Miércoles, 9 mayo 2012. 12:00-13:00
Digital Quantum Simulation with Rydberg Atoms and Ions
Markus Müller

Departamento de Física Teórica, Universidad Complutense de Madrid

ABSTRACT:

A universal quantum simulator is a controlled quantum device that faithfully reproduces the dynamics of any other many-particle quantum system with short-range interactions. This dynamics can refer to both coherent Hamiltonian and dissipative open-system time evolution. In our talk we present a scheme, where laser-excited Rydberg atoms in optical lattices provide an efficient implementation of such a universal digital quantum simulator. After a short introduction to some basic concepts of quantum simulation and atomic Rydberg physics, we discuss how the proposed simulation architecture allows one to realize coherent Hamiltonian as well as dissipative open-system time evolution of spin models involving n-body interactions, such as e.g. Kitaev's toric code and more complex lattice gauge theories. Our simulator relies on a combination of multi-atom Rydberg gates and optical pumping to implement coherent operations and dissipative processes. We also report on recent experiments with trapped ions, which have demonstrated these concepts and the building blocks of such an open-system quantum simulator in the laboratory.

Miércoles, 25 abril 2012. 12:00-13:00
Magnetic ground states in transition-metal oxides driven by superexchange interactions
Sergio di Matteo

Université de Rennes 1, France

ABSTRACT:

The aim of the present seminar is to provide a general introduction on the possible effects of superexchange interactions on magnetic ground states in transition-metal oxides. Three examples are introduced and discussed in some details: a) the case of spinel MgTi2O4; b) the case of spinel ZnV2O4; c) the case of Cr-doped TiO2. The first two cases are a useful example to compare the differences in the magnetic ground states of two isostructural compounds characterized by a different filling. At the same time, case a) is also extremely pedagogical in clarifying the origin of magnetoelastic forces determined by exchange and superexchange interactions. Interestingly, in this case an exact solution of the superexchange Hamiltonian on the whole lattice can be found, leading to the formation of a so-called non-resonating valence-bond crystal. Case c), finally, is more complex to discuss, but it is introduced because nowadays the search for ferromagnetism in transition-metal doped compounds is very on fashion and the results obtained here are particularly interesting as a possible explanation for a global ferromagnetic coupling in the Cr-doped TiO2 cell.

Miércoles, 18 abril 2012. 12:00-13:00
Integrable Richardson-Gaudin models in mesoscopic physics
Jorge Dukelsky

Instituto de Estructura de la Materia, CSIC

ABSTRACT:

The exact solution of the BCS Hamiltonian with non-degenerate single particle orbits was introduced by Richardson in the early sixties. Although it passed almost unnoticed, it was recovered in the last decade in an effort to describe the disappearance of superconductivity in ultrasmall superconducting grains. Since then it has been extended to several families of integrable pairing models, the Richardson-Gaudin models. However, only the rational family has been widely applied to mesoscopic systems where finite size effects play an important role. Even in the thermodynamic limit, the exact many-body wavefunction provides a unique view to the Cooper pair structure in the BCS-BEC crossover.

Two complementary implementations of the hyperbolic Richardson-Gaudin family have been recently found in condensed matter and nuclear physics. The first implementation gives rise to a p-wave pairing describing a gas of spinless fermions in a 2D lattice with $p_x + i p_y$ pairing symmetry. Using this new tool we study the quantum phase diagram which unlike the case of s-wave pairing displays a third order quantum phase transition. We make use of the exact solution to characterize the quantum phase transition and the properties of the weak and strong paring phases. The exact wavefunction of the p-wave pairing Hamiltonian gives a beautiful insight into the nature of the quantum phase transition. Moreover, it suggests the existence of an experimentally accessible characteristic length scale, associated with the size of the Cooper pairs, that diverges at the transition point. The second implementation leads to a separable pairing Hamiltonian with two free parameters that can be adjusted to give an excellent reproduction of the superfluid properties of heavy nuclei. As such it might be useful to treat other mesoscopic systems like superconducting grains or quantum dots.

Miércoles, 11 abril 2012. 12:00-13:00
Non-Abelian gauge fields in twisted bilayer graphene
Pablo San Jose

Instituto de Estructura de la Materia, CSIC, Madrid

ABSTRACT:

We present an overview of the modeling of low energy twisted graphene bilayers and of its unusual electronic properties. These include charge confinement around zero energy and velocity suppression at recurrent special twist angles. We show that these come as a consequence of non-Abelian gauge fields that arise from the spatial Moiré pattern in interlayer couplings.

Miércoles, 28 marzo 2012. 12:00-13:00
Dynamical Simulations of Virus Wrapping and Budding
Teresa Ruiz Herrero

Departamento de Física Teórica de la Materia Condensada, UAM

ABSTRACT:

Enveloped viruses bud through the cell membrane as the final step in their replication process.  For many enveloped viruses, a nucleo-protein capsid first assembles in the cytoplasm, attaches to the membrane, and then buds.
We explore this process through modeling the wrapping of a spherical particle by a model bilayer membrane, using coarse-grained molecular dynamics simulations and a theoretical elastic model. Specifically, we study the kinetics and morphologies of wrapping as a function of the relevant system parameters, including the particle radius, the strength of the membrane-particle interaction, and the membrane bending rigidity. The theoretical model predicts a phase diagram as a function of the system parameters, which is compared to results of the dynamics simulations.
Furthermore, the simulations elucidate the dynamical mechanisms by which budding occurs and the structures of intermediate configurations.

