Strong Phonon Coupling Research Articles

GaInX3 (X = S, Se, Te): Ultra-low thermal conductivity and excellent thermoelectric performance

Abstract Seeking intrinsically low thermal conductivity materials is a viable strategy in the pursuit of high-performance thermoelectric materials. Here, by using first-principles calculations and semiclassical Boltzmann transport theory, we systemically investigate the carrier transport and thermoelectric properties of monolayer Janus GaInX3 (X = S; Se; Te). It is found that the lattice thermal conductivities can reach values as low as 3.07 Wm−1K−1, 1.16 Wm−1K−1 and 0.57 Wm−1K−1 for GaInS3, GaInSe3, and GaInTe3, respectively, at room temperature. These notably low thermal conductivity is attributed to strong acoustic-optical phonon coupling caused by the presence of low-frequency optical phonons in GaInX3 materials. Furthermore, by integrating the characteristics of electronic and thermal transport, the dimensionless figure of merit ZT can reach maximum values of 0.95, 2.37, and 3.00 for GaInS3, GaInSe3, and GaInTe3, respectively. Our results suggest monolayer Janus GaInX3 (X = S; Se; Te) are promising candidates for thermoelectric and heat management applications.

The Nonradiative Properties of Self‐Trapped Holes in Ultra‐Wide Bandgap Gallium Oxide Film

The photoluminescence (PL) of self‐trapped holes (STH) in ultra‐wide bandgap β‐Ga2O3 is commonly its most dominant light emission and is an inherent property. Thus, gaining knowledge of the crystal dynamics that impact the PL properties is vital to sensor and other technologies. The PL, Raman‐phonons, and their interactions are studied at an extreme temperature range of 77–622 K. The PL is studied up to the bandgap value of ≈5 eV. It is found that the high‐energy Raman modes provide a major route to the nonradiative process of the PL via STH–phonon interaction with an activation energy of 72 meV. This dynamic is modeled with the configurational coordinate scheme at the strong phonon coupling limit. The exceptionally broad Gaussian PL linewidth manifests this coupling. The weak temperature response of the PL energy peak position indicates that the STH has characteristics of a deep‐level defect. This contrasts with the large redshift of ≈220 meV of the optical gap of the film, ascertained from transmission. Unlike the temperature response of the high‐energy phonons, the behavior of the low‐energy phonons is found to follow the Bose–Einstein population increase, indicating no strong interaction with the STH.

Monolayer indium selenide: an indirect bandgap material exhibits efficient brightening of dark excitons

Atomically thin indium selenide (InSe) exhibits a sombrero-like valence band, leading to distinctive excitonic behaviors. It is known that the indirect band gap of atomically thin InSe leads to a weak emission from the lowest-energy excitonic state (A peak). However, the A peak emission of monolayer (ML) InSe was observed to be either absent or very weak, rendering the nature of its excitonic states largely unknown. Intriguingly, we demonstrate that ML InSe exhibits pronounced PL emission because of the efficient brightening of the momentum-indirect dark excitons. The mechanism is attributed to acoustic phonon-assisted radiative recombination facilitated by strong exciton-acoustic phonon coupling and extended wavefunction in momentum space. Systematic analysis of layer-, power-, and temperature-dependent PL demonstrates that a carrier localization model can account for the asymmetric line shape of the lowest-energy excitonic emission for atomically thin InSe. Our work reveals that atomically thin InSe is a promising platform for manipulating the tightly bound dark excitons in two-dimensional semiconductor-based optoelectronic devices.

The effect of optical-acoustic phonon coupling on the thermal conductivity of 2D MgI2.

2D MgI2 has a large phonon band gap and strong coupling of optical and acoustic phonons, and it is difficult to accurately predict thermal conductivity by considering only three-phonon scattering. Thus, in this study, the effect of four-phonon scattering on the thermal conductivity of a 2D MgI2 lattice was investigated using first-principles calculations combined with Boltzmann transport theory. The results show that with increasing temperature, four-phonon scattering induces an increase in the scattering of phonons at the optical and acoustic phonon coupling (2 THz), as well as in the vicinity of the optical phonon branch (4.5 THz), which leads to the enhancement of the anharmonicity of phonon transport and results in a decrease in the thermal conductivity of the 2D material. At 700 K, the thermal conductivity of MgI2 decreases by over half, from 0.47 W m-1 K-1 to 0.23 W m-1 K-1, when considering both three- and four-phonon scattering, compared to considering only three-phonon scattering. This study confirms the need to consider the role of four-phonon scattering to enhance optical and acoustic phonon coupling to accurately predict the thermal conductivity of 2D materials with larger phonon band gaps.

