Nonequilibrium Phonons Research Articles

Theoretical investigation of decoherence channels in athermal phonon sensors.

The creation and evolution of nonequilibrium phonons is central in applications ranging from cosmological particle searches to decoherence processes in qubits. However, the fundamental understanding of decoherence pathways for athermal phonon distributions in solid-state systems remains an open question. Using first-principles calculations, we investigate the primary decay channels of athermal phonons in two technologically relevant semiconductors - Si and GaAs. We quantify the contributions of anharmonic, isotopic, and interfacial scattering in these materials. From this, we construct a model to estimate the thermal power in a readout scheme as a function of time. We discuss the implication of our results on noise limitations in current phonon sensor designs and strategies for improving coherence in next-generation phonon sensors.&#xD.

Simulation study of phonon transport at the GaN/AlN superlattice interface: Ballistic and non-equilibrium phenomena

GaN/AlN superlattice structures have potential for high electron mobility transistors (HEMTs) applications; however, nonequilibrium phonon transport mechanisms within the superlattice have not been systematically investigated. Therefore, we investigate in detail the non-Fourier thermal conductivity of superlattice structures in the perpendicular interface direction and in the parallel interface direction around the phonon BTE numerical solution in this paper, taking the kinetic parameter based on the first principle as the input parameter of the equations and based on the DOM numerical solution scheme. The study reveals that mode temperature non-equilibrium in superlattices arises from ballistic transport within the layers and interface scattering between the layers. The thermal conductivity (TC) of superlattices can be modulated by scale effects, and in superlattices with small thicknesses, phonons exhibit quasi-ballistic transport. The interface thermal resistance predominantly originates from contributions by acoustic phonon (AP) and the first optical phonon (OP1). With increasing interface density, the combined effects of interface scattering and boundary scattering lead to strong phonon scattering, resulting in a decrease in TC parallel to the interfaces. This work provides valuable insights into the thermal transport properties of GaN/AlN superlattice semiconductors and offers useful theoretical guidance for thermal management through phonon particle properties.

A momentum-resolved view of polaron formation in materials

An ab-initio computational methodology for interrogating the phonon contribution to polaron formation in real materials is developed that can be directly compared to experiment. Using LiF as an example, we show that the recent ab-initio theory of Sio et al.1 makes predictions of the momentum- and branch dependent phonon amplitudes in polaron quasiparticles that are testable using ultrafast electron diffuse scattering (UEDS) and related techniques. The large electron polaron in LiF has UEDS signatures that are qualitatively similar to those expected from a simple isotropic strain field model, but the small hole polaron exhibits a profoundly anisotropic UEDS pattern that is in poor agreement with an isotropic strain field. We also show that these polaron diffuse scattering signatures are directly emblematic of the underlying polaron wavefunction. The combination of new time and momentum resolved experimental probes of nonequilibrium phonons with novel computational methods promises to complement the qualitative results obtained via model Hamiltonians with a first principles, material-specific quantitative understanding of polarons and their properties.

Effective electronic properties and coupling for two-temperature model-molecular dynamics simulation of ultrafast laser ablation of nickel

ABSTRACT The accuracy of hybrid two temperature model-molecular dynamics (TTM-MD) simulations in predicting the ultrafast laser ablation phenomena in metals is influenced by the functional definition of subsystem properties and electronic-lattice coupling. In this work, laser ablation of nickel is simulated using hybrid TTM-MD with empirical and density function theory (DFT) derived electronic and coupling parameter sets. The investigation revealed that the empirically derived temperature-dependent parameterisation of electronic specific heat, thermal conductivity, and coupling factor enhanced the fidelity of electron–phonon nonequilibrium dynamics when used in conjunction with simple optical absorption models, aligning closely with prior experimental observations for nickel specimens. The TTM-MD setup is then coupled utilising two widely used coupling techniques – the temperature difference scaled and Langevin thermostat. It is found that the former exhibits increased tensile stress due to artificial enhancement of the collision cascade owing to deterministic nature of the coupling force. While the probabilistic nature of the Langevin thermostat offers more accurate predictions of the ablation threshold/yield. Under these conditions, the TTM-MD scheme is tested for a range of absorbed laser fluence, with ablation threshold, and the onset of phase explosion is determined as 115 and 270 mJ/cm2, respectively for a 1 picosecond (ps-) laser pulse.

