Acoustic Phonon Modes Research Articles

Strain-Induced Ultrahigh Electron Mobility and Thermoelectric Figure of Merit in Monolayer α-Te.

In line with the classic phonon-glass electron-crystal (PGEC) paradigm, semiconducting and semimetallic multinary compounds remain the cornerstone of the state-of-the-art thermoelectric materials. By contrast, elemental PGEC is very rare. In this work, we report a thermoelectric study of monolayer α-Te by first-principles calculations and solving the parameter-free Boltzmann transport equation. It is found that monolayer α-Te possesses high electron mobility (about 2500 cm2 V-1 s-1) at room temperature due to small effective mass, low phonon frequencies, and thus a restricted phase space for electron-phonon scattering. In monolayer α-Te, the electrons near the conduction band edge are mainly scattered by the heavily populated quadratically dispersing out-of-plane acoustic (ZA) phonon modes. The thermoelectric figure of merit (ZT) for n-type monolayer α-Te is 0.55 at 300 K and 1.46 at 700 K. Notably, tensile strain stiffens the ZA modes, yielding a linear energy-momentum dispersion relation and the removal of the diverging thermal population of ZA phonons. Consequently, the electron mobility is enhanced. At a 4% tensile strain, the electron mobility can reach up to 8000 cm2 V-1 s-1 at room temperature while the thermal conductivity is almost unaffected, yielding a state-of-the-art ZT value of 0.94 and 2.03 in n-type monolayer α-Te at 300 and 700 K, respectively. For completeness, the thermoelectric study of p-type monolayer α-Te is also conducted. These results beckon further experiments toward high-performance α-Te-based thermoelectric materials via doping, alloying, and compositing.

Liquidlike Cu atom diffusion in weakly ionic compounds Cu2S and Cu2Se

Copper sulphide (${\mathrm{Cu}}_{2}\mathrm{S}$) and copper selenide (${\mathrm{Cu}}_{2}\mathrm{Se}$) are known to exist in solid (S/Se)-liquid (Cu) hybrid phases which exhibit favorable thermoelectrics properties. The diffusion characteristics and its mechanism in these systems are therefore of significant interest. In this paper, we analyze these properties through examining the atomic radial distributions, mean-square displacements, and velocity autocorrelations obtained from ab initio molecular dynamics simulations. Exceptionally high Cu diffusion coefficients with values over ${10}^{\ensuremath{-}5}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}$/s are obtained, indicating the unexpected liquidlike behavior of Cu in these weakly ionic compounds. The diffusion mechanism obtained through analysis of the Cu atomic trajectories is found to be at variance with the previously proposed Chudley-Elliott jump diffusion model. In addition, tunability of these diffusion coefficients via small changes in the stoichiometry, namely, Cu deficiencies, is demonstrated. The higher number of low-frequency acoustic phonon modes associated with Se sublattices, compared to those with S sublattices, correlates well with the experimentally observed thermal conductivity difference between ${\mathrm{Cu}}_{2}\mathrm{S}$ and ${\mathrm{Cu}}_{2}\mathrm{Se}$.

Invoking Obscured Surface Optical, Acoustic, and Anharmonic Phonon Modes and Their Modulation by Synthesis Conditions and Arrangement of the Surface in 0D ZnS

For low-dimensional systems, symmetry breaking and surface reconstruction are critical for the observation of special vibrational properties of technical significance such as surface optical (SO), ...

Effect of biaxial strain on thermal transport in WS2 monolayer from first principles calculations

Abstract Recently, tungsten disulfide (WS2) monolayer has emerged as a two-dimensional material due to its outstanding physical properties. In this work, the thermal transports in strained WS2 monolayer are studied systematically by combining the first principles calculations and the Boltzmann transport equation. It can be found that the lattice thermal conductivity (k) decreases from 262.78 to 190.66 W m−1 K−1 for biaxial tensile strain (from 0% to 8%) and from 262.78 to 217.40 W m−1 K−1 for biaxial compressive strain (from 0% to −4%), respectively. The softened transverse/longitudinal acoustic phonon mode and the stiffened flexural acoustic phonon mode in strained WS2 monolayer change the phonon group velocities. We attribute the reduction of k in tensilely strained WS2 monolayer to the declining phonon group velocity, heat capacity and phonon lifetime of acoustic phonon modes. The comprehensive competition among the increasing phonon group velocity, heat capacity and the decreasing phonon lifetime suppresses the lattice thermal conductivity of compressively strained WS2 monolayer. Furthermore, the increasing phase space proves that the enhancing anharmonic phonon scattering, which results in the suppression of the k in tensilely strained WS2 monolayer. Additionally, the critical phonon mean free path (MFP) under different biaxial strain levels is calculated to demonstrate the contribution of the phonons with different MFPs to the total k of WS2 monolayer. This work provides a phonon behavior analysis of the thermal conduction in WS2 monolayer and paves the path for potential applications of WS2-based devices.

