Seminarios y Eventos

Seminars and Events

Teoría y Simulación de Materiales: Seminarios Alternativos

Coordinadores: Eduardo Hernández, Rafael Roldán


30 de noviembre de 2017, 12:00 h. Salón de Actos

SOME APPLICATIONS OF THE PHYSICS OF EXCEPTIONAL POINTS IN 3D TOPOLOGICAL SEMIMETALS

José González Carmona
Instituto de Estructura de la Materia CSIC

We will reexamine the topological protection of surface states in Dirac, Weyl and nodal-line semimetals by characterizing them as evanescent waves for complex values of the momentum. We will find in this way a sequence of exceptional points —that is, branch points with real energy in the complex spectrum— allowing us to identify the set of surface states decaying into the 3D semimetal. The topological protection of the surface states stems in this approach from the fact that the branch cuts in the spectrum cannot be closed by small perturbations. In the case of Weyl semimetals under circularly polarized light, we will see that new exceptional points arise within the gap opened between Floquet bands, as a signature of new surface states produced by the radiation. We will also show that the exceptional points survive when nodal-line semimetals are placed in a strong magnetic field, leading to Landau surface states and the appearance of chiral currents at the surface of the 3D material.


*Work in collaboration with R. A. Molina



16 de noviembre de 2017, 12:00 h. Sala de Seminarios, 182

Persistent Spin Textures in Semiconductor Nanostructures

John Schliemann
Universität Regensburg

Device concepts in semiconductor spintronics make long spin lifetimes desirable, and the requirements put on spin control by schemes of quantum information processing are even more demanding. Unfortunately, due to spin-orbit coupling electron spins in semiconductors are generically subject to rather fast decoherence. In two-dimensional quantum wells made of zinc-blende semiconductors, however, the spin-orbit interaction can be engineered in such a way that persistent spin structures with
extraordinarily long spin lifetimes arise even in the presence of disorder and imperfections. We review experimental and theoretical developments on this subject both for $n$-doped and $p$-doped structures, and we discuss possible device applications.


John Schliemann,
Persistent Spin Textures in Semiconductor Nanostructures,
Rev. Mod. Phys. 89, 011001 (2017).

John Schliemann, J. Carlos Egues, and Daniel Loss,
Non-ballistic spin field-effect transistor,
Phys. Rev. Lett. 90, 146801 (2003).



02 de noviembre de 2017, 12:00 h. Sala de Seminarios, 182

Decoupling and decoherence for single-spin cavity QED

Bill Cosh
McGill Univ, Dept Phys, Montreal, PQ H3A 2T8, Canada

Microwave-frequency superconducting resonators are ideally suited to perform dispersive qubit readout, to mediate two-qubit gates, and to shuttle states between distant quantum systems. A single electron spin in a double quantum dot exposed to a spatially inhomogeneous magnetic field can couple to such a resonator through an artificial spin-orbit coupling. I will discuss our theoretical analysis of a device geometry [1] that can be used to optimize this coupling and new strategies to suppress unwanted additional decoherence sources introduced by the magnetic-field gradient. In addition, I will present and characterize a novel dynamical decoupling protocol [2] that minimizes the influence of inhomogeneous broadening due to hyperfine coupling to nuclear spins or charge noise, while inhibiting “heating” of the resonator. These theoretical ideas can help in the preparation and control of quantum states in spin-resonator systems. The very recent experimental demonstration of the strong-coupling limit in such a system [3] suggests that exciting applications may be just around the corner.

[1] F. Beaudoin, D. Lachance-Quirion, W. A. Coish, M. Pioro-Ladrière, Nanotechnology 27, 464003 (2016)
[2] F. Beaudoin, A. Blais, W. A. Coish, New Jour. Phys. 19, 023041 (2017)
[3] X. Mi et al., arXiv:1710.03265



26 de octubre de 2017, 12:00 h. Sala de Seminarios, 182

Imaging, controlling and harnessing non-collinear magnetism in perovskite oxides

Manuel Bibes
Unité Mixte de Physique CNRS/Thales, Palaiseau (FRANCE)

In magnetic perovskite oxides ABO3, first-neighbour antiferromagnetic super-exchange interactions usually dominate, but may coexist with other terms such as ferromagnetic double-exchange or Dzyaloshinskii-Moriya interactions at B-O-B and A-O-A bonds. This often produces non-collinear spin configurations leading to weak ferromagnetism or to spatially modulated spin structures. A prototypical non-collinear magnetic oxide is multiferroic BiFeO3 that shows a cycloidal order with a 64 nm period in the bulk. In this talk, I will show how epitaxial strain can be used to tailor the magnetic order of BiFeO3 thin films1,2 and present real-space images of the cycloidal structure, as well as its manipulation by an electric field3. In a second part I will discuss how magnetic spin texture can influence the Hall response in strongly correlated oxide thin films.



