Phonon Band Research Articles

Insight into the structural, optoelectronic, elastic and thermodynamic properties of new lead free double halides perovskites Cs2XCuF6 (X = Sc, Y): a first principle study

We conducted a thorough investigation of Cs2XCuF6 (X = Sc, Y) using a first-principles approach, exploring a wide range of material properties. We began by confirming the structural and thermodynamic stability of these compounds, employing analyses such as formation energy calculations, examination of the phonon band structure, and the utilization of the Birch-Murnaghan equation of state (EOS) curve. A noteworthy finding was the tunability of the band gaps in these double perovskite materials, achieved by substituting Sc with Y, resulting in a band gap range from 2.67 to 2.62 eV. Our analysis extended to the mechanical stability of these compounds, characterized by elastic constants and revealing mechanical anisotropy and ductility. Additionally, we explored the optical properties, highlighting their broad absorption band from the infrared (IR) to visible regions, which holds significant promise for diverse optoelectronic applications. To provide a comprehensive understanding of these materials, we delved into their thermodynamic properties, encompassing thermal expansion coefficients (κ), heat capacities, entropy (S), volume, and Debye Temperature (θ D). This investigation spanned a wide pressure range from 0 to 30 GPa and a temperature range from 0 to 1400 K, contributing to a holistic grasp of the fundamental characteristics of Cs2XCuF6 (X = Sc, Y).

Numerical simulation studies of the new quaternary MAX phase as future engineering applications: The case study of the Nb2ScAC2 (A = Al, Si) compounds

Recently, MAX phases have attained considerable technological interest owing to their two inherent properties metallic and ceramic properties. This study extensively examined Nb2ScAC2 MAX phases using DFT, to assess the structural, mechanical, electronic, and Thermal characteristics. Firstly, the stability of these two compounds was confirmed through the formation energy, elastic constants (Cij), and phonon band structure, which confirmed their thermodynamic, mechanical, and dynamical stability. The optimized lattice parameters of these compounds were examined and then utilized to calculate the physical properties of the Nb2ScAC2 compound. Our compounds are brittle due to their Pugh’s ratio of less than 1.75. The covalent bonding of the structure revealed by the Poisson ratio is less than 0.25 for the two compounds. The Nb2ScAC2 material is anisotropic, and Nb2ScAlC2 is harder than Nb2ScSiC2.The metallic character of the materials was affirmed by the electronic band structure analysis. Calculated thermal properties such as Debye temperature and minimum and lattice thermal conductivity reveal that both compounds have the potential to enhance their deployment in thermal barrier coating materials. On the other hand, the high melting temperatures indicate that our compounds could potentially be utilized in demanding or severe conditions. Finally, the thermodynamic characteristics, comprising the isochoric heat capacity (Cv) and Debye temperature (ϴD) were analyzed subjected to high temperatures and pressures. The optical constants such as real and imaginary parts of the dielectric function, refractive index and reflectivity, are investigated. The current study recognizes these two compounds as promising candidates for utilization in modern technologies and diverse industries.

Photo-physical characteristics of color N3-center in diamond studied via UV femtosecond-laser pumped luminescence.

Micro-joule UV-range (350-415 nm) femtosecond-laser pulses generated via frequency-doubled parametric conversion of 525-nm 150-fs pulses of Yb-glass laser were used for "hot" photoluminescence excitation in a diamond plate enriched by blue-emitting N3-centers (zero-phonon line, ZPL, at 415 nm). Photoluminescence spectra acquired in the range of 400-500 nm exhibited wavelength-independent well-resolved ZPL and phonon progression bands, where the involved phonons possessed the only energies of 0.09 eV (LA-phonons) and 0.15 eV (softened LO/TO-phonons), potentially, as a result of a Clemens decay mechanism. Photoluminescence yield in the ZPL and other phonon bands exhibited the power slope of 1.8 at lower energies and ≈1 at higher energies. The transition zone at fluence ∼1014-15 photons/cm2 was related to the saturation of the pumped resonance transition and the slower non-radiative vibrational relaxation to the ZPL-related excited electronic state and the nanosecond spontaneous photoluminescence transition to the ground state. As a result, the absorption cross section σ(370-390 nm) ≈1·10-15 cm2 and concentration [N3] ≈6·1014 cm-3 were determined along with the ZPL absorption cross section σ(415 nm) ≈2.5·10-15 cm2, and the non-radiative vibrational relaxation rate was estimated, providing altogether the crucial information on lasing possibilities in N3-doped diamonds.

