Marc Zajac1,Tao Zhou1,Sujit Das2,Tiannan Yang3,Yue Cao1,Burak Guzelturk1,Vladimir Stoica3,Mathew Cherukara1,John Freeland1,Venkatraman Gopalan3,Ramamoorthy Ramesh4,Lane Martin4,Long-Qing Chen3,Martin Holt1,Stephan Hruszkewycz1,Haidan Wen1
Argonne National Laboratory1,Indian Institute of Science2,The Pennsylvania State University3,University of California, Berkeley4
Marc Zajac1,Tao Zhou1,Sujit Das2,Tiannan Yang3,Yue Cao1,Burak Guzelturk1,Vladimir Stoica3,Mathew Cherukara1,John Freeland1,Venkatraman Gopalan3,Ramamoorthy Ramesh4,Lane Martin4,Long-Qing Chen3,Martin Holt1,Stephan Hruszkewycz1,Haidan Wen1
Argonne National Laboratory1,Indian Institute of Science2,The Pennsylvania State University3,University of California, Berkeley4
Ultrafast light-matter interactions can synthesize novel metastable states of matter [1,2]. For example, when nanoscale phase mixtures of polar vortices and a<sub>1</sub>/a<sub>2</sub> ferroelectric domains in (PbTiO<sub>3</sub>)/(SrTiO<sub>3</sub>) (PTO/STO) superlattices are driven by a 400 nm wavelength optical excitation with sufficient fluence, a metastable 3D ordered phase or “supercrystal” is created [2]. It remains unknown how this photoinduced metastable state nucleates, grows, and is affected by the initial heterogeneity in real space. To gain microscopic insight, we used the recently enabled laser-pump, nano x-ray diffraction imaging capability with 25 nm spatial resolution to perform in-situ hard x-ray diffraction with controlled optical excitation [3].<br/>We show that an intermediate phase characterized by in-plane ordered stripe domains forms when excited by an optical excitation with a wavelength of 343 nm at a repetition rate of 54 kHz. The nanoscale x-ray diffraction imaging reveals that this intermediate phase nucleates within the a<sub>1</sub>/a<sub>2</sub> ferroelectric domains after five seconds of optical exposure at a fluence of 3 mJ/cm<sup>2</sup>. Upon longer exposure times, the intermediate phase expands into and consumes the polar vortex domains, and after about 1000 seconds of exposure, finally transforms into a 3D nanostructure consistent with the previously reported supercrystal phase. Because 343 nm light excites both PTO and STO layers while 400 nm light only excites the PTO layer, the distinct intermediate phase can arise from different charge screening dynamics at the superlattice interface. Our work of imaging nanoscale phase evolution shows how new light-induced ferroelectric phases with unique optical and electronic properties form and transform in real space. This will lead to new ways of optically controlling phase transformations in emerging ferroelectric technologies.<br/><br/><b>References:</b><br/><br/>[1] V.A. Stoica, N. Laanait, C. Dai, Z. Hong, Y. Yuan, Z. Zhang, S. Lei, M.R. McCarter, A. Yadav, A.R. Damodaran, S. Das, G.A. Stone, J. Karapetrova, D.A. Walko, X. Zhang, L.W. Martin, R. Ramesh, L.Q. Chen, H. Wen, V. Gopalan, J.W. Freeland, <i>Optical creation of a supercrystal with three-dimensional nanoscale periodicity</i>, Nat. Mater. <b>18</b> (2019) 377–383.<br/>[2] D. N. Basov, R. D. Averitt, D. van der Marel, M. Dressel, and K. Haule, <i>Electrodynamics of Correlated Electron Materials</i>, Rev. Mod. Phys. <b>83</b>, 471 (2011).<br/>[3] Y. Ahn, M. J. Cherukara, Z. Cai, M. Bartlein, T. Zhou, A. DiChiara, D. Walko, M. Holt, E. Fullerton, P. Evans, and H. Wen, <i>Nanoscale Ultrafast Imaging of Photoinduced Structural Phase Transition in FeRh by X-Ray Diffraction Microscopy</i>, PNAS, <b>119</b>, 19 (2022).