Yayama H., Nishimura Yu., Uchiyama H., Kawai H., van Woensel J.P., Hafez A.G. «Electric response induced by second sound in superfluid helium» Физика низких температур, 44, № 10, с. 1386-1393 (2018)

The electric response of superfluid helium was measured when a second sound standing wave was generated in a resonator cavity. The results were qualitatively in agreement with that of other research laboratories, but the normalized signal strength was one order of magnitude larger reflecting the difference in electrode structure. The temporal phase difference between the electric oscillation and the temperature oscillation was measured and compared with the analysis. The result excluded a hypothesis that the electric response was induced by the velocities of the relative motion of normal and superfluid components of liquid helium. We suggested a hypothetical explanation of the electric response based on the oscillation of chemical potential of electrons in helium atoms. The effect of an external dc electric field was examined and no effect was observed. The heater power dependence of the temperature oscillation and the electric oscillation showed the qualitative agreement with the original experiment.

Yurii Z., Djafari-Rouhani B., Krokhin A. «Dynamical effective parameters of elastic superlattice with strong acoustic contrast between the constituents» Физика низких температур, 44, № 12, с. 1639-1644 (2018)

Analytical formulas are obtained for frequency-dependent effective elastic modulus and effective mass density for a periodic layered structure. The proposed homogenization procedure is valid at sufficiently high frequencies well above the lowest band gap in the acoustic spectrum of the structure. It is shown that frequency-dependent effective parameters may take negative values either in different regions of frequencies or in the same quite narrow region. This property demonstrates that 1D elastic structure may behave in the limit of small Bloch wave vectors as a double-negative acoustic metamaterial.