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arXiv 0806.2665cond-mat.str-el
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高$T_c$超导铜氧化物中环电流序起始附近的热力学性质

Thermodynamic Properties near the onset of Loop-Current Order in high-$T_c$ superconducting cuprates

M. S. Gronsleth, T. B. Nilssen, E. K. Dahl, E. B. Stiansen, C. M. Varma, A. Sudbo

摘要:

我们对二维广义Ashkin-Teller模型进行了大规模Monte Carlo模拟,计算了临界区附近的热力学性质。Ashkin-Teller模型在每个位置都有一对伊辛自旋,它们通过成对和4自旋相互作用与相邻的自旋相互作用。该模型表示轨道电流回路之间的相互作用,在$Cu O_2$ -高$T_c$铜氧化物,其顺序与交错的磁化$\Mso$内的每个单元格中的欠掺杂区域的相图低于温度$T^*(x)$,这取决于掺杂。每个晶胞的伊辛自旋对代表了电流环中电流的方向。的概括是包括各向异性的成对最近的邻居电流-电流耦合与对称性的正方形晶格和下一个最近的邻居成对耦合一致。我们使用Binder累积量来估计相关长度指数$ν$和序参数指数$β$。我们的主要结果是,在一定的参数范围内,Ashkin-Teller模型以及它的推广有一个顺序参数的敏感性,发散为$T \to T^*$和一个顺序参数低于$T^*$。然而,重要的是,比热没有发散。这使得模型的性质与欠掺杂铜氧化物的实验结果相一致。我们还计算了临界区域比热中的“凸点”的大小,以限制其可观测性。最后,我们表明,交错磁化耦合到均匀磁化$M_0$,使后者有一个弱的奇异性在$T^*$,也显示了一个广泛的临界区域,也符合最近的实验。

英文摘要:

We have performed large-scale Monte Carlo simulations on a two-dimensional generalized Ashkin-Teller model to calculate the thermodynamic properties in the critical region near its transitions. The Ashkin-Teller model has a pair of Ising spins at each site which interact with neighboring spins through pair-wise and 4-spin interactions. The model represents the interactions between orbital current loops in $Cu O_2$-plaquettes of high-$T_c$ cuprates, which order with a staggered magnetization $\Mso$ inside each unit-cell in the underdoped region of the phase diagram below a temperature $T^*(x)$ which depends on doping. The pair of Ising spins per unit-cell represent the directions of the currents in the links of the current loops. The generalizations are the inclusion of anisotropy in the pair-wise nearest neighbor current-current couplings consistent with the symmetries of a square lattice and the next nearest neighbor pair-wise couplings. We use the Binder cumulant to estimate the correlation length exponent $ν$ and the order parameter exponent $β$. Our principal results are that in a range of parameters, the Ashkin-Teller model as well as its generalization has an order parameter susceptibility which diverges as $T \to T^*$ and an order parameter below $T^*$. Importantly, however, there is no divergence in the specific heat. This puts the properties of the model in accord with the experimental results in the underdoped cuprates. We also calculate the magnitude of the "bump" in the specific heat in the critical region to put limits on its observability. Finally, we show that the staggered magnetization couples to the uniform magnetization $M_0$ such that the latter has a weak singularity at $T^*$ and also displays a wide critical region, also in accord with recent experiments.

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