A strong correlation between magnetic interaction and topological symmetries leads to unconventional magneto-transport behavior. Weyl fermions induce topologically protected spinmomentum locking, which is closely related to spin-wave gap formation in magnetic crystals. Ferromagnetic SrRuO3, regarded as a strong candidate for Weyl semimetal, inherently possesses a nonzero spin-wave gap owing to its strong magnetic anisotropy. In this paper, we propose a method to control the spin-wave dynamics by nanolayer designing of the SrRuO3/SrTiO3 superlattices. In particular, the six-unit-cell-thick SrRuO3 layers within the superlattices undergo a phase transition in crystalline symmetry from orthorhombic to tetragonal, as the thickness of the SrTiO3 layers is modulated with atomic-scale precision. Consequently, the magnetic anisotropy, anomalous Hall conductivity, and spin-wave gap could be systematically manipulated. Such customization of magnetic anisotropy via nanoscale heterostructuring offers a novel control knob to tailor the magnon excitation energy for future spintronic applications, including magnon waveguides and filters. Our nanolayer approach unveils the important correlation between the tunable lattice degrees of freedom and spin dynamics in topologically non-trivial magnetic materials.

Excitonic-insulating phases can be realized in semimetals or narrow bandgap semiconductors where the bandgap is smaller than the excitonbinding energy (E ex ). [2,3] Since the theoretical suggestion of excitonic insulator in 1960s, [1][2][3] recent experimental studies have successfully demonstrated evidence of the excitonic-insulating phases in real crystal systems. [7][8][9][10][11][12][13][14][15][16] Among them, Ta 2 NiSe 5 has been investigated intensively because of its relatively higher excitonic transition temperature (T c ) at ≈326 K. [13,14] Ta 2 NiSe 5 consists of three layers with alternating chemically bonded Ta and Ni atoms sandwiched by two Se layers that are further stacked by van der Waals interaction along the (020) direction. [13][14][15] An orthorhombic structure, space group of Cmcm, with a direct bandgap at Γ point in the Brillouin zone is energetically stable at high temperature above T c . [11][12][13][14][15] The exciton-binding energy (E ex ) is larger than the bandgap of orthorhombic phase. The excitonic-insulating phase transition occurs with structural transformation into monoclinic (space group C2/c) when T is lower than T c . [15] The excitonicinsulating transition in Ta 2 NiSe 5 is mainly driven by strong Coulomb interaction between electron and hole and, moreover, does not involve charge density wave (CDW), [11][12][13][14][15] which is well contrasted to emergence of CDW in 1T-TiSe 2 excitonic Excitonic insulators exhibit intriguing quantum phases that further attract numerous interests in engineering the electrical and optical properties of Ta 2 NiSe 5 . However, tuning the electronic properties such as spin-orbit coupling strength and orbital repulsion via pressure in Ta 2 NiSe 5 are always accompanied with electron-hole pair breaking, which is a bottleneck for further applications. Here, the robust excitonic-insulating states invariant with electron-doping concentrations in Ta 2 NiSe 5 are demonstrated. The electron doping is conducted by substituting Cu into Ni site (Ta 2 Ni 1-x Cu x Se 5 ). The majority carrier of pristine sample is a hole-type and is converted to electrontype with a doping concentration over x = 0.01, whose carrier density can be controlled by varying the Cu concentration. The excitonic transition temperature (T c ) does not significantly alter with electron-doping concentrations, which is stark contrast with the declining T c as the hole-type dopant of Fe or Co increases. The optical conductivity data also demonstrate the invariant excitonic-insulating states in Cu-doped Ta 2 NiSe 5 . The findings of invariant excitonic-insulating states in n-type Cu-substituted Ta 2 NiSe 5 can be utilized for further electronic device applications by using excitons.