Distinguishing insulators by the dominant type of interaction is a central problem in condensed matter physics. Basic models include the Bloch-Wilson and the Peierls insulator due to electron-lattice interactions, the mott and the excitonic insulator caused by electron-electron interactions, and the Anderson insulator arising from electron-impurity interactions. In real materials, however, all the interactions are simultaneously present so that classification is often not straightforward. Here, we show that time-and angle-resolved photoemission spectroscopy can directly measure the melting times of electronic order parameters and thus identify-via systematic temporal discrimination of elementary electronic and structural processes-the dominant interaction. specifically, we resolve the debates about the nature of two peculiar charge-density-wave states in the family of transition-metal dichalcogenides, and show that Rb intercalated 1T-Tas 2 is a Peierls insulator and that the ultrafast response of 1T-Tise 2 is highly suggestive of an excitonic insulator.

Photo-induced switching between collective quantum states of matter is a fascinating rising field with exciting opportunities for novel technologies. Presently very intensively studied examples in this regard are nanometer-thick single crystals of the layered material 1T-TaS2, where picosecond laser pulses can trigger a fully reversible insulator-to-metal transition (IMT). This IMT is believed to be connected to the switching between metastable collective quantum states, but the microscopic nature of this so-called hidden quantum state remained largely elusive up to now. Here we determine the latter by means of state-of-the-art x-ray diffraction and show that the laser-driven IMT involves a marked rearrangement of the charge and orbital order in the direction perpendicular to the TaS2layers. More specifically, we identify the collapse of inter-layer molecular orbital dimers, which are a characteristic feature of the insulating phase, as a key mechanism for the non-thermal IMT in 1T-TaS2, which indeed involves a collective transition between two truly long-range ordered electronic crystals.The layered transition metal dichalcogenides (TMDs) form a vast class of materials hosting diverse non-trivial quantum phenomena such as spin-valley polarization [1], Ising-superconductivity [2] or intertwined electronic orders [3,4]. All these intriguing electronic effects along with the natural suitability of TMDs for the preparation of quasi two-dimensional (2D) nano-sheets render them highly appealing for next-generation technologies [5][6][7][8].1T-TaS 2 is a particularly interesting and extensively studied TMD in which external tuning parameters like temperature, pressure or chemical substitution span a particularly complex electronic phase diagram. Apart from several charge density waves (CDWs) this phase diagram also features pressure-induced superconductivity and a so-called Mott-phase, which stands out due to its semiconducting electronic transport properties [3,9].Remarkably, besides the aforementioned states that can be reached in thermal equilibrium, femto to picosecond optical or electrical pulses can launch a nonequilibrium IMT into a previously hidden and persistent metallic CDW-state [7,10,11]. The discovery of this so-called hidden CDW (HCDW) has sparked wide excitement as it might provide a new platform for memory device applications. Accordingly, in recent years, a significant number of experimental and theoretical studies aimed at pinning down the microscopic mechanism of this non-equilibrium IMT that is believed to be connected to a reorganization of the CDW-order. However, despite significant efforts to determine the microscopic processes underlying this novel IMT have been made [12][13][14][15][16][17], a clear picture remains elusive.In this article we address this open issue directly by means of high-resolution synchrotron x-ray diffraction (XRD) in combination with laser pumping. Our experiments enable examination of the laser-driven transition and in-particular the HCDW-order in 1T-TaS 2 nanosheets wi...

Strongly correlated systems exhibit intriguing properties caused by intertwined microscopic interactions that are hard to disentangle in equilibrium. Employing non-equilibrium time-resolved photoemission spectroscopy on the quasi-two-dimensional transition-metal dichalcogenide 1T -TaS2, we identify a spectroscopic signature of double occupied sites (doublons) that reflects fundamental Mott physics. Doublon-hole recombination is estimated to occur on time scales of one electronic hopping cycleh/J ≈ 14 fs. Despite strong electron-phonon coupling the dynamics can be explained by purely electronic effects captured by the single band Hubbard model, where thermalization is fast in the small-gap regime. Qualitative agreement with the experimental results however requires the assumption of an intrinsic hole-doping. The sensitivity of the doublon dynamics on the doping level provides a way to control ultrafast processes in such strongly correlated materials.

