Gonzalo Usaj

Multiterminal Conductance of a Floquet Topological Insulator

We report on simulations of the dc conductance and quantum Hall response of a Floquet topological insulator using Floquet scattering theory. Our results reveal that laser-induced edge states lead to quantum Hall plateaus once imperfect matching with the nonilluminated leads is lessened. The magnitude of the Hall plateaus, however, is not directly related to the number and chirality of all the edge states at a given energy, as usual. Instead, the plateaus are dominated by those edge states adding to the time-averaged density of states. Therefore, the dc quantum Hall conductance of a Floquet topological insulator is not directly linked to topological invariants of the full Floquet bands.

Localized spins on graphene.

The problem of a magnetic impurity, atomic or molecular, absorbed on top of a carbon atom in otherwise clean graphene is studied using the numerical renormalization group. The spectral, thermodynamic, and scattering properties of the impurity are described in detail. In the presence of a small magnetic field, the low-energy electronic features of graphene make it possible to inject spin-polarized currents through the impurity using a scanning tunneling microscope. Furthermore, the impurity scattering becomes strongly spin dependent and for a finite impurity concentration it leads to spin-polarized bulk currents and a large magnetoresistance. In gated graphene the impurity spin is Kondo screened at low temperatures. However, at temperatures larger than the Kondo temperature, the anomalous magnetotransport properties are recovered.

From single to multiple Ag-layer modification of Au nanocavity substrates: a tunable probe of the chemical surface-enhanced Raman scattering mechanism.

We present experimental and computational results that enlighten the mechanisms underlying the chemical contribution to surface-enhanced Raman scattering (SERS). Gold void metallic arrays electrochemically covered either by a Ag monolayer or 10-100 Ag layers were modified with a self-assembled monolayer of 4-mercaptopyridine as a molecular Raman probe displaying a rich and unexpected Raman response. A resonant increase of the Raman intensity in the red part of the spectrum is observed that cannot be related to plasmon excitations of the cavity-array. Notably, we find an additional 10-20 time increase of the SERS amplification upon deposition of a single Ag layer on the Au substrate, which is, however, almost quenched upon deposition of 10 atomic layers. Further deposition of 100 atomic Ag layers results in a new increase of the SERS signal, consistent with the improved plasmonic efficiency of Ag bulk-like structures. The SERS response as a function of the Ag layer thickness is analyzed in terms of ab initio calculations and a microscopic model for the SERS chemical mechanism based on a resonant charge transfer process between the molecular HOMO state and the Fermi level in the metal surface. We find that a rearrangement of the electronic charge density related to the presence of the Ag monolayer in the Au/Ag/molecule complex causes an increase in the distance between the HOMO center of charge and the metallic image plane that is responsible for the variation of Raman enhancement between the studied substrates. Our results provide a general platform for studying the chemical contribution to SERS, and for enhancing the Raman efficiency of tailored Au-SERS templates through electrochemical modification with Ag films.

Anomalous Josephson Current in Junctions with Spin Polarizing Quantum Point Contacts

We consider a ballistic Josephson junction with a quantum point contact in a two-dimensional electron gas with Rashba spin-orbit coupling. The point contact acts as a spin filter when embedded in a circuit with normal electrodes. We show that with an in-plane external magnetic field an anomalous supercurrent appears even for zero phase difference between the superconducting electrodes. In addition, the external field induces large critical current asymmetries between the two flow directions, leading to supercurrent rectifying effects.

Rhombohedral Multilayer Graphene: A Magneto-Raman Scattering Study

Y. Henni, H. P. Ojeda Collado, 3 K. Nogajewski, M. R. Molas, G. Usaj, 3 C. A. Balseiro, 3 M. Orlita, M. Potemski, ∗ and C. Faugeras † Laboratoire National des Champs Magntiques Intenses, CNRS, (UJF, UPS, INSA), BP 166, 38042 Grenoble, Cedex 9, France Centro Atmico Bariloche and Instituto Balseiro, Comisin Nacional de Energa Atmica, 8400 S. C. de Bariloche, Argentina Consejo Nacional de Investigaciones Cientficas y Tcnicas (CONICET), Argentina (Dated: March 15, 2016)Graphene layers are known to stack in two stable configurations, namely, ABA or ABC stacking, with drastically distinct electronic properties. Unlike the ABA stacking, little has been done to experimentally investigate the electronic properties of ABC graphene multilayers. Here, we report on the first magneto optical study of a large ABC domain in a graphene multilayer flake, with ABC sequences exceeding 17 graphene sheets. ABC-stacked multilayers can be fingerprinted with a characteristic electronic Raman scattering response, which persists even at room temperatures. Tracing the magnetic field evolution of the inter Landau level excitations from this domain gives strong evidence for the existence of a dispersionless electronic band near the Fermi level, characteristic of such stacking. Our findings present a simple yet powerful approach to probe ABC stacking in graphene multilayer flakes, where this highly degenerated band appears as an appealing candidate to host strongly correlated states.

Tunable charge and spin Seebeck effects in magnetic molecular junctions

The Seebeck effect refers to the generation of a charge current (or a voltage drop) by a temperature gradient applied across a metal [1]. The spin-Seebeck effect [2–4], concerns the thermal generation of pure spin currents. Applications of the Seebeck and spin-Seebeck effects at the nano-scale, with potential impact on a variety of new technologies, could profit from the scalability and tunability properties of nano-devices but still require a better understanding of thermopower effects in nanostructures and of the effect of strong electron-electron correlations on them. The recent experimental observation of the Seebeck effect in different nano-structures, in particular in molecular junctions [5] and quantum dots (QDs) [6], opened new routes to study these phenomena. Here we show that junctions with spin-1 molecules, like the Co(tpy-SH)2 complex [7] or the C60 buckyballs [8], give rise to a large and controllable enhancement of the Seebeck and spin Seebeck effects at low temperatures and low magnetic fields. We also show that thermoelectric experiments in these systems give access to valuable information on the residual interaction between quasiparticles in the low temperature Fermi-liquid regime [9], information that cannot be obtained by conventional techniques. This offers the opportunity to experimentally study these effects on the smallest possible lengths scales looking for the basic mechanisms of thermomagnetic effects in strongly correlated systems [10–13]—in particular in the regime where energy transfer is governed by spin fluctuations.

