Transport properties of a few nanometer-thick TiSe₂ films grown by molecular-beam epitaxy

Layered materials are known to exhibit a variety of charge-density wave (CDW) phases due to their quasi-two dimensional nature. Of particular interest is the CDW phase in a prototypical layered transition-metal dichalcogenide (TMDC) TiSe₂, where the CDW is known to form with commensurate 2a × 2a × 2c structural distortion at T = 200 K (where a and c are the lattice parameters). Recent experimental studies have revealed intriguing aspects of this material as represented by the emergence of superconductivity upon electron doping and possible existence of the excitonic insulator phase, making TiSe₂ attractive as a model material for investigation of collective phenomena in TMDC. However, the evolution of the CDW phase at nanometer-scale thickness, at least below 10 monolayers (6 nm), has not been well investigated yet, in particular from transport viewpoints, presumably due to difficulty in fabrication of such ultrathin samples by conventional approaches. Here, we report the transport properties of a few nanometer-thick highly crystalline TiSe₂ epitaxial thin films grown on insulating Al₂O₃ substrates by molecular-beam epitaxy, demonstrating robust CDW transitions down to 5 monolayers (3 nm). We also clarify an interesting aspect of van der Waals epitaxy, a “self-rotational” growth without strain, which should be realized only in a system having a weak substrate-film interaction.