The electronic structure, lattice dynamics, and the electron-phonon coupling ͑EPC͒ in hole ͑p͒-do... more The electronic structure, lattice dynamics, and the electron-phonon coupling ͑EPC͒ in hole ͑p͒-doped and electron ͑n͒-doped diamonds have been extensively investigated using ab initio methods with the virtual crystal approximation. The calculations of p-doped diamond correctly reproduced all the essential properties of B-doped diamond such as the increase of lattice constant and the redshift of the Raman spectrum with increasing dopant concentration, and the pressure-induced decrease of T c . The analysis of the spectral function for p-type diamond has shown that optical phonon modes dominate the EPC. From the theoretical prediction of n-doped diamond, it is indicated that the metallic n-doped diamond might be a good superconductor. It is found that the in n-doped diamond increases with the dopant concentration, resulting from the softening of optical phonon modes and the increase of density of states at Fermi level. At a doping level Ͼ2%, the in n-doped diamond is higher than that in p-doped diamond. Phonon linewidth and frozen phonon calculations in n-doped diamond suggested that the longitudinal optical phonon mode contributes mainly to the EPC. The possible mechanism of the predicted superconductivity in n-doped diamond has been discussed.
The dynamics of Kr atoms as guests in dense clathrate hydrate structures are investigated using s... more The dynamics of Kr atoms as guests in dense clathrate hydrate structures are investigated using site specific 83 Kr nuclear resonant inelastic x-ray scattering (NRIXS) spectroscopy in combination with molecular dynamics simulations. The dense structure H hydrate and filled-ice structures are studied at high pressures in a diamond anvil high-pressure cell. The dynamics of Kr in the structure H clathrate hydrate quench recovered at 77 K is also investigated. The Kr phonon density of states obtained from the experimental NRIXS data are compared with molecular dynamics simulations. The temperature and pressure dependence of the phonon spectra provide details of the Kr dynamics in the clathrate hydrate cages. Comparison with the dynamics of Kr atoms in the low-pressure structure II obtained previously was made. The Lamb-Mossbauer factor obtained from NRIXS experiments and molecular dynamics calculations are in excellent agreement and are shown to yield unique information on the strength and temperature dependence of guest-host interactions.
Insertion of Li atoms into a B-substituted carbon cage produces two superhard compounds with rela... more Insertion of Li atoms into a B-substituted carbon cage produces two superhard compounds with relatviely low density: LiBC11 and Li2B2C10. For each structure, phonon frequencies across the whole Brillouin zone are positive, indicating dynamical stability. Electronic structure calculations indicate that they are semiconductors under ambient conditions. Estimates of the Vickers hardness, based on a semi-empirical model, highlight the incompressible nature of these two compounds. We then performed calculations on the ideal strengths of these two structures to confirm the hardness and invertigate origins of the mechanical properties. Strikingly, both LiBC11 and Li2B2C10 can be classed as superhard materials, with hardness of 49 GPa and 41 GPa, respectively. The current results shed light on the properties of new superhard carbon cage structures more generally.
Insertion of Li atoms into a B-substituted carbon cage produces two superhard compounds with rela... more Insertion of Li atoms into a B-substituted carbon cage produces two superhard compounds with relatviely low density: LiBC11 and Li2B2C10. For each structure, phonon frequencies across the whole Brillouin zone are positive, indicating dynamical stability. Electronic structure calculations indicate that they are semiconductors under ambient conditions. Estimates of the Vickers hardness, based on a semi-empirical model, highlight the incompressible nature of these two compounds. We then performed calculations on the ideal strengths of these two structures to confirm the hardness and invertigate origins of the mechanical properties. Strikingly, both LiBC11 and Li2B2C10 can be classed as superhard materials, with hardness of 49 GPa and 41 GPa, respectively. The current results shed light on the properties of new superhard carbon cage structures more generally.
Guided by a simple strategy in searching of new superconducting materials we predict that high te... more Guided by a simple strategy in searching of new superconducting materials we predict that high temperature superconductivity can be realized in classes of high-density materials having strong sp 3 chemical bonding and high lattice symmetry. We examine in detail sodalite carbon frameworks doped with simple metals such as Li, Na, and Al. Though such materials share some common features with doped diamond, their doping level is not limited and the density of states at the Fermi level in them can be as high as that in the renowned MgB 2 . Together with other factors, this boosts the superconducting temperature (T c ) in the materials investigated to higher levels compared to doped diamond. For example, the superconducting T c of sodalite-like NaC 6 is predicted to be above 100 K. This phase and a series of other sodalite-based superconductors are predicted to be metastable phases but are dynamically stable. Owing to the rigid carbon framework of these and related dense carbon-materials, these doped sodalite-based structures could be recoverable as potentially useful superconductors.
