Planar MoRe-based direct current nanoSQUID

Faley M.I., Reith P., Satrya C.D., Stolyarov V.S., Folkers B., Golubov A.A., Hilgenkamp H., Dunin-Borkowski R.E.

Abstract We have developed Josephson junctions between the d-wave superconductor YBa2Cu3O7−x (YBCO) and the s-wave Mo0.6Re0.4 (MoRe) alloy superconductor (ds-JJs). Such ds Josephson junctions are of interest for superconducting electronics making use of incorporated π-phase shifts. The I(V)-characteristics of the ds-JJs demonstrate a twice larger critical current along the [100] axis of the YBCO film compared to similarly-oriented ds-JJs made with a Nb top electrode. The characteristic voltage I c R n of the YBCO–Au–MoRe ds-JJs is 750 μV at 4.2 K. The ds-JJs that are oriented along the [100] or [010] axes of the YBCO film exhibit a 200 times higher critical current than similar ds-JJs oriented along the [110] axis of the same YBCO film. A critical current density J c = 20 kA cm−2 at 4.2 K was achieved. Different layouts of π-loops based on the novel ds-JJs were arranged in various mutual coupling configurations. Spontaneous persistent currents in the π-loops were investigated using scanning SQUID microscopy. Magnetic states of the π-loops were manipulated by currents in integrated bias lines. Higher flux states up to ±2.5Φ0 were induced and stabilized in the π-loops. Crossover temperatures between thermally activated and quantum tunneling switching processes in the ds-JJs were estimated. The demonstrated ability to stabilise and manipulate states of π-loops paves the way towards new computing concepts such as quantum annealing computing.

Bagani K., Sarkar J., Uri A., Rappaport M.L., Huber M.E., Zeldov E., Myasoedov Y.

Scanning nanoscale superconducting quantum interference devices (SQUIDs) are gaining interest as highly sensitive microscopic magnetic and thermal characterization tools of quantum and topological states of matter and devices. Here we introduce a novel technique of collimated differential-pressure magnetron sputtering for versatile self aligned fabrication of SQUID on tip (SOT) nanodevices, which cannot be produced by conventional sputtering methods due to their diffusive, rather than the required directional point-source, deposition. The new technique provides access to a broad range of superconducting materials and alloys beyond the elemental superconductors employed in the existing thermal deposition methods, opening the route to greatly enhanced SOT characteristics and functionalities. Utilizing this method, we have developed MoRe SOT devices with sub-50 nm diameter, magnetic flux sensitivity of 1.2 $\mu\Phi_0/Hz^{1/2}$ up to 3 T at 4.2 K, and thermal sensitivity better than 4 $\mu K/Hz^{1/2}$ up to 5 T, about five times higher than any previous report, paving the way to nanoscale imaging of magnetic and spintronic phenomena and of dissipation mechanisms in previously inaccessible quantum states of matter.

Holzman I., Ivry Y.

Superconducting quantum interference devices (SQUIDs) are used for applications ranging from sensitive magnetometers to low-temperature electronics and quantum computation. Miniaturizing SQUIDs is technologically attractive for increasing spin sensitivity as well as device integration and circuit speed. We introduce a planar nano SQUID that was made with a single lithographic step out of NbN films as thin as 3 nm on a Si chip. The fabrication process of weak links that are 45 nm in width, and 165 nm in length, which were designed to account for overcoming current crowding are presented. Operation at a temperature range of 20 mK to 5 K as well as at 1 T parallel, and 10 mT perpendicular magnetic fields is demonstrated, while potential operation higher than 8 T has also been shown. The broad range of applicability of a single device as well as its scalability are promising for on-chip integrability that may open technological possibilities, including in quantum and electro-optical circuiting.

Polturak E.

