Hidden superconductivity revealed

Materials World magazine
1 Apr 2018

A hidden state of superconductivity has been discovered, taking research a step closer to the design of superconductors at room temperature. Kathryn Allen reports.

A hidden state of electronic order has been discovered in a compound of lanthanum, barium, copper, and oxygen (LBCO). The discovery – made by researchers from Germany’s Max Planck Institute for the Structure and Dynamics of Matter and the University of Hamburg, Brookhaven National Laboratory, USA, and the University of Oxford, UK – offers insight into superconductivity in LBCO and cuprates – compounds comprising copper and oxygen layers between other elements. 

The ultimate aim of this research is to design materials with superconducting properties at higher and room temperatures. These materials could be used to design high-temperature superconductors, which do not require cooling systems, and improve the efficiency of commercial power transmission. 

High-temperature superconductors 

For electrons to flow through traditional superconductors with zero resistance, they need to be frozen to 0 kelvin (K). But, high-temperature superconductors conduct electricity without resistance at relatively high temperatures. When cooled to 4K, LBCO compounds with certain concentrations of barium, conduct electricity without resistance. 

However, theoretical models of some high-temperature superconductors show that elements of superconductivity remain at temperatures even higher than the transition temperature – the point at which superconductivity is acquired. 

Srivats Rajasekaran, an author of the paper Probing optically silent superfluid stripes in cuprates, published in Science, said, ‘Depending on the doping level [the introduction of impurities to modify electrical properties] of the [LBCO] compound, superconductivity occurs for temperatures in the range from 3K to 35K (-270°C to -238°C). The compound we studied [La1.885Ba0.115CuO4, with a doping of 11.5%] has a superconducting temperature of 13K (-260°C). However, we have measured signatures of a superconducting state up to 55K (-218°C).’

Electron pairing

Superconducting order forms within the compound’s copper-oxygen layers, or planes. A stripe phase appears for certain doping levels in LBCO – when lithium cobalt oxide compounds are infused with dopant barium atoms – within these planes. This stripe phase is formed of one-dimensional rivers of charge, separated by areas of magnetism in which the electron spins alternate in opposite directions – oppositely phased antiferromagnetism. 

Rajasekaran said, ‘When these stripes are composed of superconducting pairs of electrons instead of normal electrons, the electrical transport parallel to the planes would be mediated through the superconducting pairs and the resistance would be zero.’ This can occur at temperatures above LBCO’s transition temperature. But, when the electrons move perpendicular to the planes, resistance occurs. 

Theories suggest that this is because the amplitude of the superconducting state varies between positive and negative, from one stripe to the next, resulting in an unusual spatial modulation of the superconductivity. As the stripe pattern alters by 90° from layer to layer, it was thought that the superconducting electron pairs were inhibited. 

Bringing it to light 

Until now, experimental evidence to directly test this theory was insufficient. But, using terahertz spectroscopy, the team revealed the superconducting state. 

In order to grow LBCO crystals, precise temperature, atmosphere, and chemical conditions are needed. The starting materials, in the form of a cylindrical rod, were melted in an infrared image furnace. Infrared light was then directed at the crystals in a direction perpendicular to the planes. The crystals reflected back the same frequency of light that they received, as was expected, but the team also recorded a signal three times higher than the frequency of the incident light. Rajasekaran explained, ‘This nonlinear process, termed third harmonic generation, is a signature of the superconducting state.’

John Tranquada, group leader of the Neutron Scatter Group in the Condensed Matter Physics and Materials Science Department at the Brookhaven National Laboratory, commented, ‘For samples with three-dimensional superconductivity, the superconducting signature can be seen at both the fundamental frequency and at the third harmonic. For a sample in which charge stripes block the superconducting current between layers, there is no optical signature at the fundamental frequency.

‘However, by driving the system out of equilibrium with the intense infrared light, the scientists induced a net coupling between the layers, and the superconducting signature shows up in the third harmonic,’ commented Tranquada. The results suggest the presence of a spatially modulated superconducting state, hypothesised as a pair density wave state.  

Due to their relatively low superconducting temperatures, LBCO crystals can’t be used for commercial power generation, however Rajasekaran points out that ‘understanding the essentials for superconductivity in LBCO will help design materials with superconducting properties at higher and possibly room temperatures’.

To read Probing optically silent superfluid stripes in cuprates, visit bit.ly/2EOFA6