The low and glasslike thermal conductivity of metal doped semiconductor clathrate compounds makes... more The low and glasslike thermal conductivity of metal doped semiconductor clathrate compounds makes them potentially high efficiency thermoelectric materials. The cause of this unique and remarkable property has been postulated to be due to resonant scattering of lattice phonons by localized vibrations of the dopants. We present theoretical evidence in support of this hypothesis through the analysis of electronic and vibrational interactions between dopant atoms with the host framework. In particular, the contrasting behavior of two clathrates: the glasslike thermal conductivity in Na 8 Si 46 and the normal behavior in Cs 8 Sn 44 can be rationalized.
The mechanisms for strong electron-phonon coupling predicted for hydrogen-rich alloys with high s... more The mechanisms for strong electron-phonon coupling predicted for hydrogen-rich alloys with high superconducting critical temperature (Tc) are examined within the Migdal-Eliashberg theory. Analysis of the functional derivative of Tc with respect to the electron-phonon spectral function shows that at low pressures, when the alloys often adopt layered structures, bending vibrations have the most dominant effect. At very high pressures, the H-H interactions in two-dimensional (2D) and three-dimensional (3D) extended structures are weakened, resulting in mixed bent (libration) and stretch vibrations, and the electron-phonon coupling process is distributed over a broad frequency range leading to very high Tc.
Experimental evidences are presented showing unusually large and highly anisotropic vibrations in... more Experimental evidences are presented showing unusually large and highly anisotropic vibrations in the ''simple cubic'' (SC) unit cell adopted by calcium over a broad pressure ranging from 30-90 GPa and at temperature as low as 40 K. X-ray diffraction patterns show a preferential broadening of the (110) Bragg reflection indicating that the atomic displacements are not isotropic but restricted to the [110] plane. The unusual observation can be rationalized invoking a simple chemical perspective. As the result of pressure-induced s R d transition, Ca atoms situated in the octahedral environment of the simple cubic structure are subjected to Jahn-Teller distortions. First-principles molecular dynamics calculations confirm this suggestion and show that the distortion is of dynamical nature as the cubic unit cell undergoes large amplitude tetragonal fluctuations. The present results show that, even under extreme compression, the atomic configuration is highly fluxional as it constantly changes.
High-pressure phonon density of states of Fe 2 O 3 across structural and electronic transitions h... more High-pressure phonon density of states of Fe 2 O 3 across structural and electronic transitions has been investigated by nuclear resonant inelastic X-ray scattering and firstprinciples calculations, together with synchrotron Mössbauer, X-ray diffraction, and Xray emission spectroscopies. Drastic changes in elastic, thermodynamic, and vibrational properties of Fe 2 O 3 occur across the Rh 2 O 3 (II)-type structural transition at 40-50 GPa, whereas the Mott insulator-metal transition occurring after the structural transition only causes nominal changes in the properties of the Fe 2 O 3 . The observed anomalous mode softening behavior of the elastic constants is associated with the structural transition at 40-50 GPa, leading to substantial changes in the Debye-like part of the PDOS in the terahertz acoustic phonons. Our experimental and theoretical studies provide new insights into the effects of the structural and electronic transitions in the transition metal oxide compounds.
We perform a thorough first-principles study on superconductivity in yttrium carbide halide Y2X2C... more We perform a thorough first-principles study on superconductivity in yttrium carbide halide Y2X2C2 (X=Cl, Br, I) whose maximum transition temperature (Tc) amounts to ∼10 K. A detailed analysis on the optimized crystal structures reveals that the Y2C2 blocks are compressed uniaxially upon the halogen substitution from Cl, Br to I, contrary to the monotonic expansion of the lattice vectors. With a nonempirical method based on the density functional theory for superconductors within the conventional phonon mechanism, we successfully reproduce the halogen dependence of Tc. Anomalously enhanced coupling of one C2 libration mode is observed in Y2I2C2, which imply possible departure from the conventional pairing picture. Utilizing the Wannier representation of the electron-phonon coupling, we show that the halogen electronic orbitals and ionic vibrations scarcely contribute to the superconducting pairing. The halogen dependence of this system is hence an indirect effect of the halogen ions through the uniaxial compressive force on the superconducting Y2C2 blocks. We thus establish a quantitatively reliable picture of the superconducting physics of this system, extracting a unique effect of the atomic substitution which is potentially applicable to other superconductors.
