Coordinators: Eduardo Hernández, Rafael Roldán

05 July 2018, 12:00 h. Sala de Seminarios, 182

Universidad de Budapest

Weyl semimetals, three dimensional materials with gapless excitations characterized by an effective Weyl Hamiltonian, are the tree dimensional generalizations of graphene. In connection with the recent rush to find condensed matter realizations of this new phase of matter another equally intriguing novel topological phase has been discovered: nodal line semimetals. In these systems the conduction and valence band touch each other along a line. This line can take a form of a loop or extend trough the whole Brillouin zone. Systems with many lines entangled with each other were also predicted to exist.

In this talk we shall first review how this novel phase was discovered in the turbulent rush towards finding novel topological phases. We conclude by examining experimental signatures characteristic to this exotic state of matter mainly focusing on magnetic oscillation spectra.

28 June 2018, 12:00 h. Sala de Seminarios, 182

ICMM

The study of open quantum systems has a long story, however most of the advances are related with oscillator bath models (typically describe phonons, photons, magnons, electron-hole pairs, etc). Another universality class corresponds to spin bath models, where the bath is described by a discrete set of localised n-level systems. Importantly, in certain regimes, the behaviour of the spin bath can be radically different from the oscillator bath models (some examples of bath spins are lattice defects, dislocation modes, spin impurities, dangling bonds nuclear spins and localised phonons/vibrons).

In this talk I will motivate the importance of these models in present experiments, discuss some well known results and some of the important questions not yet fully addressed. I will also discuss some of the different approaches that one can use to study these models and their limitations.

07 June 2018, 12:00 h. Sala de Seminarios, 182

Instituto de Ciencia de Materiales de Madrid ICMM-CSIC

The recently observed insulating and superconducting states upon doping a graphene bilayer with a small twist angle have created a lot of excitement in the scientific community[1,2]. The stacking misorientation creates a moiré pattern with a superlattice modulation corresponding to thousands of atoms per unit cell. The insulating states, arise when the charge per moiré cell is ± 2. Superconductivity emerges from one of these insulating states.

Understanding the nature of the insulating states is key to uncover the origin of the superconductivity. The filling at which the insulating character appears is consistent with the ones of a Mott insulator and suggests a posible relation with the physics of cuprate high-Tc superconductors.

In the seminar I will review some aspects of the experimental results and introduce the physics and properties of Mott insulators. Common wisdom about Mott insulators is generally built upon techniques which only take into account local correlations. We will see that the expectations including only local correlations are not compatible with the experimentally observed behaviour of the insulating states with temperature and magnetic field [3]. I will then argue that including non-local correlations can reverse these predictions and discuss other possible consequences of non-local correlations.

[1] Cao et al, Nature 556, p. 80-84 (2018)

[2] Cao et al, Nature 556, p. 43-50 (2018)

[3] J.M. Pizarro, M.J. Calderón, E. Bascones, arXiv:1805.07303

31 May 2018, 12:00 h. Sala de Seminarios, 182

Instituto de Ciencia de Materiales de Madrid ICMM-CSIC

Iron superconductors were discovered 10 years ago giving rise to the second family of high temperature superconductors with unknown superconducting mechanism. Most undoped pnictides present columnar magnetism and a puzzling electronic nematic phase that upon doping or pressure become superconducting. Over these years the spin scenario has shown to be a strong candidate to explain the mechanism of antiferromagnetism, nematicity and superconductivity. Notably exception is FeSe lacking the magnetic phase and presenting a peculiar orbital ordering. This situation has motivated the orbital ordering scenario for FeSe. In our work, we have taken a different route: we have derived a low energy effective model– the orbital selective spin fluctuations (OSSF)- model that allows to address spin-orbital interplay. [1,2]

So far, with this model we have been able to give a mechanism to understand (i) the difference between magnetism and nematicity in pnictides and FeSe, [1] (ii) the odd orbital ordering observed in ARPES in FeSe [3] (iv) The enigmatic anisotropy of the superconducting gaps in FeSe revealed by STM and ARPES experiments [4](vi) the renormalization of the velocity and the scattering rate due to self-energy effects reflected in the resistivity anisotropy. [5]

[1] L. Fanfarillo, A. Cortijo, and B. Valenzuela, Phys. Rev. B 91, 214515 (2015)

[2] L. Fanfarillo, L. Benfatto, and B. Valenzuela, Phys. Rev. B 97, 121109(R) (2018).

