Phase transitions in the adsorption system Li/Mo(112)

Journals published by the American Physical Society can be found at http://journals.aps.org/Experimental studies of the phase transitions in the adsorption system Li/Mo(112) are presented. This system is a model system for highly anisotropic interactions. From measurements of the half-widths of the low-energy electron diffraction spot profiles a phase diagram is derived for the whole submonolayer region of coverage in the temperature range 100-500 K. The commensurate low-coverage phases below theta=0.6 form chains normal to the troughs of the substrate. The commensurate p(4X1) phase, which is completed at a coverage, theta, of 0.25 monolayers (ML), seems to he truly long range ordered, whereas the p(2x1) phase at theta=0.5 still contains domain boundaries even at the lowest temperature of 100 K. Both undergo temperature driven order-disorder phase transitions. In contrast, the incommensurate phases existing in the coverage range theta=0.66-0.90 form chains along the troughs, which are only weakly coupled normal to the troughs of the substrate. These phases exhibit two coverage-driven phase transitions from rectangular to oblique units cells and back at critical coverages of 0.66 and 0.85, respectively, and represent floating solids. As a function of temperature, they undergo a two-dimensional melting transition. Close to the critical coverages, the melting temperatures show a sharp drop below the temperature range accessible in our experiments. Both functional dependences of the angular deviation from 90 degrees and of the melting temperature on coverage are in good agreement with a phenomenological theoretical model, assuming an instability of the shear modulus of the adsorbate unit cell at the critical coverages

Reliability of low-energy electron diffraction for studies of surface order-disorder phenomena

It is shown that a determination of critical exponents in surface phase transformations based on a kinematic analysis of LEED peak intensities is subject to errors caused by multiple scattering that are large enough to prevent a clear assignment to a known universality class. The multiple-scattering contribution arises from short-range flucutations and has its maximum value at the transition temperature. The specific-heat exponent of the surface phase can be measured directly from the variation of the integral-order-beam intensity with temperature that is caused by the multiple scattering

A vitrifying structure transition in the Dy/Mo(112) adsorption system

Annealing-driven irreversible structural transitions are studied by low-energy electron diffraction in submonolayer Dy films adsorbed on the Mo(112) surface. In a wide coverage range θ>0.07, Dy overlayers deposited at low temperatures (T≈100 K) are ordered and keep their structure upon annealing up to 350–600 K. Near θ=0.68, the overlayers are stable to high-temperature annealing (Ta=1000 K) as well, whereas the denser films are metastable and transform to more stable ordered structures. An unusual annealing effect is found for θ<0.58: the initially ordered metastable phases are replaced by phases having no extended order, which are assumed to be two-dimensional glasses. © 2001 American Institute of Physics

A vitrifying structure transition in the Dy/Mo(112) adsorption system

Annealing-driven irreversible structural transitions are studied by low energy electron diffraction in submonolayer Dy films adsorbed on the Mo(112) surface. In a wide coverage range θ > 0.07, Dy overlayers deposited at low temperatures (T≈ 100 K) are ordered and keep their structure upon annealing up to 350-600 K. Near θ = 0.68, the overlayers are stable to high- temperature annealing (Ta = 1000 K) as well, whereas the denser films are metastable and transform to more stable ordered structures. An unusual annealing effect is found for θ < 0.58: the initially ordered metastable phases are replaced by phases having no extended order, which are assumed to be two-dimensional glasses

Metastable structures of Dy layers adsorbed on Mo(112) and their transformations

Ordering of dysprosium on Mo(112) up to 1.5 monolayers has been investigated by LEED and work function analysis after adsorption at 100 K and annealing between 200 and 1000 K. At low annealing temperatures (< 350-600 K) ordered structures are found, which are changed or even destroyed irreversibly by annealing steps to higher temperatures. At coverages, θ, up to 0.3 monolayer a $(6\times 1)$ not strictly commensurate chain structure is seen, which coexists up to $\theta = 0.58$ with a one-dimensionally incommensurate c($1.56\times 2$) structure. At higher coverages up to the physical monolayer at $\theta \approx 0.77$, incommensurate ($n \times 2$) followed by oblique ($n \times 1$) structures are seen with n continuously variable with coverage. The second layer forms a p$(1.33 \times 1)$ structure. Annealing to higher temperatures causes irreversible structural transitions with strongly coverage dependent properties. Up to $\theta = 0.58$, only a glass-like disordered phase is formed, which cannot be ordered again. In contrast, the rectangular incommensurate structures between $0.58 < \theta < 0.68$ remain unchanged upon annealing, whereas the structures at higher coverages and those of the second layer are transformed into commensurate $(s \times 1)$ structures with integer s. Geometrical models are presented for the non-annealed structures and possible origins for the two-dimensional concentration dependent vitrification of the Dy layers are discussed

Irreversible structure transitions in Gd monolayers on Mo(112)

68.43.Fg Adsorbate structure, 68.35.Rh Phase transitions and critical phenomena,

Interaction of oxygen with submonolayer beryllium films on Mo(1 1 2)

Interaction of oxygen with the Mo(1 1 2) surface precovered by submonolayer beryllium films with various coverage degrees (θBe < 1) has been investigated by Auger electron spectroscopy, LEED and contact potential difference techniques. We have studied the effect of Be coverage degree on the oxygen adsorption kinetics, atomic structure and electronic properties of the O/Be/Mo(1 1 2) system. Contrary to the case of full-monolayer Be precoverage (θBe = 1), beryllium submonolayers can speed up the initial adsorption kinetics of oxygen by a factor of 10. The high sticking coefficient of oxygen on the Be/Mo(1 1 2) surface at θBe < 1 can be explained by the existence of areas on Mo(1 1 2) that are free of Be and provide fast oxygen adsorption, with O adatoms migrating further to the areas covered with Be (the spillover effect). The creation of beryllium oxide even on limited surface areas substantially decreases the oxygen sticking coefficient. This effect may originate from the surface deformation due to a structural misfit between the Be/O layer and the substrate. The coadsorbed Be/O layers with θBe < 1 modify the work function to values between those specific of the O/Be/Mo(1 1 2) systems with θBe = 0 and 1, which complies with the heterogeneous adlayer model