Biomedical subjects

Javier Junquera

Publications and source records attributed to Javier Junquera.

3 recordsLinked to original sources

PubMed2005

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.

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PubMed2003

Efficient mixed-force first-principles molecular dynamics.

We present an efficient method to mix well converged ab initio forces with simpler and faster ones in molecular dynamics. While the cheap forces are evaluated every time step, the converged ones correct the trajectory only every n time steps. For convenience, both types of forces are calculated with the same basic scheme, using density functional theory, norm-conserving pseudopotentials, and a basis set of numerical atomic orbitals. The cheap forces are evaluated with a short-range minimal basis set and the non-self-consistent Harris functional. Since these evaluations are hundreds of times faster than those of the converged forces, they add a negligible cost, and the boost in computational efficiency is approximately a factor n. Our results indicate that one can use values of n of up to 10, without affecting significantly the calculated structural and dynamical magnitudes.

Journal Article↗

PubMed2003

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 A (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 (approximately 24 A). 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.

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