Pressure-Induced Superconductivity in Elemental Ytterbium Metal

Ytterbium (Yb) metal is divalent and nonmagnetic ($4f^{14}$ configuration). Under pressure its valence increases significantly leading to the expectation that magnetic instabilities and other highly correlated electron effects may appear before a stable trivalent state is reached ($4f^{13}$ configuration). We carried out electrical resistivity and ac magnetic susceptibility measurements to 179 GPa over the temperature range 1.4–295 K. No evidence for magnetic order is observed. However, Yb becomes a superconductor at 86 GPa with $T_{\mathrm{c}}\simeq 1.4\mathrm{K}$, increasing to 4.6 K at 179 GPa. X-ray absorption spectroscopy shows that Yb remains mixed valent to at least 125 GPa, pointing to an active role of $f$ electrons in the emergence of superconductivity in this simple, elemental solid.

Figure 1

(a) Resistance of Yb in run R1 versus temperature from 1.4 to 295 K for pressures to 179 GPa at low temperature. The maximum value of the residual resistance $\sim 430\mathrm{m}\mathrm{\Omega }$ corresponds to a resistivity of approximately $10\mu \mathrm{\Omega }\mathrm{cm}$. Inset shows smooth $R(T)$ dependence below 25 K; note that residual resistance at 4 K has been subtracted off. Inset inside inset shows sharp break in slope of $R(T)$ for Nd signaling magnetic ordering at $T_o\simeq 44\mathrm{K}$ [12]. (b) Resistance data from (a) for temperatures 1.4 to 7 K showing superconducting transitions for pressures 86 GPa and above. Data at 86 GPa are from run R2. Here $T_c$ is defined (see, for example, data at 150 GPa) as temperature where the straight red line through data hits the temperature axis; vertical tick marks temperature where $R(T)$ vanishes.

Figure 2

(a) Real part of the 1st harmonic of ac susceptibility versus temperature for Yb in run X3 showing strong diamagnetic shielding of superconducting transition for pressures above 92 GPa at low temperature. Background signal from nonsuperconducting Yb at 72 GPa has been subtracted from data. Inset shows superconducting transition at 116 GPa in raw data. (b) For Yb at 116 GPa, temperature dependence of the 1st harmonic of real and imaginary parts of ac susceptibility with background was subtracted as in (a) and compared to the 3rd harmonic of the imaginary part of ac susceptibility where no background subtraction was necessary. All three susceptibilities clearly define the superconducting transition temperature.

Figure 3

Superconducting transition temperature of Yb versus pressure for all resistivity and ac susceptibility measurements. In the legend “R” and “X” are values from resistivity and ac susceptibility measurements, respectively. For resistivity $T_c$ is defined as temperature where $R(T)$ extrapolates to zero, lower error bar where $R(T)$ actually reaches zero. In ac susceptibility $T_c$ is defined as the temperature at the transition midpoint for the real part of the 1st harmonic, upper and lower error bars giving temperatures where the straight lines through data intersect, as in the upper panel of Fig. 2. The long straight line gives the best estimate of $T_c$ versus pressure with slope $50\mathrm{mK}/\mathrm{GPa}$. Structures at the top of the graph for Yb follow the structure sequence at room temperature [21, 22]: fcc(I) to bcc at 4 GPa, to hcp at 26 GPa, to fcc(II) at 53 GPa, to hP3 at 96 GPa.

Figure 4

Pressure dependence of $L_3$ x-ray absorption data at ambient temperature showing that Yb metal remains mixed valent under pressure to at least 125 GPa. Yb in ${\mathrm{YbF}}_3$ is fully trivalent [41].