Fermi liquids (FLs) display a quadratic temperature (T) dependent resistivity. This can be caused by electron-electron (e-e) scattering in presence of inter-band or Umklapp scattering. However, dilute metallic SrTiO 3 was found to display T 2 resistivity in absence of either of the two mechanisms. The presence of soft phonons as possible scattering centers raised the suspicion that T 2 resistivity is not due to e-e scattering. Here, we present the case of Bi 2 O 2 Se, a layered semiconductor with hard phonons, which becomes a dilute metal with a small single-component Fermi surface upon doping. It displays T 2 resistivity well below the degeneracy temperature in absence of Umklapp and inter-band scattering. We observe a universal scaling between the T 2 resistivity prefactor (A) and the Fermi energy (E F), an extension of the Kadowaki-Woods plot to dilute metals. Our results imply the absence of a satisfactory understanding of the ubiquity of e-e T 2 resistivity in FLs.

Bismuth oxyselenide (Bi2O2Se) attracts great interest as a potential n-type complement to p-type thermoelectric oxides in practical applications. Previous investigations were generally focused on polycrystals. Here, we performed a study on the thermoelectric properties of Bi2O2Se single crystals. Our samples exhibit electron mobility as high as 250 cm2 V−1 s−1 and thermal conductivity as low as 2 W m−1 K−1 near room temperature. The maximized figure of merit is yielded to be 0.188 at 390 K, higher than that of polycrystals. Consequently, a rough estimation of the phonon mean free path (ℓph) from the kinetic model amounts to 12 Å at 390 K and follows a T−1 behavior. An extrapolation of ℓph to higher temperatures indicates that this system approaches the Ioffe–Regel limit at about 1100 K. In light of the phonon dispersions, we argue that the ultralow ℓph is attributed to intense anharmonic phonon–phonon scattering, including the Umklapp process and acoustic to optical phonon scattering. Our results suggest that single crystals provide a further improvement of thermoelectric performance of Bi2O2Se.

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