Electronically driven phase transitions in a quasi-one-dimensional adsorbate system

A quasi-1D system is prepared using the Pt(110) surface as a template. The electronic surface resonance structure is studied by angle-resolved photoemission spectroscopy for the clean surface as well as for different Bromine coverages. A Fermi surface mapping reveals saddle points at the Fermi level in the interior of the surface Brillouin zone. Correspondingly, a maximum in the static response function chi(q, 0) at the connecting vector q is expected. With 1/2G(x) <q <2/3G(x) one observes indeed a 3-fold periodicity around defects and a 2-fold periodicity at low temperature for I similar to(Br) = 0.5 ML. Cooling of a defect-free c(2x2)-Br/Pt(110) preparation counter-intuitively results in a loss of long-range order. Motivated by DFT calculations this is attributed to an anomalous order-order phase transition into the (2x1) phase accompanied by intense, strongly anisotropic fluctuations within a temperature range of similar to 200 K. The peculiar behaviour is rationalised in terms of a competition between inter-adsorbate repulsion and an adsorbate triggered 2k(F) interaction in the substrat