Experimental Phonon Research Articles

Molecular Mechanism of the Piezoelectric Response in the β-Phase PVDF Crystals Interpreted by Periodic Boundary Conditions DFT Calculations.

A theoretical approach based on Periodic Boundary Conditions (PBC) and a Linear Combination of Atomic Orbitals (LCAO) in the framework of the density functional theory (DFT) is used to investigate the molecular mechanism that rules the piezoelectric behavior of poly(vinylidene fluoride) (PVDF) polymer in the crystalline β-phase. We present several computational tests highlighting the peculiar electrostatic potential energy landscape the polymer chains feel when they change their orientation by a rigid rotation in the lattice cell. We demonstrate that a rotation of the permanent dipole through chain rotation has a rather low energy cost and leads to a lattice relaxation. This justifies the macroscopic strain observed when the material is subjected to an electric field. Moreover, we investigate the effect on the molecular geometry of the expansion of the lattice parameters in the (a, b) plane, proving that the rotation of the dipole can take place spontaneously under mechanical deformation. By band deconvolution of the IR and Raman spectra of a PVDF film with a high content of β-phase, we provide the experimental phonon wavenumbers and relative band intensities, which we compare against the predictions from DFT calculations. This analysis shows the reliability of the LCAO approach, as implemented in the CRYSTAL software, for calculating the vibrational spectra. Finally, we investigate how the IR/Raman spectra evolve as a function of inter-chain distance, moving towards the isolated chain limit and to the limit of a single crystal slab. The results show the relevance of the inter-molecular interactions on the vibrational dynamics and on the electro-optical features ruling the intensity pattern of the vibrational spectra.

Inelastic Neutron Scattering Study of Phonon Dispersion Relation in Higher Manganese Silicides

We report inelastic neutron scattering (INS) measurements of the phonon dispersion relation in higher manganese silicides (HMSs). A large ingot of HMS is synthesized using a slow cooling method, which is found to have Mn15Si26 as the primary phase. The sample is composed of highly oriented crystallites as confirmed by a neutron pole-figure study and thermal conductivity data. Our INS results are mostly consistent with earlier experimental and theoretical phonon studies in HMS, including the presence of a low-lying twisting mode. However, some discrepancies are also observed. Most notably, a 5 meV gap at the zone center and the softer dispersion relation of the low-lying twisting mode. We discuss the potential origins of these observations and their implications for the thermal properties of HMS.

Crystal effects in the vibrational spectra of one-dimensional molecular spin crossover crystals using molecular dynamics simulations

We use molecular dynamics simulations to model the iron phonon density of states (pDOS) of a spin crossover (SCO) crystal built from 39-nuclear [Fe(atrz)_3Cl_2] chains (atrz = 4-amino-1,2,4-triazole). This is possible by using an atomistic potential that depends on the spin state of the Fe atoms. While quantum-chemical methods are able to provide data on isolated SCO molecules, the computational costs necessitate the use of classical simulations to calculate crystal effects on the spectra. Data are provided for a high-spin (HS) crystal, in which all Fe atoms are in their HS state, and a low-spin (LS) crystal. HS and LS crystals show distinctively different spectra that are in agreement with experimental data. Compared to the single-molecule spectra, the crystal spectra show several distinctive features: (i) The spectra are smoother, and the main peaks are slightly reduced in amplitude. (ii) The effect of intermolecular interactions shows up in additional structure at long-wavelengths. Good agreement between experimental and simulated phonon spectra is observed. The lattice-dynamics analysis of the low-energy pDOS allows to extract the temperature dependence of the vibrational entropy of the LS and HS crystals. Our study thus provides novel information about the crystal effects in SCO crystals.

Physical Insights on the Phonon Dispersion of TiS2

AbstractTitanium disulfide is a quasi‐2D transition‐metal dichalcogenide relevant for various potential applications. To exploit its technological capabilities, it is important to determine its fundamental properties including the lattice dynamics. The TiS2 phonon dispersion curves available to date do not reproduce properly the experimental data for several reasons: i) all available experimental data are not taken into consideration, thus poor theory‐experiment agreements have been obtained; ii) the (unknown) frequency of the infrared mode A2u is erroneously assumed; and iii) such incorrect assignment has propagated in the literature, particularly in phonon dispersion calculations. It is presented here a thorough density functional theory analysis to determine the phonon dispersion curves of TiS2, accounting for the frequencies of all experimental phonon data available to date. These include the frequencies of nine zone‐edge Raman active modes of Ag‐intercalated TiS2, in addition to infrared, Raman, and neutron scattering data. An incorrect frequency assignment of the A2u mode in the literature is thoroughly discussed. Moreover, results of attenuated total reflection terahertz spectroscopy applied to TiS2 are provided. A self‐intercalation paradigm is presented to give a rationale for the temperature dependence of the poorly understood Raman features observed in pristine TiS2 at frequencies above the A1g mode.

