Electron Phonon Coupling, Decoupled
More than a century after its discovery—having powered simple batteries to smart phones along the way—the electron still captivates researchers with its properties and potential. Among the dreams for many physicists is the realization of room-temperature superconductivity: the ability to make electric current run with flawless conduction in everyday environments. To get to that point, however, requires a deeper understanding of how electrons interact.
In work just published in Physical Review Letters, UT’s condensed matter physicists report on the first measurements of electron-phonon coupling in nanoscale artificial superlattices using resonant inelastic x-ray scattering. Essentially, they’ve taken advantage of ever-improving scientific tools to measure and analyze fundamental interactions involving electrons in materials more than a thousand times thinner than a sheet of paper. Every such result is another step toward figuring out the properties of electrons, and how those properties can be tuned.
In solids, one of the most important phenomena is electron-phonon coupling, or EPC. In materials with a crystal structure, atoms are arranged in a periodically ordered pattern called a lattice. Vibrations in the lattice are called phonons. Interestingly, when they couple with electrons, they can cause the latter to slow down, leading to resistance and energy dissipation, or pair up, moving electric current along with no resistance: hence, superconductivity. EPC clearly plays a vital role yet is not always easy to spot.
“Usually what happens is that you have something that’s not even conductive,” explained Assistant Professor Jian Liu. “Typically, you start with something that’s insulating and even if there is electron-phonon coupling, you don’t know, because there are no electrons flowing around.”
To get around this problem, scientists typically “dope” the sample: adding either more electrons or empty spaces where electrons might be.
“As you inject more and more electrons, eventually the material becomes conductive, and then if there’s electron-phonon coupling, interesting things can happen—for example, superconductivity,” he said.
This approach, however, is not without problems. Liu explained that questions can be raised as to whether the material you ended up with after doping is still the one you started with. With doping, he said, you’re making chemical substitutions, and “that also creates a lot of problems—for example, disorders.
“In some sense, doping changes a lot of things, and it’s usually very difficult to disentangle them,” he said.
A key finding of the results published in PRL is that researchers were able to separate doping from electron-phonon coupling in ultra-thin materials made from combinations of strontium, iridium, titanium, and oxygen. With doping out of the equation, they were able to get a more refined benchmark for EPC interactions.
“You don’t want the electron-phonon coupling to be too strong or too weak,” Liu explained. “Electrons talk to each other through the lattice vibration. If you have no EPC, superconductivity will not happen, because electrons don’t talk to each other.” If the EPC is too strong, however, “the electrons only couple to the lattice, and then they get trapped. So there is a sweet spot. If you can control the electron-phonon coupling from a structural point of view, then you can perhaps combine it with doping and optimize the materials.”
This more precise control of the structure offers the possibility of tuning a material’s electronic properties.
Another interesting element in the research was the use of materials in a “superlattice” heterostructure—a largely unexplored route to study electron-phonon coupling. Liu explained that the basic idea behind a heterostructure is about holding everything together.
“Both materials were originally three-dimensional,” he said, “but when you shrink one dimension then it becomes more like a 2-D kind of situation. You need something to hold onto it,” sort of like a Lego block needs a different block to lock onto, or a sandwich needs a slice of ham between two identical slices of bread.
The advent of better scientific tools makes heterostructures good candidates for study. In this case, scientists turned to resonant inelastic x-ray scattering, better known as RIXS. Researchers make sample materials by using a laser to deposit layers of atoms on a substrate, then scatter an x-ray beam off the materials’ electrons. They measure the beam’s angles, energy, and momentum to learn more about the electrons’ arrangement and interactions. Liu said the resolution of RIXS measurements has increased dramatically in the past few years, making it a valuable tool for getting a more detailed picture of nanoscale systems.
Jian Liu, Steve Johnston, Ken Nakatsukasa, Lin Hao, and Junyi Yan.
Along with Liu, other UT Physicists involved in the work were Assistant Professor Steve Johnston, Graduate Students Ken Nakatsukasa and Junyi Yan, and Postdoc Lin Hao. The data were measured with a beamline at the Swiss Light Institute at the Paul Scherrer Institute. Other collaborators are with Brookhaven National Laboratory, Oak Ridge National Laboratory, Argonne National Laboratory, and Lawrence Berkeley National Laboratory. The results are outlined in “Decoupling carrier concentration and electron-phonon coupling in oxide heterostructures observed with resonant inelastic x-ray scattering” in PRL.
The research was built on previous work where UT's physicists studied superlattices to learn more about magnetism at the nanoscale. That research was chosen as a highlight in APS Science 2017: Research and Engineering Highlights from The Advanced Photon Source at Argonne National Laboratory.