Spin-Triplet Excitonic Insulator in the Ultraquantum Limit of HfTe₅

Excitonic insulators are the condensate of the paired electrons and holes due to Coulomb interactions. They have been observed in various classes of materials, including quantum Hall bilayers, graphite, transition metal chalcogenides, and more recently in moire superlattices. In these excitonic insulators, an electron and a hole with the same (electron) spin bind together, and the resulting exciton is a spin singlet. In this talk, we show the experimental observation of a spin triplet excitonic insulator in the ultra-quantum limit of a three-dimensional topological material HfTe₅. We observe that the spin-polarized zeroth Landau bands dispersing along the field direction cross each other beyond a characteristic magnetic field in HfTe₅, 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 (electron) 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 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.