近日，美国加州理工学院的David Hsieh及其研究小组取得一项新进展。经过不懈努力，他们对光掺杂反铁磁莫特绝缘体中的哈伯德激子流体进行了研究。相关研究成果已于2023年9月14日在国际知名学术期刊《自然—物理学》上发表。
本文报道了在反铁磁莫特绝缘体Sr2IrO4中，利用超快太赫兹电导率瞬态形成哈伯德激子流体的研究。光激发后，研究人员观察到了从德鲁德金属响应到绝缘响应的快速光谱权重转移。后者的特征是由激子内跃迁产生的有限能量峰，其分配由扩展的哈伯德模型的数值模拟证实。峰的寿命很短（大约1皮秒），并且随着莫特间隙的大小呈指数级增长，这意味着与磁振子模式的耦合非常强。
据悉，未掺杂的反铁磁莫特绝缘体自然地具有每个晶格格位一个载流子。当掺杂额外载流子时，它们对自旋涨落介导的库珀对以及其他非常规类型的电荷、自旋和轨道电流有序都不稳定。光激发可以产生空穴(空穴子)和双占据(双占子)格位形式的载流子，这些载流子也可能表现出电荷不稳定性。有证据表明，反铁磁相关性增强了空穴子和双占子之间的相互吸引作用，然后形成被称为哈伯德激子的束缚对，这些束缚对可能会自组织成绝缘的哈伯德激子流体。然而，这种不平衡现象还没有被实验检测到。
附：英文原文
Title: A Hubbard exciton fluid in a photo-doped antiferromagnetic Mott insulator
Author: Mehio, Omar, Li, Xinwei, Ning, Honglie, Lenari, Zala, Han, Yuchen, Buchhold, Michael, Porter, Zach, Laurita, Nicholas J., Wilson, Stephen D., Hsieh, David
Issue&Volume: 2023-09-14
Abstract: The undoped antiferromagnetic Mott insulator naturally has one charge carrier per lattice site. When it is doped with additional carriers, they are unstable to spin-fluctuation-mediated Cooper pairing as well as other unconventional types of charge, spin and orbital current ordering. Photo-excitation can produce charge carriers in the form of empty (holons) and doubly occupied (doublons) sites that may also exhibit charge instabilities. There is evidence that antiferromagnetic correlations enhance attractive interactions between holons and doublons, which can then form bound pairs known as Hubbard excitons, and that these might self-organize into an insulating Hubbard exciton fluid. However, this out-of-equilibrium phenomenon has not been experimentally detected. Here we report the transient formation of a Hubbard exciton fluid in the antiferromagnetic Mott insulator Sr2IrO4 using ultrafast terahertz conductivity. Following photo-excitation, we observe rapid spectral-weight transfer from a Drude metallic response to an insulating response. The latter is characterized by a finite-energy peak originating from intraexcitonic transitions, whose assignment is corroborated by our numerical simulations of an extended Hubbard model. The lifetime of the peak is short (approximately one picosecond) and scales exponentially with the Mott gap size, implying extremely strong coupling to magnon modes.
DOI: 10.1038/s41567-023-02204-2
Source: https://www.nature.com/articles/s41567-023-02204-2