Capturing the currents in CuO

Orbital currents have been observed for the first time in CuO, a material that contains the same building blocks as cuprate high-T_c superconductors [Scagnoli et al., Scienceexpress (2011) doi:10.1126/science.1201061].

High-T_c superconducting materials are extraordinary in that they can lose their resistance to electric currents, albeit at fairly low temperatures, making them potential candidates for the transmission of electricity. However, despite the fact that they were discovered 25 years ago, there is still no definite theory for the microscopic origin of their superconductivity. Now, x-ray diffraction experiments carried out by researchers based in Switzerland and the UK have provided some evidence for the existence of orbital currents in CuO, a simple high-T_c superconductor.

 

X-ray diffraction is widely used in the study of crystal structures, but it is possible to obtain more information using resonant x-ray diffraction (RXD). RXD can enhance signals from magnetic moments, enabling researchers to study the magnetic properties of materials at the atomic scale, as Valerio Scagnoli explains. “Information on the physical properties of the sample can be obtained by carefully choosing the energy (wavelength) of the incident x-rays. Performing the diffraction experiment at the absorption edge of one of the elements present in the sample is like using a microscope that shows details that are out of our eye’s reach. One of these details is the (possible) presence of the orbital currents. Such a signal is expected to be very small and in our case its observation has been possible since it couples with the signal coming from the (anti-ferro)magnetism present in the material.”

 

The larger periodicity of anti-ferromagnetic structures means that light is diffracted at angles away from the reflections due to the crystallographic unit cell, allowing orbital currents to be detected.

 

“Our work presents the first microscopic observation of an object that changes sign with either space or time inversion,” remarks Scagnoli. “Such an observation is already remarkable as we observe the presence of toroidal moments (orbital currents) on the microscopic level at the atomic scale. Moreover we observe them in CuO; a material that despite its simple chemical formula has very interesting multiferroic properties, and [which] shares the same building block with high T_c cuprate superconductors, a copper oxygen plaquette.”