Spin-density-wave order controlled by uniaxial stress in ${\mathrm{CeAuSb}}_2$

The tetragonal heavy-fermion compound ${\mathrm{CeAuSb}}_2$ (space group $P4/nmm$) exhibits incommensurate spin-density wave (SDW) order below $T_N\approx 6.5\text{K}$ with the propagation vector ${\mathbf{q}}_A=({\delta }_A,{\delta }_A,1/2)$. The application of uniaxial stress along the [010] direction induces a sudden change in the resistivity ratio ${\rho }_a/{\rho }_b$ at a compressive strain of $\epsilon \approx -0.5$%. Here we use neutron scattering to show that the uniaxial stress induces a first-order transition to a SDW state with a different propagation vector $(0,{\delta }_B,1/2)$ with ${\delta }_B=0.25$. The magnetic structure of the new (B) phase consists of Ce layers with ordered moments alternating with layers with zero moment stacked along the $c$ axis. The ordered layers have an up-up-down-down configuration along the $b$ axis. This is an unusual situation in which the loss of spatial inversion in a metallic system is driven by the magnetic order. We argue that the change in SDW wave vector leads to Fermi-surface reconstruction and a concomitant change in the transport properties.

Figure 1

Phase diagram of ${\mathrm{CeAuSb}}_2$ determined from transport measurements in Refs. [6, 8, 9] for a magnetic field applied along the [001] and a compressive strain ${\epsilon }_{\langle 100\rangle }$ along the $\langle 100\rangle$-type direction.

Figure 2

DC SQUID magnetization measurements for up and down temperature sweeps for the sample measured with neutron diffraction (shown in inset). A field of 0.5 T was applied in the $ab$ plane. The inset plot shows the derivative $d\left(MT\right)/dT$ for the up-sweep data—for an antiferromagnet at constant field this quantity is proportional to the heat capacity $C_p\left(T\right)$ [19]. The transition into the SDW-A phase occurs at $T_{\mathrm{N}}\approx 6.5$ K.

Figure 3

(a) Neutron-diffraction data in the $(H,K,0.5)$ plane for three values of compressive stress along [010], ${\sigma }_{010}$, at two temperatures (5.5 and 1.7 K). The data are smoothed and a planar background has been subtracted. It can be seen that compressive uniaxial stress along [010] induces a monodomain of single-$\mathbf{q}$ order with a modulation vector rotated by ${45}^{\circ }$ with respect to the zero stress phase. (b) (top) Resistivity for a current applied parallel and perpendicular to the compressive strain as a function of strain at 1.5 K reproduced from Ref. [8]. (middle, bottom) Integrated intensity of magnetic Bragg peaks in the A & B phase as a function of stress (after background subtraction) for $T=1.7$, 5.5 K. Vertical blue lines represent the approximate phase boundary determined from the crossing of the A phase and B phase curves. (c) Magnetic Bragg peaks in the A phase (top) and B phase (bottom) at the minimum and maximum stress at which data were collected at 1.7 K. Solid lines are fits to the peak with a Gaussian convoluted with a back to back exponential, the fitted lattice parameter is indicated by dashed line (which does not coincide with the peak maximum). (d) Refined value of ${\delta }_A$ (top) and ${\delta }_B$ (bottom) at two temperatures 1.7 and 5.5 K as a function of compressive stress along $b$. Solid line is a linear fit.

Figure 4

A phase structure. (top) Observed and calculated structure factors from the refinement of the nuclear and magnetic structure at 0 MPa and 1.7 K using jana2006. The nuclear structure refinement has $R=11.9\%$ and $R_w=12.6\%$ and the magnetic structure refinement has $R=12.1\%$ and $R_w=14.1\%$. (bottom) drawing of the nuclear and magnetic structure at 0 MPa and 1.7 K (only cerium atoms shown in magnetic structure for clarity). The magnetic structure shows a single ${\mathbf{q}}_1=({\delta }_A,{\delta }_A,0.5)$ domain.

Figure 5

B phase structure. (top) Observed and calculated structure factors from the refinement of the nuclear and magnetic structure at 440 MPa and 1.7 K using jana2006. The nuclear structure refinement has $R=9.5\%$ and $R_w=9.4\%$ and the magnetic structure refinement has $R=5.6\%$ and $R_w=6.4\%$ (commensurate $A_{b}em2$ structure). (bottom) drawing of the nuclear and magnetic structure at 440 MPa and 1.7 K (only cerium atoms shown in magnetic structure for clarity).

Figure 6

(a) The intensity of (003) and (102) nuclear peaks plotted as a function of $d$ spacing for various values of compressive stress along the $b$ axis (curves are offset for clarity). Solid lines are fits with a back-to-back exponential convoluted with a Gaussian. The (003) peak was measured at higher resolution; at nonzero stress the peak exhibits a shoulder at the zero stress position indicating a portion of the sample ($\approx 20$%) was not stressed. The peak positions shift due to larger $d$ spacing with increasing compression along the $b$ axis as the $a$- and $c$-axis lattice parameters increase. (b) The $a$ and $c$ lattice parameters at 1.7 and 5.5 K as a function of compressive stress along the $b$ axis, ${\sigma }_{010}$, and the predicted $b$-axis lattice parameter for two values of the Young's modulus ($b$ could not be measured directly). The lattice parameters were refined from the $d$-spacing of eight nuclear peaks.