Phonon Frequencies Research Articles

A first-principle study of [formula omitted]- and [formula omitted]-graphyne and its BN and BC[formula omitted]N analogs

We investigated the structural, energetics, and electronic properties of the pristine/doped α- and γ-(graphyne, BNyne and BC2Nyne), based on density functional theory. Our results regarding the stability of these structures indicate that the formation energy is increased by doping the α- and γ-(graphyne or BNyne) structures with boron-nitrogen (B–N) or carbon (C) atoms, respectively. The lowest formation energy for the α- and γ-BC2Nyne structures were those with the highest number of B–N and C–C bonds, and hexagonal C rings bonded by B–N bonds, respectively. This is due to the low energy associated with the triple bond between B–N atoms, compared to the triple bond between C atoms. An examination of dynamic stability using phonon frequencies revealed that certain α- and all γ-structures studied exhibit stability. The structures with the lowest formation energies exhibited positive phonon frequencies. From the perspective of the electronic structure, it can be observed that the band gaps for α-structures are directly proportional to the number of B–N bonds present within the unit cell. For γ-structures, the band gaps depend on the composition of the hexagons present in the unit cell and the types of atoms that bond them.

Investigating the electronic structure, elastic, magnetic, and thermoelectric nature of NiV X Sc1−X Sb quaternary half-Heusler alloys

The structural, electronic, magnetic, elastic, and thermoelectric properties of NiV x Sc1−x Sb half Heusler alloys with different compositions were investigated employing a self-consistent first-principles-based calculation that uses the full-potential linearized-augmented-plane-wave method. The structural characteristics, such as the bulk modulus and lattice constants, are examined with various vanadium concentrations. The accurately modified Becke Johnson potential was used to calculate the band gap energies. The equilibrium lattice parameter of the NiScSb type-I structure has the lowest energy and seems to be most stable among the other configurations, with a lattice constant value of 6.04 Å, which deviates from the experimental results by up to 0.5%. The bulk modulus rises as the lattice constant decreases. The ground states of the studied alloy structures are dynamically stable, as concluded by the non-existence of negative phonon frequencies. The band structure of NiScSb (for x = 0) was predicted as a non-magnetic semiconductor with an indirect band nature and an energy gap value of 0.244 eV along (Γ-point > X). This tendency was further supported by the symmetrical shape of the curves that reflect the densities of states for these configuration channels. The thermoelectric characteristics of these various combinations were also thoroughly investigated and discussed.

Terahertz parametric amplification as a reporter of exciton condensate dynamics.

Condensates are a hallmark of emergence in quantum materials such as superconductors and charge density waves. Excitonic insulators are an intriguing addition to this library, exhibiting spontaneous condensation of electron-hole pairs. However, condensate observables can be obscured through parasitic coupling to the lattice. Here we employ nonlinear terahertz spectroscopy to disentangle such obscurants through measurement of the quantum dynamics. We target Ta2NiSe5, a putative room-temperature excitonic insulator in which electron-lattice coupling dominates the structural transition (Tc = 326 K), hindering identification of excitonic correlations. A pronounced increase in the terahertz reflectivity manifests following photoexcitation and exhibits a Bose-Einstein condensation-like temperature dependence well below the Tc, suggesting an approach to monitor the exciton condensate dynamics. Nonetheless, dynamic condensate-phonon coupling remains as evidenced by peaks in the enhanced reflectivity spectrum at select infrared-active phonon frequencies, indicating that parametric reflectivity enhancement arises from phonon squeezing. Our results highlight that coherent dynamics can drive parametric stimulated emission.

Thermal conductivity study of 2D Si4C8 materials by anharmonic phonon renormalization.

