Phonon States Research Articles

Temperature-dependent lattice dynamics in iridium

The characterization of simple elemental systems is key to benchmarking first-principles modeling of electronic and vibrational behaviors of materials. A large body of literature has been built for most elemental systems in the periodic table. However, surprisingly little neutron work has been performed to probe the vibrational properties of iridium, likely due to its large neutron absorption cross section. Nonetheless, iridium is of significant scientific and technological interest due to large relativistic electron effects and electron-phonon coupling, particularly in strongly correlated iridate compounds. Here, we report temperature-dependent inelastic neutron scattering measurements of the vibrational properties of iridium, from which we extract key thermodynamic properties. To overcome the challenge of the large neutron absorption of iridium, we developed a simple postprocessing correction procedure. The measured densities of phonon states compare well with quasiharmonic density functional theory calculations, although the obtained experimental phonon Gr\uneisen parameters are much larger than expected, reaching as high as $\ensuremath{\gamma}=4.5$, indicating substantial anharmonicity.

Topological phonons in oxide perovskites controlled by light.

Perovskite oxides exhibit a rich variety of structural phases hosting different physical phenomena that generate multiple technological applications. We find that topological phonons-nodal rings, nodal lines, and Weyl points-are ubiquitous in oxide perovskites in terms of structures (tetragonal, orthorhombic, and rhombohedral), compounds (BaTiO3, PbTiO3, and SrTiO3), and external conditions (photoexcitation, strain, and temperature). In particular, in the tetragonal phase of these compounds, all types of topological phonons can simultaneously emerge when stabilized by photoexcitation, whereas the tetragonal phase stabilized by thermal fluctuations only hosts a more limited set of topological phonon states. In addition, we find that the photoexcited carrier concentration can be used to tune the topological phonon states and induce topological transitions even without associated structural phase changes. Overall, we propose oxide perovskites as a versatile platform in which to study topological phonons and their manipulation with light.

Atomic Relaxation and Vibration Properties of the Cu(111)–(√3 × √3)R30°–Cr Surface

The structural and vibration properties of the (√3 × √3)R30°–Cr adsorption structure on the Cu(111) surface and the Cu(111)–(√3 × √3)R30°–Cr two-dimensional alloy incorporated into the S – 1 substrate layer are studied using the tight binding approximation. Surface relaxation is discussed along with the dispersion of surface phonons and the local density of vibration states of adatoms and substrate atoms. It has been demonstrated that the presence of chromium atoms modifies the structural and vibration characteristics of the substrate and leads to the appearance of new phonon states in it. The analysis of the entire set of data has shown that the Cu(111)–(√3 × √3)R30°–Cr subsurface two-dimensional alloy is a more stable configuration.

Magnon polaron formed by selectively coupled coherent magnon and phonon modes of a surface patterned ferromagnet

Strong coupling between two quanta of different excitations leads to the formation of a hybridized state which paves a way for exploiting new degrees of freedom to control phenomena with high efficiency and precision. A magnon polaron is the hybridized state of a phonon and a magnon, the elementary quanta of lattice vibrations and spin waves in a magnetically-ordered material. A magnon polaron can be formed at the intersection of the magnon and phonon dispersions, where their frequencies coincide. The observation of magnon polarons in the time domain has remained extremely challenging because the weak interaction of magnons and phonons and their short lifetimes jeopardize the strong coupling required for the formation of a hybridized state. Here, we overcome these limitations by spatial matching of magnons and phonons in a metallic ferromagnet with a nanoscale periodic surface pattern. The spatial overlap of the selected phonon and magnon modes formed in the periodic ferromagnetic structure results in a high coupling strength which, in combination with their long lifetimes allows us to find clear evidence of an optically excited magnon polaron. We show that the symmetries of the localized magnon and phonon states play a crucial role in the magnon polaron formation and its manifestation in the optically excited magnetic transients.

