The three types of glue for superconducting electrons: lattice vibrations (top), electron spin (middle), and fluctuations between two electron orbitals (zx and yz) (bottom). The yellow spheres represent Cooper pairs of electrons. |
The
debate over the mechanism that causes superconductivity in a class of
materials called the pnictides has been settled by a research team from
Japan and China. Superconductivity was discovered in the pnictides only
recently, and they belong to the class of so-called ‘high-temperature
superconductors’. Despite their name, the temperature at which they
function as superconductors is still well below room temperature.
Realizing superconductivity at room temperature remains a key challenge
in physics; it would revolutionize electronics since electrical devices
could operate without losing energy.
Superconductivity
in a material arises when two electrons bind together into so-called
Cooper pairs. This pairing leads to a gap in the energy spectrum of the
superconducting material, which makes the electrons insensitive to the
mechanisms causing electrical resistance. Electrons can bind into Cooper
pairs in different ways, leading to different categories of
superconductors.
Until
the work of Takahiro Shimojima from The University of Tokyo and his
colleagues, including researchers from the RIKEN SPring-8 Center in
Harima, superconducting materials were classified into two broad
categories. In classical superconductors, which function at very low
temperatures, vibrations of atoms in the crystal lattice of the material
provide the necessary glue for the pairing. In cuprates, the original
high-temperature superconductor compounds, magnetic interactions based
on an electron’s spin generate the superconductive pairing. In
the pnictide high-temperature superconductors, physicists assumed that
the underlying mechanism was similar to that for the cuprates, but
conflicting experimental results meant that the precise mechanism was
controversial.
To
investigate this debated pairing mechanism of pnictides, the
researchers studied the properties of the material’s electronic gap.
Thanks to a unique set of high-energy lasers based on very rare laser
crystals available to only a few laboratories, their experiments
resolved these states with unprecedented detail.
Shimojima
and colleagues were surprised to discover that interactions between
electron spins do not cause the electrons to form Cooper pairs in the
pnictides. Instead, the coupling is mediated by the electron clouds
surrounding the atomic cores. Some of these so-called orbitals have the
same energy, which causes interactions and electron fluctuations that
are sufficiently strong to mediate superconductivity.
This
could spur the discovery of new superconductors based on this
mechanism. “Our work establishes the electron orbitals as a third kind
of pairing glue for electron pairs in superconductors, next to lattice
vibrations and electron spins,” explains Shimojima. “We believe that
this finding is a step towards the dream of achieving room-temperature
superconductivity,” he concludes.
Excitation Order Research Team, RIKEN SPring-8 Center
