Anisotropic magneto-Coulomb effect versus spin accumulation in a ferromagnetic single-electron device

We investigate the magneto-transport characteristics of nanospintronics single-electron devices. The devices consist of single non-magnetic nano-objects (nanometer size nanoparticles of Al or Cu) connected to Co ferromagnetic leads. The comparison with simulations allows us attribute the observed magnetoresistance to either spin accumulation or anisotropic magneto-Coulomb effect (AMC), two effects with very different origins. The fact that the two effects are observed in similar samples demonstrates that a careful analysis of Coulomb blockade and magnetoresistance behaviors is necessary in order to discriminate them in magnetic single-electron devices. As a tool for further studies, we propose a simple way to determine if spin transport or AMC effect dominates from the Coulomb blockade I-V curves of the spintronics device

Spin injection in a single metallic nanoparticle: a step towards nanospintronics

We have fabricated nanometer sized magnetic tunnel junctions using a new nanoindentation technique in order to study the transport properties of a single metallic nanoparticle. Coulomb blockade effects show clear evidence for single electron tunneling through a single 2.5 nm Au cluster. The observed magnetoresistance is the signature of spin conservation during the transport process through a non magnetic cluster.Comment: 3 page

Spectroscopic Evidence for Anisotropic S-Wave Pairing Symmetry in MgB2

Scanning tunneling spectroscopy of superconducting MgB$_2$ ($T_c = 39$ K) were studied on high-density pellets and c-axis oriented films. The sample surfaces were chemically etched to remove surface carbonates and hydroxides, and the data were compared with calculated spectra for all symmetry-allowed pairing channels. The pairing potential ($\Delta_k$) is best described by an anisotropic s-wave pairing model, with $\Delta_k = \Delta_{xy} \sin ^2 \theta_k + \Delta_z \cos ^2 \theta_k$, where $\theta_k$ is the angle relative to the crystalline c-axis, $\Delta_z \sim 8.0$ meV, and $\Delta_{xy} \sim 5.0$ meV.Comment: 4 pages and 3 figures. Submitted to Physical Review Letters. Corresponding author: Nai-Chang Yeh (e-mail: [email protected]

The parameter space of graphene chemical vapor deposition on polycrystalline Cu

A systematic study on the parameter space of graphene CVD on polycrystalline Cu foils is presented, aiming at a more fundamental process rationale in particular regarding the choice of carbon precursor and mitigation of Cu sublimation. CH4 as precursor requires H2 dilution and temperatures ≥1000°C to keep the Cu surface reduced and yield a high quality, complete monolayer graphene coverage. The H2 atmosphere etches as-grown graphene, hence maintaining a balanced CH4/H2 ratio is critical. Such balance is more easily achieved at low pressure conditions, at which however Cu sublimation reaches deleterious levels. In contrast, C6H6 as precursor requires no reactive diluent and consistently gives similar graphene quality at 100-150°C lower temperatures. The lower process temperature and more robust processing conditions allow the problem of Cu sublimation to be effectively addressed. Graphene formation is not inherently self-limited to a monolayer for any of the precursors. Rather, the higher the supplied carbon chemical potential the higher the likelihood of film inhomogeneity and primary and secondary multilayer graphene nucleation. For the latter, domain boundaries of the inherently polycrystalline CVD graphene offer pathways for a continued carbon supply to the catalyst. Graphene formation is significantly affected by the Cu crystallography, i.e. the evolution of microstructure and texture of the catalyst template form an integral part of the CVD process.S.H. acknowledges funding from ERC grant InsituNANO (n°279342) and from EPSRC (Grant Nr. EP/H047565/1). P.R.K. acknowledges funding from the Cambridge Commonwealth Trust and C.D. acknowledges funding from Royal Society.This is the accepted manuscript. The final version is available from ACS at http://pubs.acs.org/doi/abs/10.1021/jp303597m

Conductance switching at the nanoscale of diarylethene derivatives self-assembled monolayers on La0.7_{0.7}Sr0.3_{0.3}MnO3_3

We report on the phosphonic acid route for the grafting of functional molecules, optical switch (dithienylethene diphosphonic acid, DDA), on La0.7Sr0.3MnO3 (LSMO). Compact self-assembled monolayers (SAMs) of DDA are formed on LSMO as studied by topographic atomic force microscopy (AFM), ellipsometry, water contact angle and X-ray photoemission spectroscopy (XPS). The conducting AFM measurements show that the electrical conductance of LSMO/DDA is about 3 decades below that of the bare LSMO substrate. Moreover, the presence of the DDA SAM suppresses the known conductance switching of the LSMO substrate that is induced by mechanical and/or bias constraints during C-AFM measurements. A partial light-induced conductance switching between the open and closed forms of the DDA is observed for the LSMO/DDA/C-AFM tip molecular junctions (closed/open conductance ratio of about 8). We show that, in the case of long-time exposition to UV light, this feature can be masked by a non-reversible decrease (a factor of about 15) of the conductance of the LSMO electrode.Comment: Full paper with supporting informatio

Quantum well confinement and competitive radiative pathways in the luminescence of black phosphorus layers

Black phosphorus (BP) stands out from other 2D materials by the wide amplitude of the band-gap energy (Delta(Eg)) that sweeps an optical window from Visible (VIS) to Infrared (IR) wavelengths, depending on the layer thickness. This singularity made the optical and excitonic properties of BP difficult to map. Specifically, the literature lacks in presenting experimental and theoretical data on the optical properties of BP on an extended thickness range. Here we report the study of an ensemble of photoluminescence spectra from 79 passivated BP flakes recorded at 4 K with thicknesses ranging from 4 nm to 700 nm, obtained by mechanical exfoliation. We observe that the exfoliation steps induce additional defects states that compete the radiative recombination from bound excitons observed in the crystal. We also show that the evolution of the photoluminescence energy versus thickness follows a quantum well confinement model appreciable from a thickness predicted and probed at 25 nm. The BP slabs placed in different 2D heterostructures show that the emission energy is not significantly modulated by the dielectric environment. Introduction Confinement effectsComment: 11 pages, 3 figures - Main text 12 pages, 5 figures - Supporting informatio

Measuring the nonlinear refractive index of graphene using the optical Kerr effect method

© 2016 Optical Society of America.By means of the ultrafast optical Kerr effect method coupled to optical heterodyne detection (OHD-OKE), we characterize the third-order nonlinear response of graphene and compare it to experimental values obtained by the Z-scan method on the same samples. From these measurements, we estimate a negative nonlinear refractive index for monolayer graphene, n2 = -1.1 × 10-13 m2/W. This is in contradiction to previously reported values, which leads us to compare our experimental measurements obtained by the OHD-OKE and the Z-scan method with theoretical and experimental values found in the literature and to discuss the discrepancies, taking into account parameters such as doping