NMR Study of the Mott Transitions to Superconductivity in the Two ${\mathrm{Cs}}_3{\mathbf{C}}_{60}$ Phases

We report a NMR and magnetometry study on the expanded intercalated fulleride ${\mathrm{Cs}}_3{\mathrm{C}}_{60}$ in both its $A15$ and face centered cubic structures. NMR allowed us to evidence that both exhibit a first-order Mott transition to a superconducting state, occurring at distinct critical pressures $p_c$ and temperatures $T_c$. Though the ground state magnetism of the Mott phases differs, their high $T$ paramagnetic and superconducting properties are found similar, and the phase diagrams versus unit volume per ${\mathrm{C}}_{60}$ are superimposed. Thus, as expected for a strongly correlated system, the interball distance is the relevant parameter driving the electronic behavior and quantum transitions of these systems.

Figure 1

(a) The $A15$ ${}^{133}\mathrm{Cs}$ NMR spectrum taken at ambient pressure displays a seven-peak powder NMR spectrum (arrows), with a quadrupole splitting of $\sim 39\mathrm{kHz}$ at 100 K, and a slight asymmetric spectral shape originating from a Knight shift anisotropy. (b) ${}^{133}\mathrm{Cs}$ NMR spectra for the fcc phase displaying the octahedral $O$ and tetrahedral $T$ and $T^{\prime }$ site peaks. (c),(d) The spectra are found nearly unmodified at 7.6 kbar when both phases are metallic (see text).

Figure 2

Ambient pressure data taken in samples $A1$ and $F1$ for the $A15$ and fcc phases. (a) Similar variations above 100 K of the anisotropic shift contribution ${}^{13}K^{\mathrm{ax}}$ to the ${}^{13}\mathrm{C}$ spectra (displayed in the inset). The scale chosen on the right for $\chi$ (black dots, SQUID data on $A1$) emphasizes the linear relation with ${}^{13}K^{\mathrm{ax}}$. (b) ${}^{133}\mathrm{Cs}$ $(T_{1}T{)}^{-1}$ data for the fcc phase $T$ site and the $A15$ phase have similar high $T$ variations but differ markedly near and below their ordered magnetic states. The spin freezing at $T_f$ in the fcc phase is monitored in the inset at the onset of decrease of the ${}^{13}\mathrm{C}$ signal intensity (see text). Open symbols are $(T_{1}T{)}^{-1}$ data for $T$.

Figure 3

SC diamagnetism is detected at a lower $p_c$ for the sample $F1$ than for $A2$ (nearly pure $A15$). The arrows point out the values of $T_c(p)$ deduced by extrapolation of the sharp $T$ dependent diamagnetism (slightly smaller than the onset). The diamagnetic magnetization at 10 K (extrapolated for some $p$ values) is displayed in the inset for both samples, together with the AFM volume fraction for the $A15$ phase (see 14).

Figure 4

Phase diagram reporting the critical temperatures (a) versus $p$ and (b) versus $V_{{\mathrm{C}}_{60}}$. The pressures $p_c$ of the Mott transitions shown as dotted lines in (a) merge into the gray area in (b). There we reported for comparison the former data on $A15$ phase [10] as a dotted line, and on the other fcc $A_3{\mathrm{C}}_{60}$ as open downward triangles [21].

Figure 5

$T$ variation above $p_c$ of $(T_{1}T{)}^{-1}$ for ${}^{133}\mathrm{Cs}$ and ${}^{13}\mathrm{C}$ (filled symbols) and the ${}^{133}\mathrm{Cs}$ shift $K$ (open diamonds) in the $A15$ phase. The full and dotted lines point out the $T_c$ values in the applied field for the two phases.