Real-time dynamics of angular momentum transfer from spin to acoustic chiral phonon in oxide heterostructures.

Chiral phonons have recently been explored as a novel degree of freedom in quantum materials. The angular momentum carried by these quasiparticles is generated by the breaking of chiral degeneracy of phonons, owing to the chiral lattice structure or the rotational motion of ions of the material. In ferromagnets, a mechanism for generating non-equilibrium chiral phonons has been suggested, but their temporal evolution, which obeys Bose-Einstein statistics, remains unclear. Here we report the real-time dynamics of thermalized chiral phonons in an artificial superlattice composed of ferromagnetic metallic SrRuO3 and non-magnetic insulating SrTiO3. Following the photo-induced ultrafast demagnetization in the SrRuO3 layer, we observed the appearance of a magneto-optic signal in the superlattice, which is absent in the SrRuO3 single films. This magneto-optic signal exhibits thermally driven dynamic properties and a clear correlation with the thickness of the non-magnetic SrTiO3 layer, implying that it originates from thermalized chiral phonons. We use numerical calculations considering the magneto-elastic coupling in SrRuO3 to validate our experimental observations and the angular momentum transfer mechanism between the lattice and spin systems in ferromagnetic systems and also to the non-magnetic system.

Chirality-Induced Phonon-Spin Conversion at an Interface.

We consider spin injection driven by nonequilibrium chiral phonons from a chiral insulator into an adjacent metal. Phonon-spin conversion arises from the coupling of the electron spin with the microrotation associated with chiral phonons. We derive a microscopic formula for the spin injection rate at a metal-insulator interface. Our results clearly illustrate the microscopic origin of spin current generation by chiral phonons and may lead to a breakthrough in the development of spintronic devices without heavy elements.

Non-thermal phonon dynamics and a quenched exciton condensate probed by surface-sensitive electron diffraction

Interactions among and between electrons and phonons steer the energy flow in photo-excited materials and govern the emergence of correlated phases. The strength of electron–phonon interactions, decay channels of strongly coupled modes and the evolution of three-dimensional order are revealed by electron or X-ray pulses tracking non-equilibrium structural dynamics. Despite such capabilities, the growing relevance of inherently anisotropic two-dimensional materials and functional heterostructures still calls for techniques with monolayer sensitivity and, specifically, access to out-of-plane phonon polarizations. Here, we resolve non-equilibrium phonon dynamics and quantify the excitonic contribution to the structural order parameter in 1T-TiSe2. To this end, we introduce ultrafast low-energy electron diffuse scattering and trace strongly momentum- and fluence-dependent phonon populations. Mediated by phonon–phonon scattering, a few-picosecond build-up near the zone boundary precedes a far slower generation of zone-centre acoustic modes. These weakly coupled phonons are shown to substantially delay overall equilibration in layered materials. Moreover, we record the surface structural response to a quench of the material’s widely investigated exciton condensate, identifying an approximate 30:70 ratio of excitonic versus Peierls contributions to the total lattice distortion in the charge density wave phase. The surface-sensitive approach complements the ultrafast structural toolbox and may further elucidate the impact of phonon scattering in numerous other phenomena within two-dimensional materials, such as the formation of interlayer excitons in twisted bilayers.

Thermal resistance across Si–SiGe alloy interface from phonon distribution mismatch

Interfacial thermal resistance has often been attributed to the mismatch of phonon spectra between two materials and resulting phonon-interface scattering. However, we use the solution of Peierls–Boltzmann transport equation to reveal a substantial nonequilibrium thermal resistance across the interfaces of Si and SiGe alloys at room temperature, despite their nearly identical phonon dispersion and negligible phonon-interface scattering. The Kapitza length of the Si–Si0.99Ge0.01 interface is approximately 600 nm of Si. This originates from the mismatch in phonon distribution between Si and SiGe alloys due to their distinct scattering rates. The mismatch is relaxed by phonon scattering over a region of 1 μm around the interface, corresponding to the upper bound of mean free path Λx of heat-carrying phonons. The relaxation process leads to the significant entropy generation and increased thermal resistance. Introducing a gradual variation in Ge concentration near the interface markedly reduces thermal resistance when implemented over the 1 μm period. Our finding demonstrates that the interfacial thermal resistance can be significant due to the nonequilibrium phonon distribution, even in the absence of phonon-interface scattering. In addition, among various phonon modes with a wide range of Λx, the relaxation of the nonequilibrium is predominantly governed by the phonons with long Λx.

