Hydrostatic pressure study of pure and doped ${\mathrm{La}}_{1-x}{R}_x\mathrm{Ag}{\mathrm{Sb}}_2$ $(R=\mathrm{Ce},\mathrm{Nd})$ charge-density-wave compounds

The intermetallic compound $\mathrm{La}\mathrm{Ag}{\mathrm{Sb}}_2$ displays two charge-density-wave (CDW) transitions, which were detected with measurements of electrical resistivity $\left(\rho \right)$, magnetic susceptibility, and x-ray scattering; the upper transition takes place at $T_1\approx 210\mathrm{K}$, and it is accompanied by a large anomaly in $\rho \left(T\right)$, whereas the lower transition is marked by a much more subtle anomaly at $T_2\approx 185\mathrm{K}$. We studied the effect of hydrostatic pressure $\left(P\right)$ on the formation of the upper CDW state in pure and doped ${\mathrm{La}}_{1-x}{R}_x\mathrm{Ag}{\mathrm{Sb}}_2$ $(R=\mathrm{Ce},\mathrm{Nd})$ compounds by means of measurements of $\rho \left(T\right)$ for $P⩽23\mathrm{kbar}$. We found that the hydrostatic pressure, as well as the chemical pressure introduced by the partial substitution of the smaller Ce and Nd ions for La, results in the suppression of the CDW ground state—e.g., the reduction of the ordering temperature $T_1$. The values of $d{T}_1∕dP$ are $\approx 2–4$ times higher for the Ce-doped samples as compared to pure $\mathrm{La}\mathrm{Ag}{\mathrm{Sb}}_2$, or even ${\mathrm{La}}_{0.75}{\mathrm{Nd}}_{0.25}\mathrm{Ag}{\mathrm{Sb}}_2$ with a comparable $T_1$ $(P=0)$. This increased sensitivity to pressure may be due to increasing Ce hybridization under pressure. The magnetic-ordering temperature of the cerium-doped compounds is also reduced by pressure, and the high-pressure behavior of the Ce-doped samples is dominated by Kondo impurity scattering.

Figure 1

(Color online) $\rho \left(T\right)$ curves for $\mathrm{La}\mathrm{Ag}{\mathrm{Sb}}_2$ in the hydrostatic pressures $P_{300\mathrm{K}}=0$, 2.5, 7.0, 11.1, 15.0, 19.4, and $21.2\mathrm{kbar}$. The onset of CDW order is marked by the break in the linear behavior of $\rho \left(T\right)$ at high temperatures. Note that the value of $P$ is reduced somewhat upon cooling.

Figure 2

(Color online) Pressure dependence of $T_{\mathit{CDW}}$ (solid circles) and $\Delta {\rho }_{\mathit{max}}∕{\rho }_{\mathit{CDW}}$ (open squares) for $\mathrm{La}\mathrm{Ag}{\mathrm{Sb}}_2$. The $P$ values shown are corrected to account for the $P$ reduction upon cooling, and they represent the $P$ values at $T_{\mathit{CDW}}$. The values of $\Delta \rho$ for $\Delta {\rho }_{\mathit{max}}∕{\rho }_{\mathit{CDW}}$ are extracted from the maximum differences between $\rho$ for $T<T_{\mathit{CDW}}$ and the linearly extrapolated values of $\rho$ from $T>T_{\mathit{CDW}}$ to $T<T_{\mathit{CDW}}$.

Figure 3

(Color online) Pressure dependence of $\Delta \rho ∕\rho$ (see text) at 50 and $300\mathrm{K}$, well below and well above $T_{\mathit{CDW}}$, respectively. The $P$ values at $50\mathrm{K}$ have been corrected to reflect the reduction in pressure with temperature. The lines are from linear fits to the data.

Figure 4

(Color online) Normalized change in resistivity below $T_{\mathit{CDW}}$ as a function of effective temperature (see text). These data were extracted from the data shown in Fig. 1. The pressure values shown are corrected to reflect the estimated values at $T_{\mathit{CDW}}$. For clarity, only a small subset of the data is shown.

Figure 5

(Color online) Normalized curves of $\rho ∕{\rho }_{300\mathrm{K}}$ vs $T$ for ${\mathrm{La}}_{0.75}{\mathrm{Nd}}_{0.25}\mathrm{Ag}{\mathrm{Sb}}_2$ and $\mathrm{La}\mathrm{Ag}{\mathrm{Sb}}_2$ at $P_{300\mathrm{K}}=0$ and $21.2\mathrm{kbar}$, respectively. The similar values of $T_{\mathit{CDW}}$ and resistive anomalies below $T_{\mathit{CDW}}$ suggest similar degrees of nesting of the Fermi surface.