The minimum energy path (MEP) and transition state are two key parameters in the investigation of... more The minimum energy path (MEP) and transition state are two key parameters in the investigation of the mechanisms of chemical reactions and structural phase transformations. However, determination of transition paths in solids is challenging. Here, we present an evolutionary method to search for the lowest energy path and the transition state for pressure-induced structural transformations in solids without any user input or prior knowledge of possible paths. Instead, the initial paths are chosen stochastically by connecting randomly selected atoms from the initial to final structure. The MEP of these trials paths were computed and ranked in order of their energies. The matrix particle swarm optimization algorithm is then used to generate improved transition paths. The procedure is repeated until the lowest energy MEP is found. This method is validated by reproducing results of several known systems. The new method also successfully located the MEP for the direct low-temperature pres...
Earth is a dynamic system. The thermodynamics conditions of Earth vary drastically depending on t... more Earth is a dynamic system. The thermodynamics conditions of Earth vary drastically depending on the depth, ranging from ambient temperature and pressure at the surface to 360 GPa and 6600 K at the core. Consequently, the physical and chemical properties of Earth's constituents (e.g., silicate and carbonate minerals) are strongly affected by their immediate environment. In the past 30 years, there has been a tremendous amount of progress in both experimental techniques and theoretical modeling methods for material characterization under extreme conditions. These advancements have elevated our understanding of the properties of minerals, which is essential in order to achieve full comprehension of the formation of this planet and the origin of life on it. This article reviews recent computational techniques for predicting the behavior of materials under extreme conditions. This survey is limited to the application of the first-principles molecular dynamics (FPMD) method to the investigation of chemical and thermodynamic transport processes relevant to Earth Science.
The mechanisms for strong electron-phonon coupling predicted for hydrogen-rich alloys with high s... more The mechanisms for strong electron-phonon coupling predicted for hydrogen-rich alloys with high superconducting critical temperature (Tc) are examined within the Migdal-Eliashberg theory. Analysis of the functional derivative of Tc with respect to the electron-phonon spectral function shows that at low pressures, when the alloys often adopt layered structures, bending vibrations have the most dominant effect. At very high pressures, the H-H interactions in two-dimensional (2D) and three-dimensional (3D) extended structures are weakened, resulting in mixed bent (libration) and stretch vibrations, and the electron-phonon coupling process is distributed over a broad frequency range leading to very high Tc.
Insertion of Li atoms into a B-substituted carbon cage produces two superhard compounds with rela... more Insertion of Li atoms into a B-substituted carbon cage produces two superhard compounds with relatively low density: LiBC11 and Li2B2C10.
CALYPSO predicts two novel structures of solid HBr with a relatively high superconductivity at su... more CALYPSO predicts two novel structures of solid HBr with a relatively high superconductivity at sufficiently high pressure.
From first-principles calculations, a high-pressure metallic phase of SnH 4 with a novel layered ... more From first-principles calculations, a high-pressure metallic phase of SnH 4 with a novel layered structure intercalated by ''H 2 '' units is revealed. This structure is stable at pressure between 70 and 160 GPa. A remarkable feature of this structure is the presence of soft modes in the phonon band structure induced by Fermi surface nesting and Kohn anomalies that lead to very strong electron-phonon coupling. The application of the Allen-Dynes modified McMillan equation with the calculated electron-phonon coupling parameter shows that a superconducting critical temperature close to 80 K can be achieved at 120 GPa.
Two structural transitions in covalent aluminum hydride AlH 3 were characterized at high pressure... more Two structural transitions in covalent aluminum hydride AlH 3 were characterized at high pressure. A metallic phase stable above 100 GPa is found to have a remarkably simple cubic structure with shortest first-neighbor H-H distances ever measured except in H 2 molecule. Although the high-pressure phase is predicted to be superconductive, this was not observed experimentally down to 4 K over the pressure range 120-164 GPa. The results indicate that the superconducting behavior may be more complex than anticipated.
The lattice distortion of hcp solid He under pressure is calculated using semiempirical and first... more The lattice distortion of hcp solid He under pressure is calculated using semiempirical and first-principle approaches. While three-body forces tend to flatten the lattice at all compressions, the effect of pair forces changes from the flattening at small compression to elongation at large one. At large compressions, the lattice distortion due to the triple forces is more than twice as large as those due to pair forces and the lattice is slightly flattened. First-principles results show that over approximately fivefold compressions higher-order, many-body forces become important.
The crystal, spin, and electronic structure of the "111"-type LiFeAs superconductor has been inve... more The crystal, spin, and electronic structure of the "111"-type LiFeAs superconductor has been investigated by first-principles calculations based on the density-functional theory. It is found that the crystal structure and weak magnetism of metallic LiFeAs can be described by general gradient approximation and local spin-density approximation ͑LSDA͒. Both methods show LiFeAs is a weak correlated system with a striped antiferromagnetic ground state and the easy axis of magnetization is along the b direction of the magnetic unit cell. The magnetic unit cell is distorted from the tetragonal into an orthorhombic structure. The spin moment/Fe calculated by the LSDA method is 0.121 B .
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