Lattice thermal conductivity reduction in Ca3AlSb3 and Ca5Al2Sb6 by manipulating the covalent tetrahedral chain.

Understanding the structural and physical origins of low thermal conductivity is critical to improving and designing efficient thermoelectric materials. For two distinct Zintl Ca-Al-Sb compounds with different stoichiometric ratios (Ca3AlSb3 and Ca5Al2Sb6), experimental measurements suggest the low lattice thermal conductivities (∼1.43 W mK-1 for Ca3AlSb3 and 1.52 W mK-1 for Ca5Al2Sb6 at 300 K). In order to understand the physical origin of the low thermal conductivity, we present the first-principles studies on the lattice dynamics and phonon-transport properties. The theoretically calculated lattice thermal conductivity of Ca3AlSb3 and Ca5Al2Sb6 is ∼1.61 W mK-1 for Ca3AlSb3 and 1.85 W mK-1 for Ca5Al2Sb6 at 300 K, which is in reasonable agreement with the experimental measurements. The low lattice thermal conductivity is attributed to the low acoustic Debye temperature and strong optical-acoustic phonon couplings in the two Ca-Al-Sb compounds. It is worth noting that the thermal conductivity of Ca3AlSb3 and Ca5Al2Sb6 along the x direction (along the Al-Sb chain) is obviously higher than that along the y/z direction (perpendicular to the chain). The high lattice thermal conductivity along the Al-Sb chain is due to the strong Al-Sb covalent bond. From the phonon density of states (PDOS), the obviously frequency regions dominated by different atoms suggest that forming defects with one atom would only shift its related PDOS and might not affect the PDOS of others. Based on the understandings of the crystal structure, PDOS and atomic displacement parameter, we represent a methodology to further lower their lattice thermal conductivity: substituting heavier atoms along the Al-Sb chain to strongly scatter phonons. When using Tl to substitute Al, the vibration frequency of the Tl dopant is only 1/3 of that of the substituted Al atom. The significantly decreased vibration frequency will introduce a low phonon band within the PDOS, which will suppress the lattice thermal conductivity. Our work not only elucidates the physical mechanism of low lattice thermal conductivity in Ca3AlSb3 and Ca5Al2Sb6 Zintl compounds, but also offers an efficient approach (breaking the covalent tetrahedral chains) to further block the heat transport.

Room-temperature wavelike exciton transport in a van der Waals superatomic semiconductor.

The transport of energy and information in semiconductors is limited by scattering between electronic carriers and lattice phonons, resulting in diffusive and lossy transport that curtails all semiconductor technologies. Using Re6Se8Cl2, a van der Waals (vdW) superatomic semiconductor, we demonstrate the formation of acoustic exciton-polarons, an electronic quasiparticle shielded from phonon scattering. We directly imaged polaron transport in Re6Se8Cl2 at room temperature, revealing quasi-ballistic, wavelike propagation sustained for a nanosecond and several micrometers. Shielded polaron transport leads to electronic energy propagation lengths orders of magnitude greater than in other vdW semiconductors, exceeding even silicon over a nanosecond. We propose that, counterintuitively, quasi-flat electronic bands and strong exciton-acoustic phonon coupling are together responsible for the transport properties of Re6Se8Cl2, establishing a path to ballistic room-temperature semiconductors.