The Kibble–Zurek (Kibble in J Phys A 9:1387–1398, 1976; Zurek in Nature 317:505–508, 1985) scenario predicts that the outcome of a second-order phase transition from a disordered system into an ordered one depends on the quench rate. The emerging order parameter in the ordered state is not spatially uniform, containing topological defects. The faster the transition, the larger the density of defects. In the case of a conductor–superconductor transition, these defects are flux quanta (vortices). To investigate this scenario, we developed a high-resolution magneto-optical imaging system capable of resolving single flux quanta. Using this system, we imaged arrays of spontaneously created vortices in a Nb film. These vortices were formed after the film was rapidly cooled into the superconducting state at rates around 109 K/s. The internal correlations within the vortex array are important in order to differentiate between competing models. In the Kibble–Zurek scenario, neighboring vortices should have a different polarity, while in Hindmarsh–Rajantie (Hindmarsh and Rajantie in Phys Rev Lett 85:4660–4663, 2000) model the polarity should be the same. Our results favor the Kibble–Zurek scenario.

Hazra D.

Nano-scale superconducting quantum interference devices (nano-SQUIDS) where the weak-links are made from nano-bridges --- i.e., nano-bridge--SQUIDs (NBSs) --- are one of the most sensitive magnetometers for nano-scale magnetometry. Because of very strong non-linearity in the nano-bridge--electrode joints, the applied magnetic flux ($\Phi_{a}$) -- critical current ($I_{c}$) characteristics of NBSs differ very significantly from conventional tunnel-junction-SQUIDs, especially when nano-bridges are long and/or the screening parameter is large. However, in most of the theoretical descriptions, NBSs have been treated like conventional tunnel-junction-SQUIDs, which are based on d.c. Josephson effect. Here, I present a model demonstrating that for long nano-bridges and/or large screening parameter the $I_{c}(\Phi_{a})$ of a NBS can be explained by merely considering the fluxoid quantization in the NBS loop and the energy of the NBS; it is not necessary to take the Josephson effect into consideration. I also demonstrate that using the model, we can derive useful expressions like modulation depth and transfer function. I also discuss the role of kinetic inductance fraction ($\kappa$) in determining $I_{c}(\Phi_{a})$.

Kazakova O., Puttock R., Barton C., Corte-León H., Jaafar M., Neu V., Asenjo A.

Since it was first demonstrated in 1987, magnetic force microscopy (MFM) has become a truly widespread and commonly used characterization technique that has been applied to a variety of research and industrial applications. Some of the main advantages of the method includes its high spatial resolution (typically ∼50 nm), ability to work in variable temperature and applied magnetic fields, versatility, and simplicity in operation, all without almost any need for sample preparation. However, for most commercial systems, the technique has historically provided only qualitative information, and the number of available modes was typically limited, thus not reflecting the experimental demands. Additionally, the range of samples under study was largely restricted to “classic” ferromagnetic samples (typically, thin films or patterned nanostructures). Throughout this Perspective article, the recent progress and development of MFM is described, followed by a summary of the current state-of-the-art techniques and objects for study. Finally, the future of this fascinating field is discussed in the context of emerging instrumental and material developments. Aspects including quantitative MFM, the accurate interpretation of the MFM images, new instrumentation, probe-engineering alternatives, and applications of MFM to new (often interdisciplinary) areas of the materials science, physics, and biology will be discussed. We first describe the physical principles of MFM, specifically paying attention to common artifacts frequently occurring in MFM measurements; then, we present a comprehensive review of the recent developments in the MFM modes, instrumentation, and the main application areas; finally, the importance of the technique is speculated upon for emerging or anticipated to emerge fields including skyrmions, 2D-materials, and topological insulators.

Golod T., Kapran O.M., Krasnov V.M.

We propose a magnetic scanning-probe sensor based on a single-planar Josephson junction with a magnetic barrier. The planar geometry together with the high magnetic permeability of the barrier faci ...

Buchner M., Höfler K., Henne B., Ney V., Ney A.