Vibrational properties of 1D- and 3D polynuclear spin crossover Fe(II) urea-triazoles polymer chains and quantification of intrachain cooperativity

The vibrational dynamics of the iron centres in 1D and 3D spin crossover Fe(II) 4-alkyl-urea triazole chains have been investigated by synchrotron based nuclear inelastic scattering. For the 1D system, the partial density of phonon states has been modelled with density functional theory methods. Furthermore, spin dependent iron ligand distances and vibrational modes were obtained. The previously introduced intramolecular cooperativity parameter H coop (Rackwitz et al, Phys. Chem. Chem. Phys. 2013, 15, 15450) has been determined to −31 kJ mol−1 for [Fe(n-Prtrzu)3(tosylate)2] and to +27 kJ mol−1 for [Fe(n-Prtrzu)3(BF4)2]. The change of sign in H coop is in line with the incomplete and gradual character of the spin transition for the former as well as with the sharp transition for the latter reported previously (Rentschler and von Malotki, Inorg. Chem., Act. 2008, 361, 3646). This effect can be ascribed to the networks of intramolecular interactions in the second coordination sphere of the polymer chains, depending on the spin state of the iron centres. In addition, we observe a decreased coupling and coherence when comparing the system which displays a sharp spin transition to the system with an incomplete soft transition by analyzing molecular modes involving a movement of the iron centres.

Persistent charge and spin currents in the 1D Holstein-Hubbard ring at half filling and at away from half filling by Bethe-ansatz approach

The persistent charge and spin currents are studied by considering the Holstein-Hubbard model for one dimensional mesoscopic ring. Conventional Lang-Firsov transformation is used to eliminate the phonon coordinates and then the zero phonon state is applied to obtain the effective electronic Hamiltonian which is then treated exactly by the Bethe-Ansatz technique. The coupled transcendental equations that arise in this method are solved numerically and the persistent charge and spin currents are studied with respect to the on-site electron-electron Coulomb correlation (U), on-site electron-phonon interaction strength (g0) and inter-site electron-phonon interaction strength (g1) at half-filling and away from half-filling.

Energy transfer process of Nd3+/Ho3+ co-doped fluoride halide glasses with anion substituted multi-wavelength tunable mid-infrared luminescence

Ho3+ doped ZBLAN glass with 2.0 and 2.9 μm emission was prepared. In order to further improve the luminescence of Ho3+, halogen ions (Cl, Br, I) were introduced to reduce the maximum phonon energy and phonon state density of the sample. At the same time, Nd3+ was introduced to transfer the energy to Ho3+ pumped with a 793 nm laser (Nd3+:4F5/2,4F3/2→Ho3+:5I6). The effect of different halogen ion on the luminescent properties of the fluoride halide glass was compared. The results show that the luminescent intensity of infrared increases with the introduction of different halogen ions. By comparison, it is found that the sample with I– has the strongest luminescence of 1064 nm, 2.0 μm and 2.9 μm. This is consistent with the calculated J-O intensity parameters. In addition, the 2.0 and 2.9 μm emission of Ho3+ pumped with a 450 nm laser will not disappear. A mid-infrared sample with multi-wavelength excitation and multi-wavelength emission can be obtained. Nd3+/Ho3+ co-doped fluoride halide glasses with 1064 nm, 2.0 μm and 2.9 μm luminescence were prepared by melt quenching method. The luminescent mechanism and the energy transfer process between the two ions of Nd3+/Ho3+ co-doped fluoride halide glass were studied. The J-O parameters, luminescence lifetime and absorption emission cross-sectional area of Ho3+ and Nd3+ were calculated, respectively. It is found that the value of Ω2 in the glass matrix increases with the introduction of different halogen ions, while Ω4 and Ω6 do not change obviously in different glass compositions. This is because the environment of the crystal field around the rare earth ions changes. The crystal phase and phonon energy of the sample were analyzed by X-ray diffraction pattern and a Fourier transform infrared spectrometer, respectively. Based on the above spectra and data (phonon energy is 634.71 cm−1), it can be predicted that Nd3+/Ho3+ co-doped fluoride halide glass is a potential mid-infrared luminescent material.

