Dimensional crossover of the charge density wave transition in thin exfoliated VSe ₂

Abstract

The capability to isolate one to few unit-cell thin layers from the bulk matrix of layered compounds 1 opens fascinating prospects to engineer novel electronic phases. However, a comprehensive study of the thickness dependence and of potential extrinsic effects are paramount to harness the electronic properties of such atomic foils. One striking example is the charge density wave (CDW) transition temperature in layered dichalcogenides whose thickness dependence remains unclear in the ultrathin limit 2-5 .Here we present a detailed study of the thickness and temperature dependences of the CDW in VSe 2 by scanning tunnelling microscopy (STM). We show that mapping the real-space CDW periodicity over a broad thickness range unique to STM provides essential insight 6 . We introduce a robust derivation of the local order parameter and transition temperature based on the real space charge modulation amplitude. Both quantities exhibit a striking non-monotonic thickness dependence that we explain in terms of a 3D to 2D dimensional crossover in the FS topology. This finding highlights thickness as a true tuning parameter of the electronic ground state and reconciles seemingly contradicting thickness dependencies determined in independent transport studies.Following the ground-breaking exfoliation of graphite into one atom thin carbon sheets, an increasing number of layered compounds can now be isolated from their bulk matrix in the form of one to few unit-cell thin layers. These sheets often feature unique 7-9 or enhanced 2,10 properties in comparison to their parent bulk compounds. They depend on material thickness and can be further tuned through doping, electrostatic gating and assembly of distinct layers into complex heterostructures. Transition metal dichalcogenides (TMDs) are of particular interest in this context. They can be readily exfoliated into thin flakes down to the single unitcell limit 11 and offer a unique playground for studying the thickness dependence of their electronic properties. For example, in MoS 2 , photo active transitions become available in the

Pseudogap, Fermi arc, and Peierls-insulating phase induced by 3D–2D crossover in monolayer VSe2

“…3(c) in which one can recognize a two-peaked structure persisting up to T = 120 K. Since the two-peaked structure disappears above 140 K, it is inferred that the gap starts to open at around this temperature. It is remarked that the observed TCDW (140 K) of monolayer VSe2 is in agreement with the recent study on a MBE-grown monolayer film [16], but is apparently higher than those of both bulk (~ 110 K) and exfoliated multilayer thin films (< 130 K) [14,15] As shown in Fig. 3(b), the EDC of b pocket (point B) at T = 40 K exhibits a leading-edge shift similarly to the a pocket, whereas the overall spectral feature is much broader and a pseudogap-like structure with an energy scale of ~200 meV is observed as indicated by a black circle in Fig.…”

“…Recently it was reported that exfoliation of bulk crystal leads to an intriguing change in TCDW; i.e. TCDW gradually decreases with reducing the number of layers [14], but suddenly exhibits a characteristic upturn at around the film thickness of 10 nm, reaching ~130 K much higher than bulk TCDW (110 K) [15]. Further, very recently, monolayer VSe2 was fabricated and the low energy electron diffraction (LEED), angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) have revealed that the TCDW is enhanced in monolayer (~140 K) compared to bulk [16,17,18].…”

Strain engineering a 4a×√3a charge-density-wave phase in transition-metal dichalcogenide 1T−VSe2Zhang1, Ha2, Baek3 et al. 2017Phys. Rev. Materials543340View full textAdd to dashboardCite

“…This corresponds to a wave vector of Q 1 = (1/4, 0, 2/3) (in units of reciprocal lattice vectors). This wave vector and the equivalent ones, related by symmetry, suggest a CDW periodicity of 4 a × 1 a × 3 c , 1 a × 4 a × 3 c , or 4 a × 4 a × 3 c , in agreement with reports for bulk VSe 2 [83,84], and unstrained thin layered samples [51]. We have checked the phonon frequencies at the Γ point for these phases and find that they are non-imaginary.…”

“…First-principles calculations identify this additional CDW state to be consistent with soft modes in the normal-state phonon dispersion and as a competing low energy state to the conventional 4 a ×4 a CDW. We note that a conventional 4 a ×4 a CDW was observed in thin (10 nm) exfoliated VSe 2 layered samples [51]. This indicates the strain in our epitaxial film plays an important role in the rise of the unconventional 4 a ×√3 a CDW.…”

Controlling the Charge Density Wave Transition in Monolayer TiSe2: Substrate and Doping Effects

“…In some cases, the monolayer of these materials may exhibit new properties compared to their bulk counterpart, for example, ferromagnetic ordering has been observed in monolayer VSe 2 while the bulk is paramagnetic to low temperatures . Moreover the CDW order temperature may be different for mono‐ and few‐layer samples compared to that of the bulk …”