Multiple charge density wave states at the surface of TbTe3

Abstract

We studied TbTe 3 using scanning tunneling microscopy (STM) in the temperature range of 298 -355 K. As seen in previous STM measurements on RTe 3 compounds, our measurements detect a unidirectional charge density wave state (CDW) in the surface Te-layer with a wavevector consistent with that of the bulk, = 0.30 ± 0.01 * . However, unlike previous STM measurements, and differing from measurements probing the bulk, we detect two perpendicular orientations for the unidirectional CDWs with no directional preference for the inplane crystal axes (a-or c-axis) and no noticeable difference in wavevector magnitude. In addition, we find regions in which the bidirectional CDW states coexist. We propose that observation of two CDW states indicates a decoupling of the surface Te-layer from the rare-earth block layer below, and that strain variations in the Te surface layer drive the local CDW direction to the specific unidirectional or, in rare occurrences, bidirectional CDW orders observed. This indicates that similar driving mechanisms for CDW formation in the bulk, where anisotropic lattice strain energy is important, are at play at the surface. In our bias-dependent measurements, we find no contrast inversion for the CDW state between occupied and empty states. This finding differs from other quasi 2-dimensional materials containing a "hidden" 1dimensional character which leads to a favorable Fermi surface nesting scenario. Our temperature-dependent measurements provide evidence for localized CDW formation above the bulk transition temperature, T cdw . Introduction:Charge density wave (CDW) states are broken-symmetry states prevalent in condensed matter systems, where they are often found to coexist and/or compete with other orders. In particular, in the intensely-studied graphene-related systems [1], layered chalcogenides [2,3], organic compounds [4], cuprate high-temperature superconductors [5][6][7][8][9], and BiS 2

Interplay of charge density wave states and strain at the surface of CeTe2

“…Further, and following our previous work on TbTe3, we identify peak "2" as the CDW wavevector, qcdw, and peak "4" as the first harmonic, 2qcdw. Peaks "3" and "5" occur at qatom -2qcdw and qatomqcdw respectively, and we identify these peaks as resulting from wavevector mixing, the nature of which is discussed in detail elsewhere [17,22,23]. As such, the origin of these four peaks is consistent with an incommensurate, unidirectional CDW established on the surface of CeTe2.…”

“…These CDW-associated wavevectors are very close to those determined by STM measurements on the related RTe3 compounds. Consequently, we label the peaks in Figure 2d as "2", "3", "4", and "5", given that their wavevector values are close to the ~2/7, ~3/7, ~4/7, and ~5/7qatom CDW-associated wavevectors reported for STM measurements of the RTe3 compounds [17,22,23]. Further, and following our previous work on TbTe3, we identify peak "2" as the CDW wavevector, qcdw, and peak "4" as the first harmonic, 2qcdw.…”

“…Our previous work on TbTe3 illustrates that the surface Te layer is only weakly bound to the bulk below and, as such, can host CDW states which differ from that observed in the bulk. [17] As a consequence, when the surface terminal layer is a Te plane, STM measurements probe the fundamental physics contained within a single Te plane, but the plane is subjected to outside influences such as local strain fields. Interestingly, while a weakly-bound surface Te layer atop a block layer may appear physically similar for both the RTe2 and RTe3 compounds, one might still expect slightly different band-fillings between the two [2] which would lead to differing Fermi surfaces.…”

“…[25,26] In 1T-TiSe2, STM measurements show that Cu intercalation leads to the formation of striped CDW order instead of the typical 2 x 2 CDW order observed without intercalation; strain was suggested as a possible mechanism. [27] Our recent work on TbTe3 [17] demonstrates that the material surface can host at least two distinct CDW states. In TbTe3, these states can exist separately or coexist with one another.…”

“…One goal of our studies is to image, in real space, the CDW state hosted by CeTe2, and to compare our extracted 𝑞 𝐶𝐷𝑊 to those previously found in the RTe2 compounds as well as to that found in the related RTe3 compounds. Given the expected sensitivity of RTe2 compounds to external factors such as strain [2,16], and given the demonstrated sensitivity of CDW states in RTe3 to local [17,18] and global strain [14,19,20], a second goal is to provide a nanoscale view of the interplay of local lattice strain and the locally-established CDW state(s) in CeTe2.…”