The complex electronic properties of ZrTe5 have recently stimulated in-depth investigations that assigned this material to either a topological insulator or a 3D Dirac semimetal phase. Here we report a comprehensive experimental and theoretical study of both electronic and structural properties of ZrTe5, revealing that the bulk material is a strong topological insulator (STI). By means of angle-resolved photoelectron spectroscopy, we identify at the top of the valence band both a surface and a bulk state. The dispersion of these bands is well captured by ab initio calculations for the STI case, for the specific interlayer distance measured in our x-ray diffraction study. Furthermore, these findings are supported by scanning tunneling spectroscopy revealing the metallic character of the sample surface, thus confirming the strong topological nature of ZrTe5.The discovery of topological insulators (TIs), characterized by metallic spin-polarized surface states connecting the bulk valence and conduction bands [1], has stimulated the search for novel topological phases of matter [2][3][4][5][6]. ZrTe 5 has recently emerged as a challenging system with unique, albeit poorly understood, electronic properties [7][8][9][10][11][12][13][14][15][16][17]. Magneto-transport [11], magneto-infrared [13] and optical spectroscopy [14] studies describe ZrTe 5 in terms of a 3D Dirac semimetal. Theoretical calculations have predicted its bulk electronic properties to lie in proximity of a topological phase transition between a strong and a weak TI (STI and WTI, respectively), where only the former displays topologically protected surface states at the experimentally accessible (010) surface [10]. The monolayer is also computed to be a 2D TI [10] and scanning tunneling microscopy/spectroscopy (STM/STS) experiments suggest the existence of topologically protected states at step edges [18,19]. However, the unambiguous identification of the topological phase of ZrTe 5 is still lacking.In this Letter we report on the STI character of the bulk ZrTe 5 by combining ab initio calculations and multiple experimental techniques, at temperature both above and below the one of the resistivity peak, T * ∼ 160 K [7][8][9]15]. Angleresolved photoelectron spectroscopy (ARPES) experiments in the ultraviolet (UV) and soft x-ray (SX) energy ranges reveal the presence of two distinct states at the top of the valence band (VB). On the basis of photon energy dependent studies, we ascribe the origin of these two states to the bulk and crystal surface, respectively. We have performed ab initio calculations of the topological phase diagram of ZrTe 5 , as a function of the interlayer distance b/2. Our measured band dispersion is in agreement with the calculations and it is consistent with the STI case for b/2 = 7.23 ± 0.02Å. This value has been confirmed for our specimen by x-ray diffraction (XRD) measurements. Furthermore, the 3D Dirac semimetal phase is not protected by crystalline symmetries, and it manifests only for the specific b/2 = 7.35Å at the boundar...

We report on the temperature dependence of the ZrTe5 electronic properties, studied at equilibrium and out of equilibrium, by means of time and angle resolved photoelectron spectroscopy (tr-ARPES). Our results unveil the dependence of the electronic band structure across the Fermi energy on the sample temperature. This finding is regarded as the dominant mechanism responsible for the anomalous resistivity observed at T * ∼ 160 K along with the change of the charge carrier character from hole-like to electron-like. Having addressed these longlasting questions, we prove the possibility to control, at the ultrashort time scale, both the binding energy and the quasiparticle lifetime of the valence band. These experimental evidences pave the way for optically controlling the thermo-electric and magneto-electric transport properties of ZrTe5.

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A novel ultrafast photoemission technique unveils the Mottness of antinodal quasiparticles in superconducting copper oxides.

MoTe2 has recently been shown to realize in its low-temperature phase the type-II Weyl semimetal (WSM). We investigated by time-and angle-resolved photoelectron spectroscopy (tr-ARPES) the possible influence of the Weyl points in the electron dynamics above the Fermi level EF, by comparing the ultrafast response of MoTe2 in the trivial and topological phases. In the low-temperature WSM phase, we report an enhanced relaxation rate of electrons optically excited to the conduction band, which we interpret as a fingerprint of the local gap closure when Weyl points form. By contrast, we find that the electron dynamics of the related compound WTe2 is slower and temperature-independent, consistent with a topologically trivial nature of this material. Our results shows that tr-ARPES is sensitive to the small modifications of the unoccupied band structure accompanying the structural and topological phase transition of MoTe2.The recent discovery of Weyl fermions as low-energy quasiparticles in TaAs [1][2][3] and other related compounds [4,5] has boosted the interest in topological semimetals (TSMs) [6]. Type-II Weyl semimetals (WSMs) are a novel class of materials that host fermions violating Lorentz invariance [7]. These quasiparticles are realized in the strongly tilted cones that form in momentum space around special Weyl points (WPs) where the valence and conduction bands touch. WTe 2 [7] and MoTe 2 [8-10] have been proposed as possible type-II WSMs. While the topological phase of WTe 2 is still under debate, the existence of the WSM phase in the low temperature non-centrosymmetric structure of MoTe 2 is supported by the observation of surface Fermi arcs [11][12][13][14][15]. The Weyl points, however, are located above the Fermi level E F and this hinders a direct observation by conventional angle-resolved photoelectron spectroscopy (ARPES). To circumvent this difficulty, one would need a probe that is sensitive to the presence/absence of small energy band gaps in the unoccupied density of states.In this Letter we show that time-and angle-resolved photoelectron spectroscopy (tr-ARPES) can provide such information. We report a shortening of the relaxation time in the gapless type-II Weyl phase of MoTe 2 , which reflects the enhanced interband scattering from the conduction band (CB) to the valence band (VB) mediated by electron -electron scattering along the Weyl cone. These scattering processes are active only when the band gap is locally closed. This conclusion is supported by the observation of a slower, temperatureindependent dynamics in WTe 2 , indicative of a local direct band gap, which acts as an effective bottleneck for the electron relaxation.MoTe 2 is a layered material. It can be cleaved to expose large flat (001) terraces, ideal for ARPES studies. Figure 1 illustrates the WSM phase, which is only realized in the lowtemperature orthorhombic (space group Pmn2 1 ) structurehereafter referred to as the 1T' phase [17] -where inversion symmetry is broken. The crystal structure is sketched in Fig. 1 (a), along wi...