Manipulation of Stacking Order in Td-WTe₂ by Ultrafast Optical Excitation

Abstract

Subtle changes in stacking order of layered transition metal dichalcogenides may have profound influence on the electronic and optical properties. The intriguing electronic properties of Td -WTe 2 can be traced to the break of inversion symmetry resulting from the ground-state stacking sequence. Strategies for perturbation of the stacking order are actively pursued for intentional tuning of material properties, where optical excitation is of specific interest since it holds the potential for integration of ultrafast switches in future device designs. Here we investigate the structural response in Td -WTe 2 following ultrafast photoexcitation by time-resolved electron diffraction. A 0.23 THz shear phonon, involving layer displacement along the b axis, was excited by a 515 nm laser pulse. Pump fluences in excess of a threshold of ∼1 mJ/cm 2 result in formation, with an ∼5 ps time constant, of a new stacking order by layer displacement along the b axis in the direction toward the centrosymmetric 1 T * phase. The shear displacement of the layers increases with pump fluence until saturation at ∼8 pm. We demonstrate that the excitation of the shear phonon and the stabilization of the metastable phase are decoupled when using an optical pump as evidenced by observation of a transition also in samples with a pinned shear phonon. The results are compared to dynamic first-principles simulations and the transition is interpreted in terms of a mechanism where transient local disorder is prominent before settling at the atomic positions of the metastable phase. This interpretation is corroborated by results from diffuse scattering. The correlation between excitation of intralayer vibrations and interlayer interaction demonstrates the importance of including both short- and long-range interactions in an accurate description of how optical fields can be employed to manipulate the stacking order in 2-dimensional transition metal dichalcogenides.

Ultrafast Ferroelectric Ordering on the Surface of a Topological Semimetal MoTe2

“…Transient changes of lattice symmetry in such atomically thin two-dimensional (2D) ferroelectric semimetals, and thus the suppression of ferroelectric polarizations, has been realized in T d -WTe 2 and T d -MoTe 2 on subpicosecond scales via the excitation of phase-determining interlayer shearing mode by intense visible to THz pulses. − Ultrafast symmetry switching in centrosymmetric 1T’-MoTe 2 has also been achieved via excitation of the interlayer shearing mode . Besides promising applications in ultrafast switching, − the corresponding inversion symmetry breaking in these materials also splits their degenerate Dirac fermions into paired topologically protected Weyl fermions of opposite chirality. − They are effectively separated monopoles of Berry curvatures that cause exotic physics such as axial magnetoelectric coupling , and chiral anomaly, which can be precisely controlled via tuning of their lattice symmetry and thus the ferroelectric polarizations .…”

Giant Shear Displacement by Light-Induced Raman Force in Bilayer Graphene

“…We find giant values for Q 0 in our leadingorder theory considering realistic experimental values for laser intensity and carrier doping. The saturation value of shear phonon displacement measured to be around Q max ∼ 8pm in layered WTe 2 using intense infrared laser with electric field strength E 0 ∼ 7.5MV/cm [34] and E 0 ∼ 10MV/cm [38]. Although the earlier experiment [34] does not support a Raman mechanism, the later [38] measures a linear power dependence of the shear displacement consistent with the Raman force F D ∝ E 2 0 .…”

“…The stimulated Raman effect is an efficient mechanism to excite Ramanactive vibrational modes [32,33]. The photo-induced structural transition in layered quantum materials such as multilayer graphene, WTe 2 , and MoTe 2 is rapidly evolving using the ultrafast pump-probe setup and time-resolved second-harmonic-generation spectroscopy [34][35][36][37][38]. Shear phonon dynamics can manipulate the staking order of layers [35,38] and the electronic topology [34].…”

“…The photo-induced structural transition in layered quantum materials such as multilayer graphene, WTe 2 , and MoTe 2 is rapidly evolving using the ultrafast pump-probe setup and time-resolved second-harmonic-generation spectroscopy [34][35][36][37][38]. Shear phonon dynamics can manipulate the staking order of layers [35,38] and the electronic topology [34]. A displacive coherent excitation of the Raman-active shear phonon in MoTe 2 cause a first-order phase transition from inversion symmetric 1T structure to the noncentrosymmetric 1T d phase [35,36].…”

Large change of interlayer vibrational coupling with stacking in Mo$_{1-x}$W$_{x}$Te$_{2}$

“…The a-axis interlayer shear mode (ISM) has been studied for its relevance in identifying the T d phase and in modulating its Weyl semimetal properties. These studies include Raman spectroscopy in MoTe 2 [13][14][15][16][17] and WTe 2 [18][19][20], and various ultrafast spectroscopy techniques in MoTe 2 [21][22][23][24] and WTe 2 [23,[25][26][27][28][29][30]. Raman spectroscopy, however, is limited to measuring the zonecenter energy hω m (i.e., the maximum of the ISM dispersion), and only in the T d phase (for bulk samples) is this mode Raman-active.…”