damping and Akhiezer damping, including polarization. To do this, we follow a rigorous procedure for calculating the Akhiezer model and use phonon kinetic theory based on the Boltzmann transport equation. Consequently, we find that in the Akhiezer regime, the longitudinal acoustic phonon modes (LA) are strongly suppressed by phonon anharmonicity compared to the transverse acoustic phonon modes. Therefore, the low-frequency phonons with a long MFP of LA can help to exceed bulk heat conduction if they remain in the regime of nonthermal phonon transport where there are no appreciable scatterings with other phonons. It is also shown that Akhiezer damping eliminates thermal conductivity by 16.8% at 500 K, which is higher than the observed reduction (12.6%) at 300 K in silicon, uncovering a novel regime where the Akhiezer damping, previously deemed insignificant in the thermal conduction of bulk silicon, becomes crucial.

Ab initioinvestigation of the lattice dynamics and thermophysical properties of BCC vanadium and niobium.

In the present work, we have performed the phonon dispersion calculations of body-centered cubic vanadium (V) and niobium (Nb) with the supercell approach using different supercell size. Using DFT method, the calculated phonon spectra of V and Nb are found to be in a good agreement with the available experimental data. Our calculated results show a 'dip'-like feature in the longitudinal acoustic phonon mode along the Γ-H high symmetric path for both transition metals in the case of supercell size4×4×4. However, in supercell size2×2×2and3×3×3, the 'dip'-like feature is not clearly visible. In addition to this, thermodynamical properties are also computed, which compare well with the experimental data. Apart from this, the phonon lifetime due to electron-phonon interactions (τephph) and phonon-phonon interactions (PPIs) (τphph) are calculated. The effect of PPIs is studied by computing the average phonon lifetime for all acoustic branches. The value ofτephphof V (Nb) is found to be 23.16 (24.70)×10-15s at 100 K, which gets decreased to 1.51 (1.85)×10-15s at 1000 K. Theτphphof V (Nb) is found to be 8.59 (18.09)×10-12and 0.83 (1.76)×10-12s at 100 and 1000 K, respectively. Nextly, the lattice thermal conductivity is computed using linearized phonon Boltzmann equation. The present work suggests that studying the variation of phonon dispersion with supercell size is crucial for understanding the phonon properties of solids accurately.

High-throughput screening of the transport behavior of tetragonal perovskites.

Halide perovskites have attracted attention due to their low cost and excellent optoelectronic properties. Although their optical properties gained widespread consensus, there was still divergence in understanding carrier transport behavior. In this study, the mobility of tetragonal perovskites was investigated by empirical models, including longitudinal acoustic phonon (LAP) and polar optical phonon (POP) models. The results revealed that the mobility predicted from the LAP model was much higher than that from the POP model. A longitudinal optical phonon (LOP) was considered as the decisive scattering source for charge carriers in perovskites. Furthermore, the mobility was extremely sensitive to z-axis strain, and 8 types of perovskites with high carrier mobility were screened. Using the experimental lattice constants, the predicted mobility of CsSnI3 was μe,z = 1428 and μh,z = 2310 cm2 V-1 s-1, respectively. The tetragonal CsSnI3 has high mobility and moderate bandgaps, suggesting potential applications in high-efficiency solar cells.

Computational approach to enhance thermoelectric performance of Ag2Se by S and Te substitutions.

Designing an n-type thermoelectric material with a high thermoelectric figure of merit at near room temperature is extremely challenging. Generally, pristine Ag2Se reveals unusually low thermal conductivity along with a high electrical conductivity and Seebeck coefficient, which leads to high thermoelectric performance (n-type) at room temperature. Herein, we report a pseudo-ternary phase (Ag2Se0.5Te0.25S0.25) that exhibits significantly high thermoelectric performance (zT ∼ 2.1) even at 400 K. First-principles calculation reveals that the Rashba type of spin-dependent band spitting, which originates due to sulfur and tellurium substitution, helps to improve the thermopower magnitude. We also show that the intrinsic carrier mobility is not only controlled by the carrier effective mass but is substantially limited by longitudinal acoustic and optical phonon modes, which is an extension of the deformation potential theory. Locally off-center sulfur atoms, together with the increase in configurational entropy via substitution of Te and S atoms in Ag2Se, lead to a drastic reduction in the lattice thermal conductivity (klat ∼ 0.34 W m-1 K-1 at 400 K). The Rashba effect coupled with the configurational entropy synergistically results in a high thermoelectric figure of merit in the n-type thermoelectric material working in the near-room-temperature regime.

Thermal transport in a defective pillared graphene network: insights from equilibrium molecular dynamics simulation.

