  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.
