Strong electron-lattice coupling as the mechanism behind charge density wave transformations in transition-metal dichalcogenides

Abstract

We consider single band of conduction electrons interacting with displacements of the transitional ions. In the classical regime strong enough coupling transforms the harmonic elastic energy for an ion to the one of the well with two deep minima, so that the system is described in terms of Ising spins. Inter-site interactions order spins at lower temperatures. Extension to the quantum regime is discussed. Below the CDW-transition the energy spectrum of electrons remains metallic because the structural vector Q and the FS sizes are not related. Large values of the CDW gap seen in the tunneling experiments correspond to the energy of the minima in the electron-ion two-well complex. The gap is defined through the density of states (DOS) inside the electronic bands below the CDW transition. We focus mainly on electronic properties of transition-metal dichalcogenides.

Charge density waves and phonon-electron coupling inZrTe3

“…In this line of thinking, it is only natural to recognize the essential role of phonon-electron coupling in the formation of CDW order, as suggested by several authors [30,32]. This idea has been explicitly tested in quasi-2D materials both theoretically [29,[33][34][35][36] and experimentally [36][37][38], and has proved quite successful in explaining the q cdw that deviates from q n . In quasi-1D materials, the importance of phonon-electron coupling relative to that of FSN has remained largely untested, as the good agreement between q n and q cdw in 1D cases might appear to render such tests unnecessary.…”

Ultrafast Dynamics of Charge Density Waves in4Hb−TaSe2Probed by Femtosecond Electron Diffraction

“…If the elastic energy cost to modulate atomic positions is lower than the electronic energy gain, the CDW state is the preferred ground state. Recently, this classical picture has been challenged [3][4][5], since (a) the nesting condition derived from the topology of the Fermi surface (2k F ) and the observed CDW modulation vectors (q CDW ) are not generally equal, and (b) the diverging susceptibility at q = 2k F is exceedingly fragile with respect to temperature, scattering or imperfect nesting [3]. Contrasting the standard Fermi surface nesting scenario the transition from the metallic into a CDW state was argued to occur due to strong qdependent electron-phonon coupling [3], particularly in transition-metal dichalcogenides [4].…”

“…Recently, this classical picture has been challenged [3][4][5], since (a) the nesting condition derived from the topology of the Fermi surface (2k F ) and the observed CDW modulation vectors (q CDW ) are not generally equal, and (b) the diverging susceptibility at q = 2k F is exceedingly fragile with respect to temperature, scattering or imperfect nesting [3]. Contrasting the standard Fermi surface nesting scenario the transition from the metallic into a CDW state was argued to occur due to strong qdependent electron-phonon coupling [3], particularly in transition-metal dichalcogenides [4].Only the concerted interplay of electronic and lattice degrees of freedom make the CDW formation possible. The two modulations can be individually examined by scanning tunneling microscopy [6], angular resolved photoemission spectroscopy [5] and electron, x-ray or neutron diffraction techniques [7].…”

Condensation of collective charge ordering in chromium

“…The way in which the dimensionality of the problem affects the timescale is debated 9 , and the role of inherent inhomogeneities in these materials remains elusive 11 . Moreover, recent theoretical 12,13 and experimental studies 9 indicate that Fermi-surface nesting, which is often used to describe the origin of CDW 4,14,15 , may not be sufficient to describe the charge ordering and that electron-phonon coupling plays an indispensable role.…”