Phonon Topology and Winding of Spectral Weight in Graphite.

The topology of electronic and phonon band structures of graphene is well studied and known to exhibit a Dirac cone at the K point of the Brillouin zone. Here, we applied inelastic x-ray scattering (IXS) along with abinitio calculations to investigate phonon topology in graphite, the 3D analog of graphene. We identified a pair of modes that form a very weakly gapped linear anticrossing at the K point that can be essentially viewed as a Dirac cone approximant. The IXS intensity in the vicinity of the quasi-Dirac point reveals a harmonic modulation of the phonon spectral weight above and below the Dirac energy, which was previously proposed as an experimental fingerprint of the nontrivial topology. We illustrate how the topological winding of IXS intensity can be understood in terms of atomic displacements and highlight that the intensity winding is not in fact sensitive in telling quasi- and true Dirac points apart.

Mechanical, electronic and dynamical properties of T2-Al2MgC2 under pressure

The presence of T2-Al2MgC2 compound has significant influence on the mechanical properties of magnesium alloys. The mechanical, electronic and dynamical properties of T2-Al2MgC2 under high pressure are investigated by plane wave pseudopotential method based on the first-principles calculation. The results show that T2-Al2MgC2 is mechanically stable under high pressure, the elastic constants except [Formula: see text] increase with the increase of pressure. This compound presents brittleness and the brittleness decreases with increasing pressure when [Formula: see text][Formula: see text]GPa, but it displays ductility when [Formula: see text][Formula: see text]GPa. T2-Al2MgC2 has strong anisotropy, and the anisotropy decreases with increasing pressure. T2-Al2MgC2 is an indirect bandgap semiconductor material. Our calculated bandgap is 1.893[Formula: see text]eV at ambient pressure, the bandgap first increases and then decreases with increasing pressure. The phonon band structure as well as the total and partial phonon density of the state under different pressures had been analyzed. The constant volume heat capacity [Formula: see text] and entropy S of T2-Al2MgC2 increase with increasing pressure.

Investigation of effects of interlayer interaction and biaxial strain on the phonon dispersion and dielectric response of hexagonal boron arsenide

In this study, the effects of interlayer interaction and biaxial strain on the electronic structure, phonon dispersion and optical properties of monolayer and bilayer BAs are studied, using first-principles calculations within the framework of density functional theory. The interlayer coupling in bilayer BAs causes the splitting of out-of-plane acoustic (ZA) and optical (ZO) mode. For both structures, positive phonon modes across the Brillouin zone have been observed under biaxial tensile strain from 0 to 8%, which indicate their dynamical stability under tensile strain. Also, the phonon band gap between longitudinal acoustic (LA) and longitudinal optical (LO)/transverse optical (TO) modes for monolayer and bilayer BAs decreases under tensile strain. An appreciable degree of optical anisotropy is noticeable in the materials for parallel and perpendicular polarizations, accompanied by significant absorption in the ultraviolet and visible regions. The absorption edge of bilayer BAs is at a lower energy with respect to the monolayer BAs. The results demonstrate that the phonon dispersion and optoelectronic properties of BAs sheet could as well be tuned with both interlayer interaction and biaxial strain that are promising for optoelectronic and thermoelectric applications.