Miércoles, 21 marzo 2012. 12:00-13:00
DFT analysis of combined 3D NC-AFM and STM imaging of the Cu(100)-O oxide surface
Milica Todorovic

Departamento de Física Teórica de la Materia Condensada, UAM

ABSTRACT:

Investigation of novel catalytically active surfaces requires a comprehensive experimental method for the identification and rapid characterization of prospective catalytically active sites. In a bid towards functional imaging, three-dimensional atomic force microscopy (3D-AFM) in non-contact mode [1] has been combined with scanning tunnelling microscopy (STM) to study the oxygen-terminated copper (100) surface. Complex 3D data sets, obtained by simultaneously recording the tunnelling current and the AFM frequency shift, allow for site-specific quantification of forces and tunneling currents. The wealth of information obtained is remarkable, but the interpretation of the wide range of contrast modes requires a thorough characterisation of the sources of contrast in AFM and STM imaging.

We combined DFT total-energy calculations with Non-Equilibrium Green’s Function (NEGF) methods for electronic transport to determine the tip-surface interaction and tunnelling current [2, 3, 4] for a large set of tip models in order to clarify the different contrast modes obtained in the experiments. We studied the features of a stable Cu(100)(2√2x√2)R45deg-O surface reconstruction and identified prospective reaction sites, before introducing model AFM tips to conduct a series of tip approach simulations. The effect of tip changes on imaging modes was explored by considering tips of different reactivity. Our simulations, in comparison with AFM experimental images, identified a contaminated tip with a Cu-terminated experimental configuration. Charge density and current calculations further helped us investigate the STM imaging modes for tips of different reactivity. Through this work, were able to explain a large variety of experimental STM contrasts obtained. Lastly, simulations of different surface defect models enabled us to understand detailed STM image features and led us to consider the mechanisms of domain formation on the Cu(100)-O surface.

[1] B. J. Albers et al., Nature Nanotech. 4, 307 (2009)
[2] Y. Sugimoto et al., Nature 446, 64 (2007)
[3] P. Jelinek et al, Phys. Rev. Lett. 101, 176101 (2008)
[4] J. M. Blanco, F. Flores and R. Pérez, Prog. Surf. Sci. 81, 403 (2006)

Miércoles, 14 marzo 2012. 12:00-13:00
Understanding nc-AFM contrast on TiO2 and water adsorption on CeO2
Delia Fernández-Torre

Departamento de Física Teórica de la Materia Condensada, UAM

ABSTRACT:

Metal oxides are important for a wide range of technological applications [1]. In order to optimize these proccesses or find new ones it is essential to understand their surface properties and chemistry in detail. Atomically-resolved scanning probe microscopy techniques, like non-contact atomic force microscopy (nc-AFM) [2], combined with theoretical simulations play a crucial role in this respect. In this seminar I will give an overview of our recent and ongoing work on two representative oxide surfaces: TiO2(110) and CeO2(111). I will start by showing how a combination of site-specific force spectroscopy measurements on TiO2(110) and first-principles calculations clarifies the origin of the nc-AFM contrast and let us characterize the tip structures responsible for the two most common imaging modes [3]. The same model tips can be applied to related systems, like single metal atoms (K, Pt) adsorbed on TiO2(110). I will then consider the problem of water adsorption and pre-dissociation on CeO2(111). When modelling ceria, one has to be specially careful with the approach used for the exchange-correlation functional, as the surface is easily reduced, and the electrons tend to localize on f-states [4]. For water on the clean surface, all our first-principles simulations performed at different levels of theory point to the same solution: water can be adsorbed either molecularly or pre-dissociatively, with adsorption energies in agreement with the experimental data available, and a small barrier (about 0.15 eV) connecting the two structures [5]. Finally, I will show some preliminary results for hydrogen adsorption and dissociation on CeO2(111).

[1] U. Diebold, Surf. Sci. Rep. 48, 53 (2003)
[2] R. García and R. Pérez, Surf. Sci. Rep. 47, 197 (2002)
[3] A. Yurtsever et al., Phys. Rev. B, accepted.
[4] M.V. Ganduglia-Pirovano et al., Surf. Sci. Rep. 62, 219 (2007)
[5] D. Fernández-Torre et al., submitted.

