Ordering in realistic elongated box-shape crystals of the molecular magnet Mn_12 Ac is investigated with the site-resolved mean-field approximation that does not assume a uniform ordering. It is shown that ferromagnetic ordering should not occur in crystals with the aspect ratio up to 12. Instead, for the aspect ratio about 6, the inner and outer regions of the crystal order in different directions, uniformly along its length. Finding ordering temperature by extrapolating the inverse susceptibility curve does not provide a correct T_C.
Theory of Two-Step Magnetic Ordering Phenomena in a Geometrically Frustrated Heisenberg Pyrochlore Antiferromagnet with Long Range Dipolar Interactions
利用平均场论研究了具有交换作用和偶极-偶极相互作用的烧绿石晶格上的海森堡反铁磁体模型。在处理偶极通过使用埃瓦尔德方法，一个软（临界）模式与一个独特的有序波矢量被选择为所有强度的偶极偶极耦合。对于弱偶极子，选择部分有序的三个子晶格自旋结构（P态），其中q_{ord} =（1/2 1/2 1/2）。一个完全有序的四个子晶格自旋结构（F态），q =（000），与P态竞争，并随着温度的降低而成为稳定的结构。我们的结果进行了比较，对其他的理论计算和连接到最近的实验结果Gd 2 Ti 2 O 7进行了讨论。
The kinetic mechanisms underlying bottom-up assembly of colloidal particles have been widely investigated in efforts to control crystallization pathways and to direct growth into targeted superstructures for applications including photonic crystals. Current work builds on recent progress in the development of kinetic theories for crystal growth of body-centered-cubic crystals in systems with short-range inter-particle interactions, accounting for a greater diversity of crystal structures and the role of the longer-ranged interactions and orientational degrees of freedom arising in polar system…
Multiple Field-Induced Phase Transitions in a Geometrically-Frustrated Dipolar Magnet - Gd2Ti2O7
场驱动的相变通常源于塞曼能量与交换或晶场各向异性之间的竞争。在这里，我们提出了一个受挫的烧绿石磁体Gd 2 Ti 2 O 7，其中晶体场分裂是小的偶极能量相比，相图。我们发现零温度临界场和那些从平均场模型得到的很好的协议。在这里，偶极相互作用耦合的真实空间和自旋空间，所以在Gd 2 Ti 2 O 7的转变产生的场诱导的“合作各向异性”，反映了破碎的烧绿石晶格的空间对称性。
We present results of computer simulations at low temperature of a two-dimensional system of dipolar bosons, with dipole moments aligned at an arbitrary angle with respect to the direction perpendicular to the plane. The phase diagram includes a homogeneous superfluid phase, as well as triangular and striped crystalline phases, as the particle density and the tilt angle are varied. In the striped solid, no phase coherence among stripes and consequently no ``supersolid" phase is found, in disagreement with recent theoretical predictions.
We study near-neighbour and dipolar Ising models on a lattice of corner-sharing octahedra. In an extended parameter range of both models, frustration between antiferromagnetism and a spin-ice-like three-in-three-out rule stabilises a Coulomb phase with correlated dipolar and quadrupolar spin textures, both yielding distinctive neutron-scattering signatures. Strong further-neighbour perturbations cause the two components to order independently, resulting in unusual multi-$k$ orders. We propose experimental realisations of our model in rare-earth antiperovskites.
A discrete time crystal (DTC) is a robust phase of driven systems that breaks the discrete time translation symmetry of the driving Hamiltonian. Recent experiments have observed DTC signatures in two distinct systems. Here we show nuclear magnetic resonance (NMR) observations of DTC signatures in a third, strikingly different system: an ordered spatial crystal. We use a novel DTC echo experiment to probe the coherence of the driven system. Finally, we show that interactions during the pulse of the DTC sequence contribute to the decay of the signal, complicating attempts to measure the intrinsi…
Two-dimensional mixtures of dipolar colloidal particles with different dipole moments exhibit extremely rich self-assembly behaviour and are relevant to a wide range of experimental systems, including charged and super-paramagnetic colloids at liquid interfaces. However, there is a gap in our understanding of the crystallization of these systems because existing theories such as integral equation theory and lattice sum methods can only be used to study the high temperature fluid phase and the zero-temperature crystal phase, respectively. In this paper we bridge this gap by developing a density…
We study equilibration and ordering in the classical dipolar kagome Ising antiferromagnet, which we show behaves as a disorder-free fragile spin glass. By identifying an appropriate order parameter, we demonstrate a transition to the ordered state proposed by Chioar et al. [Phys. Rev. B 93, 214410 (2016)] with a 12-site unit cell that breaks time-reversal and sublattice symmetries, and further provide evidence that the nature of the transition is first order. Upon approaching the transition, the spin dynamics slow dramatically. The system readily falls out of equilibrium, overshooting the tran…
The process of crystallization is often understood in terms of the fundamental microstructural elements of the crystallite being formed, such as surface orientation or the presence of defects. Considerably less is known about the role of the liquid structure on the kinetics of crystal growth. Here atomistic simulations and machine learning methods are employed together to demonstrate that the liquid adjacent to solid-liquid interfaces presents significant structural ordering, which effectively reduces the mobility of atoms and slows down the crystallization kinetics. Through detailed studies o…
Active matter exhibits striking behaviour reminiscent of living matter and molecular fluids, and has promising applications in drug delivery or mixing at the micron scale. Active colloidal systems provide important models with simple and controllable interactions amenable to theory and computer simulation. Experimental work is dominated by (quasi) two--dimensional (2d) systems at relatively low concentration, and rather less is known of the 3d case at concentrations pertinent to motility induced phase separation or to mimic morphogenesis. Here we investigate a 3d experimental system of active …
Soft dipolar spin ice physics and the ordered phase of the frustrated Tb2Sn2O7 pyrochlore magnet
从一个微观模型的烧绿石反铁磁体Tb 2Sn 2 O 7，包括晶体场的哈密顿量和角动量之间的相互作用，我们计算了一个有效的赝自旋-1/2哈密顿量Heff$，纳入微扰的有效相互作用的激发晶体场水平的影响。我们得到的半经典基态的Heff和找到一个区域的参数空间与两个在/两个出自旋冰配置上的每个四面体与有序波矢量q=0和自旋倾斜远离当地伊辛轴中发现Tb 2 Sn 2 O 7。这种基态也可以从偶极自旋冰模型中得到，其中伊辛约束被软化。后一种模型的蒙特卡罗模拟揭示了一个区域的相图与自旋冰状冻结和另一个过渡到Tb 2Sn 2 O 7型长程序。我们评论Tb_2Sn_2O_7和令人困惑的自旋液体Tb_2Ti_2O_7之间的差异。
