Physicists have observed a feature known as a "pseudogap" - something that has only previously been seen in high-temperature superconductors - in a magnetic material for the first time. The result means that pseudogaps may not be unique to high-temperature superconductors, as previously thought, and that interactions between electrons and vibrations of the crystal lattice could play an important role in these materials after all, again contrary to current thinking (Nature 438 474).
Superconductivity is the complete absence of electrical resistance in a material. It is observed in certain materials when they are cooled to below their superconducting transition temperature, and occurs when electrons overcome their mutual Coulomb repulsion to form “Cooper pairs”. In the Bardeen-Cooper-Schrieffer (BCS) theory of low-temperature superconductivity, the electrons are held together because of their interactions with phonons – quantized vibrations of the crystal lattice.
Most high-temperature superconductors consist of layers of copper and oxygen, separated by metal atoms such as yttrium and barium, and they can have transition temperatures as high as 138 Kelvin. Developing a theory to explain high-temperature superconductivity, which was discovered in these cuprate materials in 1986, has long been one of the outstanding challenges in condensed-matter physics. However, most theorists have assumed that electron-phonon interactions would not play an important role in any successful theory.
One of the defining characteristics of a superconductor is the energy gap – the energy that is needed to break a Cooper pair into two free electrons. However, in the mid-1990s physicists discovered evidence for a similar gap – which they called a pseudogap – in so-called underdoped materials at temperatures well above the transition temperature. It was also found that other electronic properties of the cuprates change with direction in momentum space. These two features – the pseudogap and the variation of electronic properties with direction – have long been considered as hallmarks of high-temperature superconductivity.
However, Zhi-Xun Shen of Stanford University and colleagues in the US, Canada, Japan and the Netherlands have now observed a pseudogap in a completely different material – a metallic manganite compound containing lanthanum, strontium, manganese and oxygen. This material exhibits “colossal magnetoresistance”, becoming a ferromagnet when it is cooled below a certain critical temperature. This phase change, which is thought to result from interactions between electrons and phonons, is accompanied by a huge drop in electrical resistance.
Shen’s team used a technique called angle-resolved photoemission spectroscopy (ARPES) to measure the electron velocity and scattering rate as a function of energy. These spectra reveal that the electron motion in the ferromagnet phase is strongly linked to the vibrations of the crystal lattice. Moreover, the spectra vary with direction in momentum space and display evidence for a pseudogap, which is similar to the behaviour seen in high-temperature superconductors.
The results suggest that the pseudogap is a general feature of all transition metal oxides, not just cuprate superconductors, and that theorists may have to rethink the role played by phonons in high-temperature superconductivity.
