We study the 2×2 charge density wave (CDW) in epitaxially-grown monolayer TiSe2. Our temperaturedependent angle-resolved photoemission spectroscopy measurements indicate a strong-coupling instability, but reveal how not all states couple equally to the symmetry-breaking distortion, with an electron pocket persisting to low temperature as a non-bonding state. We further show how the CDW order can be suppressed by a modest doping of around 0.06(2) electrons per Ti. Our results provide an opportunity for quantitative comparison with a realistic tight-binding model, which emphasises a crucial role of structural aspects of the phase transition in understanding the hybridisation in the ground state. Together, our work provides a comprehensive understanding of the phenomenology of the CDW in TiSe2 in the 2D limit.

Reducing the thickness of a material to its two-dimensional (2D) limit can have dramatic consequences for its collective electronic states, including magnetism, superconductivity, and charge and spin ordering. An extreme case is TiTe2, where a charge density wave (CDW) emerges in the single-layer, which is absent for the bulk compound, and whose origin is still poorly understood. Here, we investigate the electronic band structure evolution across this CDW transition using temperature-dependent angle-resolved photoemission spectroscopy. Our study reveals an orbital-selective band hybridisation between the backfolded conduction and valence bands occurring at the CDW phase transition, which in turn leads to a significant electronic energy gain, underpinning the CDW transition. For the bulk compound, we show how this energy gain is almost completely suppressed due to the three-dimensionality of the electronic band structure, including via a kz-dependent band inversion which switches the orbital character of the valence states. Our study thus sheds new light on how control of the electronic dimensionality can be used to trigger the emergence of new collective states in 2D materials.

Spin-valley locking is ubiquitous to transition-metal dichalcogenides (TMDs) with local or global inversion asymmetry, in turn stabilising properties such as Ising superconductivity, and opening routes towards 'valleytronics'. The underlying valley spin splitting is set by spin-orbit coupling, but can be tuned via application of external magnetic fields or through proximity coupling. However, only modest changes have been realised to date. Here, we investigate the electronic structure of the V-intercalated TMD V 1/3 NbS2 using microscopic area spatially-and angle-resolved photoemission spectroscopy. Our measurements and corresponding density-functional theory calculations reveal that the bulk magnetic order induces a giant valley-selective Ising coupling exceeding 50 meV in the surface NbS2 layer, equivalent to application of a ∼ 250 T magnetic field. This is of comparable magnitude to the intrinsic spin-orbit splittings, and indicates how coupling of local magnetic moments to itinerant states of a TMD monolayer provides a powerful route to controlling their valley spin splittings.