Departure from the Wiedemann-Franz Law in WP2 Driven by Mismatch in T-square Resistivity Prefactors

We report on a study of the electrical, \sigma, and thermal, \kappa, conductivities in bulk WP2 single crystals. The Wiedemann-Franz law was found to hold at 2 K. A downward deviation rapidly emerges upon warming and by 13 K, the thermal-to-electrical-conductivity (Lorenz) ratio becomes one-fourth of the Sommerfeld number. This is an exceptionally large mismatch compared to what was reported in other metals. Both electrical and thermal resistivities display a T^2 term arising from electron-electron scattering. Phonon scattering generates a detectable T^5 (T^3) term in electrical (thermal) resistivity. We found that the large mismatch between the amplitude of the two T-square prefactors drives the Lorenz ratio downward. This identifies electron-electron scattering as the origin of the finite-temperature violation of the Wiedemann-Franz law. We argue that this may be caused either by abundant small-angle-momentum-relaxing scattering or by momentum-conserving scattering. The latter solely generates thermal resistivity as in the case of normal-state liquid He3.

Alexandre Jaoui, Benoît Fauqué, Carl Willem Rischau, Alaska Subedi, Chenguang Fu, Johannes Gooth, Nitesh Kumar, Vicky Süß, Dmitrii L. Maslov, Claudia Felser, Kamran Behnia

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