Bulk Phonon Research Articles

P-TRANS: A Monte Carlo ray-tracing software to simulate phonon transport in arbitrary nanostructures

P-TRANS is a software to simulate phonon transport in arbitrary nanostructures based on the Boltzmann transport theory. The core part of the software is a Monte Carlo ray-tracing solver that uses the stochastic method to sample phonon transport in various nanostructures. The solver is written in FORTRAN with well-organized modules and utilizing the OpenMP and MPI parallelization to achieve high performance. The software can handle a typical calculation of nanostructures with a feature size of ∼1000 nm on a normal PC within seconds, which is useful when exploring the wide variety of structures, for instance for structural optimization or high-throughput screening in material informatics. The software takes the bulk phonon properties obtained from the first-principles anharmonic lattice dynamics calculations as inputs. The database of bulk phonon properties is available on the software webpage, which currently contains more than 60 materials and will increase in the future. We also provide a graphical user interface to design the nanostructures and to run the simulation for non-expert users. In this paper, we introduce the theoretical background, the technical details and implementation, and the applications of the software. Program summaryProgram title:P-TRANSCPC Library link to program files:https://doi.org/10.17632/n76jpj3fnz.1Developer's repository link:https://github.com/TEEL-UTokyo/P-TRANSCode Ocean capsule:https://codeocean.com/capsule/9387119Licensing provisions: GPLv3Programming language: FortranSupplementary material:http://www.phonon.t.u-tokyo.ac.jp/p-trans/Nature of problem: Knowing the effective thermal conductivity of nanostructures is important for numerous applications. Phonons are the dominate heat carriers in dielectrics and their transport is influenced by the voids, grain boundaries, and free surfaces in the nanostructure. Here we are focusing on the effective thermal conductivities of nanostructures with complex geometries and boundaries.Solution method: Monte Carlo ray tracing simulations.

Thermal Probes of Phonon-Coupled Kitaev Spin Liquids: From Accurate Extraction of Quantized Edge Transport to Anyon Interferometry

Recent theoretical studies inspired by experiments on the Kitaev magnet $\alpha$-RuCl$_3$ highlight the nontrivial impact of phonons on the thermal Hall conductivity of chiral topological phases. Here we introduce mixed mesoscopic-macroscopic devices that allow refined thermal-transport probes of non-Abelian spin liquids with Ising topological order. These devices feature a quantum-coherent mesoscopic region with negligible phonon conductance, flanked by macroscopic lobes that facilitate efficient thermalization between chiral Majorana edge modes and bulk phonons. We show that our devices enable $(i)$ accurate determination of the quantized thermal Hall conductivity, $(ii)$ identification of non-Abelian Ising anyons via the temperature dependence of the thermal conductance, and most interestingly $(iii)$ single-anyon detection through heat-based anyon interferometry. Analogous results apply broadly to phonon-coupled chiral topological orders.

First - principle studies of surface phonon modes in GeSe Multi‐Layers

The surface phonon dispersion spectra of GeSe layered crystals were investigated by first‐principles calculations within the density functional perturbation theory (DFPT). We model the GeSe surface as an arrangement of periodically placed slabs with a varying number of layers. To minimize the artificial interaction between the periodic images of the slabs, large enough vacuum space is taken in the perpendicular direction, and the slabs extend to infinity in the other directions. Detailed first‐principles analysis of modes revealed surface phonons outside the bulk frequency zones in addition to vibrations in the bulk frequency range. To obtain the phonon states, the derivative of the forces with respect to the atomic coordinates were calculated at equilibrium positions in DFPT. In the phonon spectra, vibrational modes localized at the surface appear above the bulk phonon bands and inside the bulk phonon gap. The lowest frequency modes are reminiscent of Rayleigh surface waves. The thickness dependence of the phonon spectra was explored by comparing the results of the slabs with 1–8 layers. A few suggestions are provided to verify our findings in future experiments.

