In three dimensions, quasi-one-dimensional (Q1D) transport has traditionally been associated with systems featuring a Q1D chain structure. Here, based on first-principle calculations, we go beyond this understanding to show that the Q1D transport can also be realized in certain three-dimensional (3D) altermagnetic (AM) metals with a topological nodal net in momentum space but lacking Q1D chain structure in real space, including the existing compounds 𝛽−Fe2⁢(PO4)⁢O, Co2⁢(PO4)⁢O, and LiTi2⁢O4. These materials exhibit an AM ground state and feature an ideal crossed 𝑍3 Weyl nodal line in each spin channel around Fermi level, formed by three straight and flat nodal lines traversing the entire Brillouin zone. These nodal lines eventually lead to an AM 𝑍3 nodal net. Surprisingly, the electronic conductivity 𝜎𝑥⁢𝑥 in these topological nodal net metals is dozens of times larger than 𝜎𝑦⁢𝑦 and 𝜎𝑧⁢𝑧 in the up-spin channel, while 𝜎𝑦⁢𝑦 dominates transport in the down-spin channel. This suggests a distinctive Q1D transport signature in each spin channel, and the principal moving directions for the two spin channels are orthogonal, resulting in Q1D direction-dependent spin transport. This novel phenomenon cannot be found in both conventional 3D bulk materials and Q1D chain materials. In particular, the Q1D spin transport gradually disappears as the Fermi energy moves away from the nodal net, further confirming its topological origin. Our Letter not only enhances the comprehension of topological physics in altermagnets but also opens a new direction for the exploration of topological spintronics.

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.146602