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2. Observation and characterization of titanium-like nano-filament in TiO2 memristor using superconducting electrode(s) and Andreev spectroscopy.

Author

Moško, Martin, Koscelanská, Mária, Mošková, Antónia, Vidiš, Marek, Volkov, Serhii, Gregor, Maroš, Poláčková, Magdaléna, Roch, Tomáš, Grančič, Branislav, Satrapinskyy, Leonid, Kúš, Peter, Plecenik, Andrej, and Plecenik, Tomáš

Subjects
SEMICONDUCTOR thin films, ELECTRON microscope techniques, ANDREEV reflection, BALLISTIC conduction, REFLECTANCE spectroscopy
Abstract

A thin TiO2 semiconductor film embedded between two metal electrodes works as a memristor after being formed by soft breakdown. The forming creates a nano-filament that penetrates through the poorly conducting TiO2 film and connects the electrodes conductively. While previous works characterized the nano-filament properties (shape, composition, and resistivity) by electron microscopy techniques, we present a characterization by electrical measurements. In a typical memristor, both electrodes are made of normal metals. We study the metal/TiO2/metal memristors with a bottom electrode made of a superconducting NbN layer and a top electrode made of a normal (Pt) or superconducting (Nb) metal. The nano-filament connecting the electrodes touches the bottom electrode as a point contact, thus allowing us to perform point-contact Andreev reflection spectroscopy of the NbN superconductor. The spectra, measured below the critical temperature (15 K) of NbN, are analyzed theoretically. The analysis reveals the presence of one nano-filament and determines the nano-filament resistance, Sharvin resistance of the point contact, and Maxwell resistance of the electrodes. Moreover, it shows that the nano-filament is a conical-shaped Ti-like metal point contact with a tip diameter of ∼3–5 nm, Fermi velocity of 2 × 10 6 m / s , and low-temperature resistivity of ∼ 10 − 8 – 10 − 7 Ω m. Thus, the nano-filament in our device is not the Ti4O7 phase observed in previous works. Remarkably, the point contact spectrum of the superconducting NbN layer shows the Andreev peak typical for ballistic transport. This is because the point contact probes the NbN layer through a thin Al layer that mimics superconductivity of NbN via the proximity effect and eliminates the effects of tunneling and disorder. [ABSTRACT FROM AUTHOR]

Published
2024
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7. Competing length scales and 2D versus 3D dimensionality in relatively thick superconducting NbN films

Author

Mikhail Belogolovskii, Magdaléna Poláčková, Elena Zhitlukhina, Branislav Grančič, Leonid Satrapinskyy, Maroš Gregor, and Tomáš Plecenik

Subjects
Medicine, Science
Abstract

Abstract Magneto-transport characteristics of 2D and 3D superconducting layers, in particular, temperature and angular dependences of the upper critical field H c2, are usually considered to be fundamentally different. In the work, using non-local resistance measurements at temperatures near the normal-to-superconducting transition, we probed an effective dimensionality of nm-thick NbN films. It was found that in relatively thick NbN layers, the thicknesses of which varied from 50 to 100 nm, the temperature effect on H c2 certainly pointed to the three-dimensionality of the samples, while the angular dependence of H c2 revealed behavior typical for 2D samples. The seeming contradiction is explained by an intriguing interplay of three length scales in the dimensionally confined superconducting films: the thickness, the Ginzburg–Landau coherence length, and the magnetic-field penetration depth. Our results provide new insights into the physics of superconducting films with an extremely large ratio of the London penetration depth to the Ginzburg–Landau coherence length exhibiting simultaneously 3D isotropic superconducting properties and the 2D transport regime.

Published
2023
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19. Inhomogeneous Resistivity of Transparent Superconductor Films Revealed by the Van Der Pauw Technique.

Author

Shapovalov, A., Menesenko, D., Zhitlukhina, E., Parra, A., Aliev, A., Shamaev, V., Gregor, M., and Plecenik, T.

Subjects
SUPERCONDUCTING transitions, SUPERCONDUCTING films, THIN films, SUPERCONDUCTIVITY, SUPERCONDUCTORS
Abstract

Detection of structural and transport inhomogeneities in superconducting thin films using the standard four-probe method requires multiple measurements and may be inaccurate. A new approach to extract the information about the inhomogeneous transition into the superconducting state using van der Pauw technique is presented. The proposed method is applied to an electrochemically-reduced In-Sn-oxide (ITO) films known to be simultaneously transparent and superconducting. Relationship between the processing parameters and superconducting characteristics of the ITO films is considered, and prospects for using such samples in integrated photonic-superconducting chips for quantum-information processing are discussed. [ABSTRACT FROM AUTHOR]

Published
2024
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34. Point contact spectroscopy of superconductors via nanometer scale point contacts formed by resistive switching

Author

Mária Dvoranová, Tomáš Plecenik, Martin Moško, Marek Vidiš, Maroš Gregor, Tomáš Roch, Branislav Grančič, Leonid Satrapinskyy, Peter Kúš, and Andrej Plecenik

Subjects
Physics, QC1-999
Abstract

Point contact spectroscopy is commonly used to investigate electronic properties of superconductors. Here we show that nanometer scale point contacts, which enable to study the superconductor properties locally, can be created by means of the resistive switching phenomenon. Our experiments were performed on sandwiched MgB2/Al/TiO2/Pt structures, where multiple bipolar resistive switching cycles were conducted. The differential conductance as a function of voltage was measured at temperatures below the critical temperature of the MgB2 superconductor. In the low-resistance state the MgB2 and Pt electrodes are connected by an ultrathin metallic filament which creates at the MgB2 electrode the Sharvin point contact with diameter below 10 nm. In this case the differential conductance data demonstrate the Andreev reflections due to the carrier transport between the superconducting MgB2 electrode and filament. From these data the two-gap superconductivity of MgB2 is clearly visible which also confirms the fit by the Blonder-Tinkham-Klapwijk model. If the bottom electrode is made of a superconductor with known gap, our approach allows us to estimate from the Andreev reflection spectrum the resistance of both the filament and point contact. We can then determine from the Sharvin formula the cross-section size of the point contact and thus also the filament cross-section size. In the high resistance state when the filament is ruptured, the differential conductance data demonstrate the spectrum typical for tunneling between two normal metals, with a zero-bias anomaly due to the Altshuler-Aronov effect. This suggests that the filament is not ruptured at the superconducting MgB2 electrode but elsewhere.

Published
2018
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