Evidence for a Quasi-One-Dimensional Charge Density Wave in CuTe by Angle-Resolved Photoemission Spectroscopy

We report the electronic structure of CuTe with a high charge density wave (CDW) transition temperature ${\mathrm{T}}_c=335\mathrm{K}$ by angle-resolved photoemission spectroscopy. An anisotropic charge density wave gap with a maximum value of 190 meV is observed in the quasi-one-dimensional band formed by Te $p_x$ orbitals. The CDW gap can be filled by increasing the temperature or electron doping through in situ potassium deposition. Combining the experimental results with calculated electron scattering susceptibility and phonon dispersion, we suggest that both Fermi surface nesting and electron-phonon coupling play important roles in the emergence of the CDW.

Figure 1

(a) Crystal structure of CuTe. Arrows indicate the movement of Te atoms in the CDW phase. (b) LEED pattern at 80 K shows Bragg peaks (yellow circle) and reconstruction peaks (red circle). (c) Temperature dependent resistivity of CuTe. The inset shows a picture of a typical single crystal of a few millimeters in size. (d) Bulk Brillouin zone and calculated Fermi surface for the normal phase of CuTe with high-symmetry points labeled. (e)–(j) Intensity maps at constant energies from ${\mathrm{E}}_F$ to $-0.5\mathrm{eV}$ measured at 20 K, with photon energy of 80 eV and $p$ polarization. The black arrow indicates the nesting wave vector, and the red arrows mark the edges of the ungapped area.

Figure 2

(a)–(g) Band dispersions along the $k_x$ direction at fixed $k_y=0$, 0.15, 0.25, 0.3, 0.45, 0.55, $0.77{\AA }^{-1}$, respectively. The CDW gap size is labeled. (h) Band dispersion along the quasi-1D line segment at $k_x=0.4{\AA }^{-1}$. The locations of (a)–(h) momentum cuts are marked by blue dashed lines in (i). Gap size extracted from (a)–(g) are appended as symbols. (j) Extracted gap size as a function of $k_y$. The error bars are smaller than the markers. (k)–(o) Evolution of band dispersions at $k_y=0.47{\AA }^{-1}$ with temperature from 20 K to 350 K with photon energy of 60 eV. (p) Extracted gap size as a function of temperature. The gray dashed line is the fitting curve using a BCS-type gap equation.

Figure 3

(a) Evolution of K 3p core-level spectra upon surface deposition of K atoms. (b)–(d) Band structure evolution of CuTe at $k_y=0.45{\AA }^{-1}$ [cut 5 in Fig. 2] with potassium doping at different time. (e)–(g) Constant energy maps from $E_F$ to $-0.15\mathrm{eV}$ before doping at 20 K. (h)–(j) Constant energy maps from $E_F$ to $-0.15\mathrm{eV}$ after doping at 20 K. The constant energy maps in Fig. 3 were measured by rotating the sample plane (azimuthal angle) by 90 degrees relative to Fig. 1 in order to align the analyzer slit parallel to cut 5 direction, while maintaining other experimental conditions.

Figure 4

(a) Calculated phonon spectrum along the high symmetry directions. The red arrows indicate the Kohn anomaly along the $\mathrm{\Gamma }–X$ and $Z–U$ directions. (b) Schematic drawing of scattering between $\alpha$ and $\beta$ bands. (c) and (d) Real part and Imaginary part of the susceptibility between different bands along the $\mathrm{\Gamma }–X$ direction.