1. Sando, D. et al. Crafting the magnonic and spintronic response of BiFeO3 films by epitaxial strain. Nature Mater. 12, 641–6 (2013).
2. Agbelele, A. et al. Strain and Magnetic Field Induced Spin-Structure Transitions in Multiferroic BiFeO3. Adv. Mater. 29, 1602327 (2017).
3. Gross, I. et al. Real-space imaging of non-collinear antiferromagnetic order with a single spin magnetometer. Nature 459, 252 (2017)



05 de octubre de 2017, 12:00 h. Sala de Seminarios, 182

From molecules to frameworks: The reticular approach for the discovery of materials

Felipe Gándara
Instituto de Ciencia de Materiales de Madrid ICMM-CSIC

Reticulated materials are built up by the linkage of building blocks through strong bonds, forming extended, crystalline frameworks with predefined structural and topological features [Yaghi et al. Nature 2003, 423, 705]. The rise of reticular chemistry during the last two decades has endowed material scientists with new tools for the design and preparation of new functional materials with precise structural control. Metal-organic frameworks (MOFs) [Furukawa et al. Science 2013, 341, 974], and covalent-organic frameworks (COFs) [Waller et al. Acc. Chem. Research 2015, 48, 3053], have thus emerged as versatile reticulated porous materials with interest and application in multiple fields, such as gas storage, drug delivery, catalysis, electronic, etc.
In this talk, I will start with a general, basic overview of the implementation of the reticular chemistry principles in the design of functional materials, showing relevant and illustrative examples that have contributed to the development of the field. Following, I will focus on new avenues that have been opened by reticular chemistry to achieve materials with higher levels of complexity. In this context, MOFs and COFs appear as advantageous platforms thanks to the possibility of combining order and heterogeneity in a same material, for example by the incorporation of multiple metal cations within a same crystal, by forming arrangements of controllable composition and disposition [Castillo-Blas et al. Science Advances, 2017, 3, e1700773].



28 de septiembre de 2017, 12:00 h. Sala de Seminarios, 182

Polaritons for Chemistry and Materials Science

Francisco J. Garcia-Vidal
Departamento de Física Teórica de la Materia Condensada & IFIMAC, UAM

Exciton transport plays a crucial role in natural phenomena such as photosynthesis and artificial devices such as organic solar cells, but is inefficient in many organic materials. We will discuss how the formation of collective polaritonic modes can dramatically enhance exciton conductance when the molecules are strongly coupled to an electromagnetic mode [1], which can be exploited to harvest and direct excitations to specific positions by tuning the spatial distribution of the EM mode [2]. We show that in systems with a discrete EM mode spectrum, strong-coupling-enhanced exciton transport can proceed through “dark” modes that have no photonic component, but which nonetheless acquire a delocalized character in the strong-coupling regime [3].
In the second part, we discuss the influence of strong coupling on molecular structure and chemical reactions. While most models of strong coupling are based on simple two-level models, pioneering experiments have shown modifications of chemical rates under strong coupling [4]. We have developed a first-principles model that fully takes into account both electronic and nuclear degrees of freedom [5]. We discuss the applicability of the Born-Oppenheimer approximation, challenged by the introduction of the new intermediate timescale of energy exchange between the molecule and the field. We show how photochemical reactions such as photoisomerization can be almost completely suppressed under strong coupling [6]. Finally, we show how polaritons can also lead to the formation of a polaritonic super-molecule involving the degrees of freedom of all molecules, opening a reaction path on which all involved molecules undergo a chemical transformation [7].



14 de septiembre de 2017, 12:00 h. Sala de Seminarios, 182

Quantum Hall ferromagnetism in multicomponent systems

Angelika Knothe
Université Paris Sud

Within the framework of Quantum Hall Ferromagnetism [1, 2] we theoretically investi- gate the properties of different two-dimensional electron systems in the quantum Hall regime in which the electrons may be endowed with multiple discrete degrees of freedom. Different orderings of these electronic degrees of freedom, treated as different spins and isospins, characterise different possible phases of the system. Using Hartree Fock mean field theory we treat several examples: We analyse how the structure of ground and excited states in a finite piece of monolayer graphene, where the electrons carry besides the electronic spin also a valley-isospin degree of freedom, is influenced by the presence of a boundary with respect to the bulk properties [3]. The properties of the resulting edge state structure is discussed. For a graphene bilayer, where an additional orbital isospin comes into play due to a degeneracy between the n=0 and the n=1 Landau level, we identify the various different ground state spin and isospin phases which emerge as func- tions of external electric and magnetic fields and investigate the properties of phases that exhibit non-trivial coherence properties of the isospin degrees of freedom [4]. Finally, as an outlook, possible extensions to two-dimensional surface states of three-dimensional crystals are discussed.
1] Yafis Barlas, Kun Yang, and A. H. MacDonald. Nanotechnology, 23:052001, 2012. [2] Sankar Das Sarma and Aron Pinczuk. John Wiley & Sons, New York, 1996.
[3] Angelika Knothe and Thierry Jolicoeur. Physical Review B, 92:165110, 2015.
[4] Angelika Knothe and Thierry Jolicoeur. Physical Review B, 94:235149,



30 de junio de 2017, 12:00 h. Sala de Seminarios, 182

Magnetism beyond the 3d transition metal oxides

Priya Mahadevan
S.N. Bose National Centre for Basic Sciences. Saltlake. Kolkata. India.

The small bandwidths and the large Hund’s exchange interaction strength lead to magnetism being found among the 3d transition metal oxides, with few examples among the 4d and the 5d transition metal oxides. It was therefore a surprise when a 4d transition metal oxide, SrTcO3, was discovered with an ordering temperature much higher than its 3d counterpart at the same d-band filling. In this talk, I will highlight our work based on a combination of abinitio as well as model Hamiltonian calculations to understand the high magnetic or- dering temperatures in this class of compounds. We find that this is a generic result associated with a half-filling of the band. Further, in contrast to usual expectations, the ordering temperature is enhanced in the limit of a large band- width and a small Hund’s exchange interaction. This could result in examples among 5d oxides to have higher magnetic ordering temperatures than their 3d counterparts. Doping electrons into these 4d and 5d oxides offers a route to realizing antiferromagnetic metals in contrast to doping in 3d oxides which lead to a ferromagnetic metallic state. The role of the large Hund’s rule in dictat- ing the magnetism in the metallic state of the doped systems will be discussed. This is work done in collaboration with Srimanta Middey, Ashis Nandy, Shishir Pandey and D.D. Sarma.