Time-and angle-resolved extreme ultraviolet photoemission spectroscopy is used to study the electronic structure dynamics in BaFe2As2 around the high-symmetry points Γ and M . A global oscillation of the Fermi level at the frequency of the A1g(As) phonon mode is observed. It is argued that this behavior reflects a modulation of the effective chemical potential in the photoexcited surface region that arises from the high sensitivity of the band structure near the Fermi level to the A1g phonon mode combined with a low electron diffusivity perpendicular to the layers. The results establish a novel way to tune the electronic properties of iron pnictides: coherent control of the effective chemical potential. The results further suggest that the equilibration time for the effective chemical potential needs to be considered in the ultrafast electronic structure dynamics of materials with weak interlayer coupling.PACS numbers: 74.25. Jb,74.70.Xa, Time-resolved optical and photoemission spectroscopies have become important tools to probe the microscopic details of electron-phonon coupling. The prime example is the reliable determination of the coupling parameter from measured relaxation times of excited electrons [1][2][3][4][5][6]. More recently, time-resolved spectroscopies have provided novel insights into the transient behavior of electronically ordered phases, specifically chargedensity waves and superconductivity, in which electronphonon coupling plays a prominent role [7][8][9][10][11][12][13][14].A particularly intriguing aspect of electron-phonon coupling often observed in pump-probe spectroscopy is the generation of coherent optical phonons [15] and their subsequent modulation of electronic properties. This effect not only provides a powerful means to study femtosecond lattice dynamics [16], but can also be used to coherently control the electronic structure of materials. Through time-and angle-resolved photoemission spectroscopy (trARPES), coherent phonon-induced oscillations of electron binding energies are now well known [7,8,[17][18][19][20], and in a recent study on the semimetal Bi it was also shown how the underlying momentumdependent deformation potential can be determined from such oscillations with the help of density functional theory (DFT) [19]. Since the electrons with the lowest binding energies determine material properties and collective phenomena, the physics will become particularly interesting if transient band shifts and renormalizations are induced near the chemical potential, which itself may then have to adjust to preserve charge neutrality. However, transient band renormalization effects in the vicinity of the chemical potential have so far only been reported for charge-density-wave systems [7,8,12,13].Iron pnictides should provide a fertile field for the study of coherent phonon-induced electronic effects near the chemical potential. Firstly, their electronic, magnetic, and superconducting properties are well known to be highly sensitive to the distance between the iron and pnictogen pl...

Time-and angle-resolved photoelectron spectroscopy with 13 fs temporal resolution is used to follow the different stages in the formation of a Fermi-Dirac distributed electron gas in graphite after absorption of an intense 7 fs laser pulse. Within the first 50 fs after excitation a sequence of time frames is resolved which are characterized by different energy and momentum exchange processes among the involved photonic, electronic, and phononic degrees of freedom. The results reveal experimentally the complexity of the transition from a nascent non-thermal towards a thermal electron distribution due to the different timescales associated with the involved interaction processes. 63.20.kd, 81.05.ue, 81.05.uf The extraordinary nonlinearities and optical response times of graphitic materials suggest useful applications in photonics and electronics including light harvesting [1, 2], ultrafast photodetection [3,4], THz lasing [5,6], and saturable absorption [7,8]. Both characteristics are closely linked to the ultrafast dynamics of photoexcited carriers which for this material class is governed by weakly screened carrier-carrier scattering and carrier-phonon interaction. Fundamental aspects related to these processes were addressed in different time-domain studies in the past [9][10][11][12][13]. Because of limitations in the time resolution, most of these studies were restricted, however, to the characteristic timescales of electron-lattice equilibration, i.e., timescales ranging from ≈100 fs to ≈10 ps. The primary processes directly after photoexcitation are, in contrast, still largely unexplored and were investigated experimentally only in a few studies so far [14][15][16]. The dynamics in this strongly non-thermal regime is determined by phenomena such as transient population inversion, carrier multiplication, Auger recombination, but also phonon-mediated carrier redistribution [17][18][19][20]. The challenge is to decode the relative importance and temporal sequence of these processes that drive the electronic system from a nascent non-thermal distribution as generated by photoexcitation towards a Fermi-Dirac (FD) distribution within only ≈ 50 fs [14,15]. It is obvious that such investigations rely on experiments capable of sampling this time window at an adequate time resolution of the order of 10 fs, as well as high energy and momentum resolution. This letter reports on the non-thermal carrier dynamics in highly-oriented pyrolytic graphite (HOPG) as probed in a time-and angle-resolved photoemission spectroscopy (trARPES) experiment that is operated near the transform limit at a resolution of 13 fs (FWHM of the pumpprobe cross correlation) [21]. Over the first 100 fs, we monitor the different stages in the temporal evolution of an initially non-thermal carrier distribution generated by the absorption of a 7 fs near-infrared pulse. We are able to dissect the non-thermal to thermal transition into FIG. 1. (a) WL-pump/XUV-probe cross correlation signal of the experiment. For details see Refs. [21] and [24]. (b) ...