Logarithmic Fermi-liquid breakdown in [formula omitted

Abstract

The d-electron low-temperature magnet NbFe 2 can be tuned to the threshold of magnetism by varying pressure, magnetic field or composition. In particular, it is possible to select between ferromagnetism, long-wavelength spin-density-wave (SDW) (helical) order or a paramagnetic low-temperature state by adjusting the precise stoichiometry within the Nb 1 - y Fe 2 + y homogeneity range. Preparing samples close to the composition where magnetic order vanishes—the quantum critical point—enables the first detailed investigation of the thermodynamic and transport signatures of Fermi liquid breakdown in a transition metal compound. We report measurements of the heat capacity C and of the resistivity ρ in stoichiometric and slightly Nb-rich NbFe 2 samples, including a single crystal with the composition Nb 1.02 Fe 1.98 , which on the phase diagram is located very close to the quantum critical point ( T N ≃ 2.8 K ) . Both the resistivity and the heat capacity of the nearly quantum-critical single crystal display striking, robust non-Fermi liquid temperature dependences: while the heat capacity coefficient γ = C / T diverges weakly as C / T ∼ log T from 4 K down to 0.1 K, in line with theoretical predictions for 3-D ferromagnetic quantum criticality, the resistivity follows a T 3 / 2 power-law, familiar from the case of MnSi and naively predicted for the proximity of an antiferromagnetic quantum critical point.