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

Gate-Tunable Critical Current of the Three-Dimensional Niobium Nanobridge Josephson Junction

Recent studies have shown that the critical currents of several metallic superconducting nanowires and Dayem bridges can be locally tuned by using a gate voltage (Vg). Here, we report a gate-tunable Josephson junction structure constructed from a three-dimensional (3D) niobium nanobridge junction (NBJ) with a voltage gate on top. Measurements up to 6 K showed that the critical current of this structure can be tuned to zero by increasing Vg. The critical gate voltage was reduced to 16 V and may possibly be reduced further by reducing the thickness of the insulation layer between the gate and the NBJ. Furthermore, the flux modulation generated by Josephson interference of two parallel 3D NBJs can also be tuned by using Vg in a similar manner. Therefore, we believe that this gate-tunable Josephson junction structure is promising for superconducting circuit fabrication at high integration levels.

Geometric Scaling of the Current-Phase Relation of Niobium Nanobridge Junctions

The nanobridge junction (NBJ) is a type of Josephson junction that is advantageous for the miniaturization of superconducting circuits. However, the current-phase relation (CPR) of the NBJ usually deviates from a sinusoidal function, which has been explained by a simplified model with correlation only to its effective length. Here, we investigated both measured and calculated CPRs of niobium NBJs of a cuboidal shape with a three-dimensional bank structure. From a sine-wave to a sawtooth-like form, we showed that deviated CPRs of NBJs can be described quantitatively by its skewness Δθ. Furthermore, the measured dependence of Δθ on the critical current I0 from 108 NBJs turned out to be consistent with the calculated dependence derived from the change in geometric dimensions. This suggested that the CPRs of NBJs can be tuned by their geometric dimensions. In addition, the calculated scaling behavior of Δθ versus I0 in 3D space was provided for the future design of superconducting circuits of a high integration level by using niobium NBJs.

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.

3D nano-bridge-based SQUID susceptometers for scanning magnetic imaging of quantum materials

We designed and fabricated a new type of superconducting quantum interference device (SQUID) susceptometers for magnetic imaging of quantum materials. The 2-junction SQUID sensors employ 3D Nb nano-bridges fabricated using electron-beam lithography. The two counter-wound balanced pickup loops of the SQUID enable gradiometric measurement and they are surrounded by a one-turn field coil for susceptibility measurements. The smallest pickup loop of the SQUIDs were 1 μm in diameter and the flux noise was around 1 μФ₀/√Hz at 100 Hz. We demonstrate scanning magnetometry, susceptometry and current magnetometry on some test samples using these nano-SQUIDs.

Magnetic Field Characteristics of Multiple Niobium Three-dimensional Nano-bridge Junctions in Parallel

The superconducting device of multiple Josephson junctions in arrays has increasingly attracted interest in both applications and fundamental research. The challenge of array integration and scaling is a wide concern. The present study investigated superconducting devices of multiple niobium three-dimensional nano-bridge junctions (3D-NBJs) in parallel. We fabricated evenly and unevenly spaced devices of three to six 3D-NBJs in parallel. We measured the critical current as a function of the magnetic field and voltage to magnetic field transfer function of each device. The derivative of voltage with respect to the magnetic field at the sensitive point increased linearly with the number of junctions. A maximal derivative of 97.3 V/T was achieved by our device with six unevenly spaced junctions in parallel. Furthermore, we carried out numerical simulations on devices of three and four junctions in parallel using the current-phase relation of a single 3D-NBJ. The CPR was determined by comparing the measured and simulated magnetic flux modulations of nano-SQUID. Qualitative agreement between the numerical simulation and experimental measurement suggests that it is possible to use 3D-NBJs to build SQUID arrays or SQIFs with high integration density.

Grooved Dayem Nanobridges as Building Blocks of High-Performance YBa2Cu3O7-δ SQUID Magnetometers

We present noise measurements performed on a YBa₂Cu₃O_7-δ nanoscale weak-link-based magnetometer consisting of a superconducting quantum interference device (SQUID) galvanically coupled to a 3.5 × 3.5 mm² pick-up loop, reaching white flux noise levels and magnetic noise levels as low as [Formula: see text] and 100 fT/[Formula: see text] at T = 77 K, respectively. The low noise is achieved by introducing grooved Dayem bridges (GDBs), a new concept of a weak link. A fabrication technique has been developed for the realization of nanoscale grooved bridges, which substitutes standard Dayem bridge weak links. The introduction of these novel key blocks reduces the parasitic inductance of the weak links and increases the differential resistance of the SQUIDs. This greatly improves the device performance, thus resulting in a reduction of the white noise.

Meissner effect measurement of single indium particle using a customized on-chip nano-scale superconducting quantum interference device system

As many emergent phenomena of superconductivity appear on a smaller scale and at lower dimension, commercial magnetic property measurement systems (MPMSs) no longer provide the sensitivity necessary to study the Meissner effect of small superconductors. The nano-scale superconducting quantum interference device (nano-SQUID) is considered one of the most sensitive magnetic sensors for the magnetic characterization of mesoscopic or microscopic samples. Here, we develop a customized on-chip nano-SQUID measurement system based on a pulsed current biasing method. The noise performance of our system is approximately 4.6 × 10^-17 emu/Hz^1/2, representing an improvement of 9 orders of magnitude compared with that of a commercial MPMS (~10^-8 emu/Hz^1/2). Furthermore, we demonstrate the measurement of the Meissner effect of a single indium (In) particle (of 47 μm in diameter) using our on-chip nano-SQUID system. The system enables the observation of the prompt superconducting transition of the Meissner effect of a single In particle, thereby providing more accurate characterization of the critical field H_c and temperature T_c. In addition, the retrapping field H_re as a function of temperature T of single In particle shows disparate behavior from that of a large ensemble.