In this investigation, we employed the anharmonic phonon renormalization method to analyze the thermal conductivity of two-dimensional (2D) carbon materials, while also examining the influence of quartic (fourth-order) scattering on heat transport within this class of materials. Our study centered on a representative silicon-carbon (Si-C) 2D system, Si4C8. Notably, conventional Boltzmann transport equation (BTE) calculations with harmonic phonons are inadequate for estimating the thermal conductivity of these materials due to the emergence of imaginary frequencies. Consequently, to elucidate the primary contributors to its heat transport, we employed an integrated yet novel computational framework rooted in a first-principles methodology. This approach combines self-consistent phonon (SCP) theory and the BTE to scrutinize the thermal conduction behavior; the BTE is resolved in conjunction with SCP theory to comprehensively address the quartic anharmonic effects, encompassing both four-phonon (4ph) scatterings and the temperature-induced shift of phonon frequencies. Based on the calculation results, it is evident that the meticulous incorporation of anharmonicity renormalization is pivotal for precise evaluation of the thermal conductivity of 2D Si4C8 and establishing coherent temperature dependency. Through this comprehensive examination, we aim to establish a systematic methodology for investigating the thermal transport mechanisms of 2D Si-C phases with similar bonding networks, offering insights into the intricate relationships between their structural, mechanical, electronic, and thermal properties.

Emergence of Weyl points and large anomalous Hall conductivity in layered Bi2TeMnI2.

In recent years, narrow band gap layered materials were reported as an interesting candidate for energy efficient devices. Here, we chose BiTeI, a layered material that has significant Rashba spin splitting, for charge modification with the purpose of exploring the electronic, magnetic and topological properties. Chemical doping with an Mn atom is done to the Te site in BiTeI. On the basis of density functional theory calculations, we found that the parent material BiTeI is a semiconductor with an indirect band gap of ∼0.46 eV within full-relativistic mode. The orbital contributions around the Fermi level are found to be mainly from the Bi-6p, I-5p and Te-5p states in the electronic structure. Upon chemical doping by Mn to Bi, Te and I separately, doping to the Te site is energetically favorable with a ferromagnetic ground state and a semimetallic behaviour. The doped material, i.e., Bi2TeMnI2, is found to be a magnetic Weyl semimetal with six Weyl points close to the Fermi level (around 100 meV in the conduction region). Our calculations suggest Bi2TeMnI2 as a probable candidate of a Weyl semimetal. The emergence of Weyl points gives rise to a large intrinsic anomalous Hall conductivity of up to ∼750 Ω-1 cm-1. The calculated negative value of formation energy (-0.233 eV) and the positive phonon frequency suggests Bi2TeMnI2 to be thermodynamically favorable and dynamically stable. This work deserves a transport experiment to confirm our claim, which might provide insights towards discovering new quantum materials suitable for high-speed electronics, spintronics and quantum computing.

Effect of atomic substitution and structure on thermal conductivity in monolayers H-MN and T-MN (M = B, Al, Ga).

Finding materials with suitable thermal conductivity (κ) is crucial for improving energy efficiency, reducing carbon emissions, and achieving sustainability. Atomic substitution and structural adjustments are commonly used methods. By comparing the κ of two different structures of two-dimensional (2D) IIIA-nitrides and their corresponding carbides, we explored whether atomic substitution has the same impact on κ in different structures. All eight materials exhibit normal temperature dependence, with κ decreasing as the temperature rises. Both structures are single atomic layers of 2D materials, forming M-N bonds, with the difference being that H-MN consists of hexagonal rings, while T-MN consists of tetragonal and octagonal rings. 2D IIIA-nitrides provide a good illustration of the impact of atomic substitution and structure on κ. On a logarithmic scale of κ, it approximates two parallel lines, indicating that different structures exhibit similar trends of κ reduction under the same conditions of atomic substitution. We analyzed the mechanisms behind the decreasing trend in κ from a phonon mode perspective. The main reason for the decrease in κ is that heavier atoms lower lattice vibrations, reducing phonon frequencies. Electronegativity increases, altering bonding characteristics and increasing anharmonicity. Reduced symmetry in complex structures decreases phonon group velocities and enhances phonon anharmonicity, leading to decreased phonon lifetimes. It's noteworthy that we found that atomic substitution and structure significantly affect hydrodynamic phonon transport as well. Both complex structures and atomic substitution simultaneously reduce the effects of hydrodynamic phonon transport. By comparing the impact of κ on two different structures of 2D IIIA-nitrides and their corresponding carbides, we have deepened our understanding of phonon transport in 2D materials. Heavier atomic substitution and more complex structures result in reduced κ and decreased hydrodynamic phonon transport effects. This research is likely to have a significant impact on the study of micro- and nanoscale heat transfer, including the design of materials with specific heat transfer properties for future applications.

