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Applied Physics Letters

3D nanoSQUID based on tunnel nano-junctions with an energy sensitivity of 1.3 h at 4.2 K

Matthias Schmelz, Antonio Vettoliere, V. Zakosarenko, Natascia De Leo, Matteo Fretto, Ronny Stolz, C. Granata

Published Jul 17, 2017• 18 citations
Publisher

Abstract

We report the performance of a reliable three-dimensional nanometer-sized Superconducting QUantum Interference Device (SQUID). The use of superconductor-isolator-superconductor Nb/Al-AlOx/Nb Josephson tunnel junctions together with small SQUID loop dimensions permits a high modulation depth of the SQUID's critical current and thus leads to very low intrinsic flux noise of the device. In particular, we present electrical characterization including detailed noise investigations. At 4.2 K, two-stage noise measurements with a SQUID as a low noise preamplifier result in a white flux noise of 51 nΦ0/Hz1/2, which is equivalent to an energy resolution of 1.3 h, with h being Planck's constant. Simulation of spin sensitivities results in about 1 μB/Hz1/2 for an electron spin positioned directly above the SQUID ring.

DOI
10.1063/1.4986655

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We report the performance of a reliable three-dimensional nanometer-sized Superconducting QUantum Interference Device (SQUID). The use of superconductor-isolator-superconductor Nb/Al-AlOx/Nb Josephson tunnel junctions together with small SQUID loop dimensions permits a high modulation depth of the SQUID's critical current and thus leads to very low intrinsic flux noise of the device. In particular, we present electrical characterization including detailed noise investigations. At 4.2 K, two-stage noise measurements with a SQUID as a low noise preamplifier result in a white flux noise of 51 nΦ0/Hz1/2, which is equivalent to an energy resolution of 1.3 h, with h being Planck's constant. Simulation of spin sensitivities results in about 1 μB/Hz1/2 for an electron spin positioned directly above the SQUID ring.
We investigated, at temperature 4.2 K, electric transport, flux noise, and resulting spin sensitivity of miniaturized Nb direct current superconducting quantum interference devices (SQUIDs) based on submicron Josephson junctions with HfTi barriers. The SQUIDs are either of the magnetometer-type or gradiometric in layout. In the white noise regime, for the best magnetometer we obtain a flux noise SΦ1/2=250nΦ0/Hz1/2, corresponding to a spin sensitivity Sμ1/2≥29μB/Hz1/2. For the gradiometer we find SΦ1/2=300nΦ0/Hz1/2 and Sμ1/2≥44μB/Hz1/2. The devices can still be optimized with respect to flux noise and coupling between a magnetic particle and the SQUID, leaving room for further improvement towards single spin resolution.
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 μΦ 0 /Hz 1/2 . 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.
We investigate electric transport and noise properties of microstrip-type submicron direct current superconducting quantum interference devices (dc SQUIDs) based on Nb thin films and overdamped Josephson junctions with a HfTi barrier. The SQUIDs were designed for optimal spin sensitivity Sμ1/2 upon operation in intermediate magnetic fields B (tens of mT), applied perpendicular to the substrate plane. Our, so far, best SQUID can be continuously operated in fields up to B≈±50 mT with rms flux noise SΦ,w1/2≤250 nΦ0/Hz1/2 in the white noise regime and spin sensitivity Sμ1/2≤29 μB/Hz1/2. Furthermore, we demonstrate operation in B = 0.5 T with high sensitivity in flux SΦ,w1/2≈680 nΦ0/Hz1/2 and in electron spin Sμ1/2≈79 μB/Hz1/2. We discuss strategies to further improve the nanoSQUID performance.
We report on the development of highly sensitive SQUIDs featuring sub-micrometer loop dimensions. The integration of high quality and low capacitance SIS Nb/AlO x /Nb cross-type Josephson tunnel junctions results in white flux noise levels as low as 66 n Φ 0 Hz −1/2 , well below state-of-the-art values of their Nb-based counterparts based on constriction type junctions. Estimation of the spin sensitivity of the best SQUIDs yield S μ 1/2 < 7 μ B Hz −1/2 in the white noise region, suitable for the investigation of small spin systems. We discuss fabrication challenges, show results on the electrical characterization of devices with various pickup loops, and describe options for further improvement, which may push the sensitivity of such devices even to single spin resolution.
We report results about an optimized three-dimensional nanoSQUID based on niobium tunnel nanojunctions having the loop suspended to reduce the parasitic capacitance. The SQUID loop has a size of 400 × 600 nm2while the dimension of the square tunnel nanojunctions is 150 × 150 nm2with a density of the critical current of about 35 × 103A/cm2. The nanodevice has been characterized at liquid helium temperature; it has shown nonhysteretic current-voltage characteristics and smooth voltage- magnetic flux characteristics resulting in a very stable operation in a wide range of bias points. The spectral density of the magnetic flux noise in the white region, measured with a two-stage noise measurement setup, was as low as 300 nΦ0/Hz1/2corresponding to a spin sensitivity of few Bohr magnetons for bandwidth unit.
We have fabricated superconducting quantum interference devices (SQUIDs) with an inductance of 140 pH by using sub- µm Nb/Al–AlO x –Al/Nb edge junctions. The small capacitance of the junction resulted in the very large voltage swing of 210 µV and gradient dV/ dΦ of 940 µV/Φ0. Consequently, the very low flux noise of 0.61 µΦ0/ Hz0.5 corresponding to the energy sensitivity ε of 8.6h at the white noise level was obtained by using a conventional flux-locked loop configuration. The low-frequency noise was also very low, 2.7 µΦ0/ Hz0.5 corresponding to ε= 170 h at 1 Hz. We could obtain very low flux noise for the SQUID which had the practical inductance using only a conventional readout circuit.