Direct observation of strong momentum-dependent electron-phonon coupling in a metal.

Phonon scattering in metals is one of the most fundamental processes in materials science. However, understanding such processes has remained challenging and requires detailed information on interactions between phonons and electrons. We use an ultrafast electron diffuse scattering technique to resolve the nonequilibrium phonon dynamics in femtosecond-laser-excited tungsten in both time and momentum. We determine transient populations of phonon modes which show strong momentum dependence initiated by electron-phonon coupling. For phonons near Brillouin zone border, we observe a transient rise in their population on a timescale of approximately 1 picosecond driven by the strong electron-phonon coupling, followed by a slow decay on a timescale of approximately 8 picosecond governed by the weaker phonon-phonon relaxation process. We find that the exceptional harmonicity of tungsten is needed for isolating the two processes, resulting in long-lived nonequilibrium phonons in a pure metal. Our finding highlights that electron-phonon scattering can be the determinant factor in the phonon thermal transport of metals.

Ultrafast electron diffuse scattering as a tool for studying phonon transport: Phonon hydrodynamics and second sound oscillations.

Hydrodynamic phonon transport phenomena, like second sound, have been observed in liquid helium more than 50 years ago. More recently second sound has been observed in graphite at over 200 K using transient thermal grating (TG) techniques. In this work, we explore signatures of phonon hydrodynamic transport and second sound oscillations in ultrafast electron diffuse scattering patterns, which can provide time, momentum, and branch resolved information on the state-of-excitation of the phonon system beyond that available through TG experiments. We use the density functional theory and solve the Boltzmann transport equation to determine time-resolved non-equilibrium phonon populations and model phonon transport in graphite. This model also provides the information necessary to calculate the time evolution of one-phonon structure factors and diffuse scattering patterns during thermal transport covering ballistic, diffusive, and hydrodynamic regimes where the effect of a second sound oscillation on the phonon distribution is observed. Direct measurements of how the phonon distribution varies in time and space in various thermal transport regimes should yield new insights into the fundamental physics of the underlying processes.

Entropy-driven nonequilibrium phonon-stimulated electron-phonon coupling in tin dioxide nanorods.

Nonequilibrium (NEQ) phonon fluctuation in a nanosystem has been studied through the statistical assessment of the entropy-production and -consumption events in ultrasmall tin dioxide (SnO_{2}) nanorods. Size- and shape-dependent alteration in free energy leading to modulation of the probability distribution function of the phonon dynamics has been observed from the x-ray diffraction and Raman scattering characterizations. The Gallavotti-Cohen nonequilibrium fluctuation theorem has been utilized to qualitatively describe the aforementioned behaviors under the influence of a global flux. The observation of entropy consumption and thermodynamically favorable entropy-production events indicates the presence of NEQ fluctuations in the phonon modes. The effective energy scale of fluctuation in driven phonon modes, dissipating energy faster than relaxation time, is quantified on the order of nanojoules. From optical absorption and photoluminescence studies, the observation of the electron-phonon coupled state confirms the interaction of the NEQ phonons with electrons. The strength of the coupling has been estimated from the temperature-independent Barry center shift and found to be enhanced to 5.35. Valence band x-ray photoelectron spectroscopy and Fourier transformed infrared spectroscopy analyses reconcile NEQ phonon mediated alteration of the valence band density of states, activation of silent phonon modes, and superior excitonic transitions, suitable for the new generation of ultrafast quantum device applications.

Hidden phonon highways promote photoinduced interlayer energy transfer in twisted transition metal dichalcogenide heterostructures.

Vertically stacked van der Waals (vdW) heterostructures exhibit unique electronic, optical, and thermal properties that can be manipulated by twist-angle engineering. However, the weak phononic coupling at a bilayer interface imposes a fundamental thermal bottleneck for future two-dimensional devices. Using ultrafast electron diffraction, we directly investigated photoinduced nonequilibrium phonon dynamics in MoS2/WS2 at 4° twist angle and WSe2/MoSe2 heterobilayers with twist angles of 7°, 16°, and 25°. We identified an interlayer heat transfer channel with a characteristic timescale of ~20 picoseconds, about one order of magnitude faster than molecular dynamics simulations assuming initial intralayer thermalization. Atomistic calculations involving phonon-phonon scattering suggest that this process originates from the nonthermal phonon population following the initial interlayer charge transfer and scattering. Our findings present an avenue for thermal management in vdW heterostructures by tailoring nonequilibrium phonon populations.

