Quantum Coherence in Loopless Superconductive Networks

Measurements indicating that planar networks of superconductive islands connected by Josephson junctions display long-range quantum coherence are reported. The networks consist of superconducting islands connected by Josephson junctions and have a tree-like topological structure containing no loops. Enhancements of superconductive gaps over specific branches of the networks and sharp increases in pair currents are the main signatures of the coherent states. In order to unambiguously attribute the observed effects to branches being embedded in the networks, comparisons with geometrically equivalent-but isolated-counterparts are reported. Tuning the Josephson coupling energy by an external magnetic field generates increases in the Josephson currents, along the above-mentioned specific branches, which follow a functional dependence typical of phase transitions. Results are presented for double comb and star geometry networks, and in both cases, the observed effects provide positive quantitative evidence of the predictions of existing theoretical models

Evidence of long-range coherence in superconducting networks

Systematic experimental investigations are reported of charge transport in double-comb and star-shaped planar arrays obtained by coupling superconducting islands via Josephson tunnel junctions. The fabrication of the structures is based on a standard niobium trilayer technology with superconducting transition temperature close to 9 K. Noticeable enhancements of Josephson supercurrents and energy gap are recorded. Complementarity between experimental data and theoretical predictions is employed as a tool to clarify the role of different graph topologies in conditioning the singular behavior of synthetic graph-shaped networks. The predictions of the theoretical models are based on the Bose-Einstein topological condensation and a de Gennes-Alexander approach for granular superconductors. Evidence is shown of the dependence, for both gap and Josephson critical currents, on node-to-node connections in the networks, namely on array topology

Optical characteristics of nanostructured aluminium/diamond composite systems in the visible range

The inclusion of aluminium (Al) nanoparticles (NPs) in chemical vapor deposition (CVD) diamond structures was achieved by depositing Al thin films on commercial CVD single-crystal diamond plates, and then covering them by a CVD diamond thin film to encapsulate the metal NPs formed by the dewetting occurring during the CVD process. Morphology and composition are investigated, showing a peculiar structure formed by an Al/diamond composite with both Al NPs and Al2O3 islands included and surrounded in the diamond matrix, respectively. A mosaic-patterned homoepitaxial growth occurs for the capping diamond layer. The experimentally measured reflectivity matches the simulation of a system where the thickness of the Al/diamond composite layer is 1.80 ± 0.05 μm and the composition is 95 ± 2 % diamond and 5 ± 2 % Al. Simulations of the plasmonic response of Al NPs embedded in the diamond layer suggest that the decrease in transmission of the sample in the blue region of the spectrum is unlikely to be due to plasmonic absorption by the NPs. It is concluded that the shape of the transmission spectrum follows a Rayleigh-like scattering induced by the nanoporous diamond film. Ultrafast transient absorption measurements allow us to identify a sharp feature at 700 nm which can be associated with a modification of an interband transition in Al due to heating after photon absorption at 380 nm

Dynamics of the Bulk-to-Topological State Scattering of Photoexcited Carriers in Bi₂Se₃ Thin Films

Carrier dynamics in polycrystalline Bi2Se3 topological insulator thin films were investigated by femtosecond transient absorption spectroscopy (FTAS) at 77 K, by using an infrared pump photon of 0.62 eV energy and a white supercontinuum probe ranging from the near infrared to ultraviolet regions (0.9–3.5 eV). The Bi2Se3 samples were grown by vapor solid deposition, a quick, inexpensive, and easy-to-control growth technique to obtain films of different thicknesses, endowed with topological properties. FTAS spectra present several absorption bleaching signals, which can be attributed to electronic transitions involving both bulk and surface states present in the complex Bi2Se3 band structure. We observe clear differences in the rise times of several bleaching signals, differences that can be attributed to different band filling dynamics. Fast rise times are observed for transitions only involving bulk states, while a delayed onset of the bleaching signal has been observed for transitions involving surface topological states, which are more efficiently populated by carrier–phonon scattering of bulk electrons and holes, rather than by direct photoexcitation. The observed features shed fresh insights into the properties that allow these materials to be employed as innovative, low-cost, and wide-range photodetectors

Reevaluation of Photoluminescence Intensity as an Indicator of Efficiency in Perovskite Solar Cells

The photoluminescence (PL) intensity is often used as an indicator of the performance of perovskite solar cells and indeed the PL technique is often used for the characterization of these devices and their constituent materials. Herein, a systematic approach is presented to the comparison of the conversion efficiency and the PL intensity of a cell in both open-circuit (OC) and short-circuit (SC) conditions and its application to multiple heterogeneous devices. It is shown that the quenching of the PL observed in SC conditions is a good parameter to assess the device efficiency. The authors explain the dependence of the PL quenching ratio between OC and SC on the cell efficiency with a simple model that is also able to estimate the carrier extraction time of a device