Origins of the structural phase transitions in MoTe2 and WTe2

Abstract

Layered transition metal dichalcogenides MoTe2 and WTe2 share almost similar lattice constants as well as topological electronic properties except their structural phase transitions. While the former shows a first-order phase transition between monoclinic and orthorhombic structures, the latter does not. Using a recently proposed van der Waals density functional method, we investigate structural stability of the two materials and uncover that the disparate phase transitions originate from delicate differences between their interlayer bonding states near the Fermi energy. By exploiting the relation between the structural phase transitions and the low energy electronic properties, we show that a charge doping can control the transition substantially, thereby suggesting a way to stabilize or to eliminate their topological electronic energy bands. Because of the layered structures of TMDs, several polymorphs can exist and show characteristic physical properties depending on their structures 8 . A typical TMD shows the trigonal prismatic (2H) or the octahedral (1T ) structures 9-12 . For MoTe 2 and WTe 2 , the 2H structure (α-phase, P 6 3 /mmc) is a stable semiconductor while the 1T form is unstable 7,13 . The unstable 1T structure turns into the distorted octahedral one (1T ′ ) 7,14 . The stacked 1T ′ single layer forms a threedimensional bulk with the monoclinic structure (β-phase, P 2 1 /m) or the orthorhombic one (γ-phase, P mn2 1 ) (see Fig. 1) [15][16][17] . Interestingly, the β phase with a few layers is a potential candidate of QSH insulator 7 and the bulk γ phase shows type-II Weyl semimetalic energy bands 4,18,19 , respectively. Since the structural differences between β and γ phases are minute (∼4• tilting of axis along out-of-plane direction in β phase with respect to one in γ phase), the sensitive change in their topological low energy electronic properties is remarkable and the transition between different structures can lead to alternation of topological properties of the system.A phase transition between the β-and γ-phase in the layered TMDs has been known for a long time 16,20 . MoTe 2 shows a first-order transition from the β-to γ-structure at around 250 K 20 when temperature decreases. WTe 2 , however, does not show any transition and stays at the γ-phase 21,22 . Since the structural parameters of a single layer of 1T ′ -MoTe 2 and 1T ′ -WTe 2 are almost the same 15,17,23 and Mo and W belong to the same group in the periodic table, the different phase transition behaviors are intriguing and origins of the contrasting features are yet to be clarified. To understand the phase transition, the proper treatment of long and short range interlayer interaction in TMDs is essential. Most of the theoretical studies, however, fail to reproduce the experimental crystal structures of the two phases of MoTe 2 and WTe 2 so do their topological electronic structures using crystal structures obtained from ab initio calculations [24][25][26][27][28][29][30] . Instead, the atomic structures from experiment data are r...

Bulk Fermi surface of the Weyl type-II semimetallic candidate γ−MoTe2

“…4(a). Notice that part of the discrepancy is attributable to the inter-planar coupling which is not well captured by the DFT calculations [48]. DFT suggests that this compound is van der Waals like by predicting several two-dimensional (i.e.…”

Ultrafast dynamics of the low frequency shear phonon in 1T′-MoTe2

“…A possible mechanism for the light−induced structural phase transition from 1T to T d phases is the transient displacement of the nuclear equilibrium position of the ν 1 mode, which is induced and stabilized by electron doping via photoexcitation, as has been recently proposed by Kim et al [36]. The pump−induced temperature rise (∆T l ) may partly contribute to the decrease in the peak amplitude [ Fig.4(a)] since higher temperature will destroy the T d phase [36]. The transient phase transition between the normal semimetal 1T' and the WSM T d phases observed in the sub−picosecond time domain is expected to be a key phenomenon in controlling the process of the creation and annihilation of Weyl fermions and allows the development of ultrafast phase switching device applications at RT.…”

Dimensionality-driven orthorhombic MoTe2 at room temperature

“…Recently, Kim et al has calculated the electronic structure of bulk MoTe2 across this phase transition [25]. Starting in phase, they find that there are two hole pockets that cross the Fermi energy in the out-of-plane, Γ to A direction.…”