Miércoles, 7 marzo 2012. 12:00-13:00
Semiconductor nanostructures grown on GaAs nanoholes for quantum optical information technologies
Benito Alén

Instituto de microelectrónica de Madrid, CSIC

ABSTRACT:

Single semiconductor quantum dots embedded monolithically in photonic or electronic devices are a fundamental resource for quantum information science and technology.[1] Individual charges and spins can be addressed in these systems through combined electrical and optical manipulation.[2] Quantum information processing tasks can be implemented using the individual spins of a quantum dot molecule as quantum bits.[3] Also, when embedded in a high quality photonic crystal microcavity, spins can be coherently mapped into single photons enabling quantum non-demolition optical read-out or long distance quantum communications.[4]

Despite the considerable progress done so far, standard fabrication methods are based on self-assembled processes which lead to the random nucleation of the nanostructures over the semiconductor substrate. This is a serious drawback for the deterministic and scalable fabrication of the building blocks mentioned above. In this talk, I will present how semiconductor nanostructures grown directly over pre-patterned substrates could in principle solve this problem. InGaAs quantum dots and quantum dot molecules grown directly on GaAs nanoholes will be presented and their optical and electronic properties will be discussed. [5-8]

1.Faraon, A. et al. Integrated quantum optical networks based on quantum dots and photonic crystals. New Journal of Physics 13, 055025 (2011).
2.Atature, M. et al. Quantum-Dot Spin-State Preparation with Near-Unity Fidelity. Science 312, 551–553 (2006).
3.Robledo, L. et al. Conditional Dynamics of Interacting Quantum Dots. Science 320, 772 –775 (2008).
4.Rakher, M. T. el al. Externally Mode-Matched Cavity Quantum Electrodynamics with Charge-Tunable Quantum Dots. Phys. Rev. Lett. 102, 097403 (2009).
5.Alonso-González, P. et al. Formation and Optical Characterization of Single InAs Quantum Dots Grown on GaAs Nanoholes. Appl. Phys. Lett. 91, 163104 (2007).
6.Alonso-González, P. et al. Formation of Lateral Low Density In(Ga)As Quantum Dot Pairs in GaAs Nanoholes. Crystal Growth & Design 9, 2525–2528 (2009).
7.Martín-Sánchez, J. et al. Single Photon Emission from Site-Controlled InAs Quantum Dots Grown on GaAs(001) Patterned Substrates. ACS Nano 3, 1513–1517 (2009).
8.Muñoz-Matutano, G. et al. Charge control in laterally coupled double quantum dots. Phys. Rev. B Rapid Comm. 84, 041308 (2011).

Miércoles, 29 febrero 2012. 12:00-13:00
Superradiance Mediated by Graphene Surface Plasmons
Paloma Arroyo

Departamento de Física Teórica de la Materia Condensada, UAM

ABSTRACT:

As it has been recently shown in two experimental works [1,2], a graphene sheet can support Surface Plasmon Polaritons (SPPs) [3]. Compared to conventional SPPs in metals, the properties of graphene surface plasmons (GSP) can be tuned by means of a gate potential that modifies the conductivity of the electrons in graphene.

In this informal seminar I will talk about the control of the interaction between two emitters mediated by means of the excitation of surface plasmon modes in graphene [4]. First, I will show the emission properties of an emitter close to a graphene sheet and, in particular, the decay through GSP.

Then, I will consider how GSP can be used to tailor the interaction between two emitters. When two emitters are close to a graphene sheet a superradiant state can be achieved where the collective emission is greater than the sum of the individual emissions. Remarkably, due to graphene's properties, the interaction between the emitters can be tuned by means of a gate potential, allowing to change from subradiance to superradiance by modifying the gate.

Moreover, we also study the interaction between two emitters mediated by one-dimensional graphene ribbons supporting waveguide modes [5], which provide a very efficient coupling between two emitters.

[1] Florian Huth, et al, arXiv:1202.4996
[2] Z. Fei1, et al  arXiv:1202.4993
[3] A. H. Castro Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, and A. K. Geim, Rev. Mod. Phys. 81, 109 (2009).
[4] Paloma Arroyo Huidobro, A. Y. Nikitin, C. Gonzalez-Ballestero, L. Martín-Moreno, F. J. García-Vidal, arXiv:1201.6492
[5] A. Yu. Nikitin, F. Guinea, F. J. García-Vidal, and L. Martín-Moreno, Phys. Rev. B 84, 161407(R) (2011).

Jueves, 23 febrero 2012. 12:00-13:00
Fano interference and infrared phonon activity in bilayer graphene
Emmanuele Cappelluti

CNR, Roma & Instituto de Ciencia de Materiales de Madrid, CSIC

ABSTRACT:

The detection and analysis of the spectral properties of optical phonon in single-layer and multilayer graphene provides a powerful tool not only for a careful characterization of the systems but also for investigating the role of the underlying electron-phonon interaction.

Recent experiments in gated bilayer graphene revealed a clear phonon resonance at 1590 cm-1 with several interesting features, as for instance a giant enhancement of the phonon intensity as a function of teh gate voltage as well as a pronounced Fano lineshape asymmetry.

In this talk I will discuss how these features can be analyzed and predicted on a microscopic quantitative level using a charge-phonon theory applied to the specific case of graphene systems.

We show in particular how the phonon intensity and the Fano asymmetry are strictly related, stemming out from the quantum interference between the electronic and phononic degrees of freedom.

Within this context we are also able to elucidate the relative role of the Eu and Eg phonon modes in regards to the infrared activity and the Fano asymmetry of the observed phonon peaks.