Diffusion of fluorine adatoms on doped graphene

We calculate the diffusion barrier of fluorine adatoms on doped graphene in the diluted limit using Density Functional Theory. We found that the barrier Δ strongly depends on the magnitude and character of the graphenes doping (δn): it increases for hole doping (δnu2009 u20090). Near the neutrality point the functional dependence can be approximately by Δu2009=u2009Δ0 – αδn, where α ≃ 6u2009×u200910−12u2009meVu2009cm2. This effect leads to significant changes of the diffusion constant with doping even at room temperature and could also affect the low temperature diffusion dynamics due to the presence of substrate induced charge puddles. In addition, this might open up the possibility to engineer the F dynamics on graphene by using local gates.

Electronic transport through magnetic molecules with soft vibrating modes

The low-temperature transport properties of a molecule are studied in the field-effect transitor geometry. The molecule has an internal mechanical mode that modulates its electronic levels and renormalizes both the interactions and the coupling to the electrodes. For a soft mechanical mode, the spin fluctuations in the molecule are dominated by the bare couplings, while the valence changes are determined by the dressed energies. In this case, the transport properties present an anomalous behavior and the Kondo temperature has a weak gate voltage dependence. These observations are in agreement with recent experimental data. DOI: 10.1103/PhysRevB.76.241403 PACS numbers: 72.15.Qm, 73.22.f The recent development of molecular transistors MTs has created new scenarios for the study of correlation effects in nanoscopic systems. These devices have attracted a lot of interest due to their potential application in nanoelectronics and their rich variety of behavior. Molecular transistors consist of a small molecule connecting two electrodes and, in most cases, a gate electrode is used to control the molecule’s charge electrostatically. The transport properties of MTs show such signatures of strong electronic correlations as Coulomb blockade 1 and the Kondo effect, 2‐5 similar to those observed in quantum dot QD devices. 6,7 A remarkable difference between QDs and MTs is the coupling, on the latter, of the electronic degrees of freedom with a discrete set of mechanical modes. 8‐10 For the simplest case of a linear modulation of the molecule’s electronic levels by a single vibration mode, several effects are predicted to occur. The Franck-Condon renormalization of the molecule-electrode coupling is expected to produce a suppression in the sequential and cotunneling transport through the molecule. 11‐13 The reduction of the effective Coulomb repulsion in the molecule may lead to an effective e-e attraction 14‐16 and a strong sensitivity to gate voltage. In the repulsive e-e interaction regime, the spin-Kondo effect dominates the low-temperature physics. The Kondo effect generates in these devices an increase of the conductance with decreasing temperature and a zero-bias peak in the differential conductance. These observations are a direct consequence of the formation of the Abrikosov-Suhl or Kondo resonance below the Kondo temperature TK. 17 In some MTs based on organometallic molecules, an anomalous gate voltage dependence of the transport properties has been reported. 1,2,5 In the transition metal complexes studied by Yu et al., 5 TK depends weakly on the applied gate voltage and shows a rapid increase only close to the charge degeneracy points. Moreover, the edges of the Coulomb blockade diamonds are not well defined in the Kondo charge state. Such behavior is inconsistent with the usual theory based on the Anderson model. In this Rapid Communication we present a study of the Anderson-Holstein model showing results obtained with the numerical renormalization group NRG. 18,19 We find that, as the frequency of the vibrating mode decreases, an anomalous gate dependence of TK and of the transport properties emerges. This effect arises because the soft vibrating modes in the MT drive the system into a new regime where the characteristic energy scales for spin and charge fluctuations are not related as in the conventional theory of the Kondo effect. The model Hamiltonian is H= HM+ HE+ HME, where the first two terms describe the isolated molecule and the electrodes, respectively, and the last term describes their coupling. We have H M = d n d + Un d↑ n d↓ x7f n d x7f1 a + a † + 0 a † a, 1

Tuning the Nonlocal Spin-Spin Interaction between Quantum Dots with a Magnetic Field

We describe a device where the nonlocal spin-spin interaction between quantum dots (QDs) can be turned on and off with a small magnetic field. The setup consists of two QDs at the edge of two two-dimensional electron gases (2DEGs). The QDs spins are coupled through a RKKY-like interaction mediated by the electrons in the 2DEGs. A magnetic field B(z) perpendicular to the plane of the 2DEG is used as a tuning parameter. When the cyclotron radius is commensurate with the interdot distance, the spin-spin interaction is amplified by a few orders of magnitude. The sign of the interaction is controlled by finely tuning B(z). Our setup allows for several dots to be coupled in a linear arrangement and it is not restricted to nearest-neighbor interaction.

Bilayer graphene under pressure: Electron-hole Symmetry Breaking, Valley Hall Effect, and Landau Levels

Financiamiento Basal para Centros Cientificos y Tecnologicos de Excelencia nFB 0807 nFondecyt n1150806 nAmerican Physical Society nANPCyT nPICTs 2013-1045 nBicentenario 2010-1060 nCONICET nPIP 11220110100832 nSeCyT-UNC n06/C415 nICTP associateship program nSimons Foundation