Phonon Excitations Research Articles

Theory for shear displacement by light-induced Raman force in bilayer graphene

Coherent excitation of shear phonons in van der Waals layered materials is a non-destructive mechanism to fine-tune the electronic state of the system. We develop a diagrammatic theory for the displacive Raman force and apply it to the shear phonon's dynamics. We obtain a rectified Raman force density in bilayer graphene of the order of ${\cal F}\sim 10{\rm nN/nm^2}$ leading to a giant shear displacement $Q_0 \sim 50$pm for an intense infrared laser. We discuss both circular and linear displacive Raman forces. We show that the laser frequency and polarization can effectively tune $Q_0$ in different electronic doping, temperature, and scattering rates. We reveal that the finite $Q_0$ induces a Dirac crossing pair in the low-energy dispersion that photoemission spectroscopy can probe. Our finding provides a systematic pathway to simulate and analyze the coherent manipulation of staking order in the heterostructures of layered materials by laser irradiation.

Imaging Vibrational Excitations in the Electron Microscope

Vibrational spectroscopy is a powerful tool for probing atomic motions in molecules and extended solids. Recent advances in aberration-corrected, monochromated-scanning transmission electron microscopy (STEM) now allow imaging of individual phonon modes at the nanoscale, overcoming the spatial limit imposed by diffraction-limited optical spectroscopy techniques. In this Perspective, we summarize the recent progress in high-resolution electron energy-loss spectroscopy (EELS), which now enables vibrational spectroscopy to be performed in the electron microscope. The high spatial and energy resolution of low-loss EELS can be used to directly image phonons inside or near inorganic and organic materials without considerable sample damage. Monochromated EELS can furthermore resolve isotope specific vibrational signatures and phonon modes arising from single-atom dopants. In parallel with these advances, we highlight the ability of spatial- and momentum-resolved EELS to measure the phonon dispersions at material interfaces, obtaining valuable knowledge about interfacial thermal and electrical transport phenomena. Before concluding, we illustrate how imaging infrared (IR) plasmons at high spatial resolution deepens our understanding of how plasmons interact with molecular vibrations. Taken together, these diverse studies illustrate the capability of monochromated STEM-EELS to image low-energy phonon and plasmon excitations at the nanoscale and the new insights from these studies can be leveraged to engineer the specific material properties needed for next-generation devices.

Making a case for femto-phono-magnetism with FePt.

In the field of femtomagnetism, magnetic matter is controlled by ultrafast laser pulses; here, we show that coupling phonon excitations of the nuclei to spin and charge leads to femto-phono-magnetism, a powerful route to control magnetic order at ultrafast times. With state-of-the-art theoretical simulations of coupled spin, charge, and lattice dynamics, we identify strong nonadiabatic spin-phonon coupled modes that dominate early time spin dynamics. Activating these phonon modes that we show leads to an additional (up to 40% extra) loss of moment in iron-platinum occurring within 40 femtoseconds of the pump laser pulse. Underpinning this enhanced ultrafast loss of spin moment, we identify a physical mechanism in which minority spin current drives an enhanced intersite minority charge transfer, in turn promoting increased on-site spin flips. Our finding demonstrates that the nuclear system, often assumed to play the role of an energy and angular momentum sink, when selectively preexcited, can play a profound role in controlling femtosecond spin dynamics in materials.

Phonon induced pseudospin rotation in graphene

Pseudospin, a degree of freedom in a two-level quantum system of two sublattices, is a basic quantum number that poses a fundamental symmetry prone to the electronic nature of graphene. We propose an extended Hamiltonian that involves the SU(2) rotation of the pseudospin for electron-phonon scattering, which has a significant advantage over the conventional second-quantized polar coupling. A phonon satellite accompanied by an electron linearly pumped to the upper Dirac cone corresponds to a rotated pseudospin within a scheme of the time-resolved photoemission spectroscopy (TRPES), which is found to carry nonzero angular momenta and induce nonvanishing Berry curvatures through the dichroic mode of TRPES. A phonon bringing the pseudospin rotation is identified to be an elliptically polarized one, which compensates for the angular momenta delivered to the pseudospin. This is an indication of the chiral phonon excitation without the circularly polarized pulse pumping. Our findings suggest that the controlled generation of phonon builds up a new roadmap for engineering the Berry curvature and topology in Dirac semiconductors.

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.