In the field of nanomagnetism and spintronics, integral magnetometry is nowadays challenged by samples with low magnetic moments and/or low coercive fields. Commercial superconducting quantum interference device magnetometers are versatile experimental tools to magnetically characterize samples with ultimate sensitivity as well as with a high degree of automation. For realistic experimental conditions, the as-recorded magnetic signal contains several artifacts, especially if small signals are measured on top of a large magnetic background or low magnetic fields are required. In this Tutorial, we will briefly review the basic principles of magnetometry and present a representative discussion of artifacts which can occur in studying samples like soft magnetic materials as well as low moment samples. It turns out that special attention is needed to quantify and correct the residual fields of the superconducting magnet to derive useful information from integral magnetometry while pushing the limits of detection and to avoid erroneous conclusions.

Keijers W., Baumans X.D., Panghotra R., Lombardo J., Zharinov V.S., Kramer R.B., Silhanek A.V., Van de Vondel J.

As the most sensitive magnetic field sensor, the superconducting quantum interference device (SQUID) became an essential component in many applications due to its unmatched performance. Through recently achieved miniaturization, using state-of-the-art fabrication methods, this fascinating device extended its functionality and became an important tool in nanomaterial characterization. Here, we present an accessible and yet powerful technique of targeted atom displacement in order to reduce the size of the weak links of a DC nano-SQUID beyond the limits of conventional lithography. The controllability of our protocol allows us to characterize in situ the full superconducting response after each electromigration step. From this in-depth analysis, we reveal an asymmetric evolution of the weak links at cryogenic temperatures. A comparison with time resolved scanning electron microscopy images of the atom migration process at room temperature confirms the peculiar asymmetric evolution of the parallel constrictions. Moreover, we observe that when electromigration has sufficiently reduced the junction's cross section, superconducting phase coherence is attained in the dissipative state, where magnetic flux readout from voltage becomes possible.

Godfrey T., Gallop J.C., Cox D.C., Romans E.J., Chen J., Hao L.

Superconducting QUantum Interference Devices (SQUIDs) based on nanobridge junctions have shown increasing promise for single particle detection. This paper describes the development of the fabrication of improved and reproducible nanobridge junctions fabricated by focused ion beam (FIB) milling from niobium thin films. Although the very low noise properties of nanobridge SQUIDs are well known, the nature of the milling process is little understood at the level of local superconducting properties. In this paper, we report the results for nanobridge Josephson devices and SQUIDs, which we believe are the first to be made by Xenon (Xe) FIB milling. Temperature-dependent current–voltage behavior, microwave-induced Shapiro steps, and SQUID response to magnetic fields have been measured. We make preliminary comparisons with nominally identical devices milled from Nb thin films using either Xe or Ga ions.

Halbertal D., Ben Shalom M., Uri A., Bagani K., Meltzer A.Y., Marcus I., Myasoedov Y., Birkbeck J., Levitov L.S., Geim A.K., Zeldov E.

Watching electrons lose steam in graphene Although graphene can be fabricated to be extremely clean, it still has a nonzero electrical resistance. Resistance is associated with turning electrons' energy into heat, but how exactly does this happen? Halbertal et al. used a tiny scanning temperature probe based on a superconducting quantum interference device to investigate this problem. As the current flowed through a square-shaped sample of graphene, electrons lost energy predominantly in the vicinity of atomic-scale defects, which were few and far between in the bulk but much more common on the edges of the sample. Science, this issue p. 1303 A scanning nanoscale thermometer reveals the mechanism for energy dissipation in ultrapure samples of graphene. Conversion of electric current into heat involves microscopic processes that operate on nanometer length scales and release minute amounts of power. Although central to our understanding of the electrical properties of materials, individual mediators of energy dissipation have so far eluded direct observation. Using scanning nanothermometry with submicrokelvin sensitivity, we visualized and controlled phonon emission from individual atomic-scale defects in graphene. The inferred electron-phonon “cooling power spectrum” exhibits sharp peaks when the Fermi level comes into resonance with electronic quasi-bound states at such defects. Rare in the bulk but abundant at graphene’s edges, switchable atomic-scale phonon emitters provide the dominant dissipation mechanism. Our work offers insights for addressing key materials challenges in modern electronics and enables control of dissipation at the nanoscale.