Thermal and electrostatic tuning of surface phonon-polaritons in LaAlO3/SrTiO3 heterostructures

Phonon polaritons are promising for infrared applications due to a strong light-matter coupling and subwavelength energy confinement they offer. Yet, the spectral narrowness of the phonon bands and difficulty to tune the phonon polariton properties hinder further progress in this field. SrTiO3 – a prototype perovskite oxide - has recently attracted attention due to two prominent far-infrared phonon polaritons bands, albeit without any tuning reported so far. Here we show, using cryogenic infrared near-field microscopy, that long-propagating surface phonon polaritons are present both in bare SrTiO3 and in LaAlO3/SrTiO3 heterostructures hosting a two-dimensional electron gas. The presence of the two-dimensional electron gas increases dramatically the thermal variation of the upper limit of the surface phonon polariton band due to temperature dependent polaronic screening of the surface charge carriers. Furthermore, we demonstrate a tunability of the upper surface phonon polariton frequency in LaAlO3/SrTiO3 via electrostatic gating. Our results suggest that oxide interfaces are a new platform bridging unconventional electronics and long-wavelength nanophotonics.

Significant Enhancement of Near-Field Radiative Heat Transfer by Misaligned Bilayer Heterostructure of Graphene-Covered Gratings

Abstract The near-field radiative heat transfer of heterostructure consisting of SiC gratings and graphene is investigated in this work. The rigorous coupled-wave analysis is employed to calculate the spectral heat flux. Nevertheless, monolayer heterostructure and nonmisaligned bilayer heterostructure consistently suffer from a lack of spectral heat flux. In this work, we investigate the prominent effect of misaligned bilayer heterostructure in enhancing near-field radiative heat transfer by plotting energy transmission coefficients and electromagnetic fields. The results show that when the misalignment reaches half a period, the bilayer heterostructure exhibits optimal performance with a total heat flux of 3.5 × 104 W/m2. Besides the well-known coupled surface phonon polaritons supported by SiC gratings, the surface plasmon polaritons supported by graphene dominate the enhancement of heat flux from 0.01 × 1014 rad/s to 1.5 × 1014 rad/s. Due to the spatial misalignment of the upper and lower gratings, the lower layer graphene surface plasmon polaritons are intensified, compensating for the lack of spectral heat flux. Meanwhile, the graphene surface plasmon polaritons and SiC surface phonon polaritons can be hybridized to form surface plasmon-phonon polaritons. In addition, the dynamic modulation of near-field radiative heat transfer in the misalignment state is achieved by manipulating the Fermi level of graphene. We finally show that the superiority of misaligned heterostructure is robust with respect to the frequency shift in the phonon band, providing an effective way to improve the near-field radiative heat transfer in different configuration.

Angular-Momentum Transfer Mediated by a Vibronic-Bound-State.

The notion that phonons can carry pseudo-angular momentum has many major consequences, including topologically protected phonon chirality, Berry curvature of phonon band structure, and the phonon Hall effect. When a phonon is resonantly coupled to an orbital state split by its crystal field environment, a so-called vibronic bound state forms. Here, a vibronic bound state is observed in NaYbSe2 , a quantum spin liquid candidate. In addition, field and polarization dependent Raman microscopy is used to probe an angular momentum transfer of ΔJz = ±ℏ between phonons and the crystalline electric field mediated by the vibronic bound stat. This angular momentum transfer between electronic and lattice subsystems provides new pathways for selective optical addressability of phononic angular momentum via electronic ancillary states.

