arXivDaily arXiv每日学术速递 周一至周五更新
arXiv 1901.01799cond-mat.dis-nn
收藏

随机场场自旋玻璃中的Specific heat和非线性极化率

Specific heat and non-linear susceptibility in spin glasses with random fields

M. V. Romitti, F. M. Zimmer, C. V. Morais, S. G. Magalhaes

摘要:

基于LiHo $_x$ Y $_{1-x}$ F $_4$在弱横场作用下产生无规场的假设,研究了自旋玻璃SG体系在无规场作用下的磁性.我们考虑了一个团簇自旋模型,它允许团簇之间的长程无序相互作用和团簇内部的短程相互作用,除了一个本地RF为每个自旋以下的高斯分布与标准偏差$Δ$。我们采用一步复制对称破缺(RSB)方法得到一个精确可解的单簇问题。讨论了不同无序组态的序参量、比热$C_{m}$、非线性极化率$χ_3$和相图的行为。在没有RF的情况下,$χ_3$在$T_f$处表现出发散,而$C_{m}$在比$T_f$高约30 $\%$的温度$T^{**}$处显示出宽的最大值,如对于常规SG系统所预期的。RF的出现改变了这种情况。$C_{m}$仍在$T^{**}$处显示最大值,其对$Δ$的依赖性较弱。然而,$T_f$被置换到较低的温度,显著提高了$T^{**}/T_f$的比率。此外,$χ_3$中的发散被在温度$T^{*}$下的四舍五入的最大值所取代,随着$Δ$的增强,该温度变得越来越高于$T_f$。因此,顺磁相在三个区域展开:(i)常规顺磁性($T>T^{**}$);(ii)具有冻结自旋的短程有序形成的区域($T^{*}<T<T^{**}$);(iii)具有在SG转变之前减慢自旋动力学的自由能势垒缓慢增长的区域($T_f<T<T^{*}$),这表明在SG状态之前存在中间格里菲斯相。我们的研究结果定性地再现了LiHo $_x$ Y $_{1-x}$ F $_4$的一些发现,作为RF触发的$χ_3$行为的四舍五入最大值。

英文摘要:

We study magnetic properties of spin glass SG systems under a random field (RF), beased on the suggestion that RFs can be induced by a weak transverse field in the compound LiHo$_x$Y$_{1-x}$F$_4$. We consider a cluster spin model that allows long-range disordered interactions among clusters and short-range interactions inside the clusters, besides a local RF for each spin following a Gaussian distribution with standard deviation $Δ$. We adopt the one-step replica symmetry breaking (RSB) approach to get an exactly solvable single-cluster problem. We discuss the behavior of order parameters, specific heat $C_{m}$, nonlinear susceptibility $χ_3$ and phase diagrams for different disorder configurations. In the absence of RF, the $χ_3$ exhibits a divergence at $T_f$, while the $C_{m}$ shows a broad maximum at a temperature $T^{**}$ around 30$\%$ above $T_f$, as expected for conventional SG systems. The presence of RF changes this scenario. The $C_{m}$ still shows the maximum at $T^{**}$ that is weakly dependent on $Δ$. However, the $T_f$ is displaced to lower temperatures, enhancing considerable the ration $T^{**}/T_f$. Furthermore, the divergence in $χ_3$ is replaced by a rounded maximum at a temperature $T^{*}$, which becomes increasingly higher than $T_f$ as $Δ$ enhances. As a consequence, the paramagnetic phase is unfolded in three regions: (i) a conventional paramagnetism ($T>T^{**}$; (ii) a region with formation of short-range order with frozen spins ($T^{*}<T<T^{**}$); (iii) a region with slow growth of free-energy barriers slowing down the spin dynamics before the SG transition ($T_f<T<T^{*}$) suggesting an intermediate Griffiths phase before the SG state. Our results reproduce qualitatively some findings of LiHo$_x$Y$_{1-x}$F$_4$ as the rounded maximum of $χ_3$ behavior triggered by RF.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

↑