Raman- and Infrared-Active Phonons in Nonlinear Semiconductor AgGaGeS4

AgGaGeS4 is an emerging material with promising nonlinear properties in the near- and mid-infrared spectral ranges. Here, the experimental phonon spectra of AgGaGeS4 single crystals synthesized by a modified Bridgman method are presented. The infrared absorption spectra are reported. They are obtained from the fitting of reflectivity to a model dielectric function comprising a series of harmonic phonon oscillators. In the Raman spectra, several modes are registered, which were not detected in previous works. The analysis of the experimental vibrational bands is performed on the basis of a comparison with reported data on structurally related binary, ternary, and quaternary metal chalcogenides. The temperature dependence of the Raman spectra between room temperature and 15 K is also investigated.

Lattice vibrational modes in changchengite from Raman spectroscopy and first principles electronic structure

AbstractWe measured room‐temperature phonon Raman spectra of changchengite (IrBiS) and compared the experimental phonon wavenumbers to the theoretical ones obtained by means of the ab initio density‐functional‐theory calculations in the presence and absence of the spin‐orbit coupling effects. Combining two different excitation photon energies all the symmetry predicted Raman modes are experimentally observed. The electronic properties of IrBiS are found to be similar to the recently studied isostructural compound IrBiSe showing a large Dresselhaus spin‐orbit valence band splitting. A good agreement between the experimental and theoretically predicted Raman phonon wavenumbers is found only when the lattice parameter is constrained to the experimental value. The inclusion of the spin orbit coupling does not significantly affect the phonon wavenumbers.

Thermal scattering law for ice based on neutron time-of-flight experiments carried out at the SEQUOIA spectrometer at the Oak Ridge National Laboratory

Precise estimation of neutron thermalization in moderators relies on high-fidelity thermal scattering cross-section data governed by the thermal scattering law (TSL). The Institut de Radioprotection et de Sûreté Nucléaire (IRSN) has been working on the development of improved TSL for light water ice. Many polymorphic phases of light water ice exist depending on the thermodynamic conditions. The most common type of ice at standard pressure and temperature below water freezing point (273.15 K) is ice-Ih. It is essential to have high-resolution experimental double differential data for developing and/or validating TSL for moderator materials.Existing experimental double-differential scattering data for ice-Ih are extremely sparse and of limited quality. New high-quality double-differential measurements for ice-Ih over multiple temperatures and incident neutron energies would directly support the validation and improvement of ice-Ih TSL models for criticality safety applications. Series of time-of-flight (TOF) inelastic neutron scattering experiments on ice-Ih at temperatures starting at 271 K and down to 6 K, have been carried out at the SEQUOIA spectrometer at the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL), United States. The experiments have been performed for incident neutron energies, Ei= 11, 55, 160, 250, and 600 meV, to explore different excitation energies in the vibrational phonon spectrum. This paper presents the thermodynamic conditions and the details of the TOF measurements on ice-Ih and the derived phonon spectrum from the experimentally measured double differential data. A study of the variation of the phonon spectrum of ice-Ih as a function of temperature is highlighted, and a preliminary TSL evaluation is developed based on the experimental phonon spectrum.

Origin of the Acoustic Bandgaps in Hypersonic Colloidal Phononics: The Role of the Elastic Impedance.

How phonons propagate in nanostructures determines the flow of elastic and thermal energy in dielectric materials. However, a reliable theoretical prediction of the phonon dispersion relation requires experimental verification both near to and far from the Brillouin zone of the nanostructure. We report on the experimental hypersonic phonon dispersion of hard (SiO2) and soft (polymer) fcc colloidal crystals infiltrated in liquid polydimethylsiloxane with different elastic impedance contrast using Brillouin light spectroscopy. We discuss the distinct differences with first-principles full elastodynamic calculations involving a multiple-scattering theory. Interparticle contacts strongly impact the long-wavelength speed of sound and the nature of the particle vibration resonance-induced hybridization hypersonic bandgap. The absence of the order-induced Bragg bandgap in SiO2 and its presence in soft opals cannot be fully accounted for by the theory, limiting its predictive power. Bridging the elasticity of the two colloidal crystals with suitable SiO2 core–shell (polymer) particles reveals an unprecedented crossover behavior in the dispersion relation. In view of many conversational parameters, the control tuning of phonon propagation in soft matter-based hypersonic phononics remains challenging.

