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Coordinators:
Leni Bascones

María José Calderón
Seminars of the Research Lines
Theory and Simulation of Materials
Alternative Seminars
12 July 2013, 12:00 h. Sala de Seminarios, 182
INTRINSIC DECOHERENCE & GRAVITATION
P.C.E Stamp
PITP & Physics Dept, University British Columbia; Math Inst., Oxford University
Conventional decoherence (usually called ‘Environmental Decoherence’) involves correlations established between some quantum system and its environment. ‘Intrinsic decoherence’ is hypothesized as being an essential feature of Nature – its existence entails a breakdown of quantum mechanics.
I will begin by briefly reviewing (a) the fundamental conflict between Quantum Mechanics and General Relativity, and (b) environmental decoherence, noting in particular that it can and does involve decoherence without dissipation (ie., pure phase decoherence). I then discuss a theory in which correlations exist between different branches of the wave-function, mediated by gravitation (the ‘GR-’ theory). This theory is in principle testable in condensed matter experiments, some of which should be feasible in the very near future. The weak field regime of this theory (ie., the regime relevant to experimental predictions) is discussed in detail, along with predictions of intrinsic decoherence rates, for experiments in optomechanical systems.
27 June 2013, 12:00 h. Sala de Seminarios, 182
Unbalanced Fermi-Fermi mixtures in one dimension
Christian Recher
Duisburg-Essen University (Germany) and Inst Ciencia Materiales Madrid
We study a one-dimensional quantum particle that interacts with a Fermi--sea via repulsive contact interaction. The eigenfunctions of the Bethe-Ansatz solvable model can be expressed as a determinant. This allows us to calculate zero temperature Greens function, density--density correlations and expectation values. In the hardcore limit the Greens function of the extra particle can be analyzed further using Painlevé V transcendents. It is found that depending on the extra particles momentum its Greens function undergoes a transition of that for hardcore Bosons to that of free Fermions.
25 June 2013, 12:00 h. Sala de Seminarios, 182
Irradiated Graphene as a Floquet Topological Insulator
H.A. Fertig
Indiana University
Graphene supports a number of remarkable electronic properties, some of which make it a candidate for certain microelectronic applications. The challenge, however, of opening a gap in its electronic spectrum has limited its use for basic circuit elements such as transistors. In this talk I will review recent work in which an analog of such a gapped spectrum is induced by a time-dependent potential. The resulting system turns out to have electronic structure with non-trivial topology, and is an example of a "Floquet Topological Insulator." It supports surprising fundamental behaviors -- including a quantized Hall effect with no magnetic field -- but there are fundamental challenges to predicting its electronic behavior in settings where it can be measured. I will present results of numerical calculations in which we meet some of these challenges, and show what should be found in the simplest possible measurement geometry, a two-terminal conductor. I will discuss the features of the results that demonstrate the unusual topology of the electronic structure, as well as surprising properties that are unique to the time-dependent nature of the system.
20 June 2013, 12:00 h. Sala de Seminarios, 182
Flat bands in graphene
Bruno Uchoa
University of Oklahoma
In this talk, I will address the issue of formation of time reversal symmetric flat bands in graphene. I will show that when graphene is supported on a Boron Nitride substrate, quantum interference gives rise to local potentials which modulate with the period of a Moire pattern supercell. Those potentials can give rise to a remarkable real space network of zero energy modes in the form of quantum rings, containing nearly flat bands separated by energy gaps. I will show that the size of those gaps can be tuned with an electric field effect and can reach the order of magnitude needed to confine electrons at room temperature. In the second part of my talk, I will address a different problem: the theory of superconductivity in flat bands. I will explicitly describe the problem of Dirac fermion superconductivity in the presence of time reversal symmetric Landau levels, which can be generated by strain in graphene. I will show that this is probably the most promising route towards the observation of intrinsic superconductivity in graphene.
18 June 2013, 12:00 h. Sala de Seminarios, 182
AC control of atom dynamics and transport in Bose-Einstein condensates
Martin Heimsoth
Universidad Complutense de Madrid
Periodically driven quantum many-body systems can show complex motion over a large variety of dynamical regimes. I will introduce an extended version of the so-called tt -formalism, a mathematical tool for the study of time-periodic equations of motion. We will pay special attention to the peculiarities related to its application to the Heisenberg equation of motion for field operators, and use it to study the role of particle interactions in resonantly driven Bose-Einstein condensates.
As a result, an effective static Hamiltonian involving a few single-particle modes can be obtained for these systems. The structure of the effective Hamiltonian and the characteristics of the participating modes led to the term orbital Josephson effect [1]. We present a detailed study of the dynamical regimes for the specific case of a Hamiltonian quantum ratchet. The effective description is validated by comparing it to exact simulations using the multi-configurational time-dependent Hartree for bosons (MCTDHB) algorithm.

