Quantum-critical scale invariance in a transition metal alloy

Non-Fermi liquid (NFL) behavior is a common feature in iron-based high-temperature superconductors, often linked to quantum critical points (QCPs). Understanding the role of quantum-critical fluctuations in shaping anomalous finite-temperature properties is challenging due to the presence of high-temperature superconductivity. This study focuses on Ba(Fe1/3Co1/3Ni1/3)2As2, a non-superconducting iron pnictide, to investigate the influence of quantum criticality without the complexities introduced by superconductivity. The researchers hypothesized that this material, due to its counter-doping, would exhibit a unique quantum critical ground state characterized by scale invariance in its thermodynamic and transport properties. Understanding these phenomena is crucial for advancing our knowledge of strongly correlated electron systems and potentially revealing new insights into unconventional superconductivity mechanisms. The study's importance lies in its potential to clarify the interplay between quantum critical fluctuations and unconventional metallic behavior in the absence of superconducting effects, offering a simpler model system for theoretical investigation.

Previous research on iron-based superconductors has established a strong link between NFL behavior and the proximity to antiferromagnetic (AFM) or structural phase transitions. The "strange metal" behavior, often characterized by linear-in-T resistivity, is observed near QCPs. However, the presence of superconductivity and nematic phases in many iron pnictides complicates the study of the underlying quantum critical behavior. Related materials like cobalt-based oxypnictides and Co-based intermetallic arsenides offer insights into competing magnetic interactions (ferromagnetic, FM, and AFM), influencing the electronic properties. The authors highlight the subtle yet significant effect of substituting Fe, Co, and Ni in BaCo2As2, modifying the electronic structure and ground state. This counter-doping approach serves as a unique strategy to explore nearly FM systems while disrupting specific spin correlations.

Single crystals of Ba(Fe1/3Co1/3Ni1/3)2As2 were grown using a self-flux method. Magnetotransport measurements were conducted using a 3He-4He dilution refrigerator and a high magnetic field facility. Heat capacity measurements employed the thermal relaxation method. Magnetic susceptibility was measured using vibrating sample magnetometry and SQUID techniques. Pressure-dependent transport measurements utilized a nonmagnetic piston-cylinder pressure cell. Angle-resolved photoemission spectroscopy (ARPES) was performed at the BESSY II synchrotron radiation center to study the electronic structure. Detailed analysis of the data involved extracting scattering rates, carrier densities, and effective masses. Fitting procedures were used to determine various parameters and to test scaling relations. The authors performed careful analysis to rule out alternative explanations for the observed phenomena such as Mooij correlations and quantum interference.

The resistivity of Ba(Fe1/3Co1/3Ni1/3)2As2 exhibits a quasi-linear T dependence over a wide temperature range, indicative of NFL behavior. This behavior is strongly suppressed by magnetic fields, leading to a recovery of Fermi liquid (FL) behavior at low temperatures. Magnetoresistance (MR) shows sublinear field dependence. The inelastic scattering rate displays a universal scaling relation involving temperature and magnetic field, suggesting a connection to Planckian dissipation. Notably, the magnetotransport is isotropic, independent of the applied field orientation with respect to the crystallographic axes. Thermodynamic properties (magnetic susceptibility and electronic specific heat) also exhibit NFL behavior, showing power-law divergences that are suppressed by magnetic fields. The magnetic susceptibility varies as χ ∝ T−1/3, and the electronic specific heat coefficient C_el/T shows a power-law divergence C_el/T ~ T−0.25. Temperature-field scaling relations are observed in both magnetization and specific heat, consistent with a QCP at zero temperature and field. The Hall resistivity shows a sign change at low temperatures and fields, indicating the dominance of hole-like carriers near the QCP. ARPES measurements revealed a unique electronic structure, with a large hole-like pocket and electron-like pockets. The scattering rate obtained from ARPES measurements varies linearly with kinetic energy, consistent with Planckian dissipation. The observed NFL behavior is robust against pressure and replacement of Ba with Sr. The measured critical exponents are unusual and do not match predictions from existing theories of quantum criticality.

The observed scale invariance in transport and thermodynamic properties, along with the isotropic MR and the dominance of hole-like carriers near the QCP, strongly support the presence of a unique quantum critical point in Ba(Fe1/3Co1/3Ni1/3)2As2. The unusual critical exponents obtained do not conform to predictions from standard theories of either clean or dirty ferromagnetic QCPs. The possibility of a quantum Griffiths phase was considered, but the inconsistencies in the extracted critical exponents rule out this explanation. The robustness of the NFL behavior against pressure and chemical substitution points to a critical behavior that is not directly tied to specific magnetic interactions or lattice parameters. Instead, it emphasizes the crucial role of the equal ratios of Fe:Co:Ni in stabilizing this unique quantum critical ground state. This behavior contrasts with that of related compounds lacking this precise stoichiometry. The pressure insensitivity suggests the quantum criticality is an intrinsic property of the electronic structure, not strongly influenced by lattice effects.

This study reveals a unique type of quantum criticality in the non-superconducting iron pnictide Ba(Fe1/3Co1/3Ni1/3)2As2, characterized by scale invariance in transport and thermodynamic properties. The observed behavior differs significantly from known theoretical models of quantum criticality, highlighting the need for further theoretical investigation. The robustness of the quantum criticality against pressure and substitution suggests a fundamental role for the electronic structure in driving this unusual behavior. Future work might explore the microscopic mechanisms underlying this unique quantum critical state and investigate whether this system can provide insights into the broader physics of strongly correlated electron systems and possibly unconventional superconductivity.

While the study provides compelling evidence for a unique quantum critical point, it does not fully elucidate the microscopic origin of the unusual scaling behavior and the unusual critical exponents obtained. The theoretical understanding of this system requires more advanced theoretical work. The precise role of disorder introduced by the counter-doping, despite the analysis ruling out certain effects, might still contribute subtly to the observed behavior. Further investigation into related materials with slight variations in composition or structural parameters could further clarify this point.