13 de junio de 2017, 12:00 h. Sala de Seminarios, 182

Charge Reconfiguration in Arrays of Quantum Dots

J. C. Bayer
Institut fur Festkorperphysik, Leibniz Universitat Hannover

Quantum dots are known as promising candidates for semiconductor qubit architectures. The potential scalability of lateral quantum dot structures has already been demonstrated. With increasing number of quantum dots however, the tuning complexity increases dramatically. Repro- ducible and non-complex methods for tuning arrays of quantum dots into any desired configuration while preserving the high tunability are thus required. Decoupling the quantum dot array from the reservoirs is one approach to achieve this. The presentation will cover a few experimental basics and will focus on the discussion of features occurring in isolated quantum dot arrays with up to four quantum dots. Experimental observations are complemented by model calculations and the shuttling of individual electrons between quantum dots along chosen paths is demonstrated.



08 de junio de 2017, 12:00 h. Sala de Seminarios, 182

Characterisation and simulation of the structure of glasses

Francisco Muñoz
Instituto de Cerámica y Vidrio (ICV-CSIC)

Glasses that are produced through quenching of a melt are generally homogenous and isotropic transparent solids characterized by its glass transition temperature. The transition from the undercooled liquid to the rigid solid occurs without discontinuities or phase transitions and is reversible. Most properties of glasses, such as density or refractive index, are a direct function of the proportions of oxide components and constitute the group of additive properties. However, there are cases in which the properties show non-linear trends due to particular structural variations with composition. This is typically the case of the transport properties of glasses that depend on the diffusion of matter throughout the glass, and two examples of non-linear structural influence on the glass properties are the boric anomaly and the mixed alkali effect (Navarro, El vidrio, CSIC, Madrid 2003). The analysis of the structure of glasses have thus become of a very high importance in the design of glasses with new and improved properties. This talk will give an introduction to the glassy state and the main theories that have dealt with the structure of glasses from the first work by Zachariasen, back in 1932 (Zachariasen, J. Am. Chem. Soc. 54 (1932) 3841). Then, some examples of the study of glass structures will be presented, such as the case of borosilicate glasses, P2O5-containing borosilicate glasses (Muñoz et al. J. Non-Cryst. Solids 352 (2006) 2958), phosphate glasses (Yaqoot et al. J. Mat. Chem. B 3 (2015) 9360) and oxynitride phosphate glasses (Muñoz et al. J. Non-Cryst. Solids (2013)).



01 de junio de 2017, 12:00 h. Sala de Seminarios, 182

Control of Hole Spin in Lateral Gated Quantum Dot Devices

Marek Korkusinski
Security and Disruptive Technologies, National Research Council, Ottawa, Canada

In semiconductor materials, the wave functions of holes are built from p-type atomistic orbitals. This leads to a weaker hyperfine interactions of the hole spin with nuclear lattice spins and thus promises longer coherence times compared to those of electron spins [1,2]. However, holes are also subject to much stronger spin-orbit interactions (SOI). This talk explores the new physics brought about by the SOI for few carrier systems and discusses how it influences the magneto-transport spectra of GaAs lateral double quantum dots (DQD).
We start with a systematic analysis of the properties of a single hole in the DQD potential. We map out the spin-conserving and spin-flipping tunneling (SFT) processes, of which the latter is due to the SOI. In contrast with silicon hole devices, where the SOI are much weaker [3], in our GaAs heterostructure both tunneling processes are of similar magnitude. This allows us to understand our single-hole DQD as a hybrid spin-charge system, in which all degrees of freedom can be controlled electrically. This is in clear contrast to single-electron devices, where coherent rotations of single electron spin typically require non-uniform or time-dependent magnetic field. By analyzing the tunneling current through our system under microwave-frequency modulation of gate voltages (photon-assisted transport) we extract the characteristic coherence and relaxation times for our device.




30 de mayo de 2017, 12:00 h. Sala de Seminarios, 182

Orbital Selective Physics of Iron Superconductors emerging from strong- and weak-correlated approaches

Laura Fanfarillo
SISSA/ISAS (International School of Advanced Studies), Trieste.

Understanding the nature and strength of correlations in iron based superconductors (IBS) is key to unveil the nature of the various phases that appear in the phase diagram of these systems. The analysis of correlation effects in IBS however is not an easy task since it is challenged by the multi-orbital character of the electronic band structure close to the Fermi level [1]. Contrasting evidences of weak or strong regime of electronic correlation forced the use of low-energy effective models [2] or more correlated approach e.g. multiorbital version of the Hubbard model [3].

In this seminar I will focus on the analysis of the nematic phase of IBS. I will show how using both a strong- [4] and weak-correlated approach [5, 6] complementary and consistent results can be found. In particular the orbital selective character of the interactions both at low- and high-energy is found as a common aspect emerging from weak- and strong-correlated approaches. This result opens new questions about the relevance of the orbital-selectivity in the pairing mechanism.

[1] L. De Medici, Springer Series in Materials Science, Vol. 211, 409 (2015)
[2] R. Fernandes, A.V. Chubukov, Rep. Prog. Phys. 80, 014503 (2017)
[3] L. Fanfarillo and E. Bascones, Phys. Rev. B 92, 075136 (2015)
[4] L. Fanfarillo, G. Giovannetti, M. Capone and E. Bascones, Phys. Rev. B 95, 144511 (2017).
[5] L. Fanfarillo, et al., Phys. Rev. B 91, 214515 (2015)
[6] L. Fanfarillo, et al., Phys. Rev. B 94, 155138 (2016)



25 de mayo de 2017, 12:00 h. Sala de Seminarios, 182

Antimonene: Experiments and theory of surface conductivity

Juan José Palacios
Dpto. Física de la Materia Condensada. Universidad Autónoma de Madrid.

