Large variation of interlayer coupling and electron hopping in 1 T − TaSe 2 resolved by angle-resolved photoemission spectroscopy

One fascinating aspect of correlated materials is that minor changes in system parameters may lead to huge variations in material properties. Here, utilizing small-spot angle-resolved photoemission spectroscopy (ARPES), we studied the diverse electronic phases that emerge on the surface of a correlated two-dimensional (2D) material $1T\text{\ensuremath{-}}{\mathrm{TaSe}}_{2}$. Three electronic phases were resolved including a large-gap insulating phase, a small-gap insulating phase, and a metallic phase. We characterized both the in-plane and out-of-plane band dispersions for three different phases. We found that the large- and small-gap insulating phases exhibit 2D properties. While the large-gap insulating phase could be attributed to a Mott insulator where the hopping of electrons across the two nearest central Ta atoms is prohibited due to Coulomb repulsion, the small-gap insulating phase could be attributed to a band insulator where the highest occupied band is fully filled due to an even number stacking of ${\mathrm{TaSe}}_{2}$ layers. For the metallic phase, we observed a dispersive band along the out-of-plane direction, indicating its three-dimensional property. The in-plane band dispersion is highly anisotropic, suggesting that an anisotropic electron hopping is turned on in the metallic phase across different ${\mathrm{TaSe}}_{2}$ layers. Our detailed characterization of three different phases of $1T\text{\ensuremath{-}}{\mathrm{TaSe}}_{2}$ indicates that the formation of star-of-David superstructures in the charge-density-wave (CDW) state of $1T\text{\ensuremath{-}}{\mathrm{TaSe}}_{2}$ not only leads to a correlated insulating phase within one ${\mathrm{TaSe}}_{2}$ layer but also constructs a distinct interlayer coupling that is highly sensitive to the CDW stacking order across different ${\mathrm{TaSe}}_{2}$ layers. As the stacking order changes, the interlayer coupling changes drastically, which directly determines how electrons hop in $1T\text{\ensuremath{-}}{\mathrm{TaSe}}_{2}$. Our observation explains the emergence of diverse electronic phases in $1T\text{\ensuremath{-}}{\mathrm{TaSe}}_{2}$. It also highlights that the dimensionality and electronic transport properties of $1T\text{\ensuremath{-}}{\mathrm{TaSe}}_{2}$ are highly tunable. They can be tuned by manipulating the phase difference between two adjacent CDW layers. Such manipulation does not break the lattice continuity and could be potentially used in developing nanoscale functional devices.