Time-domain classification of charge-density-wave insulators

Abstract

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.

Distinguishing attosecond electron–electron scattering and screening in transition metals

“…Using femtosecond lasers in combination with advanced spectroscopies, it is possible to measure the lifetime of excited charges and spins directly in the time domain (1). To date, such studies have been applied to a wide variety of materials, including noble metals and semiconductors (1)(2)(3)(4), ferromagnetic metals (5)(6)(7)(8), strongly correlated materials (9) and high-T c superconductors (10,11). These studies have significantly improved our understanding of the fastest coupled interactions and relaxation mechanisms in matter.…”

“…These studies have significantly improved our understanding of the fastest coupled interactions and relaxation mechanisms in matter. However, to date experimental investigations of electron dynamics have been limited to femtosecond timescale processes in materials with low charge densities (9)(10)(11)(12) or to Fermi-liquid metals with low excitation energies (<3.0 eV above E F , where E F is the Fermi energy) (3)(4)(5), due to the visible-to-UVwavelength photon energies used in these experiments. In this region, two fundamental electron interactions-electron-electron scattering and charge screening due to a rearrangement of adjacent charges-contribute to the signal, making it challenging to independently probe these dynamics.…”

Atomistic origin of an ordered superstructure induced superconductivity in layered chalcogenides

“…However, the 12 electrons occupy the states below the distortion-induced gap and the thirteenth one dominates the states above the gap, resulting in a Mott insulating state that accounts for the high resistivity of the CCDW phase [14][15][16] , (2) the neighbouring NCCDW phase contains the star-of-David clusters as well albeit that they are less homogeneously arranged 7 , (3) with the rise of temperature, the Mott phase melts into the NCCDW phase with an extremely fast charge response and a sudden drop in resistivity, where several tens of stars organize into roughly hexagonal domains 10,17 . These reproduce locally the CCDW Mott phase, signifying that the CCDW mechanism is beyond the framework of the Peierls picture [18][19][20][21][22] . Definitely, none of them have supported a seemly counter-intuitive origin in understanding the low-dimensional quantum system, that is, the crucial role of atomic reconstruction due to the well-established concept: if electrons can move, atoms do not have to.…”

Optical freezing of charge motion in an organic conductor

“…1a) in Mott insulators [4][5][6][7][8][9][10] , chargeordered (CO) systems [11][12][13][14] and charge/spin density wave materials 6,[15][16][17][18][19][20] . Recently, the excitation of coherent phonons has been established as a leading strategy for the melting/ constructing of electronic orders 8,9,[15][16][17][18][19][20] . On the other hand, the development of strong electric fields (4MV cm À 1 ) of few-cycle optical pulses and recent theoretical studies using dynamical mean-field theory suggest that extreme non-equilibrium electronic states, such as Floquet states, negative temperatures and superconducting states 21,22 , can be achieved.…”