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arXiv 2408.02789cond-mat.str-el
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无限范围横场Ising模型中的量子临界性

Quantum Criticality in the infinite-range Transverse Field Ising Model

Nicholas Curro, Kaeshav Danesh, Rajiv R. P. Singh

AI总结:

本文通过数值对角化研究无限范围横场Ising模型的量子临界性,发现其有限温度相图类似平均场理论,但激发能隙在临界点趋于零且存在长程多体纠缠,低能量子涨落可持续至交换常数数量级的温度。

摘要:

我们研究了无限范围横场Ising模型(Transverse-Field Ising Model)中的量子临界性。我们发现其与众所周知的单格点平均场理论存在细微差异,特别是在能隙、纠缠和量子临界性方面。这些计算基于对包含多达数千个自旋的哈密顿量进行数值对角化。这得益于模型对称性的增强,这些对称性将哈密顿量划分为许多块对角扇区。有限温度相图和热容的特征性跃变与平均场理论中的行为非常相似。然而,平均场理论中的激发总是有能隙的,而无限范围模型中的激发能隙在从顺磁侧和铁磁侧趋近量子临界点时均趋于零。此外,与平均场理论相反,在量子临界点处量子Fisher信息变得很大,这意味着存在长程多体纠缠。我们发现温度的主要作用是将统计权重从一个守恒扇区转移到另一个扇区。然而,每个扇区中的低能激发仅在量子临界点附近出现,这意味着低能量子涨落只能在量子临界场附近产生,并且可以持续到温度达到交换常数数量级。

英文摘要:

We study quantum criticality in the infinite range Transverse-Field Ising Model. We find subtle differences with respect to the well-known single-site mean-field theory, especially in terms of gap, entanglement and quantum criticality. The calculations are based on numerical diagonalization of Hamiltonians with up to a few thousand spins. This is made possible by the enhanced symmetries of the model, which divide the Hamiltonian into many block-diagonal sectors. The finite temperature phase diagram and the characteristic jump in heat capacity closely resemble the behavior in mean-field theory. However, unlike mean-field theory where excitations are always gapped, the excitation gap in the infinite range model goes to zero from both the paramagnetic side and from the ferromagnetic side on approach to the quantum critical point. Also, contrary to mean-field theory, at the quantum critical point the Quantum Fisher Information becomes large, implying long-range multi-partite entanglement. We find that the main role of temperature is to shift statistical weights from one conserved sector to another. However, low energy excitations in each sector arise only near the quantum critical point implying that low energy quantum fluctuations can arise only in the vicinity of the quantum critical field where they can persist up to temperatures of order the exchange constant.

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