In Situ Observation of (PbTiO₃)₁₄/(SrTiO₃)₁₆[hl1] Superlattice Structural Transition Under Electric Field

(PbTiO₃)_m/(SrTiO₃)_n (PTO_m/STO_n) superlattice thin film heterostructures exhibit exotic nanoscale domain configurations such as polar skyrmions. A key feature of these nanodomains is that external stimulus such as an electric field can perturb the energy balance consequently resulting in polarization modulation. In-situ X-ray diffraction with an application of electric field can provide the insight into the atomic scale mechanisms of field-driven domain configuration dynamics of polar skyrmions. From a thermodynamic perspective, skyrmions under positive bias are expected to shrink with more coherent distribution, while skyrmions under negative bias expand and coalesce leading to continuous c-domain matrix randomly embedded by stripe-like skyrmions.^[1] However, the field-driven domain evolution of polar skyrmions have not been experimentally explored yet. In this study, we probe the evolution of polarization configuration in polar skyrmions in (PbTiO₃)₁₄/(SrTiO₃)₁₆ (PTO₁₄/STO₁₆) superlattice thin film by measuring the x-ray reflections under the electric field.<br/><br/>To figure out the electric field induced polar skyrmion evolution paths of PTO₁₄/STO₁₆ superlattice system, quantitative analysis of polar skyrmions and c-domain structures under electric field is necessary. In-situ time-resolved x-ray microdiffraction enables the simultaneous probing of in-plane domain structure and out-of-plane superlattice structure while applying electric field. A spatial resolution of few tens of micrometers achieved by focused x-ray beam enables probing the structural evolution below the electrode where electric field is directly applied. Based on the scattering intensity of domain and superlattice structures provided from reciprocal space maps, the domain configuration of polar skyrmions under electric field can be investigated.<br/><br/>We conducted in-situ time-resolved x-ray microdiffraction at 9C beamline of Pohang Accelerator Laboratory. Superlattice peaks from out-of-plane ordering and satellite peaks from in-plane ordering with Q_y ordering of ~0.1 Å^-1 were observed simultaneously under electric field. Superlattice peaks showed peak shift about 0.3% when ±15 V is applied which corresponds to previous reports, while integrated intensity shows asymmetry with the polarity of the electric field. ^[1] The position of satellite peaks showed no shift in Q_z direction which indicates no in-plane ordering change under electric field while integrated intensity is decreased to 30% under15 V compared to pristine state, while intensity decreased 15% under -15 V, indicating asymmetric behavior of polar skyrmions with the polarity of the electric field.<br/><br/>In conclusion, we could observe asymmetric intensity behavior of both superlattice and satellite peaks with the polarity of electric field which relates to the size of domain configuration under electric field. We could propose new polar skyrmion evolution model related to polar skyrmion domains that the polar skyrmions maintain its configuration after applying higher voltage than the coercive voltage. By unveiling field-driven domain configuration of polar skyrmions, this study provides new insight into structural manipulation of superlattice.<br/><br/><br/>[1] S. Wang <i>et al.,</i> Nat Commun 15, 1374 (2024)