Magnetic anisotropy and slow relaxation of magnetisation in double salts containing four- and six-coordinate cobalt(II) complex ions.

Four novel Co(II) coordination compounds 1-4 of the general formula [Co(Ln)2][Co(NCY)4]·mCH3CN (where Ln are tridentate ligands L1 = 2,6-bis(1-hexyl-1H-benzimidazol-2-yl)pyridine for 1 and 2; L2 = 2,6-bis(1-octyl-1H-benzimidazol-2-yl)pyridine for 3; L3 = 2,6-bis(1-dodecyl-1H-benzimidazol-2-yl)pyridine for 4, Y = O for 1, 3, and 4 and Y = S for 2; m = 0 for 1 and 3, m = 0.5 for 2 and m = 2 for 4) were prepared and characterised. The molecular structures of all four compounds consist of the hexacoordinate complex cation [Co(Ln)2]2+ and tetracoordinate complex anion [Co(NCY)4]2-, with distorted octahedral and tetrahedral symmetry of coordination polyhedra, respectively. The electronic structures of all compounds feature an orbitally non-degenerate ground state well-separated from the lowest excited state, which allows the analysis of the magnetic anisotropy by the spin Hamiltonian model. ZFS parameters, derived from both CASSCF-NEVPT2 calculations and magnetic data analysis, indicate that tetrahedral anions [Co(NCY)4]2- exhibit small axial parameters |D| spanning the range of 2.2 to 7.7 cm-1, while octahedral cations [Co(Ln)2]2+ display significantly larger |D| parameters in the range of 37 to 95 cm-1. For 1-3, the Fourier-transform infrared magnetic spectroscopy (FIRMS) revealed a reasonable transmission with a magnetic absorption around the expected value for the ZFS accompanied by features allowing to identify phonon frequencies and simulate spin-phonon couplings. Dynamic magnetic investigations unveiled the field-induced slow relaxation of magnetisation, with maximal relaxation times (τ) of 92(2) μs for 2 at 2 K and BDC = 0.3 T. The temperature evolution of τ was analysed using a combination of Orbach, direct and Raman relaxations (Ueff = 8(1) K (5.6 cm-1)) or Orbach, direct and spin-phonon induced relaxations (Ueff = 10.3(9) K (7.2 cm-1)). The rest of the complexes, namely 1, 3, and 4 show field-induced slow relaxation of magnetisation with τ smaller than 16 μs.

Study on lattice dynamics and thermal conductivity of fluorite AF2 (A = Ca, Sr, Ba) based on first principles calculations.

Fluorite materials have received particular attention in electron optics due to their favorable optical properties. However, further exploration of these materials in the thermoelectric (TE) field is hampered by the lack of studies on their lattice thermal transport properties. In this work, we use first-principles calculations, combined with self-consistent phonon theory, compressive sensing lattice dynamics and the Boltzmann transport equation, to study the microscopic mechanism of lattice thermal transport properties in AF2 (A = Ca, Sr, Ba) with a fluorite structure. We investigate the effects of three-phonon and four-phonon scattering and quartic anharmonic renormalization of phonon frequencies on this system. The results show that the bonding strength of atoms A (Ca, Sr, and Ba) plays an important role in the thermal transport process, and the third-order anharmonicity also plays an important role in this system. Meanwhile, the role of the quartic anharmonicity cannot be ignored. Our findings not only fill in the gaps in the study of lattice thermal transport of fluorite materials, but also deepen the comprehensive understanding of the high κL value of fluorite materials.

Self-consistent phonon calculations and quartic anharmonic lattice thermal conductivity in 2D InS monolayer

Low lattice thermal conductivity (κl) is a crucial factor for higher figure-of merit and hence the efficiency of thermoelectric generators. There are several reports on intrinsically low κl values in two-dimensional (2D) van der Waals materials using density functional theory and molecular dynamics simulations. In general, phonon dispersions are studied at absolute zero temperature using the finite-displacement approach within harmonic approximations. In addition, the κlis calculated using the third-order cubic interatomic force constants (IFCs) by solving the Boltzmann transport equation for phonons. In these calculations, we use quartic IFCs to solve self-consistent phonon equations to obtain dynamical dispersion relations at a finite temperature of 300 K using finite-temperature Green’s function for Bosonic systems in 2D indium sulfide (InS) monolayer. The cubic and quartic IFCs are calculated using machine learning algorithms, namely, the ordinary least square fitting and the least absolute shrinkage and selection operator. It was found that there is a lowering of the renormalized anharmonic phonon frequencies in the dispersion relations at 300 K upon the inclusion of quartic IFCs and anharmonic terms in the case of the InS monolayer. Thus, the κlvalue is reduced to 0.6 W/mK as compared to 0.9 W/mK obtained using cubic IFCs.

