Understanding high temperature superconductivity (high Tc) has been a long-standing challenge since its discovery in copper oxide compounds in 1986. A key issue in addressing this problem has involved the study of phases found near superconductivity, typically at temperatures in excess of Tc or at doping levels lower than those needed to achieve this state. Now a team led by scientists from the Max Planck Institute for Structure and Dynamics (MPSD) in Hamburg has provided the first evidence of a hidden superconducting state above the superconducting transition temperature. The team’s findings are published in Science.
Such a state has been elusive to detect since it is invisible to most experimental probes. CUI member Dr. Srivats Rajasekaran and coworkers used new terahertz techniques to detect this striped superconductivity above the superconducting transition temperature in cuprates. It occurs because the fluid carrying lossless currents is not homogenously distributed, but arranges itself in stripes: one-dimensional rivers that rotate by ninety degrees along one axis of the crystal.
Mechanism remains a mystery
Superconductivity, a phenomenon involving the lossless transport of electricity, was first observed in metals cooled almost to absolute zero (~ ‑270 degrees Celsius). Over the last few decades other ceramic materials such as doped copper oxides (cuprates) have been discovered, where superconductivity occurs at much higher temperatures (Tc ~ -150 degrees Celsius). The mechanism behind this high temperature superconductivity remains a mystery.
Cooled cuprates not only develop superconductivity but also host other states that interact with the superconducting fluid. Especially interesting is a state that appears to be intertwined with the superconductor, where the fluid is arranged in a spatially modulated, rather than a homogeneous fashion. Although such striped superconductors do not carry supercurrents efficiently, their manipulation may be used to enhance the temperature for superconductivity.
Driving strong tunneling currents
Cuprate superconductors are in fact layered materials with alternating superconducting and insulating planes. The superfluid phase is formed in each layer, and three-dimensional superconductivity is made possible by quantum mechanical tunneling in the direction perpendicular to the planes.
In a striped superconductor, tunneling currents are identically zero. That is, these currents flow at each point in space, but the sum over space vanishes identically. Hence, it is difficult ot detect the presence of the striped superconductor. Dr. Srivats Rajasekaran, Prof. Andrea Cavalleri and coworkers tackled this problem by driving strong tunneling currents, far in excess of the levels usually used to diagnose the presence of a superfluid. In their geometry, the tunneling was observed to be no longer zero even in case of striped superconductivity.
Solid theoretical foundation
Together with theory colleagues from CUI member Prof. Ludwig Mathey’s group at Universität Hamburg, the MPSD scientists have also provided a quantitative description of the results, which now rest on a solid theoretical foundation.
This study, largely related to previous works in the Cavalleri group aimed at manipulating hidden superconductivity in complex materials, opens up new possibilities to study other types of hidden orders in many other condensed matter systems. Text: MPSD, ed.
Rajasekaran S. et al.
Probing optically silent superfluid stripes in cuprates