Magnetic mixed valent semimetal ${\mathrm{EuZnSb}}_2$ with Dirac states in the band structure

We report discovery of antiferromagnetic semimetal ${\mathrm{EuZnSb}}_2$, obtained and studied in the form of single crystals. Electric resistivity, magnetic susceptibility, and heat capacity indicate antiferromagnetic order of Eu with $T_N$ = 20 K. The effective moment of ${\mathrm{Eu}}^{2+}$ inferred from the magnetization and specific heat measurement is 3.5 ${\mu }_B$, smaller than the theoretical value of ${\mathrm{Eu}}^{2+}$ due to presence of both ${\mathrm{Eu}}^{3+}$ and ${\mathrm{Eu}}^{2+}$. Magnetic-field-dependent resistivity measurements suggest dominant quasi-two-dimensional Fermi surfaces whereas the first-principle calculations point to the presence of Dirac fermions. Therefore, ${\mathrm{EuZnSb}}_2$ could represent the first platform to study the interplay of dynamical charge fluctuations, localized magnetic $4f$ moments, and Dirac states with Sb orbital character.

Figure 1

(a) Crystal structure of ${\mathrm{EuZnSb}}_2$ projected approximately along [100] with anisotropic displacement parameters drawn at the 90% probability level and with unit cell outlined. Eu atoms are depicted as red crossed ellipsoids, Zn atoms are shown with blue ellipsoids, and the Sb1 and Sb2 are shown with yellow ellipsoids, respectively. (b) Top view of the Sb square nets. EXAFS oscillation with the model fit (c), Fourier transform magnitudes (d), and a close view of the Eu coordination polyhedron (e) at the room temperature. (f) Powder x-ray diffraction pattern of ${\mathrm{EuZnSb}}_2$ confirms phase purity; vertical tick marks denote reflections in the $P4/nmm$ space group. Inset in panel (f) shows average crystal size on a mm scale. (g) Temperature-dependent magnetic susceptibility taken in 1 T magnetic field both in the zero field cooling and field cooling mode for H//$ab$ and H//$c$, respectively. Inset shows field-dependent magnetization at 2 K. (h) XPS core-level spectra of Eu (3$d$) in ${\mathrm{EuZnSb}}_2$.

Figure 2

(a) Temperature dependence of specific heat for ${\mathrm{EuZnSb}}_2$. The red line represents the fit. Inset shows the low-temperature specific heat in various magnetic field. (b) Magnetic field dependence of Hall resistivity ${\rho }_{xy}$ at different temperatures; two-band-model fits (see text) are shown by red lines. Inferred carrier concentration (c) and mobility (d) from the two-band fitting.

Figure 3

(a) Temperature dependence of the in-plane resistivity of ${\mathrm{EuZnSb}}_2$ in 0 and 9 T, respectively. (b) Angle $\theta$ dependence of resistivity at 2 and 30 K, respectively measured in 9 T. [(c), (d)] Magnetoresistance of ${\mathrm{EuZnSb}}_2$ for $\theta$ = 0. Green lines show fits using two-band orbital magnetoresistance obtained using carrier concentrations and mobilities from Figs. 2 and 2; we note that such fits are possible for carrier concentration and mobility variations for up to $\pm 10$% and $\pm 20$% from values reported in Fig. 2. (e) Magnetoresistance at different tilt angles $\theta$ between $B$ and $c$ axis. Data were measured at 2 K and the current was always perpendicular to the magnetic field.

Figure 4

[(a)–(e)] The antiferromagnetic structures used in the first-principles total energy calculations. (f) The electronic band structure calculated with the AAF3-$x$ structure (e) in GGA$+U+\mathrm{SO}$. Red thick circles denote the $5p_x$/$5p_y$ band derived from the Sb2 square lattices. (g) The calculated Fermi surface of ${\mathrm{EuZnSb}}_2$. Warm colors denote hole carriers and cold colors denote electrons. Electron-like bands are located near the $\mathrm{\Gamma }$ point and $X$ point; hole bands are along the (0.0)–($\pi ,\pi$) line excluding the $\mathrm{\Gamma }$ point states.