Multiple Properties of Quantum Materials Probed Simultaneously at Nano Resolution

Researchers at Columbia University and the University of California, San Diego, have used a combination of measurement techniques to simultaneously examine the electrical, magnetic, and optical properties of quantum materials. The work was inspired by “multi-messenger” astrophysics, a technique that employs simultaneous measurements from different instruments, including infrared, optical, x-ray, and gravitational-wave telescopes, to deliver a more robust physical picture than would be possible using one form of measurement alone.

Using the multi-messenger combination of atomic force microscopy, cryogenic scanning near-field optical microscopy, magnetic force microscopy, and ultrafast laser excitation, the researchers demonstrated both “writing” and “erasing” of a metastable ferromagnetic metal phase in strained films of manganite with nanometer-scale precision. By tracking both optical conductivity and magnetism at the nanoscale, the researchers were able to uncover how strain-coupling underlays the dynamic growth, spontaneous nanotexture, and first-order melting transition of this hidden photoinduced metal.

The discovery of multi-messenger nanoprobes allows scientists to simultaneously probe multiple properties of quantum materials at nanometer-scale spatial resolutions. Courtesy of Ella Maru Studio.

To perform the experiment, the researchers focused laser light onto the sharp tip of a needle probe coated with the magnetic material. When thin films of metal oxide are subject to a unique strain, ultrafast light pulses can trigger the material to switch into an unexplored phase of nanometer-scale domains. This change is reversible. By scanning the probe over the surface of the thin film sample, the researchers were able to trigger the change locally and simultaneously manipulate and record the electrical, magnetic, and optical properties of these light-triggered domains at the nanoscale.

“We have brought a technique from the intergalactic scale down to the realm of the ultrasmall,” professor Dmitri Basov said. “Equipped with multimodal nanoscience tools, we can now routinely go places no one thought would be possible as recently as five years ago.”

The researchers proposed a Ginzburg-Landau description to rationalize the co-active interplay of strain, lattice distortions, and magnetism nanoresolved in the strained manganite films, thus guiding future functional engineering of epitaxial oxides into the regime of phase-programmable materials. The study demonstrates how unanticipated properties can emerge in long-studied quantum materials at ultrasmall scales when scientists tune them by strain.

“It is relatively common to study these nanophase materials with scanning probes,” researcher A. S. McLeod said. “But this is the first time an optical nanoprobe has been combined with simultaneous magnetic nano-imaging, and all at the very low temperatures where quantum materials show their merits. Now, investigation of quantum materials by multimodal nanoscience offers a means to close the loop on programs to engineer them.”

The research was published in Nature Materials (www.doi.org/10.1038/s41563-019-0533-y).