Nano Superconducting Quantum Interference Device Research Articles

Granular aluminum nano-superconducting quantum interference device

Granular aluminum (grAl) is an applied quantum material. We present nano-superconducting quantum interference devices (nanoSQUIDs) based on grAl thin films. These devices exhibit non-hysteretic behavior, allowing conventional SQUID readout down to temperatures well below the critical temperature as well as detection properties comparable to those of Dayem bridge-based devices of greater complexity. Despite being much longer than the coherence length, the current–phase relation of these grAl constrictions appears to be single valued at least down to half their critical temperature. This suggests that grAl thin films should be described as a network of inter-grain Josephson junctions.

High transfer coefficient niobium nano-SQUID integrated with a nanogap modulation flux line

Nano-superconducting quantum interference devices (nano-SQUIDs) with high energy sensitivity and spatial resolution are essential in many applications such as single spin detection, nano-electromechanical vibration detection and microscale magnetic imaging. This paper studies a Dayem-type niobium nano-SQUID using focus ion beam milling technology. The device has two 42 nm × 60 nm nano-bridges and an integrated on-chip Nb modulation flux line located beside the SQUID loop with a 100 nm nanogap. The non-hysteretic temperature range of the nano-SQUID is about 1.4 K from 4.6 K to 6.0 K, which could broaden the operation temperature range of the device. The maximal transfer coefficient V Φ and peak-to-peak voltage ΔV are 8.53 mV/Φ 0 and 430 μV at 4.8 K, respectively.

Enhanced Electron–Phonon Coupling and Superconductivity in Two-dimensional BC2N via Lithium Deposition: a First-Principles Investigation

Two-dimensional (2D) superconductors are important both for the basic understanding of pairing mechanism and potential applications in nanosuperconducting quantum interference devices. In this work, we explore the phonon-mediated superconductivity of hole-doped and lithium-deposited monolayer BC2N by first-principles calculations. Our results reveal that the hole-doped planar BC2N cannot superconduct due to weak electron–phonon coupling (EPC) with λ = 0.07. When lithium is deposited on the monolayer, the EPC constant is enhanced significantly to 0.52 and the superconducting transition temperature Tc is determined to be 3.58 K. Because more phonons, in particular the out-of-plane modes, are triggered to participate in the EPC process, the Tc is higher than 1.4 K of Ca-deposited graphene, 1.3 K of Li-deposited stanene, and 3 K of doped WS2 and NbSe2. It is also found that tensile biaxial strain weakens the EPC and leads to a decreased superconducting transition temperature. Our findings will provide a new guideline for designing novel 2D superconductors and enrich the potential application of 2D BC2N.

Miniaturization of the Superconducting Memory Cell via a Three-Dimensional Nb Nano-superconducting Quantum Interference Device.

Scalable memories that can match the speeds of superconducting logic circuits have long been desired to enable a superconducting computer. A superconducting loop that includes a Josephson junction can store a flux quantum state in picoseconds. However, the requirement for the loop inductance to create a bistate hysteresis sets a limit on the minimal area occupied by a single memory cell. Here, we present a miniaturized superconducting memory cell based on a three-dimensional (3D) Nb nano-superconducting quantum interference device (nano-SQUID). The major cell area here fits within an 8 × 9 μm2 rectangle with a cross-selected function for memory implementation. The cell shows periodic tunable hysteresis between two neighboring flux quantum states produced by bias current sweeping because of the large modulation depth of the 3D nano-SQUID (∼66%). Furthermore, the measured current-phase relations (CPRs) of nano-SQUIDs are shown to be skewed from a sine function, as predicted by theoretical modeling. The skewness and the critical current of 3D nano-SQUIDs are linearly correlated. It is also found that the hysteresis loop size is in a linear scaling relationship with the CPR skewness using the statistics from characterization of 26 devices. We show that the CPR skewness range of π/4-3π/4 is equivalent to a large loop inductance in creating a stable bistate hysteresis for memory implementation. Therefore, the skewed CPR of 3D nano-SQUID enables further superconducting memory cell miniaturization by overcoming the inductance limitation of the loop area.

