Soft Phonon Mode Research Articles

Phonon transport manipulation in TiSe2 via reversible charge density wave melting

Titanium diselenide (TiSe2) is a layered material that under a critical temperature of Tc ≈ 200 K features a periodic modulation of the electron density, known as charge density wave (CDW), which finds applications in quantum information and emerging electronic devices. Here, we present first-principles calculations showing the suppression of the CDW via photoexcitation and consequent stabilization of the undistorted high-temperature phase, in agreement with experimental observations. Interestingly, the unfolded CDW melting is accompanied by a sizable reduction in the thermal conductivity, κ, of up to 25% and a large entropy increase of ~10 J K−1 kg−1. The significant κ variation is almost entirely originated from photoinduced changes in the phonon–phonon scattering processes involving a high-symmetry soft phonon mode. Our results open new possibilities in the design of devices for thermal management and phonon-based logic, and suggest original applications in the of context solid-state cooling.

Ultralow Lattice Thermal Conductivity and Large Glass-Like Contribution in Cs3Bi2I6Cl3: Rattling Atoms and p-Band Electrons Driven Dynamic Rotation.

Understanding the origin of ultralow lattice thermal conductivity κL of halide perovskites is of great significance in the energy conversion field. The soft phonon modes and the large anharmonicity corresponding to the dynamic rotation of halogen atoms play an important role in limiting the thermal transport of halide perovskites. The dynamic rotation has long been thought to originate from the electrostatic repulsion of lone pairs around halogen atoms. Here, by studying the layered perovskite Cs3Bi2I6Cl3, it is found that the interaction between the lone pairs contributed by the s bands of halogen atoms is short-range, and the dynamic rotation is really driven by the occupied p-band electrons. It dominates Cs3Bi2I6Cl3 with ultralow κL, < 0.2 W mK-1 at 300 K. Moreover, soft optical phonons are presented ≈1 and 2.2 THz that constitute relatively flat and dense bands due to the rattling Cs and Cl atoms, contributing a large glass-like component to the κL. The results have important implications for understanding the origin of the ultralow κL in halide perovskites and for designing novel perovskites to serve the energy conversionfield.

Investigation of phase stability and martensitic transformation of all-d-metal Heusler alloys Fe2VIr and Fe2TaIr in high pressure: A first-principles study

First-principles calculations are used to investigate the phase stability and martensitic transformation of two new all-d-metal Heusler alloys Fe2VIr and Fe2TaIr at 0–50 GPa. The difference in phase stability between Fe2VIr and Fe2TaIr is due to the effect of d-d orbital hybridisation between different atoms and the appearance of triply degenerate in the pressure-induced phonon optical modes. The softening behaviour of the transverse phonon mode along the high symmetry point X is responsible for the martensitic transformation of Fe2TaIr. We also observed the softening of the shear elastic constant C′ = (C11-C12)/2. Strong Fe-Fe magnetic coupling, softening of transverse acoustic phonon modes and anomalous softening of C′ awaken the martensitic transformation. Low-frequency phonon hybridisation of the heavy elements Ta and Ir is the driving force behind the softening. A surprising phenomenon is found during tetragonal deformation of the structure of Fe2TaIr at 45 GPa, whereby the structure break through the energy barrier of the leap and produced a tetragonal martensitic phase with a much lower energy. It may imply that pressure-induced Fe2TaIr produced a premartensitic phase. This work presents a detailed analysis of the anomalous softening behaviour of Fe2TaIr and the mechanism of the pressure-induced martensitic transformation. A new idea is proposed for an in-depth study of the martensitic phase transition precursor reaction.

Observation of the multiple magnetic phases in double perovskite Pr1.8La0.2CoFeO6