Very recently antimony has been demonstrated to be amenable to standard exfoliation procedures opening the possibility of studying the electronic properties of isolated few-layers flakes of this material, a.k.a. antimonene [1]. Antimony is a topological semimetal, meaning that its electronic structure presents spin-split helical states (or Dirac cones) on the surface, but it is still trivially metallic in bulk. Antimonene, on the other hand, may present a much reduced electronic bulk contribution for a small number of layers. A novel technique to make electrical contacts on the surface of individual thin flakes (5-10 monolayers) has allowed us to measure the (surface) conductivity of these in ambient conditions. Our measurements show a high conductivity in the range of $1-2 , e2/h$, which we attribute to the surface helical electrons. We have also carried out theoretical work to address the origin of this value, in particular, we have studied the role scattering for the helical electrons and the bulk bands. Our calculations are based on density functional theory for the electronic structure and Kubo formalism for the conductivity, the latter considering random disorder and the presence of water.

[1] P. Ares et al., Advanced Materials 28, 6515 (2016)



18 de mayo de 2017, 12:00 h. Sala de Seminarios, 182

Atomic scale imaging of strongly correlated electronic states

Ana Isabel Maldonado Cid


The development of scanning probe techniques has allowed to obtain information about the local electronic structure of different materials [1]. Among them, those which show strong electronic correlations are especially interesting to study using these tools, as their electronic structure at low energies is, in many cases, not yet fully understood [2]. In this talk, I will give an overview about the use of scanning tunneling microscopy/spectroscopy (STM/S) to explore the local electronic structure of different strongly correlated electron systems. In particular, I will show a study of of a noncentrosymmetric superconductor, BiPd, using STM/S in combination with macroscopic measurements and relativistic first-principles calculations [3]. Later, I will describe the visualization of the superconducting vortex lattice of CeCu2Si2, the first heavy-fermion superconductor discovered, by means of STM/S [4]. Moreover, I will present a recent study of the local low-energy electronic structure of SmB6, a Kondo insulator which is candidate to show topologically protected surface states, using STM/S and DFT calculations [5]. Finally, I will discuss some open questions and future prospects in these systems.

[1] R. Wiesendanger, Scanning probe microscopy and spectroscopy: methods and applications, Cambridge University Press, Cambridge, UK, 1994.
[2] A. Yazdani, E. Da Silva Neto and P. Aynajian, Ann. Rev. Condens. Matter Phys. 7, p. p. 11-33 (2016).
[3] Z. Sun, M. Enayat, A. Maldonado et al, Nature Communications 6, p. 6633 (2015).
[4] M. Enayat, Z. Sun, A. Maldonado et al., "Superconducting gap and vortex lattice of the heavy-fermion compound CeCu2Si2", Physical Review B 93, p.



11 de mayo de 2017, 12:00 h. Sala de Seminarios, 182

Unconventional magnetotransport in Weyl semimetals

Alberto Cortijo
Instituto de Ciencia de Materiales de Madrid CSIC

In this talk I will review some transport properties in Weyl semimetals that are connected to the geometrical properties of the bandstructure in these systems, like the (now) famous B^2 dependent negative magnetoresistance, observed in these materials[1]. Within this context, I will describe other transport and optical phenomena that were supposed not to be allowed in these materials[2], like the linear magnetochiral effect[3]. I will describe how this effect also allows for the presence of second order non linear optical responses in inversion symmetric Dirac semimetals under the presence of an external magnetic field[4].

[1].D. T. Son and B. Z. Spivak."Chiral anomaly and classical negative magnetoresistance of Weyl metals". Phys. Rev. B 88, 104412 (2013)
[2]. A. A. Zyuzin and A. A. Burkov. "Topological response in Weyl semimetals and the chiral anomaly" Phys. Rev. B 86, 115133 (2012)
[3] A. Cortijo. "Linear magnetochiral effect in Weyl semimetals". Phys. Rev. B 94, 241105(R) (2016)
[4] A. Cortijo "Magnetic-field-induced nonlinear optical responses in inversion symmetric Dirac semimetals". Phys. Rev. B 94, 235123 (2016)



04 de mayo de 2017, 12:00 h. Salón de Actos

Fermi velocity renormalization of graphene (RELOADED)

Tobias Stauber
ICMM-CSIC

Fermi velocity renormalization in graphene was first predicted more than 20 years ago and it was recently verified, experimentally. Whereas there is strong renormalization especially for suspended samples, the optical response is almost unaltered compared to its non-interacting value. This issue has become a controversy for almost 10 years, now.

In this talk, I will address these topics based on a tight-binding model and thus avoiding possible ambiguities related to the band cutoff and/or chiral anomalies inherent to effective field theories. Making use of a topological invariant that protects the chirality of the Dirac electrons around the nodal points, we reduce the numerical cost to present detailed Hartree-Fock calculations for the first time. In particular, I will show that self-screening effects and finite electronic densities are crucial in order to describe the experimental data. Regarding the optical conductivity, we connect two apparently opposing results and thus (hopefully) settling this controversy after almost 10 years. We complement our study with state-of-the-art quantum Monte-Carlo calculations that show good agreement with our mean-field approach.

