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Description
SQUID-on-tip (SOT) is the most sensitive detector of small magnetic moments to date [1]. We analyze the performance of such nano-sized SOT (Fig. 1) in the presence of the magnetic field, using the state-of-the-art three dimensional (3D) simulations within the phenomenological Ginzburg-Landau (GL) theory. Based on the observed behavior of the superconducting order parameter in the SOT, the distribution of the Cooper-pair density and the behavior of the circulating supercurrents, we propose engineering solutions at the nanoscale to improve the sensitivity of the device. By introducing the constriction in the arms of the SQUID loop (Fig.1b), the gradient of phase of the order parameter is largely enhanced in the constriction of the SOT in the Meissner and first vortex state. This is expected to facilitate the interference pattern of the critical current of the SOT and reduce the noise. Furthermore, wedge shaped three junction loop of SOT (3JSOT) (Fig. 2) enables the tunability of the device and its selective response to both the in-plane and out-of-plane components of magnetic field [2]. We further discuss the other realizations of the SOT with engineered 3D shapes of the SQUID itself [3] and show the relation of those geometries to sensitivity to 3D magnetic field.
[1] D. Vasyukov, Y. Anahory et al., A scanning superconducting quantum interference device with single electron spin sensitivity, Nature Nanotech 8, 639-644 (2013). [2] Y. Anahory, J. Reiner et al., Three-Junction SQUID-on-Tip with Tunable In-Plane and Out-of-Plane Magnetic Field Sensitivity, Nano Letters 14, 648-6487 (2014). [3] C. Granata, & A. Vettoliere, Nano Superconducting Quantum interference device: A powerful tool for nanoscale investigations, Physics Reports, In press (2015).
