Search Results (20 total)

Under pressure, Ba₈Si₄₆ is found to undergo an isostructural transition, as observed by Raman spectroscopy, extended x-ray-absorption fine structure, and x-ray diffraction. Rietveld analysis of the x-ray diffraction data shows a homothetic contraction of the host lattice after the structural transition at 17  GPa. Using the Rietveld and maximum-entropy methods, we have performed an analysis of high resolution x-ray diffraction patterns collected from ambient to 30  GPa obtained in a diamond anvil cell using He as a quasihydrostatic pressure transmitting medium. The results indicate unambiguously that the homothetic phase transition at about 17  GPa is due to an extensive rehybridization of the Si atoms leading to a transfer of valence electrons from the bonding to the interstitial region. Consequently, the Si–Si bonds are weakened substantially at high density, leading to an abrupt collapse of the unit cell volume without a change in crystalline structure. The transition pressure and the change in the chemical bonding are remarkably similar to that observed in elemental Si–V.

A linear scale method for calculating electronic properties of large and complex systems is introduced within a local density approximation. The method is based on the Chebyshev polynomial expansion and the time-dependent method, which is tested on the calculation of the electronic structure of a model n-type GaAs quantum dot.

We study the effect of a phase shift on the amount of transferable two-spin entanglement in a spin chain. We consider a ferromagnetic Heisenberg or XY spin chain, both numerically and analytically, and two mechanisms to generate a phase shift, the Aharonov-Casher effect, and the Dzyaloshinskii-Moriya interaction. In both cases, the maximum attainable entanglement is shown to be significantly enhanced, suggesting its potential usefulness in quantum information processing.

We propose an atomistic model for the pressure-induced isostructural phase transition of metal-doped silicon clathrates Ba₈Si₄₆ and K₈Si₄₆ that has been observed at 14 and 23 GPa, respectively. The model successfully explains the equation of state, transition pressure, change of Raman spectra, and dependence on the doped cations as well as the effects of substituting Si(6c) atoms with noble metals.

The nature of isostructural transformations of a type-I Ba₈Si₄₆ clathrate has been studied by in situ high-pressure angle-dispersive x-ray powder diffraction using liquid He as pressure transmitting medium. The good quality of the diffraction data permitted refinement of structural and thermal parameters from Rietveld analysis. The results show that the first transition at 7 GPa is caused by the displacement of the Ba atoms in the Si₂₄ cages. The cause of the second transition at 15 GPa, characterized by a dramatic reduction of cell volume, is not as clear. Theoretical calculations predicted an electronic topological transition of Fermi surface at this pressure. Analysis of the anomalously large Si thermal parameters suggested a highly disordered Si framework. This disordering is probably static and may be due to the presence of Si vacancies. The latter hypothesis is supported by electronic calculations on model disordered systems.

A recent report on the Raman observation of a high-pressure broken-symmetry phase of Li at 120 megabar pressures is examined with first-principles electron structure and phonon calculations. The results show that the observed very high-frequency Li-Li vibration, approximately 1800 cm⁻¹, cannot be reproduced by the “dimer” structure proposed earlier or a recently found energetically competitive orthorhombic structure. Model calculations show that such a high Li vibrational frequency could, in principle, be achieved at a Li-Li contact less than 1.2 Å in a linear chain. However, no stable structure was found in the megabar pressure range that satisfies this condition.

The electronic and vibrational structures of Ba₈Si₄₆ and Ba₈M_nSi_46−n, where some of the framework Si were replaced by metal (M=Ag or Au) have been studied. Theoretical calculations show that the nature of chemical bonding of the substituted clathrates and their vibrational properties are fundamentally different from Ba₈Si₄₆. Low-frequency vibrations associated with the motions of Ba in the large cages and the framework metals have been identified from their Raman spectra. A surprisingly large contribution of the low-frequency modes, particularly those which arise from the Ba vibrations in the large cages, to the electron-phonon coupling parameter in Ba₈Si₄₆ was found.

We derive an estimate of the statistical error in calculating the trace of a large matrix by using random vectors, and show that the random phase vector gives the results with the smallest statistical error for a given basis set. This result supports use of random phase vectors in the calculation of density of states and linear response functions of large quantum systems.

The structural, electronic, and spectroscopic properties of a high-pressure phase of methane hydrate are studied by first-principles electronic structure calculations. A detailed analysis of the atomic positions suggests that ionization of hydrogen-bonded water molecules occurs around 40 GPa and centering or symmetrization of hydrogen bonds occurs around 70 GPa. These pressures are much lower compared to ionization around 55 GPa and centering around 100 GPa in pure ice. The transition may be observed with low-temperature IR/Raman spectroscopy of OH stretching modes, neutron diffraction, or ¹H-NMR.

