Y. Zempo

The semiconductor-metal transition in fluid selenium : an ab initio molecular-dynamics simulation

The semiconductor-metal transition in fluid selenium is investigated by means of an ab initio molecular-dynamics simulation using the generalized-gradient-corrected density functional theory. It is found that the chain-like structure persists even in the metallic state, although the chain structure is substantially disrupted. The average chain length decreases with increasing temperature, in agreement with the experimentally observed tendency. The detailed investigation of the time change of the chain structure shows that the interaction between the Se chains is crucially important for bond breaking, and that bond breaking and rearrangement of the Se chains occur more frequently at higher temperatures. It is important to note that when the Se-Se bonds break, the anti-bonding states above the Fermi level are stabilized while the bonding or non-bonding states below the become unstable, and, therefore, the gap at disappears at high temperatures.

Temperature dependence of the atomic structure of liquid As2Se3: ab initio molecular dynamics simulations

The structural and electronic properties of liquid As 2 Se 3 mixtures are studied by ab initio molecular dynamies simulations, We detail the temperature dependence of the atomic structure by calculating the correlation functions in a real space as well as in a reciprocal space. It is found that the network structure changes to a chain-like structure by bond breaking with increasing temperature, and that there are triangular atomic configurations in the chain-like structure at higher temperatures. We also investigate the relation between the semiconductor-metal transition and the structural change.

Ab initio Molecular-Dynamics Simulations of Anomalous Structural Change in Liquid Tellurium under Pressure

The pressure dependence of the structural and electronic properties of liquid Te is investigated by means of ab initio molecular-dynamics simulations. The anomalous pressure-induced structural change is successfully reproduced by our simulations being in agreement with the observations by the recent synchrotron X-ray-diffraction experiments. Our analyses including the overlap populations reveal that there are two stages in the compression process of the liquid Te. In the first stage under pressure up to about 6 GPa, the elongation of the nearest-neighbor distance is induced by forming a weak covalent bonding state between two Te atoms. In the second stage for further compression, the anisotropy of atomic configuration around each Te atom is reduced with increasing pressure, and the electronic states have a weak pressure dependence.

Intermediate-range order in liquid and amorphous As2S3 by ab initio molecular-dynamics simulations

Abstract The structural properties of liquid and amorphous As2S3 are investigated by means of ab initio molecular-dynamics simulations. It is shown that the first-sharp diffraction peak (FSDP) exists at about k=1.2 A−1 in the calculated structure factor for lower densities, and disappears under pressure, which is in agreement with the experimental observations. The effects of hydrostatic compression on the local atomic configuration are investigated in detail, and the relation between the structural change and the density dependence of the first-sharp diffraction peak is discussed.

Effects of spin polarization on the structural and electronic properties of supercritical fluid selenium: ab initio molecular-dynamics simulations

The effects of spin polarization on the atomic and electronic structures of supercritical fluid selenium are investigated by means of ab initio molecular-dynamics simulations using the fully spin-polarized and generalized-gradient-corrected density functional theory. Detailed investigations of the density dependence of the electronic density of states and the pair distribution functions show that the spin polarization plays a crucially important role in determining the properties of supercritical fluid selenium. It is confirmed from the spatial distribution of the spin density that there is a strong correlation between the chain structure and the amount of spin polarization around each Se atom.

Photo-induced bond breaking in the ring: an ab initio molecular-dynamics simulation

The microscopic mechanism of the recently observed bond breaking in the ring illuminated by a pulsed laser is investigated by means of an ab initio molecular-dynamics simulation. It is found that a bond in the ring is easily broken immediately after an electron in the highest occupied molecular orbital is excited to the lowest unoccupied molecular orbital. From the time dependence of the eigenvalues and the wave functions obtained by the simulation, it is established that the bond breaks in order to stabilize the anti-bonding states occupied by the excited electron.

The microscopic mechanism of the semiconductor-metal transition in liquid arsenic triselenide

We have clarified the microscopic mechanism of the semiconductor-metal transition in liquid with increasing temperature and pressure by means of ab initio molecular-dynamics simulations. It is shown that, at higher temperatures, a major part of this liquid mixture consists of twofold chain-like structures, while, at lower temperatures, it mainly has a network structure, where As and Se atoms have threefold and twofold coordinations, respectively. Accompanying this structural change, p-like non-bonding states that are almost half-filled are generated around the twofold-coordinated As atoms, and cause the metallic state of liquid .

Atomic and electronic structures in liquid arsenic telluride by ab initio molecular dynamics simulations

The temperature dependence of atomic and electronic structures in the liquid As2Te3 is studied by means of ab initio molecular dynamics simulations. The microscopic mechanism of the semiconductor–metal transition in this liquid mixture with increasing temperature and pressure is discussed compared with that in the liquid As2Se3. It is shown that the structure of the liquid As2Te3 does not change qualitatively with increasing temperature in the sense that most of the As and Te atoms continue to have three-and twofold coordination, respectively, which is contrast with the structural change from the network structure to the chain-like structure seen in the liquid As2Se3 at high temperatures. It is clarified that the electronic states around Te atoms contribute importantly to the metallization.

Effects of Semicore Electrons on the Structure of Liquid Rubidium : an ab initio Molecular-Dynamics Simulation

The effects of semicore electrons on the structural and electronic properties of liquid rubidium are studied by an ab initio molecular-dynamics simulation based on the density functional theory in the local-density approximation and on the ultrasoft pseudopotential. It is shown that the repulsive interaction between ions due to the overlap of the wavefunctions of the semicore 4 p electrons plays an important role in the structure of liquid rubidium as well as in the electronic density of states under high pressures.

Electronic and atomic structures of supercritical fluid selenium: ab initio molecular dynamics simulations

Abstract The electronic and atomic structures of supercritical fluid selenium are investigated by means of ab initio molecular dynamics simulations using the fully-spin-polarized and generalized-gradient-corrected density functional theory. It is shown that the calculated structure factors are in good agreement with recent X-ray diffraction experiments over a wide range of temperature and pressure. It is demonstrated how fluid Se changes from the chain structure to an assembly of Se 2 dimers with decreasing density in the supercritical region. The relation between the structural change and the spatial distribution of the spin density is discussed.