Phonon Dynamics Research Articles

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.

Weak interatomic interactions induced low lattice thermal conductivity in 2D/2D PbSe/SnSe vdW heterostructure

Inspired by the excellent thermoelectric performance of two-dimensional (2D) PbSe and SnSe monolayers, the thermal and electronic transport properties of 2D/2D PbSe/SnSe heterostructure is theoretically evaluated using first-principles calculations and Boltzmann transport theory. An indirect bandgap of 1.06 eV for the PbSe/SnSe heterostructure is calculated by the combination of Heyd-Scuseria-Ernzerhof (HSE06) functional and spin-orbit coupling (SOC) effect. The mechanical, thermal and dynamic stabilities of the PbSe/SnSe heterostructure are further demonstrated by the elastic modulus, ab initio molecular dynamics (AIMD) simulations and phonon dispersion curves, respectively. The honeycomb-like wrinkled structure introduce the asymmetry at the interface of the PbSe/SnSe heterostructure, thereby leading to enhanced phonon boundary scattering. Concurrently, the antibonding states originated from the weak interatomic interactions within PbSe/SnSe heterostructure also lead to strong anharmonicity. This synergistic effect of pronounced scattering and heightened anharmonicity contributes to the low lattice thermal conductivity of PbSe/SnSe heterostructure. The thermoelectric properties of PbSe/SnSe heterostructure are further investigated along the x- and y-direction, which reveals that the p-type PbSe/SnSe heterostructure attains an optimal dimensionless figure-of-merit (ZT) of 2.6 at 900 K along the y-direction. Our present study not only offers the fundamental insights into the thermal and electronic transport properties of the PbSe/SnSe heterostructure, but also provides an effective and feasible strategy for the design of 2D layered heterostructures as promising thermoelectric materials.

Two-dimensional III-nitrides: A comprehensive DFT and thermodynamics studies

We used DFT simulations for a comprehensive investigation of the stability, corrected electronic band gap, thermodynamic properties and bonding nature of the 2D III-nitrides. The geometry relaxation indicates that all the structures are stable, verified by phonon and molecular dynamics (MD) simulations. The phonon plot shows that all the vibrations are real and there is no imaginary vibration observed. The MD simulations at 500 K for the period of 1ps signify stable structures without any structural distortion. The AIM analysis demonstrates that the electron density, Laplacian and energy density values for X–N (X = B, Al, Ga and In) bonds are in the range of 0.10–0.185 au, 0.36–0.64 au and −0.029 to −0.16 au, which evidenced the partial ionic bonding nature of III-nitrides. The Hirshfeld density analysis shows that significant charge transfer occurred between the X and N atoms. The band gap study performed by HSE06, shows that the BN, AlN, GaN and InN have a band gap value of 5.65, 4.9, 3.45 and 2.0 eV, which are in good agreement with the experimental values compared to other reported theoretical results. This study would be helpful for scientists in the future exploration of 2D III-nitrides for energy, catalysis, optics applications.

Hybridization-mediated quasiparticle and phonon dynamics in single crystal cerium films

Hybridization-mediated quasiparticle and phonon dynamics in single crystal cerium films

Gate-tunable circular phonon dichroism effect in bilayer graphene

Circular phonon dichroism effect has been proposed in two-dimensional materials; however, the lack of tunability hinders the exploration of the effect. Here, we investigate the role of dual-gating-induced inversion symmetry breaking in the circular phonon dichroism effect in bilayer graphene. We find that the introduction of inversion symmetry breaking modifies the response in the layer-symmetric and layer-antisymmetric channels, and results in the occurrence of phonon dichroism in the cross-channel. In the layer representation, the inversion symmetry breaking breaks the equality of intralayer circular phonon dichroism and enhances the interlayer response. Our results suggest that layer degree of freedom provides possibilities to tune phonon dynamics, which paves a way toward different physics and applications of two-dimensional acoustoelectronics and layertronics.

