A team of Japanese scientists from the RIKEN Discovery Research Institute in Wako, AIST and the University of Tokyo are gaining deeper insight into how electron–electron interactions in an unconventional metal evolve in a magnetic field. Their results are reported in Physical Review Letters (1).
The properties of simple metals and semiconductors can be explained by models that assume the electrons are non-interacting. However, for certain materials—notably high temperature superconductors—this approximation fails. Often, the electrons and spins in these ‘strongly correlated’ electronic materials form ordered phases that are highly sensitive to changes in temperature or magnetic field.
Looking for examples of this behavior is important for testing theoretical models. This is why Hidenori Takagi and Tetsuo Hanaguri and their team are studying Sr3Ru2O7, which is a non-magnetic metal. However, experimental evidence suggests that at a high magnetic field, Sr3Ru2O7 may become ferromagnetic—meaning the electron spins prefer to point in the same direction.
To understand the microscopic interactions that lead to this so-called ‘metamagnetic’ phase transition, it is useful to look at electronic excitations close to where it occurs. Hanaguri and co-workers therefore used a tunneling spectroscopy probe in which a sharp metal tip is brought close to the material surface—in this case, freshly cleaved Sr3Ru2O7—and a voltage is applied between the tip and the surface to excite electrons in the material.
The team measured these electronic excitations as they increased an applied magnetic field, looking for changes near the metamagnetic phase transition. They found that the excitations evolve in ways that can not be explained by a model that ignores the interaction between electrons. Commenting on the impact of this result, Hanaguri notes that “If theorists want to describe the properties of this material, the many-body interaction must be taken into account.”
The team also observed an unexpected feature on the Sr3Ru2O7 surface. By scanning the tip over the surface, they can locate the positions of individual atoms. On every other atomic site they found extra electron density (Fig. 1) indicating a purely electronic ordering that has so far, only been observed on the surface of Sr3Ru2O7.
It is unclear whether the electronic ordering that the team observed at the surface is related to their other findings, yet both results could be useful to theorists seeking to understand the unusual properties of Sr3Ru2O7 and other strongly correlated electron materials.
Iwaya, K., Satow, S. Hanaguri, T, Shannon, N., Yoshida, Y., Ikeda, S. I., He, J. P., Kaneko, Y., Tokura, Y., Yamada, T. & Takagi, H. Local tunneling spectroscopy across a metamagnetic critical point in the bilayer ruthenate Sr3Ru2O7. Physical Review Letters 99, 057208 (2007).
