Revealing Charge Density Wave Formation in theLaTe2System by Angle Resolved Photoemission Spectroscopy

Abstract

We present the first direct study of charge density wave (CDW) formation in quasi-2D single layer LaTe2 using high-resolution angle resolved photoemission spectroscopy (ARPES) and low energy electron diffraction (LEED). CDW formation is driven by Fermi surface (FS) nesting, however characterized by a surprisingly smaller gap (≈ 50meV) than seen in the double layer RTe3 compounds, extending over the entire FS. This establishes LaTe2 as the first reported semiconducting 2D CDW system where the CDW phase is FS nesting driven. In addition, the layer dependence of this phase in the tellurides and the possible transition from a stripe to a checkerboard phase is discussed.The physics of the charge density wave (CDW) state remains among the most actively studied phenomena in solid-state physics due to its competition and even coexistence with superconductivity [1]- [6], its potential role in the superconducting cuprate phase diagram [7,8], and its important insights into electron-phonon physics. The origin of a CDW is most commonly traced to a Fermi surface (FS) nesting, i.e. the matching of sections of FS to others by a single wave vector, q N . For higher dimensional FSs the CDW phase tends to remain metallic, either due to imperfect nesting which leaves regions of the FS ungapped [9, 10], or to residual electron pockets formed by the CDW formation [11,12]. This is in contrast to quasi-1D CDW systems, where a perfect nesting can be realized and the FS is fully gapped, explaining why all known quasi-1D CDW materials are semiconductors in the CDW phase [1,13,14]. While we can find a few examples of non-metallic 2D CDW systems, the origin of the CDW phase is not due to true FS nesting but Mott physics [15,16]) or other non-nesting phenomena [17]. Therefore, a natural question is whether there exists any proven instance of a 2D CDW system where the CDW phase is driven by FS nesting, yet non-metallic. The 2D CDW rare earth ditellluride system LaTe 2 [18]- [20], formed by square tiled Te layers separated by RTe slabs (R = rare earth), is ideal to address this question, having been previously shown to be non-metallic [20,21]. The CDW phase is well established, first by transmission electron microscopy (TEM) measurements which identified a modulation wave vector 0.5a * [18], which we will refer to as q 1 , and later by single crystal X-ray diffraction [19], which proposed a 2 × 2 × 1 superstructure. Recently TEM measurements have reported another CDW vector with q=.6a * +.2b * [20] which we will refer to as q 2 . While ARPES has successfully studied other tellurides such as RTe 3 , showing its CDW phase to be FS driven, only recently has it been used for the LaTe 2 system [20], although a complete study of the CDW phase is missing.In this Letter, we present the first detailed ARPES study of the band structure and charge density wave formation in LaTe 2 . Like other tellurides, the CDW phase is driven by FS nesting, with q 1 =.53a* . However it is characterized by a surprisingly small gap (≈ 50 meV measured from the leading ed...

Angle-resolved and resonant photoemission spectroscopy study of the Fermi surface reconstruction in the charge density wave systemsCeTe2andPrTe2

“…The CDW gaps are found to open only partially in some part of the FS, which is consistent with the semimetallic nature of RTe 2 [17,27,28]. The overall similarity of the FS/CE maps of CeTe 2 and PrTe 2 , as well as those of LaTe 2 [15], implies that the contribution of the R 4f (R = Ce, Pr) electrons to the states at E F is minor. Nevertheless, Figs.…”

“…These features in CeTe 2 are reminiscent of those in LaTe 2 [15]. But the zigzag features in LaTe 2 were detected for the CE map with deeper BE of ∼100-160 meV.…”

“…Ito et al [18] argued the observed intensity modulation in the FS along the k z axis to support the strong 3D nature in CeTe 2 . In contrast, the 3D nature is considered to be negligible in LaTe 2 [12,15].…”

“…Despite the extensive ARPES studies for the similar CDW systems of RTe 3 [8,15,16], there are not many ARPES reports on RTe 2 [8,13,15,17,18]. Shin et al [8] reported that the FS topology of CeTe 2 in the k x -k y plane is different from that of LaTe 2 [15], and that the magnitude of the CDW gap g varies around the FS with the minimum value of 600 meV, which is much larger than that found in another ARPES study ( g ≈ 100 meV) [13,17]. Ito et al [18] argued the observed intensity modulation in the FS along the k z axis to support the strong 3D nature in CeTe 2 .…”

Observation of two separate charge density wave transitions in Gd2Te5 via transmission electron microscopy and high-resolution X-ray diffraction

“…In this regard, the rare earth (R) tellurides RTe 2 and RTe 3 , based on single and double square Te planes respectively, are especially revealing, and have attracted particular interest given the combination of their simple electronic structure [3][4][5], simple lattice modulation [6][7][8], easy "tunability" (both in terms of band filling [9,10] and also chemical pressure [11]), and finally the large magnitude of their CDW gap [12]. This renders these materials appropriate for several powerful and complementary probes of the electronic and physical structure [13][14][15][16]. Several studies have established these as prototypical nesting-driven CDW systems, with CDW wave vectors determined by well-defined peaks in the Lindhard susceptibility [2,4,17], although questions remain about the role played by a possible incipient lattice instability [18].…”