Instability of the Electron Gas in an Expanding Metal

We have measured x-ray diffraction and small-angle x-ray scattering of fluid rubidium by reducing electron density down to the range where the compressibility of the interacting electron gas has been theoretically predicted to become negative. Negative compressibility is closely associated with a negative value of the static dielectric function, which makes the screened Coulomb interaction among like charges overall attractive. It was clearly observed that the interatomic distance decreases in spite of the fact that mean interatomic distance increases with expansion, suggesting that an attractive interaction among like charges, ions in this case, is enhanced. These findings indicate that the observed structural features are evidence of the compressional instability of the 3D electron gas.

Figure 1

Structure factors $S(Q)$ for expanded fluid rubidium at various temperatures and pressures. Temperature, pressure and density are indicated on the upper right-hand side of each data set. The dots represent the experimental data and the full curves show the Fourier transforms of $g(R)$ in Fig. 2. The broken line indicates the position at the first peak and second one of $S(Q)$ at 373 K, respectively. The inset shows the phase diagram of rubidium ($T_c=2017\mathrm{K}$, $P_c=12.45\mathrm{MPa}$) [14] in which dot-and-dash line indicates the path of the conditions at which the present measurements were performed.

Figure 2

Pair distribution function $g(R)$ of expanded fluid rubidium derived from Fourier transform of $S(Q)$. The broken line denotes the peak position of the first maximum of $g(R)$ at 373 K. Temperature, pressure and density are indicated on the upper right-hand side of each curve.

Figure 3

Density dependence of local structure of fluid rubidium. (a) Density dependence of interatomic distance $R_1$. (b) Density dependence of coordination number. The critical density is denoted ${\rho }_c$. The corresponding scale of $r_s^{*}$ is shown on the upper axis of the graph of $R_1$.

Figure 4

Density dependence of correlation length and $S(0)$ for expanded fluid rubidium up to the critical region. Dots represent the data measured at the conditions as shown in the inset in Fig. 1. Open circles indicates the data measured at the pressure of 5 MPa.