Figure 1

Overview of the resistivity data as a function of pressure and temperature. The phase boundaries (taken from Refs. 4, 5) are shown. The lower figure shows the same data but only up to a maximum of 3 K, to show clearly the superconducting region.Reuse & Permissions

Figure 2

The superconducting temperature $T_c$ plotted as a function of fractional volume shift $\Delta V/V_0$, where $\Delta V=V_0-V(p)$, with $V_0$ the atomic volume at ambient pressure. $T_c$ is determined as the midheight of the transition. The inset shows the change of resistivity as a function of applied magnetic field at an atomic volume corresponding to the maximum within the Am IV phase. The shaded areas represent the transition between the different phases.Reuse & Permissions

Figure 3

The critical field $H_c(T)$ for various pressures with the extrapolations to $H_c(0)$. The inset shows the extrapolated critical field to $T=0$, $H_c(0)$, as a function of pressure. Note that the $H_c(0)$ has been deduced with the empirical $H_c(0)\times (1-t^2)$ law with $t=T/T_c(p)$. The dependence of this extrapolation will change depending on the type of superconductivity assumed. Assuming Am as the type II superconductor and using the Werthamer-Helfand-Hohenberg approximation [22] to determine the orbital critical field $H_{c2}(0)$ leads to similar values. Therefore, the exact value of $H_c(0)$ [or $H_{c2}(0)$] is not crucial; whatever form is used $H_c(0)$ [or $H_{c2}(0)$] increases dramatically as a function of pressure.Reuse & Permissions

Figure 4

Upper curves: (Sommerfeld) coefficients deduced from the superconducting properties. Open triangles (circles) assuming type-I (type-II) superconductivity. The solid diamond is derived in Ref. 13 from the superconducting properties. The solid square is the value obtained in Ref. [13, 20] by direct specific-heat measurements. The lower curves show the mean-free path ($l$) (open triangles) and the coherence lengths (${\xi }_0$, ${\xi }_{\mathrm{G}\mathrm{L}}$) as deduced assuming (1) BCS (solid squares) in the clean limit and (2) Ginzburg-Landau in the dirty limit (solid circles). The drastic decrease of $l$ with pressure (2 orders of magnitude) is directly related to the huge increase of resistivity at low temperature ($l\sim 1/\rho$ in the free electron approximation [17]). This implies an important scattering process that leads to an increase of magnetic penetration depth (${\lambda }_L$) and a decrease of superconducting coherence length (${\xi }_0$), and therefore to a possible change in the type of superconductivity [18].Reuse & Permissions