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Prediction of a Dynamically Stable New Half-Metallic Phase for the BaN and BaC Compounds
摘要： The structural, magnetic, elastic, mechanical, and thermodynamic properties of BaC and BaN compounds in different phases were studied using first-principle calculations based on spin-polarized density functional theory within the generalized gradient approximation (GGA-PBEsol) and the modified Becke–Johnson approach (mBJ-GGA-PBEsol) for the exchange-correlation energy and potential. The following phases—rock-salt (NaCl), CsCl, zinc blende (ZB), NiAs- and WZ-type hexagonal, tetragonal (P4/nmm), and orthorombic (Pnma) phases of BaC and BaN compounds—were considered. We obtained that Pnma phase has the lowest energy configuration as a function of the volume for both the BaN and BaC compounds. The ferromagnetic phase is energetically favored with respect to the non-magnetic phase in the BaN and BaC compounds, except for the CsCl phase in the BaC compound. Considering the phonon dynamics of BaN and BaC compounds in the Pnma, NaCl, ZB, and WZ phases, we observed that the BaN and BaC compounds in the Pnma, NaCl, and ZB phases are dynamically stable. The calculated elastic properties for the Pnma, NaCl, and ZB phases show that they are elastically stable. The Pnma phase for the BaN and BaC compounds, which is a new phase was found to be dynamically and elastically stable. The BaN and BaC compounds exhibit half-metallic behavior in the Pnma, NaCl, and ZB phases. The half-metallic and magnetic character found in the BaN and BaC compounds are attributed to the presence of spin-polarized 2p orbitals of the nitrogen and carbon atoms, respectively. We found that BaN and BaC compounds are half-metallic ferromagnets with magnetic moment of 1 μB and 2 μB per formula unit, respectively. Using the GGA-PBEsol (mBJ-GGA-PBEsol) approach, our calculated half-metallic gaps for BaN and BaC compounds are 0.22 eV (0.54 eV) and 0.32 eV (0.44 eV) in the Pnma phase, 0.23 eV (1.32 eV) and 0.35 eV (1.00 eV) in the NaCl phase, and 0.38 eV (1.54 eV) and 0.50 eV (1.57 eV) in the ZB phase, respectively.
关键词： BaN,Density functional theory,Half-metallic,BaC,Phase stability,Phonon dynamics
更新于2025-09-23 15:21:01
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Determination of size dependent carrier capture in InGaN/GaN quantum nanowires by femto-second transient absorption spectroscopy: effect of optical phonon, electron-electron scattering and diffusion
摘要： Understanding the ultrafast processes corresponding to carrier capture, thermalization and relaxation is essential to design high-speed optoelectronic devices. Here, we have investigated size dependent carrier capture process in InGaN/GaN 20 nm, 50 nm nanowires and quantum well systems. Femto-second transient absorption spectroscopy reveals that the carrier capture is a two-step process. The carriers are captured in the barrier by polar optical phonon scattering. They further scatter into the active region by electron-electron and polar optical phonon scatterings. The capture is found to slow down for quantum confined structures. A significant number of carriers are found to disappear from the barrier during the diffusion process. All the experimental observations are explained in a simulation framework depicting various scattering mechanisms.
关键词： nanowire,polar optical phonon scattering,electron-electron scattering,transient absorption spectroscopy,carrier capture,electron and phonon dynamics
更新于2025-09-19 17:15:36
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Anharmonicity of Bi <sub/>2</sub> Se <sub/>3</sub> revealed by fs transient optical spectroscopy
摘要： We investigate the anharmonic effects in Bi2Se3 crystals using femtosecond transient optical spectroscopy at 5–280 K. The re?ectivity time series consist of exponential decay due to hot carriers and decaying oscillations due to the A1 1g phonon vibration. Vibration frequency and dephasing time of this optical phonon mode are obtained as a function of temperature, decreasing with increasing temperature; both the red shift in frequency and the increased dephasing rate induced by heating can be well described using the anharmonicity model including lattice thermal expansion and phonon-phonon coupling.
关键词： Bi2Se3,femtosecond transient optical spectroscopy,phonon dynamics,anharmonicity,thermal expansion,phonon-phonon coupling
更新于2025-09-11 14:15:04
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: <i>Ab initio</i> lattice dynamical perspective
摘要： The hydration and ammonization of ZrW2O8 is known to lead to positive and negative thermal expansion behavior, respectively. We report ab initio calculations to understand this anomalous behavior. We identify the crucial low-energy phonon modes involving translations, rotations, and distortions of WO4 and ZrO6 polyhedra, which lead to negative thermal expansion in ZrW2O8 in pure and ammoniated forms; however, the rotation and distortion motions get inhibited on hydration and lead to positive thermal expansion. We demonstrate that the thermal expansion coef?cient could be tailored by engineering the phonon dynamics of a material.
关键词： ZrW2O8,hydration,phonon dynamics,ab initio calculations,ammonization,thermal expansion
更新于2025-09-04 15:30:14