High-resolution angle-resolved photoemission studies of YbRh$_2$Si$_2$ using 7 eV laser

We present angle-resolved photoemission spectra of prototypical heavy fermion compound YbRh$_2$Si$_2$ measured with 7 eV ultraviolet laser. Much improved energy and momentum resolutions enable us to resolve the sharp weakly dispersing peaks at the lowest energy of single-electron spectra. This coherent state grows in intensity and weight as temperature decreases below a characteristic temperature. The characteristic temperature is not only different from the single-ion Kondo temperature of YbRh$_2$Si$_2$ derived from thermodynamic measurements, it is of the same scale as the energy and the lifetime of the coherent state.