Liu Jinyu, Subramanyan Varsha, Welser Robert, McSorley Timothy, Ho Triet, Graf David, Pettes Michael T, Saxena Avadh, Winter Laurel E, Lin Shi-Zeng, Jauregui Luis A
University of California, Department of Physics and Astronomy, Irvine, California 92697, USA.
Los Alamos National Laboratory, Theoretical Division T-4, Los Alamos, New Mexico 87545, USA.
Phys Rev Lett. 2025 Jul 25;135(4):046601. doi: 10.1103/bj2n-4k2w.
More than 50 years ago, excitonic insulators formed by the pairing of electrons and holes due to Coulomb interactions were first predicted [A. N. Kozlov and L. A. Maksimov, Sov. J. Exp. Theor. Phys. 21, 790 (1965); L. V. Keldysh and Y. V. Kopaev, Sov. Phys. Solid State 6, 2219 (1965)SPSSA70038-5654; D. Jérome, T. M. Rice, and W. Kohn, Phys. Rev. 158, 462 (1967)PHRVAO0031-899X10.1103/PhysRev.158.462]. Since then, excitonic insulators have been observed in various classes of materials, including quantum Hall bilayers, graphite, transition metal chalcogenides, and more recently in moiré superlattices. In these excitonic insulators, an electron and a hole with the same spin bind together, and the resulting exciton is a spin singlet. Here, we report the experimental observation of a spin-triplet excitonic insulator in the ultra-quantum limit of a three-dimensional topological material HfTe_{5}. We observe that the spin-polarized zeroth Landau bands dispersing along the field direction cross each other beyond a characteristic magnetic field in HfTe_{5}, forming the one-dimensional Weyl mode. Transport measurements reveal the emergence of a gap of about 250 μeV when the field surpasses a critical threshold. By performing the material-specific modeling, we identify this gap as a consequence of a spin-triplet exciton formation, where electrons and holes with opposite spin form bound states, and the translational symmetry is preserved. The system reaches charge neutrality following the gap opening, as evidenced by the zero Hall conductivity over a wide magnetic field range (10-72 T). Our finding of the spin-triplet excitonic insulator paves the way for studying novel spin transport including spin superfluidity, spin Josephson currents, and Coulomb drag of spin currents in analogy to the transport properties associated with the layer pseudospin in quantum Hall bilayers.
50多年前，人们首次预言了由于库仑相互作用使电子和空穴配对而形成的激子绝缘体[A. N. 科兹洛夫和L. A. 马克西莫夫，《苏联实验与理论物理杂志》21, 790 (1965年); L. V. 凯尔迪什和Y. V. 科帕耶夫，《苏联物理固态》6, 2219 (1965年)SPSSA70038-5654; D. 热罗姆、T. M. 赖斯和W. 科恩，《物理评论》158, 462 (1967年)PHRVAO0031-899X10.1103/PhysRev.158.462]。从那时起，在各类材料中都观测到了激子绝缘体，包括量子霍尔双层、石墨、过渡金属硫族化合物，以及最近在莫尔超晶格中。在这些激子绝缘体中，具有相同自旋的一个电子和一个空穴结合在一起，形成的激子是自旋单重态。在此，我们报告在三维拓扑材料HfTe₅的超量子极限下对自旋三重态激子绝缘体的实验观测。我们观察到，在HfTe₅中，沿场方向色散的自旋极化零朗道带在超过一个特征磁场时相互交叉，形成一维外尔模式。输运测量表明，当场超过一个临界阈值时，会出现约250 μeV的能隙。通过进行针对该材料的建模，我们确定这个能隙是自旋三重态激子形成的结果，其中具有相反自旋的电子和空穴形成束缚态，且平移对称性得以保留。在能隙打开后，系统达到电荷中性，这在很宽的磁场范围(10 - 72 T)内零霍尔电导率得到了证明。我们对自旋三重态激子绝缘体的发现为研究新型自旋输运铺平了道路，包括自旋超流、自旋约瑟夫森电流，以及类似于与量子霍尔双层中的层赝自旋相关的输运性质的自旋电流的库仑拖拽。
