Ping Huai

Theory for photoinduced ionic-neutral structural phase transition in quasi one-dimensional organic molecular crystal TTF-CA

Using a quasi one-dimensional extended Peierls-Hubbard model, we theoretically study the photoinduced ionic-neutral structural phase transition in an organic molecular chain crystal TTF-CA, by means of the adiabatic approximation and the mean-field theory. This model includes strong intra-chain Coulomb interactions which nonlinearly depend on the distance between TTF and CA molecules, and it also includes a weak inter-chain interaction. We will be concerned with the case that the ionic phase is just below the neutral one, and their energies and charge (spin) distribution are investigated, as well as their energy band structures. Moreover, we calculate the optical absorption spectrum by the classical Monte-Carlo method, including the exciton effect and the thermal lattice fluctuations. We also study the nonlinear lattice relaxation of a charge transfer (CT) exciton, and clarify the adiabatic path from its Franck-Condon state to a macroscopic neutral domain. This domain, being separated from the CT exciton ...

Molecular dynamics study of damage production in single-walled carbon nanotubes irradiated by various ion species

The irradiation-induced damage production in single-walled carbon nanotubes (CNTs) by several types of ions is investigated using the molecular dynamics method with analytical potentials. We found that, in the incident energy range 25-1000 eV, the bonding action or the chemical effect of the ions could significantly enhance their damage capabilities to CNTs relative to that of non-bonding ions, and the dependence of damage yield on the ion mass is no longer monotonic. This is contrary to the previous viewpoint that the chemical aspect of the interaction is of no importance to the ion-induced defect production mechanism in CNTs. The bonding interaction of ions with CNTs also increases their implantation probabilities into CNTs. The chemical erosion effect of incident ions remarkably intensifies the sideward recoil from CNTs under irradiation while the downward recoil is still governed by the physical collision effect.

Effects of tube diameter and chirality on the stability of single-walled carbon nanotubes under ion irradiation

Using molecular dynamics method, we investigated the influence of tube diameter and chirality on the stability of single-walled carbon nanotubes (CNTs) under ion irradiation. We found that in the energy range below 1 keV, the dependence of CNT stability on the tube diameter is no longer monotonic under C ion irradiation, and the thinner (5, 5) CNT may be more stable than the thicker (7, 7) CNT, while under Ar irradiation, the CNT stability increases still monotonically with the CNT diameter. This stability behavior was further verified by the calculations of the threshold ion energies to produce displacement damage in CNTs. The abnormal stability of thin CNTs is related to their resistance to the instantaneous deformation in the wall induced by ion pushing, the high self-heating capacity, as well as the different interaction properties of C and Ar ions with CNT atoms. We also found that under ion irradiation the stability of a zigzag CNT is better than that of an armchair CNT with the same diameter. This is because of the bonding structure difference between the armchair and the zigzag CNTs with respect to the orientations of graphitic networks as well as the self-healing capacity difference

A comparative first-principles study of the electronic, mechanical, defect and acoustic properties of Ti2AlC and Ti3AlC

Ti2AlC and Ti3AlC are both considered as candidate materials for structural applications in harsh environments. In this paper, we adopt density functional theory to study the electronic, elastic, acoustic and defect properties of Ti2AlC and Ti3AlC. It was found that the mechanical properties of Ti2AlC and Ti3AlC vary little; the bulk modulus at equilibrium of Ti3AlC is 10% higher than that of Ti2AlC. However, we found that their defect properties were very different. The migration barrier of an Al vacancy in Ti2AlC is 0.834 eV compared with 4.518 eV in Ti3AlC. The difference by a factor of six in the migration barriers is explained by the bond angle variance of the C-centred octahedral. Using the calculated elastic constants, the slowness surface of the acoustic waves is obtained using the Christoffel equation. Since Ti2AlC and Ti3AlC are often presented as a second Ti–Al–C phase in each other, the individual properties calculated can be used to assess their characterizations in experiments.

