Objectives

The goal is to study new properties of modern materials and devices from a theoretical point of view. We aim at understanding experimental data and making experimental
predictions, with the major goal of being able to quickly react to new developments and to tackle novel materials and problems in a very agile way.

Funding

Projects Financed by the Industry

1. Computación y modelado en la nanoscala: Asesoramiento científico.
Period: 1/1/2006 - 31/12/2009
Financing Agency: Fundación PHANTOMS
Total Amount (euros): 20.153
Project Leader: Serena Domingo, P.A.
Students: Peláez Machado, S.

Projects Financed by CICYT, SEUID, MEC-MICINN

1. Light control on nanoscale. Nanolight.

CONSOLIDER. (CDS-2007 00046).

Periodo: 1/12/2007 - 30/11/2012

Fuente de financiación: MEC

Importe total (euros): 6.800.0000

Coordinador: N.F. Van Hulst, Instituto Ciencias

Fotónicas, Barcelona

Investigador principal subproyecto: Nieto-Vesperinas, M.

Importe (euros): 223.850

Investigadores: Gomez Medina, R.

Becarios y Doctorandos: Valdivia Valero, F.J.

2. Efectos de la correlación electrónica en materiales y en sistemas mesoscópicos (FIS2005-05478-C02-01)
Period: 31/12/2005 - 30/06/2009
Financing Agency: MEC
Total Amount (euros): 171.360
Project Leader: López Sancho, M. Pilar
Scientists: Guinea, F.; Vozmediano, M.A.H.; Gomez Santos, G.; Bascones, E.; Valenzuela, B.; Stauber, T.
Students: Roldán, R.; Seoanez, C.; Cortijo, A.

3. Nuevos conceptos y nuevos materiales para su utilización en espintrónica y nanoelectrónica (MAT2006-03741).
Period: 1/1/2007 - 31/12/2009
Financing Agency: MEC
Total Amount (euros): 121.000
Project Leader: Aguado Sola, R.
Scientists: Brey Abalo, L.; Vergés Brotons, J.A.; Calderón Prieto, M.J.;
Students: Salafranca Laforga, J.; Marcos de la Torre, D.

4. Propagación de electrones y fotones en estructuras complejas con heterogeneidades nano-y micrométricas (ELFO). (FIS2006-

11170-C02-01)

Periodo: 1/10/2006 - 30/9/2009

Fuente de financiación: MEC

Importe total (euros): 62.000

Investigador principal: Serena Domingo, P.A.

Investigadores: Nieto-Vesperinas, M. Gomez Medina R.

Becarios y Doctorandos: Peláez Machado, S.; García

Pomar, J.L., Valdivia valero, F.J.

5. Transporte electrónico de carga y espín en nanodispositivos semiconductores (MAT2005_00644).
Period: 2006 - 2009
Financing Agency: MEC
Project Leader: Platero Coello, G.
Scientists: Iñarrea, J.
Students: Sánchez, R.; Pérez-Monís, C.

6. Dinámica y estructura de fluidos complejos y sus interfases (FIS2007-65869-C03).
Period: 1/10/2007 - 3/8/2010
Financing Agency: MEC
Total Amount (euros): 21.780
Project Leader: Chacón Fuertes, E.

7. Simulación mecano-cuántica conjunta de electrones y núcleos atómicos en sólidos y moléculas (FIS2006-12117-C04-03).
Period: 1/10/2006 - 30/9/2009
Financing Agency: MEC
Total Amount (euros): 30.734
Project Leader: Herrero Aísa, C.
Scientists: Ramírez Merino, R.

Projects Financed by the Autonomous Community
of Madrid

1. Propiedades mecánicas, eléctricas y catalíticas de nanoobjetos: síntesis, caracterización y modelización (FIS2006 S-0505/MAT/0303).
Period: 01/10/2006 - 31/12/2009
Financing Agency: Comunidad de Madrid
Total Amount (euros): 53.684
Project Leader: Serena Domingo, P.A.
Students: Peláez Machado, S.

