Effects of impurities on the phase transition and critical phenomena of a chemisorbed overlayer

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Abstract
We have studied the effects of nitrogen impurities (≲5% monolayer, ML) on domain structures, phase transitions, and critical phenomena of the W(112)(2×1)-O system at ∼0.5 ML coverage [which belongs to the two-dimensional (2D) Ising class] using a high-resolution low-energy electron-diffraction technique. We find that the nitrogen impurities break the long-range order in the ground state, round the ordered (2×1)-O to disordered (1×1)-O transition, and lower the effective critical temperature. The effective critical exponents are reduced from the values associated with the pure overlayer with increasing amounts of impurities. These observations clearly indicate the destruction of the transition by impurities and are consistent with the predictions of a 2D random-field Ising model (RFIM). The line shape of the superlattice (1/20) beam in the impurity-doped ground state can be very well described by a form consisting of a Lorentzian squared plus a Lorentzian as derived within the framework of RFIM.