Observation of Ferro-Quadrupole Quantum Criticality in Iron Pnictide Superconductor Ba(Fe1−xCox)2As2

• Published in: Journal of the Physical Society of Japan Vol. 92; no. 12; p. 1
• Main Authors: Sato, Haruyasu, Akatsu, Mitsuhiro, Kurihara, Ryosuke, Kobayashi, Yoshiaki, Nemoto, Yuichi
• Format: Journal Article
• Language: English
• Published: Tokyo The Physical Society of Japan 15-12-2023
• Subjects: Angular momentum; Attenuation coefficients; Broken symmetry; Critical point; Group 5A compounds; Iron; Jahn-Teller effect; Order parameters; Phase transitions; Quadrupole interaction; Quadrupoles; Relaxation time; Superconductivity; Temperature dependence; Transformations (mathematics); Transition temperature; Ultrasonic attenuation
• ISSN: 0031-9015; 1347-4073
• DOI: 10.7566/JPSJ.92.124710

We have performed ultrasonic experiments on the iron pnictide superconductor Ba(Fe1−xCox)2As2 for x = 0.056 and 0.057 to investigate the criticality originating from degenerate orbitals at the ferro-quadrupole (FQ) quantum critical point (QCP). At this point, the FQ ordering that causes a structural phase transition disappears, and the superconducting transition temperature Tsc reaches its maximum. From analyzing the temperature dependence of the elastic soft mode C66, we found that the Jahn–Teller energy ΔQ and Weiss temperature ΘQ near the QCP are larger than those of samples away from the QCP. This indicates a significant increase in the quadrupole-strain interaction HQS, and in the quadrupole interaction HQQ obtained from the canonical transformation of HQS. Additionally, the ultrasonic attenuation coefficient α66 of the hexadecapole susceptibility for x = 0.056 diverges toward the superconducting transition temperature Tsc = 24.8 K with decreasing temperature. We discovered that the temperature dependence of the relaxation time τ obtained from α66 was fitted by τ ∝ |T − Tc|−zν with the unconventional critical exponent zν = 3 near the QCP. This result is significantly different from the critical exponent zν = 1 in the mean-field approximation observed in x = 0.071. These observations of ΔQ, ΘQ, and zν suggest that the quantum fluctuations originating from a non-Kramers doublet with two quadrupoles Ox′y′ and Ox′2y′2 and one orbital angular momentum lz can be enhanced near the QCP, where Tsc is at a maximum. The attractive interaction mediated by the order parameter fluctuations that restore the symmetry breaking of the structural phase transition probably plays a key role in the superconductivity.