Research

1. Doped Si-based quantum computing
We have analyzed some aspects that are relevant to the quantum control of donor bound electrons in silicon. In particular, we have studied the effect of the Si conduction band valley degeneracy on the manipulation of donor bound electrons close to a Si/SiO2 interface, as a function of the valley-orbit coupling at the interface. The interference between the valleys leads to oscillations (as a function of the distance between donor and interface) on the tunneling time required to take the electron from the donor to the interface by means of an external electric field. These oscillations disappear when the ground-state is degenerate (namely, when the valley-orbit coupling at the interface is zero) [1]. As a result, a precise positioning of donors may be required for the practical implementation of a doped Si quantum computer.

1. M.J. Calderón, B. Koiller, and S. Das Sarma, Phys. Rev. B, 77, 155302, 2008
Proyectos: Programa Ramón y Cajal, MAT2006-03741 (MICINN)

3. Effect of the tetrahedral distortion on the electronic properties of iron pnictides
The discovery of high-temperature super-conductivity in iron pnictides in February 2008 marks a new hit in the history of superconductivity. As in cuprates when doping an antiferromagnetic phase superconductivity sets. Moreover all the relevant physics seems to happend in the arsenic-iron planes. However, in pnictides the antiferromagnetic phase is metallic. We have studied another important difference between the two compounds. The iron environment due to the arsenic is tetrahedral instead tetragonal as in cuprates. This makes that this compound is very sensitive to structural changes. Experimentally it has been seen that the magnetic and superconducting properties vary with the distance between the arsenic to the iron plane. We have studied the effect of the tetrahedral distortion with the tight-binding approximation and we have seen that the distortion has a strong influence in the electronic properties. At present we are including correlations to study the effect of the distortion of the tetrahedron in magnetism and superconductivity.
1. M.J. Calderón, B. Valenzuela and E. Bascones (arXiv:0810.0019, aceptado en New Journal of Physics)
Proyectos: 1) Efectos de correlación electrónica en materiales y en sistemas mesoscópicos, Project Leader: Pilar López Sancho. 2) Efectos de la correlación electrónica en sistemas de baja dimensionalidad, CCG07-CSIC/ESP-2323. Investigadora principal: Elena Bascones.

3. Competing scenario in high temperature superconductors -cuprates-
Cuprates, discovered in 1986, are quasi-2D antiferromagnetic Mott insulators that with doping become high temperature superconductors. Between the Mott insulator and the superconducting phase there is an enigmatic phase called pseudogap. All the relevant physics seems to occur in the copper-oxygen planes. Though these systems are among the best characterized materials, the nature of the pseudogap and the mechanism of superconductivity is still unknown. In our work we study a scenario where the pseudogap competes with superconductivity and we calculate the spectra in angle resolved photoemission and electronic Raman scattering. For electron doped cuprates the competing phase is antiferromagnetism and for hole doped cuprates the competing phase is less conventional: a Luttinger surface. Curiosly, due to the different truncation induced by the pseudogap, the results are very different for the two superconductors. The obtained results agree with the experimental data. We have also shown that the used formalism to study the pseudogap is able to reproduced the phenomenology observed in STM experiments, in particular in the so-called checkerboard pattern.
1. B. Valenzuela and E. Bascones, Phys. Rev. B 78, 174522 (2008)
2. E. Bascones and B. Valenzuela, Phys. Rev. B 77, 024527 (2008)
Proyectos: 1) Efectos de correlación electrónica en materiales y en sistemas mesoscópicos, Investigadora principal: Pilar López Sancho. 2) Efectos de la correlación electrónica en sistemas de baja dimensionalidad, CCG07-CSIC/ESP-2323. Investigadora principal: Elena Bascones.

4. All-manganite heterostructures
It has been observed that interfaces between different oxides can show properties very different from the bulk (electronic reconstruction). We have focused on all-manganite heterostructures, where the components have different properties. In a trilayer with a thin antiferromagnetic and insulating layer sandwiched between two ferromagnetic and metallic layers, we have found that the electronic structure of the intermediate layer depends on the relative orientation of the magnetization on the electrodes, leading to a large magnetoresistance [1]. We have also shown that a metallic and ferromagnetic electron gas can appear at the interface between two antiferromagnetic and insulating manganites of different composition [2].
1. J. Salafranca, M.J. Calderón, and L. Brey, Phys. Rev. B, 77, 014441, 2008
2. M.J. Calderón, J. Salafranca, and L. Brey, Phys. Rev. B, 78, 024415, 2008
Proyectos: Programa Ramón y Cajal, MAT2006-03741 (MICINN)

