Surface phases of the transition-metal dichalcogenide $\mathrm{IrT}{\mathrm{e}}_2$

Transition-metal dichalcogenide $\mathrm{IrT}{\mathrm{e}}_2$ has attracted attention because of its striped lattice, charge ordering, and superconductivity. We have investigated the surface structure of $\mathrm{IrT}{\mathrm{e}}_2$, using low-energy electron diffraction and scanning tunneling microscopy. A complex striped lattice modulation as a function of temperature is observed, which shows hysteresis between cooling and warming. While the bulk 5 × 1 and 8 × 1 phases appear at high temperatures, the surface ground state has the 6 × 1 phase, not seen in the bulk, and the surface transition temperatures are distinct from the bulk. The broken symmetry at the surface creates a quite different phase diagram, with the coexistence of several periodicities resembling devil's staircase behavior.

Figure 1

Crystal structure of $\mathrm{IrT}{\mathrm{e}}_2$: (a) bulk structure, where two Te atoms labeled in purple and green are equivalent. (b) Bulk truncated surface with the undistorted 1 × 1 unit cell. (c) Reconstructed surface lattice with the 5 × 1 structure with Ir-Ir dimers shown in red shades. (d) Surface 6 × 1 structure with the blue dashed line indicating the glide-line symmetry.

Figure 2

(a)–(e) Simulated LEED patterns for different structures with integer spots in white circles: (a) undistorted 1 × 1 structure, (b) 5 × 1 structure with single domain, (c) 5 × 1 structure with multiple domains, (d) 8 × 1 structure, and (e) 6 × 1 structure. (f)–(o) LEED patterns at indicated temperatures during the cooling and warming processes. The patterns are taken at 80 eV; (p) 1 × 1 structure at the same temperature as (f), but with 122 eV beam energy.

Figure 3

(a) LEED pattern at 160 K and 80 eV taken after a rotation and lateral translation of ∼1 mm from Fig. 2; (b) LEED pattern at 45 K and 55 eV taken at the same position as Fig. 2; (c) LEED pattern at 213 K and 70 eV with a slightly different position from Fig. 2. The corresponding domains are marked by the arrows at the bottoms of (c) and (d).

Figure 4

Topographic STM images of $\mathrm{IrT}{\mathrm{e}}_2$ surface. (a) Atomically resolved image of a freshly cleaved surface at 300 K, scanned at $V_{\mathrm{sample}}=1\mathrm{V},I=80\mathrm{pA}$. The surface shows uniform 1 × 1 hexagonal structure. (b) Large area image at 80 K, scanned at $V_{\mathrm{sample}}=2.0\mathrm{V},I=80\mathrm{pA}$. It shows the coexistence of a mixed striped area and a uniform striped area. (c) Atomically resolved image representative of the mixed striped area, scanned at $V_{\mathrm{sample}}=-0.6\mathrm{V},I=50\mathrm{pA}$. (d) Atomically resolved image representative of the uniform striped area with 6 × 1 reconstruction, scanned at $V_{\mathrm{sample}}=-5\mathrm{mV},I=5\mathrm{nA}$.

Figure 5

(a) LEED pattern at 288 K, 55 eV during the warming process. (b) Zoom-in of the area indicated by the red square in (a). (c) Line profile data from the cuts in (b). The coordinates of the $x$ axis are normalized for all data based on the image pixels.

Figure 6

Comparison of surface and bulk structures at different temperatures for (a) cooling process and (b) warming process.

Figure 7

LEED patterns of $\mathrm{IrT}{\mathrm{e}}_2$ at 45 K and different electron-beam energies. These patterns show the emergence of fractional spot (0.5,1), indicated by the red circle. (a) 66 eV. (b) 87 eV.

Figure 8

Analysis of the 8 × 1 reconstructed pattern. (a) A reproduction of Fig. 3. (b) Line profile based on the red line in (a). The Lorentzian fittings of the peaks are also shown. (c) The peak positions of each spot with their peak numbers. The existing spots and missing spots are indicated by black and red colors, respectively. The positions can be fit by a straight line. (d) LEED pattern at 271 K and 80 eV in the warming process. The sample is at the same position as Fig. 2 but rotated.