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...

We report on the nonequilibrium dynamics of the electronic structure of the layered semiconductor Ta_{2}NiSe_{5} investigated by time- and angle-resolved photoelectron spectroscopy. We show that below the critical excitation density of F_{C}=0.2  mJ cm^{-2}, the band gap narrows transiently, while it is enhanced above F_{C}. Hartree-Fock calculations reveal that this effect can be explained by the presence of the low-temperature excitonic insulator phase of Ta_{2}NiSe_{5}, whose order parameter is connected to the gap size. This work demonstrates the ability to manipulate the band gap of Ta_{2}NiSe_{5} with light on the femtosecond time scale.

We apply ultrafast x-ray diffraction with femtosecond temporal resolution to monitor the lattice dynamics in a thin film of multiferroic BiFeO 3 after above-band-gap photoexcitation. The sound-velocity limited evolution of the observed lattice strains indicates a quasi-instantaneous photoinduced stress which decays on a nanosecond time scale. This stress exhibits an inhomogeneous spatial profile evidenced by the broadening of the Bragg peak. These new data require substantial modification of existing models of photogenerated stresses in BiFeO 3 : the relevant excited charge carriers must remain localized to be consistent with the data. Multiferroics have a great potential for application due to their possible coupling of ferroelectricity and magnetism [1][2][3]. BiFeO 3 (BFO) is one of the few room temperature multiferroics today [4][5][6][7][8], and of these, the only one that is a stable phase. Its relatively small band gap of approximately 2.7 eV [9] renders BFO an ideal candidate for applications in spintronics and memory devices [5] with a perspective for ultrafast optical switching similar to purely ferroelectric [10] or magnetic materials [11]. The photovoltaic effect in this complex material and the underlying ultrafast carrier dynamics after above-band-gap femtosecond (fs) optical excitation have been studied thoroughly [12][13][14]. The photoinduced currents in BFO lead to THz emission [15,16] and to a photostrictive response [17]. Alloptical experiments showed that the rapid photoinduced mechanical stress excites coherent phonons [18,19]. The dynamics of photoinduced strains were directly and quantitatively measured in a recent synchrotron-based ultrafast x-ray diffraction (UXRD) study with a temporal resolution of 100 ps [20]. Combined optical measurements revealed a linear dependence of the transient strain and the number of excited carriers over several nanoseconds (ns). This led to the conclusion that depolarization field screening (DFS) including macroscopic transport of the carriers to the surface and interface could be the dominant stress generating process, although the effect of excited antibonding orbitals was not ruled out [20].In this Letter, we report complementing UXRD experiments at a laser-driven plasma x-ray source (PXS) in order to monitor the coherent and incoherent lattice dynamics in a BFO thin film sample with subpicosecond (ps) temporal resolution after above-band-gap excitation. We observe a sound-velocity limited evolution of the structural response within 10 ps indicating a quasi-instantaneous stress. The substantial Bragg peak broadening is a direct evidence of an inhomogeneous spatial stress profile. It appears quasiinstantaneously and decays on nanosecond time scales as reconfirmed by new synchrotron-based UXRD data recorded at the Advanced Photon Source (APS). We obtain quantitative agreement of the transient peak shift and broadening measured with both setups and can firmly conclude that the photogenerated stress driving the film expansion has a strongly inhomogeneous sp...