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Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics

Ferromagnetic phase transition in random Potts spin chains

Jian-Cheng Lin, Philip L. Taylor, Rafael Rangel

Published Feb 1, 1993• 3 citations
Publisher

Abstract

We study the ferromagnetic phase transition in a model consisting of one-dimensional three-state Potts spin chains with random intrachain couplings and comparatively weak ferromagnetic interchain couplings. Mean-field theory is employed to decouple approximately the interchain couplings. The transfer-matrix method is then used to study the resulting effectively one-dimensional random-bond Potts model. The free energy as a function of the ferromagnetic order is calculated numerically and a first-order ferromagnetic-paramagnetic phase transition is found over a wide range of degree of randomness.

DOI
10.1103/physreve.47.981

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We study the ferromagnetic phase transition in a model consisting of one-dimensional three-state Potts spin chains with random intrachain couplings and comparatively weak ferromagnetic interchain couplings. Mean-field theory is employed to decouple approximately the interchain couplings. The transfer-matrix method is then used to study the resulting effectively one-dimensional random-bond Potts model. The free energy as a function of the ferromagnetic order is calculated numerically and a first-order ferromagnetic-paramagnetic phase transition is found over a wide range of degree of randomness.
We study phase transitions of the ferromagnetic qqq-state Potts chain with random nearest-neighbor couplings having a variance ${\mathrm{\ensuremath{\Delta}}}^{2}$ and with homogeneous long-range interactions, which decay with distance as a power ${r}^{\ensuremath{-}(1+\ensuremath{\sigma})}, \ensuremath{\sigma}>0$. In the large-qqq limit the free-energy of random samples of length $L\ensuremath{\le}2048$ is calculated exactly by a combinatorial optimization algorithm. The phase transition stays first order for $\ensuremath{\sigma}<{\ensuremath{\sigma}}_{c}(\mathrm{\ensuremath{\Delta}})\ensuremath{\le}0.5$, while the correlation length becomes divergent at the transition point for ${\ensuremath{\sigma}}_{c}(\mathrm{\ensuremath{\Delta}})<\ensuremath{\sigma}<1$. In the latter regime the average magnetization is continuous for small enough $\mathrm{\ensuremath{\Delta}}$, but for larger $\mathrm{\ensuremath{\Delta}}$---according to the numerical results---it becomes discontinuous at the transition point, thus the phase transition is expected of mixed order.
The ordering of weakly coupled random antiferromagnetic S=1/2S=1/2S=1/2 chains, as relevant for the recent experimentally investigated spin chain materials, is considered theoretically. The one-dimensional isotropic Heisenberg model with random exchange interactions is treated numerically on finite chains with the density-matrix renormalization-group approach as well as with the standard renormalization analysis, both within the mean-field approximation for interchain coupling ${J}_{\ensuremath{\perp}}$. Results for the ordering temperature TN{T}_{N}TN​ and for the ordered moment m0{m}_{0}m0​ are presented and are both reduced with increasing disorder, agreeing with the experimental observations. The most pronounced effect of the random singlet concept appears to be a very large span of local ordered moments, becoming wider with decreasing ${J}_{\ensuremath{\perp}}$, consistent with the $\ensuremath{\mu}$-SR experimental findings.
We investigate the phase transition of the three-state Potts model in an analytical approach—the generalized cumulant expansion with the effective mean field hypothesis. We find a first order phase transition in the three-dimensional three-state Potts model with ferromagnetic nearest neighbor coupling. For the model with antiferromagnetic next-to-nearest neighbor coupling, we find a first order transition when the relative strength of the -coupling is fixed to . The critical values given by this method are also in agreement with the recent high statistics Monte Carlo results.
We present a numerical study of two-dimensional random-bond Potts ferromagnets. The model is studied both below and above the critical value Qc=4, which discriminates between second- and first-order transitions in the pure system. Two geometries are considered, namely cylinders and square-shaped systems, and the critical behavior is investigated through conformal invariance techniques that were recently shown to be valid, even in the randomness-induced second-order phase transition regime Q>4. In the cylinder geometry, connectivity transfer matrix calculations provide a simple test to find the range of disorder amplitudes that is characteristic of the disordered fixed point. The scaling dimensions then follow from the exponential decay of correlations along the strip. Monte Carlo simulations of spin systems on the other hand are generally performed on systems of rectangular shape on the square lattice, but the data are then perturbed by strong surface effects. The conformal mapping of a semi-infinite system inside a square enables us to take into account boundary effects explicitly and leads to an accurate determination of the scaling dimensions. The techniques are applied to different values of Q in the range 3-64.
Phase transition in the two-dimensional q-state Potts model with random ferromagnetic couplings is studied in the large-q limit by a combinatorial optimization algorithm and by approximate mappings. We conjecture that the critical behavior of the model is controlled by the isotropic version of the infinite randomness fixed point of the random transverse-field Ising spin chain and the critical exponents are exactly given by beta=(3-sqrt[5])/4, beta(s)=1/2, and nu=1. The specific heat has a logarithmic singularity, but at the transition point there are very strong sample-to-sample fluctuations. Discretized randomness results in discontinuities in the internal energy.
Abstract We derive the phase diagram of the one-dimensional three-state Potts model with an additional mean-field interaction in the canonical ensemble. The free energy is obtained by mapping the model onto the spin-1 Blume–Emery–Griffiths model and solving it by using a Hubbard–Stratonovich transformation combined with the transfer matrix method. A complex structure with lines of first-order transitions, two triple points and a critical point appears at finite temperature. The phase diagram is two-dimensional, since there are two adjustable parameters, the nearest-neighbour coupling K and the temperature T . We show that the phase diagram does not present second-order phase transition lines, due to the fact that the order parameter is not a symmetry-breaking one. Quite remarkably, we are able to determine analytically one of the first-order phase transition lines. We also prove that, when the nearest-neighbour coupling K is large and negative, the first-order transition temperature becomes asymptotically independent of K .