Anomalous low energy phonon dispersion in bulk silicon-germanium observed by inelastic x-ray scattering

We report on an anomalous mode distinct from both optical and acoustic modes in phonon dispersion curves of bulk Si1−xGex alloy with x taking the values of 0.16, 0.32, 0.45, and 0.72. The anomalous mode at approximately 13 meV was observed directly using inelastic x-ray scattering along the Γ–X ([00q]) direction. The phonon dispersion relations of the anomalous mode indicate that there was no momentum dependence, similar to those of the longitudinal and transverse optical modes (Ge–Ge, Si–Ge, and Si–Si modes). In contrast to the acoustic and optical phonon modes, the energy of the anomalous mode shows no Ge fraction dependence. The molecular dynamics simulation corroborates that the Ge–Ge pairs or Ge atom clusters, which are surrounded by Si atoms, provide the anomalous mode, which is unique to the alloy structure. It has been suggested that such a localized vibration mode with no propagation significantly affects the acoustic modes, leading to low thermal conductivity in the SiGe alloy.

Extremely low thermal conductivity from bismuth selenohalides with 1D soft crystal structure

Materials with intrinsically low thermal conductivity are of fundamental interests. Here we report a new sort of simple one-dimensional (1D) crystal structured bismuth selenohalides (BiSeX, X = Br, I) with extremely low thermal conductivity of ~0.27 W m−1 K−1 at 573 K. The mechanism of the extremely low thermal conductivity in 1D BiSeX is elucidated systematically using the first-principles calculations, neutron powder-diffraction measurements and temperature tunable aberration-corrected scanning transmission electron microscopy (STEM). Results reveal that the 1D structure of BiSeX possesses unique soft bonding character, low phonon velocity, strong anharmonicity of both acoustic and optical phonon modes, and large off-center displacement of Bi and halogen atoms. Cooperatively, all these features contribute to the minimal phonon transport. These findings provide a novel selection rule to search low thermal conductivity materials with potential applications in thermoelectrics and thermal barrier coatings.

Theoretical investigation of the thermoelectric properties of ACuO2(A = K, Rb and Cs)

The electronic, structural, mechanical, lattice dynamics and the electronic transport properties of ACuO2(A = K, Rb and Cs) are investigated using density functional theory. The calculated elastic constants and their related elastic moduli, phonon spectra and electronic transport properties of these compounds are reported here for the first time. The predicted structural parameters are in excellent agreement with the available experimental data. The obtained lattice thermal conductivities, κL, of ACuO2 (A = K, Rb and Cs) are found to display strong anisotropic features along the a, b and c directions. It is also found that the average room-temperature κL of CsCuO2 is lower than those of RbCuO2 and KCuO2, which is due to its smaller group velocities in the low frequency region i.e., 0 ~ 3 THz. Our calculations also show that the acoustic phonon modes contribute considerably to the total κL along the a and b directions. The electrical conductivity (σ) and electronic thermal conductivity (κel) of ACuO2 (A = K, Rb and Cs) show anisotropic features i.e., σ and κel along the c-axis is significantly larger than along the a and b-axes. Meanwhile, our obtained Seebeck coefficient (S) values are found to be 248, 110 and 91 μV/K for p-doped KCuO2, p-doped RbCuO2 and p-doped CsCuO2 respectively at 300 K along the b-direction. These S values are found to be of the same order of magnitude with that of well known thermoelectric (TE) material, Bi2Te3 (with S of 200 μV/K at 300 K) and the recently discovered metal oxide TE material, NaCo2O4 (with S of 100 μV/K at 300 K). However, our computed figure of merit (ZT) values of ACuO2 (A = K, Rb and Cs) are found to be very small as compared to known thermoelectric materials. For instance, our highest computed ZT value is 0.11 for p-type KCuO2 along the c-direction at 750 K, 0.15 for p-type RbCuO2 and 0.25 for p-type CsCuO2 along the a-direction at 800 K. These small ZT values are caused by large values of the lattice thermal conductivities.