Ti2XCl2 (X = C and N), TiVCCl2 and V2CFTx (Tx = F and Cl) MXenes a DFT-D study

We reported the effect of halogen elements attached to various MXenes surfaces, on their bonding, electronic (energy bands, PDOS), dynamic (phonon dispersion) and thermal properties (Debye temperature). Ti2XCl2 (X = C and N), TiVCCl2 and V2CFTx (Tx = F and Cl) displayed high binding energy values and metallic behaviour independently of the halogens. Dynamic stability of Ti2XCl2 and V2CFTx structures is attested by their phonon dispersion spectra showing real frequencies. Further Debye temperatures calculation on Ti2XCl2 (X = C and N) MXenes attest them as better thermal conductors than Ti2CO2 MXene.

Precision-controlled ultrafast electron microscope platforms. A case study: Multiple-order coherent phonon dynamics in 1T-TaSe2 probed at 50 fs-10 fm scales.

We report on the first detailed beam tests attesting the fundamental principle behind the development of high-current-efficiency ultrafast electron microscope systems where a radio frequency (RF) cavity is incorporated as a condenser lens in the beam delivery system. To allow for the experiment to be carried out with a sufficient resolution to probe the performance at the emittance floor, a new cascade loop RF controller system is developed to reduce the RF noise floor. Temporal resolution at 50 fs in full-width-at-half-maximum and detection sensitivity better than 1% are demonstrated on exfoliated 1T-TaSe2 system under a moderate repetition rate. To benchmark the performance, multi-terahertz edge-mode coherent phonon excitation is employed as the standard candle. The high temporal resolution and the significant visibility to very low dynamical contrast in diffraction signals via high-precision phase-space manipulation give strong support to the working principle for the new high-brightness femtosecond electron microscope systems.

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.

Heterogeneous attenuation of sound waves in three-dimensional amorphous solids.

Sound waves are attenuated as they propagate in amorphous materials. We investigate the mechanism driving sound attenuation in the Rayleigh scattering regime by resolving the dynamics of an excited phonon in time and space via numerical simulations. We find sound attenuation is spatiotemporally heterogeneous. It starts in localized regions, which identify soft regions within the material and correlate with low-frequency vibrational modes. As time progresses, the regions where sound is primarily attenuated invade the system via an apparent diffusive process.

Computational Design for Enhanced Hydrogen Storage on the Newly Synthesized 2D Polyaramid via Titanium and Zirconium Decoration.

2D polyaramid (2DPA) is a porous and polymeric material that has been synthesized recently. Titanium and zirconium decoration over 2DPA increases their affinity for hydrogen substantially, making them suitable for onboard and reversible hydrogen storage, particularly in light-duty vehicles. By decorating a single unit cell of 2DPA with two transition metal (TM) atoms, hydrogen storage of up to 6.422 and 6.792 wt % of H2 with average binding energies of -0.399 and -0.480 eV is predicted for 2DPA + Ti and 2DPA + Zr, respectively. The binding of Ti and Zr with 2DPA is accompanied by a flow of charge (-1.474e for Ti and -1.696e for Zr) from the TM toward the 2DPA sheet. Further, the interaction between H2 and the TM may proceed via Kubas interaction between the d orbital of the TM in 2DPA + TM and H 1s orbitals of H2, with a net flow of charge from the TM toward H2 (-0.218e for Ti and -0.391e for Zr). The desorption of H2 bound to 2DPA + Zr is endothermic (∼0.57 eV) and close in magnitude to the binding energy of the first H2 (∼-0.544 eV). The 2DPA + TM systems show structural and dynamic stability at high temperatures, as evident from ab initio molecular dynamics simulations and phonon spectra. The movement of TM atoms across the 2DPA sheet to form clusters may be hindered by the considerable barrier energy (∼4.9 eV for Ti). Through these systematic density functional theory simulations, we predict that Ti- and Zr-decorated 2DPA are high-performance hydrogen storage materials and can be explored by experimentalists.

