Discovery of an Unconventional Charge Density Wave at the Surface ofK0.9Mo6O17

Abstract

We use Angle Resolved Photoemission Spectroscopy (ARPES), Raman spectroscopy, Low Energy Electron Diffraction (LEED) and x-ray scattering to reveal an unusual electronically mediated charge density wave (CDW) in K0.9Mo6O17. Not only does K0.9Mo6O17 lack signatures of electron-phonon coupling, but it also hosts an extraordinary surface CDW, with TS CDW =220 K nearly twice that of the bulk CDW, TB CDW =115 K. While the bulk CDW has a BCS-like gap of 12 meV, the surface gap is ten times larger and well in the strong coupling regime. Strong coupling behavior combined with the absence of signatures of strong electron-phonon coupling indicates that the CDW is likely mediated by electronic interactions enhanced by low dimensionality.PACS numbers: 71.45. Lr, 68.47.Gh Most known CDW materials are mediated by strong electron-phonon (el-ph) interaction [1], as confirmed by observation of large kinks in the dispersion by ARPES [2][3][4][5][6]. Some of the best known examples are the layered transition-metal dichacogenides and tellurides [7][8][9], where charge order often coexists and competes with superconductivity, due to their common el-ph origin [4,5,8,[10][11][12][13][14]. A CDW has been discovered within the pseudogap state of the cuprates [15][16][17][18][19][20][21][22][23][24], although its origin remains unclear. The observation of phonon anomalies suggests el-ph coupling may play a role [25][26][27], however, a number of theoretical models suggest that this CDW could be electronically mediated [28][29][30][31]. Electronelectron (el-el) interactions are also thought to be responsible for the CDW found in related cuprate ladder compounds [32].The properties of materials can be dramatically altered at the surface. In CDWs often the transition temperature is enhanced at the surface [33], an effect known as an extraordinary transition [34,35]. Recently such effect was also reported in a monolayer [36][37][38]. The increased T C has been attributed to enhanced interactions due to the decreased dimensionality [39,40]. In this letter, we show that K 0.9 Mo 6 O 17 has an enhanced surface transition temperature, and a surface energy gap an order of magnitude larger than the bulk. Despite the strong coupling nature of the surface order, K 0.9 Mo 6 O 17 shows no signatures of strong el-ph coupling, either in the phonon or electronic structure, making it a new candidate for an el-el mediated CDW. K 0.9 Mo 6 O 17 belongs to a family of materials including both one dimensional (1D) and two dimensional (2D) systems [41] and has been regarded as a model quasi-2D CDW material with T B CDW ∼ 115 K [42]. Its crystal structure [43] consists of a stacking of molybdenumoxygen slabs (Mo 6 O 17 ) along the hexagonal c axis with potassium atoms intercalated between these slabs. The Mo-O layers consist of Mo 2 O 10 zigzag chains along three directions, and the 2D Fermi surface (FS) can be constructed by superimposing three sets of quasi-1D FS lines, with a weak hybridization. The quasi-1D character of the FS is likely critical to the un...

Charge Density Waves and the Hidden Nesting of Purple Bronze K0.9Mo6O17

“…We obtained {t 0 , t 1 , t 2 , t 3 , µ} = {454, −204, 136, 114, 659} meV. Figure 2(a) shows that this effective Hamiltonian precisely captures the experimental FS [4,11] and reproduces the overall features of the three occupied DFT bands Fig. 2(c).…”

“…Figure 3(a) also illustrates the high sensitivity of T c to the magnitude of V 1 , since a 10% increase in the latter causes a twofold amplification of T c . Even though this suggests that T c can be very sensitive to the local details of the interactions (screening, in particular) and might be easily placed at the values T c ≈ 220 K attributed to the surface, the associated low-temperature gap is far from the reported value of 150 meV [4]. Conversely, setting E 0…”

“…For example, in Fig. 1(c), there is a sizable hopping between α orbitals [4]. Qµ represent the three experimentally measured CDW wave vectors.…”

Various Charge-Ordered States in the Extended Hubbard Model with On-Site Attraction in the Zero-Bandwidth Limit

“…The investigations on this phenomena attract much attention due to the fact that charge-orderings can interplay with other types of electron orderings [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. This interplay can lead to either mutual enhancement or reduction of mentioned phenomena, e.g.…”

Angle-/temperature-dependence of Raman scattering in layered NbSe3 crystal

“…6 On the other hand, temperature-dependent Raman measurements were performed in K 0.9 Mo 6 O 17 in order to investigate the role of phonons in the formation of CDW. 7 The drop of electronic background intensity below 24meV and 115K indicated the opening of electronically mediated CDW gap in such materials. In fact, Raman spectroscopy experiment was also extensively used to thoroughly investigate the nature of CDW amplitude mode.…”