Quantum oscillation studies in the unconventional superconductor YFe₂Ge₂ and in the Dirac semimetal candidates NbXSb (X = Ge/Si)
We have performed experiments to probe the ground state dynamics of YFe₂Ge₂, NbGeSb and NbSiSb. An unusually large Sommerfeld coefficient (γ ~ 100 mJmol⁻¹K⁻²) has been observed in the d-electron system YFe₂Ge₂. It also shows an anomalous power law temperature dependence of the electrical resistivity (ρ = ρ0 + AT³⁄²) which indicates Fermi liquid breakdown, possibly connected to its vicinity to a quantum critical point. The materials NbXSb (X = Ge/Si) have been theoretically predicted via initial density functional theory (DFT) calculations to harbour Dirac/Weyl-like features in their electronic band structure. Quantum oscillations were measured in the range 5-18 T using the de Haas-van Alphen effect. The observed quantum oscillation frequencies have been compared to DFT calculations of the electronic structure. Several Fermi surface sheets predicted by DFT have been observed and their quasiparticle masses have been measured. The context of these measurements with regards to the electronic structure in YFe₂Ge₂ has been discussed. It was found that the quasiparticle effective mass enhancement of the detected Fermi surface sheets can account for only ~40% of the mass enhancement previously observed from specific heat measurements. Quantum oscillations were measured over the range 3-18 T using the Shubnikov-de Haas effect in NbXSb. Many frequencies have been observed in NbSiSb which also shows an extremely large magnetoresistance. We have considered the origin of parts of the quantum oscillation spectrum at high fields and briefly discussed this in relation to magnetic interactions and magnetic breakdown. One clear frequency has been seen in NbGeSb, which also shows an unusual negative magnetoresistance at low fields. Quantum oscillation frequencies in both materials have been compared to DFT calculations. The Berry phase has been estimated for the observed Fermi surface sheets in both materials, suggesting a topological nature in these materials.
Correlated electron systems, Topological semimetals
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