Specific-heat study of two-dimensional melting phenomena in the first oxygen layer on graphite

We present a heat-capacity study of the two-dimensional melting phenomena of the first physisorbed oxygen layer on graphite. The study detects several distinct melting phenomena which markedly differ in their specific-heat signals: melting of layers with a coverage 0≤ρ≤1 (melting I at temperature $T_m^{\mathrm{I}}$) reveals symmetrical δ-function-shaped specific-heat anomalies and melting of layers with 1<ρ<1.72 (melting II at temperature $T_m^{\mathrm{II}}$) reveals asymmetrical λ-shaped specific-heat anomalies. While the features of melting I do not change when a magnetic field B is applied, melting II does respond to magnetic field: the anomalies at $T_m^{\mathrm{II}}$ gain in height and shift to lower temperatures. We attribute the melting phenomenon I to a first-order melting phase transition between phases which have properties also known from three-dimensional van der Waals systems. The physical background of the λ-like specific-heat signals at $T_m^{\mathrm{II}}$ had been a puzzle until recently; with the additional information of new susceptibility and low-energy electron-diffraction measurements we now think that the shape of those anomalies results from the succession of two closely neighbored phase transitions, which border a region of two-phase equilibrium of a solid and of a phase which is neither a solid nor a liquid. That interpretation is consistent with the observed magnetic field responses of the λ-like anomalies.