Department of Chemistry, University of California, Berkeley, California 94720, USA.
Institut für Theoretische Physik, Johannes Kepler Universität Linz, Altenbergerstraße 69, 4040 Linz, Austria.
J Chem Phys. 2018 Mar 14;148(10):102338. doi: 10.1063/1.5005522.
The quantum phase transitions of dipoles confined to the vertices of two-dimensional lattices of square and triangular geometry is studied using path integral ground state quantum Monte Carlo. We analyze the phase diagram as a function of the strength of both the dipolar interaction and a transverse electric field. The study reveals the existence of a class of orientational phases of quantum dipolar rotors whose properties are determined by the ratios between the strength of the anisotropic dipole-dipole interaction, the strength of the applied transverse field, and the rotational constant. For the triangular lattice, the generic orientationally disordered phase found at zero and weak values of both dipolar interaction strength and applied field is found to show a transition to a phase characterized by net polarization in the lattice plane as the strength of the dipole-dipole interaction is increased, independent of the strength of the applied transverse field, in addition to the expected transition to a transverse polarized phase as the electric field strength increases. The square lattice is also found to exhibit a transition from a disordered phase to an ordered phase as the dipole-dipole interaction strength is increased, as well as the expected transition to a transverse polarized phase as the electric field strength increases. In contrast to the situation with a triangular lattice, on square lattices, the ordered phase at high dipole-dipole interaction strength possesses a striped ordering. The properties of these quantum dipolar rotor phases are dominated by the anisotropy of the interaction and provide useful models for developing quantum phases beyond the well-known paradigms of spin Hamiltonian models, implementing in particular a novel physical realization of a quantum rotor-like Hamiltonian that possesses an anisotropic long range interaction.
使用路径积分基态量子蒙特卡罗方法研究了受限在二维正方形和三角形晶格顶点的偶极子的量子相变。我们分析了作为偶极相互作用强度和横向电场强度函数的相图。研究揭示了一类量子偶极转子的取向相的存在，其性质由各向异性偶极-偶极相互作用的强度、施加的横向场的强度和旋转常数之间的比值决定。对于三角形晶格，在零和弱偶极相互作用强度以及施加的横向场的情况下发现的一般各向同性无序相被发现表现出向晶格平面中净极化的相的转变，随着偶极-偶极相互作用强度的增加而独立于施加的横向场的强度，除了随着电场强度的增加预期的向横向极化相的转变。还发现正方形晶格随着偶极-偶极相互作用强度的增加从无序相转变为有序相，以及随着电场强度的增加预期的向横向极化相的转变。与三角形晶格的情况相反，在正方形晶格上，在高偶极-偶极相互作用强度下的有序相具有条纹有序。这些量子偶极转子相的性质由相互作用的各向异性主导，并为开发超越自旋哈密顿模型等知名范例的量子相提供了有用的模型，特别是实现了具有各向异性长程相互作用的量子转子样哈密顿量的新颖物理实现。