[3] L. Fanfarillo, et al., Phys. Rev. B 94, 155138 (2016).

[4] L. Fanfarillo, B. Valenzuela, L. Benfatto, arXiv:1804.05800.

[5] R. Fernández, L. Fanfarillo, L. Benfatto, B. Valenzuela, arXiv:1804.07293.

17 May 2018, 12:00 h. Sala de Seminarios, 182

IFIMAC

The excited state properties of nanoscale semiconductors are dominated by the dynamics of quantum confined electron-hole pairs known as excitons. Thanks to recent advances in the size and shape control of semiconductor nanomaterials, this confinement can now be tuned with high precision which has resulted in a rapidly expanding family of high-quality excitonic building blocks. However, while extensive research has been done to understand and control the excitonic properties of the isolated building blocks, comparatively little is known about exciton dynamics in nanoscale assemblies.

In the first part of the talk, I will present some of our efforts in trying to understand and control the exciton dynamics in nanomaterial assemblies. Specifically, I will discuss transient microscopy techniques which allow us to spatially resolve exciton diffusion in colloidal quantum-dot films. In addition, I will present our findings of anomalous excitonic energy-transfer dynamics between zero-dimensional colloidal quantum-dots and two-dimensional MoS2 monolayers.

In the second part of the talk, I will present new strategies for the assembly of excitonic building blocks into high quality wavelength-scale patterns using template stripping of colloidal quantum-dot films. I will show that this technique can produce high-quality photonic structures composed solely out of colloidal quantum dots.

Finally, I will briefly highlight some recent work on the use of plasmonic antennas to structure the fluorescence of colloidal quantum-dot emitters, specifically by mapping spectral information to the polarization state of light. Based on this concept, we propose polarization-resolved spectroscopy scheme that may benefit high-speed readout of fluorescent signatures.

10 May 2018, 12:00 h. Sala de Seminarios, 182

University of Lancaster

Topological photonics aims to replicate fermionic symmetries as feats of precision engineering. Here I show how to enhance these systems via effects such as gain, loss and nonlinearities that do not have a direct electronic counterpart. This leads to a topological mechanism of mode selection [1,2,3], formation of compactons in flat band condensates [4], and topological excitations in lasers when linearized around their working point [5]. The resulting effects show a remarkable practical robustness against disorder, which arises from the increased spectral isolation of the manipulated states.

[1] Topologically protected midgap states in complex photonic lattices, H. Schomerus, Opt. Lett. 38, 1912 (2013).

[2] Selective enhancement of topologically induced interface states in a dielectric resonator chain, C. Poli, M. Bellec, U.Kuhl, F. Mortessagne, H. Schomerus, Nat. Commun. 6, 6710 (2015).

[3] Topological Hybrid Silicon Microlasers, H. Zhao et al., Nat. Commun. 9, 981 (2018)

[4] Exciton-polaritons in a two-dimensional Lieb lattice with spin-orbit coupling, C. E. Whittaker et al., Phys. Rev. Lett. 120, 097401 (2018)

[5] Topological phases in nonlinear complex-wave equations with a time-preserving symmetry, S. Malzard, E. Cancellieri, and H. Schomerus, arXiv:1705.06895

07 May 2018, 12:00 h. Sala de Seminarios, 182

Princeton University

One hallmark of topological phases with broken time reversal symmetry is the appearance of quantized non-dissipative transport coefficients, the archetypical example being the quantized Hall conductivity in quantum Hall states. Here I will talk about a new non-dissipative transport coefficients that appear in such systems - the Hall viscosity. In the first part of the talk, I will start by reviewing previous results concerning the Hall viscosity, including its relation to a topological invariant known as the shift when rotational symmetry is preserved. Next, I will show how the Hall viscosity can be computed from a Kubo formula. For Galilean invariant systems, the Kubo formula implies a relationship between the viscosity and conductivity tensors which may have relevance for experiment. In the second part of the talk, I will examine the fate of the Hall viscosity when rotational symmetry is broken. Through a combination of field theory and numerical techniques, I will show that rotational symmetry breaking allows for the introduction of a new topological quantum number characterizing quantum Hall states.

