Elsa Prada

Isolation and characterization of few-layer black phosphorus

This is the post-peer reviewed version of the following article: A. Castellanos-Gomez et al. “Isolation and characterization of few-layer black phosphorus”. 2D Matererials, 2014, 1(2) 025001 doi:10.1088/2053-1583/1/2/025001 Which has been published in final form at: http://iopscience.iop.org/2053-1583/1/2/025001

Pseudospin valve in bilayer graphene: towards graphene-based pseudospintronics

We propose a nonmagnetic, pseudospin-based version of a spin valve, in which the pseudospin polarization in neighboring regions of a graphene bilayer is controlled by external gates. Numerical calculations demonstrate a large on-off ratio of such a device. This finding holds promise for the realization of pseudospintronics: a form of electronics based upon the manipulation of pseudospin analogous to the control of physical spin in spintronics applications.

ac Josephson Effect in Finite-Length Nanowire Junctions with Majorana Modes

It has been predicted that superconducting junctions made with topological nanowires hosting Majorana bound states (MBS) exhibit an anomalous 4π-periodic Josephson effect. Finding an experimental setup with these unconventional properties poses, however, a serious challenge: for finite-length wires, the equilibrium supercurrents are always 2π periodic as anticrossings of states with the same fermionic parity are possible. We show, however, that the anomaly survives in the transient regime of the ac Josephson effect. Transients are, moreover, protected against decay by quasiparticle poisoning as a consequence of the quantum Zeno effect, which fixes the parity of Majorana qubits. The resulting long-lived ac Josephson transients may be effectively used to detect MBS.

Transport spectroscopy of NS nanowire junctions with Majorana fermions

We acknowledge the support of the CSIC JAE-Doc program and the Spanish Ministry of Science and Innovation through Grants No.FIS2008-00124/FIS (P.S.-J) and No. FIS2009-08744 (E.P. and R.A.). This research was supported in part by the National Science Foundation under Grant No. NSF PHY05-51164

Quantum pumping in graphene.

We show that graphene-based quantum pumps can tap into evanescent modes, which penetrate deeply into the device as a consequence of Klein tunneling. The evanescent modes dominate pumping at the Dirac point, and give rise to a universal response under weak driving for short and wide pumps, in close analogy to their role for the minimal conductivity in ballistic transport. In contrast, evanescent modes contribute negligibly to normal pumps. Our findings add a new incentive for the exploration of graphene-based nanoelectronic devices.

Zero landau level in folded graphene nanoribbons

Graphene nanoribbons can be folded into a double layer system keeping the two layers decoupled. In the quantum Hall regime folds behave as a new type of Hall bar edge. We show that the symmetry properties of the zero Landau level in metallic nanoribbons dictate that the zero energy edge states traversing a fold are perfectly transmitted onto the opposite layer. This result is valid irrespective of fold geometry, magnetic field strength, and crystallographic orientation of the nanoribbon. Backscattering suppression on the N=0 Hall plateau is ultimately due to the orthogonality of forward and backward channels, much like in the Klein paradox.

Measuring Majorana nonlocality and spin structure with a quantum dot

Robust zero-bias transport anomalies in semiconducting nanowires with proximity-induced superconductivity have been convincingly demonstrated in various experiments. While these are compatible with the existence of Majorana zero modes at the ends of the nanowire, a direct proof of their nonlocality and topological protection is now needed. Here we show that a quantum dot at the end of the nanowire may be used as a powerful spectroscopic tool to quantify the degree of Majorana nonlocality through a local transport measurement. Moreover, the spin polarization of dot subgap states at singlet-doublet transitions in the Coulomb blockade regime allows the dot to directly probe the spin structure of the Majorana wave function and indirectly measure the spin-orbit coupling of the nanowire.

Helical networks in twisted bilayer graphene under interlayer bias

We acknowledge financial support from the Spanish Ministry of Economy (MINECO) through the Ramon y Cajal programme, Grants No. FIS2010-21883 (E.P.), and No. FIS2011-23713, and from the European Research Council Advanced Grant, Contract No. 290846 (P.S-J.)

Band topology and the quantum spin Hall effect in bilayer graphene

Abstract We consider bilayer graphene in the presence of spin–orbit coupling, in order to assess its behavior as a topological insulator. The first Chern number n for the energy bands of single-layer graphene and that for the energy bands of bilayer graphene are computed and compared. It is shown that for a given valley and spin, n for a Bernal-stacked bilayer is doubled with respect to that for the monolayer. This implies that this form of bilayer graphene will have twice as many edge states as single-layer graphene, which we confirm with numerical calculations and analytically in the case of an armchair terminated surface. Bernal-stacked bilayer graphene is a weak topological insulator, whose surface spectrum is susceptible to gap opening under spin-mixing perturbations. We assess the stability of the associated topological bulk state of bilayer graphene under various perturbations. In contrast, we show that A A -stacked bilayer graphene is not a topological insulator unless the spin–orbit coupling is bigger than the interlayer hopping. Finally, we consider an intermediate situation in which only one of the two layers has spin–orbit coupling, and find that although individual valleys have non-trivial Chern numbers for the case of Bernal stacking, the spectrum as a whole is not gapped, so the system is not a topological insulator.

Multiple Andreev reflection and critical current in topological superconducting nanowire junctions

We study transport in a voltage-biased superconductor-normal- superconductor (SNS) junction made of semiconducting nanowires with strong spin-orbit coupling, as it transitions into a topological superconducting phase for increasing Zeeman field. Despite the absence of a fractional steady-state ac Josephson current in the topological phase, the dissipative multiple Andreev reflection current Idc at different junction transparencies is particularly revealing. It exhibits unique features related to topology, such as gap inversion, the formation of Majorana bound states and fermion-parity conservation. Moreover, the critical current Ic, which remarkably does not vanish at the critical point where the system becomes gapless, provides direct evidence of the topological transition.