First-principles investigation of a symmetry incompatible adsorbate-substrate system: PF3 on Cu(001)

The structural relaxation of the adsorption of PF3 on Cu(001), resulting in a c(4 × 2) overlayer, was studied using first-principles Density Functional Theory (DFT) calculations in the generalized gradient approximation (GGA). Unlike previous experimental results, we found no tilting of the molecules on the surface, with the C3v axis of PF3 perfectly perpendicular to the surface. Then using Density Functional Perturbation Theory (DFPT), we calculated the phonon spectra of the Cu (001): PF3 c(4 × 2) at two high symmetry points in the surface Brillouin zone, Γ¯ and M¯. By plotting the projected density of states, we found modes localized on the surface and determined how they changed upon adsorption. Our results indicate considerable mixing with bulk phonon modes at certain back-folded q-points of the bare-surface Brillouin zone. These modes have dominant vertical vibration on the topmost layer. Modes with surface covered atoms that move with the molecule as a unit can undergo softening up to 1.5 meV. However, modes with dominant motion on the second layer remain unperturbed. In addition, we studied molecular external modes (i.e. modes that arise from free motions of the PF3 molecule) that are in the range of bulk and surface modes of Cu. Depending on the symmetry of the underlying structure, PF3 vibrations can remain isolated and induce a localized surface mode or mix with bulk modes and become a resonance.

Optical Control of Bulk Phonon Modes in Crystalline Solids

AbstractLong‐lived phonons within crystalline bulk acoustic wave (BAW) resonators have become coherent carriers of information, which can be utilized for a variety of scientific and technological applications. Herein, a brief introduction to the field of phonon control is given with an emphasis on bulk phonons at high frequencies within crystalline solids. In cavity optomechanics, the photons and phonons can be confined by material boundaries, yielding large optomechanical coupling rates at gigahertz (GHz) frequencies, but the spurious heating is detrimental to robust operation. Although electromechanical techniques can be used to access long‐lived phonons in piezoelectric crystals, they are not suitable for the study of non‐piezoelectric crystals. The phase‐matched Brillouin interactions enable efficient optical access GHz frequency mechanical modes within macroscopic crystalline solids. Combining with the well‐established framework of cavity optomechanics, efficient optical control of BAWs can be obtained. For optical phonons, strong spontaneous Raman cooling and heating based on the interaction between excitons and phonons are considered. In addition, the application of optical phonons in quantum memory at room temperature is introduced. The review finishes with a discussion of the research direction for bulk optomechanical systems and exciton–phonon coupling systems in the future.

Degenerate topological line surface phonons in quasi-1D double helix crystal SnIP

Degenerate points/lines in the band structures of crystals have become a staple of the growing number of topological materials. The bulk-boundary correspondence provides a relation between bulk topology and surface states. While line degeneracies of bulk excitations have been extensively characterised, line degeneracies of surface states are not well understood. We show that SnIP, a quasi-one-dimensional van der Waals material with a double helix crystal structure, exhibits topological nodal rings/lines in both the bulk phonon modes and their corresponding surface states. Using a combination of first-principles calculations, symmetry-based indicator theories and Zak phase analysis, we find that two neighbouring bulk nodal rings form doubly degenerate lines in their drumhead-like surface states, which are protected by the combination of time-reversal symmetry {{{mathcal{T}}}} and glide mirror symmetry {bar{M}}_{y}. Our results indicate that surface degeneracies can be generically protected by symmetries such as {{{mathcal{T}}}}{bar{M}}_{y}, and phonons provide an ideal platform to explore such degeneracies.

Experimental observation of localized interfacial phonon modes

Interfaces impede heat flow in micro/nanostructured systems. Conventional theories for interfacial thermal transport were derived based on bulk phonon properties of the materials making up the interface without explicitly considering the atomistic interfacial details, which are found critical to correctly describing thermal boundary conductance. Recent theoretical studies predicted the existence of localized phonon modes at the interface which can play an important role in understanding interfacial thermal transport. However, experimental validation is still lacking. Through a combination of Raman spectroscopy and high-energy-resolution electron energy-loss spectroscopy in a scanning transmission electron microscope, we report the experimental observation of localized interfacial phonon modes at ~12 THz at a high-quality epitaxial Si-Ge interface. These modes are further confirmed using molecular dynamics simulations with a high-fidelity neural network interatomic potential, which also yield thermal boundary conductance agreeing well with that measured in time-domain thermoreflectance experiments. Simulations find that the interfacial phonon modes have an obvious contribution to the total thermal boundary conductance. Our findings significantly contribute to the understanding of interfacial thermal transport physics and have impact on engineering thermal boundary conductance at interfaces in applications such as electronics thermal management and thermoelectric energy conversion.

Measuring phonon dispersion at an interface.

