Javier Junquera

The SIESTA method for ab initio order-N materials simulation

We have developed and implemented a selfconsistent density functional method using standard norm-conserving pseudopotentials and a flexible, numerical linear combination of atomic orbitals basis set, which includes multiple-zeta and polarization orbitals. Exchange and correlation are treated with the local spin density or generalized gradient approximations. The basis functions and the electron density are projected on a real-space grid, in order to calculate the Hartree and exchange-correlation potentials and matrix elements, with a number of operations that scales linearly with the size of the system. We use a modified energy functional, whose minimization produces orthogonal wavefunctions and the same energy and density as the Kohn-Sham energy functional, without the need for an explicit orthogonalization. Additionally, using localized Wannier-like electron wavefunctions allows the computation time and memory required to minimize the energy to also scale linearly with the size of the system. Forces and stresses are also calculated efficiently and accurately, thus allowing structural relaxation and molecular dynamics simulations.An efficient method is described to handle mesh indexes in multidimensional problems like numerical integration of partial differential equations, lattice model simulations, and determination of atomic neighbor lists. By creating an extended mesh, beyond the periodic unit cell, the stride in memory between equivalent pairs of mesh points is independent of their position within the cell. This allows to contract the mesh indexes of all dimensions into a single index, avoiding modulo and other implicit index operations.

Critical thickness for ferroelectricity in perovskite ultrathin films

The integration of ferroelectric oxide films into microelectronic devices, combined with the size reduction constraints imposed by the semiconductor industry, have revived interest in the old question concerning the possible existence of a critical thickness for ferroelectricity. Current experimental techniques have allowed the detection of ferroelectricity in perovskite films down to a thickness of 40 Å (ten unit cells), ref. 3. Recent atomistic simulations have confirmed the possibility of retaining the ferroelectric ground state at ultralow thicknesses, and suggest the absence of a critical size. Here we report first-principles calculations on a realistic ferroelectric–electrode interface. We show that, contrary to current thought, BaTiO3 thin films between two metallic SrRuO3 electrodes in short circuit lose their ferroelectric properties below a critical thickness of about six unit cells (∼24 Å). A depolarizing electrostatic field, caused by dipoles at the ferroelectric–metal interfaces, is the reason for the disappearance of the ferroelectric instability. Our results suggest the existence of a lower limit for the thickness of useful ferroelectric layers in electronic devices.

Numerical atomic orbitals for linear-scaling calculations

The performance of basis sets made of numerical atomic orbitals is explored in density-functional calculations of solids and molecules. With the aim of optimizing basis quality while maintaining strict localization of the orbitals, as needed for linear-scaling calculations, several schemes have been tried. The best performance is obtained for the basis sets generated according to a new scheme presented here, a flexibilization of previous proposals. Strict localization is maintained while ensuring the continuity of the basis-function derivative at the cutoff radius. The basis sets are tested versus converged plane-wave calculations on a significant variety of systems, including covalent, ionic, and metallic. Satisfactory convergence is obtained for reasonably small basis sizes, with a clear improvement over previous schemes. The transferability of the obtained basis sets is tested in several cases and it is found to be satisfactory as well.

The SIESTA method; developments and applicability

Recent developments in and around the SIESTA method of first-principles simulation of condensed matter are described and reviewed, with emphasis on (i) the applicability of the method for large and varied systems, (ii) efficient basis sets for the standards of accuracy of density-functional methods, (iii) new implementations, and (iv) extensions beyond ground-state calculations.

Ferroelectricity and tetragonality in ultrathin PbTiO3 films

The evolution of tetragonality with thickness has been probed in epitaxial c-axis oriented PbTiO3 films with thicknesses ranging from 500 down to 24 A. High resolution x ray pointed out a systematic decrease of the c-axis lattice parameter with decreasing film thickness below 200 A. Using a first-principles model Hamiltonian approach, the decrease in tetragonality is related to a reduction of the polarization attributed to the presence of a residual unscreened depolarizing field. It is shown that films below 50 A display a significantly reduced polarization but still remain ferroelectric.

Stiff Monatomic Gold Wires with a Spinning Zigzag Geometry

Using first principles density functional calculations, gold monatomic wires are found to exhibit a zigzag shape which remains under tension, becoming linear just before breaking. At room temperature they are found to spin, what explains the extremely long apparent interatomic distances shown by electron microscopy.The zigzag structure is stable if the tension is relieved, the wire holding its chainlike shape even as a free-standing cluster. This unexpected metallic-wire stiffness stems from the transverse quantization in the wire, as shown in a simple free electron model.

Systematic generation of finite-range atomic basis sets for linear-scaling calculations

Basis sets of atomic orbitals are very efficient for density functional calculations but lack a systematic variational convergence. We present a method to optimize numerical atomic orbitals variationally, using a single parameter to control their range. The efficiency of the basis generation scheme is tested and compared with other schemes for multiple

Scaling of structure and electrical properties in ultrathin epitaxial ferroelectric heterostructures

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Finite-size effects in BaTiO3 nanowires

basis sets. The scheme is shown to be comparable in quality to other widely used schemes albeit offering better performance for linear-scaling computations.

First-principles calculation of the band offset at BaO/BaTiO3 and SrO/SrTiO3 interfaces

Scaling of the structural order parameter, polarization, and electrical properties was investigated in model ultrathin epitaxial SrRuO3∕PbZr0.2Ti0.8O3∕SrRuO3∕SrTiO3 heterostructures. High-resolution transmission electron microscopy images revealed the interfaces to be sharp and fully coherent. Synchrotron x-ray studies show that a high tetragonality (c∕a∼1.058) is maintained down to 50A thick films, suggesting indirectly that ferroelectricity is fully preserved at such small thicknesses. However, measurement of the switchable polarization (ΔP) using a pulsed probe setup and the out-of-plane piezoelectric response (d33) revealed a systematic drop from ∼140μC∕cm2 and 60pm∕V for a 150A thick film to 11μC∕cm2 and 7pm∕V for a 50A thick film. This apparent contradiction between the structural measurements and the measured switchable polarization is explained by an increasing presence of a strong depolarization field, which creates a pinned 180° polydomain state for the thinnest films. Existence of a polydomain ...