Observation of spontaneous valley polarization in a two-dimensional electron system

Memory or transistor devices based on electron's spin rather than its charge degree of freedom offer certain advantages and comprise a cornerstone of spintronics.  Recent years have witnessed the emergence of a new field, valleytronics, which seeks to exploit electron's valley index rather than its spin. An important component in this quest is the ability to control the valley index in a convenient fashion. Here we show that the valley polarization can be switched from zero to 1 by a small reduction in density, simply tuned by a gate bias, in a 2D electron system. This phenomenon, which is akin to Bloch spin ferromagnetism, arises fundamentally due to electron-electron interaction in an itinerant, dilute electron system. Essentially, the kinetic energy favors an equal distribution of electrons over the available valleys, whereas the interaction between electrons prefers single-valley occupancy below a critical density. The gate-bias-tuned transition we observe is accompanied by a sudden, two-fold change in sample resistance, making the phenomenon of interest for potential valleytronic transistor device applications. Our observation constitutes a quintessential demonstration of valleytronics in a simple experiment. Ref: Md. S. Hossain et al., Phys. Rev. Lett. 127, 116601 (2021).