Yangmu Li is a graduate student in Martin Greven’s high-transition-temperature superconductor research lab. Li manages two lab rooms and leads a subgroup that focuses on electron-doped copper oxide (cuprate) superconductors. The cuprate superconductors are ceramic materials mainly made up of copper-oxygen layers with other chemical elements forming charge reservoirs in between. By adding extra electrons/holes into the materials through chemical manipulations, novel physics properties, such as unconventional superconductivity and charge/spin modulations, emerge in these materials.
Superconductors are materials that, below a certain temperature (superconducting transition temperature), have zero electrical resistance becomes zero and expel magnetic fields from the interior. Applications include fast digital circuits, magnetic levitating trains, magnetic resonance imaging (MRI) and low-loss power cables. The cuprate superconductors, which have a transition temperature as high as 163 Kelvin (-166 Fahrenheit), have been the subject of one of the most intensive research efforts in the world over the last thirty years.
“The materials I study are fascinating because they’re strongly correlated”, Yangmu says. In cuprate superconductors, the electrons are interacting strongly with each other. Physicists believe that strong correlation is what leads to certain macroscopic quantum phenomena such as superconductivity. However the picture of how these quantum phenomena connect to each other is not clear. "Besides the superconducting state, there are some other quantum phases. We are trying to understand how the material evolves from one phase to another. In electron-doped cuprate superconductors, a solid connection between magnetic order, Fermi-surface topology and the emergence of superconductivity is observed."
In order to study these quantum phases, the members of Greven’s lab investigate fundamental magnetic, structural, and electronic properties of these materials. The group synthesizes high quality single crystals using state-of-the-art traveling-solvent floating-zone and self-flux encapsulation methods. They then characterize these samples by various techniques and instruments, including a superconducting quantum interference device (SQUID), X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, and energy-dispersive X-ray spectroscopy. In-house charge and thermal transport experiments have been performed in Professor Paul Crowell’s spin transport laboratory and Professor Chris Leighton’s electronic and magnetic materials laboratory. Neutron scattering, synchrotron X-ray scattering, high magnetic field transport and muon spin rotation/relaxation measurements have been carried out at national laboratories in North America and Europe.