Imaging Ultrafast Dynamics of Pressure-Driven Phase Transitions in Black Phosphorus and Anomalous Coherent Phonon Softening.

Applying high pressure to effectively modulate the electronic and lattice structures of materials could unravel various physical properties associated with phase transitions. In this work, high-pressure-compatible femtosecond pump-probe microscopy was constructed to study the pressure-dependent ultrafast dynamics in black phosphorus (BP) thin films. We observed pressure-driven evolution of the electronic topological transition and three structural phases as the pressure reached ∼22 GPa, which could be clearly differentiated in the transient absorption images containing spatially resolved ultrafast carrier and coherent phonon dynamics. Surprisingly, an anomalous coherent acoustic phonon mode with pressure softening behavior was observed within the range of ∼3-8 GPa, showing distinct laser power and time dependences. Density functional theory calculations show that this mode, identified as the shear mode along the armchair orientation, gains significant electron-phonon coupling strength from out-of-plane compression that leads to decreased phonon frequency. Our results provide insights into the structure evolution of BP with pressure and hold potential for applications in microelectromechanical devices.

Origin of ultralow thermal conductivity in amorphous Si thin films investigated using nanoindentation, 3ω method, and phonon transport analysis

The origin of the ultralow thermal conductivity in amorphous Si thin films was investigated by comparing their phonon transport properties with those of single-crystal Si. The group velocity and thermal conductivity were measured at 300 K using nanoindentation and the 3ω method, respectively. The phonon mean free path (MFP) and phonon frequency were determined using the measured properties and models. The scattering in the disordered structure of amorphous Si thin films caused a significant decrease in the phonon MFP with an increase in the phonon frequency, leading to ultralow thermal conductivity. However, the group velocity was unaffected by the disordered structure.

Upward vibrational frequency shift due to electron–phonon polar-coupling interaction in C12H17ClN4OS·HCl·H2O crystals

Electron–phonon (e–p) polar-coupling interactions in C12H17ClN4OS·HCl·H2O crystals exhibiting an upward frequency shift with increasing temperature are investigated using terahertz and far-infrared spectroscopies, and the structural condition required for effective e–p polar-coupling interactions in crystals are uncovered. X-ray crystallography and first-principles calculations reveal that the key structure for the e−p polar-coupling interaction is an open cavity near the considered vibration. Based on the temperature-dependent frequency shift proportional to the Bose−Einstein statistical factor, the interaction strength and characteristic frequency of phonons interacting with electrons are determined at 5.3 × 10–3 cm–1 K–1 and 76.6 cm–1, respectively. From the phonon dispersion curves exhibiting LO (longitudinal optical)–TO (transverse optical) splitting, we find that the highest frequency LO phonon participates in the polar-coupling interaction.

Lone-pair electron-induced low lattice thermal conductivity and excellent thermoelectric performance of AuX (X = S, Se, Te) monolayers

We present a detailed study on the phonon thermal transport and thermoelectric (TE) properties of gold monochalcogenide AuX (X = S, Se, Te) monolayers via first-principles calculations and the Boltzmann transport theory. At room temperature, the calculated lattice thermal conductivity (κl) values in the x-direction (y-direction) were determined to be 12.66 (3.59), 6.06 (1.23), and 3.02 (0.40) W/(m·K) for the AuS, AuSe, and AuTe monolayers, respectively. The analysis of scattering matrix elements and anharmonic scattering rates suggested that the presence of strong bond anharmonicity leads to low κl values. This anharmonicity was induced by the nonlinear electrostatic repulsive force between lone-pair electrons (LPEs) surrounding the chalcogen atom and adjacent bonding electron. Furthermore, the AuS monolayer exhibited the highest bond anharmonicity among the three monolayers. This can be attributed to the reduced strength of the electrostatic repulsive force when electronegativity decreased from S to Se and Te. Although the AuS monolayer exhibited the strongest bond anharmonicity, its κl was still the largest, demonstrating that among the factors that influence κl, harmonic factors have a greater effect than anharmonic parameters. When the atomic mass was increased, the phonon frequencies, phonon group velocities, and κl decreased. Benefiting from low κl induced by LPEs, the maximum ZT values of p-type doped AuSe and AuTe monolayers exceeded 1.0 and 2.0 at T = 800 K, respectively. These results not only confirmed that the introduction of LPEs can reduce the lattice thermal conductivity and enhance the TE performance of 2D materials, but also demonstrated the promising potential of the 2D AuX family for applications in high-temperature TE devices.

