Recently, I wrote an article about the experiment from Kläui-Lab on surface-oxidized Cu. Since we publisehd this work, I decided to “find” the orbital Rashba effect in this material system. Implications from several experiments were quite clear: There must be something with orbital. So, it was actually more a matter of “how” I find the orbital Rashba effect. The main problem was that the structure becomes disordered as a Cu film is oxidized from the surface. Mixture of the oxygen and copper form amorphous structure.

Amorpohous structures are quite hopeless to do something seriously with DFT. Well, there is community of researchers doing this, but their works are quite limited to density of state and force acting on the atoms. On the other hand, in our group, we look at transport properties. Becuase response functions are determined by very small energy scale, often down to a few meV scale, we use a full-potential DFT code for accuracy. We also use some other sophisticated techniques to do this job. Anyway, the point is that our code is quite precies but very slow 😉 So, we needed to come up with a more clever stratagy.

So, what I did was to find a prototypical model structure that is expected to caputre essential features of the surface oxidized Cu: oxygen monolayer on top of Cu(111). Fortunately I found a very, very large orbital Rashba effect. Its magnitude is found to be larger than any other film systems that I have investigated so far. To understand why, undergraduate chemistry knowledge (I didn’t take any advanced chemistry class during college years!) is sufficient. The difference of the electro-negativity between oxygen and copper is huge. It means, oxidiziation induces large electric dipole moment, which is directly related to the orbital Rashba effect. Quantum mechanically, chemical bonding between oxygen and copper is described by orbital hybridizations. Indeed, we found almost resonant-like hybridization beween oxygen p-orbitals and copper d-orbitals.

So, from our theoretical study, it became quite clear that mysterious features of the oxidized Cu in spintronics is very likely due to orbital angular momentum. We wrote a very short paper. If someone is interested in this topic, please read our preprint on arXiv.

Reference:
Orbital Rashba effect in surface oxidized Cu
Dongwook Go, Daegeun Jo, Tenghua Gao, Kazuya Ando, Stefan Blügel, and Yuriy Mokrousov
arXiv:2011.08601