Photoinduced structural change and insulator-metal transition. A, Top left, schematic representation of a thin film strained by epitaxy (O, red; Ca, green; Ru, cyan; La, magenta and Al, gray). On the right, structural phase transformation of S-Pbca (shaded) and L-Pbca (colored). Bottom left, electronic configuration of Ru d Ca orbitals2RuO4. bPhotoinduced dynamics of the 008 Bragg peak of a constrained Ca2RuO4 thin film at a pump fluence of 50 mJ cm−2. Peak shifts to lower momentum transfer qz within 3.3 ps, indicating network expansion. Line scans show a projection onto qz of the 3D reciprocal spatial volume measured by tilting the crystal. vsThe time-resolved change in normalized scattering intensity (black circles, incident pump fluence 50 mJ cm−2) to a fixed wave vector, qz= 4.089 Å−1increases by approximately 2.5 ps and persists for τ ≤ 100 ps. The time-resolved high-frequency reflectivity (red squares, E = 1.55 eV, incident pump fluence 0.14 mJ cm−2) increases rapidly, in 1 ps, presents a peak coinciding with the expansion of the network and decreases slowly in 100 ps. The signal for time-resolved low-frequency reflectivity (purple triangles, terahertz bandwidth 0.8 to 10 meV, incident pump fluence 15.1 mJ cm−2) increases in about 8 ps and persists for 100 ps. Time-resolved X-ray data and low-frequency reflectivity were measured after photoexcitation (pump) with a E = 1.55 eV femtosecond laser. The time-resolved high-frequency reflectivity was measured with a E= 1.64 eV femtosecond laser. The uncertainty of radiological data in vs shows the standard deviation of the measured intensities in the ground state for negative delays. Credit: Natural physics(2024). DOI: 10.1038/s41567-024-02396-1
Cornell-led researchers have discovered an unusual phenomenon in a metallic insulating material, providing valuable information for designing materials with new properties through faster switching between states of matter.
Mott insulators are a family of materials with unique electronic properties, including those that can be manipulated by stimuli such as light. The origin of these unique properties is not fully understood, in part because of the difficult task of imaging the material’s nanostructures in real space and capturing how these structures undergo phase changes as fast as a trillionth of a second.
A new study published in Natural physicsrevealed the physics of Mott insulation, Ca2RuO4, because it was stimulated by a laser. In unprecedented detail, the researchers observed interactions between the material’s electrons and the underlying lattice structure, using ultrafast X-ray pulses to capture “snapshots” of structural changes in the Ca.2RuO4 in the critical picoseconds after excitation with the laser.
The results were unexpected: electronic rearrangements are generally faster than lattice ones, but the opposite was observed in the experiment.
“In general, fast electrons respond to stimuli and drag slower atoms with them,” said lead author Anita Verma, a postdoctoral researcher in materials science and engineering. “What we discovered in this work is unusual: atoms react more quickly than electrons.”
Although researchers don’t know exactly why the atomic lattice can move so quickly, one hypothesis is that the nanotexture of the material gives it nucleation points that help rearrange the lattice, in the same way that supercooled ice begins to coalesce. form more quickly around an impurity in water.
The research builds on a 2023 paper in which Andrej Singer, lead author and assistant professor of materials science and engineering, and other scientists used high-power X-rays, phase recovery algorithms and machine learning to achieve real-space visualization of the same material at the nanoscale.
“The combination of the two experiments allowed us to understand that in certain materials like this we can change phase very quickly, on the order of 100 times faster than in other materials that do not have this texture ” Singer said. “We hope this effect will provide a general pathway to accelerate switching and lead to interesting applications in the future.”
Singer said that in some Mott insulators, applications include materials becoming transparent in their insulating state, then quickly becoming opaque once excited into their metallic state. The underlying physics could also have implications for faster future electronics.
Singer’s research group plans to continue using the same imaging techniques to study new phases of matter created when nanotextured thin films are excited by external stimuli.
More information:
Anita Verma et al, Picosecond volume expansion drives subsequent insulator-to-metal transition in a nano-textured Mott insulator, Natural physics(2024). DOI: 10.1038/s41567-024-02396-1
Provided by Cornell University
Quote: Surprise physics in insulating materials offers a path to faster technology (February 9, 2024) retrieved February 9, 2024 from
This document is subject to copyright. Apart from fair use for private study or research purposes, no part may be reproduced without written permission. The content is provided for information only.