The occurrence of charge-density waves in three selected layered transition-metal dichalcogenides-1T-TaS(2), 2H-TaSe(2) and 1T-TiSe(2)-is discussed from an experimentalist's point of view with a particular focus on the implications of recent angle-resolved photoelectron spectroscopy results. The basic models behind charge-density-wave formation in low-dimensional solids are recapitulated, the experimental and theoretical results for the three selected compounds are reviewed, and their band structures and spectral weight distributions in the commensurate charge-density-wave phases are calculated using an empirical tight-binding model. It is explored whether the origin of charge-density waves in the layered transition-metal dichalcogenides can be understood in a unified way on the basis of a few measured and calculated parameters characterizing the interacting electron-lattice system. It is found that the predictions of the standard mean-field model agree only semi-quantitatively with the experimental data and that there is not one generally dominant factor driving charge-density-wave formation in this family of layer compounds. The need for further experimental and theoretical scrutiny is emphasized.

Intense femtosecond (10(-15) s) light pulses can be used to transform electronic, magnetic and structural order in condensed-matter systems on timescales of electronic and atomic motion. This technique is particularly useful in the study and in the control of materials whose physical properties are governed by the interactions between multiple degrees of freedom. Time- and angle-resolved photoemission spectroscopy is in this context a direct and comprehensive, energy- and momentum-selective probe of the ultrafast processes that couple to the electronic degrees of freedom. Previously, the capability of such studies to access electron momentum space away from zero momentum was, however, restricted owing to limitations of the available probing photon energy. Here, using femtosecond extreme-ultraviolet pulses delivered by a high-harmonic-generation source, we use time- and angle-resolved photoemission spectroscopy to measure the photoinduced vaporization of a charge-ordered state in the potential excitonic insulator 1T-TiSe(2 )(refs 12, 13). By way of stroboscopic imaging of electronic band dispersions at large momentum, in the vicinity of the edge of the first Brillouin zone, we reveal that the collapse of atomic-scale periodic long-range order happens on a timescale as short as 20 femtoseconds. The surprisingly fast response of the system is assigned to screening by the transient generation of free charge carriers. Similar screening scenarios are likely to be relevant in other photoinduced solid-state transitions and may generally determine the response times. Moreover, as electron states with large momenta govern fundamental electronic properties in condensed matter systems, we anticipate that the experimental advance represented by the present study will be useful to study the ultrafast dynamics and microscopic mechanisms of electronic phenomena in a wide range of materials.

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.

3Spontaneous symmetry breaking gives rise to a new quantum ground state featuring characteristic low-energy elementary excitations 3,11,14,[18][19][20][21][22] Ultrashort pulses in the terahertz (1 THz = 10 12 Hz) range have been used to trace electronic order via direct coupling to such excitations 22,23 . We demonstrate that THz pulses may simultaneously also track the crystalline order during an ultrafast phase transition.This idea is tested in a prominent reference system, 1T-TiSe 2 . Within the family of layered transition-metal dichalcogenides, this material has attracted special attention: Upon cooling below T c ≈ 200 K, it undergoes a transition into a commensurate CDW accompanied by the formation of a structural (2×2×2) superlattice 21 (Fig. 1a). In its high-temperature phase, TiSe 2 is a semimetal 20 with electron and hole pockets at the L and  points of the Brillouin zone, respectively 15,24 (Fig. 1b). The spatial reconstruction due to the CDW maps these two points on top of each other and leads to the partial opening of an electronic energy gap as well as a dramatic reduction of the density of free charge carriers 20 (Fig. 1b). Superconductivity emerges when the CDW is suppressed, e.g. by Cu intercalation 7 or pressure 25 . This discovery as well as novel chiral properties 26 have intensified the interest in the nature of the CDW in 1T-TiSe 2 . Yet, the microscopic mechanisms remain elusive 24,[27][28][29] . A first hypothesis assumes electron-phonon coupling based on a Jahn-Teller effect as the driving force 27 . A competing model suggests that the transition is purely electronically driven 24,28 . Coulomb attraction may render the system unstable against the formation of excitons between the electron-and hole-like Fermi pockets, leading to lattice deformation with the corresponding wave vector. Combinations of the two scenarios have also been proposed 29 . Time-resolved x-ray diffraction 16 and photoemission 10,15 experiments have separately tracked the dynamics of either structural or electronic orders. 4Evidence for both excitonic and phononic contributions was obtained in this way, leaving a controversial picture.Here we disentangle the two coupled components of the CDW order parameter by simultaneously tracing the ultrafast THz response of PLD-related phonons and electronic conductivity while a femtosecond pulse selectively melts the excitonic order. Our data reveal a transient phase in which the PLD persists in the absence of excitonic correlations. A quantummechanical theory 29 corroborates our conclusions.In TiSe 2 , the transition to the CDW ordered phase modifies the low-frequency optical response in three distinct ways: (i) The CDW-induced energy gap introduces a broad single- (Fig. 1d). Above T c , we observe a single TO phonon resonance at 17 meV. Below T c , back-folding of the uppermost acoustic branch from the L to the  point 21 yields an additional IR-active in-plane mode at 19 meV. The weaker peak at 22 meV likely originates from a folded optical branch at the M point 21 . 5W...

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