Discovery of superconductivity and electron-phonon drag in the non-centrosymmetric Weyl semimetal LaRhGe₃

We present an exploration of the effect of electron-phonon coupling and broken inversion symmetry on the electronic and thermal properties of the semimetal LaRhGe₃. Our transport measurements reveal evidence for electron-hole compensation at low temperatures, resulting in a large magnetoresistance of 3000% at 1.8 K and 14 T. The carrier concentration is on the order of 10²¹/cm³ with high carrier mobilities of 2000 cm²/Vs. When coupled to our theoretical demonstration of symmetry-protected almost movable Weyl nodal lines, we conclude that LaRhGe₃ supports a Weyl semimetallic state. We discover superconductivity in this compound with a T_c of 0.39(1) K and B_c(0) of 2.2(1) mT, with evidence from specific heat and transverse-field muon spin relaxation. We find an exponential dependence in the normal state electrical resistivity below  ~50 K, while Seebeck coefficient and thermal conductivity measurements each reveal a prominent peak at low temperatures, indicative of strong electron-phonon interactions. To this end, we examine the temperature-dependent Raman spectra of LaRhGe₃ and find that the lifetime of the lowest energy A₁ phonon is dominated by phonon-electron scattering instead of anharmonic decay. We conclude that LaRhGe₃ has strong electron-phonon coupling in the normal state, while the superconductivity emerges from weak electron-phonon coupling. These results open up the investigation of electron-phonon interactions in the normal state of superconducting non-centrosymmetric Weyl semimetals.