Acoustic Phonon Modes Research Articles

Lattice thermal conductivity in bulk and nanosheet NaxCoO2

In this paper we present the results of calculations of the lattice thermal conductivity of layered complex metal oxide NaxCoO2 within the Green–Kubo theory. Using NaxCoO2 we identify the two competing mechanisms responsible for the favorable scaling properties of the Green–Kubo method for calculating the lattice thermal conductivity. The artificial correlations of the heat flux fluctuations due to the finite size of the supercells are partially cancelled by the missing long wavelength acoustic phonon modes. We compute the lattice thermoelectric properties of bulk NaxCoO2 with varying stoichiometry, structural defects, and temperature. We also calculate the thermal conductivity of NaxCoO2 in the nanosheet geometry. While the dependence of thermal conductivity on Na fractions x in the middle range (0.5<x<0.8) is relatively weak, introducing Co vacancies results in significant lattice thermal conductivity reduction. The material exhibits strong anisotropy of lattice thermal conductivity due to a layered crystal structure and relatively weak bonding between layers. This structure leads to the possibility of manufacturing relatively large nanosheets of NaxCoO2. However, the weak inter-layer binding also results in the insensitivity of thermal conductivity to the nanosheet thickness.

High pressure phase of pnictides yttrium YAs and YBi: first-principles studies

The high pressure phase transition in pnictides yttrium YAs and YBi has been investigated by a first-principles pseudopotential plane-wave method and density functional linear-response theory. Employing total energy calculations, the predicted rocksalt → CsCl phase transition pressure is found, respectively, at 63.3 and 23 GPa for YAs and YBi. A pressure-induced soft transverse acoustic (TA) phonon mode is identified at 92 GPa for YAs and at 47 GPa for YBi. Phonon calculations suggest that YAs becomes dynamically instable with respect to the TA [ε00] and TA [0εε] phonon modes, while YBi becomes dynamically instable with respect to the TA phonon mode at the zone boundary X point. Furthermore, it is found that the increase of anti-bonding Y-4d bands significantly weakens the stability of the rocksalt phase of YAs and YBi under pressure inducing, hence a structural phase transformation.

Comment on “First-principles study of the influence of (110)-oriented strain on the ferroelectric properties of rutile TiO2”

In a recent article, Gr\"{u}nebohm et al. [Phys. Rev. B 84 132105 (2011), arXiv:1106.2820] report that they fail to reproduce the A2u ferroelectric instability of TiO2 in the rutile structure calculated with density functional theory within the PBE-GGA approximation by Montanari et al. [Chem. Phys. Lett 364, 528 (2002)]. We demonstrate that this disagreement arises from an erroneous treatment of Ti 3s and 3p semi-core electrons as core in their calculations. Fortuitously the effect of the frozen semi-core pseudopotential cancels the phonon instability of the PBE exchange-correlation, and the combination yields phonon frequencies similar to the LDA harmonic values. Gr\"{u}nebohm et al. also attempted and failed to reproduce the soft acoustic phonon mode instability under (110) strain reported by Mitev et al. [Phys. Rev. B 81 134303 (2010)]. For this mode the combination of PBE-GGA and frozen semi-core yields a small imaginary frequency of 9.8i. The failure of Gr\"{u}nebohm et al. to find this mode probably arose from numerical limitations of the geometry optimization approach in the presence of a shallow double well potential; the optimization method is not suitable for locating such instabilities.

Growth and electrical/optical characteristics of unintentional p-type BaIn2Se4 epilayers grown using hot wall epitaxy method

The epitaxial growth of photoconductive BaIn2Se4, which has orthorhombic structures, was first achieved through the hot wall epitaxy method. The electrical and optical characteristics of these epilayers were discussed. From the Hall effect measurement, in a high-temperature range of T>150K, the mobility decreased as a function of T−1 and its scattering was mainly due to the acoustic phonon mode of lattice vibrations through a deformation potential. In contrast, the mobility decreased in proportional to T0.6 in a low-temperature range of T<150K and its decrease was caused by the impurity ion scattering. Also, from the relation between the reciprocal temperature and the carrier concentration, two dominant levels 136.9 and 27.9meV were extracted out and they were estimated to be the activation energies of the shallow acceptor levels caused by the native defects of the upper edge of the valence band. In addition, from the optical absorption measurement, the bandgap variation of BaIn2Se4 epilayers extracted was well expressed by Eg(T)=2.626eV–4.99×10−3T2/(T+559).