First-principles study of the effect of phosphorus on nickel grain boundary

Based on first-principles quantum-mechanical calculations, the impurity-dopant effects of phosphorus on Σ5(012) symmetrical tilt grain boundary in nickel have been studied. The calculated binding energy suggests that phosphorus has a strong tendency to segregate to the grain boundary. Phosphorus forms strong and covalent-like bonding with nickel, which is beneficial to the grain boundary cohesion. However, a too high phosphorus content can result in a thin and fragile zone in the grain boundary, due to the repulsion between phosphorus atoms. As the concentration of phosphorus increases, the strength of the grain boundary increases first and then decreases. Obviously, there exists an optimum concentration for phosphorus segregation, which is consistent with observed segregation behaviors of phosphorus in the grain boundary of nickel. This work is very helpful to understand the comprehensive effects of phosphorus.

Difference between Photoinduced Phase and Thermally Excited Phase

We study differences between the photoinduced phase and the thermally excited one, using a two-dimensional Peierls–Hubbard model, which includes a strong electron-phonon coupling and an on-site int...

Electronic control of spin alignment in pi-conjugated molecular magnets.

Intramolecular spin alignment in pi-conjugated molecules is studied theoretically in a model of a Peierls-Hubbard chain coupled with two localized spins. By means of the exact diagonalization technique, we demonstrate that a spin singlet (S=0) to quartet (S=3/2) transition can be induced by electronic doping, depending on the chain length, the positions of the localized spins, and the sign of the electron-spin coupling. The calculated results provide a theoretical basis for understanding the mechanism of spin alignment recently observed in a diradical donor molecule.

A first-principles study on the defective properties of MAX phase Cr2AlC: the magnetic ordering and strong correlation effect

Our first-principles calculations indicate the defective properties of Cr2AlC are very different from the commonly studied Ti-based MAX phases. Al vacancies are predicted to have high formation energies while Cr vacancies have low formation energies in Cr2AlC. Compared with previously reported results of MAX phases, the formation energy of the anion antisite defect pair in Cr2AlC is the lowest (∼1.9 eV), predicting a good irradiation-resistance property. The reduction in the formation energy of the Cr–Al antisite defect is attributed to the magnetic ordering and strong correlation effect of Cr atoms. Analysis suggests that the majority spin state of the dz2 orbital of Cr is lowered in energy while the minority spin states become empty, which makes the Cr more ionic in character.

Theory of spin alignment in π-conjugated molecular magnets

Control of spin alignment between radicals by external stimuli has been theoretically investigated in π-conjugated molecular magnets. To clarify the mechanism of such type of spin alignment, we have studied the Hubbard-Peierls model including the Kondo coupling with localized spins by the exact diagonalization technique. We demonstrate that the alignment of localized spins can be controlled by charge doping in the conjugated system, if designed properly.

First-Principles Study of Vacancies in Ti3SiC2 and Ti3AlC2

MAX phase materials have attracted increased attention due to their unique combination of ceramic and metallic properties. In this study, the properties of vacancies in Ti3AlC2 and Ti3SiC2, which are two of the most widely studied MAX phases, were investigated using first-principles calculations. Our calculations indicate that the stabilities of vacancies in Ti3SiC2 and Ti3AlC2 differ greatly from those previously reported for Cr2AlC. The order of the formation energies of vacancies is VTi(a) > VTi(b) > VC > VA for both Ti3SiC2 and Ti3AlC2. Although the diffusion barriers for Ti3SiC2 and Ti3AlC2 are similar (~0.95 eV), the properties of their vacancies are significantly different. Our results show that the vacancy–vacancy interaction is attractive in Ti3AlC2 but repulsive in Ti3SiC2. The introduction of VTi and VC vacancies results in the lattice constant c along the [0001] direction increasing for both Ti3SiC2 and Ti3AlC2. In contrast, the lattice constant c decreases significantly when VA are introduced. The different effect of VA on the lattice constants is explained by enhanced interactions of nearby Ti layers.