The exceptional sensitivity of the functional electronic and magnetic properties of transition metal oxides to structural distortions has paved new ways for the design and control of their phase state via strain engineering in heterostructures. An atomically-thin oxide film on a substrate can have properties very different from its bulk form due to the mechanical strain produced by lattice mismatch at the interface between the substrate and the film.
Our group explored the possibility to dynamically control the functional properties of a thin NdNiO3 film by modifying the atomic structure of the LaAlO3 substrate with light. Neodymium nickelate is an insulating antiferromagnetic at cryogenic temperatures, becoming a paramagnetic metal above 200 Kelvin.
In a first experiment, we selectively excited a vibrational mode in the LaAlO3 substrate by mid-infrared femtosecond pulses and monitored the temporal evolution of the NdNiO3 transport properties by probing the reflectivity with time-delayed THz pulses . We observed a five-order-of magnitude increase of the nickelate conductivity following this excitation. Additional near-infrared probe experiments confirmed that this ultrafast insulator-metal transition is indeed driven by the substrate excitation.
In a following experiment, we studied the effect this substrate excitation has on the magnetic properties of the nickelate film . These changes were captured with high spatial and temporal resolution using femtosecond resonant soft x-ray diffraction at the LCLS free-electron laser. Strikingly, this diffraction experiment showed that the melting of the antiferromagnetic order in the nickelate starts locally at the substrate interface and propagates, comparable to a wave, into the NdNiO3 film. The high, likely supersonic speed, at which this wave front propagates, indicates that these dynamics are driven by local changes of the electronic structure at the interface.
Figure left: A short mid-infrared laser pulse triggers vibrations in the substrate (LaAlO3, front). This atomic motion drives melting of the magnetic order in the functional film (NdNiO3 spins, back). This process is initiated locally at the interface between the two materials and progressively moves into the interior of the film.
This picture is supported by a theoretical model that assumes the vibrationally induced creation of freely movable charge carriers at the hetero-interface, which themselves scramble the antiferromagnetic while propagating into the film.
Ultrafast Strain Engineering in Complex Oxide Heterostructures
A. D. Caviglia, R. Scherwitzl, P. Popovich, W. Hu, H. Bromberger, R. Singla, M. Mitrano, M. C. Hoffmann, S. Kaiser, P. Zubko, S. Gariglio, J.-M. Triscone, M. Först, and A. Cavalleri
Physical Review Letters, 108, 136801 (2012)
APS – Viewpoint in Physics: Ultrafast Phase Control in Oxide Thin Films
Research Highlights in Nature Materials: Beaten to Action
Spatially resolved ultrafast magnetic dynamics launched at a complex-oxide hetero-interface
M. Först, A.D. Caviglia, R. Scherwitzl, R. Mankowsky, P. Zubko, V. Khanna, H. Bromberger, S.B. Wilkins, Y.-D. Chuang, W.S. Lee, W.F. Schlotter, J.J. Turner, G.L. Dakovski, M.P. Minitti, J. Robinson, S.R. Clark, D. Jaksch, J.-M. Triscone, J.P. Hill, S.S. Dhesi, A. Cavalleri
Nature Materials,14, 883-888 (2015)
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Broadband THz spectroscopy of the insulator-metal transition driven by coherent lattice deformation at the SmNiO3/LaAlO3 interface
W. Hu, S. Catalano, M. Gibert, J.-M. Triscone, A. Cavalleri
Physical Review B, 93, 161107(R),(2016
Time-spliced X-ray diffraction imaging of magnetism dynamics in a NdNiO3 thin film
Kenneth R. Beyerlein
PNAS,115, (9), 2044-2048 (2018)