5. Pomeranchuk instability in doped graphene
Graphene a monolayer of graphite (i.e a layer of carbon atoms arranged in a hexagonal lattice) has been recently synthesized (2004). The material has many surprising properties. The fact that a strictly two dimensional system exists is by itself a big surprise. Two of the most searched properties in graphene have been ferromagnetism and superconductivity but the results have been modest due to the low density of states at the Fermi level. Recently graphene has been doped till the Van Hove level which is a critical filling with a high density of states. At this filling many instabilities can arise. In particular, we have studied that it is possible to find a Pomeranchuk instability where due to the electronic correlations the Fermi surface breaks the symmetry of the underlying ion lattice. We have also studied the competition between Pomeranchuk instability and ferromagnetism. At present we are studying the competition between superconductivity and Pomeranchuk instability.
1. B. Valenzuela and M.A.H. Vozmediano, New Journal of physics 10, 113009 (2008)
Proyectos: Efectos de correlación electrónica en materiales y en sistemas mesoscópicos, Investigadora principal: Pilar López Sancho

6. Electron-boson interaction in quantum dots: transport and quantum fluctuations
We analyze the electronic transport through a nano-electro-mechanical system (NEMS) consisting on a mobile triple quantum dot. We describe the current through the system under radiation. Photon and phonon side bands are the tunneling channels and we deduce selection rules for the current. In other work we propose a model for obtaining charge and photon fluctuations in a quantum dot coupled to electric baths. The aim is to analyze the influence of the electronic transport in the emission of photons and viceversa. We analyze the Full Counting Statistics for electrons, photons and their correlation as a function of the intensity and frequency of radiation. We propose configurations where the electron-photon correlation is maximum and spontaneous photon emission with well defined energy is regulated through electron tunneling.
1. Tunnel spectroscopy in ac-driven quantum dot nanoresonators, Villavicencio J, Maldonado I, Sánchez R., Cota E., Platero G., Applied Physics Letters,92, 192102 (2008).
2. Resonance fluorescence in driven quantum dots: Electron and photon correlations, Sánchez R.; Platero G.; Brandes T., Physical Review B, 78, 125308 (2008).
Proyectos: MAT2005-00644:Transporte Electrónico de carga y espín en nanodispositivos semiconductores.

7. Magnetotransport in a two dimensional electron gas under microwave radiation
We present a theoretical model to study the effect of microwave radiation on Weiss oscillations (WO) in a 2DEG in a perpendicular magnetic field. In our proposal WO, produced by a spatial periodic potential, are modulated by microwaves. Depending on the spatial period and the frequency of radiation, we predict that WO can reach zero resistance states (ZRS). On the other hand, ZRS induced by radiation, can be destroyed by space-dependent periodic potentials. We also analyze the effect of in-plane magnetic fields on the microwave-assisted transport in a 2DEG. We discuss recent experiments in terms of the microwave-driven harmonic motion performed by the electronic orbits and how this motion is affected by the in-plane field.
1. Driving Weiss oscillations to zero resistance states by microwave Radiation, Iñarrea J.; Platero G, Applied Phys. Letters, 93, 062104 (2008).
2. Effect of an in-plane magnetic field on microwave-assisted magneto-transport in a two-dimensional electron system, Iñarrea J.; Platero G., Physical Review B, 78, 193310 (2008).
Proyectos: MAT2005-00644:Transporte Electrónico de carga y espín en nanodispositivos semiconductores.

8. Transport in electrically tunable nanomagnets
In this research line, we study a single electron transistor (SET) based upon a II-VI semiconductor quantum dot doped with a single Mn ion. We present evidence that this system behaves like a quantum nanomagnet whose total spin and magnetic anisotropy depend dramatically both on the number of carriers and their orbital nature. Thereby, the magnetic properties of the nanomagnet can be controlled electrically. Conversely, the electrical properties of this SET depend on the quantum state of the Mn spin.
1. Optical probing of spin fluctuations in a single magnetic atom, L. Besombes, Y. Leger, H. Boukari, J. Bernos, H. Mariette, J. P. Poizat, J. Fernández-Rossier and R. Aguado, Physical Review B, 78, 125324-1-125324-9, 2008
Proyectos: Nuevos Conceptos y Nuevos Materiales para su utilización en Espintrónica y Nanoelectrónica, MAT2006-03741, MEC.