The breakdown of translational symmetry at heterointerfaces leads to the emergence of new phonon modes localized at the interface1. These modes have an essential role in thermal and electrical transport properties in devices, especially in miniature ones wherein the interface may dominate the entire response of the device2. Although related theoretical work began decades ago1,3-5, experimental research is totally absent owing to challenges in achieving the combined spatial, momentum and spectral resolutions required to probe localized modes. Here, using the four-dimensional electron energy-loss spectroscopy technique, we directly measure both the local vibrational spectra and the interface phonon dispersion relation for an epitaxial cubic boron nitride/diamond heterointerface. In addition to bulk phonon modes, we observe modes localized at the interface and modes isolated from the interface. These features appear only within approximately one nanometre around the interface. The localized modes observed here are predicted to substantially affect the interface thermal conductance and electron mobility. Our findings provide insights into lattice dynamics at heterointerfaces, and the demonstrated experimental technique should be useful in thermal management, electrical engineering and topological phononics.

Nanometric phonon spectroscopy for diamond and cubic boron nitride

Local phonon properties near heterointerfaces or lattice defects are key for controlling various material properties such as thermal conductivity and thermoelectricity. However, high-spatial-resolution measurements of bulk phonons have hardly been established, even in typical nonpolar and polar bulk materials such as diamond and cubic boron nitride (c-BN), respectively. Here, we demonstrate nanometric measurements of phonon dispersions and mapping using a heterojunction of diamond and c-BN single crystals. To achieve high spatial/energy resolution, we use a monochromated (\ensuremath{\sim}20--30 meV) electron nanoprobe, a high-sensitivity scintillator for counting inelastically scattered electrons, and the Richardson--Lucy deconvolution. The diamond phonon dispersion is first measured under a large-scattering-vector condition, particularly for the \ensuremath{\Gamma} point. Differential scattering cross sections of optical and acoustic-phonon modes in diamond and c-BN are also measured, and the totals of their modes are on the order of ${10}^{\ensuremath{-}5}\phantom{\rule{0.16em}{0ex}}\mathrm{n}{\mathrm{m}}^{2}$ for both crystals.

Coherent-interface-induced strain in large lattice-mismatched materials: A new approach for modeling Raman shift.

Strain engineering as one of the most powerful techniques for tuning optical and electronic properties of Ill-nitrides requires reliable methods for strain investigation. In this work, we reveal, that the linear model based on the experimental data limited to within a small range of biaxial strains (< 0.2%), which is widely used for the non-destructive Raman study of strain with nanometer-scale spatial resolution is not valid for the binary wurtzite-structure group-III nitrides GaN and AlN. Importantly, we found that the discrepancy between the experimental values of strain and those calculated via Raman spectroscopy increases as the strain in both GaN and AlN increases. Herein, a new model has been developed to describe the strain-induced Raman frequency shift in GaN and AlN for a wide range of biaxial strains (up to 2.5%). Finally, we proposed a new approach to correlate the Raman frequency shift and strain, which is based on the lattice coherency in the epitaxial layers of superlattice structures and can be used for a wide range of materials.Electronic Supplementary MaterialSupplementary material (Table S1: Values of bulk phonon deformation potentials and elastic constants for GaN and AlN from each reference used in Table 1, Fig. S1: Lattice parameters of SL layers using Eq. (8), and Fig. S2: Raman mapping using Eq. (7)) is available in the online version of this article at 10.1007/s12274-021-3855-4.

Effect of phonon focusing and shear waves on anisotropy of drag thermopower in potassium nanoplates

The study of the influence of phonon focusing on the electron-phonon drag has shown that, at low temperatures, the drag thermopower in rectangular-shaped cross-section monocrystalline potassium nanoplates becomes dependent on the orientation of their wide faces in the[011] temperature gradient direction. For the orientation of the wide faces {001}, it has turned out to be larger than that for the orientation {011}. As the transverse parameters of the nanoplates under conditions of competition between the boundary and bulk phonon relaxation mechanisms are increased, thedrag thermopower anisotropy varies in a non-monotonic manner. At a thickness of D≅5·10−6cm and a width of W=μD(μ=3.3), the effect magnitude first changes from 14% to a maximum of 22% at D≅3·10−5cm. Then, it monotonically decreases and disappears at D≥0.1cm. With increasing the nanoplates'width-thickness ratio, the value of the orientation anisotropy is shown to riseup to 37%.