Broad-Band Coupling With Improved Efficiency Between the Nano-SQUID and Coplanar Waveguide

Besides its well-known magnetic sensitivity, the nano superconducting quantum interference device (SQUID) has an almost instantaneous response time. However, transferring an ultrafast magnetic signal to a nano-SQUID is a technical challenge. We describe a very efficient direct-coupling structure between the SQUID and the coplanar waveguide (CPW). The center conductor of the CPW and its grounding planes are connected by a short line. We fabricated devices having direct and indirect coupling between the short line and nano-SQUIDs. The coupling efficiencies of both schemes are compared quantitatively. By injecting microwaves of frequencies 0.1-18 GHz through the CPW, we found that microwave power of 100 nW and 158 μW is required to suppress the critical current of nano-SQUIDs for the direct and indirect-coupling devices, respectively. By applying a dc current to the short line, we were able to measure the flux-modulation curves of SQUIDs. To achieve a flux quanta in SQUIDs requires currents of 0.46 and 11 mA for the direct- and indirect-coupling devices, respectively. Therefore, the direct-coupling scheme improves the efficiency by a factor of 39 from 0.1 to 18 GHz and 24 at dc, respectively. Furthermore, while sending a pulse-modulated signal of 1 μW power, a SQUID incorporating the direct-coupling scheme produced a working flux modulation.

Investigation of an anti-reflection silicon nitride layer on a superconducting NbxSi1-xfilm absorber for inductive superconducting transition edge detectors

We report research on superconducting transition temperature (Tc) tuning and the antireflective layer of NbxSi1-x films that were used as the absorbing layer in inductive superconducting transition edge detectors (ISTED). The Tc of NbxSi1-x film absorber should be tuned to be to the operating temperature range of the readout nano superconducting quantum interference devices. The composition ratio of Nb/Si was controlled by the co-sputtering powers of the Nb and Si target to adjust the Tc. To improve the detection efficiency, a 30 nm Nb95.7Si4.3 film with Tc 6.3 K was chosen to demonstrate the effect of the antireflective layer SiNx made by low temperature plasma enhanced chemical vapor deposition (LT-PECVD). According to the spectral refractive indexes and extinction coefficients of the Nb95.7Si4.3 and SiNx films, the structure parameters for 633 nm incident light were designed and the optical properties were calculated. The reflectivity measurements showed that the reflectivity was effectively reduced, which was consistent with the calculation.

Local tuning of the order parameter in superconducting weak links: A zero-inductance nanodevice

Controlling both the amplitude and the phase of the superconducting quantum order parameter ψ in nanostructures is important for next-generation information and communication technologies. The lack of electric resistance in superconductors, which may be advantageous for some technologies, hinders convenient voltage-bias tuning and hence limits the tunability of ψ at the microscopic scale. Here, we demonstrate the local tunability of the phase and amplitude of ψ, obtained by patterning with a single lithography step a Nb nano-superconducting quantum interference device (nano-SQUID) that is biased at its nanobridges. We accompany our experimental results by a semi-classical linearized model that is valid for generic nano-SQUIDs with multiple ports and helps simplify the modelling of non-linear couplings among the Josephson junctions. Our design helped us reveal unusual electric characteristics with effective zero inductance, which is promising for nanoscale magnetic sensing and quantum technologies.

Measurement of Meissner effect in micro-sized Nb and FeSe crystals using an NbN nano-SQUID

The nano-superconducting quantum interference device (SQUID) is considered one of the most sensitive magnetic sensors for the characterization of mesoscopic and microscopic magnetic moments. Therefore, it is suitable for measuring the Meissner effect in small superconductors that cannot generate large enough signals for commercial magnetometers. To achieve an optimized coupling, the sample is usually placed directly on a SQUID chip and as close to the SQUID washer as possible. Therefore, a large working temperature range of the nano-SQUID is desirable to measure a wider range of samples. Here, we achieved the measurement of the Meissner effect in a 25 μm-sized Nb and a 40 μm × 120 μm-sized FeSe crystals using a niobium nitride (NbN) nano-SQUID. This nano-SQUID has a usable magnetic flux modulation for temperatures up to 9.5 K. The flux noise is around 50–60 μΦ0 Hz−1/2 for the entire measurement system. The diamagnetic branches induced by the Meissner effect below the lower critical field were observed for both Nb and FeSe crystals with the NbN nano-SQUID device. In addition, at various temperatures, strong magnetic hysteresis arising from vortices pinning was also observed and analyzed for both Nb and FeSe crystals.

