Real-Time Observation of Phonon-Mediated sigma-pi Interband Scattering in MgB2

In systems having an anisotropic electronic structure, such as the layered materials graphite, graphene, and cuprates, impulsive light excitation can coherently stimulate specific bosonicmodes, with exotic consequences for the emergent electronic properties. Here we show that the population of E-2g phonons in the multiband superconductorMgB(2) can be selectively enhanced by femtosecond laser pulses, leading to a transient control of the number of carriers in the sigma-electronic subsystem. The nonequilibrium evolution of the material optical constants is followed in the spectral region sensitive to both the a- and c-axis plasma frequencies andmodeled theoretically, revealing the details of the sigma-pi interband scattering mechanism in MgB2.