[1] M. Heimsoth, C.E. Creffield, L.D. Carr, and F. Sols, New J. Phys. 14, 075023 (2012).
06 June 2013, 12:00 h. Sala de Seminarios, 182
Quantum Transport and Correlated Phenomena in Bilayer and Trilayer Graphene Membranes
Chun Ning (Jeanie) Lau
University of California, Riverside
Graphene, a two - dimensional single atomic layer of carbon, has recently emerged as a new model system for condensed matter physics, as well as a promising candidate for electronic materials. Though single layer graphene is gapless, bilayer and trilayer graphene have tunable band gaps that may be induced by out-of-plane electric fields or arise from collective excitation of electrons. Here I will present our results on transport measurements in bilayer and trilayer graphene devices with mobility as high as 400,000 cm2/Vs. We demonstrate the presence of an intrinsic gapped state in bilayer graphene at the charge neutrality point, evidence for quantum phase tranisition, and stacking-order dependent transport in trilayer graphene. Our results underscore the fascinating many-body physics in these 2D membranes, and have implications for band gap engineering for graphene electronics and optoelectronic applications.
04 June 2013, 12:00 h. Sala de Seminarios, 182
Emerging light-matter interactions for super-resolution imaging and optical micromanipulation
C.W. Qiu
National University of Singapore
The light-matter interaction can be manipulated to obtain a plethora of unprecedented optical phenomena. In this talk, we will present our recent developments on emerging light-matter interactions, in particular, the super-resolution imaging in farfield exploiting super-oscillation and diffraction theorem as well as novel optical micromanipulation exploiting redirection of photon momentum. The physics of super-oscillation in the context of farfield super-resolution imaging will be unveiled and various experiments have been proposed to access the implications of such techniques in the practical applications. Vector Rayleigh-Sommerfeld method is introduced and developed to design ultrathin flat surface for super imaging, while enabling the polarization control of the focal region. On the other hand, the light will exert optical force toward the object being illuminated. Manipulating light-matter interaction empowers new possibilities of achieving optical pulling force or tractor beam. We will present our recent results on how to make a Bessel beam and a plane wave to be tractor beam, with experimental verifications. Universal pulling effects and conditions are discussed in association with insight on modified far-field scattering, scattering resonances, and induced dipoles.
30 May 2013, 12:00 h. Sala de Seminarios, 182
Quantum Simulation of Dynamical Gauge Theories with Superconducting Circuits
David Marcos
Institute for Quantum Optics and Quantum Information (Innsbruck, Austria)
In this seminar, I will give a general blackboard presentation on gauge invariance and quantum circuits. An overview to lattice gauge theories as formulated by Wilson will be presented. I will also give an introduction to superconducting circuits, and show how they can be used for the simulation of dynamical gauge theories using the language of quantum-link models. In our recent work we have shown how dynamical gauge theories can be simulated with a superconducting-circuit lattice, where superconducting qubits play the role of matter fields on the sites and the gauge fields are represented by two coupled microwave resonators on each link. I will illustrate the proposal by analyzing a one-dimensional U(1) quantum-link model and describe a minimal experimental protocol for probing the physics related to string breaking effects. Our results show that despite the presence of decoherence in these systems, distinctive phenomena from high-energy physics can be visualized with state of the art technology in small superconducting-circuit arrays.
28 May 2013, 12:00 h. Sala de Seminarios, 182
Loop models and deconfined quantum criticality
Miguel Ortuño
Universidad de Murcia
In quantum phase transitions, quantum fluctuations play the role of thermal
fluctuations in standard phase transitions. In some quantum phase transitions,
called deconfined quantum transitions, the two phases involved have different
and incompatible order parameters. We will describe how these deconfined
transitions represent a new paradigm, not fitting in the Gizburg-Landau theory.
Loop models constitute an excellent tool to study this transition. We introduce these models and present results of numerical simulations on these models. The results are difficult to understand with present theoretical frameworks, and present features of both first and second order phase transitions.
23 May 2013, 12:00 h. Sala de Seminarios, 182
On magnetic-field-assisted dissipationless electric current in
nanowire systems
M. N. Chernodub
CNRS, University of Tours, France
In certain exotic systems, for example in three-dimensional Weyl semi-metals, an external magnetic field is suggested to induce an equilibrium dissipationless electric current flowing along the direction of the field. This transport phenomenon is usually associated with new topological states of matter which are characterized by non-invariance of their excitation spectra under spatial inversion symmetry. The parity breaking is a necessary condition for the realisation of this anomalous transport law. As an
