Melting of frozen electrons visualized

20 September 2016

For the first time, physicists have visualized the ‘melting’ of electrons inside a special class of insulators. It allows electrons to move freely and turns the insulator into a metal and possibly later into a superconductor. Publication in Nature Physics.

Electronic liquid

Some materials carry an electrical current more easily than others. Metals are for example world class conductors. Inside them, the electrons form an electronic liquid that flows through the atomic lattice. In specific insulators on the other hand, electrons are stuck to their place in the lattice; the electronic liquid is frozen (see image below). In these so-called Mott insulators, you can replace some atoms with different ones. Physicists call this ‘doping’. It is known that doping leads to a melting of the frozen electronic liquid, but nobody knows how this process works.

Puddles

Now, Leiden physicist Milan Allan together with lead authors Irene Battisti and Koen Bastiaans have, for the first time, visualized this melting process in a family of materials called iridates. They discovered that the melting process is very inhomogeneous, with puddles forming in between frozen areas. These puddles are only a few nanometers in size (see image below). The research group, in collaboration with theoretical physicist Jan Zaanen, publishes their results in Nature Physics.

Superconductivity

Apart from getting insight in a very fundamental process, the discovery also shines light on the mystery of superconductivity—a phenomenon where electrons move without resistance. Superconductivity is important because it allows transportation of electricity with zero energy loss. ‘We came to believe that this kind of melting is a universal prerequisite of superconductivity,’ says Allan. ‘If we would manage to melt the electronic liquid in all parts of the sample, it would likely become a new superconductor.’

Publication

I. Battisti, K.M. Bastiaans, V. Fedoseev, A. de la Torre, N. Iliopoulos, A. Tamai, E.C. Hunter, R.S. Perry, J. Zaanen, F. Baumberger, M.P. Allan, ‘Universality of pseudogap and emergent order in lightly doped Mott insulators’, Nature Physics (2016)

The melting of electrons. In the blue areas, the electrons (red dots) are stuck to the atoms in the lattice (green circles), meaning that there is no current. In the red areas, dopant atoms (black circles) are added, giving the electrons room to move and making them behave like a liquid. The researchers expect that once the whole area is molten, the material is a high-temperature superconductor.
*Left: Actual measurement. Right: Illustration of concept.*

Header: Image produced based on data from a Scanning Tunneling Microscope (STM). Red ‘mountains’ are molten electrons. The blue parts represent frozen electronic liquid.