Amplitude Higgs mode and admittance in superconductors with a moving   condensate
  We consider the amplitude (Higgs) mode in a superconductor with a condensate flow (supercurrent). We demonstrate that, in this case, the amplitude mode corresponding to oscillations $\delta |\Delta|_{\Omega} \exp(i \Omega t)$ of the superconducting gap is excited by an external ac electric field $\mathbf{E}_{\Omega} \exp(i \Omega t)$ already in the first order in $|\mathbf{E}_{\Omega}|$, so that ${\delta |\Delta|_{\Omega} \propto (\mathbf{v}_{0} \mathbf{E}_{\Omega})}$, where $\mathbf{v}_{0}$ is the velocity of the condensate. The frequency dependence $\delta |\Delta|_{\Omega}$ has a resonance shape with a maximum at ${\Omega = 2 \Delta}$. In contrast to the standard situation without the condensate flow, the oscillations of the amplitude $\delta |\Delta(t)|$ contribute to the admittance $Y_{\Omega}$. We provide a formula for admittance of a superconductor with a supercurrent. The predicted effect opens new ways of experimental investigation of the amplitude mode in superconductors and materials with superconductivity competing with other states. 
Andreas Moor, Anatoly F. Volkov, and Konstantin B. Efetov
cond-mat.supr-con