Acoustic phonon anomalies in Ca doped SrTiO3 quantum ferroelectrics as studied by Brillouin light scattering

The cubic-tetragonal structural phase transition has been investigated in 3% Ca doped SrTiO3 crystals with perovskite structure by Brillouin light scattering in the temperature range between 90 and 500 K. The frequency and the width of longitudinal acoustic (LA) and transverse acoustic phonon modes show anomalous behaviour at Ttr = 275 K. The observed anomalies are ascribed to the structural phase transition from cubic to tetragonal phases in Ca doped SrTiO3. The behaviour of LA phonon in the critical region close to Ttr is discussed within the frame work of Landau–Khalatnikov relaxation theory. The relaxation time of the order parameter, which is calculated on the basis of the Landau–Khalatnikov type of relaxation mechanism, exhibits critical slowing down towards Ttr.

Strain tunable pudding-mold-type band structure and thermoelectric properties of SnP3 monolayer

Recent studies indicated the interesting metal-to-semiconductor transition when layered bulk GeP3 and SnP3 are restricted to the monolayer or bilayer, and the SnP3 monolayer has been predicted to possess high carrier mobility and promising thermoelectric performance. Here, we investigate the biaxial strain effect on the electronic and thermoelectric properties of the SnP3 monolayer. Our first-principles calculations combined with Boltzmann transport theory indicate that the SnP3 monolayer has the “pudding-mold-type” valence band structure, giving rise to a large p-type Seebeck coefficient and a high p-type power factor. The compressive biaxial strain can decrease the energy gap and result in metallicity. In contrast, the tensile biaxial strain increases the energy gap, increases the n-type Seebeck coefficient, and decreases the n-type electrical conductivity. Although the lattice thermal conductivity becomes larger at a tensile biaxial strain due to the increased maximum frequency of the acoustic phonon modes and the increased phonon group velocity, it is still low, e.g., only 4.1 W m−1 K−1, at room temperature with 6% tensile strain. The tensile strain decreases the figure of merit, but the value is still considerable, and it can reach 2.01 for p-type doping at 700 K with 6% tensile strain. Therefore, the SnP3 monolayer is a good thermoelectric material with low lattice thermal conductivity and promising figure of merit even at 6% tensile strain.

Nematic Correlation Length in Iron-Based Superconductors Probed by Inelastic X-Ray Scattering.

Nematicity is ubiquitous in electronic phases of high-T_{c} superconductors, particularly in the Fe-based systems. We used inelastic x-ray scattering to extract the temperature-dependent nematic correlation length ξ from the anomalous softening of acoustic phonon modes in FeSe, underdoped Ba(Fe_{0.97}Co_{0.03})_{2}As_{2}, and optimally doped Ba(Fe_{0.94}Co_{0.06})_{2}As_{2}. In all cases, we find that ξ is well described by a power law (T-T_{0})^{-1/2} extending over a wide temperature range. Combined with the previously reported Curie-Weiss behavior of the nematic susceptibility, these results point to the mean-field character of the nematic transition, which we attribute to a sizable nematoelastic coupling that is likely detrimental to superconductivity.

Acoustic anomalies and relaxation dynamics of relaxor ferroelectric Na1/2Bi1/2TiO3 single crystals studied by using Brillouin spectroscopy

Acoustic anomalies of relaxor ferroelectric Na1/2Bi1/2TiO3 single crystals were investigated over a wide temperature range from −196 °C to 900 °C by using Brillouin spectroscopy. The longitudinal sound velocity, the acoustic absorption coefficient and the elastic constant C11 were determined for the acoustic phonon mode propagating in the [100] direction. Two acoustic anomalies, weaker ones at the cubic-tetragonal phase transition temperature of ~540 °C and more pronounced ones at temperatures near 315 °C near the dielectric maximum temperature, were investigated and discussed in relation with the relevant order parameters coupled to the acoustic waves. The relaxation dynamics in the cubic phase were studied based on the flattening of the mode frequency and the half width, which was observed for the first time, and a modified Arrhenius law.

Spin-flip relaxation mechanism in self-assembled quantum dots via phonon emission

The spin-flip relaxation time for electrons in self-assembled coupled quantum dots (SACQDs) due to emission of acoustic phonons is presented. We focus on arbitrary shaped quantum dots made with InAs embedded in a wetting layer InAs and surrounded by GaAs. The electron states are calculated using the 8-band strain dependent theory. The deformation potential and the piezo- electric acoustic phonon modes are employed to estimate the electron-phonon coupling. Spin-flip time is calculated by Fermi’s Golden Rule and its dependence on the shape of the QDs, the size and the lattice temperature is investigated.

