Seminars and Events

Seminars and Events

Theory and Simulation of Materials

Coordinators: Eduardo Hernández, Rafael Roldán

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

Quantum dots and superconductivity in silicon systems

Floris Zwanenburg
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

Topological materials with smooth interfaces -- what one can learn from Landau-level quantization

Mark O. Goerbig
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

Buckling, ripples and dynamical phenomena in suspended graphene

Luis Bonilla
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

Non-local conductivity of 2D Dirac cones and Quantum friction with Unruh-deWitt detectors

Pablo Rodríguez López
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

Mpemba effect in granular fluids: Can the hotter cold more quickly and can the cooler heat up more quickly?

Antonio Lasanta
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

Charge localization in hybrid superconductor-semiconductor nanowire devices

Eduardo Lee
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

Exploiting polaritonic chemistry to manipulate molecular structure and dynamics

Johannes Feist
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

Topological π-junctions from crossed Andreev reflection in the Quantum Hall regime

Francesca Finocchiaro
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

Graphene on rhodium from first principles: Tailoring electronic, structural and chemical properties

Carlos Romero Muñiz
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)


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