Topological excitonic insulators combine topological edge states and spontaneous exciton condensation, with dual functionality of topological insulators and excitonic insulators. Yet, they are very rare and little is known about their formation. In this work, we find that a mechanism dubbed as parity frustration prevents excitonic instability in usual topological insulators, and those whose band inversion is independent of spin-orbit coupling are possible candidates. We verify this by first-principles calculations on monolayer transition-metal carbides, which show a robust thermal-equilibrium exciton condensation, being sufficient for topological applications at room temperature. Such a state can be identified by angle-resolved photoemission spectroscopy and transport measurement. Our work provides not only a guide for finding more topological excitonic insulators, but also a new platform for studying the interplay between nontrivial band topology and quantum many-body effects.

Based on the first-principles calculations coupled with the Bethe-Salpeter equation, the topological exciton density wave is investigated in two-dimensional monolayer WSe_{2}. We find that the topological excitonic insulator phase can exist in monolayer WSe_{2}, and it is robust against in-plane strain. In this system, the energy minimum of exciton bands is shifted to a finite in-plane momentum, forming a Fulde-Ferrell-Larkin-Ovchinnikov-like state. Using the Gross-Pitaevskii equations, stripe-patterned exciton density waves with a nonzero velocity emerge in monolayer WSe_{2}. Our findings pave a new way for exploring the interplay between electron correlation and nontrivial topology.

LaFe₁₂B₆ is a rare itinerant-electron metamagnet, and substitution of rare-earth elements can significantly influence its field-induced phase transition and magnetocaloric effects. However, how such elemental substitution modulates its magnetoelectrical transport properties remains unclear. Here, we report electrical transport measurements on La_1−xPr_xFe₁₂B₆ (x = 0.05, 0.15) compounds between 3 and 150 K in fields up to 90 kOe using the four-wire method on characterized polycrystals containing 9.5 and 3.8 wt.% Fe₂B, respectively. Samples were thermally reset and demagnetized prior to each isothermal test. At 3 K, magnetoresistance (MR) reproduces the following magnetization features: two abrupt MR drops at 41 and 67 kOe for x = 0.05. Below the first critical field, the MR becomes positive (+20% for x = 0.05, +5% for x = 0.15) and then drops abruptly. The steps broaden and merge into a smooth response as the temperature rises. The largest negative MR values measured, −78% (x = 0.05, 40 K) and −68% (x = 0.15, 70 K), lie close to the corresponding magnetic transitions. The incomplete resistivity recovery observed after field removal—most pronounced at low temperature for both compositions and vanishing near 120 K—reveals a kinetically arrested transformation. By combining magneto-transport with magnetization measurements, the giant negative magnetoresistance (up to −78%) of La_1−xPr_xFe₁₂B₆ closely tracks the stepwise metamagnetic transition, via field-suppressed spin-disorder scattering, with the effect extending above T_C for the Pr-richer composition.