The effect of torsional deformation on thermal conductivity of mono-, bi- and trilayer graphene nanoribbon

Physical deformation and geometrical change of a low dimensional nanostructure influence the phonon transport leading to appreciable changes in the thermal characteristics. In this paper, we report the calculation of the thermal conductivity of twisted graphene nanoribbon (GNR) using nonequilibrium molecular dynamics (NEMD). A reduction in the value of thermal conductivity of GNR is observed with an increase in the angle of twist. It is observed that the value of thermal conductivity of twisted GNR is dependent on the length of GNR and the temperature at which physical deformation is taking place. Comparing with monolayer GNR, the reduction in the thermal conductivity of bilayer and trilayer GNR is less due to the interactions between the adjacent layers. Analysis of the phonon transport in twisted graphene implies that the reduced thermal conductivity of twisted graphene nanoribbon is due to the phonon softening of acoustic phonon modes.

Experimental identification of the out-of-plane phonon mode of a few layered graphene from individual Au nanoparticle-Au film junctions

We can experimentally identify the layer-layer breathing mode and the other out-of-plane transverse acoustic phonon modes of a few layered graphene via employing z-polarized incident field formed at Au nanoparticle-Au film junctions. The observed out-of-plane mode phonon at ∼150 cm−1 and other transverse acoustic phonon type defect modes from 300 cm−1 to 700 cm−1 can be readily explored. Not only for the normally used optical method using focused plane wave but also direct applying the z-directional electromagnetic field on graphene will be expected for more detailed out-of-plane phonon characterization of graphene.

Understanding the influence of grain boundary thickness variation on the mechanical strength of a nickel-doped tungsten grain boundary

Grain boundaries (GBs) in nickel (Ni)-doped tungsten (W) are found to have thickness as a function of the level of saturation of W atoms with respect to Ni atoms in the GBs. While the unsaturated Ni-doped W GBs have average thickness of approximately 0.3nm, the saturated Ni-doped W GBs have twice the average thickness (∼0.6nm). The present work examines (110)–(210) W GB mechanical strength as a function of thickness using an ab initio calculation framework based on Car–Parrinello molecular dynamics (CPMD) simulations. The atomic fraction of Ni atoms is varied to understand the influence of Ni addition and its correlation with thickness variation on the GB mechanical strength. In the case of GBs with 0.3nm thickness, the variation of peak tensile strength as a function of Ni atomic fraction variation from 5% to 50% is negligible. However, in the case of 0.6nm GB, the changes in the peak tensile strength are significant with the maximum peak tensile strength observed in the case of 58% Ni atomic fraction. Analyses examine electron density of states and phonon dispersion relations to delineate the role of atomic level bond strength in thickness dependent GB mechanical strength. The thickness dependent GB strength variation is found to be strongly correlated to the thickness dependent change in lower acoustic mode phonon frequencies. At the same time Ni atomic fraction dependent change in GB strength is found to be strongly correlated to the corresponding changes in electron density of states. Based on the analyses performed, an analytical relation to predict GB peak tensile strength as a function of atomic cohesive energy, GB thickness (level of saturation), and the Ni atomic fraction is proposed.

Temperature-driven phase transformation in Y3Co: Neutron scattering and first-principles studies

Contrary to previous studies that identified the ground state crystal structure of the entire ${R}_{3}$Co series ($R$ is a rare earth) as orthorhombic $Pnma$, we show that Y${}_{3}$Co undergoes a structural phase transition at ${T}_{t}\ensuremath{\simeq}160$ K. Single crystal neutron diffraction data reveal that at ${T}_{t}$ the trigonal prisms formed by a cobalt atom and its six nearest-neighbor yttrium atoms experience distortions accompanied by notable changes of the Y-Co distances. The formation of the low-temperature phase is accompanied by a pronounced lattice distortion and anomalies seen in heat capacity and resistivity measurements. Density functional theory calculations reveal a dynamical instability of the $Pnma$ structure of Y${}_{3}$Co. In particular, a transversal acoustic phonon mode along the $(00\ensuremath{\xi})$ direction has imaginary frequencies at $\ensuremath{\xi}&lt;1/4$. Employing inelastic neutron scattering measurements we find a strong damping of the $(00\ensuremath{\xi})$ phonon mode below a critical temperature ${T}_{t}$. The observed structural transformation causes the reduction of dimensionality of electronic bands and decreases the electronic density of states at the Fermi level that identifies Y${}_{3}$Co as a system with the charge density wave instability.

