Structural Quantum Criticality and Superconductivity in Iron-Based Superconductor Ba(Fe_1-xCo_x)₂As₂

Abstract

We investigated the elastic properties of the iron-based superconductor Ba(Fe 1Àx Co x ) 2 As 2 with eight Co concentrations. The elastic constant C 66 shows a large elastic softening associated with structural phase transition. C 66 was analyzed on the basis of the localized and itinerant pictures of Fe-3d electrons, which shows a strong electronlattice coupling and a possible mass enhancement in this system. The results are similar to those of unconventional superconductors, where the properties of the system are governed by quantum fluctuations associated with the zerotemperature critical point of long-range order, namely, the quantum critical point (QCP). In this system, the inverse of C 66 behaves just like the magnetic susceptibility in magnetic QCP systems. Although the QCPs of these existing superconductors are all ascribed to antiferromagnetism, our systematic studies on the canonical iron-based superconductor Ba(Fe 1Àx Co x ) 2 As 2 have revealed that there is a signature of ''structural quantum criticality'' in this material, which thus far has had no precedent. The elastic constant anomaly is suggested to concern with the emergence of superconductivity. These results highlight the strong electron-lattice coupling and effect of the band in this system, thus challenging the prevailing scenarios that focus on the role of iron 3d orbitals.

Effects of c/a Anisotropy and Local Crystal Structure on Superconductivity in AFe2(As1−xPx)2 (A = Ba1−ySry, Sr1−yCay and Eu)

“…Because the magnetic order, orbital order, and nematicity are intertwined with each other in iron-based superconductors, 21) it is possible that orbital order or nematicity is suppressed by structural change (not through the change of FS nesting condition) and causes the suppression of magnetic order. [22][23][24] However, there has been no report on the relationship between structural anisotropy and orbital order or nematicity, while our results are consistent with the nesting scenario and the resluts of ARPES study. 15) Therefore, in this work, we consider that the rapid change of nesting condition is the main cause of the rapid suppression of magnetic order.…”

Two-dome structure in electron-doped iron arsenide superconductors

“…Given these results, the band degeneracy appears to have an important role in emergence of the second dome. For the iron-based superconductors, there is another pairing model derived from a large softening of a shear modulus observed near the tetragonal-orthorhombic transition of parent compounds [17][18][19] . This model tells that the Fe-d orbitals are possible to order when their degeneracy in Fe-d yz,zx orbitals is removed at the structural transition and the fluctuations of this orbital ordering are shown capable of inducing superconductivity 18,19 .…”

Fermi Surface, Pressure-Induced Antiferromagnetic Order, and Superconductivity in FeSe

“…These results clearly indicate the existence of ferro-orbital fluctuation and electronic ferro-orbital order, as is recognized in other ironbased superconductors, both theoretically 8,9,12,13) and experimentally. [21][22][23] The most remarkable feature of FeSe is observed in the temperature dependence of the spin-lattice relaxation rate 1/T 1 T in NMR experiments. 24,25) With decreasing temperature, 1/T 1 T decreases and reaches a minimum at T ∼ T s .…”