Emergence of topological bands on the surface of ZrSnTe crystal

By using angle-resolved photoemission spectroscopy combined with first-principles calculations, we reveal that the topmost unit cell of ZrSnTe crystal hosts two-dimensional (2D) electronic bands of the topological insulator (TI) state, although such a TI state is defined with a curved Fermi level instead of a global band gap. Furthermore, we find that by modifying the dangling bonds on the surface through hydrogenation, this 2D band structure can be manipulated so that the expected global energy gap is most likely to be realized. This facilitates the practical applications of 2D TI in heterostructural devices and those with surface decoration and coverage. Since ZrSnTe belongs to a large family of compounds having the similar crystal and band structures, our findings shed light on identifying more 2D TI candidates and superconductor-TI heterojunctions supporting topological superconductors.

Figure 1

Crystal structure and electronic structure of ZrSnTe. (a) Crystal structure of ZrSnTe. The arrow indicates that the cleavage takes place between the adjacent ZrTe layers, which breaks the weak Zr-Te bonds indicated as vertical dashed lines. The black cuboid demonstrates the structure of the unit cell and the monolayer used in the calculation. (b) Core-level spectrum of ZrSnTe recorded at a photon energy of $h\nu =250\mathrm{eV}$. The inset shows the magnification of the $\mathrm{Zr}3d_{3/2}$ and $3d_{5/2}$ peaks. (c) Fermi surface (FS) intensity plot of ZrSnTe recorded at $h\nu =50\mathrm{eV}$, obtained by integrating the spectral weight within $\pm 10\mathrm{meV}$ with respect to $E_F$. (d) Calculated FSs of the 3D bulk crystal in the top view along (001). (e) FSs of a free-standing monolayer ZrSnTe from first-principles calculations. (f) and (g) Calculated band dispersions along the high-symmetry lines for bulk and monolayer ZrSnTe, respectively. The blue dashed curve in (g) represents the fictitious Fermi level.

Figure 2

Band structure along $\mathrm{\Gamma }\text{−}M$. (a) Photoemission intensity plot along $M\text{−}\mathrm{\Gamma }\text{−}M$ with $h\nu =77\mathrm{eV}$. (b) Calculated band structure along $M\text{−}\mathrm{\Gamma }\text{−}M$ for a seven-unit-cell-thick slab. The intensity of the red color scales the spectral weight projected to the top two unit cells. (c) Momentum distribution curve (MDC) plot of (a). (d) and (e) MDC plots of the photon-energy-dependent spectra at $E_F$ and $E_B=225\mathrm{meV}$, respectively. The dots and dashes in (c)–(e) are extracted peak positions, serving as guides to the eye.

Figure 3

Band structure around $X$. (a) Photoemission intensity plot along $\mathrm{\Gamma }\text{−}X\text{−}\mathrm{\Gamma }$ with $h\nu =55\mathrm{eV}$. (b) Two-dimensional curvature intensity plot of (a). (c) Calculated band structure along $\mathrm{\Gamma }\text{−}X\text{−}\mathrm{\Gamma }$ for a seven-unit-cell-thick slab. The intensity of the red color scales the spectral weight projected to the top two unit cells. (d) Energy distribution curves (EDCs) of (a). The blue dots are extracted peak positions, serving as guides to the eye. The vertical dashed lines indicate the gap at $X$. (e)–(h) The same as (a)–(d) but along $M\text{−}X\text{−}M$.

Figure 4

Manipulate the band structure with hydrogen adsorption. (a) Evolution of the band dispersions along $M\text{−}X\text{−}M$ as a function of the exposure time in vacuum at $T=20\mathrm{K}.t_0\text{–}{t}_7$ is the time sequence of the spectra, being separated with each other by $\sim 2\mathrm{h}$. $t_0$ and $t_7$ correspond to a fresh and a moderate hydrogenated surface, respectively. The top two black dashed lines indicate the crossing points of the electron band at $E_F$. The red dashed line is the trace of the band gap at $X$. (b) FS intensity plot at $t=t_7(h\nu =50\mathrm{eV})$ by integrating the spectral weight within $\pm 10\mathrm{meV}$ with respect to $E_F$. The intensity at $X$ originates from the valence-band top near $E_F$. (c) Photoemission intensity plot along $M\text{−}\mathrm{\Gamma }\text{−}M$ at $t=t_7$. (d) EDC plot along $\mathrm{\Gamma }\text{−}X\text{−}\mathrm{\Gamma }$ at $t=t_7$. The band dispersions are marked by black dots. (e) Extracted band dispersions along the high-symmetry lines of the “fresh” surface (blue solid triangles) and the surface with hydrogen adsorption at $t=t_7$ (red solid circles). (f) Calculated band structure along $\mathrm{\Gamma }\text{−}X\text{−}M\text{−}\mathrm{\Gamma }$ for a seven-unit-cell-thick slab with all the surface Zr ions bonded with hydrogen atoms. The size of solid circles scales the spectral weight projected to the top one unit cell. The blue dashed line indicates the opening of a global band gap.