Photon‐Assisted Landau Zener Transitions in a Tunable Driven Rabi Dimer Coupled to a Micromechanical Resonator

AbstractEmploying the multiple Davydov D2 Ansätze with the time‐dependent variational principle, photon‐assisted Landau–Zener (LZ) transitions and qubit manipulation in a hybrid quantum electrodynamics device have been investigated. Modeled as a Rabi dimer, the device comprises of two interacting transmission‐line resonators, each coupled to a qubit. The qubits, driven by independent harmonic fields, are further modulated by a micromechanical resonator mimicked by a phonon mode. The impacts of two independent driving fields on the qubit dynamics are carefully examined. The energy diagram of the system and the photon number mobilization on the resonators are analyzed to explain the behavior of the LZ transitions and qubit dynamics while taking into account the influence of the single phonon mode. Results show that low phonon frequencies can alter the qubit dynamics, particularly in the absence of the driving fields, and a strong phonon coupling strength can significantly perturb the qubit dynamics, thanks to a high influx of phonon energy. Notably, only the photon frequency affects the oscillation frequency of qubit polarization. This study unveils the imperative roles that photons and phonons play in the Rabi dimer model.

Strong Coupling of Coherent Phonons to Excitons in Semiconducting Monolayer MoTe2.

The coupling of the electron system to lattice vibrations and their time-dependent control and detection provide unique insight into the nonequilibrium physics of semiconductors. Here, we investigate the ultrafast transient response of semiconducting monolayer 2H-MoTe2 encapsulated with hBN using broadband optical pump-probe microscopy. The sub-40 fs pump pulse triggers extremely intense and long-lived coherent oscillations in the spectral region of the A' and B' exciton resonances, up to ∼20% of the maximum transient signal, due to the displacive excitation of the out-of-plane A1g phonon. Ab initio calculations reveal a dramatic rearrangement of the optical absorption of monolayer MoTe2 induced by an out-of-plane stretching and compression of the crystal lattice, consistent with an A1g -type oscillation. Our results highlight the extreme sensitivity of the optical properties of monolayer TMDs to small structural modifications and their manipulation with light.

Tunable phononic coupling in excitonic quantum emitters.

Engineering the coupling between fundamental quantum excitations is at the heart of quantum science and technologies. An outstanding case is the creation of quantum light sources in which coupling between single photons and phonons can be controlled and harnessed to enable quantum information transduction. Here we report the deterministic creation of quantum emitters featuring highly tunable coupling between excitons and phonons. The quantum emitters are formed in strain-induced quantum dots created in homobilayer WSe2. The colocalization of quantum-confined interlayer excitons and terahertz interlayer breathing-mode phonons, which directly modulates the exciton energy, leads to a uniquely strong phonon coupling to single-photon emission, with a Huang-Rhys factor reaching up to 6.3. The single-photon spectrum of interlayer exciton emission features a single-photon purity >83% and multiple phonon replicas, each heralding the creation of a phonon Fock state in the quantum emitter. Due to the vertical dipole moment of the interlayer exciton, the phonon-photon interaction is electrically tunable to be higher than the exciton and phonon decoherence rate, and hence promises to reach the strong-coupling regime. Our result demonstrates a solid-state quantum excitonic-optomechanical system at the atomic interface of the WSe2 bilayer that emits flying photonic qubits coupled with stationary phonons, which could be exploited for quantum transduction and interconnection.

Fluctuating fractionalized spins in quasi-two-dimensional magnetic V0.85PS3

Quantum spin liquid (QSL), a state characterized by exotic low energy fractionalized excitations and statistics, is still elusive experimentally and may be gauged via indirect experimental signatures. A remnant of the QSL phase may reflect in the spin dynamics as well as quanta of lattice vibrations, i.e., phonons, via the strong coupling of phonons with underlying fractionalized excitations. Inelastic light scattering (Raman) studies on ${\mathrm{V}}_{1--x}\mathrm{P}{\mathrm{S}}_{3}$ single crystals evidence the spin fractionalization into Majorana fermions deep into the paramagnetic phase reflected in the emergence of a low frequency quasielastic response, along with a broad magnetic continuum marked by a crossover temperature ${T}^{*}\ensuremath{\sim}200$ K from a pure paramagnetic state to a fractionalized spin regime qualitatively gauged via dynamic Raman susceptibility. We found further evidence of anomalies in the phonons' self-energy parameters, in particular, phonon line broadening and line asymmetry evolution at this crossover temperature, attributed to the decaying of phonons into itinerant Majorana fermions. This anomalous scattering response is thus indicative of fluctuating fractionalized spins suggesting a phase proximate to the quantum spin liquid state in this quasi-two-dimensional magnetic system.

