Freek Massee
     Laboratoire de Physique des Solides, CNRS

Atomic manipulation of the gap in Bi2Sr2CaCu2O8+x

A microscopic understanding of high temperature superconductors is one of the key open problems in condensed matter physics. Local probes, such as scanning tunneling microscopy, are ideally suited to tackle this problem due to their high spatial and energy sensitivity. However, the considerable disorder in copper oxide superconductors makes it challenging to extract to what extent the atomic structure is responsible for the remarkable electronic properties. Using the electric field of the scanning tunneling microscope itself, we discovered it is possible to selectively move individual atoms at the surface of one of the most studied high temperature superconductors, Bi2Sr2CaCu2O8+x. Since these single atom manipulations are non-invasive and reversible, they enable detailed studies of the electronic properties to be performed before and after manipulation. This technique therefore effectively removes the inhomogeneous environment and gives direct insight into the effect of single atoms on the electronic properties. They find that by moving bismuth atoms at the surface of Bi2Sr2CaCu2O8+x, the spectral gap that is associated with the local strength of superconductivity can be directly manipulated. This suggests that the single atoms locally enhance pairing, providing new insight into the pairing mechanism of these systems and the tunneling process into them.

Atomic manipulation of the gap in Bi2Sr2CaCu2O8+x
F. Massee, Y. K. Huang and M. Aprili
Science 367, 68-71 (2020)

atomic manipulation of the gapUsing the electric field of the tip, we manipulate bismuth atoms at the surface of Bi2Sr2CaCu2O8+x. The spectral gap is enhanced in the direction of the atomic shift.

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)

charge dynamics at oxygen atomsAtomically resolved current noise measurements reveal individual oxygen dopants with remarkable charge dynamics.

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)

vortex pinning due to heavy ion irradiationUpon irradiation with 249-MeV Au ions, two types of defects appear: large (columnar defects) and small (point defects), that virtually annihilate, respectively strongly suppress superconductivity.