Transverse Phonon Research Articles

Unified theoretical description of out-of-equilibrium electron intraband dynamics in gold induced by femtosecond laser pulses

The modeling of the intraband dynamics of conduction electrons in gold induced by a femtosecond laser pulse is addressed. Current approaches, based on the numerical resolution of the Boltzmann equation, are only able to describe electron excitation $\mathit{or}$ relaxation processes. In the present paper, within a single formalism, our kinetic model accounts quantitatively for both excitation and relaxation processes, i.e., photon absorption, thermal conductivity, and electron-phonon coupling coefficient. We suggest that such an approach can only be built by including Umklapp processes in the description of electron collisions. In addition to normal electronic collisional processes with phonons and other electrons, the present theoretical description further includes these mechanisms: (i) a conduction electron can be scattered in the second Brillouin zone after a collision, contributing significantly to the photon absorption, and (ii) the influence of the discrete and periodic nature of the lattice on electron collisions is considered, enabling in particular the coupling between electron and transverse phonons. A good agreement of predictions of this unified modeling with experimental observations for absorption and energy transfer from electrons to the lattice is obtained. We have investigated the influence of the Umklapp processes on the imaginary part of the dielectric function, the electron-phonon coupling parameter, and the transient shape of the electron energy distribution.

Plasmon Coupling—The Root Cause of Raman Anomaly and Laser Cooling in Nanocrystal Ge

Laser cooling of matter through anti‐Stokes photoluminescence, where the emitted frequency of light exceeds that of the impinging laser light by virtue of absorption of thermal vibrational energy, has been successfully realized in condensed media, and in particular with rare‐earth‐doped systems achieving sub‐100 K solid‐state optical refrigeration. Studies suggest that laser cooling in semiconductors has the potential of achieving temperatures down to ≈10 K and that its direct integration can usher in unique high‐performance nanostructured semiconductor devices. While laser cooling of nanostructured II–VI semiconductors has been reported recently, laser cooling of indirect bandgap semiconductors such as group IV silicon and germanium remains a major challenge. Herein, the anomalous observation of dominant anti‐Stokes photoluminescence in germanium nanocrystals principally associated with plasmon coupling is reported. Specifically, this Raman anomaly to the confluence of ultrahigh‐purity nanocrystal germanium, generation of high density of electron–hole plasma, the inherent degeneracy of longitudinal and transverse optical phonons in nonpolar indirect bandgap semiconductors, and simultaneous spatial confinement effects are attributed. At high laser intensities, plasmon‐assisted laser cooling with lattice temperature as low as ≈50 K is inferred.

Anisotropic resistivity and superconducting instability in ferroelectric metals

We propose a theoretical model of a ferroelectric metal where spontaneous electric polarization coexists with the conducting electrons. In our model we adopt a scenario when conducting electrons interact with two soft transverse optical phonons, generalize it to the case when there is a spontaneous ferroelectric polarization in the system, and show that a linear coupling to the phonons emerges as a result. We find that this coupling results in anisotropic electric transport which has a transverse to the current voltage drop. Importantly, the obtained transverse component of the resistivity has distinct linear dependence with temperature. Moreover, we show that the coupling enhances superconducting transition temperature of the ferroelectric metal. We argue that our results help to explain recent experiments on ferroelectric strontium titanate, as well as provide new experimental signatures to look for.

INFRARED REFLECTION SPECTRA OF MnxFe1–xIn2S4 SOLID SOLUTIONS

Large-block crystals of FeIn2S4, MnIn2S4 ternary compounds and MnxFe1–xIn2S4 solid solutions are grown by the method of directional crystallization (horizontal Bridgman method). The structure of the obtained crystals is determined by X-ray diffraction analysis. It is shown that both the starting compounds and solid solutions based on them crystallize in the cubic spinel structure. The IR reflection spectra in the frequency range 50–500 cm–1 of crystals of ternary compounds FeIn2S4, MnIn2S4 and solid solutions MnxFe1–xIn2S4 are studied. The frequencies of transverse (ωTO) and longitudinal (ωLO) optical phonons are determined. The concentration dependences of these parameters are plotted and the nature of their behavior is established.