Low-temperature Raman studies of graphene oxide: Analysis of structural properties

This work is devoted to the low-temperature Raman studies of a bright representative of 2D materials—graphene oxide (GO) film in the range of 5–325 K. The performed analysis of the temperature evolution of the peak positions as well as linewidths of two Raman modes D (∼1300 cm−1) and G (∼1600 cm−1) was described in terms of the anharmonic model. The temperature behavior of the G mode demonstrated a slight deviation from the anharmonic model below ∼50 K in contrast to the D mode, which could be explained by involving an additional phonon decay channel. The analysis of the linewidth of the Raman modes showed that the distribution of defects in GO is inhomogeneous and surface functionalization effectively separates neighboring layers. The average value of the distance between defects and the defect density was estimated. The obtained results can be useful for understanding phonon dynamics for the development of nanodevices based on 2D materials where confinement of propagation of phonon excitations plays a key role.

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.

Tunable bandgaps, stabilities and optical properties of X3Y (X=C, Si; Y[dbnd]N, P) monolayers with and without strain

Our study explores the various properties of a novel class of two-dimensional (2D) monolayer materials, i.e., X3Y (X = C, Si; YN, P). The materials (i.e., C3N, C3P, Si3N, Si3P) exhibit dynamic and thermal stabilities through phonon and molecular dynamics calculations. Under biaxial tensile strain of approximately 11 %, the structures undergo only deformation, indicating their ideal strength. The phonon spectra remain dynamically stable under this 11 % biaxial strain, emphasizing the high mechanical stability of the three materials. The electronic structure showed band gaps of 0.39 eV, 1.56 eV, 0.0 eV, and 0.27 eV for C3N, C3P, Si3N, and Si3P, respectively, which are crucial for electronic devices. Meanwhile, we have found the transitions from metal to semiconductor (Si3N), indirect to direct band gap (Si3P), and semiconductor to metal (C3N, Si3P) by applying compressive and tensile strains to tune the bandgaps. In addition, we have investigated the optical properties excellent absorption in the visible range for C3P, Si3N. Compressive and tensile strains also have a significant effect on these four materials, with both absorption coefficient has been enhanced under strain to some extent. These findings provide potential applications in nano-electronic and opto-electronic devices.

Prediction of new KVBi monolayer stability and electronic, magneto-optic and thermoelectric properties

ABSTRACT In this paper, we investigate the electronic, mechanical, magneto-optic and thermoelectric properties of a d0-d half-Heusler monolayer, named KVBi Heuslerene. We prove that this material is stable from static and dynamic views, using both E-V and phonon dispersion curves. We also find out this material is metal in the spin-up while it is a semiconductor in the spin-down. Moreover, it is indicated that KVBi Heuslerene is a proper candidate for magneto=optic applications. Besides, it is also illustrated that this material possesses interesting features in thermoelectric applications.

Odd‐Stoichiometry and Secondary Phase Relations in Higher Manganese Silicides

AbstractHigher manganese silicides (denoted as MnSiγ) are a prominent class of intermetallic compound for thermoelectric applications that crystallizes into a Nowotny Chimney Ladder (NCL) phase and constitutes a higher concentration of silicon with odd stoichiometry, typically represented by an irrational number. In this work, the genesis of Si odd stoichiometry and secondary phase relations in silicon‐rich and silicon‐deficient MnSix is presented to clarify the significance of labile (i.e., easily alterable) Si‐subsystem in inheriting the modulated lattice structure with characteristic anharmonicity. The transport properties show an interrelation to the presence of a secondary phase, which is driven by the odd stoichiometry, where the thermoelectric figure of merit (zT) is found to be optimal for MnSi1.74. The structural characterization of rapidly solidified melt‐spun ribbons and spark plasma sintered bulk specimens reveal a characteristic presence of MnSi and Si as secondary phases in Si‐deficient (x ≈ 1.5) and Si‐excess (x ≈ 2) compositions of nominal MnSix, respectively, that coexists with the major MnSiγ based NCL phase. This study provides the fundamental basis of phase evolution and the relevance of odd stoichiometry in silicon‐rich manganese silicides which exhibit fascinating phonon dynamics in optimal MnSi1.74 stoichiometry.

