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Physica A Statistical Mechanics and its Applications

Duality-based approximation for the critical point of the square lattice Ising ferromagnet within Tsallis statistics

L. R. da Silva, Eugene Stanley

Published Dec 1, 1996• 6 citations
Publisher

Abstract

Within the generalized thermostatistics of Tsallis, we propose for the spin-12 Ising ferromagnet a transmissivity variable which extends that defined by Tsallis and Levy for thermal magnetic systems. By using this generalized transmissivity as well as duality arguments, we calculate the q-dependence of the critical temperature corresponding to the square lattice, where q is the entropic index (q = 1 reproduces standard thermostatistics). Our approximate results are compared with those previously obtained using renormalization group and mean-field approximation.

DOI
10.1016/s0378-4371(96)00270-1

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Within the generalized thermostatistics of Tsallis, we propose for the spin-12 Ising ferromagnet a transmissivity variable which extends that defined by Tsallis and Levy for thermal magnetic systems. By using this generalized transmissivity as well as duality arguments, we calculate the q-dependence of the critical temperature corresponding to the square lattice, where q is the entropic index (q = 1 reproduces standard thermostatistics). Our approximate results are compared with those previously obtained using renormalization group and mean-field approximation.
We investigate the critical behavior and the duality property of the ferromagnetic qqq-state clock model on the square lattice based on the tensor-network formalism. From the entanglement spectra of local tensors defined in the original and dual lattices, we obtain the exact self-dual points for the model with $q \leq 5 $ and approximate self-dual points for q≥6q \geq 6q≥6. We calculate accurately the lower and upper critical temperatures for the six-state clock model from the fixed-point tensors determined using the higher-order tensor renormalization group method and compare with other numerical results.
If scaling variables and scaling expressions are chosen judiciously, ``critical'' scaling analyses do not need to be restricted to a narrow range of temperatures near the critical point. The standard scaling variable $(T\ensuremath{-}{T}_{c})/{T}_{c}$ is inadapted to wide ranges of temperature because it diverges at high temperatures. With the variable $\ensuremath{\tau}=(T\ensuremath{-}{T}_{c})/T$, in the high-dimension (mean-field) limit, the reduced susceptibility and the second-moment correlation length of the Ising ferromagnet depend on temperature as $\ensuremath{\chi}(T)={\ensuremath{\tau}}^{\ensuremath{-}1}$ and $\ensuremath{\xi}(T)={T}^{\ensuremath{-}1/2}{\ensuremath{\tau}}^{\ensuremath{-}1/2}$ exactly over the entire temperature range above the critical temperature Tc{T}_{c}Tc​. For the canonical two-dimensional square lattice near-neighbor Ising ferromagnet, it is shown that compact ``extended scaling'' expressions analogous to the high-dimensional limit form but with the appropriate exponents give accurate approximations to the true temperature dependencies, again over the entire temperature range from Tc{T}_{c}Tc​ to infinity. Within this approach, for near-neighbor interaction systems, there is no crossover temperature above which mean-field-like behavior sets in.
We present an extension of past approximations such as the Bethe approximation where we look at generalized Husimi trees. In doing so we obtain a sequence of ever more accurate approximations. The sequence is very systematic, and sequences of as few as three points can be extrapolated to give critical temperature estimates to within 0.1% or better of the exact value for our test cases. In the case of the square lattice, Ising ferromagnet, where five levels of approximation are obtained, estimates within 0.003% are obtained. Critical exponents can also be approximated but with much less accuracy. Much of our attention is devoted to the critical line of the phase diagram of the antiferromagnetic Ising model on the square lattice, where our results compare very favorably with other approximations. The method is general enough to be applicable to any spin lattice system where the spin takes on a discrete set of values.
A simple renormalisation-group approach based on self-dual clusters is proposed for the two-dimensional nearest-neighbour spin- 1 / 2 Ising model on the square lattice; it reproduces the exact critical point. The authors calculate the internal energy and the specific heat for vanishing external magnetic field, spontaneous magnetisation and the thermal (y T ) and magnetic (y H ) critical exponents. The results obtained from the first four smallest cluster sizes strongly suggest the convergence towards the exact values when the cluster size increases. Even for the smallest cluster, where the calculation is very simple, the results are quite accurate, particularly in the neighbourhood of the critical point.
For the two-dimensional ferromagnetic Ising critical point, I show that the known values of the critical exponents imply the absence of logarithms of the reduced temperature in the leading contributions to any field derivative of the free energy at zero magnetic field. For the square-lattice Ising antiferromagnet in a weak magnetic field, I compute the critical line Tc{T}_{c}Tc​(H)=Tc0{T}_{c}^{0}Tc0​(1-0.038 023 259H2{H}^{2}H2) and the leading contribution to the susceptibility \ensuremath{\chi}=0.014 718 006 6H2{H}^{2}H2ln(1/\ensuremath{\Vert}t\ensuremath{\Vert}), where t is the reduced temperature.
By using self-dual clusters (whose sizes are characterized by the numbers b=2,3,4,5b=2,3,4,5b=2,3,4,5), we calculate within a real-space renormalization-group framework, the longitudinal surface tension of the square-lattice first-neighbor \textonehalf{}-spin ferromagnetic Ising model. The exact critical temperature Tc{T}_{c}Tc​ is recovered for any value of bbb; the exact asymptotic behavior of the surface tension in the limit of low temperatures is analytically recovered; the approximate correlation-length critical exponents monotonically tend towards the exact value $\ensuremath{\nu}=1$ (which, for two dimensions, coincides with the surface-tension critical exponent $\ensuremath{\mu}$) for increasingly large cells; the same behavior is noticed in the approximate values for the surface-tension amplitude in the limit $T\ensuremath{\rightarrow}{T}_{c}$. We develop four different numerical procedures for extrapolating to $b\ensuremath{\rightarrow}\ensuremath{\infty}$ the renormalization-group (RG) results for the surface tension, and quite satisfactory agreement is obtained with Onsager's exact expression (error varying from zero to a few percent on the whole temperature domain). Furthermore, we compare the set of RG surface tensions with a set of biased surface tensions (associated with appropriate misfit seams) and find only fortuitous coincidence among them.