The physics of high-temperature superconductors in unstable, transient states is surprisingly similar to that of the same materials at equilibrium, raising hopes that these out-of-equilibrium states could be stabilized and used in practical applications. The finding, which researchers at the US Department of Energy’s SLAC National Accelerator Laboratory obtained by using a flash of light to kick-start superconductivity in materials known as cuprates, could help us better understand high-temperature superconductors and how to trigger the formation of these transient states.
Superconductors are materials that conduct electricity without any resistance when cooled to below their superconducting transition temperature, Tc. In the Bardeen-Cooper-Schrieffer (BCS) theory of conventional superconductivity, this occurs when electrons overcome their mutual repulsion and form so-called Cooper pairs that travel unimpeded through the material as a supercurrent.
The first conventional superconductors to be discovered (beginning with solid mercury in 1911) had transition temperatures only a few Kelvin above absolute zero. Beginning in the late 1980s, however, a new class of superconductors with much higher Tc began to emerge. These materials were not metals, but ceramic compounds known as cuprates that are made up of layers of copper and oxygen atoms, interleaved with atoms of other elements. The BCS theory does not apply to these high-temperature superconductors and the way their electrons pair up is not fully understood.
Non-equilibrium states
To shed more light on these materials, researchers often study them in unstable or non-equilibrium states. In the latest work, the researchers generated such a state in yttrium barium copper oxide (YBCO) by applying a laser pulse to it. In this unstable state, the material remains superconducting at temperatures much higher than its usual Tc of around 100 K.
Until now, researchers were unsure whether the properties of such unstable states bore much relation to how the materials would behave in their stable states – that is, the states that would be exploited in real-world applications. A team led by Jun-Sik Lee has now shown that in fact, these unstable states behave in a very similar way to their stable cousins.
Switching on and off
The researchers studied what happened when the normal superconducting states of YBCO were switched off using pulses of light from SLAC’s Stanford Synchrotron Radiation Light Source (SSRL) and the Pohang Accelerator Laboratory’s X-ray free-electron laser (PAL-XFEL) in Korea. They focused on a particular phase of matter in superconductors known as charge density waves (CDWs), which are wavelike patterns of higher and lower electron density. CDWs are different from ordinary waves in that they are static and they serve as markers of the transition point at which superconductivity turns on or off.
The researchers then repeated their experiments by switching off the superconductivity in YBCO using a magnetic field. This is the conventional way to study CDWs in normal equilibrium states of a high-temperature superconductor.
“Crazy experimental adventure”
The team discovered that regardless of whether they exposed the material to a magnetic field or light, similar patterns of three-dimensional CDWs appeared. Why and how this happens is, they say, still unclear, but the result does show that the states induced by either magnetic fields or laser light have the same fundamental physics. It also suggests that laser light might be a good way to create and explore transient states that could be stabilized for practical applications.
“Our result implies a common ground for normal states attained using a magnetic field and an optical pump,” Lee tells Physics World. “Inversely, with the equilibrium states, optical pump approaches can drive room temperature transient superconductivity.”
Electromagnetic cavity could enhance high-temperature superconductors
And that is not all: Lee says that it may be possible to apply an additional driver in the form of a magnetic pulse. “When three pulses, such as X-ray, magnetic and optical pulses are synchronized, it might be easier to monitor how to recover broken Cooper pairs via the magnetic field in a timely manner.”
“This would be a crazy experimental adventure, but we believe it might provide more insight into understanding high-temperature superconductivity.”
The work is detailed in Science Advances.
