Altermagnetic Even-Odd Effects in CsV${}_2$Te${}_2$O Josephson Junctions

The interplay between conventional superconductivity and unconventional magnetism offers an exciting platform for realizing exotic superconducting phenomena. Here, we investigate Josephson effects in planar and vertical junctions based on CsV${}_2$Te${}_2$O-family materials, which host hidden $d$-wave altermagnetism with G-type antiferromagnetic order. In monolayer-based planar junctions, the quasi-1D, nearly flat, spin-polarized bands of the altermagnet, when coupled to $s$-wave superconductors, produce a $\mathit{\text{fully}}$ spin-polarized supercurrent with strong directional anisotropy – a spin-selective Josephson effect. In multilayers, we uncover an $\mathit{\text{altermagnetic}}\mathit{\text{even-odd}}\mathit{\text{effect}}$: spin-polarized supercurrents persist only in odd-layer planar junctions but cancel exactly in even layers. Thus, layer parity acts as a switch for spin-polarized supercurrent. In vertical junctions, odd-layer barriers enhance equal-spin triplet transport while even layers favor opposite-spin transport, yielding a robust period-two oscillation in the total supercurrent with layer number. These layer-parity-dependent responses represent a general even-odd effect in hidden altermagnets, applicable to diverse magnetic and transport phenomena.