Multicolor Fluorescent Lead‐MOFs for White‐Light‐Emitting and Anticounterfeiting Applications

AbstractThe synergistic effect of rigid organic framework and soft lattice makes metal‐organic framework (MOF) luminescent materials more widely used. Herein, 2D lead‐MOFs are reported that produce white light through the coregulation of free excitons and self‐trapped excitons. The synthesized BIF‐142‐Cl shows outstanding white‐light emission with the CIE color coordinates of (0.312, 0.335) close to pure white light, the excellent color rendering index (CRI) of 90.4, and the gentle correlation color temperature (CCT) of 5364 K. Meanwhile, the multicolor emission from blue to white light can be adjusted by changing the excitation wavelength. Furthermore, photophysical studies confirm that the strong electronic phonon coupling in the excited state of lead halide polyhedrons is directly responsible for the generation of self‐trapped excitons. This synthetic strategy of introducing tunable dual emission centers not only offers a prospect for the development of solid‐state lighting materials but also widens the path to the fields of anticounterfeiting, display media, and information identification.

Soft phonon modes lead to suppressed thermal conductivity in Ag-based chalcopyrites under high pressure.

Pressure is a powerful way to modulate physical properties. Understanding the effect of pressure on the thermal transport properties of thermoelectric materials is of great importance for the efficient design and optimization of thermoelectric performance. In this work, based on first-principles calculations and phonon Boltzmann transport theory, we find that the lattice thermal conductivities of Ag-based chalcopyrites AgXY2 (X = Al, Ga, and In; Y = S, Se, and Te) are dramatically suppressed by applying pressure. The inherent distorted tetrahedral configuration together with highly delocalized p-orbital electrons promotes the formation of metavalent bonding. The fact of metavalent bonding with a single bonding electron and small electron transfer between neighboring atoms leads to soft low-frequency optical phonons. With the increase of pressure, the softening of acoustic and low-frequency optical phonons induces enhanced anharmonicity and scattering channels. Such strong acoustic-optical phonon coupling results in larger phonon scattering rates and thus lowers the lattice thermal conductivity. These findings not only help unveil the underlying physical mechanisms for the anomalous thermal transport behaviors under high pressure, but also pave the way for the pressure tuning of high-performance Ag-based thermoelectric materials.

Interlayer Coupling Induced Phonon-Glass–Electron-Crystal Behavior in van der Waals Heterostructure PtSe2/γ-GeSe

We report the electronic band structure and interlayer coupling induced phonon-glass–electron-crystal behavior in the van der Waals heterostructure PtSe2/γ-GeSe. The heterostructure is dynamically stable and possesses an indirect band gap of 0.63 eV (at the Heyd–Scuseria–Ernzerhof functional level) with type II band alignment. A low conduction band offset of 0.20 eV compared to the valence band offset of 0.92 eV suggests fluent electrons drive from γ-GeSe to PtSe2. Interlayer coupling induced strong phonon coupling and a unique “avoided crossing” feature between longitudinal acoustic and low-lying optical phonon modes between K and Γ points. Significant suppression of acoustic phonon modes and giant phonon scattering rates (a maximum value of 73.76 ps–1) results in a low lattice thermal conductivity of 1.20 W/(m·K) at 300 K after enforcing the mandatory rotational invariance condition. The calculated lattice thermal conductivity is 14-fold smaller than that of monolayer PtSe2 (16.97 W/(m·K)). The lattice thermal conductivity upsurges by 24% as the out-of-plane acoustic phonon dispersion undergoes linear dispersion, indicating the importance of pure quadratic flexural phonon dispersion. Our results disclose a strategy for having the phonon-glass–electron-crystal character in van der Waals heterostructures.

Zero-Dimensional (Piperidinium)2MnBr4: Ring Puckering-Induced Isostructural Transition and Strong Electron-Phonon Coupling-Mediated Self-Trapped Exciton Emission.

