José Luís Martins

A straightforward method for generating soft transferable pseudopotentials

Abstract We present a simple procedure for generating first-principles norm-conserving pseudopotentials which are very soft and therefore have good convergence properties when used in plane-wave calculations of the band structure and total-energy of materials. Using diamond as a test case, we show that our pseudopotential is more efficient for plane wave calculations than the most commonly used pseudopotentials.

Electronic States of KxC60: Insulating, Metallic, and Superconducting Character

The recent report of electrical conductivity in the alkali metal fullerides and the discovery of superconductivity at 18 K for KxC60 has raised fundamental questions about the electronic states on either side of the Fermi level, their occupancy with K intercalation, and the mechanism of superconductivity. Direct photoemission evidence is presented of filling of bands derived from the lowest unoccupied molecular orbital as a function of K incorporation for the metallic and insulating phases. This filling is not rigid band-like, and it reflects disorder in the K sites. Theoretical analysis indicates that KxC60 is a strong coupling superconductor, and we suggest that the enhanced electron-phonon interaction is related to the unique hybridization of the C sp-derived states.

Electronic properties of alkali trimers

The electronic properties of the alkali trimers Li3, Na3, and K3 are studied using the pseudopotential and the local‐spin‐density approximations. More than 100 configurations were calculated for each trimer in order to obtain a complete picture of the adiabatic Born–Oppenheimer surfaces. The equilibrium geometry of the trimers are Jahn–Teller distortions of an equilateral triangle. Although the three surfaces are quite similar, Li3 is more affected than Na3 or K3 by the dynamical character of the Jahn–Teller distortion. The calculated ionization potentials agree very well with the experimental values and the qualitative features of the Born–Oppenheimer surface are confirmed by recent ESR experiments.

Variational spherical model of small metallic particles

Abstract We study the structural and electronic properties of simple metal clusters with a model based on the density functional formalism. Our model takes into account electron relaxation effects and the lattice structure through the introduction of pseudopotentials. Results for the ionization potential and electron affinity, binding and surface energies and lattice relaxation are presented for bcc and icosahedral Na clusters having up to 350 atoms.

Analysis of occupied and empty electronic states of C60

Abstract We calculated the electronic structure of solid C 60 using a first principles pseudopotential local-density method. We present an analysis of the theoretical results and compare them to experimental photoemission and inverse photoemission spectra of solid C 60 . The agreement between theory and experiment is excellent. We give a simple interpretation of the electronic states of C 60 based on its quasi-spherical shape.

Current-induced magnetization switching in magnetic tunnel junctions

Current-induced magnetization switching (CIMS) in low-resistance magnetic tunnel junctions was shown at average critical current densities (Jc=1.33×106 A/cm2). When large vertical currents pass through the junctions, spin-transfer torque, and vortex fields can rotate the magnetization of the free layer from the initial parallel state to a vortex state, resulting in 10.8% CIMS resistance change at zero-bias current, which is about half of the resistance change (22%) induced when switching is created by an external field. A micromagnetic simulation including the spin-transfer torque and the vortex field correctly predicts the critical negative-current-inducing switching from the parallel state into the vortex state, but fails to explain the reverse switching from the vortex state into the parallel state at an approximately symmetric positive critical current. Lead fields were analyzed and found to be not the cause of the observed switching. The very small dependence of the switching currents on an external ...

First principles molecular dynamics of Li : test of a new algorithm

Abstract We have tested a new algorithm to perform first-principles molecular dynamics simulations. This new scheme differs from the Car-Parrinello method and is based on the calculation of the self-consistent solutions of the Kohn-Sham equations at each molecular dynamics timestep, using a fast iterative diagonalization algorithm. We do not use a fictitious electron dynamics, and therefore the molecular dynamics timesteps can be considerably larger in our method than in the Car-Parrinello algorithm. Furthermore, the number of basis functions is variable, which makes this method particularly suited to deal with simulations involving a cell with variable shape and volume. Applications of this method to liquid Li offers results which are in excellent agreement with experiment and indicates that it is basically comparable in efficiency to the Car-Parrinello method.

Metric tensor as the dynamical variable for variable-cell-shape molecular dynamics

We propose a variable-cell-shape molecular dynamics algorithm where the dynamical variables associated with the cell are the six independent dot products between the vectors defining the cell instead of the nine Cartesian components of those vectors. Our choice of the metric tensor as the dynamical variable automatically eliminates the cell orientation from the dynamics. Furthermore, choosing for the cell kinetic energy a simple scalar that is quadratic in the time derivatives of the metric tensor makes the dynamics invariant with respect to the choice of the simulation cell edges. Choosing the tensorial density of that scalar allows us to have a dynamics that obeys the virial theorem. We derive the equations of motion for the two conditions of constant external pressure and constant thermodynamic tension. We also show that using the metric as a variable is convenient for structural optimization under those two conditions. We use simulations for Ar with Lennard-Jones parameters and for Si with forces and stresses calculated from first principles of density-functional theory to illustrate the applications of the method. {copyright} {ital 1997} {ital The American Physical Society}

Pseudopotential spin-density-functional calculation of the electronic properties of small lithium and sodium clusters

Abstract We perform self-consistent pseudopotential calculations for the neutral clusters Lin, Nanand the singly ionized clusters Lin+, Nan+ with n ⩽ 4. We find the equilibrium geometries, and calculate the binding energies and the ionization potentials. Our results show good agreement with the existing experimental data and the available results of much more elaborate configuration interaction calculations.

Density functional calculations on the structure of crystalline polyethylene under high pressures

The geometrical structures of the crystalline polyethylene under several different external pressures up to 10 GPa are optimized by a pseudopotential plane wave density functional method. Both local density (LDA) and generalized gradient (GGA) approximations for exchange-correlation energy and potential are used. It is found that LDA heavily underestimate the geometry parameters under ambient pressure but GGA successfully correct them and get results in good agreements with the experimental geometry. The calculated GGA volume is about 94 A3 in comparison with the x-ray scattering value of about 92 A3 and the neutron scattering value of 88 A3. The bulk and Young’s modulus are calculated by means of several different methods. The Young’s modulus along the chain ranges from about 350 to about 400 GPa which is in good agreement with the experimental results. But the bulk modulus is several times larger than those of experiments, indicating a different description of the interchain interactions by both LDA and...