The structural and magnetic properties of the double perovskite Pr1.8La0.2CoFeO6 have been investigated. The X-ray diffraction study shows that the system acquires room-temperature orthorhombic phase with the Pnma space group. The X-ray photoemission spectroscopy (XPS) measurement confirmed that the B-site ions are present in mixed valence states i.e. Co has been found in Co2+ and Co3+ valance states whereas Fe has been found in Fe3+/Fe4+. Magnetic measurements of the system confirm the existence of several fascinating magnetic behaviors, such as long-range canted antiferromagnetism withTN ∼ 271 K, giant exchange bias, and re-entrant cluster glass phase (TG ∼ 33 K). Field-dependent magnetization shows a large spontaneous exchange bias; HSEB∼ 1.8 kOe and giant conventional exchange bias, HCEB∼ 2.4 kOe at 5 K. Antiferromagnetic materials with large exchange bias can be utilized in high-density spintronic devices. Temperature-dependent Raman study demonstrates that the observed phonon mode exhibits anomalous behavior close to the magnetic transition temperature. Analysis of anomalous softening of phonon mode below TN, clarifies the presence of significant spin-phonon coupling while the magnetostriction effect does not play any significant role in the observed phonon anomaly.

Ultralow Thermal Conductivity in Vacancy-Ordered Halide Perovskite Cs3Bi2Br9 with Strong Anharmonicity and Wave-Like Tunneling of Low-Energy Phonons.

Halide perovskites are of great interest due to their exceptional optical and optoelectronic properties. However, thermal conductivity of many halide perovskites remains unexplored. In this study, an ultralow lattice thermal conductivity κL (0.24W m-1 K-1 at 300 K) is reported and its weak temperature dependence (≈T-0.27) in an all-inorganic vacancy-ordered halide perovskite, Cs3Bi2Br9. The intrinsically ultralow κL can be attributed to the soft low-lying phonon modes with strong anharmonicity, which have been revealed by combining experimental heat capacity and Raman spectroscopy measurements, and first-principles calculations. It is shown that the highly anharmonic phonons originate from the Bi 6s2 lone pair expression with antibonding states of Bi 6s and Br 4p orbitals driven by the dynamic BiBr6 octahedral distortion. Theoretical calculations reveal that these low-energy phonons are mostly contributed by large Br motions induced dynamic distortion of BiBr6 octahedra and large Cs rattling motions, verified by the synchrotron X-ray pair distribution function analysis. In addition, the weak temperature dependence of κL can be traced to the wave-like tunneling of phonons, induced by the low-lying phonon modes. This work reveals the strong anharmonicity and wave-like tunneling of low-energy phonons for designing efficient vacancy-ordered halide perovskites with intrinsically low κL.

Revealing a distortive polar order buried in the Fermi sea.

Polar metals are challenging to identify spectroscopically because the fingerprints of electric polarization are often obscured by the presence of screening charges. Here, we unravel unambiguous signatures of a distortive polar order buried in the Fermi sea by probing the nonlinear optical response of materials driven by tailored terahertz fields. We apply this strategy to investigate the topological crystalline insulator Pb1-xSnxTe, tracking its soft phonon mode in the time domain and observing the occurrence of inversion symmetry breaking as a function of temperature. By combining measurements across the material's phase diagram with ab initio calculations, we demonstrate the generality of our approach. These results highlight the potential of terahertz driving fields to reveal polar orders coexisting with itinerant electrons, thus opening additional avenues for material discovery.

Suppressed Lattice Thermal Conductivity in Haeckelite Compounds for High-Performance Thermoelectric Applications.

Traditional semiconductors are known to exhibit excellent electrical properties but oversized lattice thermal conductivities, thus limiting their thermoelectric performance. Herein, we have discovered a low-energy allotrope of those traditional semiconductors. Compared with the wurtzite structure, the lattice thermal conductivity is reduced by more than five times in the haeckelite structure. This is attributed to the softening of acoustic phonon modes and concurrently enhanced anharmonicity in the haeckelite structure. Benefiting from the suppressed lattice thermal conductivity while retaining the excellent electrical properties of wurtzite structure, haeckelite compounds have been proven to be a novel category of high-performance thermoelectric materials. As an excellent representative, haeckelite CdTe exhibits a peak figure of merit approaching 1.3 at n-type doping and high temperature, which experiences a 3-fold improvement compared with its wurtzite counterpart. This work provides an alternative pathway of engineering the lattice thermal conductivities of traditional semiconductors toward superior thermoelectric properties.

