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Electric voltage from quantum mechanical electron spin - battery of the future?

Spintronics as a rapidly thriving branch of nanoelectronics deals with the intrinsic angular momentum of electrons - their electron spin (in short spin) – that can give rise to a variety of novel and fascinating phenomena. We all already unknowingly use the results of this subfield of modern physics with the latest generation of memories installed in our laptops and mobile phones. Indeed, today's density of data would be impossible to handle without the giant magnetoresistance discovered by Peter Grünberg (awarded the Nobel Prize in Physics in 2007) - a phenomenon based on a collective dynamic of electron spins.

Figure 1: The principle of electron spin-vorticity coupling in a laminar circular pipe flow. The vorticity gradient in radial direction results in the generation of a spin current in the same direction and consequently an electric current in the flow direction based on the inverse spin Hall effect (ISHE).

In their work, published in Physical Review Applied, researchers led by Prof. Christian Cierpka and Prof. Jörg Schumacher from the Institute of Thermodynamics and Fluid Mechanics at Technische Universität Ilmenau have investigated another spintronic effect in a series of microfluidic experiments. In particular, for the first time, they have verified and expanded previous investigations by a Japanese research group from Sendai (Takahashi et al. [Nat. Phys. 12, 52 (2016)] and Matsuo et al. [Phys. Rev. B. 96, 020401 (2017)]). In more detail, the work is about generating an electrical voltage by means of a collective coupling between the electron spins and the flow vorticity (see figure 1). This happens interestingly without the existence of an external magnetic field which is essential for conventional magnetohydrodynamic generators. There exist several experimental challenges in this project which should be overcome. The flow was created using a liquid metal in which electrons, as in copper or aluminum, form a "gas" of freely moving charges. As can be seen in figure 2, the flow chamber is actually a capillary tube made of special glass with a diameter of less than one millimeter, through which the liquid metal flow was generated by pressurized argon gas. Different input pressures were imposed so that laminar and turbulent regimes on the microscale flow were achieved. The measured voltages are still tiny and their magnitude reaches just several hundred nanovolts.

The experiments were carried out as part of a project funded with 120.000 € by the Volkswagen Foundation within the framework of the program „Experiment! – Auf der Suche nach gewagten Forschungsideen“.

Figure 2: A schematic illustration of the employed experimental setup for measuring the electrical voltage generated by laminar and turbulent flows of a liquid metal (GaInSn) in different narrow-glass capillary tubes.

The researchers from the Institute of Thermodynamics and Fluid Mechanics are already thinking about the next steps. Can the generated tiny voltage be increased through geometric effects or using nanostructured surfaces? Are there optimal flow vorticity configurations that enhance the electron-spin vorticity coupling?  What happens when such systems are connected in parallel? These questions show that there is still a long way to go before we may use "spintronic batteries" as a source of energy in the distant future. However, the researchers led by Prof. Cierpka and Prof. Schumacher are confident that on the way, a multitude of other interesting effects can be brought into light at this exciting junction between classical fluid mechanics and quantum physics. 

Contact person:
Technical University Ilmenau
Institute of Thermodynamics and Fluid Mechanics
Prof. Dr.-Ing. Christian Cierpka and Prof. Dr. Jörg Schumacher

Original publication (Open Access):
Hamid Tabaei Kazerooni, Alexander Thieme, Jörg Schumacher, and Christian Cierpka: Electron Spin-Vorticity Coupling in Pipe Flows at Low and High Reynolds Number. Physical Review Applied 14.1 (2020): 014002.
DOI:
https://doi.org/10.1103/PhysRevApplied.14.014002