Discovery of an Unconventional Charge Density Wave at the Surface of ${\mathrm{K}}_{0.9}{\mathrm{Mo}}_{6}{\mathrm{O}}_{17}$

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Discovery of an Unconventional Charge Density Wave at the Surface of $K_{0.9} {Mo}_{6} O_{17}$

Daixiang Mou, A. Sapkota, H.-H. Kung, Viktor Krapivin, Yun Wu, A. Kreyssig, Xingjiang Zhou, A. I. Goldman, G. Blumberg, Rebecca Flint, and Adam Kaminski

Phys. Rev. Lett. 116, 196401 – Published 13 May 2016

Abstract

We use angle resolved photoemission spectroscopy, Raman spectroscopy, low energy electron diffraction, and x-ray scattering to reveal an unusual electronically mediated charge density wave (CDW) in $K_{0.9} {Mo}_{6} O_{17}$ . Not only does $K_{0.9} {Mo}_{6} O_{17}$ lack signatures of electron-phonon coupling, but it also hosts an extraordinary surface CDW, with $T_{S_CDW} = 220 K$ nearly twice that of the bulk CDW, $T_{B_CDW} = 115 K$ . While the bulk CDW has a BCS-like gap of 12 meV, the surface gap is 10 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.

Received 8 January 2016

DOI:https://doi.org/10.1103/PhysRevLett.116.196401

Physics Subject Headings (PhySH)

Charge density waves

Angle-resolved photoemission spectroscopy Raman spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Daixiang Mou^1,2,*, A. Sapkota^1,2, H.-H. Kung³, Viktor Krapivin³, Yun Wu^1,2, A. Kreyssig^1,2, Xingjiang Zhou⁴, A. I. Goldman^1,2, G. Blumberg^3,5, Rebecca Flint^1,2, and Adam Kaminski^1,2,†

¹Division of Materials Science and Engineering, Ames Laboratory, U.S. Department of Energy, Ames, Iowa 50011, USA
²Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA
³Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
⁴National Laboratory for Superconductivity, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
⁵National Institute of Chemical Physics and Biophysics, 12618 Tallinn, Estonia

^*moudaixiang@gmail.com
^†kaminski@ameslab.gov

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Vol. 116, Iss. 19 — 13 May 2016

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Figure 1
Bulk and surface CDW gaps: (a) Measured FS at 130 K. Intensity is integrated within $E_{F} \pm 10 meV$ and data are symmetrized with sixfold symmetry. Dashed arrows indicate three nesting vectors, each connecting two quasi-1D FS sheets [51]. The red rectangle is expanded in the left-bottom inset to demonstrate the FS hybridization. (b)–(d) ARPES intensity measured along the cut (red line) shown in (a). (e) Extracted band dispersion from (d). (f) EDCs along the same cut. (g)–(i) ARPES intensity divided by Fermi function close to $E_{F}$ at 130, 75, and 45 K. (j) Temperature dependence of the EDCs at $k_{F}$ showing opening of bulk CDW gap. (k) Same as in (j), but symmetrized about $E_{F}$ .
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Figure 2
Bulk and surface CDW transition. (a) High-energy x-ray diffraction patterns of the reciprocal lattice plane ( $H K 0$ ). The CDW superstructure peaks are marked by blue arrows (logarithmic color scale). (b) High-resolution diffraction patterns of the ( $\frac{9}{2}$ 0 0) CDW peak (linear color scale). (c) Plot of the temperature dependence of the CDW peak (linear color scale). The intensity is obtained by summing up the high-resolution diffraction patterns of the ( $\frac{9}{2}$ 0 0) peak along the transverse direction in (b), and is plotted along the longitudinal direction. (d) LEED images. Red arrows point to CDW superstructure peaks.
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Figure 3
(a) Temperature dependence of the Raman response in false color image (logarithmic scale) to emphasize a general trend of spectral weight suppression upon cooling due to gapping of the Fermi surface pockets. (b) Band crossing $E_{F}$ at $T = 260 K$ along same cut as in Fig. 1. The parabolic fit to the dispersion is plotted in black. (c) Real part of self-energy extracted from (b). (d) Imaginary part of self-energy extracted from (b). Red solid line is a linear fit.
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Figure 4
Summary of the temperature-dependent CDW gap and band structure evolvement. (a) Temperature dependence of the surface (red solid circles) and bulk (blue solid circles) CDW gap. The surface gap is extracted from the back bending point of the surface band and the bulk gap is extracted from the leading edge shift of $k_{F}$ EDCs [Fig. 1]. The gray solid line is a BCS-like temperature dependence with $Δ_{0} = 12 meV$ . The integrated intensity of the CDW peak measured by x-ray diffraction [Fig. 2] is shown with yellow solid circles. Black data points represent the intensity of CDW peaks measured by LEED. Dashed line is a guide to the eye. (b) Illustration of the surface (blue line) and bulk (red line) band dispersion. (c) Illustration of surface (red) and bulk (blue) CDW formation in real space. Dashed lines represent a density distribution of conducting electrons.
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Physical Review Letters

Discovery of an Unconventional Charge Density Wave at the Surface of K0.9Mo6O17

Daixiang Mou, A. Sapkota, H.-H. Kung, Viktor Krapivin, Yun Wu, A. Kreyssig, Xingjiang Zhou, A. I. Goldman, G. Blumberg, Rebecca Flint, and Adam Kaminski

Phys. Rev. Lett. 116, 196401 – Published 13 May 2016

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Discovery of an Unconventional Charge Density Wave at the Surface of $K_{0.9} {Mo}_{6} O_{17}$