The quantum coupling of fully di erent degrees of freedom is a challenging path towards new functionalities for quantum electronics [1][2][3] . Here we show that the localized classical spin of a magnetic atom immersed in a superconductor with a twodimensional electronic band structure gives rise to a long-range coherent magnetic quantum state. We experimentally evidence coherent bound states with spatially oscillating particle-hole asymmetry extending tens of nanometres from individual iron atoms embedded in a 2H-NbSe 2 crystal. We theoretically elucidate how reduced dimensionality enhances the spatial extent of these bound states and describe their energy and spatial structure. These spatially extended magnetic states could be used as building blocks for coupling coherently distant magnetic atoms in new topological superconducting phases 4-11

International audienceSuperconducting correlations may propagate between two superconductors separated by a tiny insulating or metallic barrier, allowing a dissipationless electric current to flow(1,2). In the presence of a magnetic field, the maximum supercurrent oscillates(3) and each oscillation corresponding to the entry of one Josephson vortex into the barrier(4). Josephson vortices are conceptual blocks of advanced quantum devices such as coherent terahertz generators(5) or qubits for quantum computing(6), in which on-demand generation and control is crucial. Here, we map superconducting correlations inside proximity Josephson junctions(7) 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(8), 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

We develop a fast Magnetic Josephson Junction (MJJ)a superconducting ferromagnetic device for a scalable high-density cryogenic memory compatible in speed and fabrication with energy-efficient Single Flux Quantum (SFQ) circuits. We present experimental results for Superconductor-Insulator-Ferromagnet-Superconductor (SIFS) MJJs with high characteristic voltage I c R n of >700 V proving their applicability for superconducting circuits. By applying magnetic field pulses, the device can be switched between MJJ logic states. The MJJ I c R n product is only ~30% lower than that of conventional junction coproduced in the same process, allowing for integration of MJJ-based and SIS-based ultra-fast digital SFQ circuits operating at tens of gigahertz.High speed, low power superconducting Rapid Single Flux Quantum (RSFQ) digital circuits have already found their applications in Digital-RF systems impacting communications and signal intelligence applications [1,2]. Recently, a new energy-efficient generation of RSFQ circuits, eSFQ and ERSFQ logics, offered a way to overcome the low energy efficiency of conventional technologies for the next generation of supercomputers [3]. However, the practical applications of these superconducting digital technologies will inevitably be very limited without compatible in speed and signal levels, high-capacity, energy-efficient Random Access Memory (RAM). The largest superconducting RAM demonstrated to date, a 4 kbit RAM [4], is insufficient for practical applications and hardly compatible with SFQ-type circuits.The low density of superconducting memory is directly related to a relatively large size of memory cells based on SFQ storage loops coupled to address lines via transformers which are difficult to scale [5][6][7][8]. The required ac power posed an additional implementation problem for achieving larger capacity RAM integrated circuits [8]. In order to get around the low capacity of superconductor RAMs, hybrid superconductor-semiconductor schemes were pursued [9,10]. However, these approaches can address only limited applications and cannot satisfy the need for a fast, energy efficient memory in a close proximity to the digital circuits, preferably on the same chip. Alternatively, combining superconducting elements with ferromagnetic layers and dots was suggested to achieve higher density of superconducting memory [11,12]. However, these ideas did not go beyond initial concepts nor address compatibility with SFQ circuits.Recently, we have introduced a memory cell based on Magnetic Josephson Junction (MJJ), a Josephson switching device with ferromagnetic (F) layer(s). The MJJ critical current can change and retain its value by ferromagnet magnetization, so that a memory element size is defined by the scalable small MJJ device [13]. With achieving MJJ switching speed comparable to that of conventional JJs, both types of junctions can be integrated into a single circuit operating in an SFQ non-hysteretic switching regime, enabling a low power and high speed memory operation. Since such ...

International audienceWe 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 Delta(1) = 1.2 meV, connected to a crystalline atomic monolayer of lead with Delta(2) = 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

The dependence of the critical current density j c on the ferromagnetic interlayer thickness d F was determined for Nb/ Al 2 O 3 / Cu/ Ni/ Nb Josephson tunnel junctions with ferromagnetic Ni interlayer thicknesses from very thin films ͑ϳ1 nm͒ upward and classified into F-layer thickness regimes showing a dead magnetic layer, exchange, exchange+ anisotropy and total suppression of j c . The Josephson coupling changes from 0 to as function of d F , and-very close to the crossover thickness-as function of temperature. The strong suppression of the supercurrent in comparison to nonmagnetic Nb/ Al 2 O 3 / Cu/ Nb junctions indicated that the insertion of a F layer leads to additional interface scattering. The transport inside the dead magnetic layer was in dirty limit. For the magnetically active regime fitting with both the clean and the dirty limit theories was carried out, indicating dirty limit condition, too. The results were discussed in the framework of literature.

In this work, a new hybridization of superconducting and ferromagnetic orders is demonstrated, promising for magnonics. By measuring the ferromagnetic and spin wave resonance absorption spectra of a magnetostatically coupled permalloy/niobium bilayer at different temperatures, magnetostatic spin wave resonances with unconventional dispersion are observed. The mechanism behind the modified dispersion, confirmed with micromagnetic simulations, implies screening of the alternating magnetostatic stray fields of precessing magnetic moments in the ferromagnetic layer by the superconducting surface in the Meissner state.

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In this work, magnetization dynamics is studied in superconductor-ferromagnet-superconductor threelayered films in a wide frequency, field, and temperature ranges using the broad-band ferromagnetic resonance measurement technique. It is shown that in the presence of both superconducting layers and of superconducting proximity at both superconductor-ferromagnet interfaces a massive shift of the ferromagnetic resonance to higher frequencies emerges. The phenomenon is robust and essentially long-range: it has been observed for a set of samples with the thickness of ferromagnetic layer in the range from tens up to hundreds of nanometers. The resonance frequency shift is characterized by proximity-induced magnetic anisotropies: by the positive in-plane uniaxial anisotropy and by the drop of magnetization. The shift and the corresponding uniaxial anisotropy grow with the thickness of the ferromagnetic layer. For instance, the anisotropy reaches 0.27 T in experiment for a sample with a 350-nm-thick ferromagnetic layer, and about 0.4 T in predictions, which makes it a ferromagnetic film structure with the highest anisotropy and the highest natural resonance frequency ever reported. Various scenarios for the superconductivityinduced magnetic anisotropy are discussed. As a result, the origin of the phenomenon remains unclear. Application of the proximity-induced anisotropies in superconducting magnonics is proposed as a way for manipulations with a spin-wave spectrum.