Scanning vortex microscopy reveals thickness-dependent pinning nano-network in superconducting niobium films

Hovhannisyan R.A., Golod T., Krasnov V.M.

The utilization of Josephson vortices as information carriers in superconducting digital electronics is hindered by the lack of reliable displacement and localization mechanisms. In this Letter, we experimentally investigate planar Nb junctions with an intrinsic phase shift and nonreciprocity induced by trapped Abrikosov vortices. We demonstrate that the entrance of a single Josephson vortex into such junctions triggers the switching between metastable ±π semifluxon states. We showcase controllable manipulation between these states using short current pulses and achieve a nondestructive readout by a nearby junction. Our observations pave the way toward ultrafast and energy-efficient digital Josephson electronics. Published by the American Physical Society 2024

Vagov A., Saraiva T.T., Shanenko A.A., Vasenko A.S., Aguiar J.A., Stolyarov V.S., Roditchev D.

AbstractIn many pnictides the superconductivity coexists with ferromagnetism in an accessible range of temperatures and compositions. Recent experiments revealed that when the temperature of magnetic ordering Tm is below the superconducting transition temperature Tc, highly non-trivial physical phenomena occur. In this work we demonstrate the existence of a temperature window, situated between Tm and Tc, where these intrinsically type-II superconductors are in the intertype regime. We explore analytically and numerically its rich phase diagram characterized by exotic spatial flux configurations—vortex clusters, chains, giant vortices and vortex liquid droplets—which are absent in both type-I and type-II bulk superconductors. We find that the intertype regime is almost independent of microscopic parameters, and can be achieved by simply varying the temperature. This opens the route for experimental studies of the intertype superconductivity scarcely investigated to date.

Berti G., Torres-Castanedo C.G., Goronzy D.P., Bedzyk M.J., Hersam M.C., Kopas C., Marshall J., Iavarone M.

Niobium thin films are key components of superconducting microwave resonators. Interest in these devices has increased dramatically because of their application in quantum systems. Despite tremendous effort to improve their performance, loss mechanisms are still not well understood. Nb/substrate and Nb/air interfaces are likely culprits in contributing to decoherence and ultimately limiting the performance of superconducting devices. Here, we investigate the Nb/substrate interface by studying the effect of hydrogen-passivated H:Si(111) substrates on the local superconducting properties of ∼40 nm thick Nb films compared to Nb films grown on typical Si(001) substrates. Specifically, low-temperature scanning tunneling microscopy and spectroscopy are employed to compare nanoscale material properties. The atomically flat monohydride H:Si(111) substrates are found to yield a smoother and less defective interface with the Nb film. Correspondingly, the Nb films grown on H:Si(111) substrates present more uniform superconducting properties and exhibit less quasiparticle broadening.

Golovchanskiy I.A., Abramov N.N., Emelyanova O.V., Shchetinin I.V., Ryazanov V.V., Golubov A.A., Stolyarov V.S.

In this work, we study magnetization dynamics in superconductor-ferromagnet-superconductor thin-film structures. Results of the broad-band ferromagnetic resonance spectroscopy are reported for a large set of samples with varied thickness of both superconducting and ferromagnetic layers in a wide frequency, field, and temperature ranges. Experimentally the one-dimensional anisotropic action of superconducting torque on magnetization dynamics is established; its dependence on thickness of layers is revealed. It is demonstrated that experimental findings support the recently proposed mechanism of the superconducting torque formation via the interplay between the superconducting imaginary conductance and magnetization precession at superconductor-ferromagnet interfaces. Microwave spectroscopy studies in this work are supplemented by investigations of the crystal structure and the microstructure of studied multilayers.

Grebenchuk S.Y., Hovhannisyan R.A., Shishkin A.G., Dremov V.V., Stolyarov V.S.

The possibility of controlling Josephson vortices and fluxon states has always been an attractive area of research that can influence the development of superconducting devices and quantum computing. Here we demonstrate the capability of magnetic force microscopy to detect and manipulate Josephson vortices (JVs) and fluxons below the critical current in complex superconducting systems on the example of a dc superconducting quantum interference device. The simultaneous magnetic force microscopy and transport investigation method make it possible to generate JVs by direct current in Josephson junctions and simultaneously analyze their dynamics under various external conditions. Our results show a dynamic process of transition of $2\ensuremath{\pi}$-phase singularities of JVs and fluxons from one to another. Finally, we demonstrated that the magnetic force microscopy method could distinguish the difference in critical currents between several Josephson junctions in complex superconducting circuits without the need of direct electron transport measurements.

Stolyarov V.S., Ruzhitskiy V., Hovhannisyan R.A., Grebenchuk S., Shishkin A.G., Skryabina O.V., Golovchanskiy I.A., Golubov A.A., Klenov N.V., Soloviev I.I., Kupriyanov M.Y., Andriyash A., Roditchev D.