Optical constants of crystalline Bi2Sr2CaCu2O by Brillouin light scattering spectroscopy

Room-temperature optical constants of crystalline Bi2Sr2CaCu2O were determined using data extracted from Brillouin light scattering spectra. Optical extinction coefficient-to-refractive index ratios at a wavelength of 532 nm were obtained from bulk phonon peak linewidth and frequency shift measurements and range from for directions close to the crystallographic c-axis. These ratios, and optical extinction coefficients, absorption coefficients, and imaginary parts of the dielectric function determined from these ratios and known refractive index, are in general agreement with values found in optical reflectance studies, but are 5–7 times larger than those extracted from optical interference measurements.

Observation of a chiral wave function in the twofold-degenerate quadruple Weyl system BaPtGe

Topological states in quantum materials are defined by non-trivial topological invariants, such as the Chern number, which are properties of their bulk wave functions. A remarkable consequence of topological wave functions is the emergence of edge modes, a phenomenon known as bulk-edge correspondence, that gives rise to quantized or chiral physical properties. While edge modes are widely presented as signatures of non-trivial topology, how bulk wave functions can manifest explicitly topological properties remains unresolved. Here, using high-resolution inelastic x-ray spectroscopy (IXS) combined with first principles calculations, we report experimental signatures of chiral wave functions in the bulk phonon spectrum of BaPtGe, which we show to host a previously undiscovered twofold degenerate quadruple Weyl node. The chirality of the degenerate phononic wave function yields a non-trivial phonon dynamical structure factor, S(Q,$\omega$), along high-symmetry directions, that is in excellent agreement with numerical and model calculations. Our results establish IXS as a powerful tool to uncover topological wave functions, providing a key missing ingredient in the study of topological quantum matter.

Photoluminescence line shapes for color centers in silicon carbide from density functional theory calculations

Silicon carbide with optically and magnetically active point defects offers unique opportunities for quantum technology applications. Since interaction with these defects commonly happens through optical excitation and de-excitation, a complete understanding of their light-matter interaction in general and optical signatures, in particular, is crucial. Here, we employ quantum mechanical density functional theory calculations to investigate the photoluminescence lineshapes of selected, experimentally observed color centers (including single vacancies, double vacancies, and vacancy impurity pairs) in 4H-SiC. The analysis of zero-phonon lines as well as Huang-Rhys and Debye-Waller factors are accompanied by a detailed study of the underlying lattice vibrations. We show that the defect lineshapes are governed by strong coupling to bulk phonons at lower energies and localized vibrational modes at higher energies. Generally, good agreement to the available experimental data is obtained, and thus we expect our theoretical work to be beneficial for the identification of defect signatures in the photoluminescence spectra and thereby advance the research in quantum photonics and quantum information processing.

First-principles based analysis of thermal transport in metallic nanostructures: Size effect and Wiedemann-Franz law

Metallic nanostructures (the nanofilms and nanowires) are widely used in electronic devices, and their thermal transport properties are crucial for heat dissipation. However, there are still gaps in understanding thermal transport in metallic nanostructures, especially regarding the size effect and validity of the Wiedemann-Franz law. In this work, we perform mode-by-mode first-principles calculations combining the Boltzmann transport equation to understand thermal transport in metallic nanostructures. We take the gold (Au) and tungsten (W) nanostructures as prototypes. It is found that when the size of nanostructures is on the order of several tens of nanometers, the electronic/phonon thermal conductivity is smaller than the bulk value and decreases with size. The phonon contribution increases in nanostructures for those metals with small bulk phonon thermal conductivity (like Au), while the phonon contribution may increase or be suppressed in nanostructures for those metals with large bulk phonon thermal conductivity (like W). By assuming that the grain boundary does not induce inelastic electron-phonon scattering, the Wiedemann-Franz law works well in both Au and W nanostructures if the Lorentz ratio is estimated using electronic thermal conductivity. The Wiedemann-Franz law also works well in Au nanostructures when the Lorentz ratio is estimated by total thermal conductivity.

