Triangular lattice Hubbard model physics at intermediate temperatures
Kyungmin Lee, Prakash Sharma, Oskar Vafek, Hitesh J. Changlani
备注：
11 pages, 12 figures. v2 closely matches the published version. Sections expanded and supplemental material converted to appendices. The first two authors contributed equally
Moire systems offer an exciting playground to study many-body effects of strongly correlated electrons in regimes that are not easily accessible in conventional material settings. Motivated by a recent experiment on $\text{WSe}_2/\text{WS}_2$ moire bilayers [Y. Tang et al., Nature 579, 353-358 (2020)], which realizes a triangular superlattice with a small hopping (of approximately 10 Kelvin), with tunable density of holes, we explore the Hubbard model on the triangular lattice for intermediate temperatures $t \lesssim T < U$. Employing finite temperature Lanczos calculations, and closely following the fitting protocols used in the experiment, we recover the observed trends in the reported Curie-Weiss temperature $Θ$ with filling, using the reported interaction strength $U/t=20$. We focus on the large increase of $|Θ|$ on decreasing the density below half filling and the sign change of $Θ$ at higher fillings, which signals the onset of ferromagnetism. The increase in $|Θ|$ is also seen in the $t$-$J$ model (the low energy limit of the Hubbard model) in the intermediate temperature range, which we clarify is opposite to the trend in its high temperature limit. Differences between the low, intermediate and high temperature regimes are discussed. Our numerical calculations also capture the crossover between short-range antiferromagnetic to ferromagnetic order in the intermediate temperature regime, a result broadly consistent with the experimental findings. We find that this behavior is a finite-temperature remnant of the underlying zero temperature phase transition, which we explore with ground state density matrix renormalization group calculations. We provide evidence of ferromagnetism characterized by weak (but robust) correlations that explain the small $Θ$ seen in the experiment.
Emergent Gauge Flux and Spin Ordering in Magnetized Triangular Spin Liquids: Applications to Hofstadter-Hubbard Model
International Center for Quantum Materials, School of Physics, Peking University（北京大学物理学院量子材料中心）；Beijing Key Laboratory of Quantum Devices, Peking University（北京大学北京量子器件重点实验室）；Collaborative Innovation Center of Quantum Matter（量子物质协同创新中心）
Low-energy effective theories of the two-thirds filled Hubbard model on the triangular necklace lattice
受Mo $_3$ S $_7$（dmit）$_3$的启发，我们研究了三分之二填充的三角项链晶格上的哈伯德模型.利用二级微扰理论，我们证明了在分子极限下，该模型的基态和低能激发与自旋为1的海森堡链完全相同。后一种模型被认为是在对称保护拓扑Haldle阶段。远离这一限制，我们表明，对称性参数和密度矩阵重整化群（DMRG）计算的基础上，低能量物理的哈伯德模型的三角形项链晶格在三分之二填充被捕获的铁磁Hubbard-Kondo晶格链在一半填充。这与先前的主张一致并加强了先前的主张，即三角项链晶格上的半填充铁磁Kondo晶格模型和三分之二填充哈伯德模型也处于Half-phase。Hund规则和Nagaoka定理之间的联系也进行了讨论。
Magnetic and charge susceptibilities in the half-filled triangular lattice Hubbard model
在双费米子近似下，研究了半填充二维三角形哈伯德模型在金属、Mott绝缘和参数空间交叉区域的磁和电荷磁化率.在\textcolor{black}{insulating state}中，我们发现在低能时K点处存在强的自旋涨落，对应于\textcolor{black}{120$^{\circ}$}反铁磁序。这些自旋波动持续到金属相并移动到更高的能量。我们还提出了模拟中子能谱和\textcolor{black}{spin-lattice}弛豫时间的数据，并进行直接比较，非弹性中子能谱实验上的三角形材料Ba $_8$ CoNb $_6$ O $_{24}$和$κ$ -（ET）$_2$ Cu $_2$（CN）$_3$的弛豫时间。最后，我们提出了在不同领域的参数空间，这应该对应于动量分辨电子损失光谱测量三角形化合物的电荷亲合性。
Quantum Spin Liquid with Emergent Chiral Order in the Triangular-lattice Hubbard Model
在半填充三角格子哈伯德模型的中间相互作用区域中，自旋挫挫和电荷涨落的相互作用产生了一种奇异的量子态，而这种态的性质还存在争议.利用SU（2）$_{\rm{spin}} \otimes $ U（1）$_{\rm{charge}}$对称性实现的密度矩阵重整化群，研究了定义在直到圆周$W=6$的长圆柱几何上的三角格子哈伯德模型.一个带隙的量子自旋液体，与现场相互作用$9 \lesssim U / t \lesssim 10.75$，被确定之间的金属和反铁磁Mott绝缘相。特别是，我们发现，这种自旋液体开发了一个强大的远程自旋标量手征相关的系统长度$L$的增加，这毫不含糊地揭示了自发的时间反演对称性破缺。此外，纠缠谱的简并性支持了基态的对称性分数化和自旋边缘模。本文还讨论了这种中间自旋液体中手征序的可能起源及其与类旋光激发的关系。
High-temperature magnetization and entropy of the triangular lattice Hubbard model in a Zeeman field
我们使用强耦合展开计算的熵函数$S(T,h)$，磁化$M(T,h)$，和双占用因子$D(T,h)$的半填充三角形晶格哈伯德模型作为温度$T$和塞曼场$h$的函数，哈伯德参数比$U/t$的各种值。对于中等至大$U/t$值，这些计算在大于交换参数$J=\frac{4t^2}{U}$的温度下收敛良好。设置$μ=U/2$足以在$U/t \geq 8$研究的所有温度下获得1%以内的半填充密度。我们讨论了$U/t$的系统变化的性质。熵和双占有参数的温度依赖性显示出映射到反铁磁莫特绝缘行为在温度远高于$T=J$的$U/t\ge 10$。该系列的收敛性较弱，在中间领域，这意味着非单调变化的自旋相关性与塞曼场。我们讨论了哈伯德模型结果与Yang等人最近研究的三角晶格反铁磁材料Lu $_3$ Cu $_2$ Sb $_3$ O $_{14} $（LCSO）的相关性[Yang et al arXiv：2102.09271（2022）]。
Symmetry-Protected Topological Phases in the Triangular Majorana-Hubbard Ladder
Western Washington University（西华盛顿大学）；Stewart Blusson Quantum Matter Institute, University of British Columbia（不列颠哥伦比亚大学斯图尔特·布拉斯量子物质研究所）；Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）
Competing instabilities of the extended Hubbard model on the triangular lattice: Truncated-unity functional renormalization group and application to moiré materials
