Contains reports on two research projects.Joint Services Electronics Program (Contract DAAG29-80-C-0104

Andelman, David

Berker, A. Nihat

Catlisch, Robert G.

Walker, James S.

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23 Phase Transitions in Chemisorbed Systems6. Phase Transitions in Chemisorbed SystemsAcademic and Research StaffProf. A.N. Berker, Dr. J.S. WalkerGraduate StudentsD. Andelman, R.G. Caflisch, M. Kardar, S.R. McKay6.1 Multicritical Phase Diagrams of Chemisorbed Systems onNickelJoint Services Electronics Program (Contract DAAG29-80-C-0104)Robert G. Caflisch, A. Nihat Berker, James S. WalkerMulticritical phase diagrams are evaluated for oxygen on Ni (100), starting with adsorption-latticemodels with competing interactions, including trio interactions. We use global mean-field theory andrenormalization-group theory, which we have developed into an effective tool via prefacingtransformations onto coupled Potts-lattice-gas models. Different types of phase diagrams are found,each exhibiting disordered, 2 x 2, and /2 x J2 phases, separated by first-order transitions at lowtemperatures and second-order transitions at higher temperatures. These phase diagrams arequalitatively distinguished by tricritical points, triple points, critical end-points, and, possibly, a novelbicritical point. Our renormalization-group results carry indications of possible nonuniversality ofcritical behavior. Fitting phase diagrams from experiments currently in progress to our theoreticalresults will provide the best determination to date of microscopic interactions. The variety of phasediagrams we obtained also motivate experiments with sulfur, selenium, or tellurium on nickel.6.2 Improved Renormalization-group Calculations of CriticalExponents in Surface SystemsJoint Services Electronics Program (Contract DAAG29-80-C-0104)James S. Walker, David Andelman, A. Nihat BerkerThe Migdal-Kadanoff (MK) transformation is probably the most flexible renormalization procedure,applicable to models which are particulate and, therefore, readily connected to physical systems.However, this transformation has been at best semiquantitative. We have removed this shortcomingby developing a synthesis of this transformation and of perturbative series expansions. Theapproximate step of the transformation, instead of being performed in an ad hoc manner, is done byconserving free energies order by order. When tested on the Ising model, the results are vastlyRLE P.R. No. 12424 Phase Transitions in Chemisorbed Systemsimproved, as seen from the critical exponent values v = 1, 1.34, 1.02, from the exact, simple MK, orimproved MK calculations. The approach is also being tested on Potts models, and will be ofimportance in predicting muiticritical phenomena in surface systems.RLE P.R. No. 124

Phase Transitions in Chemisorbed Systems

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Phase Transitions in Chemisorbed Systems

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