Gallop J., Hao L.

Over the past decade, nanoscale superconducting quantum interference devices (nanoSQUIDs) have rapidly risen from nowhere to forge a new sphere of applications of these macroscopic quantum devices. New fabrication techniques have enabled these advances. In this Perspective, we highlight another recent major development in this area-the demonstration of a three-axis nanoSQUID magnetometer, which enables the vector magnetization of a nanoscale magnetic particle to be measured in the presence of an applied magnetic field. We illustrate the technological demands and developments that have driven the development of nanoSQUIDs and make suggestions for future directions for applications.

Granata C., Vettoliere A.

The magnetic sensing at nanoscale level is a promising and interesting research topic of nanoscience. Indeed, magnetic imaging is a powerful tool for probing biological, chemical and physical systems. The study of small spin cluster, like magnetic molecules and nanoparticles, single electron, cold atom clouds, is one of the most stimulating challenges of applied and basic research of the next years. In particular, the magnetic nanoparticle investigation plays a fundamental role for the modern material science and its relative technological applications like ferrofluids, magnetic refrigeration and biomedical applications, including drug delivery, hyper-thermia cancer treatment and magnetic resonance imaging contrast-agent. In this framework, several efforts have been devoted to the development of a high sensitivity magnetic nanosensor pushing sensing capability to the individual spin level. Among the different magnetic sensors, Superconducting QUantum Interference Devices (SQUIDs) exhibit an ultra high sensitivity and are widely employed in numerous applications. In the recent years, it has been proved that the magnetic response of nano-objects can be effectively measured by using a SQUID with a very small sensitive area (nanoSQUID). In fact, the sensor noise, expressed in terms of the elementary magnetic moment (spin or Bohr magneton), is linearly dependent on the SQUID loop side length. For this reason, SQUIDs have been progressively miniaturized in order to improve the sensitivity up to few spin per unit of bandwidth. With respect to other techniques, nanoSQUIDs offer the advantage of direct measurement of magnetization changes in small spin systems. In this review, we focus on nanoSQUIDs and its applications. In particular, we will discuss the motivations, the theoretical aspects, the fabrication techniques, the different nanoSQUIDs and the relative nanoscale applications.

Singh V., Schneider B.H., Bosman S.J., Merkx E.P., Steele G.A.

Superconducting microwave resonators (SMRs) with high quality factors have become an important technology in a wide range of applications. Molybdenum-Rhenium (MoRe) is a disordered superconducting alloy with a noble surface chemistry and a relatively high transition temperature. These properties make it attractive for SMR applications, but characterization of MoRe SMR has not yet been reported. Here, we present the fabrication and characterization of SMR fabricated with a MoRe 60–40 alloy. At low drive powers, we observe internal quality-factors as high as 700 000. Temperature and power dependence of the internal quality-factors suggest the presence of the two level systems from the dielectric substrate dominating the internal loss at low temperatures. We further test the compatibility of these resonators with high temperature processes, such as for carbon nanotube chemical vapor deposition growth, and their performance in the magnetic field, an important characterization for hybrid systems.

Quintana C.M., Megrant A., Chen Z., Dunsworth A., Chiaro B., Barends R., Campbell B., Chen Y., Hoi I.-., Jeffrey E., Kelly J., Mutus J.Y., O'Malley P.J., Neill C., Roushan P., et. al.

Many superconducting qubits are highly sensitive to dielectric loss, making the fabrication of coherent quantum circuits challenging. To elucidate this issue, we characterize the interfaces and surfaces of superconducting coplanar waveguide resonators and study the associated microwave loss. We show that contamination induced by traditional qubit lift-off processing is particularly detrimental to quality factors without proper substrate cleaning, while roughness plays at most a small role. Aggressive surface treatment is shown to damage the crystalline substrate and degrade resonator quality. We also introduce methods to characterize and remove ultra-thin resist residue, providing a way to quantify and minimize remnant sources of loss on device surfaces.

