Group news

In a recent paper [Journal of Physics: Condensed Matter 32, 275301 (2020)], C. Núñez et al present a thorough study of the thermoelectric properties of silicene nanoribbons in the presence of a random distribution of atomic vacancies. By using a linear approach within the Landauer formalism, they calculate phonon and electron thermal conductances, the electric conductance, the Seebeck coefficient and the figure of merit of the nanoribbons. They found a sizable reduction of the phonon thermal conductance as a function of the vacancy concentration over a wide range of temperature. At the same time, the electric properties are not severely deteriorated, leading to an overall remarkable thermoelectric efficiency. They conclude that the incorporation of vacancies paves the way for designing better and more efficient nanoscale thermoelectric devices.

In a recent paper [Physica E 116, 113769 (2019)], Yuriko Baba and Marta Saiz-Bretín study electronic transport in graphene/ferromagnetic insulator hybrid devices. The system comprises an armchair graphene nanoribbon with a lens-shaped EuO ferromagnetic insulator layer deposited on top of it. When the device supports a large number of propagating modes, the proximity exchange interaction of electrons with the magnetic ions of the ferromagnetic insulator results in electrons being spatially localized at different spots depending on their spin. They found the spin-dependent electron focusing is robust under moderate edge disorder. A spin-polarized electric current can be generated by placing a third contact in the proper place. This opens the possibility to use these effects for fabricating tunable sources of polarized electrons.

In a recent paper [Scientific Reports 9, 13572 (2019)], V. Clericó et al. report on a novel implementation of the cryo-etching method, which enabled them to fabricate low-roughness hBN-encapsulated graphene nanoconstrictions with unprecedented control of the structure edges; the typical edge roughness is on the order of a few nanometers. They characterized the system by atomic force microscopy and used the measured parameters of the edge geometry in numerical simulations of the system conductance, which agree quantitatively with their low temperature transport measurements. The quality of our devices is confirmed by the observation of well defined quantized 2e2/h conductance steps at zero magnetic field. Such an observation reports the clearest conductance quantization in physically etched graphene nanoconstrictions. The fabrication of such high quality systems and the scalability of the cryo-etching method opens a novel promising possibility of producing more complex truly-ballistic devices based on graphene.

In a recent paper [Physical Review B 100, 165105 (2019)], Yuriko Baba et al. investigate the consequences of applying electric fields perpendicularly to thin films of topological semimetals. In particular, they consider Weyl and Dirac semimetals in a configuration such that their surface Fermi arcs lie on opposite edges of the films. They develop an analytical approach based on perturbation theory and a single-surface approximation and compare their analytical results with numerical calculations. The effect of the electric field on the dispersion is twofold: It shifts the dispersion relation and renormalizes the Fermi velocity, which would, in turn, have direct effects on quantum transport measurements. Additionally, it modifies the spatial decay properties of surface states which will impact the connection of the Fermi arcs in opposite sides of a narrow thin film.

In a recent paper [Physical Review B 100, 075412 (2019)], A. Díaz-Fernández et al. propose to Floquet engineer Dirac cones at the surface of a three-dimensional topological insulator. They show that a large tunability of the Fermi velocity can be achieved as a function of the polarization, direction, and amplitude of the driving field. Using this external control, the Dirac cones in the quasienergy spectrum may become elliptic or massive, in accordance with experimental evidence. These results help one to understand the interplay of surface states and external ac driving fields in topological insulators. In this work they use the full Hamiltonian for the three-dimensional system instead of effective surface Hamiltonians, which are usually considered in the literature. Their findings show that the Dirac cones in the quasienergy spectrum remain robust even in the presence of bulk states, and therefore, they validate the usage of effective surface Hamiltonians to explore the properties of Floquet-driven topological boundaries. Furthermore, their model allows us to introduce out-of-plane field configurations which cannot be accounted for by effective surface Hamiltonians.

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