T. Stauber, P. Parida, M. Trushin, M. V. Ulybyshev, D. L. Boyda, and J. Schliemann, arXiv:1704.03747



27 de abril de 2017, 12:00 h. Sala de Seminarios, 182

THEORETICAL SIMULATIONS OF CARBON NANOSTRUCTURES: ENDOHEDRAL AND EXOHEDRAL FULLERENES

Manuel Alcamí
Departmento de Química, Universidad Autónoma de Madrid -UAM-, 28049 Madrid

In this seminar I will present our work done in the study of carbon nanostructures. A brief summary will be given in the work done on graphene deposited on metal surface and most part of the seminar will focus in our last work done on charged fullerenes and fullerene endohedral and exohedral derivatives. One of the most challenging task in fullerene research is the prediction of the most stable structures, due to the large number of isomeric forms accessible (e.g., more than 20 billion isomers for C60X8). We have developed a simple model, exclusively based on topological arguments, that allows to predict the relative stability of these fullerene species without the need for electronic structure calculations or geometry optimizations. This model also allows identifying the key structural motifs that explain the fullerene stability. We show that the subtle interplay between π delocalization, cage strain, and steric hindrance is responsible for the stability of these compounds. The most stable structures predicted by the model are in good agreement with those found in synthetic experiments performed in high-energy conditions and with high level ab inito calculations.

We have also performed Molecular Dynamics simulations to understand the interaction of charged fullerenes with He atoms in a He nanodroplet environment. Combination of these results with a simple model that describes the polarizability induced by the He atoms on the fullerene cage allows us to interpret the absorption line shifts as a function of the number of He atoms observed in recent experiments.



27 de abril de 2017, 16:00 h. Salón de Actos

Shot Noise in Single-Electron Tunneling through Quantum Dots

Rolf J. Haug
Institut für Festkörperphysik, Leibniz Universität Hannover

Electronic transport through quantum dots, i.e. quasi zero-dimensional systems in semicon-ductors, is governed by single-electron tunneling and Coulomb interactions. In studying shot noise in tunneling through quantum dots one has access to correlations in these systems in a variety of ways. In applying ultrasensitive noise and correlation measurements deep insight into the quantum mechanical properties of the systems studied is obtained [1]. Such electron transport experiments with quantum dots can be performed in very similar ways to experi-mental arrangements in quantum optics. So, e.g. electron pairs can be generated on demand and the splitting and partitioning of these electron pairs can be investigated [2]. Another scheme often applied in quantum optics is the usage of feedback to influence the quantum mechanical states. Recently, we were successfully applying feedback control in experiments with quantum dots and we were able to show that the full counting statistics of such systems can be strongly influenced [3,4]. In addition we analyzed shot noise in single-electron tunnel-ing in a variety of different ways to acquire further knowledge about our quantum dot sys-tems.

[1] N. Ubbelohde, et al; Nature Comm. 3, 612 (2012)
[2] N. Ubbelohde, F. Hohls, V. Kashcheyevs, T. Wagner, L. Fricke, B. Kästner, K. Pierz, H.W. Schumacher, R.J. Haug; Nature Nanotech. 10, 46 (2015)
[3] T. Wagner, P. Strasberg, J.C. Bayer, E.P. Rugeramigabo, T. Brandes, R.J. Haug; Nature Nanotech. 12, 218 (2017)
[4] T. Wagner, et al; Phys. Stat. Solidi B254, 1600791 (2017)



06 de abril de 2017, 12:00 h. Sala de Seminarios, 182

Complexity in some photonic systems

C. López
Materials Science Factory & Photonic Materials Department, ICMM

Border regions between physical problems involving either very few bodies with well-described interactions or immensely many (amenable to statistical description) often fall in the category associated to complexity [1]. I will try to identify trails of complexity in optical materials.

Disordered optical materials per se would not qualify as complex but when disorder is partial or merely incipient or non-linear interactions are included, a new character is added. I will offer examples of these optical systems.

On the one hand I present the optical properties of disordered materials with intentionally added defects. When a threshold concentration consistent with percolation is reached, a clear change is manifest in the spectroscopic features lineshapes.

The lack of a proper cavity renders Random Laseres non-directional, non-collimated and non-monochromatic. Some control can be attained by proper design of the scattering and gain components and, above all, the pumping scheme [2]. In this way it is possible to control the modes and their interactions and change the emission characteristics.

Certain conventional lasers have surprisingly shown signatures of replica symmetry breaking, allegedly arising from low cavity quality [3]. Thus geometrical frustration, unheard of in ordered configurations brings a simple system into the complexity realm.

Refs.

[1] Weaver, W. (1948). Science and complexity. American Scientist, 36, 536–544.

[2] Leonetti, M., et al. (2011). The mode-locking transition of random lasers. Nature Photonics, 5, 615.

[3] Basak, et al. (2016). Large fluctuations at the lasing threshold of solid- and liquid-state dye lasers. Scientific



04 de abril de 2017, 12:00 h. Sala de Seminarios, 182

Graphene: the good, the bad, the nano & the pseudo

Cristiane Morais Smith
Utrecht University. The Netherlands.