We introduce an efficient and numerically stable method for calculating linear response functions χ(q,ω) of quantum systems at finite temperatures. The method is a combination of numerical solution of the time-dependent Schrödinger equation, random vector representation of trace, and Chebyshev polynomial expansion of Boltzmann operator. This method should be very useful for a wide range of strongly correlated quantum systems at finite temperatures. We present an application to the ESR spectrum of s=1 / 2 antiferromagnet Cu benzoate.

The density and the elastic stiffness coefficients of fcc solid argon at high pressures from 1 GPa up to 80 GPa are computed by the first-principles pseudopotential method with plane-wave basis set and generalized gradient approximation (GGA). The result is in good agreement with an experimental result recently obtained with Brillouin spectroscopy by Shimizu et al. [Phys. Rev. Lett. 86, 4568 (2001)]. Solid argon becomes very hard due to its closed-shell electronic configuration. The Cauchy condition is strongly violated, indicating a large contribution from a noncentral many-body force. The present result has made it clear that the standard density functional method with periodic boundary conditions can be successfully applied for calculating the elastic properties of rare gas solids at high pressures in contrast to those at low pressures where dispersion forces are important.

We extend the recently proposed order-N algorithms for calculating linear- and nonlinear-response functions in time domain to the systems described by nonorthonormal basis sets.

We propose an efficient time-dependent algorithm for nonlinear response function that requires CPU time proportional to the system size, and study the size effects in two-photon absorption spectra of Si nanocrystallites by using this algorithm.

The quantum-size effect is investigated theoretically in model nanocrystalline/amorphous mixed-phase Si structures. We show that the peak energies of the imaginary part of the dielectric function of the model structures shift as a function of the size of nanocrystalline Si and the thickness of amorphous Si which surrounds the nanocrystalline Si. This effect cannot be explained by the conventional quantum confinement model because of the lack of an apparent potential which confines electrons within the nanocrystalline Si embedded in amorphous Si. Atomic positions of the amorphous Si are modeled by a Monte Carlo calculation with the Stillinger-Weber potential for the interaction between Si atoms. The electronic calculation is based on an empirical pseudopotential method with basis states represented in a real-space grid. Dielectric functions of modeled structures containing up to 13 824 Si atoms are calculated, which has been made possible by calculating a time-dependent representation of the linear-response function.

We emphasize the importance of choosing an appropriate correlation function to reduce numerical errors in calculating the linear-response function as a Fourier transformation of a time-dependent correlation function. As an example we take dielectric functions of silicon crystal calculated with a time-dependent method proposed by Iitaka et al. [Phys. Rev. E 56, 1222 (1997)].

A linear scaling calculation of the optical-absorption spectra is performed for hydrogenated silicon nanocrystallites up to 13 464 Si atoms. The calculation is performed in real space and is based on a time-dependent representation of the linear-response function. The empirical pseudopotential method is used for potentials of silicon and hydrogen atoms. The peaks in higher excited states in the ultraviolet region of the calculated absorption spectra are shown to shift to higher energy with a decrease in size.

An O(N) algorithm is proposed for calculating linear response functions of noninteracting electrons. This algorithm is simple and suitable to parallel and vector computation. Since it avoids O(N³) computational effort of matrix diagonalization, it requires only O(N) computational efforts, where N is the dimension of the state vector. The use of this O(N) algorithm is very effective since, otherwise, we have to calculate a large number of eigenstates, i.e., the occupied one-electron states up to the Fermi energy and the unoccupied states with higher energy. The advantage of this method compared to the Chebyshev polynomial method recently developed by Wang and Zunger [L. W. Wang, Phys. Rev. B 49, 10 154 (1994); L. W. Wang and A. Zunger, Phys. Rev. Lett. 73, 1039 (1994)] is that our method can calculate linear response functions without any storage of huge state vectors on external storage.

We introduce an explicit scheme to solve the time-dependent Schrödinger equation. The scheme is a straightforward extension of the second order differencing scheme to the fourth, sixth, and higher order accuracy. The accuracy is remarkably improved with minor changes in the second order differencing program. This method is conditionally stable. There is a trade-off between the higher order accuracy and the condition of stability. The performance is evaluated and compared to the standard methods, such as the Crank-Nicholson scheme (CN) and the Chebyshev scheme (CH). The new scheme is much more accurate than CN, almost equal to CH.

The energy distribution of the convoy electrons produced by fast highly charged ions is studied with classical-trajectory Monte Carlo calculation. We found that, in the case of small-angle incidence, the convoy peak is shifted to higher energy because of the anisotropy of the electron velocity distribution which is caused by the image potential of the incident ion.

Measurements were made, at various emission angles, of energy spectra of electrons from Al induced by grazing-angle-incident ions of N⁶⁺, Ar¹²⁺, and Xe²⁷⁺ with equal velocities corresponding to 0.98 MeV/amu. A new line which could not be explained by any of the hitherto identified mechanisms was observed at an energy obviously larger than that of an electron with a velocity equal to the projectile. The projectile and emission angle dependencies of the line are consistent with the dynamic image potential acceleration mechanism.