Toward the Use of NanoSQUIDs to Measure the Displacement of an NEMS Resonator

We discuss steps toward the readout of the position of a nanoelectromagnetic system (NEMS) beam resonator using a Nb nanosuperconducting quantum interference device (nanoSQUID). We describe our fabrication procedure for coupling the nanoSQUID and a suspended Al-coated Si3N4 NEMS resonator together by a combination of focused ion beam lithography and nanomanipulation. We discuss typical electrical characteristics of the integrated devices, and independent postfabrication atomic force microscope nanoindentation measurements of the elastic properties of the integrated resonator to estimate its natural frequencies of vibration. We compare and discuss the response of a nanoSQUID with current-carrying and superconducting screening (noncurrent carrying) modes of operation of the resonator

NanoSQUID magnetometry of individual cobalt nanoparticles grown by focused electron beam induced deposition

We demonstrate the operation of low-noise nano superconducting quantum interference devices (SQUIDs) based on the high critical field and high critical temperature superconductor YBa2Cu3O7 (YBCO) as ultra-sensitive magnetometers for single magnetic nanoparticles (MNPs). The nanoSQUIDs exploit the Josephson behavior of YBCO grain boundaries and have been patterned by focused ion beam milling. This allows us to precisely define the lateral dimensions of the SQUIDs so as to achieve large magnetic coupling between the nanoloop and individual MNPs. By means of focused electron beam induced deposition, cobalt MNPs with a typical size of several tens of nm have been grown directly on the surface of the sensors with nanometric spatial resolution. Remarkably, the nanoSQUIDs are operative over extremely broad ranges of applied magnetic field (–1 T 1 T) and temperature (0.3 K 80 K). All these features together have allowed us to perform magnetization measurements under different ambient conditions and to detect the magnetization reversal of individual Co MNPs with magnetic moments (1–30) . Depending on the dimensions and shape of the particles we have distinguished between two different magnetic states yielding different reversal mechanisms. The magnetization reversal is thermally activated over an energy barrier, which has been quantified for the (quasi) single-domain particles. Our measurements serve to show not only the high sensitivity achievable with YBCO nanoSQUIDs, but also demonstrate that these sensors are exceptional magnetometers for the investigation of the properties of individual nanomagnets.

Improved noise performance of ultrathin YBCO Dayem bridge nanoSQUIDs

We have fabricated YBa2Cu3O (YBCO) nano superconducting quantum interference devices (nanoSQUIDs), realized in Dayem bridge configuration, on films with thickness down to 10 nm. The devices, which have not been protected by a Au capping layer during the nanopatterning, show modulations of the critical current as a function of the externally applied magnetic field from 300 mK up to the critical temperature of the nanobridges. The absence of the Au shunting layer and the enhancement of the sheet resistance in ultrathin films lead to very large voltage modulations and transfer functions, which make these nanoSQUIDs highly sensitive devices. Indeed, by using bare YBCO nanostructures, we have revealed an upper limit for the intrinsic white flux noise level .

