Atomistic Engineering of Phonons in Functional Oxide Heterostructures

Real-time dynamics of angular momentum transfer from spin to acoustic chiral phonon in oxide heterostructures

Chiral phonons have recently been explored as a novel degree of freedom in quantum materials. The angular momentum carried by these quasiparticles is generated by the breaking of chiral degeneracy of phonons, owing to the chiral lattice structure or the rotational motion of ions of the material. In ferromagnets, a mechanism for generating non-equilibrium chiral phonons has been suggested, but their temporal evolution, which obeys Bose-Einstein statistics, remains unclear. Here we report the real-time dynamics of thermalized chiral phonons in an artificial superlattice composed of ferromagnetic metallic SrRuO3 and non-magnetic insulating SrTiO3. Following the photo-induced ultrafast demagnetization in the SrRuO3 layer, we observed the appearance of a magneto-optic signal in the superlattice, which is absent in the SrRuO3 single films. This magneto-optic signal exhibits thermally driven dynamic properties and a clear correlation with the thickness of the non-magnetic SrTiO3 layer, implying that it originates from thermalized chiral phonons. We use numerical calculations considering the magneto-elastic coupling in SrRuO3 to validate our experimental observations and the angular momentum transfer mechanism between the lattice and spin systems in ferromagnetic systems and also to the non-magnetic system.

Exotic Magnetic Anisotropy Near Digitized Dimensional Mott Boundary

The magnetic anisotropy of low-dimensional Mott systems exhibits unexpected magnetotransport behavior useful for spin-based quantum electronics. Yet, the anisotropy of natural materials is inherently determined by the crystal structure, highly limiting its engineering. The magnetic anisotropy modulation near a digitized dimensional Mott boundary in artificial superlattices composed of a correlated magnetic monolayer SrRuO3 and nonmagnetic SrTiO3 , is demonstrated. The magnetic anisotropy is initially engineered by modulating the interlayer coupling strength between the magnetic monolayers. Interestingly, when the interlayer coupling strength is maximized, a nearly degenerate state is realized, in which the anisotropic magnetotransport is strongly influenced by both the thermal and magnetic energy scales. The results offer a new digitized control for magnetic anisotropy in low-dimensional Mott systems, inspiring promising integration of Mottronics and spintronics.

Giant Enhancement of Electron-Phonon Coupling in Dimensionality-Controlled SrRuO3 Heterostructures

Electrons in crystals interact closely with quantized lattice degree of freedom, determining fundamental electrodynamic behaviors and versatile correlated functionalities. However, the strength of the electron-phonon interaction is so far determined as an intrinsic value of a given material, restricting the development of potential electronic and phononic applications employing the tunable coupling strength. Here, it is demonstrated that the electron-phonon coupling in SrRuO₃ can be largely controlled by multiple intuitive tuning knobs available in synthetic crystals. The coupling strength of quasi-2D SrRuO₃ is enhanced by ≈300-fold compared with that of bulk SrRuO₃ . This enormous enhancement is attributed to the non-local nature of the electron-phonon coupling within the well-defined synthetic atomic network, which becomes dominant in the limit of the 2D electronic state. These results provide valuable opportunities for engineering the electron-phonon coupling, leading to a deeper understanding of the strongly coupled charge and lattice dynamics in quantum materials.

Atomic-Scale Modulation of Synthetic Magnetic Order in Oxide Superlattices

Atomic-scale precision control of magnetic interactions facilitates a synthetic spin order useful for spintronics, including advanced memory and quantum logic devices. Conventional modulation of synthetic spin order has been limited to metallic heterostructures that exploit Ruderman-Kittel-Kasuya-Yosida interaction through a nonmagnetic metallic spacer; however, they face issues arising from Joule heating and/or electric breakdown. The practical realization and observation of a synthetic spin order across a nonmagnetic insulating spacer will lead to the development of spin-related devices with a completely different concept. Herein, the atomic-scale modulation of the synthetic spiral spin order in oxide superlattices composed of ferromagnetic metal and nonmagnetic insulator layers is reported. The atomically controlled superlattice exhibits an oscillatory magnetic behavior, representing the existence of a spiral spin structure. Depth-sensitive polarized neutron reflectometry evidences modulated spiral spin structures as a function of the nonmagnetic insulator layer thickness. Atomic-scale customization of the spin state can move the field one step further to actual spintronic applications.