Leggett mode controlled by light pulses

The discovery of symmetry-broken phases that host multiple order parameters, such as multiband superconductors1,2, has triggered an enormous interest in condensed matter physics. However, many challenges continue to hinder the fundamental understanding of how to control the collective modes corresponding to these multiple order parameters3,4. Here we demonstrate that, in full analogy with phonons, Raman-active electronic collective modes can be manipulated by intense light pulses. By tuning a sum-frequency excitation process, we selectively trigger collective excitations that can be ascribed to the relative phase fluctuations between two superconducting order parameters--the so-called Leggett mode--in the multiband superconductor MgB2. The excellent comparison between experiments and theory establishes a general protocol for the advanced control of Raman-active electronic modes in symmetry-broken quantum phases of matter. © 2019