26 April 2018, 12:00 h. Sala de Seminarios, 182

Instituto de Ciencia de Materiales de Madrid ICMM-CSIC

Following recent reports of superconductivity and Mott transitions in twisted graphene bilayers, the field of moiré patterns in stacked 2D crystals has seen a surge of interest. I will review the rich phenomenology associated to moiré patterns in various heterostructures of 2D crystals, both from an electronic and from an elastic point of view. I will focus on the case of twisted graphene bilayer and graphene on hexagonal boron nitride. The elastic aspect of the problem, which has received comparatively little attention, will be analysed within a simple continuum description. This model is sufficient to capture the formation and properties of stacking solitons, which dramatically affect the electronic properties in some systems. It will also be employed to explain and the characteristic the spontaneous rippling of some van der Waals materials, such as Franckeite and Cylindrite, whose large scale structure is thus shown to be a consequence of interlayer moiré patterns.

19 April 2018, 12:00 h. Salón de Actos

Instituto de Ciencia de Materiales de Madrid ICMM-CSIC

In 2008, Hosono’s group discovered superconductivity in the iron-based superconductors. In this family, a quasi-2D layer is formed by arranging the iron atoms in a square pattern, with pnictogen or chalcogen atoms tetrahedrally coordinating these iron positions. The superconducting phase appears when a antiferromagnetic phase is supressed by doping or by applying pressure. Superconductivity is unconventional but its origin is not well established. The phase diagram of this family is similar to the one of cuprate superconductors. However, a striking difference is that the parent compound of iron superconductors is a metal in which the Hund’s coupling seems to play an important role, while for the cuprates it is a Mott insulator. The metallic nature of the iron superconductors has led to a never ending debate about the role of local correlations on superconductivity.

In 2015 a new kind of iron superconductor with a quasi-one dimensional two leg ladder structure was discovered. BaFe2S3 becomes superconductor with T_c^max≈24K when pressure is applied ≈10GPa and an antiferromagnetic phase is supressed. In 2017, another related compound, BaFe2Se3, was also reported to be a superconductor with T_c^max≈11Kat ≈12.7GPa. Interestingly, at zero pressure these systems are insulators, and what has led some authors to propose that they are Mott insulators.

In this talk I will briefly review some of the phenomenology of iron superconductors, as well as the Hund metal paradigm. In the main part, I will present the calculations that we have performed to clarify the role of electronic interactions in the superconductor BaFe2S3.

05 April 2018, 12:00 h. Salón de Actos

Universidad del País Vasco

The inversion symmetry breaking at the interface between different materials generates strong spin-orbit coupling (SOC). We will study through theoretical models various transport phenomena in metal-metal and metal-oxide and ferromagnet-oxide junctions induced by this interaction. This type of interaction is responsible of the greatest spin-to-charge conversion. We will show that apart from this spin-to-charge conversion this SOC is also responsible of spin swapping, (spin-to-spin conversion) and anomalous Hall effects in the presence of ferromagnetic materials.

22 March 2018, 12:00 h. Sala de Seminarios, 182

Departamento de Física Teórica de la Materia Condensada (UAM) and IFIMAC

There is an increasing interest on the interface between Plasmonics and electron transport phenomena, mainly motivated by the ability of localized surface plasmons to concentrate light in sub-nanometric ìhot spotsî in a controllable manner. This offers unique opportunities in the design of novel molecular-scale optoelectronic devices exhibiting highly tuneable operativeness. Systems comprising plasmonic nanostructures bridged by tunnel junctions constitute a natural route towards the realization of such devices. Among those, simple vacuum sub-nanometric gaps in metallic nanoparticle dimers have been addressed experimentally and analysed theoretically using classical-optics prescriptions. However, only recently fully ab-initio simulations of the optical response have been carried out for such archetypical systems. In this case, the detailed atomic structure in the junction has to be necessarily considered since, for instance, small distortions of the around-gap geometry not only affect to the intensity of the photoinduced current, but also lead to qualitative changes in the optical absorption spectrum.

