PHYSICAL REVIEW B 85, 085104 (2012) Fermi surface reconstruction in CeTe 2 induced by charge density waves investigated via angle resolved photoemission J.-S. Kang, 1,* D. H. Kim, 1 H. J. Lee, 1 Jihoon Hwang, 1 Han-Koo Lee, 2 H.-D. Kim, 2 B. H. Min, 3 K. E. Lee, 3 Y. S. Kwon, 3 J. W. Kim, 4 Kyoo Kim, 4 B. H. Kim, 4 and B. I. Min 4,† 1 Department of Physics, The Catholic University of Korea, Bucheon 420-743, Korea 2 Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang 790-784, Korea 3 Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea 4 Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea (Received 3 January 2012; published 10 February 2012) Electronic structures of a charge-density wave (CDW) system CeTe 2−x Sb x (x = 0, 0.05) have been investigated by employing angle-resolved photoemission spectroscopy (ARPES) and the first-principles electronic and phonon band-structure methods. The observed Fermi surface (FS) agrees very well with the calculated FS for the undistorted CeTe 2 both in shapes and sizes. The metallic states crossing the Fermi level (E F ) are observed in ARPES even in the CDW state. The carriers near E F have mainly the Te(1) 5p character, with a negligible contribution from Ce 4f states. The supercell (shadow) bands and the corresponding very weak FSs are found to arise from band folding due to the interaction of Te(1) layers with Ce-Te(2) layers. We found that the CDW modulation vector is along Ŵ-X (Q CDW ≈ X), which is not coincident with the most prominent FS nesting vector. DOI: 10.1103/PhysRevB.85.085104 PACS number(s): 71.45.Lr, 71.18.+y, 71.20.−b, 79.60.−i I. INTRODUCTION CeTe 2 is known as a charge-density wave (CDW) system having a high CDW transition temperature of T CDW ∼ 1000 K, and intriguingly the CDW state coexists with magnetism and also with superconductivity (T C = 2.7 K) under pressure. 1 CeTe 2 crystallizes in the quasi-two-dimensional layered Cu 2 Sb-type tetragonal structure with two types of Te sites: Te(1) and Te(2). Te(1) atoms form planar square sheets, which are sandwiched along the c axis by the corrugated double layers of Ce and Te(2) atoms (Fig. 1). The ionic configuration of CeTe 2 is considered to be Ce 3+ Te(2) 2− Te(1) 1− , so that hole carriers are produced in Te(1) sheets. 2 Then the square net of Te(1) would be easily distorted by the Peierls-like mechanism 3 due to the partial filling. Underneath this picture is the assumption of trivalent Ce 3+ states. 4 Band-structure calculations indicate that the CDW instability occurs due to the nesting between the Fermi surfaces in Te(1) square sheets in the a-b plane, 5–7 which was supported experimentally. 8 However, the relevant CDW modulation vector has not been identified. Due to the difficulty in growing high-quality single crystals appropriate for the angle-resolved photoemission spectroscopy (ARPES) study, there have been only a few ARPES reports on CeTe 2 , which studied the Fermi-surface (FS) topology in the CDW state. 9,10 This is in contrast to similar CDW systems, LaTe 2 and RTe 3 (where R indicates a rare-earth element), for which more complete ARPES studies have been reported. 9,11,12 Shin et al. 9 reported that the FS topology of CeTe 2 in the k x -k y plane is different from that of LaTe 2 . 11 They conjectured that the CDW gap E g is larger than 600 meV and that the magnitude of E g varies around the FS. This minimum value of E g ≈ 600 meV is much larger than E g ≈ 100 meV, found in another ARPES study. 2 Ito et al. 10 examined the FS along the k z axis and observed some intensity modulation in the spectral weight at FS. Therefore, no good understanding has been made yet for the CDW state of CeTe 2 . As shown in Fig. 1(a), Te(1) ions in CeTe 2 form the planar square lattices, which are sandwiched by the double layers of Ce and Te(2) ions. Due to the underlying Ce-Te(2) layer, the unit cell of Te(1) square lattices is doubled in CeTe 2 ( √ 2 × √ 2) [see Fig. 1(b)]. Accordingly, the Brillouin zone (BZ) is reduced to a half of that of Te(1) square lattices, and thereby the bands are folded into the reduced BZ to produce the supercell (shadow) bands. In reality, the periodicity of the potential in the Te(1) layer depends on the hybridization strength between Te(1) and Ce-Te(2) layers. If the hybridization strength is small, the electronic structure of Te(1) ions would keep the two-dimensional nature of the planar square lattices. Otherwise, it will have a three-dimensional (3D)-like nature. The FSs, denoted with dotted lines in Fig. 1(b), come from those shadow bands. The natural question concerns the role of Ce 4f and Te 5p electrons in CeTe 2 . This includes the following issues: (i) whether the FS topology and the CDW state of CeTe 2 are the same as those of the non-f electron CDW system of LaTe 2 ; (ii) how large is the effect of the band folding, arising from the interaction between Te(1) and Ce-Te(2) layers, on the FS of Te(1) sheets; and (iii) what is the CDW modulation vector, Q CDW . We have resolved these questions by performing careful ARPES measurements for high-quality stoichiometric single crystals of CeTe 2−x Sb x (x = 0, 0.05) and also by carrying out first-principles electronic and phonon band calculations. II. EXPERIMENTALAND CALCULATIONAL DETAILS High-quality CeTe 2−x Sb x single crystals having very low residual resistivity were grown by using the self-fluxed Bridgeman method. 13 The quality and the orientation of the single crystal were checked by Laue patterns. ARPES experiments were carried out at the 3A1 beamline of the Pohang Light Source with a beam size of <50 μm and using a Scienta SES-2002 electron energy analyzer. Single crystals were cleaved in situ at T ∼ 30 K under pressure better than 5 × 10 −11 Torr, which exposed the (001) surfaces. The Fermi level and the overall instrumental resolution of the system were determined from the Fermi edge of an evaporated Cu metal. 085104-1 1098-0121/2012/85(8)/085104(6) ©2012 American Physical Society