Absence of superconductivity in electron-doped chromium pnictides ${\mathrm{ThCrAsN}}_{1-x}{\mathrm{O}}_x$

Theoretical studies predicted possible superconductivity in electron-doped chromium pnictides isostructural to their iron counterparts. Here, we report the synthesis and characterization of a ZrCuSiAs-type Cr-based compound ThCrAsN, as well as its oxygen-doped variants. All samples of ${\mathrm{ThCrAsN}}_{1-x}{\mathrm{O}}_x$ show metallic conduction, but no superconductivity is observed above 30 mK even though the oxygen substitution reaches 75%. The magnetic structure of ThCrAsN is determined to be G-type antiferromagnetic by magnetization measurements and first-principles calculations jointly. The calculations also indicate that the in-plane Cr-Cr direct interaction of ThCrAsN is robust against the heavy electron doping. The calculated density of states of the orbital occupations of Cr for ThCrAs(N,O) is strongly spin polarized. Our results suggest the similarities between chromium pnictides and iron-based superconductors should not be overestimated.

Figure 1

(a) Powder x-ray diffraction pattern and the corresponding Rietveld refinement for ThCrAsN, whose crystal structure is shown in the inset. (b) A series of XRD patterns from ${\mathrm{ThCrAsN}}_{1-x}{\mathrm{O}}_x$ ($x=0–1$) polycrystalline samples. The doping concentration $x$ for each pattern increases from bottom to top. The triangles, crosses, diamonds, and circles below the curve of $x=0.4$ mark the peaks from ${\mathrm{ThO}}_2, {\mathrm{Th}}_3{\mathrm{As}}_4, {\mathrm{Cr}}_2\mathrm{As}$, and some unidentified impurities, respectively. The $hkl$ indices in space group $P4/nmm$ are shown with black tics below the curve of $x=0$. Two green bars are sketched to show the shift of characteristic peaks of ${\mathrm{ThCrAsN}}_{1-x}{\mathrm{O}}_x$. Note that there is no phase in space group $P4/nmm$ for the fully doped sample (the top curve, $x=1$). (c) Cell parameters of ${\mathrm{ThCrAsN}}_{1-x}{\mathrm{O}}_x$ as a function of the nominal doping concentration $x$ ($0\le x\le 0.9$). The errors of data points are of the order of ${10}^{-4}$ Å, which is invisible in comparison with the lattice parameters. So the error bars are not present. Two green bars are sketched based on the data points with $x\le 0.4$, and the green dashed line is at $x=0.75$.

Figure 2

Temperature dependences of resistivity ($\rho$) for ${\mathrm{ThCrAsN}}_{1-x}{\mathrm{O}}_x$. (a) $\rho \left(T\right)$ curves with $T>2$ K. The inset zooms in the data between 2 and 20 K for $x=0.2$, 0.3, 0.4, 0.5, and 0.7. (b) Data of $\rho \left(T\right)$ with 30 mK $<T<$ 0.8 K collected in the dilution refrigerator. The specimens with the same composition exhibit different residual resistivity in panels (a) and (b), due to variations in contact resistance and electrode size across different measurements.

Figure 3

Temperature dependences of the magnetic susceptibility ($\chi$) measured at 1 T for (a) ThCrAsN and (b) ${\mathrm{ThCrAsN}}_{0.9}{\mathrm{O}}_{0.1}$. The small kink around 50 K in panel (b) is due to the oxygen contamination. $d\left(\chi T\right)/dT$ is plotted as a function of $T$ in the insets.

Figure 4

Calculated magnetic energies (black squares, left axis) and Cr spin moments (red circles, right axis) for (a) ThCrAsN and (b) ThCrAsO with different spin configurations. The notations for the magnetic structures are as follows: NM denotes nonmagnetic states; A denotes A-type AFM order (in-plane FM order and out-of-plane AFM coupling); C denotes C-type AFM order (in-plane checkerboard AFM order and out-of-plane FM coupling); G denotes G-type AFM order (in-plane checkerboard AFM order and out-of-plane AFM coupling); S denotes the structure with in-plane striped AFM order and out-of-plane FM coupling. The spin configurations are presented at the top of the figure.

Figure 5

(a),(d) Total electronic density of states of G-type AFM ordered ThCrAsN and ThCrAsO, respectively. Insets of (a) and (d) zoom in on the DOS around the Fermi energy. (b),(c),(e),(f) Projected density of states of $3d$ orbitals of Cr in ThCrAsN and ThCrAsO, respectively. The contributions of $d_{xz}$ and $d_{yz}$ are combined because they are degenerate.