alternative, in our talk we propose several geometrical designs of nanowire structures which may also support long-distance, dissipationless transfer of electric current in the presence of magnetic field. In these structures, the nanowires themselves are assumed to be made of usual, parity-even materials while the parity-broken spectrum appears due to special, parity-odd geometric design of the nanowire structures. We argue that the effect is robust against the presence of diffusion and interactions. We support our claim by explicit calculations of both the energy spectra and currents in several models.
16 May 2013, 12:00 h. Sala de Seminarios, 182
Electronic properties of the two-dimensional crystal MoS2
Rafa Roldán
Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC)
Two-dimensional crystals have recently emerged as an interesting family of materials with a large variety of electronic properties ranging from superconductors to topological insulators. Although graphene is by far the most studied two-dimensional crystal, its lack of a bandgap hampers its application in semiconducting and photonic devices. This fact has motivated the research in other 2D crystals with a large intrinsic bandgap, such as atomically thin MoS2. Single-layer MoS2 transistors have shown large in-plane mobility and a high current on/off ratio, making this material of great interest for electronic devices and sensors, possibly also in combination with graphene. Whereas bulk MoS2 is an indirect bandgap semiconductor, single-layers have a direct gap at the K and K points of the hexagonal Brilloin zone. Furthermore, MoS2 presents a large spin-orbit coupling, which splits the valence band edges in 150 meV. In this talk I will review the recent experimental progress on growing, characterization and application of single- and multi-layer MoS2. The electronic band structure will be discussed in terms of a combination of density functional theory (DFT) and tight-binding methods. The electrostatic screening by few-layer MoS2 will be studied by means of electrostatic force microscopy in combination with a non-linear Thomas-Fermi theory. I will further analyze the possible microscopic origin of the recently observed superconducting phase in this material, which admits an order parameter with opposite signs in different valleys, resembling the superconductivity found in the pnictides and cuprates.
13 May 2013, 12:00 h. Sala de Seminarios, 182
Physics of underdoped cuprate superconductors: Evolution of spin states from the Mott insulator.
Sanjoy Sarker
University of Alabama
The origin of the highly unusual behavior of a cuprate superconductor is thought to be due to its proximity to the Mott insulator. Anderson argued quite early that spin structure of the doped region is intimately connected to that of the insulator. However, attempts to construct a theory based on such a connection have run into serious difficulties. I will discuss some recent progress made in this direction. Specifically, we have derived a renormalized Hamiltonian from the parent t-J model, which is charaterized by a spin gap. The model is analyzed by continuing the known spin states from the insulator, which constraints the theory. This leads to the correct phase diagram: a strange metal, a pseudogap metal and a d-wave superconductor. These results follow directly from the symmetry of the known
valence-bond "vacuum" which is continued from half filling. Not much calculation is needed. Additionally, the two-dimensional nature of the metallic state emerges from the theory.
09 May 2013, 12:00 h. Sala de Seminarios, 182
The ab initio energetics of water clusters, ice and the bulk liquid: DFT meets quantum Monte Carlo
Mike Gillan
London Centre for Nanotechnology & Thomas Young Centre University College London
Water has probably been studied more extensively than any other substance, but its energetics remains surprisingly elusive. The properties of water clusters, ice structures and the bulk liquid are poorly reproduced by conventional density functional theory (DFT), for reasons that are still controversial. I will describe some new approaches that we are pursuing at UCL in collaboration with colleagues in Cambridge and Bristol, focusing particularly on our recent work with quantum Monte Carlo (QMC) [1,2], but emphasising also machine-learning ideas based on Gaussian Approximation Potentials [3-5]. I will show that QMC is much more accurate than standard DFT methods for the energetics of clusters and ice structures, and that it can also supply energy benchmarks for statistical samples of configurations of thermal-equilibrium nano-droplets and the bulk liquid. We are using the benchmarks to analyse the origin of errors in DFT approximations. Our analysis shows that conventional DFT approximations suffer from important errors in both the 2-body and the beyond-2-body parts of the energy. I will show how these errors can be corrected.

