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Roditchev D.,CNRS Nanosciences Institute of Paris | Roditchev D.,CNRS Physics and Materials Study Laboratory | Brun C.,CNRS Nanosciences Institute of Paris | Serrier-Garcia L.,CNRS Nanosciences Institute of Paris | And 7 more authors.
Nature Physics | Year: 2015

Superconducting correlations may propagate between two superconductors separated by a tiny insulating or metallic barrier, allowing a dissipationless electric current to flow. In the presence of a magnetic field, the maximum supercurrent oscillates and each oscillation corresponding to the entry of one Josephson vortex into the barrier. Josephson vortices are conceptual blocks of advanced quantum devices such as coherent terahertz generators or qubits for quantum computing, in which on-demand generation and control is crucial. Here, we map superconducting correlations inside proximity Josephson junctions using scanning tunnelling microscopy. Unexpectedly, we find that such Josephson vortices have real cores, in which the proximity gap is locally suppressed and the normal state recovered. By following the Josephson vortex formation and evolution we demonstrate that they originate from quantum interference of Andreev quasiparticles, and that the phase portraits of the two superconducting quantum condensates at edges of the junction decide their generation, shape, spatial extent and arrangement. Our observation opens a pathway towards the generation and control of Josephson vortices by applying supercurrents through the superconducting leads of the junctions, that is, by purely electrical means without any need for a magnetic field, which is a crucial step towards high-density on-chip integration of superconducting quantum devices. © 2015 Macmillan Publishers Limited. All rights reserved.

Cherkez V.,CNRS Nanosciences Institute of Paris | Cuevas J.C.,Autonomous University of Madrid | Brun C.,CNRS Nanosciences Institute of Paris | Cren T.,CNRS Nanosciences Institute of Paris | And 6 more authors.
Physical Review X | Year: 2014

We present a combined experimental and theoretical study of the proximity effect in an atomic-scale controlled junction between two different superconductors. Elaborated on a Si(111) surface, the junction comprises a Pb nanocrystal with an energy gap Δ1 = 1.2 meV, connected to a crystalline atomic monolayer of lead with Δ2 = 0.23 meV. Using in situ scanning tunneling spectroscopy, we probe the local density of states of this hybrid system both in space and in energy, at temperatures below and above the critical temperature of the superconducting monolayer. Direct and inverse proximity effects are revealed with high resolution. Our observations are precisely explained with the help of a self-consistent solution of the Usadel equations. In particular, our results demonstrate that in the vicinity of the Pb islands, the Pb monolayer locally develops a finite proximity-induced superconducting order parameter, well above its own bulk critical temperature. This leads to a giant proximity effect where the superconducting correlations penetrate inside the monolayer a distance much larger than in a nonsuperconducting metal.

Sun Z.,Huazhong University of Science and Technology | Sun Z.,ESPCI ParisTech | Sun Z.,CNRS Physics and Materials Study Laboratory | Chang H.,Huazhong University of Science and Technology | Chang H.,Tohoku University
ACS Nano | Year: 2014

Graphene and graphene-like two-dimensional (2D) materials have attracted much attention due to its extraordinary electronic and optical properties, which accommodate a large potential in optoelectronic applications such as photodetection. However, although much progress has been made, many challenges exist in fundamental and practical aspects hindering graphene and graphene-like 2D materials from photodetector and other photonic and optoelectronic applications. Here, we review the recent progress in photodetection based on graphene and graphene-like 2D materials and start with the summary of some most important physical mechanisms, including photoelectric, photo-Thermoelectric, and photo-bolometric regimes. Then methodology-level discussions are given from viewpoints of state-of-The-Art designs in device geometry and materials. It is worth emphasizing that emerging photodetection and photodetectors based on graphene-like 2D materials such as metal chalcogenide nanosheets are reviewed systematically. Finally, we conclude this review in a brief discussion with remaining challenges in photodetection of two-dimensional photonics and optoelectronics (2D POE) and note that complete understandings of 2D materials and 2D POE may inspire solar energy conversion and other new applications. © 2014 American Chemical Society.