Structure and Lattice Dynamics of Bi1−xNdxFeO3 and Bi1−xGdxFeO3 Ceramics near the Morphotropic Phase Boundary

The crystal structures of Bi1−xNdxFeO3 and Bi1−xGdxFeO3 solid solutions (0 ≤ x ≤ 0.2) with chemical compositions across structural transformations from the polar rhombohedral phase to the orthorhombic phase with an antipolar distortion and then to the nonpolar orthorhombic phase have been investigated using X-ray diffraction and infrared reflective spectrometry. The obtained results clarify details of the structural transitions assuming the changes that occurred in the crystal lattice dynamics of the compounds. Increase in the dopant content causes a notable change in the intensity and position of the reflectance lines at 18.2 μm and 22.6 μm (550 cm−1 and 440 cm−1) ascribed to the transverse optical phonon modes associated with Bi (Nd, Gd)–O and Fe–O bonds. In the concentration region attributed to the dominant rhombohedral phase, the chemical substitution leads to an increase in intensity of the modes A1 for solid solutions of both systems. Meanwhile, in the case of Gd doping, the mode A1 shifts towards the red side of the spectrum, but there is an opposite tendency in the case of Nd doping; the intensity of the modes E decrease regardless of both the dopant-ion type and concentration. This behavior is discussed assuming the change in mass for the chain of chemical bonds caused by different dopant ions and the structural transformations occurring in the compounds upon chemical doping.

Ultrafast Suppression of the Ferroelectric Instability in KTaO_{3}.

We use an x-ray free-electron laser to study the lattice dynamics following photoexcitation with ultrafast near-UV light (wavelength 266nm, 50fs pulse duration) of the incipient ferroelectric potassium tantalate, KTaO_{3}. By probing the lattice dynamics corresponding to multiple Brillouin zones through the x-ray diffuse scattering with pulses from the Linac Coherent Light Source (LCLS) (wavelength 1.3Å and <10 fs pulse duration), we observe changes in the diffuse intensity associated with a hardening of the transverse acoustic phonon branches along Γ to X and Γ to M. Using force constants from density functional theory, we fit the quasiequilibrium intensity and obtain the instantaneous lattice temperature and density of photoexcited charge carriers. The density functional theory calculations demonstrate that photoexcitation transfers charge from oxygen 2p derived π-bonding orbitals to Ta 5d derived antibonding orbitals, further suppressing the ferroelectric instability and increasing the stability of the cubic, paraelectric structure.

Thermal decay in the one-dimensional transient thermal grating experiment using modified Debye-Callaway model

We calculate the thermal decay in the one-dimensional transient thermal grating (TTG) experiment by solving the transient Boltzmann-Peierls transport equation (BPTE) within the framework of the single-mode relaxation time approximation and using modified Debye-Callaway model in which both longitudinal and transverse phonon modes are included explicitly. We consider surface heating of an opaque thick semiconductor (SC) crystal film that we assume to have a cubic symmetry and is treated as a continuum, elastic, isotropic, and dispersionless medium. We obtain a nonuniversal spectral suppression function (SSF) in the integrand of the effective apparent thermal conductivity that is similar to the one obtained by Chiloyan et al. [Phys. Rev. B 93, 155201 (2016)] using the standard single-mode relaxation time approximation (RTA) model. Therefore, the nonuniversal character of the SSF in the TTG experiment does not depend on the form of the collision operator approximation in the BPTE: Callaway's or standard. Moreover, the analysis of the behavior of the thermal decay rate shows how the peculiar crystal momentum shuffling effect of phonon-phonon scattering Normal processes (N processes) that is captured by Callaway's model influences the onset of the nondiffusive (quasiballistic) regime in the phonon transport process in SC crystals. This effect tends independently from the other phonon scattering processes to favor the maintenance of the phonon diffusive regime over a large length-scale range, a remarkable feature that cannot be put into light with the standard RTA model used in previous works. Hence, the implicit effect of N processes has certainly an important impact on the extraction of the phonon mean-free path spectrum distribution, especially in the high-temperature regime.

