Quantum Critical Behavior in the Asymptotic Limit of High Disorder

The NiCoCr_x alloys (with x near 1) are related to the recently discovered high-entropy-alloys, such as NiCoCrFeMn, where configurational entropy stabilizes a random distribution of elements on a face-centered cubic lattice. These alloys have been shown to be chemically homogeneous from the centimeter to the nanometer scale. Here we demonstrate that NiCoCr_x single crystal alloys are remarkable model systems for investigating QCP physics in a metallic environment with a high degree of chemical disorder. For NiCoCr_x alloys with x = 0.8, the critical exponents associated with a ferromagnetic quantum critical point (FQCP) are experimentally determined from low temperature magnetization and heat capacity measurements. All of the five exponents are in remarkable agreement with predictions of the Belitz-Kirkpatrick-Vojta (BKV) theory in the asymptotic limit of high disorder, and are quite different from Hertz mean field values. Using these critical exponents, excellent scaling of the magnetization data is demonstrated with no adjustable parameters. These results clearly demonstrate that disorder has a fundamental and profound effect on quantum critical behavior near an itinerant ferromagnetic QCP. This work also shows that entropy-stabilized alloys represent a unique platform to study quantum critical behavior in a highly tunable class of materials. Research supported by the DOE Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.