Cassette E.,CNRS Physics and Materials Study Laboratory | Mahler B.,CNRS Physics and Materials Study Laboratory | Guigner J.-M.,IRD Montpellier | Patriarche G.,CNRS Optic of Semiconductor nanoStructures Group | And 2 more authors.
ACS Nano | Year: 2012

We report the synthesis and properties of a novel class of nanocrystals with mixed dimensionality: a dot-in-plate core/shell nanostructure. This system was synthesized by growing a flat, disk-shaped, CdS shell on spherical CdSe cores. The anisotropic pressure induced by the shell drastically splits the first exciton fine structure in two: the 'heavy hole' and 'light hole' states become separated by up to 65 meV. As a result, these nanocrystals exhibit an emission strongly polarized in two dimensions, in the plane perpendicular to the wurtzite crystal c axis. We use polarization measurements on single nanocrystals and ensemble anisotropy studies to confirm the nature and position of the excitonic energy levels. These nanocrystals orient spontaneously when evaporated on a substrate, enabling a precise control of the orientation of their emission dipole. © 2012 American Chemical Society.

Zimmers A.,CNRS Physics and Materials Study Laboratory | Aigouy L.,CNRS Physics and Materials Study Laboratory | Mortier M.,CNRS Laboratory of Condensed Matter Chemistry, Paris | Sharoni A.,Bar - Ilan University | And 4 more authors.
Physical Review Letters | Year: 2013

We show that the main mechanism for the dc voltage or dc current induced insulator-metal transition in vanadium dioxide VO2 is due to local Joule heating and not a purely electronic effect. This "tour de force" experiment was accomplished by using the fluorescence spectra of rare-earth doped micron sized particles as local temperature sensors. As the insulator-metal transition is induced by a dc voltage or dc current, the local temperature reaches the transition temperature indicating that Joule heating plays a predominant role. This has critical implications for the understanding of the dc voltage or dc current induced insulator-metal transition and has a direct impact on applications which use dc voltage or dc current to externally drive the transition. © 2013 American Physical Society.

Orieux F.,Institute Pasteur Paris | Sepulveda E.,CNRS Physics and Materials Study Laboratory | Loriette V.,CNRS Physics and Materials Study Laboratory | Dubertret B.,CNRS Physics and Materials Study Laboratory | Olivo-Marin J.-C.,Institute Pasteur Paris
IEEE Transactions on Image Processing | Year: 2012

Structured illumination microscopy is a recent imaging technique that aims at going beyond the classical optical resolution by reconstructing high-resolution (HR) images from low-resolution (LR) images acquired through modulation of the transfer function of the microscope. The classical implementation has a number of drawbacks, such as requiring a large number of images to be acquired and parameters to be manually set in an ad-hoc manner that have, until now, hampered its wide dissemination. Here, we present a new framework based on a Bayesian inverse problem formulation approach that enables the computation of one HR image from a reduced number of LR images and has no specific constraints on the modulation. Moreover, it permits to automatically estimate the optimal reconstruction hyperparameters and to compute an uncertainty bound on the estimated values. We demonstrate through numerical evaluations on simulated data and examples on real microscopy data that our approach represents a decisive advance for a wider use of HR microscopy through structured illumination. © 2011 IEEE.

Mottaghizadeh A.,CNRS Physics and Materials Study Laboratory | Yu Q.,CNRS Physics and Materials Study Laboratory | Lang P.L.,CNRS Physics and Materials Study Laboratory | Zimmers A.,CNRS Physics and Materials Study Laboratory | Aubin H.,CNRS Physics and Materials Study Laboratory
Physical Review Letters | Year: 2014

We report the study of gold-SrTiO3 (STO)-gold memristors where the doping concentration in STO can be fine-tuned through electric field migration of oxygen vacancies. In this tunnel junction device, the evolution of the density of states (DOS) can be followed continuously across the metal-insulator transition (MIT). At very low dopant concentration, the junction displays characteristic signatures of discrete dopant levels. As the dopant concentration increases, the semiconductor band gap fills in but a soft Coulomb gap remains. At even higher doping, a transition to a metallic state occurs where the DOS at the Fermi level becomes finite and Altshuler-Aronov corrections to the DOS are observed. At the critical point of the MIT, the DOS scales linearly with energy N(Ïμ)∼Ïμ, the possible signature of multifractality. © 2014 American Physical Society.