Possible observation of the signature of the bad metal phase and its crossover to a Fermi liquid in κ–(BEDT–TTF)2Cu(NCS)2 bulk and nanoparticles by Raman scattering

κ‐(BEDT‐TTF)2Cu(NCS)2 has been investigated by Raman scattering in both bulk and nanoparticle compounds. Phonon modes from 20 to 1600 cm−1 have been assigned. Focusing on the unexplored low frequency phonons, a plateau in frequencies is observed in the bulk phonons between 50 and 100 K and assigned to the signature of the bad metal phase. Nanoparticles of κ‐(BEDT‐TTF)2Cu(NCS)2 exhibit anomalies at 50 K associated to the crossover from a bad metal to a Fermi liquid whose origins are discussed.

Revisiting the Reduction of Thermal Conductivity in Nano- to Micro-Grained Bismuth Telluride: The Importance of Grain-Boundary Thermal Resistance

Nanograined bulk alloys based on bismuth telluride (Bi2Te3) are the dominant materials for room-temperature thermoelectric applications. In numerous studies, existing bulk phonon mean free path (MFP) spectra predicted by atomistic simulations suggest sub-100 nm grain sizes are necessary to reduce the lattice thermal conductivity by decreasing phonon MFPs. This is in contrast with available experimental data, where a remarkable thermal conductivity reduction is observed even for micro-grained Bi2Te3 samples. In this work, first-principles phonon MFPs along both the in-plane and cross-plane directions are re-computed for bulk Bi2Te3. These phonon MFPs can explain new and existing experimental data on flake-like Bi2Te3 nanostructures with various thicknesses. For polycrystalline Bi2Te3-based materials, a better explanation of the experimental data requires further consideration of the grain-boundary thermal resistance that can largely suppress the transport of high-frequency optical phonons.

Thermal transport in nanoporous holey silicon membranes investigated with optically induced transient thermal gratings

In this study, we use transient thermal gratings—a non-contact, laser-based thermal metrology technique with intrinsically high accuracy—to investigate room-temperature phonon-mediated thermal transport in two nanoporous holey silicon membranes with limiting dimensions of 120 nm and 250 nm, respectively. We compare the experimental results with ab initio calculations of phonon-mediated thermal transport according to the phonon Boltzmann transport equation (BTE) using two different computational techniques. We find that the calculations conducted within the Casimir framework, i.e., based on the BTE with the bulk phonon dispersion and diffuse scattering from surfaces, are in quantitative agreement with the experimental data and thus conclude that this framework is adequate for describing phonon-mediated thermal transport in silicon nanostructures with feature sizes of the order of 100 nm.

Magneto-optical transport properties of parabolic potential GaAs/Ga1−Al As wells under the influence of Al concentration and hydrostatic pressure

Abstract Magneto-optical transport properties of GaAs/Ga1−κAlκAs parabolic potential quantum-wells (PQWs) under the effects of Al-contents, hydrostatic pressure, and temperature is examined by studying their effects on the absorption power (AP) for magnetophonon resonance (MPR) and on the full width at half maximum (FWHM) of the optically-detected-MPR (ODMPR) peak due to modes confinement represented by slab-mode model, Huang-Zhu (HZ) model, guided-mode model. The numerical results indicate the Al-contents, pressure, as well as temperature which have great effect on the AP and FWHM in PQWs. Moreover, the results are also compared to bulk phonon modes for both two processes of phonons (emission and absorption). The results denote that phonon absorption process is always much more dominant compared to phonon emission process for four types of above phonons. In particular, the influences of pressure, Al-concentration in Ga1−κAlκAs barriers on the magneto-phonon resonance AP and FWHM in PQWs are more pronounced as confined modes are taken into account.

Probing and steering bulk and surface phonon polaritons in uniaxial materials using fast electrons: Hexagonal boron nitride

We theoretically describe how fast electrons couple to polaritonic modes in uniaxial materials by analyzing the electron energy loss (EEL) spectra. We show that in the case of an uniaxial medium with hyperbolic dispersion, bulk and surface modes can be excited by a fast electron traveling through the volume or along an infinite interface between the material and vacuum. Interestingly, and in contrast to the excitations in isotropic materials, bulk modes can be excited by fast electrons traveling outside the uniaxial medium. We demonstrate our findings with the representative uniaxial material hexagonal boron nitride. We show that the excitation of bulk and surface phonon polariton modes is strongly related to the electron velocity and highly dependent on the angle between the electron beam trajectory and the optical axis of the material. Our work provides a systematic study for understanding bulk and surface polaritons excited by a fast electron beam in hyperbolic materials and sets a way to steer and control the propagation of the polaritonic waves by changing the electron velocity and its direction.