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RESEARCH PRODUCT

Phase Transitions in Adsorbates with Internal Quantum States

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In principle, phase transitions in realistic systems at low temperatures should be studied including quantum effects. However, a full quantum treatment of all degrees of freedom in a simulation is restricted to small systems, if possible at all. In some cases, as is demonstrated for adsorbates, some degrees of freedom can still be modelled classically even at low temperatures, whereas only for the rest a quantum treatment is unavoidable. The path-integral Monte Carlo approach allows a systematic distinction between classical and quantum degrees of freedom in many-body systems. Using this technique in combination with finite-size methods, the complex phase diagram of a two-dimensional model fluid including various coexistences, tricritical and triple points can be obtained precisely. In a second step, a highly realistic model for N2 in the \( \left( {\sqrt 3 \times \sqrt 3 } \right) \) R30° structure on graphite is investigated along similar lines. The influence of quantum fluctuations on the orientational phase transition to the “2-in” herringbone phase and the decrease of the saturation order parameter are studied and compared to quasiclassical simulations using the Feynman-Hibbs effective potential.