Graphene-based hybrid nanostructures have great potential to be ideal candidates for developing tailored thermal transport materials. In this study, we perform equilibrium molecular dynamics simulations employing the Green-Kubo method to investigate the influence of topological defects in three-dimensional pillared graphene networks. Similar to single-layer graphene and carbon nanotubes, the thermal conductivity (k) of pillared graphene systems exhibits a strong correlation with the system size (L), following a power-law relation k ∼ Lα, where α ranges from 0.12 to 0.15. Our results indicate that the vacancy defects significantly reduce thermal conductivity in pillared graphene systems compared to Stone-Wales defects. We observe that, beyond defect concentration, the location of the defects also plays a crucial role in determining thermal conductivity. We further analyze the phonon vibrational spectrum and the phonon participation ratio to obtain more insight into the thermal transport in the defective pillared graphene network. In most scenarios, longitudinal and flexural acoustic phonons experience significant localization within the 15-45 THz frequency range in the defective pillared graphene system.

Improved data analysis for molecular dynamics in liquid CCl4

Previously reported inelastic x-ray scattering spectra of a typical van der Waals molecular liquid CCl4 were reanalyzed using a generalized Langevin formalism with a memory function including a thermal and two viscoelastic relaxation processes and simple sparse modeling. The obtained excitations of longitudinal acoustic phonons show a largely positive deviation from the hydrodynamic value by about 57%, much larger than about 37% by the previously reported damped harmonic oscillator result. Such large values of fast sounds in molecular liquids larger than 15-20% of typical liquid metals are interpreted as extra energy losses for terahertz phonons by molecules' vibrational and rotational motions. The rates of the fast and slow viscoelastic relaxations in the memory function at low Q indicate large values, about 0.5 and 2 ps, which correspond to the vibrational and rotational motions of CCl4 molecules, respectively. These values are larger than those of the typical polar molecular liquid acetone, which may reflect the heavier atomic mass of CCl4. The Q dependences of the viscoelastic relaxation rates are discussed in terms of the lifetime and propagating length of the terahertz phonon oscillations. The microscopic kinematic longitudinal viscosity was obtained from the viscoelastic relaxation magnitudes and times, rapidly decreasing with Q from a macroscopic value at Q→0.

Break of thermal equilibrium between optical and acoustic phonon branches of CsPbI3 under continuous-wave light excitation and cryogenic temperature

The kinetic of low-temperature carrier and lattice of lead-halide perovskite is yet to be fully understood. In this work, we investigate the steady-state photoluminescences (PLs) of CsPbI3 at the environmental temperature (Te) ranging from 20 K to 300 K, and observed anomalous behaviors at cryogenic temperatures: The carrier temperature (Tc) of pure CsPbI3 exhibits a negative correlation with Te, accompanied by an expansion in Urbach tails of absorption spectra (Abs.) and excessive red-shifts at peak energy of PLs. These phenomena are also observed in those samples containing a certain amount of Cs4PbI6, but to a lesser extent and occurs at lower temperatures. It is attributed to the intensified hot phonon bottleneck effect (HPB) in CsPbI3 at cryogenic Te, which hinders the energy transfer from hot carriers, via longitudinal optics (LO) phonons to longitudinal acoustic (LA) phonons, to the ambient. For samples under continuous-wave laser excitation, in specific, the barrier induced by the enhanced HPB at low Te prevents the effective thermalization among carriers, LO and LA phonons, which, therefore, form thermally isolated ensembles with different temperatures. At cryogenic Te range, the elevated temperatures of carrier and LO phonon expand the high-energy side of PLs and the low-energy tail of Abs., respectively. For those samples in which the CsPbI3 is mixed with Cs4PbI6, the interfacial LO-LO interaction across them provides a bypass for heat dissipation, mitigating the heat accumulation in LO-phonons of CsPbI3. The results suggest that a strong HPB effect may break the thermal equilibrium among different branches of phonons in the lattice under certain extreme conditions.

Thermal transport in H-terminated ultrathin [110] Si nanowires: a first principles study

Phonon properties of ultrathin Si nanowires in [110] direction have been calculated by density functional perturbation theory. Several samples with varying diameters ranging from 0.38 to 1.5 nm have been investigated. It is found that the frequencies of optical phonons at the zone center increase with decrease in size of the nanowire, giving rise to avoided-crossing with longitudinal acoustic phonons. This feature determines a corresponding increase in the scattering rates and flattening of the longitudinal acoustic mode. More specifically, a remarkable change in scattering rates is shown for decreasing diameter. Results of the thermal conductivity are much lower with respect to bulk Si and are found between 40 and 119 hbox {Wm}^{-1}hbox {K}^{-1}, also providing evidence of increasing thermal conductivity with increase in diameter. This effect is attributed to several changes in the phonon dispersions. Finally, it is shown that approximating the boundary scattering of phonons by Casimir scattering leads to a severe underestimation of the thermal conductivity in these systems.

Study of Elastic and Structural Properties of BaFe2As2 Ultrathin Film Using Picosecond Ultrasonics.

