All-NbN DC-SQUID Magnetometer Based on NbN/AlN/NbN Josephson Junctions

Abstract

We have developed an all-NbN-integrated dc SQUID magnetometer for future applications in the higher operating temperature by taking advantage of NbN films' higher critical temperature about 16 K. Our NbN dc SQUID magnetometer is fabricated on a single crystal MgO (100) substrate, consisting of epitaxial NbN/AlN/NbN Josephson junctions, Ti/Pd shunt resistors, and a top 400-nm-thick NbN wiring layer. The critical current, inductance, and shunt resistance of NbN SQUID are set at 20 μA, 240 pH, and 1 Ω, respectively. We measured the current-voltage characteristics, voltage-flux characteristics and flux noise of the NbN SQUID magnetometer in a superconductive niobium shield at 4.2 K. In a flux locked loop operation mode, the NbN SQUID magnetometer exhibits excellent performances with a flux noise level of 6 μΦ 0 /ÝHz in the white region. The corner frequency is only 10 Hz. Based on the calibrated magnetic field sensitivity of 1.7 nT/Φ 0 at magnetically shielded room, the magnetic field resolution of 10 fT/ÝHz is achieved.

Size effect on the response of superconductivity in NbN nanowires to external magnetic field

“…In this paper, we report an in-depth investigation on the physical performances of NbN nanowires with further smaller sizes under magnetic field. We chose NbN material for this study because it has been widely used in superconducting micro/nano devices [15][16][17][18] and therefore has better practical significance. For the sample with a transverse size of 10 nm, the critical current density can remain almost unchanged under a high field which is 10 times of that previously reported in YBCO nanowires [8], indicating a rather robust vortexfree state.…”

Ultrathin epitaxial NbN superconducting films with high upper critical field grown at low temperature

“…For example, NbN has emerged as a preferred material for superconducting nanowire single photon detectors (SNSPD) [1][2][3][4][5]. Epitaxial NbN films have also been investigated as the superconducting electrode for Josephson junctions in superconducting quantum interference devices, superconducting qubits, and rapid single flux quantum logic circuits [6][7][8][9][10][11].…”

On-chip integrable planar NbN nanoSQUID with broad temperature and magnetic-field operation range

“…31 Amongst these materials, NbN is rather attractive because it has relatively high critical current density and can sustain high magnetic fields, while it is also commonly used for logic, and photon-sensing devices. 6,8,38,39,9,23,[32][33][34][35][36][37] However, NbN has a rather short coherence length (𝜉 ≈6 nm 40 ), challenging the weak-link fabrication, while growing ultra-thin NbN films with controllable properties is also a non-trivial task. 41 Thus far, NbN planar SQUIDs have already demonstrated sustainability to high parallel fields (≤ 8 T), 42 sensitivity to perpendicular fields (≲ 3 mT, 23 note that earlier works discuss sensitivity up to 10 mT, but without a demonstration 24 ).…”

“…7,8 NbN SQUIDs are usually 3D structures that require a very complex fabrication process. 37,43 NbN planar SQUIDs have been reported for ~15-nm films 42 (6-nm thick devices have been discussed, though no interference pattern has been demonstrated 24 ), but not for thinner films. Moreover, to increase the device density, smaller footprint (washer area) is needed, while indeed NbN SQUIDs with 750-nm inner radius have already been developed.…”