[1] M. J. Gillan, F. R. Manby, M. D. Towler and D. Alfè, Assessing the accuracy of quantum Monte Carlo and density functional theory for energetics of small water clusters, J. Chem. Phys. 136, 244105 (2012).

[2] D. Alfè, A. P. Bartók, G. Csányi and M. J. Gillan, Energy benchmarking with quantum Monte Carlo for water nano-droplets and bulk liquid water, in preparation.

[3] A. P. Bartók, M. C. Payne, R. Kondor and G. Csányi, “Gaussian Approximation Potentials: The accuracy of quantum mechanics without the electrons”, Phys. Rev. Lett., 140, 136403 (2010).

[4] A. P. Bartók, M. J. Gillan, F. R. Manby and G. Csányi, Machine learning for predictive condensed-phase simulation, arXiv: 1302.5680 (2013).

[5] M. J. Gillan, D. Alfè, A. P. Bartók and G. Csányi, First-principles energetics of water: a many-body analysis, arXiv: 1303.0751 (2013).
25 April 2013, 12:00 h. Sala de Seminarios, 182
Electronic Mach-Zehnder interferometry with co-propagating spin-resolved edge states in the quantum Hall regime
Luca Chirolli
Instituto de Ciencia de Materiales de Madrid, ICMM-CSIC
We introduce and experimentally demonstrate a new method that allows us to controllably couple co-propagating spin-resolved edge states of a two-dimensional electron gas (2DEG) in the integer quantum Hall regime. The scheme exploits Zeeman and spin-orbit modulation via a spatially periodic in-plane magnetic field that is created by an array of voltage-controlled Cobalt nanomagnets placed at the boundary of the 2DEG. A maximum charge or spin transfer of 28±1% is achieved at 250 mK. This method allows for a straightforward implementation of a Mach-Zehnder interferometer in a co-propagating architecture. Finally, we address the role of electron-electron interaction in the coherence of the interferometer.
18 April 2013, 12:00 h. Sala de Seminarios, 182
Caracterización, visualización y uso del agua en cristales fotónicos autoensamblados
Álvaro Blanco
Instituto de Ciencia de Materiales de Madrid, ICMM-CSIC
Desde su aparición como estructuras fotónicas tridimensionales autoensambladas a mediados de los años 90, los ópalos artificiales han sido ampliamente estudiados en numerosos laboratorios en todo el mundo ya que su fabricación es relativamente barata, nada sofisticada y además estos suelen presentar excelentes propiedades ópticas. Debido a la manera en la que estas estructuras se forman, suelen contener un cantidad de agua que depende de la humedad relativa del entorno pero que suele ser del orden alrededor de un 10-12% en volumen en condiciones normales. Aunque esta agua ha sido identificada en mayor o menor medida a lo largo de estos últimos años no ha sido hasta muy recientemente que se ha estudiado con más detalle, caracterizando como afecta a las propiedades ópticas de las estructuras fotónicas en las que está presente, e incluso haciendo uso de ella para modificar a voluntad algunas de estas propiedades.

En el presente seminario haré un repaso de nuestros últimos trabajos sobre este tema donde mostraré como esta agua afecta enormemente las propiedades tanto ópticas como mecánicas de los ópalos artificiales. Además es posible visualizarla a escala nanométrica sin mas que usar un microscopio electrónico. Finalmente mostráre un método basado en lo anterior para la fabricación de anillos nanométricos de algunos óxidos comunes (por ejemplo SiO2).
11 April 2013, 12:00 h. Sala de Seminarios, 182
Topological Insulators under Dissipative Dynamics
Angel Rivas
Universidad Complutense de Madrid
In this talk we will present recent results regarding the stability of topological order in the presence of dissipation. Particularly, we address the behavior of topological insulators in the presence of thermal baths. These systems present non-vanishing topological conductivity at zero temperature, as their conduction and valence bands are connected by the so-called (topologically protected) edge states. We shall explain that, in general, these edge states are no longer protected when the system is in contact with a thermal bath. However, for some kind of environments, it is possible to obtain and characterize topologically ordered phases even in the presence of thermal dissipation. We will illustrate both results with examples: the Creutz Ladder in 1D and the Haldane model in 2D.
05 April 2013, 12:00 h. Salón de Actos
Experiments on a Triple Quantum Dot Circuit
Andrew Sachrajda
National Research Council, Ottawa
In this talk I will briefly describe some recent measurements we have performed on a triple quantum dot circuit. Firstly I will show how quantum backaction manifests itself in the device. Remarkably the backaction is also a demonstration of a single phonon interferometer. Secondly I will present a holistic overview of qubits based on three interacting spins. These are dominated by Landau-Zener-Stuckelberg oscillations and an all-exchange qubit originally proposed a decade ago as a solution to the addressability issue with simple spin qubits.
21 March 2013, 12:00 h. Sala de Seminarios, 182
Magnetic nanoparticles for cancer diagnosis and treatment
Francisco Terán
IMDEA-Nanociencia
Nanomedicine is an emerging and multidisciplinary area based on the progress of different scientific disciplines at the nanometer scale such as material science or cell biology. One of the nanomedicine’s goal is to achieve personalized and more efficient platforms for detecting and treating health diseases such as cancer. In this context, superparamagnetic iron oxide nanoparticles (SPION) appear as suitable nanovectors with different physical, chemical and biological functionalities. Indeed, SPION may simultaneously act as drug delivery nanocarriers, contrast agents for magnetic resonance imaging techniques, and heating mediators when subjected to alternating magnetic fields (HAC). Recent works have shown the potential of SPION as minimally- invasive therapeutic and diagnostic platforms. For aiming intracellular heating inductors or contrast agents, SPION biocompatibility is the first requirement to satisfy. Secondly, selective targeting is searched to interact with selective cancer cell markers. SPION functionalization with highly selective peptides and/or antibodies leads to specific targeting of given cancer cells. Thirdly, the decoration of SPION with drugs or peptides provides a multimodal therapeutic approach reinforcing the cancer cell elimination avoiding tumour relapses.