Phonons in twisted bilayer graphene

We theoretically investigate phonon dispersion in AA-stacked, AB-stacked, and twisted bilayer graphene with various rotation angles. The calculations are performed using the Born--von Karman model for the intralayer atomic interactions and the Lennard-Jones potential for the interlayer interactions. It is found that the stacking order affects the out-of-plane acoustic phonon modes the most. The difference in the phonon densities of states in the twisted bilayer graphene and in AA- or AB-stacked bilayer graphene appears in the phonon frequency range 90--110 cm${}^{\ensuremath{-}1}$. Twisting bilayer graphene leads to the emergence of different phonon branches---termed hybrid folded phonons---which originate from the mixing of phonon modes from different high-symmetry directions in the Brillouin zone. The frequencies of the hybrid folded phonons depend strongly on the rotation angle and can be used for noncontact identification of the twist angles in graphene samples. The obtained results and the tabulated frequencies of phonons in twisted bilayer graphene are important for the interpretation of experimental Raman data and in determining the thermal conductivity of these material systems.

Intrinsic to extrinsic phonon lifetime transition in a GaAs–AlAs superlattice

We have measured the lifetimes of two zone-center longitudinal acoustic phonon modes, at 320 and 640 GHz, in a 14 nm GaAs/2 nm AlAs superlattice structure. By comparing measurements at 296 and 79 K we separate the intrinsic contribution to phonon lifetime determined by phonon–phonon scattering from the extrinsic contribution due to defects and interface roughness. At 296 K, the 320 GHz phonon lifetime has approximately equal contributions from intrinsic and extrinsic scattering, whilst at 640 GHz it is dominated by extrinsic effects. These measurements are compared with intrinsic and extrinsic scattering rates in the superlattice obtained from first-principles lattice dynamics calculations. The calculated room-temperature intrinsic lifetime of longitudinal phonons at 320 GHz is in agreement with the experimentally measured value of 0.9 ns. The model correctly predicts the transition from predominantly intrinsic to predominantly extrinsic scattering; however the predicted transition occurs at higher frequencies. Our analysis indicates that the ‘interfacial atomic disorder’ model is not entirely adequate and that the observed frequency dependence of the extrinsic scattering rate is likely to be determined by a finite correlation length of interface roughness.

Electron density window for best frequency performance, lowest phase noise and slowest degradation of GaN heterostructure field-effect transistors

The problems in the realm of nitride heterostructure field-effect transistors (HFETs) are discussed in terms of a novel fluctuation–dissipation-based approach impelled by a recent demonstration of strong correlation of hot-electron fluctuations with frequency performance and degradation of the devices. The correlation has its genesis in the dissipation of the LO-mode heat accumulated by the non-equilibrium longitudinal optical phonons (hot phonons) confined in the channel that hosts the high-density hot-electron gas subjected to a high electric field. The LO-mode heat causes additional scattering of hot electrons and facilitates defect formation in a different manner than the conventional heat contained mainly in the acoustic phonon mode. We treat the heat dissipation problem in terms of the hot-phonon lifetime responsible for the conversion of the non-migrant hot phonons into migrant acoustic modes and other vibrations. The lifetime is measured over a wide range of electron density and supplied electric power. The optimal conditions for the dissipation of the LO-mode heat are associated with the plasmon-assisted disintegration of hot phonons. Signatures of plasmons are experimentally resolved in fluctuations, dissipation, hot-electron transport, transistor frequency performance, transistor phase noise and transistor reliability. In particular, a slower degradation and a faster operation of GaN-based HFETs take place inside the electron density window where the resonant plasmon-assisted ultrafast dissipation of the LO-mode heat comes into play. A novel heterostructure design for the possible improvement of HFET performance is proposed, implemented and tested.

