LATTICE DISTORTION NEAR VACANCIES IN DIAMOND AND SILICON .1.

A dynamical relaxation procedure, coupled with a valence force potential, has been used to calculate the distortion around point defects in a diamond-type crystal. The method has been applied to the vacancy in diamond and silicon. The response of the lattice to symmetrized forces on the nearest neighbours to the vacancy was calculated. The results can be used in estimates of point defect properties which depend on lattice distortion, including the jahn-teller effect, and formation energies. The ratios of the atomic displacements under uniform external stresses for the perfect lattice and for the lattice with a vacancy are also determined

Microscopic theory of the nearest-neighbor valence bond sector of the spin-1/2 kagome antiferromagnet

The spin-1/2 Heisenberg model on the kagome lattice, which is closely realized in layered Mott insulators such as ZnCu$_3$(OH)$_6$Cl$_2$, is one of the oldest and most enigmatic spin-1/2 lattice model. While the numerical evidence has accumulated in favor of a quantum spin liquid, the debate is still open as to whether it is a $Z_2$ spin liquid with very short-range correlations (some kind of Resonating Valence Bond spin liquid), or an algebraic spin-liquid with power-law correlations. To address this issue, we have pushed the program started by Rokhsar and Kivelson in their derivation of the effective quantum dimer model description of Heisenberg models to unprecedented accuracy for the spin-1/2 kagome, by including all the most important virtual singlet contributions on top of the orthogonalization of the nearest-neighbor valence bond singlet basis. Quite remarkably, the resulting picture is a competition between a $Z_2$ spin liquid and a diamond valence bond crystal with a 12-site unit cell, as in the DMRG simulations of Yan, Huse and White. Furthermore, we found that, on cylinders of finite diameter $d$, there is a transition between the $Z_2$ spin liquid at small $d$ and the diamond valence bond crystal at large $d$, the prediction of the present microscopic description for the 2D lattice. These results show that, if the ground state of the spin-1/2 kagome antiferromagnet can be described by nearest-neighbor singlet dimers, it is a diamond valence bond crystal, and, a contrario, that, if the system is a quantum spin liquid, it has to involve long-range singlets, consistent with the algebraic spin liquid scenario.Comment: 11 pages, 14 figures. Revised and extended version. Results are untouched, implications have been clarified and better put in contex

Ab initio lattice dynamics of nonconducting crystals by systematic fragmentation

A systematic method for approximating the ab initio electronic energy of crystal lattices has been improved by the incorporation of long range electrostatic and dispersion interactions. The effect of these long range interactions on the optimization of the crystal structure is reported. The harmonic lattice dynamics have been evaluated to give phonon frequencies and neutron scattering intensities. Exemplary results are reported for diamond, silicon, and α-quartz using Hartree-Fock, Möller-Plesset perturbation, and coupled-cluster levels of ab initio theory

Electron-Phonon Coupling in Boron-Doped Diamond Superconductor

The electronic structure, lattice dynamics, and electron-phonon coupling of the boron-doped diamond are investigated using the density functional supercell method. Our results indicate the boron-doped diamond is a phonon mediated superconductor, con rming previous theoretical conclusions deduced from the calculations employing the virtual crystal approximation. We show that the optical phonon modes involving B vibrations play an important role in the electron-phonon coupling. Di erent from previous theoretical results, our calculated electron-phonon coupling constant is 0.39 and the estimated superconducting transition temperature Tc is 4.4 K for the boron doped diamond with 2.78% boron content using the Coulomb pseudopotential \mu*= 0.10, in excellent agreement with the experimental result.Comment: 11 pages, 4 figures, Accepted by PR

Stability of binary colloidal crystals immersed in a cholesteric liquid crystal

In this paper, we model a number of both closed-packed and non-closed-packed crystals inside a cholesteric liquid crystal (LC) with different pitch values and nematic LC through the Landau–de Gennes free-energy method. We used binary boundary conditions (normal and planar anchoring) applied on the surface of colloids as we are interested in investigating the stability of binary crystals. The results indicate that body-centered-cubic (BCC) crystals have a lower-energy lattice defect structure than the diamond crystal, and the most energetically favorable BCC lattice can be formed in a cholesteric liquid crystal with a pitch value commensurate with the lattice spacing. Furthermore, it is shown that a pair of binary colloids can be self-assemble into a stable face-centered-cubic lattice structure inside a nematic LC, as it has the lowest energy comparing to diamond and BCC crystals