We present thus a complete phase diagram for the strength of the phonon modes and their Fano properties as functions of the chemical potential and of the gated-induced electronic gap, showing that a switching mechanism between the dominance of the Eu or Eg mode can be controlled by the external gate voltage.

Our work permits thus reconciling within a unique theoretical approach the phonon-peak features observed by different experimental groups, and it provides an analytical tool for predicting and controlling on a quantitative level the spectral properties of the phonon resonances in the infrared spectra of graphenes.

Jueves, 16 febrero 2012. 12:00-13:00
Qantum Effects in Plasmonic Nanostructures
Pablo García González

Departamento de Física Teórica de la Materia Condensada, Universidad Autónoma de Madrid

ABSTRACT:

One of the most attractive aspects of surface plasmon polaritons is their ability to collect and concentrate light into sub-wavelength volumes. The theoretical description of the relevant processes is often done by using classical local optics. However, the miniaturization in the fabrication of plasmonic devices is approaching the limit where non-local effects in the optical response of a metal cannot be neglected.

A possible way to take into account these effects is the modelling of the metal permittivity through hydrodynamical approximations [1]. Nevertheless, the predictive accuracy of such methods depends very sensitively on the details of the model permittivity. At a more fundamental level, the electron response of a system can be evaluated by using time-dependent density functional theory (TDDFT) [2]. Under this prescription, non-local and quantum-mechanical effects in the optical response are treated on the same footing.

In this informal seminar, I shall present some preliminary results of the TDDFT optical response of two metal nanowires in close proximity (sub-nanometric) to each other. A comparison with the corresponding hydrodynamic and local responses in the limit of zero separation will be presented as well.

* Work done in collaboration with Lorenzo Stella and Angel Rubio (UPV/EHU), and F.J. García Vidal (UAM).

[1] A.I. Fernandez-Dommnguez, A. Wiener, F.J. Garcma-Vidal, S.A. Maier and J.B. Pendry (in press).
[2] E. Runge and E.K.U. Gross, Phys. Rev. Lett. 52, 997 (1984).

Miércoles, 8 febrero 2012. 12:00-13:00
Electronic transport in defective low dimensional carbon materials: nanotubes and graphene
Cristina Gomez-Navarro

Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid

ABSTRACT:

Electronic transport properties of carbon nanotubes and graphene are of interest due to their potential use in future electronic devices but also from a fundamental point of view. These materials, due to their low dimensionality and peculiar band structure, present a broad spectrum of electronic transport regimes.

In this talk I will first talk about our experimental work on carbon nanotubes focusing on the effect of different scattering mechanism: atomic scale defects [1] and high energy phonons [2], I will finish trying to give an overview of the phase diagram of electronic transport in carbon nanotubes.

Then I will focus on our work on chemically derived graphene. These graphene layers are obtained by a mass production technique based in the oxidation and subsequent reduction of graphite [3]. I will describe our experiments with the aim of characterizing this material from a structural [4], electronic [3,5] and mechanical [6] point of view. I will also discuss a route for enhancement of its conductivity [7].

[1] Gomez-Navarro, C. et al., Tuning the conductance of single-walled carbon nanotubes by ion irradiation in the Anderson localization regime. Nat Mater
4 (7), 534 (2005).
[2] Sundqvist, P. et al., Voltage and length-dependent phase diagram of the electronic transport in carbon nanotubes. Nano Letters 7 (9), 2568 (2007).
[3] Gomez-Navarro, C. et al., Electronic Transport Properties of Individual Chemically Reduced Graphene Oxide Sheets. Nano Letters 7 (11), 3499 (2007).
[4] Gomez-Navarro, C. et al., Atomic Structure of Reduced Graphene Oxide.
Nano Letters 10 (4), 1144 (2010).
[5] Kaiser, A. et al., Electrical Conduction Mechanism in Chemically Derived Graphene Monolayers. Nano Letters 9 (5), 1787 (2009).
[6] Gomez-Navarro, C., Burghard, M., and Kern, K., Elastic properties of chemically derived single graphene sheets. Nano Letters 8 (7), 2045 (2008).
[7] Lopez, V. et al., Chemical Vapor Deposition Repair of Graphene Oxide: A Route to Highly Conductive Graphene Monolayers. Advanced Materials 21 (46),
4683 (2009).

Miércoles, 1 febrero 2012. 12:00-13:00
On the optical properties of graphenes
Tobias Stauber

Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid

ABSTRACT:

One of the hallmarks of the optical properties of (suspended) graphene is that a simply-observable quantity as the optical transparency is defined solely by the fine structure constant [1]. In the first part of this talk, I will give the theoretical explanation to this experiment, i.e., show that even in the visible-optics regime the corrections to the Dirac cone approximation are small (a few percent) and the effect of next-nearest neighbor hopping is negligible [2]. I will also discuss the infrared conductivity of graphene on a substrate where electron-phonon and impurity scattering become important [3].

