Phonon Spectrum Research Articles

On the nonlinear rovibrational excitation of phononic frequency combs in molecules

The mechanical analog of optical frequency combs, phononic frequency combs (PFCs), has recently been demonstrated in mechanical resonators via nonlinear coupling among multiple phonon modes. However, the requisite strong nonlinear couplings need not be readily present for exciting phononic combs in molecules. To overcome this limitation, this paper introduces an alternative route for generating phononic combs in polar molecules. Theoretically, we investigated the radiation and phononic spectra generated from CO molecules possessing relatively large permanent dipole moment with density matrix formalism. By considering the rovibronic excitation of the ground-state CO molecule while avoiding the electronic excitation, the contribution of the permanent dipole moment and electric dipole polarizability to creating PFCs is demonstrated and distinguished. The finding could motivate the possible extension of combs to molecular systems to offer new avenues in molecular sciences.

Lattice thermal conductivity in CrSBr: the effects of interlayer interaction, magnetic ordering and external strain

With the continuous development of digital information and big data technologies, the ambient temperature and heat generation during the operation of magnetic storage devices play an increasingly crucial role in ensuring data security and device stability. In this study, we conducted a thorough investigation on in-plane lattice thermal conductivity of the van der Waals (vdWs) magnetic semiconductor CrSBr from bulk to monolayer using first-principles calculations and phonon Boltzmann transport equation. Our findings indicated that CrSBr show strong anisotropic thermal transport behaviors and layer number and magnetic ordering dependent lattice thermal conductivity. The lowest thermal conductivity is observed in y direction of antiferromagnetic CrSBr bilayer at all temperatures. Through the analysis of phonon spectra, phonon lifetime, heat capacity, scattering probability, phonon-phonon interaction strength, we demonstrated that out of plane acoustic phonon modes soften, the shift of Cr-Br antisymmetrical stretching vibrations, and large phonon band gap are the main factors. These results offer a comprehensive insight into phonon transport phenomena in vdWs magnetic materials.

Strain dependent properties of the Bi2Sr2CaCu2O8 superconductor: an ab initio study.

Abstract Ab initio calculations were performed to investigate the effects of strain on the structural, electronic, and vibrational properties of the Bi2Sr2CaCu2O8 (Bi-2212) compound. To accurately represent the Bi-2212 ground state, a modulation correction was applied, generating a distorted structure with lower symmetry that better represents the incommensurate superstructure observed in this compound. Phonon spectra and electronic properties were calculated under various levels of c-axis strain, ranging from -2.0% to +2.0%. For the electronic properties, minor changes were observed in the electronic density of states and band structure. However, trends could be identified by analyzing the fine features of the band structure through a tight-binding model. The most significant changes were observed in the vibrational properties, where different trends emerged for the various Raman-active modes. The changes observed in the vibrational and electronic properties can be explained by examining the distances and overlap populations of the relevant bonds, as well as the reduced mass of certain modes. This work can serve as an input for analyzing experimental measurements, helping to distinguish structural effects from others.

Molecular dynamics work on thermal conductivity of SiGe nanotubes.

SiGe nanotubes (SiGeNTs) hold significant promise for applications in nanosolar cells, optoelectronic systems, and interconnects, where thermal conductivity is critical to performance. This study investigates the effects of length, diameter, temperature, and axial strain on the thermal conductivity of armchair and zigzag SiGeNTs through molecular dynamics simulations. Results indicate that thermal conductivity increases with sample length due to ballistic heat transport and decreases with temperature as phonon scattering intensifies. Axial strain transitions from compression to tension enhance phonon propagation, improving conductivity. Chirality affects conductivity, with zigzag SiGeNTs consistently outperforming armchair structures, while diameter exhibits negligible impact. Non-equilibrium molecular dynamics simulations were conducted using the LAMMPS package with the Tersoff potential to model Si-Ge interactions. Thermal conductivity was computed via Fourier's law, with the system divided into regions for controlled heat input and dissipation. Lengths, diameters, temperatures (100-500K), and axial strains (- 6% to + 9%) were varied systematically. Phonon spectrum analysis was performed using Fourier transforms of velocity autocorrelation functions to compute.