During the presentation, I will introduce the fundamentals of this multidisciplinary discipline where biologists, chemists, physicists and engineering work together for i) synthesizing SPION with optimal colloidal and magnetic features, ii) characterizing dynamical magnetic and thermal properties of SPION, iii) functionalizing with anticancer agents, and iv) evaluating the anticancer potential of SPION in in vitro and in vivo studies.
19 March 2013, 11:00 h. Salón de Actos
Effective lattice Hamiltonian for a monolayer MoS2
Reza Asgari
School of Physics, Institute for Research in Fundamental Sciences, IPM, Tehran,
Two dimensional (2D) materials can be mostly exfoliated into individual thin layers from stacks of strongly bonded layers with weak interlayer interaction. A famous example is graphene. The 2D exfoliates versions of transition metal dichalcogenides exhibit properties that are complementary to and distinct from those in graphene. MoS2 is a hexagonal crystal layered structure with a covalently bonded S-Mo-S hexagonal quasi-two dimensional network which does not have inversion symmetry, packed by weak van der Waals interactions. The monolayer of MoS2 has provided a new material with a peculiar structure for the charge and the spin interactions. Due to the peculiar band structure, a variety of nanoelectronic applications including valleytronics, spintronics, optoelectronics and room temperature transistor are suggested for MoS2.
In this talk, I will briefly discuss about the some essential properties of MoS2 from theoretical and experimental point of views and then move on to the main part of my talk proposing an effective lattice Hamiltonian for monolayer MoS2 in order to describe the low-energy band structure and investigating the effect of perpendicular electric and magnetic fields on its electronic structure. We derive a tight-binding model based on the hybridization of d orbitals of Molybdenum and p orbitals of Sulfur atoms and then, introduce a modified two-band continuum model of monolayer MoS2. Our theory reveals a difference in electron and hole masses and provides trigonal warping effects. Furthermore, we predict a valley degeneracy breaking effect in Landau levels. Besides, we also show that application of a gate field perpendicular to the monolayer, breaks the vertical mirror symmetry and induces different potentials in the three sublayers, can slightly modify the electronic structure including the band gap and effective masses.
14 March 2013, 12:00 h. Sala de Seminarios, 182
Electrical properties and stability of single-molecule junctions formed with a scanning tunneling microscope in ambient conditions
Teresa González
IMDEA-Nanociencia
The realization and study of molecular junctions formed by a few (down to one) molecules bonded between two metallic electrodes opens the possibility of directly investigating electrical properties of compounds at the ultimate level of a single molecule. Apart from the determination of their electrical conductance, it is important to understand which factors play a role in the formation and breakage of the molecular junctions, and how stable we can expect them to be.
In this presentation, I will summarize some of the most relevant achievements in the field in the last 10 year. Then, I will present our studies on the molecular junctions formed by thiol-, amine-, and C60-terminated molecules. We use a STM (scanning tunneling microscope) to create and characterized single-molecule junctions using different techniques. We compare thiols and amines on the alkane family [1] and an oligo(phenylenethynylene) (OPE) [2,3]. We find that amines affect atomic rearrangement at the electrodes significantly less than thiols [1], while they can form very stable junctions, and have adsorption lifetimes well above a few seconds [3]. In addition, we explore the potential of C60 as terminal group for molecular junctions [4,5]. Its larger size allows us to identify isolated molecules in our STM images, which can be targeted individually guaranteeing the formation of single-molecule junctions. We also performed simultaneous conductance and thermopower measurements on junctions formed by one and two pristine C60 molecules in series [6], showing the potential of inter-molecular interactions manipulation for increasing the thermopower.

[1] C. R. Arroyo, et al., J. Am. Chem. Soc., 133, 14313 (2011).
[2] M. T. González et al., J. Phys. Chem. C, 115, 17973 (2011).
[3] M. T. González et al., J. Am. Chem. Soc (2013) accepted for publication.
[4] E. Leary et al. Nano Letters, 11, 2236 (2011).
[5] K. Gillemot et al., submitted.
[6] C. Evangeli et al., submitted.
07 March 2013, 12:00 h. Sala de Seminarios, 182
Beyond time-dependent charge transport: noise and thermoelectric effects.

