Nearly Free Electrons in the Layered Oxide Superconductor ${\mathrm{Ag}}_5{\mathrm{Pb}}_2{\mathrm{O}}_6$

We present first measurements of quantum oscillations in the layered oxide superconductor ${\mathrm{Ag}}_5{\mathrm{Pb}}_2{\mathrm{O}}_6$. From a detailed angular and temperature dependent study of the de Haas–van Alphen effect we determine the electronic structure and demonstrate that the electron masses are very light, $m^{*}\sim 1.2m_e$. The Fermi surface we observe is essentially that expected of nearly free electrons—establishing ${\mathrm{Ag}}_5{\mathrm{Pb}}_2{\mathrm{O}}_6$ as the first known example of a monovalent, nearly free electron superconductor at ambient pressure.

Figure 1

(a) Measured dHvA frequencies in ${\mathrm{Ag}}_5{\mathrm{Pb}}_2{\mathrm{O}}_6$ as the angle between the magnetic field and $c$ axis is varied. The data for the rotation study was taken at $T\sim 20\mathrm{mK}$ and shows two branches labeled $\alpha$ and $\beta$. The dashed lines represent dHvA frequencies based on the band structure calculations of Oguchi [20] which predict nearly free electron behavior. (b) The dependence of the amplitude of the $\beta$ branch at $\theta =0°$ on temperature. The line through the data is a fit to the standard Lifshitz-Kosevich formula, yielding $m^{*}=1.25\pm 0.10m_e$. The $\alpha$ branch gives $m^{*}=1.1\pm 0.2m_e$ for the same orientation (not shown). (c) A sample dHvA spectra taken at $T\sim 20\mathrm{mK}$ and $\theta \sim 20°$.

Figure 2

(a) The FS of ${\mathrm{Ag}}_5{\mathrm{Pb}}_2{\mathrm{O}}_6$ as measured by dHvA, shown in the periodic zone scheme, with the translucent box corresponding to the first Brillouin zone of the hexagonal lattice. (b) The FS predicted by the band structure calculations of Oguchi [20]. (c) The proposed FS of Brennan and Burdett [18].

Figure 3

Main panel: The evolution of the amplitude of dHvA oscillations as a function of polar angle $\theta$. The vertical axis is the Fourier amplitude of the large frequency oscillation, in arbitrary units. Insets: Examples of dHvA field sweeps on ${\mathrm{Ag}}_5{\mathrm{Pb}}_2{\mathrm{O}}_6$, taken for angles $\theta =27°$ and 43°. The vertical axis is the pickup signal (in arbitrary units, same scale for both sweeps) at the second harmonic of the excitation frequency. A large enhancement in the oscillation amplitude is seen at the Yamaji angle.