Long wavelength interdomain phonons and instability of dislocations in small-angle twisted bilayers

Abstract We develop a theory for long wavelength phonons originating at dislocations separating domains in small-angle twisted homobilayers of 2D materials such as graphene and MX2 transition metal dichalcogenides (M=Mo,W; X=S,Se). We find that both partial and perfect dislocations, forming due to lattice relaxation in the twisted bilayers with parallel and anti-parallel alignment of unit cells of the constituent layers, respectively, support several one-dimensional subbands of the interdomain phonons. We show that spectrum of the lowest gapless subband is characterized by imaginary frequencies, for wave-numbers below a critical value, dependent on the dislocation orientation, which indicates an instability for long enough straight partial and perfect dislocations. We argue that pinning potential and/or small deformations of the dislocations could stabilize the gapless phonon spectra. The other subbands are gapped, with subband bottoms lying below the frequency of interlayer shear mode in domains, which facilitates their detection with the help of optical and magnetotransport techniques.

A First-Principle Study Investigating the Half-Metallic and Mechanical Properties of Double Halide Perovskites Rb2OsX6 (X = cl, Br, and I) for Spintronic Applications.

In this work, we present a density functional calculation of the structural, electronic, and mechanical properties of cubic double halide perovskites Rb2OsX6 (X = Cl, Br, and I). Our results show that these compounds are stable in the ferromagnetic phase with lattice parameters, bulk modulus, and their first-pressure derivatives in good agreement with other available theoretical data. The negative values of cohesive energy and formation energy, along with the absence of negative or imaginary frequencies in the phonon spectrum, confirm the mechanical stability of all the compounds. The Curie temperature (Tc) is determined using a Heisenberg model in the mean-field approximation. We obtained a half-metallic character for all compounds, making them promising materials for spintronic applications. The magnetic properties indicate that the Os atoms in all compounds are responsible for the magnetism, while the positive exchange constants suggest a strong preference for ferromagnetic alignment. This indicates a stable ferromagnetic phase and potential applications in spintronics. The mechanical properties demonstrate that the compounds studied are isotropic and ductile.

First-principles assisted design of high-entropy thermoelectric materials based on half-Heusler alloys

The deformation potential theory and semi-classical Boltzmann theory were combined to predict the thermoelectric performances of half-Heusler NaCuTe alloy and Li0.5Na0.5CuSe0.5Te0.5 high-entropy half-Heusler alloy through first-principles calculations. The former was constructed via the congener substitution method from LiCuSe alloy, while the latter was designed by the high-entropy engineering concept. The phonon spectrum and ab initio molecular dynamics simulations indicated that the three alloys display stable intermetallic compounds at ambient temperature. The electrical and thermal transport properties of p-type LiCuSe, NaCuTe, and Li0.5Na0.5CuSe0.5Te0.5 alloys were computed as a function of temperature and carrier concentration. The thermoelectric figure of merit for p-type Li0.5Na0.5CuSe0.5Te0.5 alloy was 1.005 and 3.443 at room temperature and 800 K, whereas that of p-type NaCuTe alloy achieved 2.488 at 800 K, which is obviously superior to most of the recently reported p-type half-Heusler thermoelectric materials. A comprehensive analysis of the phonon lifetime, Grüneisen parameters, phonon group velocities, and primitive cell phonon spectrum revealed that high-entropy engineering could introduce non-equivalent atoms and thus enhance phonon scattering, resulting in the reduction of lattice thermal conductivity. Furthermore, numerical simulations demonstrated that high-entropy engineering could improve the thermoelectric performances of half-Heusler alloys effectively, which provides a unique approach for the optimized design of novel thermoelectric materials.

Prediction of the Topologically Nontrivial Phase in Three-Dimensional ABX Zintl Compounds.