Systematic design of 2D and 3D vibroacoustic fluidic cavity sensors using topology optimization

In this research we investigate a novel sensor concept, which utilizes a standing acoustic wave inside a liquid-filled cavity to probe volumetric properties of a fluid analyte. However, realizing a high-Q cavity resonator is a challenge because of low acoustic impedance contrast between liquids and solids. In our previous studies, we surround the cavity resonator with phononic crystal layers that provide a strong cavity resonance within the phononic band gap. The quality factor drastically depends on the geometry of the metamaterial lattice. Therefore, the main aim of this study is to find an optimal material layout of the solid domain around the cavity by employing topology optimization. We formulate the optimization problem as maximization of the Q-factor of the cavity resonance. We consider the sensor as a fully coupled vibroacoustic system, taking into account material losses and frequency-dependent material properties. Since resonance problems are very sensitive to minor variations in geometry, we apply a gray scale suppression constraint to minimize the influence of geometrical uncertainties and make the optimized designs suitable for additive manufacturing

Effects of anisotropy and disorder on the superconducting properties of niobium

We report results for the superconducting transition temperature and anisotropic energy gap for pure niobium based on Eliashberg’s equations and electron and phonon band structures computed from density functional theory. The electronic band structure is used to construct the Fermi surface and calculate the Fermi velocity at each point on the Fermi surface. The phonon bands are in excellent agreement with inelastic neutron scattering data. The corresponding phonon density of states and electron–phonon coupling define the electron–phonon spectral function, α2F(p, p′; ω), and the corresponding electron–phonon pairing interaction, which is the basis for computing the superconducting properties. The electron–phonon spectral function is in good agreement with existing tunneling spectroscopy data except for the spectral weight of the longitudinal phonon peak at ℏωLO = 23 meV. We obtain an electron–phonon coupling constant of λ = 1.057, renormalized Coulomb interaction μ⋆ = 0.218, and transition temperature Tc = 9.33 K. The corresponding strong-coupling gap at T = 0 is modestly enhanced, Δ0 = 1.55 meV, compared to the weak-coupling BCS value Δ0wc=1.78kBTc=1.43meV. The superconducting gap function exhibits substantial anisotropy on the Fermi surface. We analyze the distribution of gap anisotropy and compute the suppression of the superconducting transition temperature using a self-consistent T-matrix theory for quasiparticle-impurity scattering to describe niobium doped with non-magnetic impurities. We compare these results with experimental results on niobium SRF cavities doped with nitrogen impurities.

Structural and electronic phase transitions of thorium monoxide from first-principles calculations

Because of the innovative concepts for creating the thorium fuel cycle as an alternative nuclear energy source, thorium and its compounds have attracted a lot of attention. It is essential to study the physical properties that are necessary for the basic understanding and practical application of these materials. Thorium monoxide has already drawn researchers' attention as a metal oxide. Under different pressure conditions, the crystal structures, electronic band structures, phonon dispersions, mechanical properties, and thermodynamic properties of ThO have been determined using a particle swarm optimization structure prediction method in conjunction with first-principles calculations. Previous experiments agree with our results for the lattice constants of the phase of Fm3‾m. Moreover, a novel structure of R-3m is predicted to be thermally stable at a pressure of roughly 22 GPa. The phase is then also mechanically and dynamically stable, according to the findings of our computed elastic constants and phonon dispersion. The phase change from Fm3‾m to R-3m has not, however, been associated with any electronic transitions, and as both phases are mental, they may have excellent thermal conductivity. We further calculated the results of the Helmholtz free energy, entropy, specific heat, internal energy, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and Debye temperature based on the stable dynamics and mechanical properties, which reveal that they all could be good fuel. The results provide key insights into understanding the structural and electronic behaviors of ThO under the condition of external pressure.

Thermoelectric figure-of-merit of metastable crystalline ST12 germanium allotrope

Computational science has facilitated significant improvement in identifying stable crystal systems that are good thermoelectric materials with a high figure-of-merit (ZT). One approach for further discontinuous improvement is to explore the metastable phase of the crystals, particularly because there is an increasing need for thermoelectric materials operating at room temperature in energy harvesting applications. Here, using first-principles calculations, we find that the single-crystalline metastable ST12 germanium allotrope has an much higher ZT than DC8 stable phase. The optimal ZT for p- and n-type doping is 0.22 and 0.27 at room temperature, respectively, and the values reach 0.56 and 0.87 at 500 K. The electrical and thermal conductivities are higher and significantly lower than those of stable germanium, respectively. This is attributed to the smaller carrier effective mass and denser phonon bands with a lower group velocity and larger scattering phase space. The Seebeck coefficient remains high owing to the adequate bandgap and large derivative of the electron density of states with respect to the Fermi energy at the band edges. This work demonstrates that the metastable ST12 germanium allotrope, which has been recently synthesized, is a promising candidate to realize improvement in thermoelectric materials, and this case study encourages future exploration of metastable materials.

