Out-of-equilibrium electrons lead to record thermionic emission in LaB6 with the Jahn-Teller instability of boron cage

Materials with low electron work function are of great demand in various branches of science and technology. LaB6 is among the most effective electron-beam sources with one of the highest brightness of thermionic emission. A deep understanding of the physical mechanisms responsible for the extraordinary properties of LaB6 is required in order to optimize the parameters and design of thermionic elements for application in various electron-beam devices. Motivated by recent experiments on rare earth borides indicating a strong coupling of conduction electrons to the crystal lattice and rare earth ions, we have studied the state of electrons in the conduction band of lanthanum hexaboride by performing infrared spectroscopic, DC resistivity and Hall-effect studies of LaB6 single crystals with different ratios of 10B and 11B isotopes. We find that only a small amount of electrons in the conduction band behave as Drude-type mobile charge carriers while up to 70% of the electrons are far out of equilibrium and involved in collective oscillations of electron density coupled to vibrations of the Jahn-Teller unstable rigid boron cage and rattling modes of La-ions that are loosely bound to the lattice. We argue that exactly these non-equilibrium (hot) electrons in the conduction band determine the extraordinary low work function of thermoemission in LaB6. Our observation may guide future search for compounds with possibly lower electron work function.