Observation of microwave Higgs-Anderson modes in superconducting Titanium nanostructures

The Higgs-Anderson mode in superconductors is known to be difficult to observe because of its weak coupling to the electromagnetic field. A recent theory [1] predicted a huge increase of this coupling in the presence of a DC supercurrent, which should translate into an anomaly in the complex conductivity at frequencies of the order of twice the superconducting gap Δ. This phenomenon has been experimentally confirmed in macroscopic NbN films exposed to THz radiations at a temperature of 5K [2]. In order to better control, and investigate in more depth Higgs mode properties, it would be very useful to be able to work at much lower frequencies, thus much lower temperature. Our experiment aims at providing such a step towards detecting and manipulating Higgs mode in a microwave circuit. We studied Titanium samples for which 2Δ is of the order of 10-30 GHz and can be tuned with the sample thickness and temperature. We implemented a calibrated cryogenic microwave reflectance setup, with which we measured the complex impedance vs. frequency and temperature of superconducting wires of various dimensions. In the absence of DC current we compare our results with BCS theory at equilibrium. Adding a current results in the appearance of an anomaly at frequency 2Δ on both the real and imaginary parts of the complex impedance. This feature behaves as predicted in [1], however it is much broader in frequency.

References:

[1]A. Moor, A. F. Volkov, and K. B. Efetov, Phys. Rev. Lett. 118, 047001 (2017).

[2]S. Nakamura, Y. Iida, Y. Murotani, R. Matsunaga, H. Terai, and R. Shimano, Phys. Rev.Lett. 122, 257001 (2019)