Graphene is probably the most fascinating material ever discovered, but it has a drawback: it does not exhibit the quantum spin Hall effect. By creating honeycomb lattices of compounds other than carbon, novel materials with unexpected properties may emerge. A key question is: if we build a honeycomb lattice out of semiconducting nanocrystals, is it going to behave like graphene or like the semiconducting building blocks? I will show that these systems, which were recently experimentally synthesized[1], combine the best of the two materials. They exhibit a gap at zero energy, as well as Dirac cones at finite energies. In addition, a honeycomb lattice made of CdSe nanocrystals displays topological properties in the valence band[2], whereas for HgTe very large topological gaps are predicted to occur in the conduction p-bands[3]. These artificial materials open the possibility to engineer high-orbital physics with Dirac electrons and to realize quantum (spin) Hall phases at room-T[3]. Then, I will discuss the effect of dynamical electromagnetic interactions in massive and massless 2D systems like transition-metal-dichalcogenides and graphene. By using the pseudo-QED approach, quantized edge states emerge and give rise, respectively, to a quantum Hall Effect (massive)[4] and a quantum Valley Hall effect (massless)[5], as a consequence of the parity anomaly.

[1] M.P. Boneschanscher et al., Science 344, 1377 (2014)
[2] E. Kalesaki et al., Phys.Rev. X 4, 011010 (2014)
[3] W. Beugeling et al., Nat.Comm. 6, 6316 (2015)
[4] L.O. Nascimento et al., arXiv:1702.01573
[5] E.C. Marino et al., Phys.Rev. X 5, 011040 (2015)



30 de marzo de 2017, 12:00 h. Sala de Seminarios, 182

Unconventional Superconductivity Cast in Iron

Raymond Osborn
Materials Science Division, Argonne National Laboratory, Argonne IL 60439, USA

Unconventional superconductivity usually occurs close to where spin, charge, or nematic order is suppressed by doping or pressure. The discovery of superconductivity in a number of iron-based compounds, with phase diagrams that resemble those of the cuprate high-temperature superconductors, has refocused attention on what constitute the essential conditions for unconventional superconductivity. Unlike the cuprates, the parent compounds of the iron-based superconductors are metallic, with multiple d-orbitals contributing to the Fermi surface, but there is an ongoing debate about whether the correlations are strong enough to justify quasi-localized models of the magnetism or whether the electronic properties are best described by weakly correlated itinerant models. In our investigations of a number of hole-doped iron arsenides, we have discovered a new magnetic phase that coexists with superconductivity. It has a unique double-Q structure with a non-uniform magnetization produced by the coherent superposition of orthogonal antiferromagnetic stripes. I will discuss how this observation is incompatible with orbital order and how it establishes that the magnetic order is best described as an itinerant spin-density wave driven by Fermi surface nesting.



22 de marzo de 2017, 12:00 h. Sala de Seminarios, 182

Isolation of Highly Stable Antimonene under Ambient Conditions. Optical and Electrical Properties

Pablo Ares
Department of Condensed Matter Physics, Universidad Autónoma de Madrid

Graphene paved the way for the rising of a whole family of 2D materials. Graphene is a semimetal with zero-gap and transition metal dichalcogenides present a band gap in the range 1.5-2.5 eV [1], inappropriate for some optoelectronics applications where 0.1-1.5 eV are preferred. Few-layer black phosphorous (BP) [2] presents an energy gap within this range. However, it is highly hygroscopic [3]. In the same group in the periodic table we also find antimony, a silvery lustrous, non-hygroscopic element with a layered structure similar to that of BP. Theoretical calculations [4] point towards a band gap suitable for these optoelectronics applications. We report micromechanical and liquid-phase exfoliation of antimony down to the single-layer regime and experimental evidence of its high stability in ambient conditions [5, 6]. We also present optimized optical identification [7] and preliminary results on the conductive properties of few-layer antimonene, which point to be governed by surface states. In this context, Probe-Assisted Nanowire (PAN) lithography is introduced; a novel technique to create nanoelectrodes that has allowed probing the electrical properties of tiny few-layer antimonene flakes.

[1] Wang, et al., Nat. Nanotech. 7, 699–712 (2012).
[2] Castellanos-Gomez, J. Phys. Chem. Lett. 6, 4280–4291 (2015).
[3] Island et al., 2D Materials 2, 011002 (2015).
[4] Aktürk, et al., Phys. Rev. B 91, 235446 (2015).
[5] Ares et al., Adv. Mater. 28, 6332–6336 (2016).
[6] Gibaja et al., Angew. Chem. Int. Ed. 55, 14345-14349 (2016).
[7] Ares et al., ACS Photonics DOI: 10.1021/acsphotonics.6b00941 (2017).



09 de marzo de 2017, 12:00 h. Sala de Seminarios, 182

Stardust: A machine to produce cosmic-dust analogs analogs and to study interstellar chemistry

J. A. Martín-Gago, J. Cernicharo
ICMM-CSIC

Cosmic dust is made in evolved stars. However, the processes involved in the formation and evolution of dust remain so far unknown. NANOCOSMOS, an ERC-Synergy project, will take advantage of the new observational capabilities (increased angular resolution) of the Atacama Large Millimeter/submillimeter Array (ALMA) to unveil the physical and chemical conditions in the dust formation zone of evolved stars. These observations in combination with novel top-level ultra-high vacuum experiments and astrophysical modelling will provide a cutting-edge view of cosmic dust.
In particular, in this seminar, the machine called stardust, designed, fabricated and commissioned at the ICMM will be presented. This machine, nowadays 80% operational, produce seeds in the form of nanoparticles of different materials, which are formed in conditions similar to that in the photosphere of a giant-star. The aim of stardust is to provide to the astronomers with an experimental workbench for testing ideas about possible chemical mechanisms operating in evolved stars, Supernova-ejecta or other ideal environment. The capabilities of the machine for studying interaction of gas with nanoparticles will be discussed.
This seminar will be the scientific presentation of the stardust machine and it will include a visit to the experiment (and we will offer you a home-made wine in the Lab).



