Research ArticleAPPLIED PHYSICS

Negative resistance state in superconducting NbSe2 induced by surface acoustic waves

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Science Advances  21 Aug 2020:
Vol. 6, no. 34, eaba1377
DOI: 10.1126/sciadv.aba1377
  • Fig. 1 Experimental setup and fundamental properties of the device.

    (A) Schematic image of the device structure. The SAW (white wave) emitted from comb-shaped electrodes travels along the x direction and irradiates a NbSe2 thin film (dark gray flake). (B) Optical microscope image of the device and schematic image of the circuit. The NbSe2 film, whose thickness is about 30 nm, is attached on a LiNbO3 substrate. The film is electrically connected by Ti/Au electrodes. Each IDT consists of 10 pairs of electrodes. The resonance frequency of the IDTs is 3.25 GHz. (C) Temperature dependence of resistances of the NbSe2 and NbS2 devices measured with the four-terminal lock-in technique. The bias current is 1 μA. A small bump due to the CDW phase is observed at 33 K only for the NbSe2 device.

  • Fig. 2 I-V characteristics of NbSe2 and NbS2 devices exposed to SAWs.

    (A) dc voltage of the NbSe2 thin film exposed to the SAW with different rf powers as a function of bias current measured at 1.6 K. (B) Close-up of the I-V curve shown in (A) near zero bias current. (C) Differential resistance obtained by numerically differentiating the I-V curve shown in (A). (D) Close-up of the dV/dI curve shown in (C) near zero bias current. (E) dc voltage of the NbS2 thin film exposed to the SAW with different rf powers as a function of bias current measured at 1.6 K. (F) Close-up of the I-V curve shown in (E) near zero bias current. (G) Differential resistance obtained by numerically differentiating the I-V curve shown in (E). (H) Close-up of the dV/dI curve shown in (G) near zero bias current.

  • Fig. 3 Temperature and magnetic field dependencies of resistances of NbSe2 and NbS2 devices exposed to SAWs.

    (A) Temperature dependence of resistance of the NbSe2 device with different SAW powers. (B) Magnetic field dependence of resistance of the NbSe2 device measured at 1.6 K and with different SAW powers. (C) Temperature dependence of resistance of the NbS2 device with different SAW powers. (D) Magnetic field dependence of resistance of the NbS2 device measured at 1.6 K and with different SAW powers.

  • Fig. 4 Theoretical model to reproduce the negative resistance.

    (A) Schematics of nucleation of soliton and antisoliton pairs. When the SAW is irradiated to the CDW state, the CDW phase φ is modulated over the wavelength of the SAW. Instead of modulating the CDW over the wavelength, it is more stable to nucleate soliton and antisoliton pairs in the CDW state. In the superconducting state, local charges (−Q, +Q) are accumulated, resulting in a temporal and local capacitance C(t) and a phase difference Δφ of 2π. (B) Circuit model used in the calculation (see the Supplementary Materials for more details). We assume a resistively shunted Josephson junction (JJ), which is capacitively coupled via C(t). The change in C(t) is synchronized with the frequency of the SAW. We assume a sawtooth wave function for C(t). (C) Typical I-V curve calculated with the above capacitively coupled Josephson junction model. The horizontal and vertical axes are normalized by IC and the product of the resistance R of the normal state and IC, respectively.

Supplementary Materials

  • Supplementary Materials

    Negative resistance state in superconducting NbSe2 induced by surface acoustic waves

    Masahiko Yokoi, Satoshi Fujiwara, Tomoya Kawamura, Tomonori Arakawa, Kazushi Aoyama, Hiroshi Fukuyama, Kensuke Kobayashi, Yasuhiro Niimi

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    • Supplementary Text
    • Figs. S1 to S5

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