Nonequilibrium Phonon Dynamics and Its Impact on the Thermal Conductivity of the Benchmark Thermoelectric Material SnSe.

Thermoelectric materials play a vital role in the pursuit of a sustainable energy system by allowing the conversion of waste heat to electric energy. Low thermal conductivity is essential to achieving high-efficiency conversion. The conductivity depends on an interplay between the phononic and electronic properties of the nonequilibrium state. Therefore, obtaining a comprehensive understanding of nonequilibrium dynamics of the electronic and phononic subsystems as well as their interactions is key for unlocking the microscopic mechanisms that ultimately govern thermal conductivity. A benchmark material that exhibits ultralow thermal conductivity is SnSe. We study the nonequilibrium phonon dynamics induced by an excited electron population using a framework combining ultrafast electron diffuse scattering and nonequilibrium kinetic theory. This in-depth approach provides a fundamental understanding of energy transfer in the spatiotemporal domain. Our analysis explains the dynamics leading to the observed low thermal conductivity, which we attribute to a mode-dependent tendency to nonconservative phonon scattering. The results offer a penetrating perspective on energy transport in condensed matter with far-reaching implications for rational design of advanced materials with tailored thermal properties.

Dynamical Phonons Following Electron Relaxation Stages in Photoexcited Graphene.

Ultrafast electron-phonon relaxation dynamics in graphene hides many distinct phenomena, such as hot phonon generation, dynamical Kohn anomalies, and phonon decoupling, yet it still remains largely unexplored. Here, we unravel intricate mechanisms governing the vibrational relaxation and phonon dressing in graphene at a highly nonequilibrium state by means of first-principles techniques. We calculate dynamical phonon spectral functions and momentum-resolved line widths for various stages of electron relaxation and find photoinduced phonon hardening, overall increase of relaxation rate and nonadiabaticity, as well as phonon gain. Namely, the initial stage of photoexcitation is found to be governed by strong phonon anomalies of finite-momentum optical modes along with incoherent phonon production. The population inversion state, on the other hand, allows the production of coherent and strongly coupled phonon modes. Our research provides vital insights into the electron-phonon coupling phenomena in graphene and serves as a foundation for exploring nonequilibrium phonon dressing in materials where ordered states and phase transitions can be induced by photoexcitation.

Boltzmann transport equation simulation of phonon transport across GaN/AlN interface

The miniaturization of devices makes interfacial resistance to heat dissipation more and more obvious. Therefore, it is important to study interfacial thermal transport systematically. A modified higher harmonic inelastic model is proposed to describe the inelastic scattering at the interface. Combined with this interface model, the thermal transport of interface structures is studied by the Boltzmann transport equation. The results show that the introduction of inelastic scattering enhances the heat flux contribution of low-frequency phonons, changes the phonon distribution near the interface, and enhances the phonon non-equilibrium. This demonstrates the importance of inelastic interfacial scattering in simulations. By analyzing the phonon distribution, it is found that temperature and thickness affect the interfacial heat transport through the redistribution of phonons. The redistribution has the opposite effect on the contribution of inelastic scattering. By studying ballistic phonons in the interface structure, it is found that the ballistic phonons are mainly concentrated in the low-frequency region. Due to the longer mean free path, the phonons with frequencies below 2.5 THz are more likely to maintain a ballistic transport state. Similar to nanoscale hotspots, strong quasi-ballistic transport hinders heat diffusion. The energy fraction of ballistic phonons increases by 49%, and the local effective thermal conductivity drops by 52%. This work studied the mechanism of interfacial thermal transport, and will be useful for future interface simulation.

Vibrational Dichroism of Chiral Valley Phonons.

