Emergent Magnetic Degeneracy in Iron Pnictides due to the Interplay between Spin-Orbit Coupling and Quantum Fluctuations

Abstract

Recent experiments in iron pnictide superconductors reveal that, as the putative magnetic quantum critical point is approached, different types of magnetic order coexist over a narrow region of the phase diagram. Although these magnetic configurations share the same wave vectors, they break distinct symmetries of the lattice. Importantly, the highest superconducting transition temperature takes place close to this proliferation of near-degenerate magnetic states. In this Letter, we employ a renormalization group calculation to show that such a behavior naturally arises due to the effects of spin-orbit coupling on the quantum magnetic fluctuations. Formally, the enhanced magnetic degeneracy near the quantum critical point is manifested as a stable Gaussian fixed point with a large basin of attraction. Implications of our findings to the superconductivity of the iron pnictides are also discussed. Disciplines Condensed Matter Physics | Metallurgy | Physics CommentsThis article is published as Christensen, Morten H., Peter P. Orth, Brian M. Andersen, and Rafael M. Fernandes. "Emergent magnetic degeneracy in iron pnictides due to the interplay between spin-orbit coupling and quantum fluctuations." Physical Review Letters 121, no. 5 (2018) Recent experiments in iron pnictide superconductors reveal that, as the putative magnetic quantum critical point is approached, different types of magnetic order coexist over a narrow region of the phase diagram. Although these magnetic configurations share the same wave vectors, they break distinct symmetries of the lattice. Importantly, the highest superconducting transition temperature takes place close to this proliferation of near-degenerate magnetic states. In this Letter, we employ a renormalization group calculation to show that such a behavior naturally arises due to the effects of spin-orbit coupling on the quantum magnetic fluctuations. Formally, the enhanced magnetic degeneracy near the quantum critical point is manifested as a stable Gaussian fixed point with a large basin of attraction. Implications of our findings to the superconductivity of the iron pnictides are also discussed.

Magnetic phase diagram of the iron pnictides in the presence of spin-orbit coupling: Frustration between C2 and C4 magnetic phases

“…Furthermore, we present the full numerical solutions of the RG flow equations, which are in agreement with the analytical treatment presented in Ref. 49.…”

“…Here we review our results presented in Ref. 49 Let us commence with the caser x,1 r y,1 ,r z,1 , corresponding to α 1 α 2 , α 3 . The bare free energy in this case is given in Eq.…”

“…In Ref. 49 we presented the main result of this treatment: the emergence of magnetic degeneracy for a wide range of initial bare parameters near the putative QCP. Here we provide further details for the derivation of the RG flow equations in the presence of spin anisotropy.…”

Muon spin rotation and infrared spectroscopy study of Ba1−xNaxFe2As2

“…Note that such a magnetic degeneracy due to the effect of spin orbit interaction on quantum fluctuations has been predicted in Ref. [44].…”

Pressure-Induced Large Volume Collapse, Plane-to-Chain, Insulator to Metal Transition in CaMn2Bi2

“…Initially it was believed that quantum fluctuations can only be appreciable in magnetic materials where magnetic frustration is present, [7][8][9][10][11] however, it is now realized that strong spin-orbit coupling (SOC) can also produce the energies needed for quantum fluctuations. [12][13][14] As the heaviest non-radioactive element, bismuth, in main group V, is located at the Zintl border in the periodic table. [15][16][17][18][19][20] Bi has moderate electronegativity (and can commonly act as either an anion or a cation) and strong spin-orbit coupling.…”