Collapse of long-range charge order tracked by time-resolved photoemission at high momenta

Abstract

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.

Time-domain classification of charge-density-wave insulators

“…3c), essentially resulting from shifts of the topmost spectral features to higher energies, and, on the other hand, the suppression of the superstructure peak in 1T-TiSe 2 at the M point (Fig. 3b), as indicated by the transfer of spectral weight to electron states above E F , the narrowing of the spectrum at M , and the apparent switch of the band dispersion from holelike to electron-like 19 . We also note that despite the poorer energy resolution compared to the synchrotron data of Fig.…”

“…Yet, the ARPES snapshots in Fig. 3a-d display qualitative electronic structure changes happening within 65-210 fs after excitation, which is much faster than the 0.35-4 ps it takes to transfer the excess energy in the electron system to the phonon bath 16,17,19,[39][40][41] . These changes therefore do not reflect transitions to quasi-equilibrium states at elevated effective temperatures, although the band maps look fairly similar to temperature-dependent equilibrium ARPES data 9,[12][13][14] .…”

“…We now set out to answer these questions by time-resolved pump-probe ARPES [16][17][18][19]35,36 using a highharmonic-generation source 37,38 . The central idea is that the timescale on which electronic order in momentum space melts should provide a strong hint as to its predominant nature or origin.…”

“…A distinctive feature of the electronic structure dynamics of 1T-TiSe 2 is that the melting time of the superstructure peak is dependent on the excitation density, the faster melting occurring at the higher density (see energy range between − 0.6 and − 0.2 eV between 0 and 100 fs in Fig. 3f,j) 19 . The oscillatory components in the spectral responses of pristine and Rb intercalated 1T-TaS 2 are analysed in Fig.…”

“…Two prominent examples are 1T-TaS 2 and 1T-TiSe 2 . In these two compounds, the electronic structures [8][9][10][11][12][13][14][15] and ultrafast spectral responses [16][17][18][19] indicate the presence of significant electron correlation effects. In both compounds, electron-electron interactions may in fact have an important role in CDW formation [12][13][14][18][19][20] and in the emergence of superconductivity from CDW states upon intercalation 21 or under pressure 22,23 .…”