Unified deep learning network for enhanced accuracy in predicting thermal conductivity of bilayer graphene, hexagonal boron nitride, and their heterostructures

In this research, we utilized density functional theory (DFT) computations to perform ab initio molecular dynamics simulations and static calculations on graphene, hexagonal boron nitride, and their heterostructures, subjecting them to strains, perturbations, twist angles, and defects. The gathered energy, force, and virial information informed the creation of a training set comprising 1253 structures. Employing the Neural Evolutionary Potential framework integrated into Graphics Processing Units Molecular Dynamics, we fitted a machine learning potential (MLP) that closely mirrored the DFT potential energy surface. Rigorous validation of lattice constants and phonon dispersion relations confirmed the precision and dependability of the MLP, establishing a solid foundation for subsequent thermal transport investigations. A further analysis of the impact of twist angles uncovered a significant reduction in thermal conductivity, particularly notable in heterostructures with a decline exceeding 35%. The reduction in thermal conductivity primarily stems from the twist angle-induced softening of phonon modes and the accompanying increase in phonon scattering rates, which intensifies anharmonic interactions among phonons. Our study underscores the efficacy of the MLP in delineating the thermal transport attributes of two-dimensional materials and their heterostructures, while also elucidating the micro-mechanisms behind the influence of the twist angle on thermal conductivity, offering fresh perspectives for the design of advanced thermal management materials.

Terahertz electric-field-driven dynamical multiferroicity in SrTiO3

The emergence of collective order in matter is among the most fundamental and intriguing phenomena in physics. In recent years, the dynamical control and creation of novel ordered states of matter not accessible in thermodynamic equilibrium is receiving much attention1–6. The theoretical concept of dynamical multiferroicity has been introduced to describe the emergence of magnetization due to time-dependent electric polarization in non-ferromagnetic materials7,8. In simple terms, the coherent rotating motion of the ions in a crystal induces a magnetic moment along the axis of rotation. Here we provide experimental evidence of room-temperature magnetization in the archetypal paraelectric perovskite SrTiO3 due to this mechanism. We resonantly drive the infrared-active soft phonon mode with an intense circularly polarized terahertz electric field and detect the time-resolved magneto-optical Kerr effect. A simple model, which includes two coupled nonlinear oscillators whose forces and couplings are derived with ab initio calculations using self-consistent phonon theory at a finite temperature9, reproduces qualitatively our experimental observations. A quantitatively correct magnitude was obtained for the effect by also considering the phonon analogue of the reciprocal of the Einstein–de Haas effect, which is also called the Barnett effect, in which the total angular momentum from the phonon order is transferred to the electronic one. Our findings show a new path for the control of magnetism, for example, for ultrafast magnetic switches, by coherently controlling the lattice vibrations with light.

Rotational phase transitions in antifluorite-type osmate and iridate compounds

We present temperature-dependent single-crystal diffraction results on seven antifluorite-type A2MeX6 compounds with Me = Os or Ir: K2OsCl6, A 2OsBr6 with A = K, Rb, Cs and NH4, and K2IrX 6 with X = Cl and Br. The structural transitions in this family arise from MeX 6 octahedron rotations that generate a rich variety of symmetries depending on the rotation axis and stacking schemes. In order to search for local distortions in the high-symmetry phase we perform refinements of anharmonic atomic displacement parameters with comprehensive data sets. Even at temperatures close to the onset of structural distortions, these refinements only yield a small improvement indicating only small anharmonic effects. The phase transitions in these antifluorites are essentially of displacive character. However, some harmonic displacement parameters are very large reflecting soft phonon modes with the softening covering large parts of the Brillouin zone. The occurrence of the rotational transitions in the antifluorite-type family can be remarkably well analyzed in terms of a tolerance factor of ionic radii.

Computational predictions of optoelectronic energy materials Cs2SiBr6 Cs2GeBr6 & Cs2SnBr6 for phenomenal photovoltaic applications; a first principles study