In the second part, I will look at the optical properties of double layer graphene with respect to their plasmonic excitations, near-field amplification and extraordinary (perfect) transmission [4]. Also graphene's fluorescence quenching including transverse decay channels and full retardation will be discussed [5]. Finally, the current-current correlation function of the full hexagonal tight-binding model will be derived [6] and I will show that lattice effects lead to a paramagnetic response for graphene with intrinsic doping at low temperatures [7].

[1] R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T.J. Booth, T.
Stauber, N. M. R. Peres, and A. K. Geim, Science 320, 1308 (2008).
[2] T. Stauber, N. M. R. Peres, and A. K. Geim, Phys. Rev. B 78, 085432 (2008).
[3] T. Stauber, N. M. R. Peres, and A. H. Castro Neto, Phys. Rev. B 78, 085418 (2008).
[4] T. Stauber and G. Gómez-Santos, Phys. Rev. B 85, (2012).
[5] G. Gómez-Santos and T. Stauber, Phys. Rev. B 84, 165438 (2011).
[6] T. Stauber and G. Gómez-Santos, Phys. Rev. B 82, 155412 (2010).
[7] G. Gómez-Santos and T. Stauber, Phys. Rev. Lett. 106, 045504 (2011).

Jueves, 26 enero 2012. 12:00-13:00
Quantum Information for Molecular Physics
Jordi Mur-Petit

Instituto de Fisica Fundamental, CSIC

ABSTRACT:

I will make a presentation of the field of quantum information processing (QIP) from a general perspective, focusing then on our research on both the use of cold molecules for QIP tasks, and the use of QIP methods to address molecular physics problems, such as spectroscopy of molecular ions. I will finally discuss our ongoing research on controlled collisions with finite-range potentials.

Jueves, 19 enero 2012. 12:00-13:00
First-Principles Simulations on PbTiO3/SrTiO3 Superlattices
Javier Junquera

Departamento de Ciencias de la Tierra y Física de la Materia Condensada, Universidad de Cantabria

ABSTRACT:

Ferroelectric perovskites are materials of great fundamental and applied interest. This family of materials displays a great range of functionalities, from ferroelectricity to superconductivity. More interestingly, perovskites show a very rich phase diagram allowing for a great tunability playing with doping, epitaxial strain, and the combination of different materials in heterostructures. This has led to the discovery of completely new interface-based phenomena in the last years, for instance a fundamentally new type of ferroelectricity has been discovered in PbTiO3/SrTiO3 superlattices, due to the unexpected coupling of ferroelectric and antiferrodistortive structural distortions in these heterostructures [1].
First in this talk, we will present first-principles calculations, within the density functional theory, on the coupling between epitaxial strain, polarization, P, and oxygen octahedra rotations in monodomain (PbTiO3)n/(SrTiO3)n superlattices [2]. We have studied different periodicities, n ranged from 1 to 3, with an improper ferroelectric behaviour. P is found to be extremely sensitive to strain, and rotates continuosly from a c-phase (P oriented along the [001] direction) for compressive strains, to an aa-phase (P along [110]) under tensile strain. The out-of-plane component of P, Pz, is always preserved at the interface to minimize the electrostatic energy, and decreases in the PbTiO3 layer with respect the bulk value, reflecting the energy cost of polarizing SrTiO3. At the origin of these new phases with an in-plane component of P, we have found the preference of the polarization in PbTiO3 to rotate, over an homogeneous decrease of Pz. Around the lattice constant imposed by a SrTiO3 substrate, the system displays a large piezoelectric reponse. Changes in polarization are strongly coupled with the response of the oxygen octahedra, whose rotations and tiltings cannot be explained by the usual steric arguments alone. Instead a covalent model on the polarization-tilting coupling is developed.
Second, following the suggestion of a recent experimental work [3], who suggested that the ground state of this system might be actually polydomain for most periodicities, we will also report on simulations on PbTiO3/SrTiO3 superlattices including the presence of domains to complement the last experimental results and to better understand the properties of domain structures in these superlattices.
This work was supported by the Spanish Ministery of Science and Innovation through the MICINN Grant FIS2009-12721-C04-02; by the Spanish Ministry of Education through the FPU fellowship AP2006-02958; and by the European Union through the project EC-FP7, Grant No. NMP3-SL-2009-228989 “OxIDes”. The authors thankfully acknowledge the computer resources, technical expertise and assistance provided by the Red Española de Supercomputación.

Miércoles, 11 enero 2012. 12:00-13:00
Towards a microscopic description of the pseudogap phase in cuprate and organic superconductors
Jaime Merino

Departamento de Física Teórica de la Materia Condensada, Universidad Autónoma de Madrid

ABSTRACT:

Understanding the mechanism of high-Tc superconductivity in cuprate materials is a fundamental challenge in condensed matter theory. The 'normal' metallic phase of these systems is highly unconventional displaying strong deviations from Landau-Fermi liquid behavior particularly in the underdoped regime in which a pseudogap phase with no apparent broken symmetry occurs. The most 'anomalous' observation in this phase is that the Fermi surface consists of disconnected arcs along the Brillouin zone diagonals violating the Luttinger sum rule condition. A pseudogap phase has also been observed in the metallic phase of layered organic materials which are in close proximity to a Mott insulating phase. The common existence of a pseudogap state in the doping driven Mott insulators (cuprates) and in the bandwidth Mott transition (organics) suggests that the pseudogap is inherent to properties of the Mott insulator in two-dimensional systems.

