[论文解读] Superconducting dome in doped quasi-2d organic Mott insulators: a paradigm for strongly-correlated superconductivity

Charles-David Hébert, P. Sémon+1 more|arXiv (Cornell University)|Sep 23, 2015

Physics of Superconductivity and Magnetism被引用 3

一句话总结

本研究采用晶胞单元动力学平均场理论与连续时间量子蒙特卡洛方法，表明掺杂准二维有机莫特绝缘体中的超导性源于伪能隙金属与关联金属之间的的一阶相变，而非反铁磁量子临界点。与半满情况相比，有限掺杂时超导穹顶的转变温度提高了约25%，且U/t范围更宽，该相变与莫特相变在掺杂区的延续相关，与铜氧化物物理行为一致。

ABSTRACT

Layered organic superconductors of the BEDT family are model systems for the interplay of the Mott transition with superconductivity, magnetic order and frustration. Recent experimental studies on a hole-doped version of BEDT compounds reveal an enhancement of superconductivity and a rapid crossover between two different conducting phases above the superconducting dome. One of these phases is a Fermi liquid, the other not. Using plaquette cellular dynamical mean field theory with state of the art continuous-time quantum Monte Carlo calculations, we study this problem with the two-dimensional Hubbard model on the anisotropic triangular lattice. Phase diagrams as a function of temperature $T$ and interaction strength $U/t$ are obtained for anisotropy parameters $t'=0.4t$, $t'=0.8t$ and various fillings. As for cuprates, we find, at finite doping, a first-order transition between two normal-state phases. For $T$ above the critical point of the first-order transition, there is a Widom line where crossovers occur. The results are in broad agreement with experiment. This suggests that for compounds with intermediate to high frustration, very light-doping should reveal the first-order transition and associated crossovers. These crossovers could leave traces in the superconducting phase. We also predict that destroying the superconducting phase by a magnetic field should reveal the first-order transition. Finally, we predict that electron-doping should also lead to an increased range of $U/t$ for superconductivity but with a reduced maximum $T_c$. This work also clearly shows that the superconducting dome here is tied to the Mott transition and its continuation as a transition separating pseudogap phase from correlated metal in doped compounds, as in the cuprates. Contrary to heavy fermions for example, the maximum $T_c$ is definitely not attached to an antiferromagnetic quantum critical point.

研究动机与目标

• 理解掺杂准二维有机莫特绝缘体（特别是BEDT基空穴掺杂化合物）中超导穹顶的起源。
• 确定实验观测到的增强超导性与正常态交叉是否与反铁磁量子临界性相关，或与莫特相变相关。
• 研究电子关联阻挫（通过t′/t比值）对抑制磁序及调控超导与正常态相的影响。
• 预测在超导态中及磁场抑制下，一阶相变的特征信号。
• 比较空穴掺杂与电子掺杂体系在强关联超导背景下的行为。

提出的方法

• 采用晶胞单元动力学平均场理论（CDMFT）求解在各向异性三角晶格上的二维 Hubbard 模型。
• 利用连续时间量子蒙特卡洛（CTQMC）计算以高精度获得自能与格林函数。
• 计算温度T、关联强度U/t、填充因子n及阻挫参数t′/t（0.4t，0.8t）的相图。
• 通过热力学与谱学分析，识别伪能隙相与金属相之间的一阶相变。
• 在T–U/t平面中绘制代表交叉行为的威多姆线（Widom line）。
• 对比电子掺杂与空穴掺杂的影响，以评估超导行为的对称性。

实验结果

研究问题

• RQ1 掺杂有机莫特绝缘体中的超导穹顶是否源于反铁磁量子临界点，或源于莫特相变的延续？
• RQ2 阻挫（t′/t）如何影响磁序的抑制与超导稳定性的调控？
• RQ3 在正常态中，伪能隙相与金属相之间的一阶相变是否可通过施加磁场破坏超导性来探测？
• RQ4 为何掺杂化合物中超导穹顶更宽且Tc更高，相较于半满情况？
• RQ5 电子掺杂如何影响最大Tc与超导性存在的U/t范围？

主要发现

• 最大超导转变温度Tc,max在有限掺杂时相比半满时提升了约25%。
• 超导出现的U/t范围在掺杂体系中显著拓宽，尤其在较高阻挫（t′/t = 0.8）时更为明显。
• 在有限掺杂与低温下，伪能隙相与关联金属之间发生一阶相变，且与半满时的莫特相变连续相连。
• 超导穹顶与反铁磁量子临界点无关，这由最优掺杂附近不存在长程反铁磁序所证实。
• 在轻度掺杂且高度阻挫的体系中，通过施加磁场抑制超导性，应可揭示正常态中伪能隙相与金属相之间的一阶相变。
• 预测电子掺杂将降低最大Tc，但仍可扩展超导存在的U/t范围，表明其与铜氧化物存在不同的掺杂不对称性。