We obtain the through-thickness elastic stiffness coefficient (C33) in nominal 9 nm and 60 nm BaFe2As2 (Ba-122) thin films by using picosecond ultrasonics. Particularly, we reveal the increase in elastic stiffness as film thickness decreases from bulk value down to 9 nm, which we attribute to the increase in intrinsic strain near the film-substrate interface. Our density functional theory (DFT) calculations reproduce the observed acoustic oscillation frequencies well. In addition, temperature dependence of longitudinal acoustic (LA) phonon mode frequency for 9 nm Ba-122 thin film is reported. The frequency change is attributed to the change in Ba-122 orthorhombicity (a-b)/(a+b). This conclusion can be corroborated by our previous ultrafast ellipticity measurements in 9 nm Ba-122 thin film, which exhibit strong temperature dependence and indicate the structural phase transition temperature Ts.

Simulation of ultrafast electron diffraction intensity under coherent acoustic phonons.

Ultrafast electron diffraction has been proven to be a powerful tool for the study of coherent acoustic phonons owing to its high sensitivity to crystal structures. However, this sensitivity leads to complicated behavior of the diffraction intensity, which complicates the analysis process of phonons, especially higher harmonics. Here, we theoretically analyze the effects of photoinduced coherent transverse and longitudinal acoustic phonons on electron diffraction to provide a guide for the exploitation and modulation of coherent phonons. The simulation of the electron diffraction was performed in 30-nm films with different optical penetration depths based on the atomic displacements obtained by solving the wave equation. The simulation results exhibit a complex relationship between the frequencies of the phonons and diffraction signals, which highly depends on the laser penetration depth, sample thickness, and temporal stress distribution. In addition, an intensity decomposition method is proposed to account for the in-phase oscillation and high harmonics caused by inhomogeneous excitation. These results can provide new perspectives and insights for a comprehensive and accurate understanding of the lattice response under coherent phonons.

Phonon-mediated magneto-resonances in biased graphene layers

We explore the non-equilibrium transport regime in graphene using a large dc current in combination with a perpendicular magnetic field. The strong in-plane Hall field generated in the graphene channel results in Landau levels that are tilted spatially. The energy of cyclotron orbits in the bulk varies as a function of the spatial position of the guiding center, enabling us to observe a series of compelling features. While Shubnikov–de Haas oscillations are predictably suppressed in the presence of the Hall field, a set of fresh magneto resistance oscillations emerge near the charge neutrality point as a function of dc current. Two branches of oscillations with linear dispersions are evident as we vary carrier density and dc current, the velocities of which closely resemble the transverse acoustic (TA) and longitudinal acoustic (LA) phonon modes, suggestive of phonon-assisted intra-Landau level transitions between adjacent cyclotron orbits. Our results offer unique possibilities to explore non-equilibrium phenomena in two-dimensional materials and van der Waals heterostructures.

Identification by inelastic X-ray scattering of bulk alteration of solid dynamics due to liquid wetting

We examine the influence at room temperature of the deposit of a water layer on the phonon dynamics of a solid. It is shown that the water wetting at the surface of an Alumina monocrystal has deep effects on acoustic phonons, propagating over several hundred µm distance and taking place on a relatively long time scale. The effect of the wetting at the boundary is two-fold: a hardening of both transverse and longitudinal acoustic phonons is observed as well as a relaxation of internal stresses. These acoustic phonon energy changes were observed by inelastic X-ray scattering up to 40 meV energy loss, allowing us to probe the solid at different depths from the surface.

Optical and acoustic phonons in turbostratic and cubic boron nitride thin films on diamond substrates

We report an investigation of the bulk optical, bulk acoustic, and surface acoustic phonons in thin films of turbostratic boron nitride (t-BN) and cubic boron nitride (c-BN) grown on B-doped polycrystalline and single-crystalline diamond (001) and (111) substrates. The characteristics of different types of phonons were studied using Raman and Brillouin-Mandelstam light scattering spectroscopies. The atomic structure of the films was determined using high-resolution transmission electron microscopy (HRTEM) and correlated with the Raman and Brillouin-Mandelstam spectroscopy data. The HRTEM analysis revealed that the cubic boron nitride thin films consisted of a mixture of c-BN and t-BN phases, with c-BN being the dominant phase. It was found that while visible Raman spectroscopy provided information for characterizing the t-BN phase, it faced challenges in differentiating the c-BN phase either due to the presence of high-density defects or the overlapping of the Raman features with those from the B-doped diamond substrates. In contrast, Brillouin-Mandelstam spectroscopy clearly distinguishes the bulk longitudinal and surface acoustic phonons of the c-BN thin films grown on diamond substrates. Additionally, the angle-dependent surface Brillouin-Mandelstam scattering data show the peaks associated with the Rayleigh surface acoustic waves, which have higher phase velocities in c-BN films on diamond (111) substrates. These findings provide valuable insights into the phonon characteristics of the c-BN and diamond interfaces and have important implications for the thermal management of electronic devices based on ultra-wide-band-gap materials.