Zintl compounds have garnered research interest due to their diverse technological applications. Utilizing first-principles calculations, we performed a systematic study of ABX (A = Li, Na, K, Rb, or Cs; B = Si, Ge, Sn, or Pb; and X = P, As, Sb, or Bi) Zintl materials with the P63 mc KSnSb-type structure. Notably, six ABX Zintl compounds (RbSiBi, CsSiBi, LiGeBi, KGeBi, RbGeBi, and CsGeBi) were found to have topologically nontrivial phases, as demonstrated by the Z 2 invariant computed using the hybrid functional HSE06. Among them, RbGeBi and CsGeBi were identified as topological insulators with nontrivial bandgaps of 28 and 116 meV, respectively. The topological phase transition arises as a result of spin-orbit coupling, as demonstrated in the representative material, CsGeBi. Additionally, the existence of gapless surface states further confirmed the topologically nontrivial phases of the six materials. Moreover, phonon spectra and formation energy calculations verified that all identified nontrivial materials under the hybrid functional are dynamically and structurally stable, except LiGeBi which exhibited imaginary phonon frequencies. Finally, the thermodynamic stability of the representative material CsGeBi was verified through elastic constants and ab initio molecular dynamics simulations. These results provide foundational insights that could drive further experimental research and synthesis, potentially enabling the application of ABX Zintl compounds in electronic technologies such as quantum computing or spintronics.

Valley splitting of monolayer Hf3C2O2 by the spin-orbit coupling effect: first principles calculations using the HSE06 methods.

Electrons have not only charge and spin degrees of freedom, but also additional valley degrees of freedom. The search for valleytronic materials with large valley splits is important for the development of valleytronics. In this work, we applied first principles computations to calculate 1 L Hf3C2O2 at the level of HSE06. When the spin-orbit coupling (SOC) effect is considered, 1 L Hf3C2O2 is an indirect bandgap semiconductor with a bandgap of 0.952 eV. Meanwhile, valley splitting occurs between the conduction bands Γ and K, with a valley splitting value as high as 98.228 meV. Bader charge analysis was used to determine that Hf-O and Hf-C are ionic bonds. The computed elastic constants and phonon spectra proved that 1 L Hf3C2O2 is mechanically and dynamically stable. In addition, the Berry curvature of 1 L Hf3C2O2 is non-zero. In the work the effect of the electronic properties of 1 L Hf3C2O2 was also calculated with respect to the biaxial strain. The results show that the biaxial strain can well regulate the band gap and valley splitting. Finally, we calculated the effects of hole and electron doping on the band gap and valley splitting of 1 L Hf3C2O2. The results show that the band gap and valley splitting are linearly related to the doping concentration. Our study shows that 1 L Hf3C2O2 is a promising two-dimensional valleytronics material.

Mechanisms of interlayer friction in low-dimensional homogeneous thin-wall shell structures and its strain effect.

Due to the multi-factor coupling effect, the rule of interlayer friction in low-dimensional homogeneous thin-wall shell structures is still unclear. Double walled carbon nanotubes (DWCNTs) having a typical low-dimensional homogeneous thin-wall shell structure are selected for this study. The interlayer friction of numerous chiral DWCNTs is investigated using molecular dynamics simulations to systematically analyze and understand the coupling mechanisms of various factors in interlayer friction. To eliminate the influence of the edge effect, a high-speed pure rotation model is used. The results demonstrate that DWCNTs with varying mismatch angle, interlayer distance, and interfacial radius exhibit distinct interlayer frictions and strain effects, due to the differences in interlayer interaction, atomic vibrational amplitudes, and lattice periods. Theoretical analysis is conducted on the interlayer friction and its strain effect based on the theoretical model. Based on phonon spectrum analysis, the vibrational modes and energy dissipation of DWCNTs with different chiral combinations under various strains are demonstrated. Based on the analysis, physical insight into the variation in friction forces is provided. The findings of this work deepen the understanding of the interlayer friction and strain mechanism and provide a theoretical basis for further exploitation of the strain effect to realize the regulation of nanoelectromechanical devices.

The stability of CsGeX3 (X = I, Br, Cl) selectively tuned by the crystal structures and halide ions as inferred from the calculated phonon spectrum.

