Research news

María José Calderón

 Silicon electronics

We have performed effective mass theory calculations for donors and pairs of donors in Si including the central cell correction. This correction takes account of the fact that the donor potential is not properly screened very close to the donor position. The length scale involved, the central cell radius, is an empirical parameter fixed to reproduce the experimental binding energy of the neutral donor.  Once this empirical parameter is fixed for a particular donor species, we can reproduce with very good accuracy the energy of ionized donors and pairs of donors.  The details of the model and the calculations can be found in arXiv:1407.8224.  Our results have been used in the identification of a donor molecule in a silicon MOSFET device in Nano Letters 4, 5672 (2014)arXiv:1312.4589. This work is also the first report of a measurable exchange coupling in these devices. This research has been done in collaboration with researchers at the Universidade Federal do Rio de Janeiro (Belita Koiller, André Saraiva, Alejandra Baena), University of Wisconsin-Madison (André Saraiva), University of Cambridge (Andrew Ferguson, Dominik Heiss), and Hitachi Cambridge Laboratory (Fernando González-Zalba).

Individual donors  in Si can be now imaged by STM techniques. We have shown that the effective mass theory gives very good account of the experimental observations, not only for the prediction of the binding energies, but also for the shape of the wave-function (in particular, the anisotropy of the envelope function as appears in the original Kohn-Luttinger theory.  From the STM images it was also possible to estimate the position of the conduction band minima. The preprint can be found in arXiv:1508.02772. This work has been done in collaboration with researcher at the Universidade Federal do Rio de Janeiro (André Saraiva, Rodrigo Capaz and Belita Koiller) and at University of New South Wales (J. Salfi, J. Bocquel, B. Voisin and Sven Rogge).

With my colleague Belita Koiller at the KITPC, Beijing, July 2014.
With my colleague Belita Koiller at the KITPC, Beijing, July 2014.

Iron superconductors

Leni Bascones, Belén Valenzuela and I have been working on these materials since they were discovered in 2008.

We have recently submitted a review on magnetic interactions in Fe superconductores (arXiv:1503.04223).

We have studied the effect of correlations, doping and interband transitions on the optical conductivity of iron superconductors. Optical conductivity measurements are widely used to study the electronic properties of strongly correlated systems. We show how the interpretation of these measurements is very much complicated in multiorbital systems, where correlations can affect each band differently. The consequences for iron superconductors are reported in Phys. Rev. B 90, 115128 (2014) (arXiv:1407.6935). This work has been done in collaboration with Luca de’ Medici at ESPCI-ParisTech.

 Oxide interfaces

In collaboration with colleagues from Universidad Complutense de Madrid and ICMM, we have studied the properties of a multilayer formed by a metallic ferromagnetic manganite oxide (La0.7Sr0.3MnO3) and the insulating SrTiO3. Magnetoresistance measurements as a function of the relative angle between the magnetic field and the interface plane have shown an unexpected in-plane peak. Calculations of resistivity in a model system including spin-orbit coupling reveal that the unexpected in-plane maximum is due to transport through a two-dimensional system formed at the manganite interface. This is consistent with ab-initio calculations and XLD experiments. The magnetoresistance measurements thus expose the character of the electronic reconstruction occurring in this multilayer.

“Signatures of a two-dimensional ferromagnetic electron gas at the La0.7Sr0.3MnO3/SrTiO3 interface arising from orbital reconstruction”. Norbert Marcel Nemes, María José Calderón, Juan Ignacio Beltrán, Flavio Yair Bruno, Javier García-Barriocanal, Zouhair Sefrioui, Carlos León, Mar García-Hernández, Carmen Muñoz, Luis Brey, and Jacobo Santamaría. Advanced Materials. Published online Oct 18th, 2014.