Noisy defects in a high temperature superconductor

Dopants and impurities are crucial in shaping the ground-state of host materials: semiconducting technology is based on their ability to donate or trap electrons, and in many correlated electron systems they are used to transform materials of little interest into exotic matter. A prime example is the copper oxide high temperature superconductors, which are insulators without the addition of dopants. To understand the role of dopants in this transformation at the microscopic level, local atomic scale techniques such as scanning tunnelling microscopy are crucial. However, due to limited time resolution, most of these studies focus on the effect of dopants on the electronic properties averaged over time. Using newly developed circuitry, we were able to study the dynamics of optimally doped Bi2Sr2CaCu2O8+x using current-noise measurements. We visualize sub-nanometre sized objects where the tunnelling current-noise is enhanced by at least an order of magnitude and show that these objects are previously undetected oxygen dopants whose ionization and local environment leads to unconventional charge dynamics resulting in correlated tunnelling events. The ionization of these dopants opens up new routes to dynamically control doping at the atomic scale, enabling the direct visualization of local charging on e.g. high-Tc superconductivity.

Noisy defects in the high-Tc superconductor Bi2Sr2CaCu2O8+x
F. Massee, Y. K. Huang, M. S. Golden and M. Aprili
Nature Communications 10, 544 (2019)

Visualizing the effect of ion irradiation on superconductivity and vortex pinning

Maximizing the sustainable supercurrent density, JC, is crucial to high-current applications of superconductivity. To achieve this, preventing dissipative motion of quantized vortices is key. Irradiation of superconductors with high-energy heavy ions can be used to create nanoscale defects that act as deep pinning potentials for vortices. This approach holds unique promise for high-current applications of iron-based superconductors because JC amplification persists to much higher radiation doses than in cuprate superconductors without significantly altering the superconducting critical temperature. Using spectroscopic imaging scanning tunnelling microscopy, we visualize the atomic-scale effects of irradiating the Fe(Se,Te) superconductor with high-energy heavy ions. Simultaneous imaging of defects, superconducting order parameter and vortex configuration reveals how columnar and point defects pin quantum vortices allowing high critical current density in this system.

Imaging atomic-scale effects of high-energy ion irradiation on superconductivity and vortex pinning in Fe(Se,Te)
F. Massee, P. O. Sprau, Y. -L. Wang, J. C. Davis, G. Ghigo, G. Gu, W. -K. Kwok
Science Advances 1, e1500033 (2015)