Magnetic Field‐Induced Polar Order in Monolayer Molybdenum Disulfide Transistors

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In semiconducting monolayer transition metal dichalcogenides (ML‐TMDs), broken inversion symmetry and strong spin‐orbit coupling result in spin‐valley lock‐in effects so that the valley degeneracy may be lifted by external magnetic fields, potentially leading to real‐space structural transformation. Here, magnetic field ( B )‐induced giant electric hysteretic responses to back‐gate voltages are reported in ML‐MoS 2 field‐effect transistors (FETs) on SiO 2 /Si at temperatures < 20 K. The observed hysteresis increases with | B | up to 12 T and is tunable by varying the temperature. Raman spectroscopic and scanning tunneling microscopic studies reveal significant lattice expansion with increasing | B | at 4.2 K, and this lattice expansion becomes asymmetric in ML‐MoS 2 FETs on rigid SiO 2 /Si substrates, leading to out‐of‐plane mirror symmetry breaking and the emergence of a tunable out‐of‐plane ferroelectric‐like polar order. This broken symmetry‐induced polarization in ML‐MoS 2 shows typical ferroelectric butterfly hysteresis in piezo‐response force microscopy, adding ML‐MoS 2 to the single‐layer material family that exhibits out‐of‐plane polar order‐induced ferroelectricity, which is promising for such technological applications as cryo‐temperature ultracompact non‐volatile memories, memtransistors, and ultrasensitive magnetic field sensors. Moreover, the polar effect induced by asymmetric lattice expansion may be further generalized to other ML‐TMDs and achieved by nanoscale strain engineering of the substrate without magnetic fields.