A High-Performance Nb Nano-Superconducting Quantum Interference Device with a Three-Dimensional Structure

A superconducting quantum interference device (SQUID) miniaturized into the nanoscale is promising in the inductive detection of a single electron spin. A nano-SQUID with a strong spin coupling coefficient, a low flux noise, and a wide working magnetic field range is highly desired in a single spin resonance measurement. Nano-SQUIDs with Dayem bridge junctions excel in a high working field range and in the direct coupling from spins to the bridge. However, the common planar structure of nano-SQUIDs is known for problems such as a shallow flux modulation depth and a troublesome hysteresis in current-voltage curves. Here, we developed a fabrication process for creating three-dimensional (3-D) niobium (Nb) nano-SQUIDs with nanobridge junctions that can be tuned independently. Characterization of the device shows up to 45.9% modulation depth with a reversible current-voltage curve. Owning to the large modulation depth, the measured flux noise is as low as 0.34 μΦ0/Hz1/2. The working field range of the SQUID is greater than 0.5 T parallel to the SQUID plane. We believe that 3-D Nb nano-SQUIDs provide a promising step toward effective single-spin inductive detection.

Magnetic field imaging of a tungsten carbide film by scanning nano-SQUID microscope

We present the results of magnetic field imaging by scanning nano-superconducting quantum interference device (SQUID) microscopy on a tungsten carbide (W-C) film fabricated using focused-ion-beam chemical vapor deposition. We have investigated magnetic field change by a W-C film in an external magnetic field using a scanning nano-SQUID microscope system. We have found that the reduction of the magnetic field above the W-C film was 0.9%, indicating the penetration of vortices in the W-C at an external magnetic field of 0.171 mT.

Three-Axis Vector Nano Superconducting Quantum Interference Device.

We present the design, realization, and performance of a three-axis vector nano superconducting quantum interference device (nanoSQUID). It consists of three mutually orthogonal SQUID nanoloops that allow distinguishing the three components of the vector magnetic moment of individual nanoparticles placed at a specific position. The device is based on Nb/HfTi/Nb Josephson junctions and exhibits line widths of ∼250 nm and inner loop areas of 600 × 90 and 500 × 500 nm(2). Operation at temperature T = 4.2 K under external magnetic fields perpendicular to the substrate plane up to ∼50 mT is demonstrated. The experimental flux noise below [Formula: see text] in the white noise limit and the reduced dimensions lead to a total calculated spin sensitivity of [Formula: see text] and [Formula: see text] for the in-plane and out-of-plane components of the vector magnetic moment, respectively. The potential of the device for studying three-dimensional properties of individual nanomagnets is discussed.

Niobium NanoSQUIDs Based on Sandwich Nanojunctions: Performance as a Function of the Temperature

In this paper, an experimental investigation of the main characteristics of a niobium nano Superconducting QUantum Interference Device (nanoSQUID) as a function of the temperature (9-0.3 K) is presented. The nanosensor consists of a niobium superconducting loop (0.4 × 1.0 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) interrupted by two sandwich nanojunctions (Nb/Al-AlOx/Nb) having an area of about (300 × 300) nm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . These nanodevices have been fabricated by means of a focused ion beam sculpting method, which is used as a lithographic technique to define the various elements of the SQUID. We have performed measurements of current-voltage, critical current-magnetic flux characteristics, and switching current distributions from the zero voltage state for different temperatures. The high critical current modulation depths and the low intrinsic dissipation exhibited by these devices ensure a suitable sensitivity for nanoscale applications in the whole temperature range investigated.

Vertical nano superconducting quantum interference device based on Josepshon tunnel nanojunctions for small spin cluster detection

The ultra high sensitivity exhibited by Superconducting Quantum Interference Device (SQUIDs) could be the key to explore new field of nanoscience such as the investigation of small cluster of elementary magnetic moments. In this paper, an ultra high sensitive niobium nanoSQUID based on submicron Josephson tunnel junction is presented. It has been fabricated in a vertical configuration by using a three-dimensional focused ion beam sculpting technique. In such a configuration, the nanosensor loop (area of 0.25μm2) is perpendicular to the substrate plane allowing to drastically reduce the spurious effects of the external magnetic field employed to excite the nano-objects under investigation. Main device characteristics have been measured at T=4.2K by using a low noise readout electronics. Due to high voltage responsivity, the nanosensor has exhibited a spectral density of the magnetic flux noise as low as 1.6μΦ0/Hz1/2.