In spite of the considerable advancements achieved in enhancing the power conversion efficiency (PCE) of lead-based all-inorganic perovskite solar cells, there persists a need for materials that are both more stable and environmentally friendly. This investigation systematically explores the structural and thermodynamic stability, and electronic properties of Ge-based all-inorganic perovskite CsGeX3 (X = Cl, Br, I) in two space groups, Pm3̄m and R3m, utilizing first-principles calculations. Introducing the novel concept of the "imaginary frequency coefficient" alongside the tolerance factor and stabilizing the chemical potential window, we collectively characterize the stability of CsGeX3 based on the phonon spectrum and phonon density of states calculations. The findings reveal that the stable phase of the Ge-based perovskite differs from that of lead-based systems, with the R3m structure of CsGeX3 being the most stable in the rhombohedral phase. Moreover, the stability of R3m-CsGeX3 can be manipulated by adjusting the halide composition with a gradual increase in stability observed as halogen atoms shift from I to Cl. This comprehensive approach, integrating the phonon spectrum, innovative measurement indicators, and tolerance factor, presents an effective strategy for designing materials that are both non-toxic and stable.

Possible coexistence of superconductivity and topological electronic states in 1T-RhSeTe

Abstract Transition metal dichalcogenides (TMDs), exhibit a range of crystal structures and topological quantum states. The 1T phase, in particular, shows promise for superconductivity driven by electron-phonon coupling (EPC), strain, pressure, and chemical doping. In this theoretical investigation, we explore 1T-RhSeTe as a novel type of TMD superconductor with topological electronic states. The optimal doping structure and atomic arrangement of 1T-RhSeTe are constructed. Phonon spectrum calculations validate the integrity of the constructed doping structure. The analysis of the electron-phonon coupling using the Electron-phonon Wannier (EPW) method has confirmed the existence of a robust electron-phonon interaction in 1T-RhSeTe, resulting in total EPC constant λ = 2.02, the logarithmic average frequency ω log = 3.15 meV and Tc = 4.61 K, consistent with experimental measurements and indicative of its classification as a BCS superconductor. The band structure analysis revealed the presence of Dirac-like band crossing points. The topological non-trivial electronic structures of the 1T-RhSeTe are confirmed via the evolution of Wannier charge centers (WCCs) and time-reversal symmetry-protected topological surface states (TSSs). These distinctive properties underscore 1T-RhSeTe as a possible candidate for a topological superconductor, warranting further investigation into its potential implications and applications.

Carrier-Induced Room-Temperature Half-Metallicity in an Exfoliable Two-Dimensional Cr(TCNB)2 Metal-Organic Framework.

Half-metallicity, enabling 100% spin polarization, is pivotal for spintronics but remains challenging to achieve in low-dimensional materials. Using first-principles calculations, we theoretically propose an experimentally feasible two-dimensional (2D) metal-organic framework (MOF) magnetic semiconductor, Cr(TCNB)2 (TCNB = 1,2,4,5-tetracyanobenzene). This monolayer can be exfoliated from a Ag(100) substrate due to its low exfoliation energy of 0.14 J/m2. Phonon spectra and ab initio molecular dynamics confirm its dynamical and thermal stability up to 600 K. Cr(TCNB)2 exhibits a ferrimagnetic ground state stabilized by direct p-d magnetic interactions. Notably, carrier doping induces half-metallicity, transforming it into a fully spin-polarized conductor. Monte Carlo simulation predicts that doping elevates the Curie temperature above room temperature. This work introduces a novel 2D MOF magnet with carrier-tunable half-metallicity, offering promising potential for flexible and nanoscale spintronic devices.

Molecular dynamic studies of gold nanoparticles in a dental material TEGDMA.

In the context of biomaterials, triethylene glycol dimethacrylate (TEGDMA) is a widely used monomer in dental resins due to its favorable mechanical properties and ease of polymerization. However, improving its structural stability and enhancing its performance in biological applications remain crucial goals. This study examines the impact of incorporating gold (Au) nanoparticles into the TEGDMA matrix, focusing on their potential to improve mechanical, thermal, and optical properties for biomedical applications. Gold is known for its bio-compatibility, antimicrobial properties, and ability to improve material conductivity, making it an attractive addition for dental and tissue engineering composites. By introducing Au nanoparticles at varying concentrations (0%, 3%, 4%, and 5%), this research aims to optimize TEGDMA's performance in medical grade materials, particularly in dental composites where enhanced strength and durability are vital. Molecular dynamics (MD) simulations were employed to investigate the structural, thermal, and mechanical properties of TEGDMA with varying concentrations of Au nanoparticles. LAMMPS software was used to compute cohesive energy, surface tension, viscosity, and mechanical moduli. Quantum chemical calculations were conducted using Gaussian and RDKit to optimize the molecular structure and obtain electronic properties, including UV-Vis and IR spectra. Phonon spectra and lattice energy were analyzed to evaluate the vibrational properties and thermal stability of the Au-doped TEGDMA matrix. The results were benchmarked against experimental data, offering a comprehensive computational analysis of the impact of Au nanoparticles on the material's performance. Functional data analysis techniques were applied to correlate nanoparticle concentration with the computed properties.