More interesting is the case of hybrid systems, where metallic nanostructures are bridged by atomic or molecular junctions. In this talk, we shall discuss the effects of different single-atom junctions on the optical properties of bridged nanoparticle dimers, as well as the corresponding plasmon-induced electric currents through the junctions. We show that, besides the appearance of well-defined signatures in the infrared absorption spectrum associated to a photoinduced quantized electric current, the plasmonic response is affected by such a current. A deeper understanding of the physical process is given via a simple model system.

02 March 2018, 10:30 h. Sala de Seminarios, 182

University of Twente. The Netherlands.

Ge/Si core/shell nanowires are proposed candidates for observing Majorana fermions where a hard superconducting gap is essential for topological protection at zero energy. In double quantum dots, we observe shell filling of new orbitals and corresponding Pauli spin blockade. In nanowires with superconducting Al leads we create a Josephson junction via proximity-induced superconductivity. A gate-tuneable supercurrent is observed with a maximum of ~60 nA. We identify two different regimes: Cooper pair tunnelling via multiple subbands in the open regime and, while near depletion a supercurrent is carried by single-particle levels of a quantum dot operating in the few-hole regime.

Secondly, we create ambipolar quantum dots in silicon nanoMOSFETs. After passivation of charge defects we can electrostatically define hole quantum dots up to 180 nm in length. In recent devices, we have characterized the conformity of aluminium, titanium and palladium nanoscale gates by means of transmission electron microscopy (TEM). Subsequently, we have defined low-disorder quantum dots with Pd gates. Finally, we have made depletion-mode hole quantum dots in intrinsic silicon. We use fixed charge in a SiO2/Al2O3 dielectric stack to induce a 2DHG at the Si/SiO2 interface. This depletion-mode design avoids complex multilayer architectures requiring precision alignment and allows directly adopting best practices already developed for depletion dots in other material systems.

01 March 2018, 12:00 h. Sala de Seminarios, 182

Laboratoire de Physique des Solides. Université Paris Saclay.

Motivated by experimental findings [1], we study theoretically smooth topological interfaces, i.e. interfaces between a topological and a normal insulator. In addition to the usual topologically protected chiral surface states, which do not depend on the specific form of the interface, several massive states appear if the interface width is larger than a particular intrinsic length (given by the bulk gap and the Fermi velocity). These states, first described by Volkov and Pankratov in the 1990ies [2], are intrinsically relativistic and can be related to Landau bands of relativistic fermions. We show that the gap variation can be interpreted precisely as a vector potential that is affected by an additional electric field in a relativistic manner [3]. The electric field can thus be used not only to dope electronically these massive surface states, but they become even more accessible due to the reduction of the Landau gap in the presence of an electric field. The effect is at the origin of an oscillating resistance measured as a function of the electric field in high-frequency experiments [1]. We finish with a short discussion of how this "Landau-level approach" can also be used in the framework of Weyl semimetals and the description of Fermi arcs that play the role of chiral Landau bands here [4]

[1] A.Inhofer et al. PRB 96, 195104 (2017)

[2] V.Volkov and O. Pankratov, JETP Lett 42, 178 (1985)

[3] S.Tchoumakov et al. PRB 96, 201302 (2017)

[4] S.Tchoumakov et al. PRB 95, 125306 (2017)

22 February 2018, 12:00 h. Sala de Seminarios, 182

Universidad Carlos III

Recent scanning tunneling microscope (STM) experiments have shown that a soft rippled flat state coexists with a hard buckled state in suspended graphene sheets. For small values of the STM current, the transition between these states is reversible, whereas it becomes irreversible when the STM current surpasses a certain threshold. We present phenomenological models of graphene as a membrane coupled to pseudo-spins that undergo Glauber dynamics. These models allow us to understand the STM induced transitions between rippled flat and buckled states as driving the spin-membrane system through a first order phase transition.

