Quantum fluctuations can enhance the coupling between bosonic particles through suppression of an antiferromagnetic (AFM) order parameter in the vicinity of a quantum critical point (QCP). Such enhancement has been associated with many interesting quantum ground states including the unconventional superconductivity (SC). While quantum critical behaviors in AFM superconductors can be characterized by thermodynamic measurements, it often remains hidden under the SC dome. Inelastic neutron scattering (INS) can directly study quantum criticality via spin fluctuations and dynamic spin susceptibilities provide information regarding dynamic correlation length, the energy-temperature scaling, etc.
We previously studied a putative AFM QCP by performing systematic neutron diffraction measurements on Ba(Fe1−xCux)2As2 and inelastic neutron scattering measurements on non-superconducting Ba(Fe1−xCux)2As2 with x = 0.043. We found that the dynamic spin-spin correlation length increases rapidly, ξ ~ T−0.32(5) as T → 0, and we observed the ω/T scaling behavior, the hallmark of quantum criticality at the AFM QCP. The theory for magnetic quantum phase transitions for spin-density wave transitions in a 3D system predicts the scaling as ξ ~ T−3/4 and no ω/T scaling of spin fluctuation spectra at/near an AFM QCP. In contrast, in a 2D system, the theory predicts temperature- and energy-independent dynamic correlation length and ω/T scaling at/near an AFM QCP. Likewise, comparing the information with theoretical predictions in model systems will unveil the quantum criticality in AFM superconductors.
Our Research
Our project is studying spin fluctuations in two related systems with uniquely different AFM behaviors: a) the rare-earth oxypnictide superconductors where the interaction between 4f and 3d magnetism enhances SC; b) the ‘122’ pnictide superconductors with three transition metal elements that exhibit quantum criticality without SC. We will focus on understanding the quantum criticality and its relation to unconventional SC in the pnictide superconductors.
- M. G. Kim, W. Ratcliff, D. M. Pajerowski, J.-W. Kim, J.-Q. Yan, J. W. Lynn, A. I. Goldman, and A. Kreyssig, “Magnetic ordering and structural distortion in a PrFeAsO single crystal studied by neutron and x-ray scattering,” Phys. Rev. B 103, 174405 (2021).
- Min Gyu Kim et al., “Magnetic structure of Nd in NdFeAsO compound studied by x-ray resonant magnetic scattering,” Phys. Rev. B 100, 224401 (2019).
- M. Wang, M. Yi, H. L. Sun, P. Valdivia, M. G. Kim, Z. J. Xu, T. Berlijn, A. D. Christianson, Songxue Chi, M. Hashimoto, D. H. Lu, X. D. Li, E. Bourret-Courchesne, Pengcheng Dai, D. H. Lee, T. A. Maier, R. J. Birgeneau, “Experimental elucidation of the origin of the ‘double spin resonances’ in Ba(Fe1-xCox)2As2,” Phys. Revi. B 93, 205149 (2016).
- Min Gyu Kim et al., “Spin dynamics near a putative antiferromagnetic quantum critical point in Cu-substituted BaFe2As2 and its relation to high-temperature superconductivity,” Phys. Rev. B 92, 214404 (2015).
- Min Gyu Kim et al., “Magnonlike Dispersion of Spin Resonance in Ni-doped BaFe2As2,” Phys. Rev. Lett.110, 177002 (2013).
Related research
– M. G. Kim, T. W. Heitmann, S. R. Mulcahy, E. D. Bourret-Courchesne, R. J. Birgeneau, ” Structural and antiferromangetic properties of Ba(Fe1-x-yCoxRhy)2As2 compounds,” Phys. Rev. B 93, 094520 (2016).
– M. G. Kim, J. Lamsal, T. Heitmann, G. S. Tucker, D. K. Pratt, S. N. Khan, Y. B. Lee, A. Alam, A. Thaler, N. Ni, S. L. Bud’ko, K. J. Marty, M. D. Lumsden, P. C. Canfield, B. N. Harmon, D. Johnson, A. Kreyssig, R. J. McQueeney, and A. I. Goldman, “Effects of transition metal substitutions onthe incommensurability and spin fluctuations in BaFe2As2 by elastic and inelastic neutron scattering”, Phys. Rev. Lett. 109, 167003 (2012).