Effects of Boron Doping on the Bulk and Surface Acoustic Phonons in Single-Crystal Diamond.

We report the results of the investigation of bulk and surface acoustic phonons in the undoped and boron-doped single-crystal diamond films using the Brillouin-Mandelstam light scattering spectroscopy. The evolution of the optical phonons in the same set of samples was monitored with Raman spectroscopy. It was found that the frequency and the group velocity of acoustic phonons decrease nonmonotonically with the increasing boron doping concentration, revealing pronounced phonon softening. The change in the velocity of the shear-horizontal and the high-frequency pseudo-longitudinal acoustic phonons in the degenerately doped diamond, as compared to that in the undoped diamond, was as large as ∼15% and ∼12%, respectively. As a result of boron doping, the velocity of the bulk longitudinal and transverse acoustic phonons decreased correspondingly. The frequency of the optical phonons was unaffected at low boron concentration but experienced a strong decrease at the high doping level. The density-functional-theory calculations of the phonon band structure for the pristine and highly doped samples confirm the phonon softening as a result of boron doping in diamond. The obtained results have important implications for thermal transport in heavily doped diamond, which is a promising material for ultra-wide-band-gap electronics.

Evaluation of vibrational properties and local structure change during phase transition in Ge2Sb2Te5 and In3SbTe2 phase change materials

Temperature-dependent local structural details of In3SbTe2 (IST) and Ge2Sb2Te5 (GST) phase change materials are explored with the aid of Raman scattering and x-ray photoelectron spectroscopy (XPS) techniques. Significant temperature-induced changes occur in the local structure of the phase change material, facilitating the amorphous to crystalline phase transformation. These two phases exhibit a large resistivity contrast, which is utilized to store data in the phase change memory application. The Raman spectra recorded for IST material suggest that the as-deposited sample (amorphous phase) first crystallized around 300 annealing temperature with two binary phases InTe and InSb formations. InTe and InSb phases were obtained at ∼85.5 cm−1 and ∼180 cm−1 Raman shift, respectively, having vibration modes associated with B1g symmetry and transversal optical phonons with Г15 symmetry. Further annealing at a higher temperature of 400 , a ternary IST phase is obtained at ∼163 cm−1 Raman shift indicating the transformation to a fully crystalline state. On the other hand, the amorphous GST material forms a metastable face-centred cubic phase and stable hexagonal (HEX) phase upon crystallization. The Raman findings demonstrate the changes in vibration modes of Ge–Te and Sb–Te bonds during phase switching without any phase separation. Crystalline GST (HEX phase) comprises rising peaks of GeTe4 and GeTe4−n Ge n (n = 1, 2) corner-sharing tetrahedra with A1 mode at ∼104.5 cm−1 and ∼137 cm−1 Raman shift, respectively, and a declining Sb2Te3 peak at ∼175 cm−1, having A1g (2) vibration mode. Furthermore, the XPS analysis displays the changes in the bonding mechanism of the elements present in the phase change material during the amorphous to crystalline phase transition, firmly supporting the Raman observations.