Functionalization of single-layer TaS2 and formation of ultrathin Janus structures

Abstract

Temporal superoscillations of subterahertz coherent acoustic phonons

We observe coherent acoustic phonon superoscillations at sub-terahertz frequencies. The superoscillations result from the interference of optically excited coherent longitudinal acoustic phonon modes in a GaAs/AlGaAs superlattice. The superoscillations are seen in local temporal regions and their frequency is up to 60% larger than the highest frequency excited/detected phonon mode. Such superoscillatory phonon pulses may potentially be used for high-resolution acoustic measurement in strong scattering media. Coherent acoustic phonons with frequencies in the 100s of GHz range, which are also known as acoustic nanowaves, are a powerful tool for investigating nano-scale objects with high temporal and spatial resolution. The established technique, known as picosecond acoustics, uses ultrafast (femtosecond) pulsed lasers to optically generate and detect coherent acoustic phonons with frequencies up to ~1 THz [1]. Such acoustic nanowaves can be used to probe the structural and mechanical properties of matter with sub-nm axial resolution and sub-µm transverse resolution [2]. Examples of the applications include studies of various nano-objects [3, 4], interfaces [5], defects in solids [6], soft condensed matter [7], liquids [8] and biological matter (single cells) [9] (for a review see also [10]). Acoustic nanowaves have also been used for ultrafast control of photonic [11], electronic [12], and spintronic [13] devices. Despite the successes of the picosecond acoustics technique, it suffers from a significant practical drawback: the strong attenuation and/or scattering, i.e. short mean free path, of high frequency (>~100s of GHz) phonons in many of the objects of interest. Causes of phonon attenuation and scattering include: propagation through non-ideal interfaces [14], e.g. between a stick-on optoacoustic transducer and the object under investigation; propagation in polycrystalline materials and crystals with a high defect density [15]; viscous damping [16]; natural isotope (point mass defect) scattering [17], which occurs even in perfect crystals. At ambient, room, temperature, the mean free path of THz acoustic phonons in high-quality (semiconductor grade) crystals is only a few µm [18]. The result of these is that the mean free path decreases strongly with increasing phonon frequency with the net effect of imposing a bandwidth limit, restricting the highest frequency of the phonons that can be propagated in the system. This determines the highest phonon frequency that can be used in a picosecond acoustic measurement and hence the best resolution that could normally be obtained. Bandwidth limitations also occur in measurements using electromagnetic waves and the search for ways to overcome these has led to renewed interest in the phenomenon of superoscillation. Superoscillations occur in a band-limited function due the interference of a series of harmonic waves, Fourier series, with the appropriate amplitudes and phases. They are oscillations found in local temporal regions having rates arbitrarily higher than the highest frequency Fourier component [19]. Superoscillations have relevance in all areas of physics where waves are superposed, e.g. in quantum mechanics [20]. It has also been proposed and demonstrated that superoscillations could be exploited

Thermal and mechanical properties of U3Si2: A combined ab-initio and molecular dynamics study

Triuranium disilicide (U3Si2) has been identified as an accident resistant alternative to uranium dioxide (UO2) based fuels. While experimental studies of U3Si2 are not new, only few studies on the thermal conductivity has been reported. Moreover, theoretical studies of the thermal properties of U3Si2 are scarce. We found that inaccuracies in experimental data were a source of discrepancies in recently reported theoretical results, as such need to be addressed. In this work, the structural, thermal, and mechanical properties of, the non-magnetic and magnetic phases, of U3Si2 were investigated using Hubbard corrected density functional theory (DFT+U) and molecular dynamics (MD) simulations. The lattice and electronic thermal properties were determined based on Boltzmann transport equation (BTE) approaches. A considerable anisotropy was determined the lattice and electronic contributions to the thermal conductivity from DFT+U/BTE results, in which the electronic contribution is predominant. A relative good agreement between the lattice contributions to the thermal conductivity, predicted from DFT+U and MD computations, was associated to similarities in the acoustic phonon modes. The present computational results significantly enhance the understanding of the thermal properties of U3Si2. The good agreement between DFT+U/BTE results with experimental data confirm the accident tolerant characteristic of U3Si2, but also further demonstrating the accuracy of the DFT(+U)/BTE approach for the investigation of thermal properties of advanced nuclear fuels.

Dynamical properties of organo lead-halide perovskites and their interfaces to titania: insights from Density Functional Theory.

