A joint research team from Korea and abroad has theoretically demonstrated that a one-dimensional helical nanowire twisted like a screw can act as a "quantum pump" that moves electrons in one direction by a fixed amount. Just as ancient Greece's Archimedes pump draws water upward through rotational motion, the finding means that charge can be moved in the nano world by using geometric shape.
A team led by Professor Park No-jeong of the Department of Physics at the Ulsan National Institute of Science and Technology (UNIST) revealed on the 4th, through joint research with the team of Professor Binghai Yan of Pennsylvania State University in the United States, that a one-dimensional helical material can function as a "topological charge pump." Dr. Esmaeil Taghizadeh Sisakht participated as the first author of the study, and the findings were published in the international academic journal Nano Letters.
The Archimedes pump is a classical device that draws water from a low point upward by rotating a screw mounted inside a cylinder. The research team proved that this principle can operate similarly in the quantum mechanical world.
When a material with a helical structure is used, charge moves in a constant direction simply by rotating the direction of an external electric field through one full turn. At this time, the amount of charge that moves per rotation cycle is fixed to a determined value, which is called a "topological charge pump" or a "Thouless quantum pump."
This differs from the ordinary method of applying voltage to send current through. It is a physical phenomenon that safely moves charge by cycling the quantum-mechanical state of electrons through one full turn in an insulating state. While existing topological pump systems required precise control of two or more complex external control factors, the helical material discovered this time is drawing attention in academia because pumping is possible with only "a single condition" — the direction of the electric field.
The research team analyzed the time-dependent electron states for a helical hydrocarbon model and trigonal selenium nanowires. As a result, they confirmed that as charge moves along the helical structure, "orbital angular momentum," which corresponds to the orbital direction of electrons, is generated together.
In particular, in the selenium nanowire, a "spin-orbit coupling" phenomenon was observed in which this alignment of orbital properties leads to the "spin property," which is the rotational direction of the electron. A series of mechanisms was confirmed in which the movement of charge induces the alignment of orbitals, and this in turn connects to a spin polarization phenomenon.
Professor Park No-jeong of the UNIST Department of Physics said, "This study is the first to prove that topological charge pumping, orbital angular momentum, and spin polarization are connected as one unified mechanism in nanowires with handedness." He added, "Going forward, it will serve as a foundation for new methodologies that can topologically control the charge, orbital, and spin properties of electrons in the field of nano devices."
This study was conducted with support from the National Research Foundation of Korea's basic research program under the Ministry of Science and ICT and the KIAT human resource development program under the Ministry of Trade, Industry and Energy.
