金刚石装饰方格上自旋-1/2 Heisenberg模型宏观简并四聚体-二聚体相的热力学性质
Thermodynamic properties of the macroscopically degenerate tetramer-dimer phase of the spin-1/2 Heisenberg model on the diamond-decorated square lattice
AI总结：
本文结合精确对角化、无符号问题量子Monte Carlo及有效格气模型，研究金刚石装饰方格自旋-1/2 Heisenberg反铁磁体的二聚体-四聚体相，揭示其宏观简并、相界附近反常热力学及绝热退磁冷却效应。
摘要：
在磁场存在时，金刚石装饰方格上的自旋-1/2 Heisenberg反铁磁体表现出多种量子相，除平凡的完全饱和态外，还包括Lieb-Mattis亚铁磁相、二聚体-四聚体相、单体-二聚体相和自旋倾斜相。本文采用若干互补的解析与数值方法研究该模型的热力学性质，例如至多40个自旋体系的精确对角化、有效的单体-二聚体描述、至多180个自旋且无符号问题的量子Monte Carlo模拟，以及退耦近似。我们特别关注有利于二聚体-四聚体相的参数区域。该基态可由方格上的经典硬二聚体模型表示，并且即使在磁场下仍保持宏观简并。然而，要描述宏观简并的二聚体-四聚体相与非简并的单体-二聚体相边界附近的低温热力学，需要一个扩展的经典单体-二聚体格气模型。本文详细研究了二聚体-四聚体相附近出现的反常热力学性质。在绝热退磁过程中，我们检测到增强的磁热效应；若体系在零场下达到二聚体-四聚体基态，该效应可促进向绝对零度的高效冷却。
英文摘要：
The spin-1/2 Heisenberg antiferromagnet on the diamond-decorated square lattice in the presence of a magnetic field displays various quantum phases including the Lieb-Mattis ferrimagnetic, dimer-tetramer, monomer-dimer, and spin-canted phases, in addition to the trivial fully saturated state. Thermodynamic properties of this model are investigated using several complementary analytical and numerical methods such as exact diagonalization up to the systems of 40 spins, an effective monomer-dimer description, sign-problem-free quantum Monte Carlo simulations for up to 180 spins, and a decoupling approximation. Our particular attention is focused on the parameter region favoring the dimer-tetramer phase. This ground state can be represented by a classical hard-dimer model on the square lattice and retains a macroscopic degeneracy even under a magnetic field. However, the description of the low-temperature thermodynamics close to the boundary between the macroscopically degenerate dimer-tetramer and the non-degenerate monomer-dimer phases requires an extended classical monomer-dimer lattice-gas model. Anomalous thermodynamic properties emerging in the vicinity of the dimer-tetramer phase are studied in detail. Under the adiabatic demagnetization we detect an enhanced magnetocaloric effect promoting an efficient cooling to absolute zero temperature, provided that the system reaches the dimer-tetramer ground state at zero field.
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