Experimental band structure of potassium as measured by angle-resolved photoemission

The bulk band structure of potassium along the [110] direction was measured using angle-resolved photoemission from an epitaxial potassium film several thousand angstroms thick grown on a Ni(100) substrate. We find the occupied bandwidth to be 1.60±{}0.05 eV, which is narrower than the free-electron bandwidth of 2.12 eV and agrees with recent calculations of the quasiparticle self-energy. A narrow peak near the Fermi level which did not disperse with photon energy was observed for photon energies which, according to the nearly-free-electron model, should yield no direct transitions. A comparison of the binding energy and intensity of the anomalous peak as functions of photon energy is made to the calculations of Shung and Mahan [Phys. Rev. B 38, 3856 (1988)]. The discrepancies found are discussed in terms of an enhanced surface photoeffect in the photon energy range 20{≤}{}{ω}{≤}30 eV. For low photon energies, a bulk peak was also observed due to a surface umklapp process with an intensity comparable to the standard bulk (110) peak. The possible contributions to this strong surface umklapp process from a shear instability at the first few (110) atomic planes is discussed.