Janine Splettstoesser
Aachen University
Nanoscale systems driven by time-dependent signals, such as quantum pumps, have recently attracted a lot of attention since they can serve as controlled sources of single particles [1]. Furthermore it can be shown, that time-dependent transport provides an intriguing spectroscopy tool, revealing quantum effects that are not accessible from a stationary state measurement [2].

In this talk I will present different examples for these particular characteristics of time-dependently driven quantum dot devices. Due to the smallness of these setups many-body effects like the Coulomb interaction, as well as quantum fluctuations play an important role for the transport properties and their signatures are observable in charge transport.
In addition to the transported charge I will discuss the transport noise induced by the time-dependent modulation[3]. Interestingly, there can be pumping noise even in the absence of charge pumping, which gives additional insight into the underlying transport mechanism.

Finally, I will talk about the thermoelectric performance of driven quantum dots [4]. Under certain conditions not only quantized charge pumping can be realized, but also the heat current exhibits plateaus, related to the spin degeneracy of the system. This renders possible the operation of time-dependently driven quantum dot devices as nanoscale engines, in particular as battery chargers, cooling devices or heat engines.


[1] G. Feve et al., Science 316, 1169 (2007); M. D. Blumenthal, et al., Nature Physics 3, 343 (2007); V. F. Maisi, et al., New J. Phys. 11, 113057 (2009).
[2] F. Reckermann, J. Splettstoesser, and M. R. Wegewijs, Phys. Rev. Lett. 104, 226803
(2010).
[3] R.-P. Riwar, J. Splettstoesser, and J. Konig, arxiv:1212.3545.
[4] S. Juergens, F. Haupt, M. Moskalets, and J. Splettstoesser, in preparation.
28 February 2013, 12:00 h. Sala de Seminarios, 182
Graphene electronics and photonics
Tony Low
IBM TJ Watson Research Center
Graphene possesses unique properties for electronic and photonic applications, such as gate tunability, high carrier mobility, wide-band optical absorption extending into terahertz regime and compatibility with silicon processing technologies. Drawing upon theoretical and modeling studies of in-house experiments, I will discuss several key issues related to the performances of graphene electronic and photonic devices. First, I will discuss how deformation and morphological structures found in large scale growth graphene can serve as dominant electronic scattering centers, compromising performance in high-speed electronic devices and the possibility for novel graphene electronics by strain engineering. Second, I will discuss how substrate polar phonons can provide energy dissipation pathway for optically excited carriers, and suppressing these energy loss channels would allow for more efficient graphene photodetectors. In addition, coupling of collective electronic excitations with these phonons can lead to modified plasmon dispersions and losses, where long-lived hybrid plasmon-phonon coupled mode can be utilized for tunable plasmonic-based bolometers.
26 February 2013, 12:00 h. Sala de Seminarios, 182
Optical Phonon Lasing in Semiconductor Double Quantum Dots
Rin Okuyama
Faculty of Science and Technology, Keio University, Japan
We propose optical phonon lasing for a double quantum dot (DQD) fabricated in a semiconductor substrate. We show that the DQD is weakly coupled to only two LO phonon modes that act as a natural cavity. The lasing occurs for pumping the DQD via electronic tunneling at rates much higher than the phonon decay rate, whereas an antibunching of phonon emission is observed in the opposite regime of slow tunneling. Both effects disappear with an effective thermalization induced by the Franck-Condon effect in a DQD fabricated in a carbon nanotube with a strong electron-phonon coupling.
21 February 2013, 12:00 h. Sala de Seminarios, 182
Anomalies and transport properties of chiral fermions
Karl Landsteiner
Instituto de Física Teórica UAM/CSIC
Quantum anomalies are among the most characteristic and fundamental properties of relativistic quantum field theory. They are known since the late 60s and play major role in high energy physics. But only recently it has be realized that anomalies have profound impact on the transport properties of relativistic fluids. In my talk I will discuss the classification of the anomaly induced transport coefficients from the viewpoints of hydrodynamics and Kubo formulae. I will give an overview of the role they play in the physics of heavy ion collisions, focusing on the so-called chiral magnetic effect (CME). Finally I will make some (possibly unqualified) comments on applications to the physics of Weyl-semi metals.
19 February 2013, 12:00 h. Sala de Seminarios, 182
Rydberg-Atom Quantum Simulation of a Topological Mott Insulator
Markus Müller
Universidad Complutense de Madrid
In this talk we consider a system of spinless fermions with nearest and next-to-nearest neighbor repulsive Hubbard interactions on a honeycomb lattice within a mean-field theory treatment, and propose and analyze a realistic scheme for analog quantum simulation of this model with cold atoms in a two-dimensional hexagonal optical lattice. Within the mean-field-theory treatment, the system exhibits besides a semi-metallic and a charge-density-wave ordered phase a quantum anomalous Hall phase. The latter is generated dynamically, i.e. purely as a result of the repulsive fermionic interactions and in the absence of any external gauge fields. We establish the topological nature of this dynamically created Mott insulating phase by the numerical calculation of a Chern number, and study the possibility of coexistence of this phase with the other phases characterized by local order parameters. Based on the knowledge of the mean-field phase diagram, we then discuss in detail how the interacting Hamiltonian can be engineered effectively by state-of-the-art experimental techniques for laser-dressing of cold fermionic ground-state atoms with electronically excited Rydberg states that exhibit strong dipolar interactions.