Ab-initio investigation of the physical properties of BaAgAs Dirac semimetal and its possible thermo-mechanical and optoelectronic applications

BaAgAs is a ternary Dirac semimetal which can be tuned across a number of topological orders. In this study we have investigated the bulk physical properties of BaAgAs using density functional theory based computations. Most of the results presented in this work are novel. The optimized structural parameters are in good agreement with previous results. The elastic constants indicate that BaAgAs is mechanically stable and brittle in nature. The compound is moderately hard and possesses fair degree of machinability. There is significant mechanical/elastic anisotropy in BaAgAs. The Debye temperature of the compound is medium and the phonon thermal conductivity and melting temperature are moderate as well. The bonding character is mixed with notable covalent contribution. The electronic band structure calculations reveal clear semimetallic behavior with a Dirac node at the Fermi level. BaAgAs has a small ellipsoidal Fermi surface centered at the Γ-point of the Brillouin zone. The phonon dispersion curves show dynamical stability. There is a clear phonon band gap between the acoustic and the optical branches. The energy dependent optical constants conform to the band structure calculations. The compound is an efficient absorber of the ultraviolet light and has potential to be used as an anti-reflection coating. Optical anisotropy of BaAgAs is moderate. The computed repulsive Coulomb pseudopotential is low indicating that the electronic correlations in this compound are not strong.

First principle insights into the physical properties of Ti-based 211-MAX phase nitrides Ti2AN (A = Tl and Pb)

We have used the theoretical ab initio approach to scrutinize the electronic and other physical properties of Ti2AN (A = Tl and Pb). Geometrical optimization has been carried out to obtain accurate lattice constants and internal coordinates. The formation energies of Ti2TlN and Ti2PbN are found to be negative, which confirms their stability. The aforementioned compounds are found to be metallic because of their zero-band gaps. The metallicity f m (x 10−3) of Ti2TlN and Ti2PbN phases were determined to be 1.77 and 2.11, respectively. In addition, we evaluate the elastic constant C ij , which obeys the Born-Huang mechanical stability criterion. We used the Voigt-Reuss-Hill approximation for the analysis of Young’s modulus, shear modulus, and bulk modulus successfully. Furthermore, Ti2TlN is found to be brittle, but Ti2PbN is close to the brittle-ductile boundary line according to Pugh’s and Poisson’s ratios. The Debye temperature, melting temperature, and minimum thermal conductivity have all been rigorously studied to examine the potential scenarios of genuine high-temperature applications. Lower Young’s modulus, the minimum thermal conductivity (Ti2TlN and Ti2PbN), and Debye temperature values reveal that Ti2PbN might be used as a thermal barrier coating application. A study of elastic anisotropy demonstrates that Ti2PbN has a higher degree of anisotropy than Ti2TlN, according to the universal anisotropy index. We confirmed the dynamic stability (i.e., no negative frequencies at the gamma point) of predicted compounds by performing phonon DOS and phonon band structures. Finally, the temperature-dependent thermodynamic properties of Ti2TlN and Ti2PbN have been thoroughly analyzed, where the entropy (S), free energy, and internal energy (E) vary with respect to temperature. Moreover, the convergence of specific heat capacity is observed at constant volume to the Dulong-Petit limit at higher temperatures.

