Researchers at the Center for Quantum Devices publish in Nature Physics

Tekst: Researchers at the Center for Quantum Devices, Copenhagen University, and in Israel, Italy and the US have observed a “Wigner molecule” consisting of two electrons or two holes. Theoretical predictions about such delicate electronic states date back to the 1930s, but an unambiguous experimental verification requires extremely clean materials at very low temperatures, and strong electron interactions.

“We fabricated tiny gate electrodes underneath the carbon nanotube before growing the nanotube itself, which resulted in an exceptionally clean system that we could then manipulate at 0.1 Kelvin above absolute zero”, says Ferdinand Kuemmeth, who fabricated the device and performed the measurements. Together with Shahal Ilani and colleagues at Cornell University and the CNR-NANO Research Centre in Italy, the researchers were able to add exactly two additional electrons to the nanotube and study how they responded to electric and magnetic fields. Surprisingly, the two electrons formed a highly correlated state in which each electron avoided the electric charge density of the other, nearly independent of their spin states. This result has already stimulated new experiments that are now performed in the group of Shahal Ilani at the Weizmann Institute in Israel.

The researchers published their work in Nature Physics doi:10.1038/nphys2692.

Figure: Two electrons confined to a box behave drastically different depending on the strength of their mutual Coulomb interaction.?Top left: In the absence of interactions a large spin excitation energy (X) occurs when changing an antiparallel spin state (symmetric orbital state, S) to a parallel spin state (anti-parallel orbital state, AS), because the first excited single-particle state (K) has to be occupied. For very strong interactions, however, it has been predicted that the two electrons move in highly correlated orbits, resulting in a molecular charge density (bottom left) that is independent of the electrons’ spin configuration. Right panel: By measuring the magnetic spectra of symmetric (red) and anti-symmetric orbitals (blue) in clean, suspended carbon nanotubes, it has now been discovered that the excitation energy X can be surprisingly small (comparable to the relativistic corrections due to spin-orbit coupling SO), indicating a highly correlated two-electron state (Wigner molecule).