Dark matter direct detection from the single phonon to the nuclear recoil regime

In most direct detection experiments, the free nuclear recoil description of dark matter scattering breaks down for masses $\lesssim$ 100 MeV, or when the recoil energy is comparable to a few times the typical phonon energy. For dark matter lighter than 1 MeV, scattering via excitation of a single phonon dominates and has been computed previously, but for the intermediate mass range or higher detector thresholds, multiphonon processes dominate. We perform the first calculation of the scattering rate via multiphonon production for the entire keV-GeV dark matter mass range, assuming a harmonic crystal target. We provide an analytic description that connects the single phonon, multiphonon, and the nuclear recoil regimes. Our results are implemented in the public package $\texttt{DarkELF}$.

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.

Hyperuniform jammed sphere packings have anomalous material properties.

A spatial distribution is hyperuniform if it has local density fluctuations that vanish in the limit of long length scales. Hyperuniformity is a well known property of both crystals and quasicrystals. Of recent interest, however, is disordered hyperuniformity: the presence of hyperuniform scaling without long-range configurational order. Jammed granular packings have been proposed as an example of disordered hyperuniformity, but recent numerical investigation has revealed that many jammed systems instead exhibit a complex set of distinct behaviors at long, emergent length scales. We use the Voronoi tessellation as a tool to define a set of rescaling transformations that can impose hyperuniformity on an arbitrary weighted point process, and show that these transformations can be used in simulations to iteratively generate hyperuniform, mechanically stable packings of athermal soft spheres. These hyperuniform jammed packings display atypical mechanical properties, particularly in the low-frequency phononic excitations, which exhibit an isolated band of highly collective modes and a band gap around zero frequency.

Coexistence of multifold and multidimensional topological phonons in KMgBO3

Topological interpretations of phonons facilitate a new platform for novel concepts in phonon physics. Though there are ubiquitous set of reports on topological electronic excitations, the same for phonons are extremely limited. Here, we propose a new candidate material, KMgBO 3 , which showcase the co-existence of several multifold and multidimensional topological phonon excitations, which are protected by spatial and non-spatial symmetries. This includes zero dimensional double, triple and quadratic Weyl phonon nodes, one dimensional nodal line/loop and two dimensional doubly degenerate nodal surface states. Nodal line/loop emerges from the spin- 12 phonon nodes, while the two dimensional doubly degenerate nodal surface arises from a combination of two fold screw rotational and time reversal symmetries. Application of strain breaks the C 3 rotational symmetry, which annihilates the spin-1 double Weyl nodes, but preserves other topological features. Interestingly, strain helps to create two extra single Weyl nodes, which in turn preserve the total chirality. Alloying also breaks certain symmetries, destroying most of the topological phonon features in the present case. Thus, KMgBO 3 is a promising candidate which hosts various Weyl points, large Fermi arcs with a very clean phonon spectra and tunable topological phonon excitations, and hence certainly worth for future theoretical/experimental investigation of topological phononics.

Phononic origin of structural lubrication

Atomistic mechanisms of frictional energy dissipation have attracted significant attention. However, the dynamics of phonon excitation and dissipation remain elusive for many friction processes. Through systematic fast Fourier transform analyses of the frictional signals as a silicon tip sliding over a graphite surface at different angles and velocities, we experimentally demonstrate that friction mainly excites non-equilibrium phonons at the washboard frequency and its harmonics. Using molecular dynamics simulations, we further disclose the phononic origin of structural lubrication, i.e., the drastic reduction of friction force as the contact angle between two commensurate surfaces changes. In commensurate contacting states, friction excites a large amount of phonons at the washboard frequency and many orders of its harmonics that perfectly match each other in the sliding tip and substrate, while for incommensurate cases, only limited phonons are generated at mismatched washboard frequencies and few low order harmonics in the tip and substrate.

Laser-induced THz piezomagnetism and lattice dynamics of antiferromagnets [formula omitted] and [formula omitted

The piezomagnetic effect manifests itself in emergence of a net magnetic moment in a mechanically-stressed magnetically ordered material. Up to now, this effect was theoretically analyzed and experimentally studied for the case of static or low frequency deformations of antiferromagnets, and in our brief overview we present the recent progress in exploring the piezomagnetic effect at the THz frequencies. We discuss particular lattice distortions in the model piezomagnetic antiferromagnets MnF2 and CoF2 that are associated with the Raman-active phonons. We show that the phonons of the Eg and B2g symmetry should support emergence of a dynamic magnetic moment oscillating at the corresponding THz phonon frequencies. We further analyze conditions for coherent excitations of these modes by using ultrashort laser pulses via the mechanism of impulsive stimulated Raman scattering. We show that observation of transverse piezomagnetic effect is, in principle, feasible for the Eg phonon. We also discuss experimental results demonstrating laser-induced excitation of coherent Eg phonon in MnF2 and CoF2, as well as speculate about possibilities to detect piezomagnetic effect at the THz frequencies.