An ab initio study of vertical heterostructures formed by CdO and SnC monolayers

Assembling two dimensional (2D) materials in vertical heterostructures is one of the main techniques for tuning electronic and optical properties. In most cases, known as van der Waals heterostructures (vdWHs), the interlayer distances are larger than typical covalent bond lengths resulting in weak interlayer interactions. It has been shown that reducing the distance between the layers can significantly alter the properties of separated layers, which is not so noticeable in vdWHs and thus creates a new platform for controlling the physical properties of 2D materials. Motivated by enhanced properties of 2D vertical heterostructures, employing ab-initio calculations based on density functional theory we examined CdO/SnC systems in four different configurations. Our results reveal that in spite of thermodynamic and mechanical stabilities of all considered structures, according to the calculated phonon frequencies, only the structure formed by placing the Sn atom on the O atom and the C atom on the Cd atom is dynamically stable at zero temperature. This structure has an interlayer distance of 2.52 Å which is smaller than the interlayer distance in typical vdWHs. We investigated the electronic and optical properties of this dynamically stable structure utilizing GW approximation and solving Bethe–Salpeter equation. Unlike the monolayer CdO which possesses a single optical absorption peak close to the red light energy, the considered CdO/SnC structure has an optical band gap of 1.14 eV, and it can absorb 13% of incident light in the blue light region.

1T’-[formula omitted] hybrid monolayer as a novel magnetic material: A first principles study

The advent of two-dimensional (2D) materials, such as transition metal dichalcogenides (2D-TMD), has led to an extensive amount of interest amongst scientists and engineers alike in search of major breakthroughs in the electronic, magnetic, and thermodynamic properties of 2D materials, generating interest in novel device applications. This paper discusses a theoretical study of the structural, electronic, magnetic, stability, phonon dispersion, and thermodynamic properties of the monoclinic RuWTe2 monolayer, a type of TMD in its 1T’ phase. The study uses computational modeling, employing the Density Functional Theory (DFT) and Density Functional Perturbation Theory (DFPT) formalisms, using GGA approximation and pseudopotentials to replace nucleus electrons in each atomic species. Just for bandgap estimation, it was used a single point energy of HSE06 hybrid functional which resulted in a small gap semiconductor (≈ 0.4eV). Even after adding the DFT+U correction to PBE functional, it presented magnetic properties with a trend toward ferrimagnetism. It presents structural and energetic stability through calculations of cohesive, formation energy, and phonon dispersion, since it does not present virtual phonon frequency. Through thermodynamics, the free energy (F) indicates that the synthesis process for 1T’-RuWTe2 would be spontaneous even at low temperatures. These results demonstrate that the monoclinic 1T’-RuWTe2 hybrid monolayer is a promising candidate suitable for applications in magnetic and thermal devices.

Origin of gamma surface asymmetries in body-centered cubic refractory high entropy alloys

