Interface controlled Berry phase and anisotropic spin-charge conversion in altermagnet-topological insulator bilayers

We propose an altermagnet-topological insulator bilayer as a platform to engineer Berry phase driven spin-charge responses using an interfacial buffer layer. Using a momentum-space lattice model and linear-response theory, we investigate a $d$-wave altermagnet coupled to a topological insulator and highlight the crucial role of spin-flip tunneling in shaping its electronic and transport properties. Interfacial hybridization strongly modifies the band structure, leading to anisotropic Rashba-Edelstein and Hall responses. The spin-flip component of the coupling induces an inverse $d$-wave spin texture in the altermagnetic bands, signaling the onset of an altermagnetic topological phase. This coupling also renders the Rashba-Edelstein effect strongly in-plane anisotropic, enhancing the transverse response relative to ferromagnetic or antiferromagnetic analogues. These results establish interfacial spin-flip tunneling as a practical control knob for direction-sensitive, stray-field-free spin-charge conversion in correlated topological heterostructures.