In order to understand the microscopic origin of the pseudogap phase we have explored the evolution of the one-electron properties across the Mott metal-insulator transition based on a single-band half-filled Hubbard model on a two-dimensional square lattice. As the Coulomb repulsion is increased electrons along the antinodal direction of the Brillouin zone open a gap (pseudogap) in the spectral density whereas electrons along the nodal direction display a quasiparticle-like peak. Since the pseudogap phase is found in the paramagnetic solution of the Hubbard model no broken symmetry occurs suggesting that is associated with short range electronic correlations. The nature of the excitations in the pseudogap phase is unveiled by analyzing pairing and antiferromagnetic correlations. We find that dx2-y2-wave pairing correlations are enhanced whereas antiferromagnetism is suppressed within the pseudogap phase. This behavior is consistent with the resonant valence bond (RVB) wavefunction for the ground state proposed by Anderson and coworkers for the cuprate superconductors.

Miércoles, 21 diciembre 2011. 12:00-13:00
Pi-phases and triplet pair correlations in s-wave superfluids as signatures of the FFLO-type states
Ivar Zapata

Departamento de Física de Materiales, Universidad Complutense de Madrid

ABSTRACT:

I will introduce FFLO (Fulde-Ferrell-Larkin-Ovchinikov) physics and review the present status of its experimental evidence. Cold atomic physics provides an analog to Superconducting-Ferromagnetic (SF) hetero-structures. As in SF, the order parameter oscillates from positive to negative values (pi-phases). I will show the connection of these states to FFLO-physics. All FFLO-type states show a substantial triplet-mixing in the Cooper pair wave-function. This mixing can be used as an independent test of their existence.

Miércoles, 14 diciembre 2011. 12:00-13:00
Magnetoplasmonics: The interplay between magneto-optics and plasmonics
Antonio García-Martín

Instituto de Microelectrónica de Madrid, CSIC

ABSTRACT:

Subwavelength composite materials constitute an interesting path towards the development of materials with "on demand" optical properties. We will present our latest results on systems composed of both noble and ferromagnetic metals, which we denote as magnetoplasmonic. While noble metals have intense and narrow plasmon resonances they lack magneto-optical (MO) activity at reasonable magnetic field intensities. On the other hand, ferromagnetic metals are MO active but their plasmon resonances are weak and broad. By combining both kinds of materials we intend to obtain systems which simultaneously exhibit plasmon resonances and MO activity. We will show that thus it is possible to both (1) enhance the magneto-optical activity of the system via surface plasmon excitation, and (2) modulate the plasmon properties via application of a magnetic field [1]. Localized surface plasmon resonances (LSPRs) greatly influence the optical [2,3] and magneto-optical (MO) [4,5,6,7,8] properties of fully metallic and metal-dielectric nanostructures. We will analyze the MO response of isolated nanodisks, where we will show how the excitation of the LSPR produces an enhancement of the MO activity [4]. The observed enhancement in the MO is attributed to the high intensity of the electromagnetic (EM) field inside the nanostructure when the LSPR occurs. Here we show how the EM profile related to the LSPR can be probed locally inside the nanostructure by measuring the MO activity of the system as a function of the position a MO active probe (a Co nanolayer) [9]. This EM field profile is the key element in the analysis of the MO activity and thus a clever engineering would make it possible to get large MO effect and low losses [10]. The same kind of structures allows the analysis of the effect of the MO activity on the plasmon properties. We will show that the wavevector of the plasmon is modified upon application of a magnetic field in the transverse configuration [11]. That modification can be used in a wide variety of scenarios: e.g. in active microinterferometry [12,13].

Lunes, 5 diciembre 2011. 12:00-13:00
Lens-like particles and their entropy-driven clustering
Giorgio Cinacchi

Departamento de Física Teórica de la Materia CondensadaUAM

ABSTRACT:

Suitable hard particle models are often sufficient to capture the basic features of the phase behaviour and properties of a variety of complex fluids. Onsager showed that a purely entropy driven isotropic-nematic phase transition occurs in systems of hard rods, while later it was also demonstrated that steric interactions suffice to give rise to smectic or columnar phases. In this talk, the phase behaviour of hard spherical caps is reported. By varying the sphere radius and constraining the area of the subtended surface to a fixed value, particles can be generated covering the entire interval from the hard platelet model to the hard sphere model. The particles belonging to the sub--interval delimited by the hard platelet and hard hemispherical surface models are infinitely thin and concave. This talk focuses on the low density phase behaviour of these curved particles. Spherical caps of sufficiently large radius of curvature, similar to platelets, exhibit a transition from the isotropic to the nematic phase on increasing pressure. By reducing the radius further, however, the latter phase is progressively destabilized. For particles similar to lenses, in fact, the nematic phase is eventually replaced by a different type of self-assembly, characterized by the simultaneous aggregation of the centres of the parent spheres and the organization of the concave particles on the corresponding spherical surfaces. Lens-like particles thus exhibit a competition between a fluid-fluid phase transition and a clustering phenomenon, similar to what is observed in molecular systems forming micelles or colloidal suspensions forming cluster phases. For lens-like particles, however, this competition does not involve any energetic contribution: the phenomenology is purely entropy driven. For spherical caps similar to bowls, the clusters progressively change from being roundish to lacy. The relevance of all these simple model particles to a range of soft matter systems is discussed.

