Evidence for superconductivity in Rb metal above 55 GPa pressure

The only alkali metal known to be superconducting at ambient pressure is Li at 0.4 mK. Under 30 GPa pressure $T_{\text{c}}$ for Li rises to 14 K. In addition, nearly 50 years ago the heavy alkali metal Cs was reported to become superconducting near 1.3 K at 12 GPa. In the present experiment the superconductivity of Cs under pressure is confirmed. In addition, strong evidence is presented in electrical resistivity measurements that neighboring Rb also becomes superconducting near 2 K at 55 GPa as it enters the oC16 phase, as for Cs, where $T_{\text{c}}$ decreases under the application of pressure. It would seem likely that under the right temperature/pressure conditions all alkali metals, including metallic hydrogen, will join the ranks of the superconducting elements. With the addition of Rb, 55 of the 92 naturally occurring elements are superconducting at ambient or high pressure.

Figure 1

Schematic drawing of pressure cell with Re gasket used in the present four-point electrical resistivity experiments on pure Cs and Rb. Unlike the standard configuration (see Fig. 1 in Ref. [19]), here a bowl is formed in the central cBN/epoxy section to contain the soft alkali metals. The opposing diamond anvil presses downward from above generating pressures to $\sim 80$ GPa.

Figure 2

Dependence of room-temperature resistance of Rb on pressure to 75 GPa. The arrows give the direction of pressure change. Beginning at the lowest pressure, data are taken along the black line to point 7, followed by a pressure decrease to 34 GPa. The red line then tracks a second increase in pressure to point 16, followed by a decrease to point 17. Beginning with data point 1 on the black line, all further data points $R\left(T\right)$ were measured between 295 and 1.3 K. For blue data points superconductivity was observed near 2 K; for black data points no evidence for superconductivity was found above 1.3 K. Numbers give the order of measurement for blue data points first on the black line, then on the red line. Crystal structures at the top of graph are from Ref. [22].

Figure 3

Resistance of Rb vs temperature at three different pressures where a superconducting transition is seen at low temperature. Red numbers give the order of measurement from Fig. 2. The inset shows a transition at 2.1 K for 56 GPa. The value of the transition temperature is determined by the intersection point of two straight lines.

Figure 4

Resistance Rb vs temperature to 4 K at 71 GPa for four different values of magnetic field 0, 240, 360, and 480 G. The superconducting transition shifts to lower temperatures with increasing magnetic field. The inset shows the dependence of the transition temperature on magnetic field at 71 and 77 GPa pressure. The fit of the data allows an estimate of critical field $H_0$ at 0 K (see text). The number “13” gives the order of data points from Fig. 2.

Figure 5

Resistance of Rb vs temperature to 3 K showing superconducting transitions at various pressures. Black numbers give the value of pressure for each transition. Red numbers give the order of measurement from Fig. 2. The smaller size of the transition at 66 GPa gives evidence the sample is in a mixed phase.

Figure 6

Dependence of transition temperature $T_{\text{c}}$ of Rb on pressure to 88 GPa in two separate experiments (black $•$, blue $\blacksquare )$. The broad gray straight line gives a rough fit to the data with slope $d{T}_{\text{c}}/dP\simeq -39$ mK/GPa.