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PGI-1 Seminar: Prof. Dr. Javier Junquera

Simulations of ferroelectric capacitors under different electrical boundary conditions using Numerical Atomic Orbitals

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04 Feb 2015 11:30
venue
PGI Lecture Hall

Univerdisad de Cantabria Santander, Spain

Abstract:

Ferroelectric oxide nanostructures have been a very attractive research topic in the last few years. The combination of theoretical methods with recent advances in ultrathin film growth and characterization techniques have led to the discovery of new phenomena in the nanoscale regime, and have opened the door to the design of oxide nanostructures with new and promising fuctionalities. First-principles simulations are playing an important role in this new research avenue, allowing to ascertain the behaviour of interfaces between different materials, and to gauge the relative influence of the different boundary conditions and the local chemical environment at the interface. In the theoretical description of the interfaces, the distortions of the
crystalline cells, the atomic displacements, the electronic structure, and, in particular, the band gap have to be accurately described simultaneously.
It is well known that the LDA and GGA usually lead to a significant understimation of the band gaps of the Kohn- Sham electronic band-structure. This DFT bandgap underestimate prevents accurate predictions of the barriers simultaneously for electrons and holes in metal/ferroelectric interfaces and can, in many cases, produce pathological situations where the Fermi level of the metal is erroneously aligned with the conduction bands of the ferroelectric producing unphysical population of the conduction bands of the ferroelectric. In this talk, analyzing the Projected Density Of States (PDOS) and the Local Density Of States (LDOS) around the Fermi energy, we discuss some of the fingerprints of selected pathological cases, both in unpolarized capacitors (where the ferroelectric material is in a centrosymmetric configuration), and in a polar capacitor where the underestimate of the electron barrier artificially lowers the breakdown field. Besides, we explore the possibility of performing first-principles simulations on ferroelectric nano structures at desired values of the dielectric displacement field using the "constrained-sigma" method, including some external charges at the surface using the virtual crystal approximation approach.
Some simulations on ferroelectric capacitors carried out within the Siesta method will be presented. One of the most important features that ensures the efficienty of the Siesta code is the use of a basis set of numerical atomic orbitals. The only requirement of the Siesta method with respect to basis sets is that they be made of functions that are a product of a radial function of finite support (zero beyond some radius) and a spherical harmonic. The cutoff radii can be defined at will by the user, and so can the shapes of the radial functions, (numerical table) and the number of basis functions for every angular momentum channel. The user can also defined where to put them (not necessarily centred on atoms). The flexibility is large, from minimal atomic basis sets, to generating an homogeneous grid of gaussians in space. The choices made by the user, however, can enormously affect both the efficiency and the accuracy of the calculations. The Siesta team firmly believes that the user should take responsibility on the choice of basis set. In this talk, we will give also some practical guidelines on how to generate good basis sets using the various tools available for it in Siesta.

Contact

Prof. Dr. Stefan Blügel
Phone: +49 2461 61-4249
Fax: +49 2461 61-2850
email: s.bluegel@fz-juelich.de

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