Charge carrier coupling to the soft phonon mode in a ferroelectric semiconductor

Many crystalline solids possess strongly anharmonic ``soft'' phonon modes characterized by diminishing frequency as temperature approaches a critical point associated with a symmetry breaking phase transition. While electron--soft phonon coupling can introduce unique scattering channels for charge carriers in ferroelectrics, recent studies on the nonferroelectric lead halide perovskites have also suggested the central role of anharmonic phonons bearing resemblance to soft modes in charge carrier screening. Here we apply coherent phonon spectroscopy to directly study electron coupling to the soft transverse optical phonon mode in a ferroelectric semiconductor SbSI. Photogenerated charge carriers in SbSI are found to be exceptionally long lived and are associated with a transient electro-optical effect that can be explained by interactions between charge carriers and thermally stimulated soft phonon excitations. These results provide strong evidence for the role of electron--soft phonon coupling in the efficient screening of charge carriers and in reducing charge recombination rates, both desirable properties for optoelectronics.

Probing Nieh-Yan anomaly through phonon dynamics in the Kramers-Weyl semimetals of chiral crystals

Nieh-Yan anomaly describes the non-conservation of chiral charges induced by the coupling between Dirac fermions and torsion fields. Since the torsion field is beyond general relativity, this effect remains hypothetical and its relevance to our universe is unclear in the context of high-energy physics. In this work, we propose that the phonons can induce a torsion field for the Kramers-Weyl fermions through electron-phonon interaction in a non-magnetic chiral crystal, thus leading to the occurrence of the Nieh-Yan anomaly in this condensed matter system. As a consequence, the Nieh-Yan term can strongly influence the phonon dynamics and lead to the helicity of acoustic phonons, namely, two transverse phonon modes mix with each other to form a circular polarization with a non-zero angular momentum at a finite phonon momentum and the phonon angular momentum reverses its sign for opposite momenta due to time reversal symmetry. The phonon helicity can be probed through measuring the total phonon angular momentum driven by a temperature gradient.

High-harmonic spectroscopy of coherent lattice dynamics in graphene

High-harmonic spectroscopy of solids is a powerful tool, which provides access to both electronic structure and ultrafast electronic response of solids, from their band structure and density of states to phase transitions, including the emergence of the topological edge states, to the PetaHertz electronic response. However, in spite of these successes, high-harmonic spectroscopy has hardly been applied to analyze the role of coherent femtosecond lattice vibrations in the attosecond electronic response. Here we study coherent phonon excitations in monolayer graphene to show how high-harmonic spectroscopy can be used to detect the influence of coherent lattice dynamics, particularly longitudinal and transverse optical phonon modes, on the electronic response. Coherent excitation of the in-plane phonon modes results in the appearance of sidebands in the spectrum of the emitted harmonic radiation. We show that the spectral positions and the polarization of the sideband emission offer a sensitive probe of the dynamical symmetries associated with the excited phonon modes. Our work brings the key advantage of high-harmonic spectroscopy---the combination of subfemtosecond to tens of femtoseconds temporal resolution---to the problem of probing phonon-driven electronic response and its dependence on the dynamical symmetries in solids.

Simultaneous optical resonances at visible and mid-infrared frequencies with epitaxial TiN/Al0.72Sc0.28N/TiN metal/polar-dielectric/metal multilayers

Traditionally, light-matter interactions in the visible spectral range utilize tailored plasmonic nanostructure in metals that often require advanced nanofabrication techniques. Concomitantly, polar lattice vibrations in dielectrics are used to achieve mid-to-long wavelength infrared (IR) light-matter coupling. Achieving optical resonances simultaneously in the visible and mid-to-long wavelength IR spectral range in one host medium has been challenging due to mutually conflicting material and optical property requirements. In this article, we show simultaneous light-matter coupling and development of super absorbers in the visible and mid-IR spectral ranges with a lithography-free planar wide-angle metal-(polar)-dielectric-metal (MDM) Fabry-Pérot cavity composed of ultrathin refractory metallic TiN as the top layer and polar dielectric Al0.72Sc0.28N as the spacer layer. Lattice-matched TiN/Al0.72Sc0.28N/TiN MDM cavities exhibit super absorption resonance with maximum absorptivity of ∼99%. The absorption maxima spectral position is tuned with changes in spacer layer thickness and by utilizing multiple cavity interactions. Al0.72Sc0.28N inside the MDM is also used for light coupling to the transverse optical phonon mode and to the Berremann mode near the longitudinal phonon frequency with strong selective absorption. Demonstration of refractory epitaxial TiN/Al0.72Sc0.28N/TiN MDM metamaterials mark significant progress towards developing compact optical meta-structures with on-demand optical resonances at different spectral ranges.

