• Alvarez, Carlos
• Okuno, Hanako
• Bosak, Alexei
• Sant, Roberto
• Tsipas, Polychronis
• Dimoulas, Athanasios
• Tsoutsou, Dimitra
• Renaud, Gilles
• Fragkos, Sotirios

Abstract

Despite a large number of studies [2,3] over the years since the first discovery [7] and a couple of comprehensive reviews [8,9] the actual mechanism for PLD/CDW formation is still under debate. The most recent experimental [10-13] and theoretical [14] works focus on the large area growth of the CDW phase [13] the thickness dependence , and the possible unconventional behavior in the ultimate 2D limit of a single layer TiSe 2. [10-12,14] On the other hand, the other Ti dichalcogenides namely TiS 2 and TiTe 2 did not show any clear evidence until very recently when a CDW state was reported only for 1 monolayer (ML)-thin TiTe 2 at temperatures lower than 92 K. [15] It is surprising that the CDW in TiTe 2 was found to be totally suppressed for films thicker than 1 ML, [15] unlike the case of other TMDs where 1 ML and bulk-like films both make the transition to a CDW at nearly the same temperature. The interest about TiTe 2 is continuously increasing in view of theoretical predictions [16] and more recent experimental evidence [17] about pressure induced topological phase transitions in TiTe 2. The possibility to also manipulate superconduc-tivity by external pressure as predicted [18] and more recently evidenced [19] in bulk TiTe 2 creates the prospect to explore the emergence of topological superconductivity in this material. In the latter work [19] it has been shown that under nonhydro-static pressure, a CDW-like state with estimated transition temperature above room temperature (RT) appears in bulk TiTe 2 at around 0.5-1.8 GPa. These results call for a re-examination of the possibility to obtain a CDW in multilayer TiTe 2 and indeed at RT with good potential for real world applications utilizing the properties of the CDW state. These applications include a voltage-controlled oscillator device operating at room temperature , [20] fast electronic resistance switching for nonvolatile memories, [21,22] and field-effect transistor devices potentially suitable for implementation of non-Boolean logic. [23] In this paper it is shown that multilayer films (50 ML ≈ 32 nm), as well as single layer TiTe 2 epitaxially grown on InAs(111)/ Si(111) substrates by molecular beam epitaxy exhibit, in ambient pressure conditions, a CDW distortion at room temperature which is sustained up to higher temperatures, at least 400 °C, as evidenced by reflection high energy electron diffrac-tion (RHEED) (Figure S1, Supporting Information). The results are explained in terms of anisotropic strain imposed by the substrate. The group IVB 2D transition metal dichalcogenides are considered to be stable in the high symmetry trigonal octahedral structure due to the lack of unpaired d-electrons on the metal site. It is found that multilayer epitaxial TiTe 2 is an exception adopting a commensurate 2 × 2 × 2 charge density wave (CDW) structure at room temperature with an ABA type of stacking as evidenced by direct lattice imaging and reciprocal space mapping. The CDW is stabilized by highly anisotropic strain imposed by the substrate with an out-off-plane compression which reduces the interlayer van der Waals gap increasing the coupling between TiTe 2 layers.

Topics

• density
• impedance spectroscopy
• phase
• anisotropic
• phase transition
• superconductivity
• superconductivity