Structural and magnetic phase diagram of CrAs and its relationship with pressure-induced superconductivity

We use neutron diffraction to study the structure and magnetic phase diagram of the newly discovered pressure-induced superconductor CrAs. Unlike most magnetic unconventional superconductors where the magnetic moment direction barely changes upon doping, here we show that CrAs exhibits a spin reorientation from the $ab$ plane to the $ac$ plane, along with an abrupt drop of the magnetic propagation vector at a critical pressure ($P_c\approx 0.6$ GPa). This magnetic phase transition, accompanied by a lattice anomaly, coincides with the emergence of bulk superconductivity. With further increasing pressure, the magnetic order completely disappears near the optimal $T_c$ regime ($P\approx 0.94$ GPa). Moreover, the Cr magnetic moments tend to be aligned antiparallel between nearest neighbors with increasing pressure toward the optimal superconductivity regime. Our findings suggest that the noncollinear helimagnetic order is strongly coupled to structural and electronic degrees of freedom, and that the antiferromagnetic correlations between nearest neighbors might be essential for superconductivity.

Figure 1

Pressure dependence of the magnetic structure for CrAs. (a), (b) Magnetic structures at $P=0$ and $0.6$ GPa, respectively. The red dashed lines indicate the direction of moment 1. (c) Observed (red) and calculated (green) diffraction intensities at $P=0$ GPa and 4 K. (d) The diffraction pattern at $P=0.4$ GPa and $T=4$ K. The four refined phases are crystal and magnetic structures of CrAs, the second- and third-order neutron reflections from the aluminum pressure cell. The missing data correspond to the strong first-order neutron reflections from the aluminum pressure cell. (e) The diffraction pattern at $P=0.6$ GPa and $T=50$ K.

Figure 2

Structural and magnetic phase transition temperatures at representative pressure for CrAs. All measurements were performed on warming after initially cooling to $T=4$ K. (a)–(j) Temperature-dependent diffraction profile at various pressures. The subscript letters ($M,N$) denote magnetic and nuclear peaks, respectively. (k)–(o) Temperature dependence of the peak intensity of the data obtained in (a)–(j). We note that in (l), the decrease of the ${(000\pm )}_M$ peak intensity at low temperature is due to the decrease of the phase angle ${\beta }_{12}$ on cooling, as shown in Fig. 4. The error bars indicate one standard deviation.

Figure 3

Structural and magnetic phase diagram of CrAs. (a) Temperature and pressure dependence of the magnetic propagation vector. The open and solid circles indicate the magnetic moment in the $ab$ and $ac$ plane, respectively. (b) Structural and magnetic phase diagram of CrAs. The contour propagation vector map is plotted from the data in (a) in the temperature-pressure space. The superconducting transition temperatures are adapted from Refs. [6, 7, 15]. (c) The pressure dependence of the propagation vector and the superconducting volume fraction determined by susceptibility measurements adapted from Ref. [6]. Different from Refs. [7, 15], indication of superconductivity was also observed at $0.29\le P\le 0.5$ GPa in Ref. [6] (open squares). However, in this pressure range, the superconductivity is filamentary rather than a bulk phenomenon because the superconducting volume fraction is extremely low ($\le 15\%$) and the resistivity exhibits multiple superconducting transitions [6]. The superconducting volume fraction increases significantly above 0.6 GPa, when the spin-reoriented low-vector phase is fully developed. The same color code as in (b) is used, which indicates the amplitude of propagation vector. (d) The pressure dependence of the magnetic moment at 4 K.

Figure 4

Temperature and pressure dependence of lattice constants, Cr-Cr distances, and angles between Cr moments. (a)–(c) Lattice parameter as a function of temperature and pressure. When the magnetic order is fully suppressed at 0.94 GPa and 1.5 K, the lattice parameters are exactly in the extrapolation of the curve obtained by considering the lattice parameters above $T_N$ for $P=0$, 0.4, and 0.6 GPa. (d) Pressure dependence of the lattice parameter in the magnetically ordered state. (e) Distance between $S_1$ and $S_2$ (nearest neighbor). (f) Distance between $S_2$ and $S_3$ (nearest neighbor). (g) Angle (${\beta }_{12}$) between spin $S_1$ and $S_2$. (h) Angle (${\beta }_{23}$) between spin $S_2$ and $S_3$. By symmetry, we have ${\beta }_{12}={\beta }_{34}, {\beta }_{23}={\beta }_{{41}^{\prime }}$ [Figs. 1 and 1].