Liu Yuan, Shen Tong, Zhang Hang, Rubenstein Brenda
Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States.
Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544, United States.
J Chem Theory Comput. 2020 Jul 14;16(7):4298-4314. doi: 10.1021/acs.jctc.0c00288. Epub 2020 Jun 22.
The ability to accurately predict the finite temperature properties and phase diagrams of realistic quantum solids is central to uncovering new phases and engineering materials with novel properties ripe for device applications. Nonetheless, there remain comparatively few many-body techniques capable of elucidating the finite temperature physics of solids from first principles. In this work, we take a significant step toward developing such a technique by generalizing our previous, fully finite temperature Auxiliary Field Quantum Monte Carlo (FT-AFQMC) method to model periodic solids and employing it to uncover the finite temperature physics of periodic hydrogen chains. Our chains' unit cells consist of 10 hydrogen atoms modeled in a minimal basis, and we sample 5 k-points from the first Brillouin zone to arrive at a supercell consisting of 50 orbitals and 50 electrons. Based upon our calculations of these chains' many-body energies, free energies, entropies, heat capacities, double and natural occupancies, and charge and spin correlation functions, we outline their metal-insulator and magnetic ordering as a function of both H-H bond distance and temperature. At low temperatures approaching the ground state, we observe both metal-insulator and ferromagnetic-antiferromagnetic crossovers at bond lengths between 0.5 and 0.75 Å. We then demonstrate how this low-temperature ordering evolves into a metallic phase with decreasing magnetic order at higher temperatures. In order to contextualize our results, we compare the features we observe to those previously seen in one-dimensional, half-filled Hubbard models at finite temperature and in ground state hydrogen chains. Interestingly, we identify signatures of the Pomeranchuk effect in hydrogen chains for the first time and show that spin and charge excitations that typically arise at distinct temperatures in the Hubbard model are indistinguishably coupled in these systems. Beyond qualitatively revealing the many-body phase behavior of hydrogen chains in a numerically exact manner without invoking the phaseless approximation, our efforts shed light on the further theoretical developments that will be required to construct the phase diagrams of the more complex transition metal, lanthanide, and actinide solids of longstanding interest to physicists.
准确预测实际量子固体的有限温度特性和相图的能力，对于发现新相和设计具有适合器件应用的新颖特性的工程材料至关重要。然而，能够从第一原理阐明固体有限温度物理的多体技术相对较少。在这项工作中，我们朝着开发这样一种技术迈出了重要一步，即推广我们之前的完全有限温度辅助场量子蒙特卡罗（FT-AFQMC）方法来模拟周期性固体，并使用它来揭示周期性氢链的有限温度物理。我们的链的晶胞由10个氢原子组成，采用最小基组进行建模，并且我们从第一布里渊区采样5个k点，以得到一个由50个轨道和50个电子组成的超胞。基于我们对这些链的多体能量、自由能、熵、热容量、双占据和自然占据以及电荷和自旋关联函数的计算，我们概述了它们作为H-H键距离和温度函数的金属-绝缘体和磁有序。在接近基态的低温下，我们在键长为0.5至0.75 Å之间观察到金属-绝缘体和铁磁-反铁磁转变。然后我们展示了这种低温有序如何在较高温度下随着磁有序的降低演变成金属相。为了将我们的结果置于背景中，我们将观察到的特征与先前在有限温度下的一维半填充哈伯德模型和基态氢链中看到的特征进行比较。有趣的是，我们首次在氢链中识别出波梅兰丘克效应的特征，并表明在哈伯德模型中通常在不同温度出现的自旋和电荷激发在这些系统中是不可区分地耦合的。除了以数值精确的方式定性地揭示氢链的多体相行为而不调用无相近似之外，我们的工作还阐明了构建物理学家长期感兴趣的更复杂的过渡金属、镧系和锕系固体的相图所需的进一步理论发展。