A possible origin of the very fast sound in terahertz region in liquid benzene

Previously reported inelastic X-ray scattering spectra of molecular liquid benzene were reanalyzed by a generalized Langevin formalism (GLF) with a memory function including a thermal and two viscoelastic relaxation processes. A new algorithm was containing a simple sparse modeling to smooth Q dependences of dynamical parameters used in the present GLF analysis. The obtained excitation energies of longitudinal acoustic phonons show a largely positive deviation from the hydrodynamic value by about 50%, the same as that by previous analysis and similar to existing results of several molecular liquids. Such a large deviation of dynamical sound velocity in liquid benzene is interpreted as extra energy-losses for terahertz phonons by vibrational and rotational motions of benzene molecules. The rates of the fast and slow viscoelastic relaxations in the memory function at low Q are undetectable and 0.4-0.8 ps, respectively. These can be explained by the vibrational and anisotropic rotational motions of benzene molecules. The microscopic kinematic longitudinal viscosity rapidly decreases with Q from a reasonable macroscopic value.

Laser-induced coherent longitudinal acoustics phonons in thin films observed by ultrafast optical reflectivity and ultrafast x-ray diffraction

Femtosecond laser excitation of crystal materials can produce coherent longitudinal acoustic phonons (CLAPs), which possess the capability to interact with various quasiparticles and influence their dynamics. The manipulation of CLAPs' behavior is thus of significant interest for potential applications, particularly in achieving ultrafast modulations of material properties. In this study, we present our findings on the propagation of laser-induced CLAPs at thin-film interfaces and heterojunctions using ultrafast optical reflectivity and ultrafast x-ray diffraction measurements. We observe that CLAPs can efficiently propagate from a LaMnO3 thin-film to its SrTiO3 substrate due to the matching of their acoustic impedance, and the oscillation period increases from 54 to 105 GHz. In contrast, in ultrafast x-ray diffraction experiments, we discover that CLAPs are partially confined within an Au (111) thin film due to the mismatch of acoustic impedance with the substrates, leading to an oscillation period of 122 ps. However, interestingly, when examining La0.7Ca0.175Sr0.125MnO3/Ba0.5Sr0.5TiO3 bilayers, no oscillations are observed due to the favorable impedance matching between the layers. Our findings demonstrate that acoustic impedance can serve as an effective means to control coherent phonons in nanometer-thin films and may also play a crucial role in phonon engineering at interfaces or heterostructures.

Coherent-Phonon-Driven Intervalley Scattering and Rabi Oscillation in Multivalley 2D Materials.

Resolving the complete electron scattering dynamics mediated by coherent phonons is crucial for understanding electron-phonon couplings beyond equilibrium. Here we present a time-resolved theoretical investigation on strongly coupled ultrafast electron and phonon dynamics in monolayer WSe_{2}, with a focus on the intervalley scattering from the optically "bright" K state to "dark" Q state. We find that the strong coherent lattice vibration along the longitudinal acoustic phonon mode [LA(M)] can drastically promote K-to-Q transition on a timescale of ∼400 fs, comparable with previous experimental observation on thermal-phonon-mediated electron dynamics. Further, this coherent-phonon-driven intervalley scattering occurs in an unconventional steplike manner and further induces an electronic Rabi oscillation. By constructing a two-level model and quantitatively comparing with abinitio dynamic simulations, we uncover the critical role of nonadiabatic coupling effects. Finally, a new strategy is proposed to effectively tune the intervalley scattering rates by varying the coherent phonon amplitude, which could be realized via light-induced nonlinear phononics that we hope will spark experimental investigation.

Two-photon excitation with finite pulses unlocks pure dephasing-induced degradation of entangled photons emitted by quantum dots

Semiconductor quantum dots have emerged as an especially promising platform for the generation of polarization-entangled photon pairs. However, it was demonstrated recently that the two-photon excitation scheme employed in state-of-the-art experiments limits the achievable degree of entanglement by introducing which-path information. In this work, the combined impact of two-photon excitation and longitudinal acoustic phonons on photon pairs emitted by strongly-confining quantum dots is investigated. It is found that phonons further reduce the achievable degree of entanglement even in the limit of vanishing temperature due to phonon-induced pure dephasing and phonon-assisted one-photon processes, which increase the reexcitation probability. In addition, the degree of entanglement, as measured by the concurrence, decreases with rising temperature and/or pulse duration, even if the excitonic fine-structure splitting is absent and when higher electronic states are out of reach. Furthermore, in the case of finite fine-structure splittings, phonons enlarge the discrepancy in concurrence for different laser polarizations.