We report on the synthesis, structure, and photophysical properties of a lead-free organic-inorganic hybrid halide, (Piperidinium)2MnBr4 (PipMBr). It crystallizes in a monoclinic P21/n structure, with isolated MnBr4 tetrahedra representing a zero-dimensional compound. It undergoes a reversible isostructural transition at 422/417 K in the heating/cooling cycle owing to the hydrogen-bonding rearrangement mediated by ring puckering of piperidinium cations. This compound exhibits green emission with a photoluminescence quantum yield of 51%. Interestingly, strong electron-longitudinal optical phonon coupling with γLO of 237 meV is evidenced from the broadening of the temperature-dependent emission linewidth and the Raman spectrum. Such strong electron-phonon coupling and a relatively low Debye temperature (137 K) suggest the self-trapped exciton emission in this compound.

Phonon Symphony of Stacked Multilayers and Weak Bonds Lowers Lattice Thermal Conductivity.

Controlling lattice vibrations to obtain intrinsic low thermal conductivity play a critical role in thermal management of electronic and photonic devices, energy converters, and thermal insulation, which necessitates exploring new compounds and a thorough understanding of their chemical structure, bonding, and lattice dynamics. Herein, a new chalcogenide, Ga6 Cr5 Se16 , shows intrinsic low lattice thermal conductivity κlat , which crystallizes in the monoclinic phase (C2/m) with the stacked inverse GaSe4 layers (g'), close-packed Cr3+ Se6 layers (c), GaSe4 layers (g) and loosely-stacked Cr2+ Se6 layers (c') along the c-axis. In this structure, a wide variety of chemical bonding is arranged in each layer, such as covalent Ga-Se, covalent Cr3+ -Se, and weaker Cr2+ -Se bonding, which endow it with a large phonon symphony by strong coupling of soft acoustic and low-lying optical phonons. As a result, Ga6 Cr5 Se16 realizes an intrinsic low κlat of 0.79 W m- 1 K- 1 at 323 K, which is almost four times, or twice lower than that of Cr3 Se4 (2.95 W m- 1 K- 1 ), or Cr2 Se3 (1.56 W m- 1 K- 1 ), Ga2 Se3 (1.36 W m- 1 K- 1 ) at 323 K, respectively. These insights will offer comprehensive understanding of the phonon propagation in complex layered chalcogenides, andalso shed useful light on future design of low-κlat solids.

MoSi2N4: A 2D Regime with Strong Exciton–Phonon Coupling

AbstractThe exciton and phonon strong coupling in functional materials is rich of physics and has promising applications in modern optoelectronics and photonics. However, the direct realization of strong exciton–phonon coupling without complex and well‐designed resonator at room temperature is still a challenge in this topic associated with the Born–Oppenheimer approximation and constrained by the thermalized noise. Here, the enhancement of exciton–phonon coupling in 2D MoSi2N4 material is demonstrated by the near exciton resonance technique and first realization of the strong‐coupling states of exciton and phonon in MoSi2N4 at room temperature. With the involving of three‐band terms transition processes, the phonon scattering transition probability and exciton–phonon interaction are significantly enhanced. Then the strong coupling is reached which maintains high energy transfer efficiency between exciton and phonon supporting over ten phonon replicas at room temperature without introducing harsh resonant cavity. This work identifies that the 2D MoSi2N4 should be a promising strong‐coupling system for exploring many‐body quantum states with strong correlations and further applications in quantum systems, especially for exciton tailoring and room‐temperature tunable exciton lasers.

Two-dimensional phonon polaritons in multilayers of hexagonal boron nitride from a macroscopic phonon model

Phonon polaritons (PHPs) in freestanding and supported multilayers of hexagonal boron nitride are systematically studied using a macroscopic optical-phonon model. The PHP properties such as confinement, group velocity, propagation quality factor (PQF), and wavelength scaling are studied. Owing to high-frequency screening, there is an upper frequency limit making the two-dimensional (2D) PHPs have a frequency band and also a maximum PQF occurs near the center frequency. The substrate’s dielectric response should be included to accurately calculate the PHP properties. While the simple electrostatic approximation (ESA) is valid for PHPs with frequencies ω above ω0 (e.g., ω>1.03ω0 for the 30-layers; ω0 is the Γ point optical-phonon frequency), it fails to describe the PHP properties near ω0 and the effect of retardation should be included for a proper description. The PHP wavelength vs layer thickness near ω0 deviates significantly from a linear scaling law given by the ESA due to strong coupling of photons and longitudinal optical phonons. The calculated PHP dispersion and scaling are compared with experimental data of a number of spectroscopic studies and found to be in good agreement for most of the results. While the frequency of incident light should be near the center frequency to maximize the PQF, the PHP wavelength, confinement, and propagation length can be engineered by varying the multilayer thickness and its dielectric environment.