9. Capillary waves at liquid interfaces
The presence of thermal capillary waves at free liquid surfaces produces a smoothening of the surface density profiles, increasing with the size of the sampled area, and also long-ranged transverse surface correlations, which are superimposed on the typical correlations of the bulk liquid. In previous work we have developed a procedure to explore these questions with computer simulations, through an operational definition of the intrinsic surface, which gives the instantaneous boundary between the liquid and vapour phases, in a liquid-slab simulation cell. According to the CW theory, the intrinsic density profile associated to the distance of the molecules to that intrinsic surface, eliminates the blurring effect of the capillary waves on the mean density profile. We have shown that, with the appropriate definition of the intrinsic surface, the surface layering is a quite generic property, rather than a peculiarity of metallic liquids, or systems with very low triple to critical temperature ratio. Moreover, the application to molecular liquids, like water, has shown that the intrinsic surface structure contains very useful local information which is lost in the transverse average by the capillary waves blurring effect. During that year we have applied our intrinsic sampling method to two subjects, the first one has been the Oil-Water interface and the study of the intrinsic structure of hydrophobic surfaces [1]. The second one has been directed to the extension of the method to analyze the self-diffusion, and other dynamical properties of the system, and to explore the merging of the hydrodynamic capillary waves at the long-wavelength limit, with the molecular level of description [2].
1. Physical Review Letters, 101 056102 (2008)
2. Physical Review Letters, 101 106102 (2008)

10. Molecular dynamics optimisation of metallic nanostructures
We study the appearance of high-stability structures in metallic nanocontacts and nanowires. These “magic” configurations present electronic or ionic character, and they are of fundamental interest to determine favorable nanowires configurations of potential use in future nanoelectronics. Using Molecular Dynamics we analyze the nanowire evolution under stretching conditions, explaining the experimental conductance histograms.
1. P. García-Mochales et al. Journal of Nanomaterials, 2008, 361464 (2008).
2. P. García-Mochales et al., Physica Status Solidi, 205, 1317-1323 (2008).
3. P. García-Mochales et al. Nanotechnology 19, 225704 (2008).
Proyectos: MEC FIS2006-11170-C02-01, S-0505/MAT/0303

11. Transport properties of nanoparticle arrays
The electronic transport through metallic nanoparticles is strongly influenced by the quantization of the charge. This effect is enhanced in nanoparticle arrays. During this year we have finished a detailed work on the transport properties of one-dimensional metallic nanoparticle arrays which focuses on threshold voltages and on the spatial distribution of potential drops across the array both below and above thresholds. We have studied dependences on array parameters and analyzed the roles of charge and resistance disorder. We have also started a new project to study the interplay between charging energy effects and ferromagnetism. To this end we consider several cases within two limits. The limits of long and short spin relaxation time. In the second case we asume that both the nanoparticles and the electrodes are ferromagnetic and in the first case only the electrodes are ferromagnetic.
1. E. Bascones, V. Estévez, J.A. Trinidad and A.H. MacDonald, Phys. Rev. B 77, 245422 (2008)
Proyectos: 1) Efectos de la correlación electrónica en sistemas de baja dimensionalidad CCG07-CSIC/ESP2323. 2) Efectos de correlación electrónica en materiales y en sistemas mesoscópicos FIS2005-05478-C02-01

12. Negative and anomalous refraction of electrons in graphene
We propose a device to break the valley degeneracy in graphene and produce fully valley-polarized currents that can be either split or collimated to a high degree in a experimentally controllable way. The proposal combines two recent seminal ideas: negative refraction and the concept of valleytronics in graphene. The key new ingredient lies in the use of the specular shape of the Fermi surface of the two valleys when a high electronic density is induced by a gate voltage (trigonal warping). By changing the gate voltage in a n-p-n junction of a graphene transistor, the device can be used as a valley beam splitter, where each of the beams belong to a different valley, or as a collimator. The result is demonstrated through an optical analogy of negative and anomalous refraction with two-dimensional photonic crystals.
1. J.L. Garcia-Pomar, A. Cortijo y M. Nieto-Vesperinas, Fully Valley-Polarized Electron Beams in Graphene, Physical Review Letters 100, 236801-1 - 236801-4 (2008).
Proyectos: Light control on nanoscale. Nanolight. CONSOLIDER. (CDS-2007 00046).