02 de marzo de 2017, 12:00 h. Sala de Seminarios, 182

Exotic 2D materials

Andrés Castellanos-Gómez
2D Materials & Devices group. IMDEA Nanoscience.

In this talk I will review the recent progress on the application of atomically thin crystals different than graphene on optoelectronic devices. The current research of 2D semiconducting materials has already demonstrated the potential of this family of materials in optoelectronic applications [1-4]. Nonetheless, it has been almost limited to the study of molybdenum- and tungsten- based dichalcogenides (a very small fraction of the 2D semiconductors family). Single layer molybdenum and tungsten chalcogenides present large direct bandgaps (~1.8 eV). Alternative 2D semiconducting materials with smaller direct bandgap would be excellent complements to the molybdenum and tungsten chalcogenides as they could be used for photodetection applications in the near infrared. Furthermore, for applications requiring a large optical absorption it would be desirable to find a family of semiconducting layered materials with direct bandgap even in their multilayer form.

Here I will summarize our recent results on the exploration of novel 2D semiconducting materials for optoelectronic applications: black phosphorus [5-7], TiS3 [8, 9] and franckeite [10].

References:
[1] Yin Z. et al, Single-layer MoS2 phototransistors, ACS Nano (2011)
[2] Lopez-Sanchez, O., et al., Ultrasensitive photodetectors based on monolayer MoS2, Nature Nanotech. (2013)
[3] Buscema M., et al., Large and tunable photo-thermoelectric effect in single-layer MoS2, Nano Letters (2013)
[4] Groenendijk D.J., et al., Photovoltaic and photothermoelectric effect in a doubly-gated WSe2 device, Nano Letters (2014)
[5] Castellanos-Gomez, A., et al., Isolation and Characterization of few-layer black phosphorus. 2D Materials (2014)
[6] Buscema M., et al., Fast and broadband



23 de febrero de 2017, 12:00 h. Sala de Seminarios, 182

Quantum Thermoelectricity in Single-Molecule Junctions

Nicolás Agraït
Dpto. Física Materia Condensada. Universidad Autónoma de Madrid.

Molecular junctions are promising candidates to achieve high thermoelectric efficiencies due to the discreteness of the energy levels responsible for transport and the tunability of their properties via chemical synthesis, electrostatic gates, or pressure. After a general introduction to thermoelectric effects in the nanoscale, I will present our recent results on the thermoelectric properties of fullerenes. Using a modified scanning tunneling microscope (STM), we find that in contrast with C60 (1) the endohedral fullerene Sc3N@C80 (2) forms jnctions in which the magnitude and sign of the thermopower depend strongly on the orientation of the molecule and on applied pressure. We demonstrate that the origin of this exceptional behavior is the presence of a sharp resonance near the Fermi level created by the Sc3N inside the fullerene cage, whose energetic location, and hence the thermopower, can be tuned by applying pressure. These results reveal that Sc3N@C80 is a bi-thermoelectric material, exhibiting both positive and negative thermopower, and provide an unambiguous demonstration of the importance of transport resonances in thermoelectric performance of organic materials.

(1) C. Evangeli et al, Nanoletters 2013, 13, 2141.
(2) L. Rincon-Garcia et al, Nature Materials 2016, 15, 289.



16 de febrero de 2017, 12:00 h. Sala de Seminarios, 182

Scanning tunneling microscopy studies of vortex cores in pnictide and conventional superconductors

Isabel Guillamón
Laboratorio de Bajas Temperaturas. Departamento Física Materia Condensada. UAM.

High critical temperature superconductivity, and in particular iron based superconductivity, often appears in doped materials with some sort of substitutional disorder. Getting conclusive insight into the electronic properties eventually explaining high critical temperatures, or into the vortex pinning features that explain the value of the critical current, is difficult in systems with intrinsic disorder. Recently, a new stoichiometric compound CaKFe4As4 that belongs to the so-called 1144 family was synthesized with a critical temperature Tc=35 K comparable to the “optimal” Tc obtained usually in doped pnictides. Here I will show first tunneling spectroscopy and vortex imaging in this compound. We obtain multigap superconductivity consistent with s+- pairing and a hexagonal disordered vortex lattice under magnetic field. We directly show that main vortex pinning mechanism in this compound is due to pair breaking in non-magnetic defects.

Additionally, I will present recent STM experiments under vector magnet fields showing for the first time how vortices bend beneath the surface of a superconductor. Interestingly, the Coulomb like intervortex interaction due to stray fields at the surface dominates pinning and determines the orientation of the bulk vortex lattice.
Finally, I will present a few prospects and ongoing work, such as the development of STM experiments for very high magnetic fields, efforts to grow crystals of topological materials and first quasiparticle interference results in the Weyl semimetal WTe2.

Work supported by Spanish MINECO, ERC Starting Grant and CIG Marie Curie program.



02 de febrero de 2017, 12:00 h. Sala de Seminarios, 182

Nine years of iron superconductors: what we have learnt

Elena Bascones
ICMM-CSIC

A major breakthrough in condensed matter physics happened in February 2008 when the second family of high-Tc superconductors was discovered. Superconductivity was observed in LaFeAsO1-xFx at 26 K and quite soon the critical temperature was raised up to ~60 K in related systems. It was early understood that the superconductivity in these compounds could not be explained by the electron-phonon mechanism at work in conventional superconductors. Similar to previous findings in strongly correlated systems, like cuprate and heavy fermion systems, magnetic phases appeared in proximity to superconductivity.