Group-IV pentaoctite: a new 2D material family

Abstract This study investigates the structural, mechanical, and electronic properties of novel two-
dimensional (2D) pentaoctite (PO) monolayers composed of group-IV elements (PO-C, PO-Si, PO-
Ge, and PO-Sn) using first-principles calculations. Stability is explored through phonon spectra and
ab initio molecular dynamics simulations, confirming that all proposed structures are dynamically
and thermally stable. Mechanical analysis shows that PO-C monolayers exhibit exceptional rigidity,
while the others demonstrate greater flexibility, making them suitable for applications in foldable
materials. The electronic properties show semimetallic behavior for PO-C and metallic behavior for
PO-Si, while PO-Ge and PO-Sn possess narrow band gaps, positioning them as promising candi-
dates for semiconductor applications. Additionally, PO-C exhibits potential as an efficient catalyst
for the hydrogen evolution reaction (HER), with strain engineering further enhancing its catalytic
performance. These findings suggest a wide range of technological applications, from nanoelectronics
and nanomechanics to metal-free catalysis in sustainable energy production.

Collective mode across the BCS-BEC crossover in Holstein model

We investigate the emergence of the collective mode in the phonon spectra of the superconducting state within the Holstein model by varying the electron-phonon coupling. Using dynamical mean field theory combined with the numerical renormalization group technique, we calculate the phonon spectra. In the superconducting state with a pairing gap (ΔP), the peak position of the collective mode (ωcol) evolves from the Bardeen-Cooper-Schrieffer (BCS) regime, manifesting near 2ΔP and increasing with coupling, to the Bose-Einstein condensation (BEC) regime, where ωcol decreases with increasing coupling. The decrease of ωcol matches well with the reduction of superfluid stiffness, which originates from the increasing phase fluctuations of local pairs with coupling strength. In the crossover regime with intermediate coupling, ωcol aligns with the soft phonon mode (ωs) of the normal state and decreases with increasing coupling when ωs<2ΔP. Additionally, comparing the collective mode weight to ΔP suggests that the collective mode predominantly stems from U(1) gauge symmetry breaking across all coupling strengths. Published by the American Physical Society 2024

Single‐Crystal and Polycrystal SiC Bonding for Cost‐effective Chip Fabrication

AbstractHigh‐quantity single‐crystal silicon carbide (SiC) is widely used in power electronics due to its excellent breakdown electric field strength and high thermal conductivity. However, back grinding during the chip fabrication generally results in ≈70% of single‐crystal SiC being wasted, leading to the high cost of SiC chips. In order to improve the utilization, single‐crystal SiC on polycrystal SiC (SoP‐SiC) is bonded. The challenge to achieve excellent bonding interfaces for such a system is the heterogeneous surface of polycrystals in which those grains with different orientations usually have different physical and chemical properties, making it difficult to achieve sufficiently smooth surfaces for bonding. Here, ion beam etching (IBE) is employed to activate the surface of polycrystal and single‐crystal SiC and achieve high bonding strength (up to ≈20 MPa) after annealing in the atmosphere. Sub‐nanometer‐scale electron microscopy and energy spectroscopy analysis showing the IBE method can effectively inhibit the formation of silicon oxide at the bonding interface, which is expected to reduce the interface thermal resistance according to the phonon spectrum analysis. This study provides a novel method to fabricate single‐polycrystal SiC junctions with high bonding strength and high thermal conductivity, which is valuable for the SiC industry.