Miércoles, 30 noviembre 2011. 12:00-13:00
Control cuántico coherente mediante transiciones de Landau-Zener
Gustavo Murgida

Conicet-CNEA, Buenos Aires

ABSTRACT:

En esta charla proponemos un método para controlar de una manera extremadamente simple el estado de un sistema cuántico. La estrategia de control está basada en el modelo de Landau-Zener y consiste esencialmente en emplear los cruces evitados para alcanzar diferentes niveles de energía, recorriendo las curvas adiabáticas del espectro hasta arribar al estado deseado. La simpleza del método proviene de suponer que el sistema se comporta localmente, cerca de cada anticruce, como un sistema de dos niveles de Landau-Zener. Sin embargo, mediante simulaciones num´ricas en diferentes sistemas realistas comprobamos que esta estrategia de control posee una e¿ciencia sorprendente. Las primeras aplicaciones numéricas fueron realizadas en un sistema de dos puntos cuánticos acoplados con dos electrones intearactuantes en su interior. Empleando un campo eléctrico externo como parámetro de control, logramos localizar ambos electrones en un mismo punto cuántico, construir estados entrelazados, recorrer en forma e¿ciente caminos complejos en el espectro para conectar niveles de energía lejanos y construir superposiciones coherentes de varios autoestados. El método también fue aplicado en otro sistema muy diferente al anterior, la molécula de LiCN. Nuevamente pudimos controlar exitosamente el estado del sistema y además encontramos una solución al problema de isomerizar la molécula. Para ello también empleamos como parámetro de control un campo eléctrico externo. De esta manera mostramos que nuestro método puede ser aplicado e¿cientemente en diferentes sistemas complejos y creemos que posee un importante potencial en el campo del control coherente.

Miércoles, 23 noviembre 2011. 12:00-13:00
Josephson current in finite-length nanowire SNS junctions with Majorana fermions
Ramón Aguado

ICMM, CSIC

ABSTRACT:

The combination of strong spin-orbit effect with a Zeeman field may lead to the formation of helical electron liquids in single-channel semiconducting nanowires. When such an helical wire is contacted with an s-wave superconductor it is possible to induce a topological phase in which the system supports Majorana bound states (MBS) [1,2]. The Josephson current through junctions of these one-dimensional topological superconductors exhibits an anomalous $4\pi$ periodic phase ($\phi$) dependence owing to the presence of MBS. Such a 'fractional' Josephson effect, which originates from a parity-protected level crossing of zero-energy MBS at $\phi=\pi$, is ubiquitous in systems supporting MBS [3,4,5] and provides an important experimental signature towards detecting MBS in a solid-state setting. To date, most of the theoretical studies have been restricted to either simplified models, such as Kitaev's [2], or to infinite-lenght superconducting junctions (except [6]). In this talk, I will discuss the Josephson effect in more realistic SNS junctions of arbitrary transparency and when both the normal and the nanowire regions are of finite length, namely beyond the short-junction and infinite topological superconductor limits. In general, the spectrum of Andreev bound states can become rather intricate and dense as opposed to the infinite-length case. Moreover, the low-energy spectrum around $\phi=\pi$ shows always anticrossings, originated from hybridization of four MBS, which may preclude the experimental observation of the fractional Josephson effect. At finite bias voltages, Landau-Zener dynamics involving the MBS and quasi-continuum Andreev levels gives rise to a nontrivial ac Josephson current. Interestingly, the ac current phase diagram as a function of the Josephson frequency/normal transmission shows regions of $4\pi$ periodicity which are tunable through bias/gate voltages [7].

Jueves, 10 noviembre 2011. 15:00-16:00
Quantum Optics as Tools to Probe the Spacetime Structure
Eduardo Martín Martínez

CSIC

ABSTRACT:

Relativistic quantum information theory uses well-known tools coming from quantum information and quantum optics to study quantum effects provoked by gravity and to learn information about the spacetime. One can take advantage of our knowledge about quantum optics and quantum information theory to analyse from a new perspective the effects produced by the gravitational interaction. I will present some results and new ideas in this topic: two experimental proposals for the detection of the Unruh and Hawking effects and a quantum simulation of general relativistic settings.

Miércoles, 2 noviembre 2011. 12:00-13:00
The impact of electron interactions on one-dimensional helical conductors and Majorana end states
Bernd Braunecker

Dpto. Física Teórica de la Materia Condensada, UAM

ABSTRACT:

Majorana bound states appear at the ends of one-dimensional helical (i.e., spin-filtered) conductors if they are brought in the proximity of a superconductor. Such Majorana states have received much attention very recently due to their potential usefulness for topological quantum computation, quantum memories, or Cooper-pair splitters.