Superconductivity from energy fluctuations in dilute quantum critical polar metals

Superconductivity in low carrier density metals challenges the conventional electron-phonon theory due to the absence of retardation required to overcome Coulomb repulsion. Here we demonstrate that pairing mediated by energy fluctuations, ubiquitously present close to continuous phase transitions, occurs in dilute quantum critical polar metals and results in a dome-like dependence of the superconducting Tc on carrier density, characteristic of non-BCS superconductors. In quantum critical polar metals, the Coulomb repulsion is heavily screened, while the critical transverse optical phonons decouple from the electron charge. In the resulting vacuum, long-range attractive interactions emerge from the energy fluctuations of the critical phonons, resembling the gravitational interactions of a chargeless dark matter universe. Our estimates show that this mechanism may explain the critical temperatures observed in doped SrTiO3. We provide predictions for the enhancement of superconductivity near polar quantum criticality in two- and three-dimensional materials that can be used to test our theory.

A toolbox for easy entry low wavenumber in situ atomic layer deposition transmission FTIR spectroscopy studies.

A detailed description of a flexible and portable atomic layer deposition (ALD) system is presented for conducting in situ Fourier transform infrared (FTIR) absorption spectroscopy studies during the evolution and growth of ALD films. The system is directly integrated with a commercial FTIR spectrometer (Bruker Vertex 80V) to avoid the necessity of an external optical path to the instrument, thereby mitigating complexity and optical losses. In this work, we use potassium bromide (KBr) with a 5nm layer of sputtered Si as a substrate due to higher infrared transmittance when compared to a single-side polished Si wafer. The FTIR absorption study is conducted at normal incidence in transmission mode using a deuterated L-alanine doped triglycine sulfate (DTGS) detector owing to its potential applicability for reliable measurements at wavenumbers below ∼700cm-1. We demonstrate this by measuring ex situ the transverse optical phonon of bulk Al2O3 centered at 680cm-1. The integrity and functionality of the system to track the nucleation stage are validated by conducting in situ FTIR absorption measurements of Al2O3 using tri-methyl aluminum (TMA) and H2O. The measured IR absorption spectra for the Al2O3 growth after each cycle of TMA and H2O show the formation and removal of CH3 (2800-3000cm-1) groups on the substrate surface and CH4 (3016 and 1306cm-1) molecules in the reactor, thus confirming the successful tracking of ligand exchange. Thus, this instrument, together with the choice of KBr as substrate, can enable straightforward ALD nucleation studies using a DTGS detector having sufficient signal without additional optical setup and modifications to off-the-shelf FTIR systems that allow low wavenumber experiments.

INFRARED REFLECTION SPECTRA OF SOLID SOLUTIONS (FeIn2S4)х·(In2S3)1–х

On homogeneous monocrystals of solid solutions (FeIn2S4)х·(In2S3)1–х, grown by the method of directional melt crystallization (vertical Bridgman method), IR reflection spectra in the frequency range of 50—450 cm–1 were studied. The frequencies of transverse (ωTO) and longitudinal (ωLO) optical phonons, as well as their damping coefficients, were determined. The concentration dependences of the mentioned parameters were constructed and the nature of their behavior was established.

Electron–phonon coupling in copper intercalated Bi_{2}Se_{3}

We report charge and heat transport studies in copper-intercalated topological insulator Bi_2Se_3 hybrid devices. Measured conductivity shows impact of quantum corrections, electron–electron and electron–phonon interactions. Our shot noise measurements reveal that heat flux displays a crossover between T^2 and T^4 with the increase of temperature. The results might be explained by a model of inelastic electron scattering on disorder, increasing the role of transverse acoustic phonons in the electron–phonon coupling process.

