Quantum critical behavior in the asymptotic limit of high disorder in the medium entropy alloy NiCoCr_0.8

The behavior of matter near a quantum critical point is one of the most exciting and challenging areas of physics research. Emergent phenomena such as high-temperature superconductivity are linked to the proximity to a quantum critical point. Although significant progress has been made in understanding quantum critical behavior in some low dimensional magnetic insulators, the situation in metallic systems is much less clear. Here, we demonstrate that NiCoCr_x single crystal alloys are remarkable model systems for investigating quantum critical point physics in a metallic environment. For NiCoCr_x alloys with x ≈ 0.8, the critical exponents associated with a ferromagnetic quantum critical point are experimentally determined from low temperature magnetization and heat capacity measurements. All of the five exponents (γ _T ≈ 1/2, β _T ≈ 1, δ ≈ 3/2, νz_m ≈ 2, α α _T ≈ 0) are in remarkable agreement with predictions of Belitz-Kirkpatrick-Vojta theory in the asymptotic limit of high disorder. Using these critical exponents, excellent scaling of the magnetization data is demonstrated with no adjustable parameters. We also find a divergence of the magnetic Gruneisen parameter, consistent with a ferromagnetic quantum critical point. This work therefore demonstrates that entropy stabilized concentrated solid solutions represent a unique platform to study quantum critical behavior in a highly tunable class of materials.