Reference: A. Dauphin, M. Müller, and M. A. Martin-Delgado, Phys. Rev. A 86, 053618 (2012) (arXiv:1207.6373)
14 February 2013, 12:00 h. Sala de Seminarios, 182
Multi-orbital structure of the two-dimensional electron gas in Perovskites/Perovskites heterostructures
Juan I. Beltrán
Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC)
Conductive two-dimensional electron gas (2DEG) has been found at oxide heterojunctions and was initially explained by the polarization discontinuity formed at the interface. Here, I will discuss about the results obtained using density functional theory calculations on the formation of a ferromagnetic 2DEG dxy electron sheet strictly confined to the TiO2 interface layer in LaAlO3/SrTiO3 heterostructures. The complex subband structure of the 2DEG generated at the TiO2/LaO (001) interface is universal, and almost independent of the SrTiO3 thickness. It is composed of a ladder of dxy states of light electrons and only one degenerate dxz,yz heavier subband. All the states are spin-polarized although the exchange splitting is only significant for the lowest energy dxy subband, which leads to magnetic moments ferromagnetically coupled and localized at the interface. The SrTiO3 ferroelectric-like lattice distortions determine the subband occupation and therefore their orbital character, exchange splitting and charge density profile.
07 February 2013, 12:00 h. Sala de Seminarios, 182
Advanced methods in Magnetic Force Microscopy
Agustina Asenjo
Instituto de Ciencia de Materiales de Madrid
After the development of the Scanning Probe Microscopy (SPM) an intense activity was carried out by many research groups all around the world. Consequently, the number of the SPM applications increased rapidly in the following years; this is the case of the Magnetic Force Microscopy (MFM). More than 20 years after its invention, the MFM has become a widespread tool to characterize magnetic materials and structures at nanoscale.
In this seminar we will present different MFM-based techniques developed in our laboratory. The Variable-Field MFM [1] allows to perform MFM measurements under variable magnetic fields in order to gain information about reversal magnetization processes [2,3]. In particular, the hysteresis loops of MFM probes or individual nanostructures [4] have been obtained. We have demonstrated the usefulness of the combination of Kelvin Probe Force Microscopy (KPFM) and MFM techniques to distinguish the electrostatic and magnetic tip-sample forces. Moreover, the split of both contributions in real time is crucial to study low magnetic moment materials [5,6] and devices [7]. Finally, we will present recent results about the study of magnetic dissipation processes in MFM [8].
[1] M. Jaafar et al., Ultramicroscopy 109, 693 (2009)
[2] M. Jaafar et al., 19, 285717 (2008)
[3] M. Jaafar et al., Phys. Rev. B 054439 (2010)
[4] M. Jaafar et al., Beilstein J. Nanotechnol., 2, 552-560 (2011)
[5] D. Martínez-Martín et al., Phys. Rev. Lett. 105, 257203 (2010)
[6]M. Jaafar et al., Nanoscale Research Letters, , 6 407 (2011)
[7] V. Panchal et al., IEEE Trans. Magn, accepted
[8] O. Iglesias-Freire et al., APL, accepted.
31 January 2013, 12:00 h. Sala de Seminarios, 182
Valleytronics in Silicon: The principles and the practice
André L. Saraiva
Universidade Federal do Rio de Janeiro
Alternative schemes for information processing are being pursued by the industry. Spintronics - the most prominent candidate for alternative electronics - explores an internal degree of freedom of electrons, namely the spin, in order to encode information. Recent advances in the manipulation of the valley degree of freedom of certain semiconductors are opening the possibility for a third option: Valleytronics. But in order to achieve useful control of valley, large enough valley-orbit induced valley splitting (VS) must be achieved.

