Effect of surface spin fluctuations on chemisorbed atoms

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Abstract
Using functional derivative techniques, the authors develop a systematic theory of the effect of substrate spin fluctuations on the electronic spectral density of hydrogen chemisorbed on a transition metal. The authors augment the standard Newns-Anderson model Hamiltonian to include exchange interactions (I) between the substrate d electrons as well as an exchange interaction (J) between substrate and adatom electrons. To second order in J, the adatom self energy involves the dynamic spin susceptibility of the substrate d-band. As the Stoner ferromagnetic instability is approached, the adatom energy levels are increasingly broadened due to excitation of substrate paramagnons. In a very small region above Tc, it is found that a magnetic moment ((nspin up) not=(nspin down)) may be induced on the adatom due to the large static substrate spin susceptibility. The exchange interaction (U) between electrons on the adatom is treated in the Hartree-Fock approximation. The authors show that, by itself, the energy level broadening near Tc has no effect on the net chemisorption energy. The metal spin fluctuations only contribute if there is an induced magnetic moment on the adatom.