K-point longitudinal acoustic phonons are responsible for ultrafast intervalley scattering in monolayer MoSe2

In transition metal dichalcogenides, valley depolarization through intervalley carrier scattering by zone-edge phonons is often unavoidable. Although valley depolarization processes related to various acoustic phonons have been suggested, their optical verification is still vague due to nearly degenerate phonon frequencies on acoustic phonon branches at zone-edge momentums. Here we report an unambiguous phonon momentum determination of the longitudinal acoustic (LA) phonons at the K point, which are responsible for the ultrafast valley depolarization in monolayer MoSe2. Using sub-10-fs-resolution pump-probe spectroscopy, we observed coherent phonons signals at both even and odd-orders of zone-edge LA mode involved in intervalley carrier scattering process. Our phonon-symmetry analysis and first-principles calculations reveal that only the LA phonon at the K point, as opposed to the M point, can produce experimental odd-order LA phonon signals from its nonlinear optical modulation. This work will provide momentum-resolved descriptions of phonon-carrier intervalley scattering processes in valleytronic materials.

Symmetry-enforced nodal cage phonons in Th2BC2

Exploring unique topological states in condensed-matter systems has attracted great interest especially for the topological phonons recently. Based on the unbiased structure prediction approach combined with first-principles calculations, the long-sought crystal structure of ${\mathrm{Th}}_{2}{\mathrm{BC}}_{2}$ is determined. Most importantly, we show by the symmetry analysis and the phonon tight-binding Hamiltonian that ${\mathrm{Th}}_{2}{\mathrm{BC}}_{2}$ hosts nodal surface phonons on the ${q}_{z}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}$ plane, coexisting with nodal line phonons on the ${q}_{y}=0$ and ${q}_{y}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}$ planes, consequently, forming cagelike phonons. The nodal surface phonons are protected by the screw axis ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{C}}_{2z}$, and the nodal line phonons are enforced by inversion and time-reversal symmetries, demonstrated by the codimension argument and the effective model analysis. In addition, we also investigate the phonon surface states and the isofrequency arc on the (100) surface, which benefit the confirmation of the nodal cage phonons in experiments. Our paper not only determines the long-sought crystal structure of ${\mathrm{Th}}_{2}{\mathrm{BC}}_{2}$, but also provides an ideal candidate to realize the exotic topological phonon excitations.

Thermal expansion and phonon anharmonicity of cuprite studied by inelastic neutron scattering and ab initio calculations

Inelastic neutron scattering measurements were performed with a time-of-flight chopper spectrometer to observe phonons in all parts of the Brillouin zone of a single crystal of cuprite ${\mathrm{Cu}}_{2}\mathrm{O}$. We reduced the experimental data to phonon dispersions in the high-symmetry directions, and changes between 10 and 300 K are reported. In this paper, we show ab initio quasiharmonic (QH) and anharmonic (AH) calculations of phonon dispersions. We performed all AH calculations with a temperature-dependent effective potential method. Both QH and AH calculations account for the small negative thermal expansion of cuprite at low temperatures. However, the measured temperature-dependent phonon behavior was predicted more accurately with the AH calculations than the QH ones. Nevertheless, at 300 K, the cubic AH used in this paper did not entirely account for the experimental phonon dispersions in cuprite.

Spin-Phonon Coupling in Ferromagnetic Monolayer Chromium Tribromide.

Novel 2D magnets exhibit intrinsic electrically tunable magnetism down to the monolayer limit, which has significant value for nonvolatile memory and emerging computing device applications. In these compounds, spin-phonon coupling (SPC) typically plays a crucial role in magnetic fluctuations, magnon dissipation, and ultimately establishing long-range ferromagnetic order. However, a systematic understanding of SPC in 2D magnets that combines theory and experiment is still lacking. In this work, monolayer chromium tribromide is studied to investigate SPC in 2D magnets via Raman spectroscopy and first principle calculations. The experimental Curie temperature and phonon shifts are found to be in good agreement with the numerical simulations. Specifically, it is demonstrated how magnetic exchange interactions affect phonon vibrations, which helps establish design fundamentals for 2D magnetic materials and other related devices.

