Insensitivity of the striped charge orders in ${\mathrm{IrTe}}_2$ to alkali surface doping implies their structural origin

We present a combined angle-resolved photoemission spectroscopy and low-energy electron diffraction (LEED) study of the prominent transition metal dichalcogenide ${\mathrm{IrTe}}_2$ upon potassium (K) deposition on its surface. Pristine ${\mathrm{IrTe}}_2$ undergoes a series of charge-ordered phase transitions below room temperature that are characterized by the formation of stripes of Ir dimers of different periodicities. Supported by density functional theory calculations, we first show that the K atoms dope the topmost ${\mathrm{IrTe}}_2$ layer with electrons, therefore strongly decreasing the work function and shifting only the electronic surface states towards higher binding energy. We then follow the evolution of its electronic structure as a function of temperature across the charge-ordered phase transitions and observe that their critical temperatures are unchanged for K coverages of 0.13 and 0.21 monolayer. Using LEED we also confirm that the periodicity of the related stripe phases is unaffected by the K doping. We surmise that the charge-ordered phase transitions of ${\mathrm{IrTe}}_2$ are robust against electron surface doping, because of its metallic nature at all temperatures, and due to the importance of structural effects in stabilizing charge order in ${\mathrm{IrTe}}_2$.

Figure 1

(a) Evolution of the work function of ${\mathrm{IrTe}}_2$ as a function of K coverage. The inset shows energy distribution curves (EDCs) measured at RT with a photon energy of $h\nu =6.3$ eV on a pristine crystal and 0.29 ML K-doped crystal along the AL direction. (b) Side and top views of the ${\mathrm{IrTe}}_2$ atomic structure with a $(3\times 3)$ K adlayer on the surface. The light blue isosurface refers to the missing charge and the yellow isosurface to the gained charge at $0.001e^{-}/a_0^3$ [27], where $a_0$ is the Bohr radius. (c) Band structure calculation of the pristine ${\mathrm{IrTe}}_2$ (red) and with a K adlayer as shown in (b) (black).

Figure 2

(a) The Brillouin zone of ${\mathrm{IrTe}}_2$. (b) Fermi surface of ${\mathrm{IrTe}}_2$ for $h\nu =21.22$ eV taken at 295 K. (c) ARPES spectra measured along AL direction for $h\nu =21.22$ eV for a pristine, 0.04, 0.13, 0.21, and 0.29 ML K-doped crystals. (d) EDCs for the pristine and K-doped crystals (integrated $\pm 0.09$ ${\AA }^{-1}$ around A along AL direction). (e) Binding energy shift of the surface state SS measured in ARPES as a function of K doping.

Figure 3

(a) Structural models of the Ir atomic planes for the different charge-ordered phases. (b) ARPES spectra measured along AL direction for $h\nu =21.22\mathrm{eV}$ at three different temperatures for 0.13 ML K-doped crystal of ${\mathrm{IrTe}}_2$. (c) Temperature-dependent EDCs upon cooling (integrated $\pm 0.09$ ${\AA }^{-1}$ around A along AL direction). (d) RT relative binding energy shift of the surface state SSD measured in ARPES as a function of temperature for a pristine crystal, 0.13 and 0.21 ML K-doped crystals.

Figure 4

(a) Line profiles of LEED images shown in (b). (b) Raw LEED images of a 0.13 ML K-doped ${\mathrm{IrTe}}_2$ crystal in the ($1\times 1$) phase at 295 K, ($5\times 1$) phase at 230 K, ($8\times 1$) phase at 170 K, and ($6\times 1$) phase at 30 K. All images were obtained using 64 eV electron energy.

Figure 5

$19.7\times 19.7{\mathrm{nm}}^2$ constant current mode STM image of a 0.01 ML K-doped ${\mathrm{IrTe}}_2$ sample, $V_{\text{bias}}=-200$ mV, $I=0.2$ nA.

Figure 6

(a) Lines profiles of LEED images shown in (b). (b) LEED image from a 0.13 ML K-doped crystal of ${\mathrm{IrTe}}_2$ in the ($8\times 1$) and ($6\times 1$) phases at 30 K. All images were obtained using 64 eV electron energy.

Figure 7

Temperature of the sample as a function of time during the temperature dependence study.

Figure 8

(a) ARPES spectra of a 0.13 ML K-doped ${\mathrm{IrTe}}_2$ crystal measured along AL direction with a photon energy of $h\nu =21.22\mathrm{eV}$ upon cooling. (b) Fermi surfaces of a 0.13 ML K-doped ${\mathrm{IrTe}}_2$ crystal integrated over 0.05 eV around $E_F$ at different temperatures upon cooling.