Made of a thin non-superconducting metal (N) sandwiched by two superconductors (S), SNS Josephson junctions enable novel quantum functionalities by mixing up the intrinsic electronic properties of N with the superconducting correlations induced from S by proximity. Electronic properties of these devices are governed by Andreev quasiparticles (Andreev, A. Sov. Phys. JETP 1965, 20, 1490) which are absent in conventional SIS junctions whose insulating barrier (I) between the two S electrodes owns no electronic states. Here we focus on the Josephson vortex (JV) motion inside Nb-Cu-Nb proximity junctions subject to electric currents and magnetic fields. The results of local (magnetic force microscopy) and global (transport) experiments provided simultaneously are compared with our numerical model, revealing the existence of several distinct dynamic regimes of the JV motion. One of them, identified as a fast hysteretic entry/escape below the critical value of Josephson current, is analyzed and suggested for low-dissipative logic and memory elements.

Hovhannisyan R.A., Grebenchuk S.Y., Baranov D.S., Roditchev D., Stolyarov V.S.

Lateral Josephson junctions (LJJ) made of two superconducting Nb electrodes coupled by Cu-film are applied to quantify the stray magnetic field of Co-coated cantilevers used in magnetic force microscopy (MFM). The interaction of the magnetic cantilever with LJJ is reflected in the electronic response of LJJ as well as in the phase shift of cantilever oscillations, simultaneously measured. The phenomenon is theorized and used to establish the spatial map of the stray field. Based on our findings, we suggest integrating LJJs directly on the tips of cantilevers and using them as nanosensors of local magnetic fields in scanning probe microscopes. Such probes are less invasive than conventional magnetic MFM cantilevers and simpler to realize than SQUID-on-tip sensors.

Soloviev I.I., Ruzhickiy V.I., Bakurskiy S.V., Klenov N.V., Kupriyanov M.Y., Golubov A.A., Skryabina O.V., Stolyarov V.S.

I. I. Soloviev, 2, 3, ∗ V. I. Ruzhickiy, 2, 3 S. V. Bakurskiy, 2, 4 N. V. Klenov, 2, 3 M. Yu. Kupriyanov, A. A. Golubov, 5 O. V. Skryabina, 4 and V. S. Stolyarov 2 Lomonosov Moscow State University Skobeltsyn Institute of Nuclear Physics, 119991 Moscow, Russia Dukhov All-Russia Research Institute of Automatics, Moscow 101000, Russia Physics Department of Moscow State University, 119991 Moscow, Russia Moscow Institute of Physics and Technology, State University, 141700 Dolgoprudniy, Moscow region, Russia Faculty of Science and Technology and MESA+ Institute of Nanotechnology, 7500 AE Enschede, The Netherlands (Dated: July 2, 2021)

Golod T., Hovhannisyan R.A., Kapran O.M., Dremov V.V., Stolyarov V.S., Krasnov V.M.

Phase shifter is one of the key elements of quantum electronics. In order to facilitate operation and avoid decoherence, it has to be reconfigurable, persistent, and nondissipative. In this work, we demonstrate prototypes of such devices in which a Josephson phase shift is generated by coreless superconducting vortices. The smallness of the vortex allows a broad-range tunability by nanoscale manipulation of vortices in a micron-size array of vortex traps. We show that a phase shift in a device containing just a few vortex traps can be reconfigured between a large number of quantized states in a broad [−3π, +3π] range.

Galin M.A., Rudau F., Borodianskyi E.A., Kurin V.V., Koelle D., Kleiner R., Krasnov V.M., Klushin A.M.

Phase-locking of oscillators leads to superradiant amplification of the emission power. This is particularly important for development of THz sources, which suffer from low emission efficacy. In this work we study large Josephson junction arrays containing several thousands of Nb-based junctions. Using low-temperature scanning laser microscopy we observe that at certain bias conditions two-dimensional standing-wave patterns are formed, manifesting global synchronization of the arrays. Analysis of standing waves indicates that they are formed by surface plasmon type electromagnetic waves propagating at the electrode/substrate interface. Thus we demonstrate that surface waves provide an effective mechanism for long-range coupling and phase-locking of large junction arrays.

Carbillet C., Cherkez V., Skvortsov M.A., Feigel'man M.V., Debontridder F., Ioffe L.B., Stolyarov V.S., Ilin K., Siegel M., Roditchev D., Cren T., Brun C.