Radiative recombination in zinc blende ZnSe nanocrystals ion-beam synthesized in silica

Zinc selenide nanocrystals (NCs) were successfully synthesized in silicon dioxide (silica grown on a silicon wafer) by high-fluence implantation of Zn+ and Se+ ions with subsequent rapid thermal annealing at 1000 °C for 3 min. The high crystalline quality of the zinc blende ZnSe nanoclusters was proven by transmission electron microscopy with selected area electron diffraction and Raman spectroscopy. Low-temperature photoluminescence (PL) reveals the recombination of excitons in ZnSe, which further indicates a good crystalline quality of the synthesized nanocrystals. PL analysis shows a strong coupling of phonons and excitons. The Huang–Rhys parameter of the longitudinal optical phonon in the exciton transition S is in the range of 0.6–0.7. Despite the excellent quality of the ZnSe NCs synthesized in silica, defect states inside the NCs or at the NCs/SiO2 interface with activation energies of 0.1–0.2, 0.45 and 0.67 eV play a crucial role in radiative recombination.

Phonon transport in Zintl Ba2ZnAs2 and Ba2ZnSb2: A first-principles study

Layered Zintl Ba2ZnX2 (X=As, Sb, and Bi) with intrinsically ultralow lattice thermal conductivity (κ) have attracted continuous attention because of their potential applications in thermoelectric devices. Understanding the low-κ mechanism is favorable for preserving the temperature gradient between the two ends of a material and achieving high thermoelectric performance. Herein, based on first-principles calculations, we systematically report the phonon-limited thermal conductivities of Ba2ZnX2. In the framework of the harmonic approximation, the absence of unstable vibrational modes indicates Ba2ZnAs2 and Ba2ZnSb2 are dynamically stable, while Ba2ZnBi2 is unstable with imaginary frequencies in the phonon dispersion. Remarkably, Ba2ZnSb2 exhibits low acoustic phonon group velocities (<4.5 km/s), large Grüneisen parameters, short phonon relaxation time (<5.5 ps), low-lying optical modes (∼1.5 THz), and strong optical-acoustic phonon couplings. These inherent phonon features can greatly hinder the heat transport ability and therefore give rise to an ultralow thermal conductivity of ∼ 0.11 W/mK at 300 K for Ba2ZnSb2. Furthermore, elastic properties calculated based on the density functional theory helped to gain insight into the origin of the low κ. Our study provides fundamental physical insights into the reason of low thermal conductivity of Ba2ZnX2, which is valuable toward the search for efficient thermoelectric materials based on layered Zintl phase compounds.

Strong phonon coupling induces low thermal conductivity of one-dimensional carbon boron nanotube

Recently, one-dimensional materials have drawn much attention due to their excellent performance in the field of energy storage and thermal management. As a newly synthesized representative of 1D carbon boron compounds, BC3 nanotube (BC3NT), its thermal conductivity and phonon transport mechanism are urgently required for designing electron-related devices and have been still lacking. Herein, the thermal transport properties of BC3NT are investigated using non-equilibrium molecular dynamic simulations. Compared with the carbon nanotube, the lattice analysis and differential charge density revealed that the much lower thermal conductivity of BC3NT stems from the strong phonon coupling induced by the disorders from atomic mass discrepancy and the polarized covalent bond. Furthermore, the temperature and strain effect of thermal conductivity of BC3NT are explored. Interestingly, it is found that for both CNT and BC3NT, the small tensile strain can enhance the thermal conductivity while the large tensile and compressive strain would impede the phonon transport, indicating that the strain strategy can effectively modulate the thermal conductivity of 1D systems. Our studies not only provide a newly perspective to understand phonon transport of 1D systems and could be helpful to extend phonon coupling mechanism to low-dimensional systems.