RESEARCH
Our group is at the forefront of the development of efficient methods for quantum-mechanical simulations, and also holds a very strong position in their application to a wide range of problems in Materials Physics and Chemistry, with a solid network of collaborations. Of particular relevance are the very important international impact of the SIESTA code for ab-initio simulation (developed, among others, by A. Garcia), the outstanding contributions and very wide network of collaborators of E. Canadell in the field of low-dimensional materials (including organic conductors), the world-class work of R. Rurali in nanowires, and the seminal work of M. Stengel on key fundamental issues in functional oxide systems. We maintain strong links with the Theory and Simulation Group at CIN2/ICN2.
Research Interests
Methodological developments
- Development of
the SIESTA code
The possibility of treating large systems with first-principles electronic-structure methods has opened up new research avenues in many disciplines. Since its first release in 1996, the SIESTA program has become quite popular and is increasingly being used by researchers in geosciences, biology, and engineering (apart from those in its natural habitat of materials physics and chemistry). SIESTA's efficiency stems from the use of strictly localized basis sets and from the implementation of linear-scaling algorithms which can be applied to suitable systems. A very important feature of the code is that its accuracy and cost can be tuned in a wide range, from quick exploratory calculations to highly accurate simulations matching the quality of other approaches, such as plane-wave methods. The use of atomic-like orbitals allows also a more straightforward analysis of the electronic-structure features.
The article describing the method has been recently chosen as one of the most influential papers in the 50-year history of the Journal of Physics series. As of November 2016, the paper has had more than 6000 citations, and the program has a user base in the thousands.
Applications
- First-principles modeling of complex phenomena in ferroelectric
and antiferroelectric systems
Ferroelectrics and antiferroelectrics are characterized by spontaneous electrical dipoles, which can align either parallel or antiparallel to each other in their low-temperature phase. Either type of macroscopic ordering leads to many useful functionalities (e.g. switchable remnant polarization, piezoelectricity, flexoelectricity), making these materials very interesting for applications in energy and information technologies. Perovskite oxides such as BiFeO3, BaTiO3 or PbZrO3 are the most promising compounds in this context, and by far the most intriguing: Many additional order parameters (magnetism, antiferrodistortive tilts of the oxygen octahedra) coexist, and in many cases strongly interact, with the polarization. This dramatically broadens the range of functional properties that can be achieved at bulk, interfaces and domain walls. In the past few years a wealth of phenomena has emerged where polarization, strain and antiferrodistortive degrees of freedom interact in highly nontrivial ways, and such interactions are further modified by spatial inhomogeneities in the order parameter (domain walls), in the composition (surfaces or interfaces), or in both. Notable examples include the observation of polarization vortices in ferroelectric domain structures, the emergence of polarity at ferroelastic domain boundaries, hybrid-improper mechanisms for inducing polarity and magnetoelectricity in layered systems, the mechanical control of polarization via flexoelectricity, and photovoltaic effects at phase boundaries. All these instances challenge the conventional picture of ferroelectricity, and require advanced theoretical modeling techniques in order to be properly understood.
- Low-dimensional materials
Low-dimensional materials have been and remain one of the strong research interests of our group. In the case of metallic systems, the reduced dimensionality often leads to particular topologies of the Fermi surface exhibiting nesting properties which are responsible for different electronic instabilities. Some of the materials in which we have been interest include different molecular conductors and superconductors, transition metal oxides and chalcogenides, etc. Most of our work is carried out in collaboration with experimental groups studying these systems by means of ARPES, NEXAFS, magnetoresistance, x-ray diffuse scattering, etc. On the other hand, the possibility to prepare single-layers or flakes with a very small number of layers of bulk layered materials has opened the possibility to study how the reduced electronic screening brought about by the reduction of dimensionality from bulk to layers of different thickness influences the transport, optical and other physical properties of these materials. Recently work along this line has been carried out concerning systems like 2H-NbSe2, 1T-TiSe2, MX3 (M = Ti, Zr; X = S, Se, Te), InSe, etc.
- Nanowires for novel devices
One-dimensional semiconductor nanostructures are attracting great interest for their potentially high impact in emergent nanoelectronics devices. Silicon nanowires appear to be an especially appealing choice, due to their ideal interface compatibility with conventional Si-based technology.
- Nanoscale heat transport
We use a wide range of theoretical techniques —ranging from nonequilibroum molecular dynamics, to Green’s functions and solution of the Boltzmann transport equation based on first-principles force constant calculations— to study thermal transport at the nanoscale. We focus on phononics applications, where heat flow is used to encode and transmit information. This is a challenging task, because phonons have no mass and no charge, thus their propagation cannot be modulated with an external field
Last updated: 16-XI-2016
Members of the ICMAB electronic structure group, with a guest appearance by Dr. Jordi Faraudo.
Visualization of the lone-pairs associated to the As atoms in the mineral Karibibite
Fermi surface of (PO2)4(WO3)2m (m=4). From E. Canadell and coworkers.
Thermal diode based on a telescopic nanowire. Nanowires with different diameters have different vibrational properties, thus control of their size offer an additional way of tuning the thermal transport of nanowires.