Elastic, vibrational and thermodynamic properties of [formula omitted] based group IV semiconductors and GeC under pressure

We present first-principles calculations of the structural, elastic, vibrational and thermodynamic properties of SnSi, SnGe, SnC and GeC. We employ the density-functional perturbation theory (DFPT) within the local density approximation in conjunction with the quasi-harmonic approximation. The calculated lattice parameters, which are obtained by minimizing the total energy, are in the range of those reported in the literature for the binary compounds and in good agreement with the measured ones for the elemental components. Our results for the elastic properties show that c44 softens as pressure increases for SnSi and SnGe. The phonon spectra, the density of states and the Born effective charge at zero pressure are calculated and the phonon frequencies are positive. A pressure induced soft transverse acoustic phonon mode is identified at the zone boundary X point of the Brillouin zone at pressure of 12.95 and 12.45GPa for SnSi and SnGe respectively. The linear expansion coefficient for the elemental components is calculated and compared to experiment. The temperature and pressure dependence of the thermal expansion, the overall Grüneisen parameter, the bulk modulus and the heat capacity is predicted. The thermal expansion coefficient decreases with increasing pressure and does not show any negative behavior for GeC and this is due to the positive transverse acoustic mode Grüneisen parameters. Our results for SnxGe1−x alloys using the supercell method indicate that the variation of the Grüneisen parameter and the thermal expansion with concentration has the same trend and the bulk modulus softens.

Anisotropic thermal conductivity of semiconducting graphene monoxide

The intrinsic thermal conductivity of monolayer graphene monoxide is determined via first-principles calculations. The phonon transport in graphene monoxide is anisotropic, with the lattice thermal conductivity along the armchair direction (…C-2O-C…) about five times higher than that along the zigzag (…C-C…) direction. The predicted thermal conductivity (&amp;gt;3000 Wm−1K−1 at 300 K) of graphene monoxide is 80% of that of graphene along the armchair direction for large sample lateral sizes (&amp;gt;5 μm). In addition, heat is predominantly carried by longitudinal acoustic phonons along the armchair direction, while the contribution from the transverse acoustic phonon mode is prevalent along the zigzag direction.

The vibron dressing in α-helicoidal macromolecular chains

We present a study of the physical properties of the vibrational excitation in α-helicoidal macromolecular chains, caused by the interaction with acoustical and optical phonon modes. The influence of the temperature and the basic system parameters on the vibron dressing have been analyzed by employing the simple mean-field approach based on the variational extension of the Lang—Firsov unitary transformation. The applied approach predicts a region in system parameter space where one has an abrupt transition from a partially dressed (light and mobile) to a fully dressed (immobile) vibron state. We found that the boundary of this region depends on system temperature and the type of bond among structural elements in the macromolecular chain.

Gap-Dependent Quasiparticle Dynamics and Coherent Acoustic Phonons in CaFe2As2 across Spin Density Wave Phase Transition

We report ultrafast quasiparticle (QP) dynamics and coherent acoustic phonons in undoped CaFe_2As_2 iron pnictide single crystals exhibiting spin-density wave (SDW) and concurrent structural phase transition at temperature TSDW ~ 165 K using femtosecond time-resolved pump-probe spectroscopy. The contributions in transient differential reflectivity arising from exponentially decaying QP relaxation and oscillatory coherent acoustic phonon mode show large variations in the vicinity of T_SDW. From the temperature-dependence of the QP recombination dynamics in the SDW phase, we evaluate a BCS-like temperature dependent charge gap with its zero-temperature value of ~(1.6+/-0.2)k_BT_SDW, whereas, much above T_SDW, an electron-phonon coupling constant of ~0.13 has been estimated from the linear temperature-dependence of the QP relaxation time. The long-wavelength coherent acoustic phonons with typical time-period of ~100 ps have been analyzed in the light of propagating strain pulse model providing important results for the optical constants, sounds velocity and the elastic modulus of the crystal in the whole temperature range of 3 K to 300 K.

Intrinsic electrical transport properties of monolayer silicene and MoS2from first principles

The electron-phonon interaction and related transport properties are investigated in monolayer silicene and MoS${}_{2}$ by using a density functional theory calculation combined with a full-band Monte Carlo analysis. In the case of silicene, the results illustrate that the out-of-plane acoustic phonon mode may play the dominant role unlike its close relative, graphene. The small energy of this phonon mode, originating from the weak $sp$${}^{2}$ $\ensuremath{\pi}$ bonding between Si atoms, contributes to the high scattering rate and significant degradation in electron transport. In MoS${}_{2}$, the longitudinal acoustic phonons show the strongest interaction with electrons. The key factor in this material appears to be the $Q$ valleys located between the $\ensuremath{\Gamma}$ and $K$ points in the first Brillouin zone as they introduce additional intervalley scattering. The analysis also reveals the potential impact of extrinsic screening by other carriers and/or adjacent materials. Finally, the effective deformation potential constants are extracted for all relevant intrinsic electron-phonon scattering processes in both materials.

