We report the influence of uniaxial tensile mechanical strain in the range 0-2.2% on the phonon spectra and bandstructures of monolayer and bilayer molybdenum disulfide (MoS2) two-dimensional crystals. First, we employ Raman spectroscopy to observe phonon softening with increased strain, breaking the degeneracy in the E' Raman mode of MoS2, and extract a Grüneisen parameter of ~1.06. Second, using photoluminescence spectroscopy we measure a decrease in the optical band gap of MoS2 that is approximately linear with strain, ~45 meV/% strain for monolayer MoS2 and ~120 meV/% strain for bilayer MoS2. Third, we observe a pronounced strain-induced decrease in the photoluminescence intensity of monolayer MoS2 that is indicative of the direct-to-indirect transition of the character of the optical band gap of this material at applied strain of ~1%. These observations constitute a demonstration of strain engineering the band structure in the emergent class of two-dimensional crystals, transition-metal dichalcogenides.

We directly trace the multi-THz conductivity of VO2 during an insulator-metal transition triggered by a 12-fs light pulse. The femtosecond dynamics of lattice and electronic degrees of freedom are spectrally discriminated. A coherent wave packet motion of V-V dimers at 6 THz modulates the lattice polarizability for approximately 1 ps. In contrast, the electronic conductivity settles to a constant value already after one V-V oscillation cycle. Based on our findings, we propose a qualitative model for the nonthermal phase transition.

Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic VO 2 quasi-instantaneously into a metal. Thereby, we exclude an 80 fs structural bottleneck for the photoinduced electronic phase transition of VO 2 . First-principles many-body perturbation theory calculations reveal a high sensitivity of the VO 2 band gap to variations of the dynamically screened Coulomb interaction, supporting a fully electronically driven isostructural insulatorto-metal transition. We thus conclude that the ultrafast band structure renormalization is caused by photoexcitation of carriers from localized V 3d valence states, strongly changing the screening before significant hot-carrier relaxation or ionic motion has occurred. DOI: 10.1103/PhysRevLett.113.216401 PACS numbers: 71.27.+a, 71.20.Be, 71.30.+h, 79.60.-i Since its discovery in 1959 [1], studies of the VO 2 phase transition (PT) from a monoclinic (M 1 ) insulator (Fig. 1, top left) to a rutile (R) metal at T C ¼ 340 K (Fig. 1, top right) have revolved around the central question [2][3][4][5] of whether the crystallographic PT is the major cause for the electronic PT or if strong electron correlations are needed to explain the insulating low-T phase. While the M 1 structure is a necessary condition for the insulating state below T C , the existence of a monoclinic metal (mM) and its relevance to the thermally driven PT is under current investigation [6][7][8][9][10][11][12]. In particular, the role of carrier doping at temperatures close to T C by charge injection from the substrate or photoexcitation has been increasingly addressed [6,8,[13][14][15][16].One promising approach to disentangling the electronic and lattice contributions is to drive the PT nonthermally using ultrashort laser pulses in a pump-probe scheme. Time-resolved x-ray [17,18] and electron diffraction [16,19] showed that the lattice structure reaches the R phase quasithermally after picoseconds to nanoseconds. Transient optical spectroscopies have probed photoinduced changes of the dielectric function in the terahertz [20][21][22], near-IR [9,10,17,23], and visible range [23]. The nonequilibrium state reached by photoexcitation (hereinafter transient phase) differs from the two equilibrium phases, but eventually evolves to the R phase [17][18][19][20][21][22][23][24][25][26][27][28]. The observation of a minimum rise time of 80 fs in the optical response after strong excitation (50 mJ=cm 2 ), described as a structural bottleneck in VO 2 [24], challenged theory to describe the photoinduced crystallographic and electronic PT simultaneously [15,25].Time-resolved photoelectron spectroscopy (TR-PES) directly probes changes of the electronic structure. Previous photoelectron spectroscopy (PES) studies of VO 2 used high photon energies generating photoelectrons with large kinetic energies to study the dynamics of the electronic structure; however, with a low repetition rate (50 Hz [27]) and inadequate time resolution (> 150 fs) the ultrafast dynamics of t...

The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. Here we show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in Vo 2 and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.