Exploring prospective materials for energy storage and production is one of the largest confrontation of the century. Solar optoelectronic energy is one of the utmost exceptional renewable resources now a days, due to their small Environmental impact and wide availability. The perovskite type double halides are promising materials for optoelectronic and solar applications. In this article, the optoelectronic characteristics of newly perovskite double halides Cs2SiBr6 Cs2GeBr6 and Cs2SnBr6 are addressed. The calculated band gaps 2.70 eV (Cs2SiBr6), 1.80 eV (Cs2GeBr6), 2.90 eV (Cs2SnBr6) with raising performance for optoelectronic applications. The optical properties are explored by large dielectric constants, high optical absorption coefficients, small optical loss etc. Moreover, the mechanical properties for the studied materials have been performed to confirm the mechanical stability, comactability and reliability of the materials for photovoltaic applications. The thermodynamic and vibrational stability and compatibility has been verified due to negative values of enthalpy, cohesive energies and no soft phonon mode of vibrations. These optoelectronic energy related calculations confirmed that the perovskites type double halides may Cs2SiBr6, Cs2GeBr6 and Cs2SnBr6 may be novel, stimulating potential materials and can be synthesized experimentally for next generation photovoltaic devices and related applications.

Phonon mode softening and band convergence induced significant enhancement of thermoelectric performance in strained CdI2-type SnI2 monolayer

Monolayer group IVA metal halides have attracted intensive attention for their excellent thermoelectric performance. Recently, the CdI2-type SnI2 monolayer has been successfully fabricated by molecular beam epitaxy for the first time. The universal two-dimensional growth behavior on different substrates, motivates strain engineering on its thermoelectric performance. Herein, the structure stability and thermoelectric properties of SnI2 monolayer under different strains are systematically investigated, based on first-principles calculations combined with Boltzmann transport theory. The investigation shows that the tensile strain can simultaneously induce phonon mode softening and band convergence, resulting in a significant enhancement in thermoelectric performance. The phonon mode softening for ZA and optical branches further enhances lattice anharmonicity and a reduced thermal conductivity of ∼0.14 Wm−1K−1 at 300 K (∼0.05 Wm−1K−1, 800 K) can be obtained. Additionally, band convergence in valence band improves the Seebeck coefficient and increases power factor. Under +2 % tensile strain, the optimal ZT values of SnI2 monolayer for p-type doping are substantially enhanced up to 3.05 (300 K) and 5.06 (800 K), which are increased by ∼98 % and ∼50 % compared to the case without strain. Our results reveal the close relationship between the phonon mode softening and enhanced anharmonicity, and provide the physical insights into the understanding of the modulation of thermoelectric properties of monolayer SnI2, shedding light on substantially optimizing the thermoelectric performance of monolayer group IVA metal halides family.

Hydrogenation-induced superconductivity in monolayer

Here, we construct a new two-dimensional hydrogenated transition metal dichalcogenide material, the Janus WSH monolayer, which is created by replacing the top-layer S atoms in the 2H-WS2 monolayer with H atoms. Then we use first-principles calculations to investigate its electronic structure, phonon dispersion, and superconductivity. The results show that hydrogenation breaks the reflection symmetry, which helps orbital hybridization and to flatten the electronic bands. Thus, it leads to a high electronic density of states near the Fermi level. Additionally, the electron-phonon coupling is enhanced by the softening of phonon modes from the in-plane vibrations of W. The strong interactions between electrons and phonons result in phonon-mediated superconductivity in Janus WSH monolayer. The calculated critical temperature (T c ) is approximately 23.1 K at atmospheric pressure. This T c is about twice higher than that of existing WS2-based materials.

Artificial neural network for deciphering the structural transformation of condensed ZnO by extended x-ray absorption fine structure spectroscopy

Pressure-induced structural phase transitions play a pivotal role in unlocking novel material functionalities and facilitating innovations in materials science. Nonetheless, unveiling the mechanisms of densification, which relies heavily on precise and comprehensive structural analysis, remains a challenge. Herein, we investigated the archetypal B4 → B1 phase transition pathway in ZnO by combining x-ray absorption fine structure (XAFS) spectroscopy with machine learning. Specifically, we developed an artificial neural network (NN) to decipher the extended-XAFS spectra by reconstructing the partial radial distribution functions of Zn–O/Zn pairs. This provided us with access to the evolution of the structural statistics for all the coordination shells in condensed ZnO, enabling us to accurately track the changes in the internal structural parameter u and the anharmonic effect. We observed a clear decrease in u and an increased anharmonicity near the onset of the B4 → B1 phase transition, indicating a preference for the iT phase as the intermediate state to initiate the phase transition that can arise from the softening of shear phonon modes. This study suggests that NN-based approach can facilitate a more comprehensive and efficient interpretation of XAFS under complex in-situ conditions, which paves the way for highly automated data processing pipelines for high-throughput and real-time characterizations in next-generation synchrotron photon sources.

