Electrically switchable valley polarization, spin/valley filter, and valve effects in transition-metal dichalcogenide monolayers interfaced with two-dimensional ferromagnetic semiconductors

Electron valleys in transition-metal dichalcogenide monolayers drive novel physics and allow designing multifunctional architectures for applications. We propose to manipulate the electron valleys in these systems for spin/valley filter and valve devices through band engineering. Instead of the magnetic proximity effect that has been extensively used in previous studies, in our strategy, the electron valleys are directly coupled to the spin-polarized states of two-dimensional ferromagnets. We find that this coupling results in a valley-selective gap opening due to the spin-momentum locking in the transition-metal dichalcogenide monolayers. This physics gives rise to a variety of unexpected electronic properties and phenomena including half-metallicity, electrically switchable valley polarization, spin/valley filter, and valve effects in the transition-metal dichalcogenide monolayers. We further demonstrate our idea in ${\mathrm{MoTe}}_2/{\mathrm{CoCl}}_2$ and ${\mathrm{CoCl}}_2/{\mathrm{MoTe}}_2/{\mathrm{CoCl}}_2$ van der Waals heterojunctions based on first-principles calculations. Thus, our study provides a way of engineering the electron valleys in transition-metal dichalcogenide monolayers for contemporary devices.

Figure 1

Coupling of a TMD monolayer to a 2D ferromagnet. (a) Schematic illustrations of the heterostructure (upper panel) and the electron valleys in the TMD monolayers (lower panel). (b), (c) Schematic illustrations of the band alignments of the heterostructure (left) and the band structures due to the hybridization between the states of the constituents (right). The dashed lines represent the spin-polarized bands of the 2D ferromagnet. There are two types of band alignments between the valence band maximum (VBM) of the TMD monolayer and the spin-polarized band of the substrate. The spin-polarized band of the FM monolayer can lie either (b) higher or (c) lower than the VBM of the TMD monolayer, which are defined as type-I and type-II alignments, respectively. Arrows show the spins of the electronic bands. $\mathrm{\Delta }E$ denotes the valley polarization in the TMD monolayer, i.e., $\mathrm{\Delta }E=E_{\text{VBM}}\left(K^{\prime }\right)-E_{\text{VBM}}\left(K\right)$. (d), (e) In-plane spin/valley valve effect in a FM/TMD/FM junction

Figure 2

Valley-selective gap opening in ${\mathrm{MoTe}}_2/{\mathrm{CoCl}}_2$. (a) Perspective view of the heterostructure. (b)–(d) Top views of the heterostructure in three high-symmetry stackings and (e)–(g) corresponding band structures. (h)–(m) Layer- and spin-projected band structures for the three configurations. The energy difference relative to the lowest-energy structure (C1) is given below each configuration. The dashed-dotted lines in (e) and (h) are drawn to guide the eye. The Fermi level is set to be zero.

Figure 3

Electrical switching of the valley polarization in ${\mathrm{MoTe}}_2/{\mathrm{CoCl}}_2$. (a), (b) Band structures of the heterostructure under different electric fields. (c), (d) Corresponding band structures with layer and spin projections onto ${\mathrm{MoTe}}_2$. The dashed-dotted lines in (c) and (d) are used to guide the eye. (e) Valley splitting as a function of gate voltage. (f) Phase diagram of ${\mathrm{MoTe}}_2/{\mathrm{CoCl}}_2$ under external electric fields.

Figure 4

Spin/valley valve effect in the ${\mathrm{CoCl}}_2/{\mathrm{MoTe}}_2/{\mathrm{CoCl}}_2$ junction. Spin-projected band structures around $K$ and $K^{\prime }$ for that the magnetizations of the two ${\mathrm{CoCl}}_2$ layers are aligned (a) ferromagnetically and (b) antiferromagnetically, respectively. The red and blue lines denote different spin states. The insets show the structure of the junction and the magnetizations of the ${\mathrm{CoCl}}_2$ monolayers. The Fermi level is set to be zero. (c), (d) Proposed spin and valley valve made of a FM/TMD/FM junction.