The vibrational density of states (VDOS), electronic structure and optical properties of bulk organo lead-halide perovskites, CH3NH3PbX3 (where X = Cl, I and Br), very promising and exciting candidate materials for solar-energy applications, have been studied by means of (hybrid) Density Functional Theory (DFT), with and without spin-orbit coupling, and equilibrium Born-Oppenheimer molecular dynamics (BOMD) in the constant-volume, isothermal (NVT) ensemble at 298 K. Particular emphasis has been directed towards the detailed characterisation of optimal hybrid-DFT strategies to reproduce faithfully the band gap, band structure and optical properties vis-à-vis both experiment and more computationally demanding GW calculations (i.e., those involving the single-particle Green's function, G, and the screened Coulomb interaction, W). The VDOS was found to feature intimate coupling between the lead and halide atoms, and was dominated by acoustic phonon modes – particularly so for chlorine, suggesting this as the more effective candidate material of the considered halides. Bulk optical properties were also determined. In view of promising ‘hybrid’ architectures of perovskites adsorbed on titania substrates, further simulations of lead iodide in contact with titania have been performed to assess thermal stability, as well as dynamical and structural properties of these systems. It was found that lattice strain led to some atomic layers in perovskite further from the interface adopting less crystal-like structure and less pronounced phonon spectra.

Parametric resonance of acoustic and optical phonons in a doped semiconductor superlattice in the presence of a laser field

In this paper, we analytically investigated the possibility of parametric resonance of acoustic and optical phonons. We obtained a general dispersion equation for parametric amplification and transformation of phonons. The dispersions of the resonant acoustic phonon modes and the threshold amplitude of the field for acoustic phonon parametric amplification are obtained. The parametric amplification for acoustic phonons in a doped semiconductor superlattice can occur under the condition that the amplitude of the external electromagnetic field is higher than the threshold amplitude.

Hot Phonon and Auger Heating Mediated Slow Intraband Carrier Relaxation in Mixed Halide Perovskite

A clear understanding of excited carrier relaxation to their equilibrium state via carrier-phonon, carrier-carrier interaction etc. is always required for the advancement of perovskite based device performance. Organic/inorganic lead halide perovskite have shown remarkable slow hot carrier cooling properties. Herein, we investigate the effect of hot phonon and Auger heating on hot carrier relaxation dynamics in Methyl-ammonium lead iodide-chloride (CH3NH3PbI3-XClX or mixed halide) perovskite using transient absorption spectroscopy. The reduced energy-loss rate on sub-picosecond time scale clearly shows the non-equilibrium build-up of longitudinal optical and acoustic phonon modes (carrier-phonon interaction) at low pump excitation (1018 cm-3). However at high pump excitation (1019 cm-3), dominating Auger heating process (carrier-carrier interaction) further reduces the energy-loss rate by more than two order of magnitude and delays the carrier relaxation process to tens of picoseconds which makes perovskite as a promising candidates in hot carrier photovoltaic.

Time-resolved ultrafast dynamics in triple degenerate topological semimetal molybdenum phosphide

We employ the time resolved pump probe experiment to investigate the ultrafast dynamics in a topological semimetal molybdenum phosphide (MoP), which exhibits triple degenerate points in the momentum space. Two relaxation processes with the lifetime of 0.3 and 150 ps have been observed. We attribute the fast component to the electron-phonon scattering and the slow component to the phonon-phonon scattering, respectively. Temperature dependence investigation shows that both the lifetimes of the fast and slow components enhance slightly with increasing temperature. We also successfully generate and detect a thermal-stress-induced coherent acoustic phonon mode with a frequency of 0.033 THz, which does not vary with temperature. Our ultrafast spectroscopy investigation of the quasiparticle dynamics and the coherent phonon in MoP provides useful experimental facts and information about the overall excited state dynamics and the temperature dependence of electron-phonon coupling.

Novel elastic evolution of carbide Mo2Ga2C under pressure: Ab initio theoretical investigation

The pressure dependence of elastic properties of Mo2Ga2C is studied via first-principles calculation. The present investigation shows that differing from other MAX phases, in Mo2Ga2C the [Formula: see text] is larger than [Formula: see text], because of the strong Ga–Ga interlayer bonds along [Formula: see text]-axis. Moreover, under pressure, the [Formula: see text] increases more rapidly, originating from the faster strengthening of Ga–Ga bonds. Interestingly, elastic constants [Formula: see text] soften under high pressure (more than 20 GPa). Especially, the calculated phonon structure demonstrates that transverse acoustic (TA) phonon mode also softens under pressure, implying possible phase transition. The reduction of [Formula: see text] and softening of phonon mode are attributed to significantly weakened Mo–Mo interaction in contrast to the strengthening of Ga–Ga bonds under high pressure. Our present results further indicate that Mo2Ga2C is more ductile under pressure.