Development of nano and micro SQUIDs based on Al tunnel junctions

Superconducting quantum interference devices (SQUIDs) with nano (micro)-meter dimensions are called nano (micro)-SQUIDs. The high sensitivity for flux and position of nano (micro)-SQUIDs can be applied to detect local magnetic fields induced by vortices and the magnetization of mesoscopic superconductors. Nano-SQUIDs based on carbon-nanotube junctions and niobium weak junctions are well known. However, such nano-SQUIDs are not suitable for large-scale integrated circuits and mass production. Therefore, we employ a combination of lithography using the Niemeyer-Dolan technique and the inductively coupled plasma reactive-ion etching technique to fabricate nano-SQUIDs. Here, we report the fabrication of nano (micro)-SQUIDs based on superconducting aluminum tunnel junctions and their application for vortex formation into mesoscopic chiral superconducting Sr2RuO4[1-3].

Optimizing the spin sensitivity of grain boundary junction nanoSQUIDs—towards detection of small spin systems with single-spin resolution

We present an optimization study of the spin sensitivity of nano superconducting quantum interference devices (SQUIDs) based on resistively shunted grain boundary Josephson junctions. In addition the direct current SQUIDs contain a narrow constriction onto which a small magnetic particle can be placed (with its magnetic moment in the plane of the SQUID loop and perpendicular to the grain boundary) for efficient coupling of its stray magnetic field to the SQUID loop. The separation of the location of optimum coupling from the junctions allows for an independent optimization of the coupling factor and junction properties. We present different methods for calculating (for a magnetic nanoparticle placed 10 nm above the constriction) as a function of device geometry and show that those yield consistent results. Furthermore, by numerical simulations we obtain a general expression for the dependence of the SQUID inductance on geometrical parameters of our devices, which allows to estimate their impact on the spectral density of flux noise of the SQUIDs in the thermal white noise regime. Our analysis of the dependence of and on the geometric parameters of the SQUID layout yields a spin sensitivity of a few ( is the Bohr magneton) for optimized parameters, respecting technological constraints. However, by comparison with experimentally realized devices we find significantly larger values for the measured white flux noise, as compared to our theoretical predictions. Still, a spin sensitivity on the order of for optimized devices seems to be realistic.

Ultra High Sensitive Niobium NanoSQUID by Focused Ion Beam Sculpting

The nanosuperconducting quantum interference device (nanoSQUID) is a powerful tool for nanoscience investigations. In this paper, the main features of niobium nanoSQUID based on deep submicron Josephson tunnel junctions have been investigated. The superconductive nanosensor has a rectangular loop of (1 × 0.4 μm 2) interrupted by two square Josephson junctions having a side length of 0.3 μm. The crucial steps of the fabrication process have been performed using focusing ion beam (FIB) nanosculpting technique. A full characterization of the nanosensor has been performed including the measurement of the voltage swing, the voltage responsivity, the spectral density of the magnetic flux and spin noise and the investigation of the main characteristics as a function of the temperature. Due to a very high responsivity (2.9 mV/ Φ0), the nanodevice exhibited, at T= 4.2 K, an intrinsic spectral density of the magnetic flux as low as 600 n Φ0/Hz 1/2, corresponding to a magnetic moment noise of 9 μB/Hz 1/2. For temperature less than 4.0 K, the nanodevice shows hysteretic current-voltage characteristics. However, the high critical current modulation depths ensure a suitable sensitivity for nanoscale applications.

Resistive state triggered by vortex entry in YBa2Cu3O7−δ nanostructures

We have realized YBa2Cu3O7−δ nanowires and nano Superconducting Quantum Interference Devices (nanoSQUID). The measured temperature dependence of the wire resistances below the superconducting transition temperature has been analyzed using a thermally activated vortex entry model valid for wires wider than the superconducting coherence length. The extracted zero temperature values of the London penetration depth, λ0≃270±15nm, are in good agreement with the value obtained from critical current modulations as a function of an externally applied magnetic field in a nanoSQUID implementing two nanowires.