Nazhestkin I.A., Bakurskiy S.V., Neilo A.A., Tarasova I.E., Ismailov N.G., Gurtovoi V.L., Egorov S.V., Lisitsyn S.A., Stolyarov V.S., Antonov V.N., Ryazanov V.V., Kupriyanov M.Y., Soloviev I.I., Klenov N.V., Yakovlev D.S.

The transport properties of a nanobridge superconducting quantum interference device made of Al/Pt bilayer have been studied. Measurement and approximation of the voltage‐field dependencies allow to estimate the inductance of the structure. It is found that this value significantly exceeds the expected geometric inductance and exhibits an atypical temperature dependence. To explain this effect, a microscopic model of electron transport in SN bilayers is developed, considering the proximity effect, and the available regimes of the current distribution are described. The measured properties may be indicative of the formation of high‐resistance aluminum with high values of kinetic inductance during the fabrication of Al/Pt bilayers.

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

Abstract We study the effect of electrode width on superconducting current transport in Nb–Au–Nb Josephson bridges. The critical current as well as the normal bridge resistance drop with decreasing electrode width on scales of a few μ m , which are orders of magnitude larger than the estimated coherence length of the Au strip. We consider several physical reasons for such an anomalous influence of the width W of the superconducting electrode on the critical current I c (AIWIc) and provide model fits for the resistive and superconducting properties of the bridges. The smooth dependence of the Nb–Au–Nb bridge parameters on the electrode width can be used to optimize the design of superconducting devices for specific applications.

Bakurskiy Sergey, Ruzhickiy Vsevolod, Neilo Alexey, Klenov Nikolay, Soloviev Igor, Elistratova Anna, Shishkin Andrey, Stolyarov Vasily, Kupriyanov Mikhail

We have studied the Thouless energy in Josephson superconductor – normal metal – superconductor (SN-N-NS) bridges analytically and numerically, considering the influence of the sub-electrode regions. We have discovered a significant suppression of the Thouless energy with increasing interfacial resistance, consistent with experimental results. The analysis of the temperature dependence of the critical current in Josephson junctions in comparison with the expressions for the Thouless energy may allow the determination of the interface parameters of S and N-layers.

Faley M.I.

The fabrication methods, principle of operation, microstructural, and electron transport properties, as well as the application of nanoscale superconducting quantum interference devices (nanoSQUIDs) are considered. NanoSQUIDs by definition have a submicrometer loop size, which limits dimensions of Josephson junctions to about 100 nm or less. This makes the use of tunnel junctions problematic, but encourages the use of nanobridges and other types of Josephson junctions with high critical current densities. The use of nitride superconductors helps to optimize the superconducting parameters and enhances the corrosion resistance of the nanobridge Josephson junctions and nanoSQUIDs. Additional passivation by a Si layer improves thermal sink and protects against the mechanical and electrical fragility of the devices. Nanosculpturing of cantilevers makes it possible to position the nanoSQUID at a distance of 10–100 nm from the objects under study in the nanoSQUID scanning measurement system. Several promising applications of nanoSQUIDs are briefly reviewed.

Zhukova Elena, Nekrasov Boris, Kadyrov Lenar, Melentev Aleksandr, Shaimardanov Anton, Shishkin Andrey, Golubov Alexander, Kupriyanov Mikhail, Gorshunov Boris, Stolyarov Vasily

