Structural approach to charge density waves in low-dimensional systems: electronic instability and chemical bonding

Abstract

Abstract. The charge density wave (CDW) instability, usually occurring in low-dimensional materials, has been a topic of interest for longtime. However, some very fundamental aspects of the mechanism remain unclear. Recently, a plethora of new CDW materials, a substantial fraction of which is 2D (two-dimensional) or even 3D (three-dimensional), has been prepared and characterized as bulk and/or single-layers. As a result, the need for revisiting the primary mechanism of the instability, based on the electron-hole instability established more than 50 years ago for quasi-1D (quasi-one-dimensional) conductors, has clearly emerged. In this work, we consider a large number of CDW materials to revisit the main concepts used in understanding the CDW instability, and emphasize the key role of the momentum dependent electron-phonon coupling in linking electronic and structural degrees of freedom. We argue that for quasi-1D systems, earlier weak coupling theories work appropriately and the energy gain due to the CDW and the concomitant periodic lattice distortion (PLD) remains primarily due to a Fermi surface nesting mechanism. However, for materials with higher dimensionality, intermediate and strong coupling regimes are generally at work and the modification of the chemical bonding network by the PLD is at the heart of the instability. We emphasize the need for a microscopic approach blending condensed matter physics concepts and state-of-the-art first-principles calculations with quite fundamental chemical bonding ideas in understanding the CDW phenomenon in these materials.

Room Temperature Charge Density Wave in a Tetragonal Polymorph of Gd2Os3Si5 and Study of Its Origin in the RE2T3X5 (RE = Rare Earth, T = Transition Metal, X = Si, Ge) Series

“…The observed scattering near the satellite positions in the SXRD at 400 K is diffuse scattering (Figure ) and it indicates CDW fluctuation above T CDW . Diffuse scattering at temperatures above T CDW has been observed for other CDW systems too . The symmetry of the periodic phase is given by the space group P 4/ mnc , much like RE 2 Re 3 Si 5 ( RE = Ce, Pr, Ho) , and RE 2 Ru 3 Ge 5 ( RE = Dy, Sm, Pr) .…”

“…As described above, the CDW in Gd 2 Os 3 Si 5 involves the modulation of Si atoms as opposed to Ir–Ir zigzag chains in Ho 2 Ir 3 Si 5 or Ge–Ge zigzag chains in Sm 2 Ru 3 Ge 5 . A CDW can be stabilized by one of several mechanisms . These include FSN and hidden nesting, − ,− wave vector-dependent electron–phonon coupling, − strong electron correlations, ,, and a large EDOS near the Fermi level ( E F ) having degenerate branches. , …”

“…Diffuse scattering at temperatures above T CDW has been observed for other CDW systems too. 50 The symmetry of the periodic phase is given by the space group P4/mnc, much like RE 2 Re 3 Si 5 (RE = Ce, Pr, Ho) 24,25 and RE 2 Ru 3 Ge 5 (RE = Dy, Sm, Pr). 7 Structure refinements led to an excellent fit to the SXRD data (Table 1).…”

“…7 A CDW can be stabilized by one of several mechanisms. 50 These include FSN and hidden nesting, 45−49,54−56 wave vector-dependent electron−phonon coupling, 57−60 strong electron correlations, 47,61,62 and a large EDOS near the Fermi level (E F ) having degenerate branches. 63,64 We have calculated the electronic band structure of Gd 2 Os 3 Si 5 and thus obtained the Fermi surface and the Lindhard susceptibility in the static limit (ω = 0), with real part Re{χ 0 (q, ω)} and imaginary part Im{χ 0 (q, ω)}.…”