Ultrasonic Observation of Slowing Down of Multipole Fluctuations in Sr2RuO4
Abstract
We performed ultrasonic measurements on the unconventional superconductor Sr2RuO4 to investigate the dynamical properties of the electronic states near its superconducting transition temperature, Tc = 1.4 K. We observed an increase in the in-plane transverse ultrasonic attenuation coefficient as the temperature approached Tc. The ultrasonic attenuation exhibited a Landau-Khalatnikov-type ultrasonic frequency dependence with a typical relaxation time of approximately 10-10 s. Under an applied magnetic field of 10 T, the superconducting transition was suppressed. However, the ultrasonic attenuation coefficient exhibited an increase down to low temperatures, indicating the slowing down of fluctuations associated with multipole degrees of freedom. Based on group-theoretical considerations, we propose that the electric hexadecapole plays a crucial role in the slowing down. Furthermore, we discuss the relationship between multi-component superconducting order parameters and multipole degrees of freedom.
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