Ultrahigh-Density Superhard Hexagonal BN and SiC with Quartz Topology from Crystal Chemistry and First Principles

Based on superdense C6 with a quartz (qtz) topology, new ultrahigh-density hexagonal binary phases, qtz BN and qtz SiC, were identified via full geometry structure relaxations and ground state energies using calculations based on the quantum density functional theory (DFT) with a gradient GGA exchange–correlation XC functional. Like qtz C6, with respect to diamond, the resulting binary qtz BN and qtz SiC were found to be less cohesive than cubic BN and cubic SiC, respectively, but were confirmed to be mechanically (elastic constants) and dynamically (phonon band structures) stable. Higher densities of the new phases correlate with higher hardness values compared to cubic BN and cubic SiC. In contrast to the regular tetrahedra that characterize the cubic BN and SiC phases, the corner-sharing tetrahedra in the new phases are distorted, which accounts for their exceptional density and hardness. All three qtz phases were found to be semiconducting to insulators, with reduced band gaps compared to diamond, cubic BN, and cubic SiC.

Study of vibrational modes of MeV Ni ion implanted MgO crystal

The control over various vacancy and substitutional defects by ion implantation provides many interesting properties for the modulation of materials. In this report, we studied the effect of defects and anionic vacancies produced by MeV Ni ion implanted single crystal MgO on different vibrational active modes. The breathing mode of D bands is observed for most of the Raman peaks, and simultaneously, the overlapping of D and G bands is confirmed from Raman spectra. The Fourier Transform Infrared spectra identified the stretching and bending vibrations of Mg, O, and Ni atoms and compared them with pristine. The presence of atmospheric constituents and their role in affecting the vibrational modes are well justified. The phonon band and density of states calculation in the framework of density functional theory confirms the presence of T1u and T2u Raman active mode that arises due to vibration from different vacancy and substitutional defect associated in MgO structures, agrees well with experimental results. The tunable anionic vacancies produced in MgO by ion implantation can be beneficial to form the filament in valance charge memory-based resistive random-access memory (RRAM).

Phononic band gap in random spring networks

Phononic band gap in random spring networks

Cation substitution varieties of I-II2-III-VI4 semiconductors and their effects on electronic and phononic properties

Quaternary chalcogenides from the semiconducting I-II2-III-VI4 class of systems are relatively unexplored in terms of their electronic and vibrational properties. These multinary systems can be derived from simpler zincblende and wurtzite binary materials as they can accommodate different lattice structures with diverse cation atom arrangements. Here by using first principles simulations, we investigate the electronic and phonon properties of I-II2-III-VI4 systems in terms of their structural and atomic diversity. Common to the parents as well as unique to the specific atomic composition signatures are identified. Distinct structure-property relations for the electronic and phonon band structures are found in order to make predictions for the electronic and thermal transport in these materials. Property trends associated with the average atomic number and mass dissimilarities are identified, which can be used to understand the underlying transport properties in quaternary chalcogenides derived from binary materials.

Direct observation of topological magnon polarons in a multiferroic material

Magnon polarons are novel elementary excitations possessing hybrid magnonic and phononic signatures, and are responsible for many exotic spintronic and magnonic phenomena. Despite long-term sustained experimental efforts in chasing for magnon polarons, direct spectroscopic evidence of their existence is hardly observed. Here, we report the direct observation of magnon polarons using neutron spectroscopy on a multiferroic Fe2Mo3O8 possessing strong magnon-phonon coupling. Specifically, below the magnetic ordering temperature, a gap opens at the nominal intersection of the original magnon and phonon bands, leading to two separated magnon-polaron bands. Each of the bands undergoes mixing, interconverting and reversing between its magnonic and phononic components. We attribute the formation of magnon polarons to the strong magnon-phonon coupling induced by Dzyaloshinskii-Moriya interaction. Intriguingly, we find that the band-inverted magnon polarons are topologically nontrivial. These results uncover exotic elementary excitations arising from the magnon-phonon coupling, and offer a new route to topological states by considering hybridizations between different types of fundamental excitations.