13. Quantum noise and correlations in nanoelectronics
Quantum fluctuations have to be contrasted with classical and thermal fluctuations which, as an equilibrium property, are well-understood. In contrast, transport in nanoscale-systems is almost always a non-equilibrium phenomenon, and the associated quantum noise is by now been regarded as a one of the primary tools to extract invaluable information on the transport process. Broadly speaking, quantum noise is related to (in principle arbitrarily complicated) correlation functions. Already at the lowest level, the second-order zero-frequency current-correlation function (shot noise) provides us with information beyond that contained in usual DC transport experiments. In the past few years it has turned out that a complete understanding of transport requires to go beyond shot noise and to study the full counting statistics (FCS), which yields all zero-frequency current-correlation functions at once. In the future, non-trivial cross-correlation measurements at low frequencies on multiterminal devices could give a deeper understanding of spin- and orbital entanglement. The zero frequency noise correlator, which is accessible in the experiment, is a tool to measure the joint detection probability for two particles
1. Entanglement between charge qubits induced by a common dissipative environment, L. D. Contreras and R. Aguado
Physical Review B, 77, 155420-1-155420-9, 2008.
Proyectos:Nuevos Conceptos y Nuevos Materiales para su utilización en Espintrónica y Nanoelectrónica, MAT2006-03741, MEC.
Full counting statistics and noise correlations in open qubits. Ministerio de Educación y Ciencia de España (MEC) - Deutscher Akademischer Austauschdienst (DAAD). Acción integrada: CSIC/ Technische Universität Berlin

14. Quantum-mechanical simulation of electrons and atomic nuclei in solids and molecular systems
This project deals with quantum simulations of solids and molecular systems. Our main goal focuses on the influence of the quantum nature of atomic nuclei on observable magnitudes. The most important non-trivial effects of this kind are due to the anharmonicity of the interatomic potentials. Such anharmonicity affects the molecular vibrations and phonons in solids, even at low temperatures, as well as structural (such as interatomic distances and lattice parameters), thermodynamic (as the heat capacity and bulk modulus), and electronic properties (gap renormalization). A well-suited technique to study these effects is provided by Feynman’s path integrals. This technique, combined with Monte Carlo or molecular dynamics simulations, has allowed us to study several properties at finite temperatures. Among the most interesting applications of this method, we have studied the effects of quantum nuclear fluctuations on structural and electronic properties of molecules, as well as the diffusion of light impurities in solids and phase diagrams of pure substances.
1. R. Ramírez and C.P. Herrero, J. Chem. Phys. 129, 204502 (2008).
2. R. Ramírez, C.P. Herrero, E.R. Hernández, and M. Cardona, Phys. Rev. B 77, 045210 (2008).
3. C.P. Herrero, J. Phys.: Condens. Matter 20, 295230 (2008).
Proyectos: FIS2006-12117-C04-03

15. Charge and spin transport in quantum dots
Our research consists on a theoretical analysis of electron spin dynamics and transport through double quantum dots. We have addressed different topics: we have analyzed the effect of hyperfine interaction in these systems on the transport in the spin blockade regime, we have proposed a model to analyze the dynamical nuclear polarization and its role in the electronic transport[1]. We also analyze the spin dynamics in these systems in presence of crossed dc and ac magnetic fields. We have investigated the interplay between Rabi oscillations due to interdot tunneling and coherent spin rotations induced by the ac magnetic field. We discuss how the magnetic field is able to trap electrons in the system under certain conditions[2]. We have as well analyzed the charge and spin shot noise and Fano factor in double quantum dots in the presence of time dependent electric and magnetic fields[3].
1. Role of Dynamic Nuclear Polarization on the transport through weakly coupled double quantum dots, Iñarrea J.; Platero G.,J. Phys. D: Appl. Phys. 41 195104 (2008).
2. Coherent spin rotations in open driven double quantum dots, Sánchez R., López-Monís C., Platero G., Phys. Rev. B, 77, 165312 (2008)
3. Spin correlations in spin blockade, Sánchez R.; Kohler S.; Platero G.,
New Journal of Physics 10, 115013 (2008).
Proyectos: MAT2005-00644:Transporte Electrónico de carga y espín en nanodispositivos semiconductores.

Ph.d Theses