Terahertz time-domain spectroscopy is used to perform the first detailed studies of the electrodynamic properties of MoRe (60\( \% \)/40\( \% \)) films with thicknesses ranging from 10 to 100 nm. Films are prepared by magnetron sputtering technique on silicon substrates. The critical temperatures vary from \( 6.5\, \textrm{K} \) (for 10 nm film) to \( 9.5\, \textrm{K} \) (for 100 nm film). Spectra of complex permittivity, conductivity, refraction index, surface impedance and reflection coefficient for the films are acquired at frequencies \( 0.15 - 2.4\, \textrm{THz} \) (wavenumbers \( 5 – 80\, \textrm{cm}^{-1} \)) and in the temperature interval \( T=5\,–\, 300\,\textrm{K} \). For all films, temperature dependencies of the superconducting energy gap, penetration depth, superconducting condensate plasma frequency, and normalised superfluid density are obtained on a quantitative level. It is shown that the reduction of film thickness leads to a strong decrease of the critical temperature and magnitude of the energy gap. The observed suppression of superconductivity is assigned to reduction of the superconducting order parameter due to the contribution to the free energy of the electronic energy states at the surface of superconductor. The MoRe films with the obtained characteristics can be used in designing advanced superconducting electronic devices.

Borst M., Vree P.H., Lowther A., Teepe A., Kurdi S., Bertelli I., Simon B.G., Blanter Y.M., van der Sar T.

Superconductors are materials with zero electrical resistivity and the ability to expel magnetic fields, which is known as the Meissner effect. Their dissipationless diamagnetic response is central to magnetic levitation and circuits such as quantum interference devices. In this work, we used superconducting diamagnetism to shape the magnetic environment governing the transport of spin waves—collective spin excitations in magnets that are promising on-chip signal carriers—in a thin-film magnet. Using diamond-based magnetic imaging, we observed hybridized spin-wave–Meissner-current transport modes with strongly altered, temperature-tunable wavelengths and then demonstrated local control of spin-wave refraction using a focused laser. Our results demonstrate the versatility of superconductor-manipulated spin-wave transport and have potential applications in spin-wave gratings, filters, crystals, and cavities.

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

In this paper, we present a theoretical study of electronic transport in planar Josephson Superconductor–Normal Metal–Superconductor (SN-N-NS) bridges with arbitrary transparency of the SN interfaces. We formulate and solve the two-dimensional problem of finding the spatial distribution of the supercurrent in the SN electrodes. This allows us to determine the scale of the weak coupling region in the SN-N-NS bridges, i.e., to describe this structure as a serial connection between the Josephson contact and the linear inductance of the current-carrying electrodes. We show that the presence of a two-dimensional spatial current distribution in the SN electrodes leads to a modification of the current–phase relation and the critical current magnitude of the bridges. In particular, the critical current decreases as the overlap area of the SN parts of the electrodes decreases. We show that this is accompanied by a transformation of the SN-N-NS structure from an SNS-type weak link to a double-barrier SINIS contact. In addition, we find the range of interface transparency in order to optimise device performance. The features we have discovered should have a significant impact on the operation of small-scale superconducting electronic devices, and should be taken into account in their design.

Yakovlev D.S., Nazhestkin I.A., Ismailov N.G., Egorov S.V., Antonov V.N., Gurtovoi V.L.

We study operation of a superconducting quantum interference devices (SQUIDs) based on a new bilayer material. They can be used for the ultra-sensitive detection of magnetic momentum at temperatures down to milliKelvin range. Typically, thermal origin hysteresis of the symmetric SQUID current-voltage curves limits operating temperatures to T>0.6Tc. We used a new bilayer material for SQUID fabrication, namely proximity-coupled superconductor/normal-metal (S/N) bilayers (aluminum 25 nm / platinum 5 nm). Because of the 5 nm Pt-layer, Al/Pt devices show nonhysteretic behavior in a broad temperature range from 20 mK to 0.8 K. Furthermore, the Al/Pt bilayer devices demonstrate an order of magnitude lower critical current compared to the Al devices, which decreases the screening parameter (βL) and improves the modulation depth of the critical current by magnetic flux. Operation at lower temperatures reduces thermal noise and increases the SQUID magnetic field resolution. Moreover, we expect strong decrease of two-level fluctuators on the surface of aluminum due to Pt-layer oxidation protection and hence significant reduction of the 1/f noise. Optimized geometry of Al/Pt symmetric SQUIDs is promising for the detection of single-electron spin flip.