Vitrey A.,Imm Institute Microelectronica Of Madrid Cnm Csic | Aigouy L.,CNRS Physics and Materials Study Laboratory | Prieto P.,Imm Institute Microelectronica Of Madrid Cnm Csic | Garcia-Martin J.M.,Imm Institute Microelectronica Of Madrid Cnm Csic | Gonzalez M.U.,Imm Institute Microelectronica Of Madrid Cnm Csic
Nano Letters | Year: 2014

In this work we discuss the excitation of parallel collective resonances in arrays of gold nanoparticles. Parallel collective resonances result from the coupling of the nanoparticles localized surface plasmons with diffraction orders traveling in the direction parallel to the polarization vector. While they provide field enhancement and delocalization as the standard collective resonances, our results suggest that parallel resonances could exhibit greater tolerance to index asymmetry in the environment surrounding the arrays. The near- and far-field properties of these resonances are analyzed, both experimentally and numerically. © 2014 American Chemical Society.

Gan C.H.,Ecole Polytechnique - Palaiseau | Lalouat L.,CNRS Physics and Materials Study Laboratory | Lalanne P.,Ecole Polytechnique - Palaiseau | Aigouy L.,CNRS Physics and Materials Study Laboratory
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Much effort has been recently devoted to the study of the electromagnetic field launched on a metal-dielectric interface by one-dimensional (1D) subwavelength indentations. In this work, we consider the wave launched on a semiconductor surface that does not support any surface modes, in contrast to the metallic case. Through analytical calculations performed for polarized 2D Dirac line sources, we show that the waves launched by the two orthogonal polarizations (parallel and perpendicular to the surface) have approximately the same form, differing only in phase and magnitude. This finding implies that the wave launched on the surface by an arbitrary subwavelength indentation under an arbitrary illumination, either at grazing, normal, or oblique incidence, is always the same and possesses universal characteristics. We further observe this wave on a silicon substrate with a near-field probe. Through fitting with a simple model, the measured field is found to exhibit characteristics that are reminiscent of quasicylindrical waves on metal surfaces. © 2011 American Physical Society.

Bakulin A.A.,FOM Institute for Atomic and Molecular Physics | Neutzner S.,FOM Institute for Atomic and Molecular Physics | Bakker H.J.,FOM Institute for Atomic and Molecular Physics | Ottaviani L.,Aix - Marseille University | And 2 more authors.
ACS Nano | Year: 2013

The efficiency of solution-processed colloidal quantum dot (QD) based solar cells is limited by poor charge transport in the active layer of the device, which originates from multiple trapping sites provided by QD surface defects. We apply a recently developed ultrafast electro-optical technique, pump-push photocurrent spectroscopy, to elucidate the charge trapping dynamics in PbS colloidal-QD photovoltaic devices at working conditions. We show that IR photoinduced absorption of QD in the 0.2-0.5 eV region is partly associated with immobile charges, which can be optically detrapped in our experiment. Using this absorption as a probe, we observe that the early trapping dynamics strongly depend on the nature of the ligands used for QD passivation, while it depends only slightly on the nature of the electron-accepting layer. We find that weakly bound states, with a photon-activation energy of 0.2 eV, are populated instantaneously upon photoexcitation. This indicates that the photogenerated states show an intrinsically bound-state character, arguably similar to charge-transfer states formation in organic photovoltaic materials. Sequential population of deeper traps (activation energy 0.3-0.5 eV) is observed on the ∼0.1-10 ns time scales, indicating that most of carrier trapping occurs only after substantial charge relaxation/transport. The reported study disentangles fundamentally different contributions to charge trapping dynamics in the nanocrystal-based optoelectronic devices and can serve as a useful tool for QD solar cell development. © 2013 American Chemical Society.

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