Strongly correlated materials often show unusual magnetic and electronic properties, such as high-temperature superconductivity. An example of such behavior is the formation of self-organized electronically ordered phases, which can cause charges to segregate into atomic-scale patterns and is linked to the emergence of high-temperature superconductivity.
An international research team has now illuminated the origins of the so-called “stripe phase” in which electrons become concentrated in stripes throughout a material.
“We’re trying to understand nano-scale order and how that determines materials properties such as superconductivity,” said Robert Kaindl, a physicist at Lawrence Berkeley National Laboratory (Berkeley Lab). “Using ultrafast optical techniques, we are able to observe how charge stripes start to form on a time scale of hundreds of femtoseconds.”
Kaindl, W.-S. Lee (SLAC National Accelerator Laboratory), T. Sasagawa (Tokyo Institute of Technology), and their colleagues reported the results of their work with strontium-doped lanthanum nickelate (LSNO) in the October 24 issue of Nature Communications (DOI: 10.1038/ncomms3643).
Lead author Giacomo Coslovich, a postdoctoral researcher at Berkeley Lab, said, “We chose to work with LSNO because it has essential similarities to the cuprates (an important class of high-temperature superconductors), but its lack of superconductivity lets us focus on understanding the stripe phase alone.”
In this LSNO crystal, stripes form only at cryogenic temperatures of about –168°C, yet at far higher temperatures, the team hit upon large changes in the material’s infrared reflectivity. These invisible “color” changes represent an energy threshold for electrical currents, dubbed the energetic “pseudogap,” which grows as the crystal cools, revealing progressive localization of charges around the nickel atoms.
The scientists then examined the dynamics of LSNO in pump-probe experiments, where they melted stripes with an initial ultrafast pulse of laser light and measured the optical changes with a second, delayed pulse. This allowed them to map out the early steps of charge ordering, exposing surprisingly fast localization dynamics preceding the development of organized stripe patterns. A final twist came when they probed the vibrations between nickel and oxygen atoms, uncovering a strong coupling to the localized electrons with synchronous dynamics.
Beyond the ultrafast measurements, the team also studied x-ray scattering and the infrared reflectance of the material to develop a thorough, cohesive understanding of the stripe phase and why it forms.
Having illuminated the origins of the stripe phase in LSNO, the researchers expect their results to provide new impetus to understand the pseudogap in other correlated oxides—especially in high-temperature superconductors where fluctuating stripes occur while their role in the superconductivity mechanism remains unclear.