Figure 6

(Color online) Normalized electrical resistivity $\rho ∕{\rho }_{300\mathrm{K}}$ vs $T$ for ${\mathrm{La}}_{0.75}{\mathrm{Nd}}_{0.25}\mathrm{Ag}{\mathrm{Sb}}_2$ compounds in pressures up to $22\mathrm{kbar}$. The $\rho ∕{\rho }_{300\mathrm{K}}\left(T\right)$ curves for $P>0$ are offset for clarity.

Figure 7

(Color online) $d\rho ∕dT$ as a function of temperature for ${\mathrm{La}}_{0.75}{\mathrm{Nd}}_{0.25}\mathrm{Ag}{\mathrm{Sb}}_2$ at differing applied pressures. Pressure values shown (in kbar) have been corrected for differential thermal contraction and represent estimated values at $T_{\mathit{CDW}}$. The $\rho \left(T\right)$ data have been smoothed before the derivatives were taken. The highest pressure in which $T_{\mathit{CDW}}$ can be unequivocally identified is $9.6\mathrm{kbar}$.

Figure 8

(Color online) (a) Normalized electrical resistivity $\rho ∕{\rho }_{300\mathrm{K}}$ vs $T$ for ${\mathrm{La}}_{0.9}{\mathrm{Ce}}_{0.1}\mathrm{Ag}{\mathrm{Sb}}_2$ under pressures up to $22\mathrm{kbar}$. The $\rho ∕{\rho }_{300\mathrm{K}}\left(T\right)$ curves for $P>0$ are offset for clarity. (b) and (c) Subtraction of ${\rho }_{12.9\mathrm{kbar}}\left(T\right)$ (suppressed CDW) from ${\rho }_{7.8\mathrm{kbar}}\left(T\right)$ and ${\rho }_{10.3\mathrm{kbar}}\left(T\right)$ ($T_{\mathit{CDW}}=116\mathrm{K}$ and $90\mathrm{K}$, respectively), yielding ${\rho }_{\mathit{CDW}}\left(T\right)$ behavior deconvoluted from the contribution of single-ion Kondo scattering.

Figure 9

(Color online) Normalized electrical resistivity $\rho ∕{\rho }_{300\mathrm{K}}$ vs $T$ for (a) ${\mathrm{La}}_{0.9}{\mathrm{Ce}}_{0.2}\mathrm{Ag}{\mathrm{Sb}}_2$ and (b) ${\mathrm{La}}_{0.7}{\mathrm{Ce}}_{0.3}\mathrm{Ag}{\mathrm{Sb}}_2$ in pressures up to $22\mathrm{kbar}$. The $\rho ∕{\rho }_{300\mathrm{K}}\left(T\right)$ curves for $P>0$ are offset for clarity.

Figure 10

(Color online) Pressure dependence of $T_{\mathit{CDW}}$, $T_{\mathit{min}}$, and $T_m$ for ${\mathrm{La}}_{1-x}{\mathrm{Ce}}_x\mathrm{Ag}{\mathrm{Sb}}_2$. The inset shows the detailed values of $T_m\left(P\right)$ for $x=0.2$ and 0.3. The value of $d{T}_m∕dP$ for both compositions is $\approx -0.2\mathrm{K}∕\mathrm{kbar}$.

Figure 11

(Color online) Temperature dependence of $T_{\mathit{CDW}}$ as a function of pressure for $\mathrm{La}\mathrm{Ag}{\mathrm{Sb}}_2$, ${\mathrm{La}}_{1-x}{\mathrm{Ce}}_x\mathrm{Ag}{\mathrm{Sb}}_2$ $(x=0.1,0.2)$, and ${\mathrm{La}}_{0.75}{\mathrm{Nd}}_{0.25}\mathrm{Ag}{\mathrm{Sb}}_2$. The open symbols for the doped materials represent actual $T_{\mathit{CDW}}$’s. The solid symbols for the doped materials were shifted in pressure so that their ambient pressure values match the $T_{\mathit{CDW}}$ values for pure $\mathrm{La}\mathrm{Ag}{\mathrm{Sb}}_2$, in order to emphasize the stronger detrimental effect of pressure on the CDW ground state for the doped materials, particularly for Ce doping.