In this study, we develop an understanding of the effect of high stacking fault elements in the γ-surfaces and twinnability of NbMoTaW refractory High Entropy Alloy (HEA) using first-principles density functional theory calculations. γ-surfaces developed low energy pathways leading to asymmetries by controlling the concentration of high stacking fault energy elements in the HEA. This effect was correlated to the strength of specific solid-state pair interactions using the projected crystal orbital Hamilton population analysis, and they were found to be weak except for W-Mo and W-Ta. Since the interactions involving Tungsten are stronger than the other interactions, stacking fault and twin formation are less likely to happen near tungsten-rich portions of the alloy. This suggests the formation of tungsten-deficient twin boundaries in NbMoTaW which will further improve the ductility of the system. Stable twin fault energy as low as 522 mJ/m2 is reported for the equimolar random atomic configurations of the HEA system. The ratio between twin boundary migration energy and the difference in unstable stacking fault energies (γTBMΔus) was found to be as low as 0.13 suggesting deformation twinning can happen in the HEA system. Vibrational analysis of the alloy system has indicated that tungsten and tantalum contribute to the acoustic phonon frequencies of the HEA. Fundamental insights from this study will help in engineering high entropy alloys with high strength and ductility.

Investigation of mechanical, optical, phonon frequencies, and sound velocity of nano-structured Gallium Phosphide (GaP) material under the effect of pressure

Investigation of mechanical, optical, phonon frequencies, and sound velocity of nano-structured Gallium Phosphide (GaP) material under the effect of pressure

Scrutinize the physical attributes of thermodynamically and elastically stable double perovskite oxides Ba2CdXO6 (X = Mo, U) for optoelectronics, photocatalytic and green technology

Perovskite oxides offer a rich playground for researchers due to their versatile properties, which make them valuable in diverse applications. We studied the stable phases of Ba2CdXO6 (X = Mo, U) perovskite oxides under the shadow of DFT. Optimized structures are utilized to evaluate the physical characteristics of perovskite oxides. The electronic properties are accomplished by utilizing modified Becke-Johnson potential. The impact of spin orbit coupling is also extracted, which shows a minimal influence on the electronic properties. The stability is gained via negative formation energies and positive values of phonon frequencies. A ductile character with metallic bonding is observed in both perovskite oxides. The calculation of band structures shows an indirect (L-X) bandgap of 2.94 eV and 2.93 eV for Ba2CdMoO6 and a direct (L–L) bandgap of 2.18 eV and 2.21 eV for Ba2CdUO6 using mBJ and mBJ + SOC, respectively. The estimated optical response shows the absorption in the UV region for both perovskite oxides and endorses their promising use in optoelectronic devices. The high electrical conductivities with decent ZT values and minimal thermal conductivities confirm their availability for green energy applications. The water splitting ability is achieved for Ba2CdMoO6 through photocatalytic activity, while hydrogen extraction characteristics are attained for Ba2CdUO6.

Ab-Initio stability of Iridium based newly proposed full and quaternary heusler alloys

In order to demonstrate the stability of newly proposed iridium-based Ir2Cr(In,Sn) and IrRhCr(In,Sn) heusler alloys, we present ab-initio analysis of these alloys by examining various properties to prove their stability. The stability of these alloys can be inferred from different cohesive and formation energies as well as positive phonon frequencies. Their electronic structure results indicate that they are semi-metals in nature. The magnetic moments are computed using the Slater-Pauling formula and exhibit a high value, with the Cr atom contributing the most in all alloys. Mulliken’s charge analysis results show that our alloys contain a range of linkages, mainly ionic and covalent ones. The ductility and mechanical stability of these alloys are confirmed by elastic constants viz. Poisson’s ratio, Pugh’s ratio, and many different types of elastic moduli.

Many Body Aspects on the Lattice Vibrational Properties of Germanene and Silicene along Highly Symmetry Directions

The investigation of lattice vibrational properties in monolayer two-dimensional honeycomb lattices comprising 2D group-IV semiconductor materials such as Germanene and Silicene (Low Buckled structure) holds immense significance in the realm of research, owing to their potential integration into the forthcoming electronic sector for information technology. The ductile nature of 2D group-IV semiconductor materials enables seamless integration with existing semiconductor technology on substrates, setting them apart from Graphene. Our primary objective revolves around comprehending the lattice vibrational properties of Germanene and Silicene (LB) through the examination of their honeycomb and buckled lattice structures. We use the Adiabatic Bond Charge Model with a Python program to derive vibrational frequencies at the Г points along highly symmetrical directions. Moreover, we extensively analyze the acoustical and optical contributions to the lattice vibrational frequencies. We anticipate that the phonon frequencies along the Г‒M direction for 2D atomically thin Germanene and Silicene will yield reasonably similar outcomes to those other research groups achieve.