Force and Displacement Sensing Using the Superconducting Proximity Effect
Hajime Okamoto, Tatsushi Akazaki and Hiroshi Yamaguchi
Physical Science Laboratory
Techniques for detecting small forces and displacements using semiconductor cantilevers are being applied to various sensing tools, such as acceleration sensors, gas detectors, and atomic force microscopes. The detection is based on either optical or electrical methods. The latter needs no external detectors like laser optics, which makes it very advantageous for miniaturization and integration. A notable self-sensor based on a purely electric method is the piezoresistive cantilever, which detects its deflection as a change in the electrical resistance. We have reported that piezoresistive cantilevers using semiconductor low-dimensional structures enable highly sensitive detection of force and displacement due to the enhancement of piezoresistance caused by the quantum mechanical effect . Recently, we fabricated a new type of piezoresistive cantilever that integrates a superconductor-semiconductor-superconductor junction . Here, we describe mechanical sensing using the superconducting proximity effect in this junction.
Figure 1a shows a scanning electron microscope image of the fabricated cantilever. The cantilever is 200-µm long and 60-µm wide and contains a junction formed by the superconductor (Nb) and semiconductor (InAs) (Figs. 1b and 1c). At low temperature, a supercurrent flows between the Nb electrodes via the InAs layer, which is the superconducting proximity effect. We found that the maximum supercurrent (the superconducting critical current) is modulated by the deflection of the cantilever. This indicates that, when the bias current is set to the critical current value, the deflection of the cantilever induces a transition between the superconducting state and the resistive state, providing a large resistance change, i.e., large piezoresistance. Low temperature measurements using a dilution refrigerator revealed that the piezoresistance around the critical current (〜327 mA) is larger than that in the resistive state by more than a factor of ten (Fig. 2). This suggests that small forces and displacements can be detected by using the transition between the superconducting state and resistive state.
 H. Yamaguchi, et al., Phys. Rev. Lett., 93 (2004) 036603.
 H. Okamoto, et al., Physica E, 32 (2006) 512.
Fig. 1. (a) SEM image of the fabricated cantilever and (b) a magnified view of the Nb-InAs-Nb junction. (c) Schematic of the cross section of the junction.
Fig. 2. Dependence of the resonance characteris-tics (resistance change) on the bias current at 30 mK. The quality factor (Q) is about 10,000.
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