Scientists Observe New Electron ‘Spin-Orbital Density Wave’ Physical State

Scientists at EPFL, Hannover and Bangalore have made the first observation a new physical state, the electron “spin-orbital density wave”.

Electrons move in two ways: they orbit around the atom’s nucleus, and they spin around themselves. When these two types of motion interact, they change the electron’s energy profile, and can have a knock-on effect on nearby electrons as well. This largely theoretical phenomenon is known as a “spin-orbit density wave” and lies at the heart of certain magnetic effects and the fundamentals of spintronic technologies. EPFL scientists, working with colleagues in Hannover and Bangalore, have experimentally observed, for the first time, a spin-orbit density wave forming on a material. The discovery is published in Nature Communications.

The study was lead by Leibniz Universität Hannover and it included Hugo Dil’s lab at EPFL, and the Indian Institute of Science. Using spin- and angle-resolved photoelectron spectroscopy, the researchers were able to map out the evolution of the spin-texture at various conditions. The material they explored was Pb-atomic wires on Si(557) surfaces.

The tests demonstrated electron events where electron charge, spin, and atomic order all match up perfectly and propagate across the surface of the material like a wave of tiny whirlpools. The researchers named this phenomenon a “spin-orbital density wave”. In some ways, it resembles the properties of a quasiparticle called a “skyrmion”, which also appears as tiny whirlpools across the surface of magnetic materials.

“This wave represents a new physical state,” explains Hugo Dil. “In some systems, charge and atomic order match and give rise to charge-density waves. In others, it’s the spin and atomic order, so you have spin-density waves. Here, all three match up, which is why we’re calling it a ‘spin-orbital density wave’.”

In the field of spintronics, such spin–orbit effects in semiconducting materials like the one used in this study are aimed at technological applications. In particular, the degree of freedom of electron spin has attracted much attention recently as it plays a central role in spintronic device structures, topological insulators and others.

The study shows that spin-orbital density wave can be exploited in this field, as it results in completely anisotropic conductance,” says Dil. “Along one direction the material acts as a perfectly good conductor with spin-polarized states; along the perpendicular direction it acts as an insulator.”

This study was funded by the Deutsche Forschungsgemeinschaft and the Swiss National Science Foundation.

Reference

Brand C, Pfnür H, Landolt G, Muff S, Dil JH, Das T, Tegenkamp C. Observation of correlated spin-orbit order in a strongly anisotropic quantum wire system.Nature Communications 10 September 2015. DOI: 10.1038/NCOMMS9118