Observation of well-defined quasiparticles at a wide energy range in a quasi-two-dimensional system

We report an observation of a highly anisotropic Dirac-cone structure, and its unconventional electronic behaviors, in high quality ${\mathrm{SrMnBi}}_2$ crystal by angle-resolved photoemission spectroscopy measurement. We observed a well-defined sharp quasiparticle peak in a wide energy range, linearly dispersive, forming a highly anisotropic holelike Dirac cone $(v_{F_1}/v_{F_2}\sim 5–6)$. The density of state remains linear, and the scattering rate of the quasiparticle increases linearly as a function of binding energy up to higher binding energy. The results contrast with the expectation for a two-dimensional (2D) or three-dimensional (3D) Fermi-liquid theory and rather suggest the existence of a two-dimensional electron gas system in this 3D material, making ${\mathrm{SrMnBi}}_2$ a potentially useful 2D-electron-gas-encapsulated material.

Figure 1

Structure, core levels, and Fermi surface topology of ${\mathrm{SrMnBi}}_2$ at low temperature $(T=30$ K). (a) Crystal structure of ${\mathrm{SrMnBi}}_2$ with ${\mathrm{MnBi}}_4$ layers and Bi square nets. (b) Shallow core levels of the ${\mathrm{SrMnBi}}_2$. (c) Integrated intensity plot around $E_F$ $(-20$ meV, 20 meV), which approximately illustrates the FS topology of ${\mathrm{SrMnBi}}_2$, with photon energy $h\nu =60$ eV. The primary BZ is illustrated by black lines in (c).

Figure 2

Band dispersion of anisotropic Dirac cone along two directions. (a)–(c) Intensity plot, second derivative $({\partial }^{2}I/\partial {\omega }^2)$, and the MDCs along cut 1, shown in Fig. 1, respectively. (d)–(f) The intensity plot, second derivative, and the MDCs plot along cut 2. The dashed lines in figures are peak positions to illustrate band dispersions.

Figure 3

Constant line shape as a function of binding energy with high energy and momentum resolution at $T=10$ K. (a), (d) High-resolution intensity plot and the second derivative of the intensities along the cut 1 direction up to 1.4 eV binding energy. (b), (e) Intensity plot and EDCs plot of the band dispersion with background removed. (c) The integrated intensity of (b) as a function of binding energy [$n_{\theta }\left(\omega \right)$].

Figure 4

Unusual energy dependence of the quasiparticle width. (a) MDCs and Lorentz fitting of the high-resolution intensity along cut 1, as shown in Fig. 3. (b) MDCs and Lorentz-fitting results of intensity plot along the cut 2 direction. The black MDCs are recorded at $E_F$. The blue MDCs indicate the points between double-Lorentz and single-Lorentz fitting. (c), (d) Extracted half width of half maximum (HWHM) (red circles) in momentum (left axis) and energy (right axis, ${\Gamma }_{\mathrm{EDC}}={\Gamma }_{\mathrm{MDC}}{v}_F)$ of the Dirac fermions along cut 1 and cut 2. The blue triangles show the intrinsic peak width, disposed of the momentum resolution by ${\Gamma }_0=\sqrt{{\Gamma }_{\mathrm{MDC}}^2-{\Delta }_k^2}$. The lines in (c) and (d) are a guide to the eye.