Breakdown of Raman selection rules by Fröhlich interaction in few-layer WS2

The Raman selection rules arise from the crystal symmetry and then determine the Raman activity and polarization of scattered phonon modes. However, these selection rules can be broken in resonant process due to the strong electron-phonon coupling effect. Here we reported the observation of breakdown of Raman selection rules in few-layer WS$_2$ by using resonant Raman scattering with dark A exciton. In this case, not only the infrared active modes and backscattering forbidden modes are observed, but the intensities of all observed phonon modes become strongest under paralleled-polarization and independent on the Raman tensors of phonons. We attributed this phenomenon to the interaction between dark A exciton and the scatted phonon, the so-called intraband Fr\"{o}hlich interaction, where the Raman scattering possibility is totally determined by the symmetry of exciton rather than the phonons due to strong electron-phonon coupling. Our results not only can be used to easily detect the optical forbidden excitonic and phononic states but also provide a possible way to manipulate optical transitions between electronic levels.

Generation and dynamics of entangled fermion–photon–phonon states in nanocavities

AbstractWe develop the analytic theory describing the formation and evolution of entangled quantum states for a fermionic quantum emitter coupled simultaneously to a quantized electromagnetic field in a nanocavity and quantized phonon or mechanical vibrational modes. The theory is applicable to a broad range of cavity quantum optomechanics problems and emerging research on plasmonic nanocavities coupled to single molecules and other quantum emitters. The optimal conditions for a tripartite entanglement are realized near the parametric resonances in a coupled system. The model includes dissipation and decoherence effects due to coupling of the fermion, photon, and phonon subsystems to their dissipative reservoirs within the stochastic evolution approach, which is derived from the Heisenberg–Langevin formalism. Our theory provides analytic expressions for the time evolution of the quantum state and observables and the emission spectra. The limit of a classical acoustic pumping and the interplay between parametric and standard one-photon resonances are analyzed.

Phonon Spectroscopy in Antimony and Tellurium Oxides.

α-Sb2O3 (senarmontite), β-Sb2O3 (valentinite), and α-TeO2 (paratellurite) are compounds with pronounced stereochemically active Sb and Te lone pairs. The vibrational and lattice properties of each have been previously studied but often lead to incomplete or unreliable results due to modes being inactive in infrared or Raman spectroscopy. Here, we present a study of the relationship between bonding and lattice dynamics of these compounds. Mössbauer spectroscopy is used to study the structure of Sb in α-Sb2O3 and β-Sb2O3, whereas the vibrational modes of Sb and Te for each oxide are investigated using nuclear inelastic scattering, and further information on O vibrational modes is obtained using inelastic neutron scattering. Additionally, vibrational frequencies obtained by density functional theory (DFT) calculations are compared with experimental results in order to assess the validity of the utilized functional. Good agreement was found between DFT-calculated and experimental density of phonon states with a 7% scaling factor. The Sb-O-Sb wagging mode of α-Sb2O3 whose frequency was not clear in most previous studies is experimentally observed for the first time at ∼340 cm-1. Softer lattice vibrational modes occur in orthorhombic β-Sb2O3 compared to cubic α-Sb2O3, indicating that the antimony bonds are weakened upon transforming from the molecular α phase to the layer-chained β structure. The resulting vibrational entropy increase of 0.45 ± 0.1 kB/Sb2O3 at 880 K accounts for about half of the α-β transition entropy. The comparison of experimental and theoretical approaches presented here provides a detailed picture of the lattice dynamics in these oxides beyond the zone center and shows that the accuracy of DFT is sufficient for future calculations of similar material structures.

IN THE CONTEXT OF TIME-INDEPENDENT PARAMETERS IN TWO QUANTUM SYSTEMS: QUANTUM ENTANGLEMENT AND CORRELATIONS WITH NEGATIVITY MEASUREMENT

In this paper, we consider two phonons and three-level trapped ion in configuration forming Hilbert 12-space. The negativity and quantum correlations are revealed in trapped ion two phonon states system. Three values of LDP, = 0.006, =0.06 and =0.08 are given. The effects of the time-independent coupling in terms of the system, degree of quantum entanglement are investigated. Therefore, we have found the main optimal times for obtaining the high amount of entanglement with negativity.In this paper, we consider two phonons and three-level trapped ion in configuration forming Hilbert 12-space. The negativity and quantum correlations are revealed in trapped ion two phonon states system. Three values of LDP, = 0.006, =0.06 and =0.08 are given. The effects of the time-independent coupling in terms of the system, degree of quantum entanglement are investigated. Therefore, we have found the main optimal times for obtaining the high amount of entanglement with negativity.

