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Applied Physics Letters

Granular aluminum nano-superconducting quantum interference device

Shilo Avraham, Shlomi Bachar, Aviv Glezer Moshe, Eliyahu Farber, G. Deutscher

Published Oct 23, 2023• 4 citations
Publisher

Abstract

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.

DOI
10.1063/5.0168514

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•Apr 28, 2014•0 citations
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.
Granular aluminum (grAl) is a promising high kinetic inductance material for detectors, amplifiers, and qubits. Here we model the grAl structure, consisting of pure aluminum grains separated by thin aluminum oxide barriers, as a network of Josephson junctions, and we calculate the dispersion relation and nonlinearity (self-Kerr and cross-Kerr coefficients). To experimentally study the electrodynamics of grAl thin films, we measure microwave resonators with open-boundary conditions and test the theoretical predictions in two limits. For low frequencies, we use standard microwave reflection measurements in a low-loss environment. The measured low-frequency modes are in agreement with our dispersion relation model, and we observe self-Kerr coefficients within an order of magnitude from our calculation starting from the grAl microstructure. Using a high-frequency setup, we measure the plasma frequency of the film around 70 GHz, in agreement with the analytical prediction.
Superconducting high kinetic inductance elements constitute a valuable resource for quantum circuit design and millimeter-wave detection. Granular aluminum (grAl) in the superconducting regime is a particularly interesting material since it has already shown a kinetic inductance in the range of nH/□ and its deposition is compatible with conventional Al/AlOx/Al Josephson junction fabrication. We characterize microwave resonators fabricated from grAl with a room temperature resistivity of 4×10^{3} μΩ cm, which is a factor of 3 below the superconductor to insulator transition, showing a kinetic inductance fraction close to unity. The measured internal quality factors are on the order of Q_{i}=10^{5} in the single photon regime, and we demonstrate that nonequilibrium quasiparticles (QPs) constitute the dominant loss mechanism. We extract QP relaxation times in the range of 1 s and we observe QP bursts every ∼20 s. The current level of coherence of grAl resonators makes them attractive for integration in quantum devices, while it also evidences the need to reduce the density of nonequilibrium QPs.
Nano-Superconducting Quantum Interference Devices (nano-SQUIDs) are usually fabricated from a single layer of either Nb or Al. We describe here a simple method for fabricating suspended nano-bridges in Nb/Al thin-film bilayers. We use these suspended bridges, which act as Josephson weak links, to fabricate nano-SQUIDs which show critical current oscillations at temperatures up to 1.5 K and magnetic flux densities up to over 20 mT. These nano-SQUIDs exhibit flux modulation depths intermediate between all-Al and all-Nb devices, with some of the desirable characteristics of both. The suspended geometry is attractive for magnetic single nanoparticle measurements.
We describe aluminum-niobium-tungsten trilayer Nano-Superconducting Quantum Interference Devices (NanoSQUIDs) that can be read out continuously down, in temperature, to at least 230 mK. They show voltage oscillations up to at least 20 mT in field. A voltage modulation of 500 μV, voltage sensitivity of 2 mV/Φ0, and white noise floor better than 5×10−5 Φ0/Hz have been obtained. Flux noise places them between conventional low impedance SQUIDs and standard nanoSQUIDs. High sensitivity and ease of implementation make this new kind of nanoSQUID attractive for magnetic detection schemes on the nanoscale and low temperature scanning SQUID microscopy.
The optimal design of a direct current superconducting quantum interference device (DC-SQUID) with high sensitivity requires two identical Josephson junctions, which is difficult to achieve. The design of a DC-SQUID using controlled Josephson junction technology provides a mechanism for modifying the characteristics of the device. Here we report on the nano-fabrication and measurement of a DC-SQUID utilizing two Dayem bridge weak-link Josephson junctions fabricated with a width ∼100 nm and a length of ∼200 nm. These junctions are controlled via the irradiation of hot phonons from nearby titanium/chromium normal metal constrictions which are current biased. The devices were measured at a temperature of 4.2 K and control over the critical current oscillations was observed.
Nanoscale Superconducting QUantum Interference Devices (SQUIDs) have spin sensitivities approaching that required to detect the flip of a single spin in close proximity. There is considerable interest in developing them for measuring the properties of small spin populations in magnetic systems. It is desirable that such measurements can be realised at sub-kelvin temperatures thereby allowing the study of magnetic systems that undergo phase changes at such temperatures. However, most nanoscale SQUIDs use nanobridges as the Josephson elements which limits the operating range to temperatures close to the transition temperature of the device. Well below this, the current-phase relation can become non-sinusoidal, and hot spots arising from the large critical current lead to hysteretic I-V characteristics. To extend the temperature range downwards, we have developed a range of nanoSQUIDs fabricated from alternative materials with lower transition temperatures including Ti/Au and Al/Ag bilayers patterned using lift-off and e-beam lithography (EBL). We report on their I-V characteristics, noise performance and behaviour in applied magnetic fields at temperatures down to 60 mK. We discuss theoretical analysis and computer modelling of the heat flow in the nanobridge structure, and consider the effects of bank geometry and kinetic inductance on the overall device performance. Finally, we present measurements of several magnetic systems: control lines, superconducting islands and ground planes and discuss the feasibility of magnetic measurements of novel materials of interest, including the heterointerface between lanthanium aluminate and strontium titanate.