Dynamical stability of the lanthanum dihydride under high pressure: A density functional lattice dynamics approach

We report first-principles pseudopotential method and density functional linear response theory calculations of the structural, electronic, vibrational and thermodynamical properties of LaH2 in fluoride phase at zero and high pressures. The electronic band structure and first time calculated pressure dependent lattice dynamic properties show that the LaH2 is metallic in nature at zero pressure. The pressure dependent electronic band structure calculations show that the bands are closing and reopening with pressure. The phonon mode softens at 11 GPa indicating an instability in lattice structure and pressure induced transverse acoustic phonon mode driven displacive type structural phase transition. The LaH2 also shows weak superconductivity at zero pressure. The valence charge electron density at 46 GPa shows a cage type charge contour and a space where hydrogen can be absorbed and filled easily. There is a considerable charge accumulation in the bonding region of La and H atoms, which strengthens covalent character of LaH2, and henceforth this material can be one of the best materials for hydrogen storage application at high pressures.

Electronic structure, optical properties and the mechanism of the B3–B8 phase transition of BeSe: insights from hybrid functionals, lattice dynamics and NPH molecular dynamics

We have investigated the electronic structure and the mechanism of the pressure induced phase transition of beryllium selenide (BeSe) by employing a first-principles pseudopotential method within the framework of density functional theory. Our study demonstrates that use of the hybrid PBE0 functional (PBE stands for Perdew, Burke and Ernzerhof) leads to significant improvement in the band gap calculations, compared to those using either of the common density functionals (local density approximation (LDA) and generalized gradient approximation (GGA)), which severely underestimate the band gap of BeSe. The band gap obtained from the hybrid PBE0 functional shows excellent agreement with available experimental data. A constant-pressure (NPH) first-principles molecular dynamics (FPMD) approach has been adopted to characterize the first-order pressure induced phase transition from the zinc blende (ZB) to the nickel arsenide (NiAs) structure. We have shown that the FPMD simulation overestimates the transition pressure PT (compared to static enthalpy and experimental data) due to overpressure in the simulation box. The MD simulation reveals the structural pathway (cubic → orthorhombic → monoclinic → hexagonal), leading from the ZB phase to the NiAs phase. To find an explanation for the phase transition we calculated the vibrational and elastic properties under pressure. Negative Grüneisen parameters were obtained for the transverse acoustic phonon modes at the X and L high symmetry points. However, no mechanical instability or imaginary frequencies were found at pressures near PT. Thus the transition results from a thermodynamic instability rather than an elastic/dynamical one. We have also calculated the optical properties of both the B3 and B8 phases, such as the real and imaginary parts of the dielectric constant, reflectivity, loss function and refractive index, and compared them with the existing experimental and theoretical data. An abrupt decrease is obtained from the reflectivity spectrum of the NiAs phase at PT, which is supported from the peaks in the loss function.

Lifetimes of Confined Acoustic Phonons in Ultrathin Silicon Membranes

We study the relaxation of coherent acoustic phonon modes with frequencies up to 500 GHz in ultrathin free-standing silicon membranes. Using an ultrafast pump-probe technique of asynchronous optical sampling, we observe that the decay time of the first-order dilatational mode decreases significantly from ~4.7 ns to 5 ps with decreasing membrane thickness from ~194 to 8 nm. The experimental results are compared with theories considering both intrinsic phonon-phonon interactions and extrinsic surface roughness scattering including a wavelength-dependent specularity. Our results provide insight to understand some of the limits of nanomechanical resonators and thermal transport in nanostructures.

High- and low-frequency phonon modes in dipolar quantum gases trapped in deep lattices

We study normal modes propagating on top of the stable uniform background in arrays of dipolar Bose-Einstein condensate (BEC) droplets trapped in a deep optical lattice. Both the on-site mean-field dynamics of the droplets and their displacement due to the repulsive dipole-dipole interactions (DDIs) are taken into account. Dispersion relations for two modes, viz., high- and low- frequency counterparts of optical and acoustic phonon modes in condensed matter, are derived analytically and verified by direct simulations, for both cases of the repulsive and attractive contact interactions. The (counterpart of the) optical-phonon branch does not exist without the DDIs. These results are relevant in the connection to emerging experimental techniques enabling real-time imaging of the condensate dynamics and direct experimental measurement of phonon dispersion relations in BECs.