Chiral Phase Transitions in Topological Phases at Oxide Superlattices

Over the past few years, the existence of materials capable of showing non-trivial topological textures of the polarization pattern has attracted lots of attention triggered by the presumably higher density (due to their smaller sizes) and faster response (phonon frequencies typically range in the THz regime) than their magnetic counterparts making them good candidates for next generation electronic devices. In particular, structures arising in polar oxide nanostructures, due to the delicate interplay between elastic, electrostatic and gradient energies have emerged as a fertile playground to observe novel emergent phenomena and exotic dipole textures. Most studies were performed embedding a prototype ferroelectric (PbTiO₃) in superlattices with a prototype dielectric (SrTiO₃). Theoretical predictions together with an atomically precise synthesis and characterization of materials have lead to the observation of flux-closure [1], vortices [2], skyrmions [3] or merons [4] depending on the periodicity, mechanical and electrostatic boundary conditions. This delicate balance and multiminima potential energy surface suggest a very rich phase diagram with novel phase transitions [5-7] together with exotic physical responses such as negative capacitance [8] or chirality [9]. The involved typical sizes and the will of studying the dependence of the polar texture with external stimuli such as temperature or electric fields require the development of new modelling tools. In this respect, second-principles methods [10-11] are emerging as a valuable solution for such a problems.<br/>In this talk, I shall describe the path of phase transitions occurring in the polar vortex chiral structures upon increasing temperature and characterize them with appropriate order parameters [12]. Although the overall polarization is zero throughout the process, the system suffers a first-order first transition from a chiral-crystal to a chiral-liquid of polarization vortices upon increasing temperature. Finally, a second order phase transition is found at higher temperatures where the mirror symmetry is restored and the system becomes a disordered achiral-liquid. Moreover, in the ordered polar vortex phase, under suitable mechanical (epitaxial strain) and electrostatic boundary conditions (built in potential), a deterministic and reversible control of chirality over mesoscale regions have been found under the application of homogeneous electric fields [13]. Second-principles simulations together with second-harmonic generation based circular-dichroism measurements have shown the existence of hysteresis between left- and right-handed chiral states under the application of such electric fields.<br/>We acknowledge the continuous feedback and support of the experimental groups leaded by Prof. R. Ramesh, L. W. Martin and S. Salahuddin from University of California-BerKeley and by D. A. Muller from University of Cornell. The authors acknowledge financial support from grant PGC2018-096955-B-C41 funded by MCIN/AEI/10.13039/501100011033 and by “ERDF a way of making Europe”. FGO acknowledge support from grant FPU18/04661 funded by MCIN/AEI/10.13039/501100011033.<br/>[1] Y. L. Tang et al. Science <b>348</b>, 547 (2015)<br/>[2] A. K. Yadav et al. Nature <b>530</b>, 198 (2016)<br/>[3] S. Das et al. Nature <b>568</b>, 368 (2019)<br/>[4] Y. J. Wang et al. Nat. Mater. <b>19</b>, 881 (2020)<br/>[5] Y. Nahas et al. Nature <b>577</b>, 47 (2020)<br/>[6] Y. Nahas et al, Phys. Rev. Lett. <b>119</b>, 117601 (2017)<br/>[7] L. Zhou et al. Adv. Funct. Mater. 2111392 (2022)<br/>[8] A. K. Yadav et al. Nature <b>565</b>, 468-471 (2019)<br/>[9] P. Shafer et al. Proc. of the Natl. Acad. Sci. U.S.A., <b>115</b>, 915-920 (2018)<br/>[10] J. Wojdel et al. J. of Phys.: Condens. Matter, <b>25</b>, 305401 (2013)<br/>[11] P. García-Fernández et al. Phys. Rev. B, <b>93</b>, 195137 (2016)<br/>[12] F. Gómez-Ortiz et al. Phys. Rev. B. <b>105, </b> L220103 (2022)<br/>[13] P. Behera et al. Sci. Adv. 8, eabj8030 (2022)