Microscopic evidence for strong periodic lattice distortion in two-dimensional charge-density wave systems

Abstract

In the quasi-2D electron systems of the layered transition metal dichalcogenides (TMD) there is still a controversy about the nature of the transitions to charge-density wave (CDW) phases, i.e. whether they are described by a Peierls-type mechanism or by a lattice-driven model. By performing scanning tunneling microscopy (STM) experiments on the canonical TMD-CDW systems, we have imaged the electronic modulation and the lattice distortion separately in 2H-TaS2, TaSe2, and NbSe2. Across the three materials, we found dominant lattice contributions instead of the electronic modulation expected from Peierls transitions, in contrast to the CDW states that show the hallmark of contrast inversion between filled and empty states. Our results imply that the periodic lattice distortion (PLD) plays a vital role in the formation of CDW phases in the TMDs and illustrate the importance of taking into account the more complicated lattice degree of freedom when studying correlated electron systems. PACS numbers: 71.45.Lr, 71.27.+a, 74.55.+v The transition metal dichalcogenides (TMD) have shown intriguing phenomena like the spin-valley physics [1, 2], superconductivity induced/enhanced by tuning various parameters [3][4][5][6][7][8][9], and charge-density waves (CDW) that coexist and compete with superconductivity (e.g. 2H-NbSe 2 , TaSe 2 , 1T-TaS 2 , and TiSe 2 ) [5-9]. Such competition between CDW and superconductivity is closely tied to the complicated interactions between the internal degrees of freedom, including charge, lattice and orbital. Differentiating which interactions are key requires knowing which of them are responsible for the different phases. The degree to which an electron-driven mechanism is the cause of CDWs in the trigonal prismatic structured (2H) TMDs is still under debate. Existing studies have proposed a variety of different mechanisms, including the Fermi surface nesting [10, 11], saddle band driven susceptibility divergence [12], f-wave gapping and marginal Fermi liquid [13], etc.These materials do show some evidence of the electronic nature of the CDW phases including the existence of incommensurate CDW phases [14,15], since 2k F is generally not expected to be a rational fraction of the lattice reciprocal vectors. Furthermore, the electronic origin is also supported by photoemission experiments which measured the Fermi surface of NbSe 2 and TaSe 2 and approximated the electronic susceptibility with an autocorrelation. This analysis showed peaks at wave vectors corresponding to those of the CDW [10,11]. However, inelastic X-ray scattering experiment in NbSe 2 [16] reveals that the lattice dynamics exhibit unconventional behavior and hence may be dominating the transition. Moreover, recent LDA calculations have shown the difficulties with Fermi surface nesting and have suggested that periodic lattice distortion (PLD), instead of eMod, is the essential ingredient [17,18]. In a recent real space study, Soumyanarayanan et al. [19] have shown that there is a close relationship between CDW formation and...

Interplay between multiple charge-density waves and the relationship with superconductivity inPdxHoTe3

“…However, the driving force behind the CDW phase is still under debate. [3][4][5][6][7][8][9][10][11][12][13][14][15] From a Peierls perspective, in an ideal one-dimensional (1D) system, the electronic susceptibility would develop a logarithmic divergence singularity at some sheets of the FSs spanning by nesting vectors via EPCs, and hence results in a phase transition to the CDW ground state accompanied by the commensurate/incommensurate periodic lattice distortions and the opening of energy gaps at E F .…”

Structural versus electronic distortions inIrTe2with broken symmetry

“…Nevertheless, most of the previous STM studies for CDW [31] and charge order materials [32,33] interpreted topographies as representing electronic modulations. The limitation of the STM topography would be important for a systems with a substantial structural modulation as in the present case and more generally for diverse complex systems with various different degrees of freedom entangled.…”

Dimensionality-driven phonon softening and incipient charge density wave instability in TiS 2

“…For instance 47001-p1 bulk NbSe 2 and TaSe 2 are notoriously known to exist in both a superconducting and a charge density wave (CDW) state [19]. The appearance of the CDW phase can be ascribed to several competing mechanisms such as Peierls instability, exciton insulator instability, Jahn-Teller distortion, or Fermi surface (FS) nesting [19][20][21][22][23]. All the mechanisms are closely related to each other, especially in LTMD materials, and in general they are the result of a competition between the ionic and the electronic system to reduce the total energy.…”