Enhancement of superconductivity by organic flexible substrates in NbSe2 thin flakes

The regulation of superconductivity is of great significance in basic physics and device applications. Typically superconductivity can be tuned by chemical doping, gate voltage, and pressure. The interaction between superconductivity and substrates in low-dimensional systems is concerned in recent years, but a systematic and in-depth investigation is still lacking. Here we report a systematic study concerning the influence of substrates on the performance of superconductivity in NbSe2 thin flakes. It is found that the superconducting critical temperature Tc can be enhanced when the organic flexible substrates are adopted, as compared with that using the inorganic rigid substrates. Such an enhancement is more distinct under the applied magnetic fields. The analysis combined with Raman scattering experiments shows that the improvement of superconductivity is closely related to the softening of E2g1 phonon mode, pointing out the most possible mechanism for the effect of substrates on superconductivity. Meanwhile, the involvement of organic substrate also enhances the wave number of the charge-density-wave related soft mode.

Superconductivity in unconventional metals

Based on first-principles calculations, we demonstrate that 1H/2H-phase transition metal dichalcogenides MX2 (M = Nb, Ta; X = S, Se, Te) are unconventional metals, which have an empty-site band of A1′@1e\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${A}_{1}^{{\\prime} }@1e$$\\end{document} elementary band representation at the Fermi level. The computed phonon dispersions indicate the stability of the system at high temperatures, while the presence of the soft phonon mode suggests a phase transition to the charge density wave state at low temperatures. Based on the Bardeen-Cooper-Schrieffer theory and computed electron-phonon coupling, our calculations show that the superconductivity (SC) in NbSe2 is mainly attributed to the soft phonon mode due to the half filling of the empty-site band. Accordingly, the SC has been predicted in unconventional metals TaNS monolayer and 2H-TaN2 bulk with computed TC = 10 K and 26 K respectively. These results demonstrate that the unconventional metals with partial filling of the empty-site band offer an attractive platform to search for superconductors.

Pressure-driven superconductivity in layered isostructural germanium phosphides

The discovery of superconductivity and its modulation are long-standing cutting-edge research topics in condensed matter physics. As a powerful tool, the high-pressure technique can be used to achieve novel superconductors and tune their physical properties. One typical example is binary germanium phosphides with different stoichiometries, which exhibit abundant physical properties with layered lattice structures similar to blue phosphorus. The detailed phase diagrams of the Ge–P systems are important for understanding the influence of stoichiometry on pressure-driven superconductivity, but it remains unexplored. Here, we measured and compared the detailed superconducting phase diagrams of the Ge–P systems of layered isostructural germanium phosphides GeP3 and GeP5 under pressure. Even though these two binary phosphides exhibit obviously different atomic occupations in the crystal structure due to their distinct stoichiometric ratios, the onset superconducting transition temperatures Tc of GeP3 and GeP5 both show dramatic enhancements from ∼2.5 K at 12.0 GPa to the maximum values of ∼9.0 K at 28.0 GPa, which are higher than those of other binary metal phosphides. Such pressure-enhanced superconductivity therein is accompanied by significant pressure-induced phonon mode softening, which is confirmed via in situ high-pressure Raman measurements. Our observations deepen the physical understanding of pressure-driven superconductivity in phosphorous-rich layered compounds and pave the way for potential applications in microsuperconducting devices.

Ru Prussian blue analogue-derived Ru nanoparticles composited with a trace amount of Pt as an efficacious electrocatalyst for the hydrogen evolution reaction.

