Crossover from 2-D metal to 3-D Dirac semimetal in metallic PtTe2 films with local Rashba effect

Transition metal dichalcogenides (TMDCs) formed by group 10 metals (e.g. PtSe₂, PtTe₂) have emerged as important materials with intriguing properties discovered both in bulk single crystals and atomically thin films. While bulk PtSe₂ and PtTe₂ are type-II Dirac semimetals, monolayer (ML) PtSe₂ film is a semiconductor with helical spin texture induced by local Rashba effect. However, the properties of atomically thin PtTe₂ films and the evolution with film thickness remain unexplored. Recently, Shuyun Zhou's group from Tsinghua University reported a systematic study on the electronic structure of high quality PtTe₂ thin films with thickness from 2 ML to 6 ML grown by molecular beam epitaxy (MBE). This work provides direct experimental evidence for a crossover from 2-D metal (2ML film, distinguished from the semiconducting PtSe₂ film) to 3-D Dirac semimetal in PtTe₂ films with spin texture induced by local Rashba effect.

In bulk PtTe₂ crystal, massless Dirac fermions are found to emerge at the topologically protected touching points of electron and hole pockets. The strongly tilted Dirac cone along the out-of-plane momentum direction breaks the Lorentz invariance and therefore the low energy excitations are Dirac fermions that do not have counterpart in high-energy physics. Their previous work shows that the isostructural material PtSe₂ is also a type-II Dirac semimetal, and there is a 3-D Dirac semimetal-semiconductor transition with decreasing thickness. In addition, monolayer PtSe₂ shows interesting helical spin texture induced by local Rashba effect (R-2) despite the fact that the monolayer film itself is centrosymmetric. Such hidden spin texture induced by local Rashba effect has been expected to provide an important platform for realizing novel topological superconductivity with odd parity if superconductivity with s-wave pairing can be induced. However, considering that monolayer PtSe₂ is a semiconductor with a large gap size of 1.2 eV, it is difficult to tune the Fermi energy in such a large range to make it a superconductor. It is therefore critical to find a similar centrosymmetric film with a local Rashba effect yet with a metallic property, which can provide a better opportunity for realizing topological superconductivity.

In this work, by using ARPES, Shuyun Zhou's group directly detects the electronic structure of PtTe₂ thin films with different thickness, and observed metallic band dispersion of PtTe₂ thin films even down to 2 ML. A crossover from 2-D metal (2ML, distinguished from the 2 ML semiconducting PtSe₂ film) to 3-D Dirac semimetal is also revealed. The electronic dispersion exhibits strong thickness dependent: with increasing film thickness, the V-shaped pockets at the Fermi level in 2 ML and 3 ML films move down in energy and eventually touch the hole-like pocket at higher binding energy, leading to the formation of a three-dimensional type-II Dirac fermions in 4-6 ML films, which show similar dispersion to bulk crystal and thus are effectively a topological semimetal. Further spin-ARPES measurements on PtTe₂ bulk crystal reveal a helical spin texture induced by local Rashba effect. Since the Rashba effect is determined by the crystal symmetry and bulk and thin film PtTe₂ share the same symmetry, similar spin texture by local Rashba effect is also expected in PtTe₂ thin films.

This systematic investigation on PtTe₂ thin films with controlled thickness offers a unique platform to study the metallic thin films with local Rashba effect, and opens up opportunities for further investigating the intriguing properties, e.g. doping induced superconductivity or topological superconductivity.