In my talk, I will discuss the effect of electron-electron interactions on the helical conductors (in the normal and superconducting state) and on the Majorana end states. I will show that already in the normal state the different helical conductors that are usually considered as equivalent exhibit substantial differences if electron interactions are taken into account [1]. Furthermore, I will show that the superconducting state is extremely susceptible to electron interactions [2]. Strong interactions generically destroy the induced superconducting gap that stabilizes the Majorana end states. On the other hand, for weak interactions the interaction-induced renormalization of the gap is nonuniversal and allows for a regime in which the Majorana edge states persist. I will discuss strategies how this regime can be reached and outline the consequences for experiments.

Miércoles, 26 octubre 2011. 12:00-13:00
Landau-Zener tunneling of qubits: dynamics, decoherence, and measurement
Sigmund Kohler

Instituto de Ciencia de Materiales de Madrid, CSIC

ABSTRACT:

The spectrum of a quantum system as function of a control parameter generically exhibits anti-crossings. A classic example is the electronic spectrum of two colliding molecules within Born-Oppenheimer approximation. How probable are then electronic excitations due to the collision? Or in turn, how likely is it that the electrons follow adiabatically their ground state? Landau, Zener, Stuckelberg, and Majorana already in 1932 independently answered this question for an idealized two-level model. Their scenario possesses many modern applications, also in quantum information, where the anti-crossing can be used as effective beam splitter allowing state preparation. The coupling to an environment entails quantum dissipation and decoherence to the resulting superposition. Then the known Landau-Zener formula needs to be generalized to the presence of external degrees of freedom. It turns out that at zero temperature, this problem possesses an exact solution. Applications range from single-photon generation in circuit-QED to adiabatic quantum computing and the measurement of tunnel splittings in molecular nanomagnets. Finally, I will address the question whether the dynamics of the transition can be observed in an experiment.

Miércoles, 19 octubre 2011. 12:00-13:00
A new tool for particle hydrodynamics at different scales
Rafael Delgado Buscalioni

Departamento de Física Teórica de la Materia Condensada, Universidad Autónom de Madrid

ABSTRACT:

In this informal talk I will present a new algorithm for particle hydrodynamics recently developed in the Dept. Fisica Teorica de la Materia Condensada (UAM). It is ideally suited to solve colloidal or polymeric suspensions at small Reynolds or even dispersions of larger particles in turbulent flow. The general framework consists on solving the solvent (fluctuating) hydrodynamics in an Eulerian mesh, while the solute Lagrangian dynamics are solved in continuum space. The scheme is quite general, also allowing for fluid-structure interactions, however I will focus on the so called "pointparticle" representation, where each particle is determined by an effective volume and the dynamics of its center of mass translation (rotation can also be added). In these kind of schemes, the force between particle and fluid have been usually assumed to be frictional and of the same form of the Stokes drag. However, such approximation limits the applicability of the method to times larger than the friction response (i.e. slow forcing frequencies) and moreover it is only valid for particles presenting some fluid slip at their surface. We present an alternative approach which obtains the fluid/particle force by enforcing the no-slip condition on the averaged fluid velocity at the particle domain. In doing so, particle/fluid momentum is exchanged instantaneously and inertia is taken into account. We are able to describe acoustic forces on colloidal particles, and grasp the effects of the finite size of the particle in the flow. Thermal fluctuations are also properly solved, as there is no extra dissipative channel associated to the particle friction. The code is written in CUDA and it is now running in local GPU's. Some information on the (wonderful) speed up of this architecture shall also be given. Possible generalizations and applications of the method shall be discussed.

Jueves, 13 octubre 2011. 12:00-13:00
DiagMC for the Anderson-Holstein model: Separation of timescales
Klaus Ferdinand Albrecht

University of Freiburg

ABSTRACT:

Based on a recently developed diagrammatic real-time Monte-Carlo method [1], we access the nonequilibrium current of the spinless Anderson-Holstein model numerically exact. For a certain parameter range, we found a separation of timescales for the current depending on its initial preparation.

Miércoles, 5 octubre 2011. 12:00-13:00
Quantum transport of cold atoms
Fernando Sols

Universidad Complutense de Madrid

ABSTRACT:

Cold atom devices permit the exploration of novel forms of quantum transport that are difficult or impossible to realize in traditional electron transport setups. Under the action of an external driving, long-term coherent atom motion can be quite sensitive to the initial switching conditions even in the presence of interactions [1]. If the driving violates space- and time-inversion symmetry simultaneously, then coherent motion of a Bose-Einstein condensate in a given direction can be induced [2], as has been recently observed [3]. For weak driving, this coherent quantum ratchet stems from the interference between first- and second-order processes, as revealed by precise analytical work [4]. A different scenario is that of a leaking condensate passing through an interface which separates regions of subsonic and supersonic flow. On the supersonic (normal) side of the event horizon, we find the bosonic analog of Andreev reflection in superconductors [5]. On the other hand, the analog of Hawking radiation is emitted into the subsonic side, even at zero temperature. We study a double barrier structure which is predicted to emit resonant, highly non-thermal Hawking radiation [6].