Computational investigation of the phase stability, electronic, optical, phonon spectrum, and elastic behavior of layered perovskites Ca2XO4 (X = Zr, Hf) for optoelectronic applications.

A significant class of solid-state materials, Ruddlesden-Popper (RP) perovskites are well-known for their rich chemical compositions that make them excellent electrocatalysts. Therefore, in this work, we conduct thorough analysis of RP perovskites, specifically Ca2XO4 (X = Zr, Hf). We delve into the structural, electronic, optical, and mechanical properties presenting their insights for the first time. Our investigations reveal that Ca2XO4 (X = Zr, Hf) exhibits stable tetragonal phase structures, with energies (E0) of - 10,522.93eV and - 33,520.14eV, respectively. The calculated in terms of the ground state determines to possess distinct values such as - 4.79 eV/atom for LiScTe2 and - 3.95 eV/atom for Ca2ZrO4 and Ca2HfO4. Notably, the semiconductor characteristics of Ca2ZrO4 and Ca2HfO4 are underscored by their respective wide direct band gap of 5.02eV and 5.30eV. Then, we discuss the optical parameters encompassing the dielectric function, absorption coefficient, optical conductivity, refractive index, and energy loss function. is described as ductile, whereas is defined as brittle. Our outcomes underscore remarkable ability of these materials to absorb ultraviolet radiation from the electromagnetic spectrum, positioning them as promising candidates for optoelectronic applications. We made these calculations by employing first-principles approach rooted in density functional theory (DFT) and utilizing the Perdew-Burke-Ernzerhof-Generalized Gradient Approximation (PBE-GGA) and Tran and Blaha-modified Becke-Johnson (TB-mBJ) functional within the WEIN2K framework. In this framework, Kramers-Kronig relations are used to obtain crucial optical parameters. Mechanical behavior is assessed by employing Voigt-Reuss-Hill approximation (VRH) fulfilling the Born's criteria which emphasizes that these materials are equally appropriate for a broad range of mechanical applications.

Regulation of thermal transport by cycle structures in complex networks

The regulation of thermal transport in complex networks is a challenging topic. This paper focuse on thermal transport in the Erdős-Rényi network by adjusting two coefficients: the number of cycle structures and the cycle ratio. The results indicate that the increase of cycles in Erdős-Rényi networks leads to enhanced phonon localization and reduced efficiency in thermal transport. Further more, as the average cycle ratio increases, we observe an improvement of the network’s thermal transport efficiency, accompanied by thermal rectification phenomena. Finally, through the analysis of participation ratio, eigenvalue spectra, and node vibration time series, we clarified that the variations in phonon localization and the widths of the phonon spectrum are key factors influencing the thermal transport in complex networks. This work enables control of thermal transport by adjusting the structural characteristics of the network and provides further exploration and development of phonon localization in disordered systems.

Nonlinear optical properties of glassy TeO[formula omitted]: ab initio modeling of [formula omitted] and Hyper Raman spectra

The nonlinear optical properties of amorphous tellurium oxide is studied in a framework of computational technique based on the ab initio quantum-chemical calculations. The calculated values of refractive index and third-order nonlinear susceptibility are in a very good agreement with experimental data. The origin of the high nonlinear properties of TeO2 is attributed, in general, to the hyperpolarizability of Te atoms electron lone pairs and to the hyperpolarizability of electronic density localized on bonds of the asymmetric Te-O-Te bridges in part. The computational technique is extended on Hyper Raman spectroscopy including scattering on longitudinal modes. The spectra were calculated for both crystalline and glassy TeO2 with a very good reproduction of experimental data. An extra bands inactive in classical Raman spectroscopy are studied using this approach. The most intense high-frequency band in the spectra of paratellurite is attributed to anti-symmetric stretching of long-short bonds in TeO4 disphenoid with A2 symmetry. The influence of the structural unit configuration on the band position in Hyper Raman spectra is established. The best agreement between calculated phonon spectrum and experimental data is obtained using hybrid functional approach, which explains the strong correlation on valence electron states of Te atoms. The strong dependence of LO modes intensity in Hyper Raman spectra on third order nonlinear susceptibility is obtained and explained. The reported technique can be easily extended on a big family of binary and ternary compounds.