Hidden structures: a driving factor to achieve low thermal conductivity and high thermoelectric performance.

The long-range periodic atomic arrangement or the lack thereof in solids typically dictates the magnitude and temperature dependence of their lattice thermal conductivity (κlat). Compared to crystalline materials, glasses exhibit a much-suppressed κlat across all temperatures as the phonon mean free path reaches parity with the interatomic distances therein. While the occurrence of such glass-like thermal transport in crystalline solids captivates the scientific community with its fundamental inquiry, it also holds the potential for profoundly impacting the field of thermoelectric energy conversion. Therefore, efficient manipulation of thermal transport and comprehension of the microscopic mechanisms dictating phonon scattering in crystalline solids are paramount. As quantized lattice vibrations (i.e., phonons) drive κlat, atomistic insights into the chemical bonding characteristics are crucial to have informed knowledge about their origins. Recently, it has been observed that within the highly symmetric 'averaged' crystal structures, often there are hidden locally asymmetric atomic motifs (within a few Å), which exert far-reaching influence on phonon transport. Phenomena such as local atomic off-centering, atomic rattling or tunneling, liquid-like atomic motion, site splitting, local ordering, etc., which arise within a few Å scales, are generally found to drastically disrupt the passage of heat carrying phonons. Despite their profound implication(s) for phonon dynamics, they are often overlooked by traditional crystallographic techniques. In this review, we provide a brief overview of the fundamental aspects of heat transport and explore the status quo of innately low thermally conductive crystalline solids, wherein the phonon dynamics is majorly governed by local structural phenomena. We also discuss advanced techniques capable of characterizing the crystal structure at the sub-atomic level. Subsequently, we delve into the emergent new ideas with examples linked to local crystal structure and lattice dynamics. While discussing the implications of the local structure for thermal conductivity, we provide the state-of-the-art examples of high-performance thermoelectric materials. Finally, we offer our viewpoint on the experimental and theoretical challenges, potential new paths, and the integration of novel strategies with material synthesis to achieve low κlat and realize high thermoelectric performance in crystalline solids via local structure designing.

Toward intrinsic ultra-high-temperature ferromagnetism in a CrAuTe2/graphene heterosystem.

Exploring intrinsic two-dimensional (2D) ferromagnetic (FM) materials with high Curie temperatures (TC) and large magnetic anisotropy energies (MAE) is one of the effective solutions to develop materials for high-performance spintronic applications. Using density functional theory calculations and high-throughput computations, we predict an intrinsic bimetallic FM monolayer, CrAuTe2, which has a large MAE and high TC. The results show that the value of the MAE can reach about 1.5 meV per Cr, and Monte Carlo simulations show that the TC of monolayer CrAuTe2 is about 840 K. Further analysis indicates that the joint effects of spin-orbit coupling (SOC) interaction and magnetic dipole-dipole interaction result in high in-plane magnetic anisotropy. In addition, this monolayer has good dynamic, thermal, and mechanical stabilities, which were confirmed by ab initio molecular dynamics simulations, phonon spectra, and elastic constants, respectively. In order to propose a practical synthesis approach, we built a CrAuTe2/graphene van der Waals heterostructure, and found that the heterostructure does not affect the magnetic properties of monolayer CrAuTe2. These findings appear promising for the future applications in nano-spintronics.

BlueP encapsulated Janus MoSSe as a promising heterostructure anode material for LIBs.