Disorder has different profound effects on superconducting thin films. For a large variety of materials, increasing disorder reduces electronic screening which enhances electron-electron repulsion. These fermionic effects lead to a mechanism described by Finkelstein: when disorder combined to electron-electron interactions increases, there is a global decrease of the superconducting energy gap $\Delta$ and of the critical temperature $T_c$, the ratio $\Delta$/$k_BT_c$ remaining roughly constant. In addition, in most films an emergent granularity develops with increasing disorder and results in the formation of inhomogeneous superconducting puddles. These gap inhomogeneities are usually accompanied by the development of bosonic features: a pseudogap develops above the critical temperature $T_c$ and the energy gap $\Delta$ starts decoupling from $T_c$. Thus the mechanism(s) driving the appearance of these gap inhomogeneities could result from a complicated interplay between fermionic and bosonic effects. By studying the local electronic properties of a NbN film with scanning tunneling spectroscopy (STS) we show that the inhomogeneous spatial distribution of $\Delta$ is locally strongly correlated to a large depletion in the local density of states (LDOS) around the Fermi level, associated to the Altshuler-Aronov effect induced by strong electronic interactions. By modelling quantitatively the measured LDOS suppression, we show that the latter can be interpreted as local variations of the film resistivity. This local change in resistivity leads to a local variation of $\Delta$ through a local Finkelstein mechanism. Our analysis furnishes a purely fermionic scenario explaining quantitatively the emergent superconducting inhomogeneities, while the precise origin of the latter remained unclear up to now.

Dremov V.V., Grebenchuk S.Y., Shishkin A.G., Baranov D.S., Hovhannisyan R.A., Skryabina O.V., Lebedev N., Golovchanskiy I.A., Chichkov V.I., Brun C., Cren T., Krasnov V.M., Golubov A.A., Roditchev D., Stolyarov V.S.

Josephson vortices play an essential role in superconducting quantum electronics devices. Often seen as purely conceptual topological objects, 2π-phase singularities, their observation and manipulation are challenging. Here we show that in Superconductor—Normal metal—Superconductor lateral junctions Josephson vortices have a peculiar magnetic fingerprint that we reveal in Magnetic Force Microscopy (MFM) experiments. Based on this discovery, we demonstrate the possibility of the Josephson vortex generation and manipulation by the magnetic tip of a MFM, thus paving a way for the remote inspection and control of individual nano-components of superconducting quantum circuits. Josephson vortices (JVs) play an important role in superconducting quantum devices, but they remain difficult to be observed and manipulated. Here, Dremov et al. report magnetic fingerprint of JVs in magnetic force microscopy experiments, which paves a way to generate and control JVs.

Vinnikov L.Y., Veshchunov I.S., Sidelnikov M.S., Stolyarov V.S.

The high-resolution Bitter technique for visualization (decoration) of the magnetic-flux structure in superconductors and magnets at low temperatures is presented. This method is based on the preparation of magnetic nanoparticles directly during a low-temperature experiment (in situ) by evaporating a magnetic material in the atmosphere of a buffer gas (helium) above the sample surface. An apparatus was constructed and a technique was proposed to stabilize the temperature of a sample during decoration with an accuracy of ±1 K within a wide temperature range. As an example, the results of observation of the magnetic-flux structure in single crystals of BSCCO(2212) high-temperature superconductor and EuFe2(As0.79P0.21)2 ferromagnetic superconductor at T ≤ 18 K are presented.

Di Giorgio C., Scarfato A., Longobardi M., Bobba F., Iavarone M., Novosad V., Karapetrov G., Cucolo A.M.

We present a new procedure that takes advantage of the magnetic flux quantization of superconducting vortices to calibrate the magnetic properties of tips for magnetic force microscopy (MFM). Indeed, a superconducting vortex, whose quantized flux is dependent upon Plank constant, speed of light and electron charge, behaves as a very well defined magnetic reference object. The proposed calibration procedure has been tested on new and worn tips and shows that the monopole point-like approximation of the probe is a reliable model. This procedure has been then applied to perform quantitative MFM experiments on a soft ferromagnetic thin film of permalloy, leading to the determination of the local out-of-plane component of the canted magnetization, together with its spatial variations across a few μm2 scan area.

Stolyarov V.S., Veshchunov I.S., Grebenchuk S.Y., Baranov D.S., Golovchanskiy I.A., Shishkin A.G., Zhou N., Shi Z., Xu X., Pyon S., Sun Y., Jiao W., Cao G., Vinnikov L.Y., Golubov A.A., et. al.

Adding ferromagnetism to superconductor leads to spatially patterned phases of spontaneously generated vortex-antivortex pairs. The interplay between superconductivity and magnetism is one of the oldest enigmas in physics. Usually, the strong exchange field of ferromagnet suppresses singlet superconductivity via the paramagnetic effect. In EuFe2(As0.79P0.21)2, a material that becomes not only superconducting at 24.2 K but also ferromagnetic below 19 K, the coexistence of the two antagonistic phenomena becomes possible because of the unusually weak exchange field produced by the Eu subsystem. We demonstrate experimentally and theoretically that when the ferromagnetism adds to superconductivity, the Meissner state becomes spontaneously inhomogeneous, characterized by a nanometer-scale striped domain structure. At yet lower temperature and without any externally applied magnetic field, the system locally generates quantum vortex-antivortex pairs and undergoes a phase transition into a domain vortex-antivortex state characterized by much larger domains and peculiar Turing-like patterns. We develop a quantitative theory of this phenomenon and put forth a new way to realize superconducting superlattices and control the vortex motion in ferromagnetic superconductors by tuning magnetic domains—unprecedented opportunity to consider for advanced superconducting hybrids.