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Journal of Physics C Solid State Physics

Criticality of the semi-infinite Potts ferromagnet: a renormalisation group approach

U.M.S. Costa, Constantino Tsallis, Enaldo F. Sarmento

Published Oct 30, 1985• 8 citations
Publisher

Abstract

Within a real space renormalisation group framework which uses rather sophisticated clusters, the authors discuss the phase diagram and the universality classes of a semi-infinite cubic-lattice q-state Potts ferromagnet. In particular, they study the influence, on the surface magnetism, of q and Delta identical to J S /J B -1 (where J S and J B are respectively the surface and bulk coupling constants). The exact d=2 critical temperature T c 2D is recovered for all values of q. The q-dependence of the value Delta c above which surface magnetic order can exist even if the bulk is disordered, is calculated and, through a convenient extrapolation, reliable results are obtained (for the Ising particular case, i.e. q=2, they obtain the extrapolated value Delta c =0.569, which compares satisfactorily with the series result 0.6+or-0.1 and the Monte Carlo one, 0.50+or-0.03). At the surface-bulk (SB) multicritical point they calculate the q-dependences of the critical amplitude A and the crossover exponent phi (defined in the Delta to Delta c +0 limit through (T c S ( Delta )/T c 3D -1) approximately A( Delta / Delta c -1) 1 phi /, where T c S ( Delta ) and T c 3D identical to T c S ( Delta c ) respectively are the surface and the bulk critical temperatures), as well as the critical exponent y t1 SB . For the Ising particular case they obtain phi equivalent to 0.641 (which compares satisfactorily with the epsilon -expansion (0.68) and the Monte Carlo (0.56+or-0.04) results), A equivalent to 0.4 and 1/y t1 SB equivalent to 1.623. (As far as we know, no series or Monte Carlo results are available in the literature for A or 1/y t1 SB .).

DOI
10.1088/0022-3719/18/30/010

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Unknow•Jan 1, 1989•0 citations
Within a real space renormalisation group framework which uses rather sophisticated clusters, the authors discuss the phase diagram and the universality classes of a semi-infinite cubic-lattice q-state Potts ferromagnet. In particular, they study the influence, on the surface magnetism, of q and Delta identical to J S /J B -1 (where J S and J B are respectively the surface and bulk coupling constants). The exact d=2 critical temperature T c 2D is recovered for all values of q. The q-dependence of the value Delta c above which surface magnetic order can exist even if the bulk is disordered, is calculated and, through a convenient extrapolation, reliable results are obtained (for the Ising particular case, i.e. q=2, they obtain the extrapolated value Delta c =0.569, which compares satisfactorily with the series result 0.6+or-0.1 and the Monte Carlo one, 0.50+or-0.03). At the surface-bulk (SB) multicritical point they calculate the q-dependences of the critical amplitude A and the crossover exponent phi (defined in the Delta to Delta c +0 limit through (T c S ( Delta )/T c 3D -1) approximately A( Delta / Delta c -1) 1 phi /, where T c S ( Delta ) and T c 3D identical to T c S ( Delta c ) respectively are the surface and the bulk critical temperatures), as well as the critical exponent y t1 SB . For the Ising particular case they obtain phi equivalent to 0.641 (which compares satisfactorily with the epsilon -expansion (0.68) and the Monte Carlo (0.56+or-0.04) results), A equivalent to 0.4 and 1/y t1 SB equivalent to 1.623. (As far as we know, no series or Monte Carlo results are available in the literature for A or 1/y t1 SB .).
Within a real-space renormalisation group framework, the authors treat the q-state Potts ferromagnet in a simple cubic lattice consisting of two different semi-infinite bulks (respectively characterised by the coupling constants J1and J2) separated by a (1,0,0) interface (characterised by Js). The use of a rather sophisticated two-terminal cluster allows a quite reliable discussion of the phase diagram and its universality classes. Four physically different phases are obtained, namely the paramagnetic and the double-bulk, single-bulk and surface ferromagnetic ones. The critical surface contains a multicritical line (associated with J1not=J2) which in turn contains a special point associated with J1=J2.
Within a real-space renormalisation group framework, the authors discuss the criticality of a system constituted by two (not necessarily equal) semi-infinite ferromagnetic q-state Potts bulks separated by an interface. This interface is a bond-diluted Potts ferromagnet with a coupling constant which is in general different from those of both bulks. The phase diagram presents four physically different phases, namely the paramagnetic one, and the surface, single-bulk and double-bulk ferromagnetic ones. These various phases determine a multicritical surface which contains a higher-order multicritical line. Particular attention is devoted to the discussion of the critical concentration p c . Here, p c is the concentration of the interface bonds above which surface magnetic ordering is possible even if the bulks are disordered. An interesting feature comes out which is that p c varies continuously with J 1 /J s and J 2 /J s . The standard two-dimensional percolation concentration is recovered for J 1 =J 2 =0. From the analysis of the various fixed points obtained within the present formalism, a very rich set of critical universality classes emerges.
Using a recently introduced renormalisation group method, the authors study the behaviour of the spontaneous surface and bulk magnetisations as functions of the temperature for an Ising ferromagnet in a semi-infinite cubic lattice for various J S /J B ratios (J S and J B respectively are the surface and bulk coupling constants). In particular, they study the extraordinary transition where the surface maintains its magnetisation as the bulk disorders: the authors find a discontinuity in the first derivative of the surface magnetisation at the bulk transition temperature. The criticality of the system (universality classes, critical exponents and amplitudes) is discussed as well.
Within a real space renormalisation group framework, the authors discuss the criticality of the full anisotropic (arbitrary J x , J y and J z )q-state Potts ferromagnet in the simple cubic lattice. Several previously known exact results for the d=1 and d=2 particular cases are recovered. Furthermore they obtain: (i) the q-dependence of the d=3 correlation length critical exponent nu 3 (in particular, if q to 0, nu 3 (q) approximately nu 3 (0)+ nu 3 '(0)q where the present approximate values are nu 3 (0) approximately=1.105 and nu 3 '(0) approximately=-0.66); (ii) the q-dependence of the d=2 to or from d=3 crossover critical exponent phi 23 (in particular, phi 23 varies as 1/ square root q if q to 0); (iii) through a convenient numerical extrapolation, a quite accurate proposal for the critical temperatures corresponding to arbitrary ratios, J y /J x and J z /J x and values of q.
The choice of a convenient self-dual cell within a real space renormalisation group framework enables a satisfactory treatment of the anisotropic square lattice q-state Potts ferromagnet criticality. The exact critical frontier and dimensionality crossover exponent Hf as well as the expected universality behaviour (renormalisation flow sense) are recovered for any linear scaling factor b and all values of q(q<or=4). The b=2 and b=3 approximate correlation length critical exponent nu is calculated for all values of q and compared with the den Nijs conjecture. The same calculation is performed, for all values of b for the exponent nu (d=1) associated with the one-dimensional limit and the exact result nu (d=1)=1 is recovered in the limit b to infinity .
Within a real space renormalisation group approach, the authors study the phase diagram and the universality classes of the three-state chiral clock model in a self-dual planar hierarchical lattice. They find that the chiral field Delta is relevant at the pure Potts critical point with a crossover exponent phi approximately=0.32. The critical line which separates the ferromagnetic phase from the non-ferromagnetic ones is characterised by a multicritical (Lifshitz) point located at (K, Delta ) approximately= (1.452, 0.382).