Since 2008 many new superconducting compounds have been found. Many of their electronic properties and phases are unconventional and a huge effort has been done by the scientific community to understand them. After an introduction to some concepts in superconductivity and electronic correlations, I will review the main properties of iron superconductors and their current understanding. We will see that the electronic correlations and the spin-orbital interplay lie at the heart of this complicated and fascinating problem, in which many questions are still to be answered.



27 de enero de 2017, 12:00 h. Sala de Seminarios, 182

Quantized circular photogalvanic effect in Weyl semimetals

Adolfo Grushin
Condensed Matter Theory Center, University of California at Berkeley

The circular photogalvanic effect (CPGE) is the part of a photocurrent that switches depending on the sense of circular polarization of the incident light. It has been consistently observed in systems without inversion symmetry and depends on non-universal material details. We find that in a class of Weyl semimetals (e.g. SrSi2) and three-dimensional Rashba materials (e.g. doped Te) without inversion and mirror symmetries, the CPGE trace is effectively quantized in terms of the combination of fundamental constant e^3/h^2 with no material-dependent parameters. This is so because the CPGE directly measures the topological charge of Weyl points near the Fermi surface, and non-quantized corrections from disorder and additional bands can be small over a significant range of incident frequencies. Moreover, the magnitude of the CPGE induced by a Weyl node is relatively large, which enables the direct detection of the monopole charge with current techniques.

F. de Juan, A. G. Grushin, T. Morimoto, J. E. Moore, ArXiv: 1611.05887



26 de enero de 2017, 12:00 h. Sala de Seminarios, 182

Computational modeling of materials for heterogeneous catalysis:
The example of cerium oxide based catalysts

Verónica Ganduglia-Pirovano
Instituto de Catálisis y Petroquímica

Ceria (CeO2) is the most significant of the oxides of rare-earth elements in industrial catalysis with its reducibility being essential to its functionality in catalytic applications. The complexity of real (powder) catalysts hinders the fundamental understanding of how they work. Specifically, the role of ceria in the catalytic activity of ceria-based systems is still not fully understood. To unravel it, well-defined ceria-based model catalysts are prepared experimentally or created theoretically and studied. In this talk, recent results on CeO2(111) and Ni/CeO2(111) model ceria-based catalysts will be discussed, as examples of catalysts for partial alkyne hydrogenation [1], hydrogen production [2,3], and methane dry reforming [4,5], respectively. The emphasis is here put on theoretical studies and special attention is given to the effects of ceria as catalyst support and to the ability of state-of-the-art quantum-mechanical methods to provide reliable energies and an accurate description of the electronic structure of reducible ceria-based systems [6].

1. J. Phys. Chem. C 118, 5352 (2014), J. Chem. Phys. 141, 014703 (2014).
2. J. Phys. Chem. C 117, 8241 (2013). 3. Angew. Chem. Int. Ed. 54, 3917 (2015).
4. Angew. Chem. Int. Ed. 55, 7455 (2016).
5. ACS Catal. 6, 8184 (2016).
6. Phys. Rev. Lett. 102, 026101 (2009), Phys. Rev. Lett. 106, 246801 (2011), Phys. Rev. Lett. 110, 246101 (2013), Surf. Sci. Rep. 62, 219–270 (2007).



19 de enero de 2017, 12:00 h. Sala de Seminarios, 182

ADVANCES IN MAGNETIC FORCE MICROSCOPY IMAGING: FROM PERMANENT MAGNETS TO BACTERIA

Miriam Jaafar
ICMM-CSIC

Despite decades of advances in magnetic imaging, obtaining direct, quantitative information with high spatial resolution remains an outstanding challenge. The imaging technique most widely used for local characterization of magnetic nanostructures is the magnetic force microscope (MFM), which is indeed a very active topic of investigation. Advantages of MFM include relatively high spatial resolution (down to 10 nm), simplicity in operation as well as sample preparation, and the capability to applied in situ magnetic fields to study magnetization process [1]. Recently we have also demonstrate the possibility of operate in different environments including liquid media that allow us to investigate biological samples [2]. The tip engineering [3], the quantitative measurements, the correct interpretation of the resulting MFM images, or the analysis of the loss of energy [4] are subjects of ongoing research that will be reviewed in this talk.

[1] Sci. Rep. 6. 29702 (2016); APL Materials 2, 076111 (2014); Nan. Res.Lett 6 (2011) 1
[2] Small, 11, 4731–4736 (2015)
[3] Beilstein J. Nanotechnol. 7, 1068-1074 (2016)
[4] Nanoscale 8, 16989-16994 (2016)



12 de enero de 2017, 12:00 h. Sala de Seminarios, 182

One-step generation of alloyed Core@Shell and Core@Shell@Shell nanoparticles using gas aggregation sources

Lidia Martínez
Structure of Nanoscopic Systems Group, ICMM

The investigations of the fundamental properties of nanometer size structures have been subject of extensive studies during the last decades. However, it is easy to find in the literature discrepancies on the results obtained by different research groups. The origin of these inconsistencies could arise from the different approaches to obtain the nanomaterials. The fabrication method employed for the fabrication of these nanostructures is of crucial importance on the final properties. In this sense, I will present the development of a device for the fabrication of nanoparticles from the gas phase in ultra-high vacuum. This equipment allows the fabrication of nanoparticles with controlled size, composition and structure in one single step without the need of further treatments. The purity of the nanoparticles generated makes this technique suitable for basic research studies.



       

ICMM-2017 - Sor Juana Inés de la Cruz, 3, Cantoblanco, 28049 Madrid, España. Tel: +34 91 334 9000. info@icmm.csic.es