Electronic structure of ferromagnetic semiconductor ${\mathrm{CrGeTe}}_3$ by angle-resolved photoemission spectroscopy

As one of the rare ferromagnetic semiconductors, ${\mathrm{CrGeTe}}_3$ has recently attracted a great deal of attention as a potential candidate for next-generation high-performance nano-spintronic devices. In this study, by combining density functional theory calculations and angle-resolved photoemission spectroscopy measurements, we explore the electronic structure of ${\mathrm{CrGeTe}}_3$ directly. The low-lying valence bands are centered around the $\mathrm{\Gamma }$ point and mainly consist of Te $5p$ orbitals. The majority of the bands show almost no $k_z$ dispersion, consistent with its layered crystalline structure. Due to the higher hopping integral along the out-of-plane direction, however, bands comprised of $p_z$ orbitals exhibit significant $k_z$ dispersion. Furthermore, an indirect band gap of 0.38 eV is directly measured by surface electron doping with potassium deposition.

Figure 1

(a) Schematic illustration of the crystalline structure of CrGe($\mathrm{Si}){\mathrm{Te}}_3$ from the top view and the side view. A unit cell is indicated by a black line. (b) Morphology of a fresh surface of ${\mathrm{CrGeTe}}_3$ after exfoliation. (c) In situ LEED patterns after ARPES measurements to confirm the quality of the surface. The markers indicate two sets of (11) diffraction pattern. (d) $2\mathrm{\Theta }-\omega$ x-ray diffraction scan along the (00l) direction. The insets show azimuthal $\mathrm{\Phi }$ scans of (116) diffraction peaks for ${\mathrm{CrGeTe}}_3$ and ${\mathrm{CrSiTe}}_3$. (e) ZFC-FC curves with 50 Oe external magnetic field parallel to the $c$ axis. The inset shows the isothermal magnetization curves $M\left(H\right)$ at 5 K. The saturation field is 5.0 kOe with an external field parallel to the $ab$ plane (green line) and 2.4 kOe (orange line) parallel to the $c$ axis. A zoom-in of the magnetization curve shows no significant coercive force.

Figure 2

(a) The Brillouin zone of ${\mathrm{CrGeTe}}_3$ and the high-symmetry path. (b) Calculated band structures along the high-symmetry path and total and projected density of states for spin-up and spin-down states. $E_F$ is aligned with the valence-band top. (c) ARPES isoenergy mapping in the first Brillouin zone with binding energy of 0.7 eV. The orange dashed lines are the corresponding energy contour for band set $\alpha$ and the green dashed lines for band set $\beta$. Calculated isoenergy surface for (d) band set $\alpha$ and (e) band set $\beta$ with corresponding binding energy of ARPES mapping. (The Brillouin zone has been expanded threefold along the $k_z$ axis for better illustration.)

Figure 3

ARPES spectra along the path of (0,0)-(1,0)-(0.67,0.67)-(0,0) taken with (a) He i$\alpha$ and (b) He ii$\alpha$ photons. The calculated band dispersion is superimposed. Schematic illustration of the high-symmetry path in the first Brillouin zone is shown in the inset.

Figure 4

Detailed structure of valence bands along the path of $(-1,0)-(0,0)-(1,0)$ for ${\mathrm{CrGeTe}}_3$ with (a) He i$\alpha$ and (b) He ii$\alpha$ ARPES spectra. Detailed structure of valence band along the path of $(-0.67,-0.67)-(0,0)-(0.67,0.67)$ for (c) He i$\alpha$ and (d) He ii$\alpha$ ARPES spectra. Parts (e)–(h) are the corresponding results for ${\mathrm{CrSiTe}}_3$. Crossings are the fitting points. Band set $\alpha$ is denoted by an orange dashed line and band $\beta$ by a green dashed line under parabola fitting.

Figure 5

The conduction band along the path of (0,0)-(0.67,0.67); band sets $\alpha$ and $\beta$ are denoted by an orange dashed line and a green dashed line, respectively. The conduction band is denoted by a purple dashed line.