A group of physicists headed by CAS scientists questioned the validity of the conventional definition of spin current, and proposed a new one. Their work was published by the US journal Physical Review Letters.
The research breakthrough is the result of a collaborative effort, which involves Prof. SHI Junren from the CAS Institute of Physics and his colleague, Prof. ZHANG Ping from the Institute of Applied Physics & Computational Mathematics, in cooperation with Prof. NIU Qian and his Ph.D. student XIAO Di from University of Texas at Austin, USA.
The new discipline of spintronics or spin electronics is an emerging research field in condensed matter physics, aiming at developing a new generation of information technology by using electron spin, instead of charge, as the basic carrier for the device manipulation and information transfer. In the traditional electronics, electric current is the primary concept in describing the charge transfer, while in the spin electronics, correspondingly, spin current is the most important concept in describing the spin transfer. How to properly define the spin current is a fundamental issue for this new field.
In most of the previous studies, spin current is defined as the product of the spin operator and the electron velocity, in analogy to the definition of the electric current that is the product of the electron charge and velocity. This definition, however, neglects a fundamental difference between spin and charge: unlike the charge, the spin is not conserved in general. The issue had become outstanding in the latest studies of the spin Hall effect. The research team investigated the concept of spin current in depth, and found that the conventional definition of the spin current is incomplete and unphysical in describing the spin transport. They further proposed that the proper definition of spin current be the total time derivative of the spin displacement operator. The new definition fits well into the standard near-equilibrium transport theory and is directly measurable. As a concept describing the true spin transport, it also has the desirable property to vanish in an insulator.
This work solves a fundamental issue of spin electronics, and has strong implications to the future studies. For instance, using the new definition of spin current, a Japanese research team headed by S. Murakami and N. Nagaosa re-calculated the spin-Hall effect for a few semiconductor models, and obtained the null results for systems with the non-magnetic short-range disorder only, contradicting with the previous conclusions based on the conventional definition of spin current.