Electron energy relaxation in disordered superconducting NbN films

We report on the inelastic-scattering rate of electrons on phonons and relaxation of electron energy studied by means of magnetoconductance, and photoresponse, respectively, in a series of strongly disordered superconducting NbN films. The studied films with thicknesses in the range from 3 to 33 nm are characterized by different Ioffe-Regel parameters but an almost constant product q_Tl(q_T is the wave vector of thermal phonons and l is the elastic mean free path of electrons). In the temperature range 14-30 K, the electron-phonon scattering rates obey temperature dependencies close to the power law 1/\tau_{e-ph} \sim T^n with the exponents n = 3.2-3.8. We found that in this temperature range \tau_{e-ph} and n of studied films vary weakly with the thickness and square resistance. At 10 K electron-phonon scattering times are in the range 11.9-17.5 ps. The data extracted from magnetoconductance measurements were used to describe the experimental photoresponse with the two-temperature model. For thick films, the photoresponse is reasonably well described without fitting parameters, however, for thinner films, the fit requires a smaller heat capacity of phonons. We attribute this finding to the reduced density of phonon states in thin films at low temperatures. We also show that the estimated Debye temperature in the studied NbN films is noticeably smaller than in bulk material.

Thermodynamic properties of glassy phonon states induced by strong electron correlations in 𝜃-type organic charge transfer salts

The appearance of electron correlations–induced glassy state of low-energy phonons in the non-dimeric organic charge transfer salt in [Formula: see text]-(BEDT-TTF)2CsZn(SCN)4 is considered as a strong evidence of charge lattice coupling in molecular charge transfer salts. We discuss the temperature and the magnetic field dependences of the heat capacity of this salt in terms of the soft potential model to describe the thermodynamic properties of enhanced phonons that occur in molecular glasses. The evaluated [Formula: see text] term of [Formula: see text]-(BEDT-TTF)2CsZn(SCN)4 is about 30 mJ K[Formula: see text] mol[Formula: see text], which is much larger than other charge transfer salts described by the low-temperature approximation of the Debye model. The magnetic fields dependence of the boson peak is almost negligible, but the low-energy term, for example, in the temperature-linear term of heat capacity shows a slight change, probably due to the small amount of localized spin moments. The comparison with other systems is also performed.

Spectral features of one dimensional phononic quasicrystals

We make an in-depth analysis of phonon frequencies and phononic eigenstates for one-dimensional phononic lattices. The results are analyzed for two different types of lattices, depending on whether the spring constants are uniform or aperiodic. For the first case, we find usual band structures with extended states, while several non-trivial features are obtained for the latter one. We find the eigenfrequencies by solving the set of coupled equations involving the motions of different atoms in the chain. The frequency spectrum reveals a fractal like behaviour and the fractality gradually decreases with the increase of the strength of the aperiodic modulation. The nature of different phonon states is characterized by calculating inverse participation ratio. A finite transition from conducting to non-conducting phase is obtained upon the variation of the modulation strength. We hope the results studied here can easily be tested in a suitable setup.

Long lifetime of the E1u in-plane infrared-active modes of h -BN

We present an infrared reflectivity study of the ${E}_{1u}$ in-plane phonons of hexagonal BN as a function of temperature in the 40--680 K range. The infrared reflectance spectra of high-quality lamellar single crystals are accurately fitted using Lowndes' factorized form of the dielectric response, where the longitudinal-optic (LO) frequency is an independent adjustable parameter. From this analysis we obtain reliable values for the phonon damping of the IR-active ${E}_{1u}$ phonons which couple to light and give rise to the phonon-polariton excitations in this hyperbolic material. Anharmonic coupling potentials are estimated from the temperature dependence of the damping parameters. The ${E}_{1u}(\text{LO})$ mode exhibits a substantially longer lifetime than its transverse-optic counterpart, as it is basically unaffected by isotopic-disorder scattering owing to the low density of phonon states around the ${E}_{1u}(\text{LO})$ frequency.