Near-field radiative heat transfer between two α-quartz plates having hyperbolic and double-negative-permittivity bands

Near-field radiative heat transfer (NFRHT) between two semi-infinite α-quartz plates was theoretically investigated based on the fluctuation-dissipation theorem. By setting the temperatures of the two α-quartz plates respectively equal to 300 K and 299 K, the near-field radiative heat flux (NFRHF) was calculated and compared with the cases of SiC and hexagonal boron nitride (hBN), considering the influence of optic axis (OA) orientation. The results show that the NFRHF between the two α-quartz plates can surpass 3 times that between two SiC plates and 7 times that between two hBN plates at a separation distance of 10 nm. This vastly enhanced NFRHT was attributed to excitation of phonon polaritons in the hyperbolic and double-negative-permittivity (DNP) bands of α-quartz. It is found that excited surface phonon polaritons in DNP bands can be hyperbolic-like or elliptic-like, and the NFRHF in DNP bands contributes more than half of the total NFRHF for α-quartz. Furthermore, the hyperbolic and DNP bands that distribute in a wide frequency range give α-quartz the potential to achieve much higher NFRHF than using other materials such as SiC and hBN at different temperatures. Therefore, our work is helpful to deepen the understanding of surface polaritons in hyperbolic and DNP bands and their effect on NFRHT, which may be beneficial to design of energy transfer and conversion devices based on NFRHT.

Altering Terahertz Sound Propagation in a Liquid upon Nanoparticle Immersion.

One of the grand challenges of new generation Condensed Matter physicists is the development of novel devices enabling the control of sound propagation at terahertz frequency. Indeed, phonon excitations in this frequency window are the leading conveyor of heat transfer in insulators. Their manipulation is thus critical to implementing heat management based on the structural design. To explore the possibility of controlling the damping of sound waves, we used high spectral contrast Inelastic X-ray Scattering (IXS) to comparatively study terahertz acoustic damping in a dilute suspension of 50 nm nanospheres in glycerol and on pure glycerol. Bayesian inference-based modeling of measured spectra indicates that, at sufficiently large distances, the spectral contribution of collective modes in the glycerol suspension becomes barely detectable due to the enhanced damping, the weakening, and the slight softening of the dominant acoustic mode.

Detection of roton and phonon excitations in a spin-orbit-coupled Bose-Einstein condensate with a moving barrier

We propose to detect phonon and roton excitations in a two-dimensional Bose-Einstein condensate with Raman-induced spin-orbit coupling by perturbing the atomic cloud with a weak barrier. The two excitation modes can be observed by moving the barrier along different directions in appropriate parameter regimes. Phonon excitations are identified by the appearance of solitary waves, while roton excitations lead to distinctive spatial density modulations. We show that this method can also be used to determine the anisotropic critical velocities of superfluid.

Nano‐Optical Engineering of Anisotropic Phonon Resonances in a Hyperbolic α‐MoO3 Metamaterial

AbstractLow‐dimensional α‐phase molybdenum trioxide (α‐MoO3), a layered van‐der‐Waals (vdW) semiconductor, has emerged as an attractive natural hyperbolic material supporting mid‐infrared hyperbolic phonon polaritons (PhPs), which exhibit strong spatial confinement and low loss. With the advantages of strong in‐plane optical anisotropy, many efforts have been devoted to investigating the properties of hyperbolic PhPs in α‐MoO3 using scanning near‐field optical microscopy. However, the studies of far‐field controlling of hyperbolic PhPs in α‐MoO3 have been quite limited so far. This work reports the first experimental demonstration of far‐field excitation and manipulation of hyperbolic phonon resonances in metamaterial structures consisting of α‐MoO3 nanodisks and slabs. The excited phonon resonances show a maximum spatial confinement factor (free space wavelength/period) of 32 and a quality factor of 76. It is shown that the in‐plane phonon resonances in α‐MoO3 can be selectively excited in the two Reststrahlen bands featuring hyperbolic dispersion relations along its two crystal directions, by simply controlling the incident polarization. In addition, the relative strength of resonances along different in‐plane crystal directions and the resultant field distributions can be continuously reconfigured by varying the incident polarization angle. These findings pave the way for future development of novel nanophotonic devices based on hyperbolic PhPs in vdW materials.