Mutual spin-phonon driving effects and phonon eigenvector renormalization in nickel (II) oxide

The physics of mutual interaction of phonon quasiparticles with electronic spin degrees of freedom, leading to unusual transport phenomena of spin and heat, has been a subject of continuing interests for decades. Despite its pivotal role in transport processes, the effect of spin-phonon coupling on the phonon system, especially acoustic phonon properties, has so far been elusive. By means of inelastic neutron scattering and first-principles calculations, anomalous scattering spectral intensity from acoustic phonons was identified in the exemplary collinear antiferromagnetic nickel (II) oxide, unveiling strong spin-lattice correlations that renormalize the polarization of acoustic phonon. In particular, a clear magnetic scattering signature of the measured neutron scattering intensity from acoustic phonons is demonstrated by its momentum transfer and temperature dependences. The anomalous scattering intensity is successfully modeled with a modified magneto-vibrational scattering cross-section, suggesting the presence of spin precession driven by phonon. The renormalization of phonon eigenvector is indicated by the observed "geometry-forbidden" neutron scattering intensity from transverse acoustic phonon. Importantly, the eigenvector renormalization cannot be explained by magnetostriction but instead, it could result from the coupling between phonon and local magnetization of ions.

High figure-of-merit and power generation in high-entropy GeTe-based thermoelectrics

The high-entropy concept provides extended, optimized space of a composition, resulting in unusual transport phenomena and excellent thermoelectric performance. By tuning electron and phonon localization, we enhanced the figure-of-merit value to 2.7 at 750 kelvin in germanium telluride-based high-entropy materials and realized a high experimental conversion efficiency of 13.3% at a temperature difference of 506 kelvin with the fabricated segmented module. By increasing the entropy, the increased crystal symmetry delocalized the distribution of electrons in the distorted rhombohedral structure, resulting in band convergence and improved electrical properties. By contrast, the localized phonons from the entropy-induced disorder dampened the propagation of transverse phonons, which was the origin of the increased anharmonicity and largely depressed lattice thermal conductivity. We provide a paradigm for tuning electron and phonon localization by entropy manipulation, but we have also demonstrated a route for improving the performance of high-entropy thermoelectric materials.

Resonant interaction between phonons and PbTe/PbSe (001) misfit dislocation networks

This work aims at a quantitative and mechanistic understanding of the dynamic process of the phonon-dislocation interaction in PbTe/PbSe (001) heterostructures using the Concurrent Atomistic-Continuum (CAC) method as the simulation tool. The misfit dislocation network and the atomic-scale dislocation core structure obtained in the simulations are found to agree reasonably well with the experimental observations of the PbTe/PbSe (001) interface. Through visualizing the dynamic interaction between phonons and dislocations, as well as quantifying the dislocation vibration amplitude, the phonon energy transmission, and the thermal resistance of the misfit interfaces, this work has illustrated and quantified two mechanisms for phonon-dislocation interaction: (1) phonon scattering by the strain field of dislocations, and (2) phonon scattering by dislocations that vibrate via the local modes of a dislocation network; the latter, leads to resonant phonon-dislocation interaction, which is manifested as local maxima of out-of-phase vibration of the atoms on the two sides of the slip plane, leading to local minima of the energy transmission in the heterostructure that contains one interface. The local vibrational modes are found to be excited only by shear stress induced by transverse phonons. Among various resonant modes, the one with the lowest frequency has the strongest effect. This work has also demonstrated the collective motion of dislocations under ultrafast phonon pulses. In addition, the dynamic properties of the misfit dislocation network localized within one interface are found to be significantly altered by the presence of misfit dislocations at other interfaces, thus further confirming the cooperative dynamic nature of the motion of dislocations and phonons.