| AVS 60th International Symposium and Exhibition | |
| Magnetic Interfaces and Nanostructures | Tuesday Sessions |
| Session MI+AS+NS+SP-TuA |
| Session: | Advanced Probes in Magnetic Imaging and Characterization |
| Presenter: | F.O. Schumann, Max-Planck Institut für Mikrostrukturphysik, Germany |
| Correspondent: | Click to Email |
The emergence of long range magnetic order is a consequence of the mutual interaction between electrons. A key postulate of quantum mechanics is the requirement of the wave function to be antisymmetric upon exchange. This inclusion leads to a modification of the Coulomb interaction which is termed exchange interaction. For ferromagnets this leads to parallel spins while for antiferromagnets an antiparallel alignment is prefered.
Electron pair emission from surfaces is an advanced tool to study the relation between electrons which goes beyond the capabilities of single electron spectroscopy e.g. photoemission. The power of this approach will be demonstrated by two case studies on Fe and NiO films. The angular distributions of emitted electron pairs reveal a region of reduced intensity which can be traced back to the exchange-correlation hole.[1] This concept was introduced by Wigner, Seitz and Slater more than 75 years ago. It plays an important role in modern solid state theory. We performed experiments on Fe films to unravel the spin-dependence of the exchange-correlation hole. We find that the contribution of exchange is more extended than the Coulomb contribution as suggested by Slater.[2]
The investigation of correlation effects in solids is an active field of research. In this context metal oxides like NiO are usually termed "highly correlated", because the material properties are decisively determined by the electron-electron interaction. The very existence of a finite electron pair emission requires a finite electron-electron interaction. This immediately leads to the question whether the intensity level provides insight into the correlation strength. A theoretical study of pair emission from a strongly correlated system modeled by the Hubbard Hamiltonian gives an affirmative answer.[3] We tested this conjecture and find that the coincidence intensity for NiO is roughly an order of magnitude larger compared to the Ag(100) substrate.[4] This also holds for the comparison of other transition metals and their oxide phases. This result suggests that the electron correlation strength is accessible via the pair emission intensity.
Our results demonstrate that electron pair emission is a unique tool to unravel the nature of the electron correlation in solids.
[1] F.O. Schumann, C. Winkler, and J. Kirschner,Phys. Rev. Lett. 98, 257604 (2007).
[2] F.O. Schumann, C. Winkler, J. Kirschner, F. Giebels, H. Gollisch, and R. Feder, Phys. Rev. Lett. 104, 087602 (2010).
[3] B.D. Napitu and J. Berakdar, Phys. Rev. B 81, 195108 (2010).
[4] F.O. Schumann, L. Behnke, C.H. Li, J. Kirschner, Y. Pavlyukh, and J. Berakdar, Phys. Rev. B 86, 035131 (2012)