Optical phonon mode assisted thermal conductivity in p-type ZrIrSb half-Heusler alloy: a combined experimental and computational study

Half-Heusler (HH) alloys with an 18 valence electron count have attracted significant interest in the area of research related to thermoelectrics (TEs). Understanding the novel transport properties exhibited by these systems with semiconducting ground state is an important focus area in this field. The large thermal conductivity shown by most of the HH alloys poses a major hurdle to improving the figure of merit (ZT). In addition, understanding the mechanism of thermal conduction in heavy-constituent HH alloys is an interesting aspect. Here, we have investigated the high-temperature TE properties of ZrIrSb through experimental studies, phonon dispersion and electronic band structure calculations. ZrIrSb is found to exhibit a substantially lower magnitude of resistivity and Seebeck coefficient near room temperature due to the existence of anti-site disorder between Ir/Sb and vacant sites. Interestingly, in ZrIrSb, lattice thermal conductivity is governed by a coupling between the acoustic and low-frequency optical phonon modes, which originates due to heavier Ir/Sb atoms. This coupling leads to an enhancement in the Umklapp processes due to the optical phonon excitations near the zone boundary, resulting in a lower magnitude of κ L. Our studies point to the fact that the simultaneous existence of two heavy mass elements within a simple unit cell can substantially decrease the lattice degrees of freedom.

The origin of ultra-low thermal conductivity of the Bi2Te2S compound and boosting the thermoelectric performance via carrier engineering

In this study, a series of Bi2Te2-xS1+x compounds were prepared by traditional melting-quenching-annealing process. In the range of 0.03 ≤ x ≤ 0.25, Bi2Te2-xS1+x compounds possess the Bi2Te3 structure. S atoms preferentially fully occupy the Te(2) sites, while any extra S atoms randomly occupy the Te(1) sites. Experimental results and phonon dispersion calculations demonstrate that the strong coupling of the phonon density of state contributed by Bi and Te(1) atoms results in the low avoided-crossing frequencies, sound velocities, Debye temperature and therefore the low lattice thermal conductivities. Moreover, substitutions of extra S atoms on the Te(1) site intensifies alloy phonon scattering that further suppresses the lattice thermal conductivity. An ultra-low lattice thermal conductivity of 0.49 W m−1 K−1 is achieved at 523 K. Doping with Cl increases the room temperature carrier concentration, resulting in a significantly enhanced power factor from 1.5 mW m−1 K−2 for the Bi2Te1.93S1.07 sample to 1.7 mW m−1 K−2 for the Cl-doped sample at 323 K. Furthermore, doping with Cl enhances phonon point defect scattering that leads to a suppression of the thermal conductivity to 0.41 W m−1 K−1 at 573 K. As a result of the optimization of the electronic transport properties and the reduction in the lattice thermal conductivity, Bi2Te1.93S1.065Cl0.005 sample achieved the highest ZT of 0.67 at 623 K which is superior to intrinsic Bi2Te3 in the whole temperature range.

Importance of macroscopic polarization on vibrational properties and the robust nature of (001) surface states of SnTe

Here, we study the importance of macroscopic polarization for explaining the lattice dynamics of SnTe. Along with this, the robustness of (001) surface states is discussed for the practical application of this compound. The long range Coulomb forces using nonanalytical term correction are needed for proper explanation of experimental phonon modes with the Born effective charge's value of ∼5.5 (∼-5.5) for Sn (Te) atom. The peaks' positions of phonon density of states are in good agreement with the experimental observation. The Debye temperature for SnTe is found to be ∼189 K. We have also explored the possibility of protecting the (001) surface states via mirror and time-reversal symmetry. These surface states are expected to be robust against the point and line defects.

Examining Experimental Raman Mode Behavior in Mono- and Bilayer 2H-TaSe2 via Density Functional Theory: Implications for Quantum Information Science.

Tantalum diselenide (TaSe2) is a metallic transition metal dichalcogenide whose structure and vibrational behavior strongly depend on temperature and thickness, and this behavior includes the emergence of charge density wave (CDW) states at very low temperatures. In this work, observed Raman modes for mono- and bilayer are described across several spectral regions and compared to those seen in the bulk case. These modes, which include an experimentally observed forbidden Raman mode and low-frequency CDWs, are then matched to corresponding vibrations predicted by density functional theory (DFT). The reported match between experimental and computational results supports the presented vibrational visualizations of these modes. Support is also provided by experimental phonons observed in additional Raman spectra as a function of temperature and thickness. These results highlight the importance of understanding CDWs since they are likely to play a fundamental role in the future realization of solid-state quantum information platforms based on nonequilibrium phenomena.