The vast majority of the experiments in silicon heterostructures points toward a VS of the 2DEG much too small for any practical use. Confinement of electrons in all three dimensions mitigates this problem, but the control of the VS over a useful range still requires better VS than what is available so far. In this talk we suggest innovative techniques for enhancing the VS that go beyond the usual recipe of ―sharper interfaces. We focus on Si/SiGe and Si/SiO2 heterostructures.
The industry standard (and so far most popular research platform) for Si/SiGe heterostructures is a Si quantum well embedded in SiGe alloy barriers. This leads to high quality heterostructures with low noise (less trapped charges commonly found in SiO2), but the alloy is intrinsically disordered, which reduces the VS drastically. We will show that a much better solution is to adopt Si/Ge superlattices [1], so that 1) the ideal barrier is intrinsically atomically ordered, being limited only by growth-related interdiffusion of Ge into Si; 2) the layer widths can be chosen to generate constructive interference between the scattered valley states, leading to VS as large as 8 meV. As for the noisier SiO2 barriers, there is experimental evidence of enhanced VS (up to room temperature) which seems to be specific to interfaces grown by a certain method (buried oxide - BOX) [2]. Otherwise, all other SiO2 samples have the same problem as SiGe. We will show that certain extended Shockley interface states may hybridize with the conduction states opening a new conduction channel, without forming a trap [3]. This mechanism leads to much enhanced VS, which is compatible to those observed in Ref. 3. We will also show a more detailed study of the nature of this electronic state and possible ways to detect it experimentally[4], which might confirm our prediction.

1.L. Zhang, J.-W. Luo, A Saraiva, Belita Koiller, Alex Zunger (in preparation).
2.K. Takashina, Y. Ono, A. Fujiwara, Y. Takahashi and Y. Hirayama, PRL 96, 236801 (2006).
3.A. Saraiva, Belita Koiller and M. Friesen, PRB 82, 245314 (2010).
4.A. Dusko, A. Saraiva and Belita Koiller (in preparation).
24 January 2013, 12:00 h. Sala de Seminarios, 182
Orbital polarization at complex oxide hetero-interfaces, an angular magnetoresistance study
Norbert Nemes
Departamento de Fisica Aplicada III. Univesidad Complutense de Madrid
Electronic reconstruction at the interface between complex oxides is known to give rise to new electronic ground-states with emergent responses. Charge transfer, modified orbital structure, changes (rotations and tilts) of the oxygen octahedra all strongly couple to the magnetic structure and may cause profound spin rearrangements. . At the interface between a manganite and a titanate there is a new Mn-O-Ti superexchange path through the interfacial oxygen that results in both charge transfer from the Mn4+ to the Ti2+ and an induced magnetic moment on the otherwise non-magnetic Ti ion. Furthermore, x-ray linear dichroism, reveal a substantial orbital polarization indicating that the relevant d-orbitals in this exchange are the d3z2-r2. This interfacial interaction can drive multilayers of LaMnO3/SrTiO3, made of non-magnetic and insulating constituents into a magnetic and metallic system. In multilayers of La0.7Sr0.3MnO3/SrTiO3 and La0.7Sr0.3MnO3/BaTiO3 we found an unexpected and highly anomalous form of symmetry breaking as identified from low temperature magnetoresistance sweeps as the applied magnetic field is rotated away from the surface-normal, precisely around the direction parallel to the film. This effect disappears rapidly with temperature, above 20 K, well below the Curie temperature, and with decreasing magnetic field, below 3 T, well above the saturation field. This indicates that its origin is related to the Lorentz-magnetoresistance, sensitive to the band structure and not AMR, sensitive to conventional magnetic anisotropy. The effect also saturates with increasing magnetic field, around 10 T at the lowest temperatures. We argue that this anomaly reflects the orbital polarization at the Mn/Ti interface. I will discuss the possible competition between various energy scales (thermal, Zeeman field, and eg orbital separation) hinting at interesting physics. Were this idea to find theoretical support, low temperature angular magnetoresistance could become a quick screening tool of orbital polarization at a wide variety of complex oxide hetero-interfaces: a rapidly increasing family of materials attracting great interest for both basic physics and technological applications.
17 January 2013, 12:00 h. Sala de Seminarios, 182
Gap generation in topological insulator surface states by nonferrimagnetic magnets
Alberto Cortijo
Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC)
Topological insulators in three spatial dimensions (3D TI s) display a distinctive electromagnetic response encoded in the topological magnetoelectric or axion term. This response gives rise to exotic optical properties that might be used to identify such state of matter. The key point for the observability of this topological response is opening a gap in the surface states by time reversal breaking perturbations. In this talk I will review the electromagnetic properties of 3D TI s and discuss in which ways it is possible to open a spectral gap in the mentioned electronic surface states
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