15 February 2018, 12:00 h. Sala de Seminarios, 182

Instituto de Ciencia de Materiales de Madrid ICMM-CSIC

This talk will be structured in two parts.

In the first part, we investigate the electromagnetic response of staggered two-dimensional materials of the graphene family, including graphene, silicene, germanene, and stanene, as they are driven through various topological phase transitions using external fields [1] [2]. Utilizing Kubo formalism, we compute their optical conductivity tensor taking into account the frequency and wave vector of the electromagnetic excitations, and study its behavior over the full electronic phase diagram of the materials. We compute the Plasmon dispersion relation for different phases.

In the second part of the talk, we revisit the atom-plate quantum friction and Casimir force with a full-relativistic formalism for atoms modelled as Unruh-deWitt detectors in exited, relaxed and coherent superposition close to a plate [3]. We show that, for relative velocities close to c, the quantum friction diverges while the Casimir force is almost independent of the velocity. We are able to include the effect of the finite size of the detector, then we also obtain quantum friction when the detector is isolated but follows a non-inertial trajectory and we obtain a more realistic result for short distance interactions.

Those studies open the venue to understand the role of non-local response in quantum friction.

[1] P. Rodriguez-Lopez, et al., Nature Communications 8, 14699 (2017)

[2] P. Rodriguez-Lopez, et al., Phys. Rev. Materials 2, 014003 (2018)

[3] P. Rodriguez-Lopez and E. Martin-Martinez. Casimir Forces and Quantum friction of finite-size atoms in relativistic trajectories. Submitted

08 February 2018, 12:00 h. Sala de Seminarios, 182

G. Millán Institute, Universidad Carlos III Madrid

Let us consider two identical beakers of water, initially at two different temperatures, put in contact with a thermal reservoir at subzero (on the Celsius scale) temperature. While one may intuitively expect that the initially cooler sample would freeze first, it has been observed that this is not always the case. This paradoxical behavior named the Mpemba effect (ME) [1] has been known since antiquity and discussed by philosophers like Aristotle, Roger Bacon, Francis Bacon, and Descartes [2].

There is no consensus on the underlying physical mechanisms that bring about the ME. Specifically, water evaporation, differences in the gas composition of water, natural convection, or the influence of supercooling, either alone or combined with other causes, have been claimed to have an impact on the ME. Conversely, the own existence of the ME in water has been recently put in question [3]. Notwithstanding, Mpemba-like effects have also been observed in different physical systems, such as carbon nanotube resonators or clathrate hydrates.

In this seminar, we show that the Mpemba effect and its inverse are present in granular fluids [4], both in uniformly heated and in freely cooling systems. In both cases, the system remains homogeneous, and no phase transition is present. Analytical quantitative predictions are given for how differently the system must be initially prepared to observe the Mpemba effect, the theoretical predictions being confirmed by both molecular dynamics and Monte Carlo simulations. Possible implications of our analysis for other systems are also discussed.

01 February 2018, 12:00 h. Sala de Seminarios, 182

Universidad Autónoma de Madrid

Hybrid devices that couple superconductors and semiconductor nanowires have attracted considerable attention in recent years owing to their potential to realize topological superconductivity and Majorana zero modes. The topological phase is predicted to result from a combination of induced superconductivity, spin-orbit coupling and spin polarization in the semiconductor nanowire. Crucially, the one-dimensional character of such a system must be preserved over micron-length scales for the topological phase to the established - a requirement that is not so straightforward to meet in realistic materials. In this talk, I will address experiments performed in InAs-based hybrid superconductor-semiconductor nanowire devices, in which clear signatures of charge localization are detected. I will discuss different effects associated with the resulting superconductor-quantum dot system whose signatures could be mistakenly interpreted in favor of Majorana zero modes. In particular, I will address zero-bias peaks related to the Kondo effect and the Zeeman splitting of Andreev levels. Finally, I will discuss how, even when seemingly absent, charge localization plays a crucial role in the transport properties of quasi-ballistic nanowire quantum point contacts.