In this work, we designed a straightforward and highly reproducible synthetic methodology to prepare Ru0-Pt0 composites. We report a significant improvement in the electrocatalytic performance upon compositing Ru with a very trace amount of Pt. In particular, Ru nanoparticles were derived from a Ru-Prussian blue analogue (Ru PBA) and composited with (0.1, 0.5, and 1 mmol) metallic platinum following an optimized chemical reduction method. Interestingly, the composite with 0.5 mmol of Pt (Ru@C/Pt0.5) required low overpotentials of 32 and 140 mV to achieve current densities of 10 and 100 mA cm-2, respectively. Furthermore, Ru@C/Pt0.5 exhibited a smaller Tafel slope (26 mV dec-1), robust durability with 50 hours of long-term stability and a higher turnover frequency (TOF: 5.6 s-1@η10 mA cm-2) than commercial Pt/C (TOF: 4.1 s-1@η10 mA cm-2). First-principles calculations using density functional theory (DFT) revealed that the existence of Pt islands on the Ru nanoparticles weakened the strength of the adsorption of hydrogen at the Ru interstitials due to electrostatic repulsion caused by charge retention at Ru atoms near the corner of the islands, leading to rapid dissociation of hydrogen. This created a significant impact on the improvement of the electrocatalytic HER activity of the Ru@C/Pt0.5 electrocatalyst. It appears that restricting the concentration of Pt to trace amounts is a necessary condition for the observed catalytic efficiency, as the catalytic efficiency decreases with an increasing island size due to stronger binding of atomic hydrogen on peripheral Pt atoms and stabilization of adsorbed atomic hydrogen caused by softening of phonon modes with increasing island size. This study opens up a novel avenue for the exploration of highly efficient electrocatalysts for hydrogen evolution reactions.

Correlating negative thermal expansion and thermal conductivity in two-dimensional carbon-based materials.

Negative thermal expansion (NTE) is a fascinating phenomenon wherein certain materials contract upon heating. The phonon transport properties of two-dimensional carbon-based allotropes are not yet fully understood in terms of their NTE properties. This work with a specific focus on carbon-based allotropes investigates the underlying mechanisms of the thermal conductivity (TC) and NTE of graphene, haeckelite, pentahexoctite, s-graphene, 6.6.12 and delta Graphynes (Gys). High TC is imperative for efficiently dissipating heat in electronic devices, whereas thermoelectric devices need to be thermally resistive with low TC. Delta-Gy shows the highest NTE as well as the lowest TC and vice versa is true for graphene. Graphene displays a lower degree of anisotropic TC, while s-graphene exhibits the highest level of anisotropic TC. The behaviour of their TC can be explained based on the soft-phonon modes, phonon group velocity (vg), phonon lifetime (τ) and mean free path (MFP). The acoustic and optical phonon branches play a key role in determining both the TC and NTE of the materials. Out-of-plane buckling in two-dimensional materials reduces thermal conductivity by increasing the phonon scattering. Buckling has also been shown to increase the NTE. A precise control on the pore sizes 5-7 (haeckelite), 5-6-8 (pentahexoctite), and 4-8 (s-graphene), 6-12-14 (6.6.12-Gy) and 6-14 (delta-Gy) can significantly influence their soft unit modes. This investigation not only deepens our understanding of NTE and TC but also highlights the potential of future applications of carbon-based materials with controlled thermal expansion properties in nanotechnology, composites, and beyond.

Observation of piezoelectricity in a lead-free Cs2AgBiBr6 perovskite: a new entrant in the energy harvesting arena.

Halide perovskite materials have recently been recognised as powerful ferroelectric and piezoelectric materials with applications in the energy harvesting arena, but their experimental proof is very limited. We achieved strong intrinsic piezoelectricity in the lead-free inorganic double perovskite Cs2AgBiBr6 at room temperature and utilized it for mechanical energy harvesting, with a piezoelectric co-efficient (d33) of 12.7 pC N-1. Hysteresis loop and structural analyses offered further validation for the substantial ferroelectric features of the as-synthesised double perovskite. Density functional theory (DFT) calculations revealed the presence of anharmonic phonon soft modes in tetragonal Cs2AgBiBr6 due to dynamic instability, which resulted in piezoelectricity. Under an optimal pressure of ≈25 kPa, a Cs2AgBiBr6 thin film-based piezoelectric nanogenerator device delivered instantaneous output values of ≈45 V and ≈200 nA. The strain-sensitive responses of the device were also exemplified to identify specific body motions from the detected instantaneous output values. The energy obtained from the device is shown to be effective for capacitor charging and commercial light-emitting diode (LED) lighting. Our study provides significant insights into the dielectric behaviour of materials as well as piezo- and ferroelectric behaviours, which are crucial for the development of modern electronic and energy harvesting devices.