Faley M.I., Dunin-Borkowski R.E.

We report the development of a planar 4-Josephson-junction nanoscale superconducting quantum interference device (nanoSQUID) that is self-biased for optimal sensitivity without the application of a magnetic flux of Φ0/4. The nanoSQUID contains novel NbN-TiN-NbN nanobridge Josephson junctions (nJJs) with NbN current leads and electrodes of the nanoSQUID body connected by TiN nanobridges. The optimal superconducting transition temperature of ~4.8 K, superconducting coherence length of ~100 nm, and corrosion resistance of the TiN films ensure the hysteresis-free, reproducible, and long-term stability of nJJ and nanoSQUID operation at 4.2 K, while the corrosion-resistant NbN has a relatively high superconducting transition temperature of ~15 K and a correspondingly large energy gap. FIB patterning of the TiN films and nanoscale sculpturing of the tip area of the nanoSQUID’s cantilevers are performed using amorphous Al films as sacrificial layers due to their high chemical reactivity to alkalis. A cantilever is realized with a distance between the nanoSQUID and the substrate corner of ~300 nm. The nJJs and nanoSQUID are characterized using Quantum Design measurement systems at 4.2 K. The technology is expected to be of interest for the fabrication of durable nanoSQUID sensors for low temperature magnetic microscopy, as well as for the realization of more complex circuits for superconducting nanobridge electronics.

Faley M.I., Fiadziushkin H., Frohn B., Schüffelgen P., Dunin-Borkowski R.E.

Abstract We report the fabrication and properties of titanium nitride (TiN) nanobridge Josephson junctions (nJJs) and nanoscale superconducting quantum interference devices (nanoSQUIDs) on SiN-buffered Si substrates. The superior corrosion resistance, large coherence length, suitable superconducting transition temperature and highly selective reactive ion etching (RIE) of TiN compared to e-beam resists and the SiN buffer layer allow for reproducible preparation and result in long-term stability of the TiN nJJs. High-resolution transmission electron microscopy reveals a columnar structure of the TiN film on an amorphous SiN buffer layer. High-resolution scanning electron microscopy reveals the variable thickness shape of the nJJs. A combination of wet etching in 20% potassium hydroxide and RIE is used for bulk nanomachining of nanoSQUID cantilevers. More than 20 oscillations of the V(B) dependence of the nanoSQUIDs with a period of ∼6 mT and hysteresis-free I(V) characteristics (CVCs) of the all-TiN nJJs are observed at 4.2 K. CVCs of the low-I c all-TiN nJJs follow theoretical predictions for dirty superconductors down to ∼10 mK, with the critical current saturated below ∼0.6 K. These results pave the way for superconducting electronics based on nJJs operating non-hysteretically at 4.2 K, as well as for all-TiN qubits operating at sub-100 mK temperatures.

Faley M.I., Liu Y., Dunin-Borkowski R.E.

Nanobridge Josephson junctions and nanometer-scale superconducting quantum interference devices (nanoSQUIDs) based on titanium nitride (TiN) thin films are described. The TiN films have a room temperature resistivity of ~15 µΩ·cm, a superconducting transition temperature Tc of up to 5.3 K and a coherence length ξ(4.2 K) of ~105 nm. They were deposited using pulsed DC magnetron sputtering from a stoichiometric TiN target onto Si (100) substrates that were heated to 800 °C. Electron beam lithography and highly selective reactive ion etching were used to fabricate nanoSQUIDs with 20-nm-wide nanobridge Josephson junctions of variable thickness. X-ray and high-resolution electron microscopy studies were performed. Non-hysteretic I(V) characteristics of the nanobridges and nanoSQUIDs, as well as peak-to-peak modulations of up to 17 µV in the V(B) characteristics of the nanoSQUIDs, were measured at 4.2 K. The technology offers prospects for superconducting electronics based on nanobridge Josephson junctions operating within the framework of the Ginzburg–Landau theory at 4.2 K.