Valley degrees of freedom in transition metal dichalcogenides thoroughly influence electron-phonon coupling and its nonequilibrium dynamics. We conducted a first-principles study of the quantum kinetics of chiral phonons following valley-selective carrier excitation with circularly polarized light. Our numerical investigations treat the ultrafast dynamics of electrons and phonons on equal footing within a parameter-free ab initio framework. We report the emergence of valley-polarized phonon populations in monolayer MoS2 that can be selectively excited at either the K or K' valleys depending on the light helicity. The resulting vibrational state is characterized by a distinctive chirality, which lifts time-reversal symmetry of the lattice on transient time scales. We show that chiral valley phonons can further lead to fingerprints of vibrational dichroism detectable by ultrafast diffuse scattering and persist beyond 10 ps. The valley polarization of nonequilibrium phonon populations could be exploited as an information carrier, thereby extending the paradigm of valleytronics to the domain of vibrational excitations.

Significant phonon drag effect in wide band gap GaN and AlN

Understanding the electrical and thermal transport properties of group-III nitride semiconductors is crucial for their electronic and thermoelectric applications. This study investigates the impact of the phonon drag effect, which refers to the momentum exchange between nonequilibrium phonons and electrons, on electronic transport in GaN and AlN. Results show that, even at room temperature, phonon drag significantly enhances mobility and the Seebeck coefficient. The study delves into the specific mechanisms for this.

Characterization of local non-equilibrium phonons in bulk MoS2 probed by temperature-dependent Raman scattering

This study presents an experimental implementation to determine the temperature dependence of Raman spectra of bulk 2H-MoS2 in the temperature range of 300–650 K. In addition, we have measured according to the anti-Stokes to Stokes integrated intensity ratio (). The results indicate that Raman shift of both the vibration modes and exhibits a linear temperature-dependent behavior in the heating and cooling processes. It is confirmed that the sample did not deteriorate by oxidization and decomposition at high temperatures. However, above 500 K, the measured phonon temperature deviated from the heat bath temperature. It is suggested that the does not correspond to the equilibrium population of optical phonons, thus indicating that the measurement was not in thermal equilibrium but in a thermal steady state under laser irradiation. We conclude that this is caused by the difference in relaxation times in the thermal steady state.

Thermal resistance from non-equilibrium phonons at Si–Ge interface

As nanostructured devices become prevalent, interfaces often play an important role in thermal transport phenomena. However, interfacial thermal transport remains poorly understood due to complex physics across a wide range of length scales from atomistic to microscale. Past studies on interfacial thermal resistance have focused on interface-phonon scattering at the atomistic scale but overlooked the complex interplay of phonon-interface and phonon-phonon scattering at microscale. Here, we use the Peierls-Boltzmann transport equation to show that the resistance from the phonon-phonon scattering of non-equilibrium phonons near a Si–Ge interface is much larger than that directly caused by the interface scattering. We report that non-equilibrium in phonon distribution leads to significant entropy generation and thermal resistance upon three-phonon scattering by the Boltzmann's H-theorem. The physical origin of non-equilibrium phonons in Ge is explained with the mismatches of phonon dispersion, density-of-states, and group velocity, which serve as general guidance for estimating the non-equilibrium effect on interfacial thermal resistance. Our study bridges a gap between atomistic scale and less studied microscale phenomena, providing comprehensive understanding of overall interfacial thermal transport and the significant role of phonon-phonon scattering.

Signatures of hot carriers and hot phonons in the re-entrant metallic and semiconducting states of Moiré-gapped graphene

Stacking of graphene with hexagonal boron nitride (h-BN) can dramatically modify its bands from their usual linear form, opening a series of narrow minigaps that are separated by wider minibands. While the resulting spectrum offers strong potential for use in functional (opto)electronic devices, a proper understanding of the dynamics of hot carriers in these bands is a prerequisite for such applications. In this work, we therefore apply a strategy of rapid electrical pulsing to drive carriers in graphene/h-BN heterostructures deep into the dissipative limit of strong electron-phonon coupling. By using electrical gating to move the chemical potential through the “Moiré bands”, we demonstrate a cyclical evolution between metallic and semiconducting states. This behavior is captured in a self-consistent model of non-equilibrium transport that considers the competition of electrically driven inter-band tunneling and hot-carrier scattering by strongly non-equilibrium phonons. Overall, our results demonstrate how a treatment of the dynamics of both hot carriers and hot phonons is essential to understanding the properties of functional graphene superlattices.