Pham Phu X T, Ngo V Thanh, Diep H T
Laboratoire de Physique Théorique et Modélisation, Université de Cergy-Pontoise, CNRS, UMR 8089 2, Avenue Adolphe Chauvin, 95302 Cergy-Pontoise, France.
Phys Rev E Stat Nonlin Soft Matter Phys. 2009 Jun;79(6 Pt 1):061106. doi: 10.1103/PhysRevE.79.061106. Epub 2009 Jun 9.
In the bulk state, the Ising face-centered-cubic (fcc) antiferromagnet is fully frustrated and is known to have a very strong first-order transition. In this paper, we study the nature of this phase transition in the case of a thin film as a function of the film thickness. Using Monte Carlo simulations, we show that the transition remains first order down to a thickness of four fcc cells (eight atomic layers). It becomes clearly second order at a thickness of two fcc cells, i.e., four atomic layers. It is also interesting to note that the presence of the surface reduces the ground-state degeneracy found in the bulk. For the two-cell thickness, the surface magnetization is larger than the interior one. It undergoes a second-order phase transition at a temperature TC while interior spins become disordered at a lower temperature TD. This loss of order is characterized by a peak of the interior spins susceptibility and a peak of the specific heat which do not depend on the lattice size suggesting that either it is not a real transition or it is a Kosterlitz-Thouless nature. The surface transition, on the other hand, is shown to be of second order with critical exponents deviated from those of pure two-dimensional Ising universality class. We also show results obtained from the Green's function method. A discussion is given.
在体相状态下，伊辛面心立方（fcc）反铁磁体完全受挫，并且已知具有非常强的一级相变。在本文中，我们研究了薄膜情况下这种相变的性质与薄膜厚度的函数关系。通过蒙特卡罗模拟，我们表明，直到四个fcc晶胞（八个原子层）的厚度，相变仍为一级相变。在两个fcc晶胞的厚度，即四个原子层时，它明显变为二级相变。同样有趣的是要注意到，表面的存在降低了体相中发现的基态简并度。对于两个晶胞的厚度，表面磁化强度大于内部磁化强度。它在温度TC下经历二级相变，而内部自旋在较低温度TD下变得无序。这种有序性的丧失表现为内部自旋磁化率的峰值和比热的峰值，它们不依赖于晶格大小，这表明要么它不是真正的相变，要么它具有Kosterlitz-Thouless性质。另一方面，表面相变显示为二级相变，其临界指数偏离了纯二维伊辛普适类的临界指数。我们还展示了从格林函数方法获得的结果。并给出了讨论。