In this work, the significance of BlueP-Janus MoSSe heterostructures in LIBs is explored in detail by using density functional theory calculations. The Janus MoSSe possesses two different atomic layers, and hence two different heterostructures, BlueP-SMoSe and BlueP-SeMoS, are taken into account. The heterostructure formation energies are computed to check their stability. Besides, ab initio molecular dynamics simulations and phonon studies are done to check their thermal and dynamical stabilities, respectively. The adsorption and diffusion of Li at different surfaces of both the heterostructures are calculated. Our study reveals that the heterostructures show strong Li intercalation capability with ultrafast Li diffusion barrier energies. The electronic properties of the lithiated heterointerfaces are also explored. Both the heterostructures can hold a maximum of two layers of Li ions on each side of both BlueP and MoSSe to give a large storage capacity, signifying their extraordinary potential to be appropriate as an anode material for Li-ion batteries. Additionally, due to their strong mechanical strength, the 2D BlueP-Janus MoSSe heterostructures can withstand massive volume expansion during the lithiation-delithiation reaction, which is remarkably beneficial for manufacturing flexible anodes. Based on the above findings, the newly designed heterostructures are expected to open a new avenue for the next generation of electronic devices.

The coexistence of high piezoelectricity and superior optical absorption in Janus Bi2X2Y (X = Te, Se; Y = Te, Se, S) monolayers.

Bismuth chalcogenide and its derivatives have been attracting attention in various fields as semiconductors or topological insulators. Inspired by the high piezoelectric properties of Janus Bi2TeSeS monolayer and the excellent optical absorption properties of the Bi2X3 (X = Te, Se, S) monolayers, we theoretically predicted four new-type two-dimensional (2D) monolayers Janus Bi2X2Y (X = Te, Se; Y = Te, Se, S) using the first principles combined with density functional theory (DFT). The thermal, dynamic, and mechanical stabilities of Janus Bi2X2Y monolayers were confirmed based on ab initio molecular dynamics (AIMD) simulations, phonon dispersion, and elastic constants calculations. Their elastic properties, band structures, piezoelectric, and optical properties were systematically investigated. It was found that Janus Bi2X2Y monolayers have a typical Mexican hat-shaped valence band edge structure and, therefore, have a ring-shaped flat band edge, which results in their indirect band gaps. The results show that Janus Bi2X2Y monolayers are semiconductors with moderate band gaps (0.62-0.98 eV at the HSE + SOC level). After considering the electron-phonon renormalization (EPR), the band gaps are reduced by less than 5% at 0 K under the zero-point renormalization (ZPR) and further reduced by approximately 10% at 300 K. Besides, Janus Bi2X2Y monolayers also exhibit excellent optical absorption properties in the blue-UV light region, with the peak values at the order of 8 × 105 cm-1. Particularly, the Janus Bi2Te2S monolayer was found to exhibit a piezoelectric strain coefficient d11 of up to 20.30 pm V-1, which is higher than that of most of the 2D materials. Our results indicate that Janus Bi2X2Y monolayers could be promising candidates in solar cells, optical absorption, and optoelectronic devices; especially, a Janus Bi2Te2S monolayer can also be an excellent piezoelectric material with great prospects in the fields of mechanical and electrical energy conversion.

A promising high temperature 2D thermoelectric material: novel single-layer ZrHfS4.

Based on the first-principles calculation and Boltzmann transport theory, we report the discovery of a promising candidate for high-performance two-dimensional (2D) thermoelectric material: the single-layer (1L) ZrHfS4. Through the first-principles molecular dynamics simulation and phonon calculation, 1L-ZrHfS4 is predicted to be thermodynamically stable even at high temperatures. In addition, the lattice thermal conductivity of 1L-ZrHfS4 is calculated to be 10.2 W m-1 K-1, which is smaller than that of 2D transition-metal dichalcogenides, such as MoS2 and WS2. Notably, considering not only all the transport coefficients but also the relaxation time for both acoustic and optical phonon scattering, the maximum figure of merit (ZT) of 1L-ZrHfS4 at 1200 K can achieve 1.92 at moderate n-type doping. These findings suggest 1L-ZrHfS4 as a potential material for efficient thermoelectric energy conversion, particularly in applications requiring high temperature operation.