Off-diagonal electron-phonon coupling effects on two-level system dynamics

Electromagnetically coupled two-level systems play a central role in several condensed matter components being considered for quantum information processing applications. If the states couple to phonon excitations, their electromagnetic response is altered via phonon-assisted transitions and lifetime broadening. The former has been treated extensively for a number of specific two-level systems (e.g., excitons in artificial quantum dots, localized states associated with impurities or defects, etc.), but the latter has received less attention. Here we study a microscopic model of the dipole transition of a two-level system under the influence of both diagonal and nondiagonal interactions with a bath of phonons. Our results capture both the influence of the frequency distribution of phonons on the relative spectral weight of the zero-phonon transition and the phonon sidebands, and the broadening of the zero-phonon line due to nondiagonal electron-phonon coupling. We use a formalism that includes non-Markovian effects related to the feedback mechanism between the two-level system and the phonon bath. For simplified forms of the phonon spectral functions we provide analytical expressions up to second order in the coupling strength that demonstrate the importance of including both forms of electron-phonon coupling in studies of these systems. Our formalism can be generalized to higher orders of coupling and for realistic phonon spectral functions.

Spectrally Resolving the Phase and Amplitude of Coherent Phonons in the Charge Density Wave State of 1T‐TaSe2

AbstractThe excitation and detection of coherent phonons have given unique insights into the condensed matter, in particular for materials with strong electron–phonon coupling. A study of coherent phonons is reported in the layered charge density wave (CDW) compound 1T‐TaSe2 performed using transient broadband reflectivity spectroscopy, in the photon energy range 1.75–2.65 eV. Several intense and long‐lasting (>20 ps) oscillations, arising from the CDW superlattice reconstruction, are observed allowing for detailed analysis of the spectral dependence of their amplitude and phase. For energies above 2.4 eV, where transitions involve Ta d‐bands, the CDW amplitude mode at 2.19 THz is found to dominate the coherent response. At lower energies, instead, beating arises between additional frequencies, with a particularly intense mode at 2.95 THz. Interestingly, the spectral analysis reveals a π phase shift at 2.4 eV. Results are discussed considering the selective coupling of specific modes to energy bands involved in the optical transitions seen in steady‐state reflectivity. The work demonstrates how coherent phonon spectroscopy can distinguish and resolve optical states strongly coupled to the CDW order and provide additional information normally hidden in conventional steady‐state techniques.

Probing phonon dynamics with multidimensional high harmonic carrier-envelope-phase spectroscopy

We explore pump-probe high harmonic generation (HHG) from monolayer hexagonal-boron-nitride, where a terahertz pump excites coherent optical phonons that are subsequently probed by an intense infrared pulse that drives HHG. We find, through state-of-the-art ab initio calculations, that the structure of the emission spectrum is attenuated by the presence of coherent phonons and no longer comprises discrete harmonic orders, but rather a continuous emission in the plateau region. The HHG yield strongly oscillates as a function of the pump-probe delay, corresponding to ultrafast changes in the lattice such as specific bond compression or stretching dynamics. We further show that in the regime where the excited phonon period and the pulse duration are of the same order of magnitude, the HHG process becomes sensitive to the carrier-envelope phase (CEP) of the driving field, even though the pulse duration is so long that no such sensitivity is observed in the absence of coherent phonons. The degree of CEP sensitivity versus pump-probe delay is shown to be a highly selective measure for instantaneous structural changes in the lattice, providing an approach for ultrafast multidimensional HHG spectroscopy. Remarkably, the obtained temporal resolution for phonon dynamics is ∼1 femtosecond, which is much shorter than the probe pulse duration because of the inherent subcycle contrast mechanism. Our work paves the way toward routes of probing phonons and ultrafast material structural changes with subcycle temporal resolution and provides a mechanism for controlling the HHG spectrum.

Two-dimensional correlation spectroscopy analysis of Raman spectra of NiO nanoparticles

We report two-dimensional correlation spectroscopy (2D-COS) analyses of the Raman spectra of NiO nanoparticles over a temperature range from 100 to 300 K. 2D-Raman correlation spectra suggest strong correlation of the phonon spectral intensity variation with the magnetic ordering in NiO nanoparticles. It is revealed that the antiferromagnetic ordering affects the TO phonon anisotropy in NiO nanoparticles. We elucidate the complex spectral features of two-magnon (2 M) bands by performing appropriate 2D-COS model simulations. Significant spin-phonon coupling in NiO nanoparticles is supported by our results. High energy magnon-magnon interaction tails are also found to be involved in the spin-phonon coupling. 2D-COS analyses provide rich information regarding the nature of the phonon and magnon excitations of NiO nanoparticles.

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.