25 January 2018, 12:00 h. Sala de Seminarios, 182

IFIMAC & Departamento de Física Teórica de la Materia Condensada, UAM

Strong coupling is achieved when the coherent energy exchange between a confined electromagnetic field mode and material excitations becomes faster than the decay and decoherence of either constituent. This creates a paradigmatic hybrid quantum system with eigenstates that have mixed light-matter character (polaritons). It has recently been realized that polariton formation in organic molecules also affects their internal nuclear degrees of freedom, opening the possibility to manipulate and control reactions through polaritonic chemistry. I will first discuss our theoretical approach towards modeling such systems, based on extending the well-known Born-Oppenheimer approximation to describe polaritonic potential energy surfaces. I will then show various applications, including the possibility to completely suppress reactions such as photoisomerization, which surprisingly works more efficiently when many molecules are coupled to a single light mode due to a “collective protection” effect. Finally, I will show how polaritonic chemistry can be exploited to allow many-molecule reactions triggered by a single photon. Here, the collective nature of polaritons and the resulting formation of a "supermolecule", in which a single excitation is distributed over many molecules, can enable a reaction involving the nuclear degrees of freedom of most or even all coupled molecules. This process can overcome the Stark-Einstein law that applies for most common photochemical reactions, which states that a single photon will only induce a reaction in a single molecule.

Finally, I will discuss future perspectives, open questions, and remaining challenges to fully exploit the potential of polaritonic chemistry.

18 January 2018, 12:00 h. Sala de Seminarios, 182

Instituto de Ciencia de Materiales de Madrid ICMM-CSIC and IMDEA Nanociencia

In this talk I will review some of the current achievements in the pursuit of Majorana fermions, along with the major drawbacks that stand in the way of exploiting them as fundamental qubits in the field of topological quantum computation. I will introduce a novel setting for creating Majorana bound states and performing protected non-Abelian single-qubit operations without real space braiding of Majoranas and with no fine-tuning of the control parameters. The proposed platform is a two-dimensional electron gas in the Quantum Hall regime in the presence of a Zeeman field, with the Fermi level tuned to filling factor 1. I will show that, in the presence of spin-orbit coupling, contacting the 2DEG to a narrow strip of an s-wave superconductor produces a topological superconducting gap along the contact as a result of crossed Andreev reflection processes across the strip. The sign of the topological gap depends periodically on the Fermi wavelength and strip width and can be externally tuned. An interface between two halves of a long strip with topological gaps of opposite sign implements a robust π-junction that hosts a pair of protected Majorana zero modes. Such a configuration can be exploited to perform non-Abelian tunnel-braid operations that are much simpler to execute than Majorana braidings in real space, and are nevertheless topologically protected.

11 January 2018, 12:00 h. Sala de Seminarios, 182

Universidad Autónoma de Madrid

We demonstrate the possibility of modeling recent experiments in complex graphene-metal (G-M) systems by means of Density Functional Theory (DFT) calculations. Since different modification techniques are currently applied on G -and related materials- to get new functionalities, our work does not restrict to the mere description of the G-M interface but we have to reproduce all these surface modifications. More precisely, we reveal the multi-domain structure of G grown on Rh(111), an archetypical strongly interacting substrate, in which G adopts a rippled structure with corrugations larger than 1 Å [1]. Additionally, we will present different examples of surface modification; like the evolution of G properties as a function of the oxygen coverage in the interface [2]; the atomistic mechanisms involved in this intercalation process [3]; and the tailoring of the electronic properties by means of ion implantation nitrogen doping [4,5]. Finally, we will briefly discuss about some methodological aspects of DFT calculations in this kind of G-M systems and their current limitations when dealing with very large systems (like grain boundaries, G-covered metallic steps or under-cover chemical reactions).To overcome this issue, we present preliminary results of a new approach based on highly optimized localized orbital basis set to reach high-accuracy descriptions at quantum level of systems with thousands of atoms.

[1] A. Martín-Recio et al. Nanoscale 7 (2015) 11300

[2] C. Romero-Muñiz et al. Carbon 101 (2016) 129

[3] C. Romero-Muñiz et al. (submitted)

[4] A. Martín-Recio et al. Nanoscale 8 (2016) 17686

[5] A. Martín-Recio et al. (submitted)