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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.

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.

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.

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.

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.

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