Laser‐induced phonon and magnon properties of NiO nanoparticles: A Raman study

AbstractIn the present study, the influence of laser intensity on phonon and magnon properties of nickel oxide (NiO) nanoparticles were extensively studied using room temperature Raman spectroscopy. The required NiO nanoparticles were synthesized using a simple sol–gel method. The structural and morphological properties were characterized using powder X‐ray diffraction (XRD), field emission scanning electron microscopy (FE‐SEM), and transmission electron microscopy (TEM) techniques. The rarely reported surface optical (SO) phonon modes of NiO nanoparticles were observed and well‐studied using the dielectric continuum (DC) model. A mismatch between the experimental and DC model SO phonon wavenumbers were observed and is attributed to one magnon‐induced first order phonon shift. The difference between the experimental and DC model SO phonon wavenumbers were reduced with increase in laser power, which indicates the diminishing behavior of magnon excitations. The laser power dependency of highly distinguishable two magnon (2 M) excitation of NiO nanoparticles were well studied. The 2 M modes were experienced a red shift and decrease in intensity with increase in laser power.

Ab initio study of MgO under pressure using quasi-harmonic approximation

We examine the dependence of the Gibbs free energy and entropy on pressure and density along MgO isotherm 300 K. Some theoretical works have previously predicted a drastic drop of entropy along MgO isotherms by analyzing existing experimental data and extrapolating them to high pressures. We present first-principle calculations of thermodynamic properties of MgO under pressure using density functional theory and quasi-harmonic approximation. The robustness of our calculations is verified by comparing the calculated and experimental phonon dispersion curves. The comparison with available diamond anvil cell experimental data is also provided. Our estimate for the B1–B2 phase transition is in good agreement with other experimental and theoretical studies. However, our results for entropy along isotherm 300 K do not agree with previous theoretical estimates based upon the integration of thermal expansion coefficient and isothermal bulk modulus by volume.

Study of anisotropic thermal conductivity in textured thermoelectric alloys by Raman spectroscopy.

Polycrystalline p-type Sb1.5Bi0.5Te3 (SBT) and n-type Bi2Te2.7Se0.3 (BTS) compounds possessing layered crystal structure show anisotropic electronic and thermal transport properties. This research is in pursuit of better understanding the anisotropic thermal properties using Raman spectroscopy. A systematic Raman spectroscopic study of the hot-pressed pellet of the textured p-type SBT and n-type BTS is reported in both directions: parallel (‖) and perpendicular (⊥) to the pressing axis as a function of temperature and laser power. The first-order temperature coefficient, optical thermal conductivity, and phonon lifetime are qualitatively determined from the temperature and laser power-dependent frequency and full-width half maximum (FWHM) of Raman peaks (A11g, E2g & A21g). Anisotropy in experimental phonon thermal conductivity in both directions is correlated with the approximated optical thermal conductivity, phonon lifetime and phonon anharmonicity. The anisotropy in phonon anharmonicity in both directions is explained by the modified Klemens–Hart–Aggarwal–Lax phonon decay model. In this study, the symmetric three-phonon scattering process is considered responsible for thermal transport in the temperature range of 300 to 473 K.

Strongly two-dimensional exchange interactions in the in-plane metallic antiferromagnet Fe2As probed by inelastic neutron scattering

To understand spin interactions in materials of the Cu$_2$Sb structure type, inelastic neutron scattering of Fe$_2$As single crystals was examined at different temperatures and incident neutron energies. The experimental phonon spectra match well with the simulated phonon spectra obtained from density functional theory (DFT) calculations. The measured magnon spectra were compared to the simulated magnon spectra obtained via linear spin wave theory with the exchange coupling constants calculated using the spin polarized, relativistic Korringa-Kohn-Rostoker method in Zhang et al. (2013). The simulated magnon spectra broadly agree with the experimental data although, the energy values are underestimated along the $K$ direction. Exchange coupling constants between Fe atoms were refined by fits to the experimental magnon spectra, revealing stronger nearest neighbor Fe1-Fe1 exchange coupling than previously reported. The strength of this exchange coupling is almost an order of magnitude higher than other exchange interactions despite the three-dimensional nature of the phonon interactions. The lack of scattering intensity at energies above 60 meV makes unconstrained determination of the full set of exchange interactions difficult, which may be a fundamental challenge in metallic antiferromagnets.