Phononic and photonic properties of shape-engineered silicon nanoscale pillar arrays

We report the results of Brillouin–Mandelstam spectroscopy and Mueller matrix spectroscopic ellipsometry of the nanoscale ‘pillar with the hat’ periodic silicon structures, revealing intriguing phononic and photonic—phoxonic—properties. It has been theoretically shown that periodic structures with properly tuned dimensions can act simultaneously as phononic and photonic crystals, strongly affecting the light-matter interactions. Acoustic phonon states can be tuned by external boundaries, either as a result of phonon confinement effects in individual nanostructures, or as a result of artificially induced external periodicity, as in the phononic crystals. The shape of the nanoscale pillar array was engineered to ensure the interplay of both effects. The Brillouin–Mandelstam spectroscopy data indicated strong flattening of the acoustic phonon dispersion in the frequency range from 2 GHz to 20 GHz and the phonon wave vector extending to the higher-order Brillouin zones. The specifics of the phonon dispersion dependence on the pillar arrays’ orientation suggest the presence of both periodic modulation and spatial localization effects for the acoustic phonons. The ellipsometry data reveal a distinct scatter pattern of four-fold symmetry due to nanoscale periodicity of the pillar arrays. Our results confirm the dual functionality of the nanostructured shape-engineered structure and indicate a possible new direction for fine-tuning the light–matter interaction in the next generation of photonic, optoelectronic, and phononic devices.

Unsupervised Manifold Clustering of Topological Phononics.

Classification of topological phononics is challenging due to the lack of universal topological invariants and the randomness of structure patterns. Here, we show the unsupervised manifold learning for clustering topological phononics without any apriori knowledge, neither topological invariants nor supervised trainings, even when systems are imperfect or disordered. This is achieved by exploiting the real-space projection operator about finite phononic lattices to describe the correlation between oscillators. We exemplify the efficient unsupervised manifold clustering in typical phononic systems, including a one-dimensional Su-Schrieffer-Heeger-type phononic chain with random couplings, amorphous phononic topological insulators, higher-order phononic topological states, and a non-Hermitian phononic chain with random dissipations. The results would inspire more efforts on applications of unsupervised machine learning for topological phononic devices and beyond.

The Equation of Motion Phonon Method and its application to neutron rich odd nuclei

We report on the extension to odd nuclei of a microscopic multiphonon approach known as equation of motion phonon method and its application to the odd neighbors of the neutron rich 22O. A calculation using the chiral potential NNLO opt and encompassing up to two phonon basis states provides a description of the spectroscopic properties which is good quantitatively for 23O and qualitatively for 21O and 21N. Serious discrepancies between theory and experiments occur in 23F. A possible recipe for curing them is under investigation.

Vacuum-induced surface-acoustic-wave phonon blockade

Photon blockade via a giant self-Kerr nonlinearity in an optical resonator induced by an N-type quantum system is one of the main routines towards photon-based quantum information processing. However, such an N-type system for phonons is lacking. We propose an effective N-type quantum system for phonons, forming from a superconducting transmon qubit and a microwave resonator. In the absence of any external driving, the quantum vacuum-qubit interaction causes the surface acoustic wave (SAW) phonon blockade. Such a vacuum-induced phonon blockade paves the way to prepare single SAW phonons from a weak coherent phononic state for SAW accessible superconducting quantum computation.

Lattice Dynamics of Sb2Se3 from Inelastic Neutron and X‐Ray Scattering

The lattice dynamics of orthorhombic Sb2Se3 is studied by a combination of inelastic neutron and 121Sb nuclear inelastic scattering giving access to the total and Sb partial density of phonon states (DPS). The Se partial DPS is determined from the difference between the total and Sb partial DPS. The total DPS is determined at 39, 150, and 300 K, and an analysis of the temperature‐induced mode shifts in combination with low‐temperature powder diffraction data is provided. Using an earlier reported theoretical approach, the corresponding total and partial DPS of Sb2Se3 are calculated by first‐principles calculations. Herein, a detailed analysis of the Grüneisen parameter, element‐specific and bulk Debye temperatures, and the mean force constants as derived from the experimental data and discrete Fourier transform calculations is provided. In general, the calculations underestimate the strength of the covalent SbSe bonds.