Ultrasound Evidence for a Low-Temperature Anomaly Inside the Superconducting State of 4Hb-TaS${}_2$

Abstract

We report low-temperature ultrasound measurements on single crystals of the layered van der Waals superconductor 4Hb-TaS${}_2$. Specific heat and ac magnetic susceptibility show a sharp bulk superconducting transition at $T_{\mathrm{c}}\approx 2.9$~K. Ultrasound measurements reveal an additional anomaly deep inside the superconducting state near $T^{\ast }\approx 1$~K. The most direct signature is observed in the relative ultrasonic attenuation change $\Delta \alpha$: instead of being rapidly suppressed at $T_{\mathrm{c}}$, $\Delta \alpha$ remains large throughout the intermediate superconducting regime and drops strongly only near $T^{\ast }$. This loss of acoustic dissipation is accompanied by a pronounced anomaly in the relative sound velocity change $\Delta v/v$, indicating strong coupling to the lattice. The low-temperature anomaly is rapidly suppressed by magnetic field and by Se substitution, suggesting a possible superconducting origin of the anomaly. We speculate that this feature may be related to induced superconductivity in the 1T layers.

6 pages, 4 figures