Manipulation of Gap Nodes by Uniaxial Strain in Iron-Based Superconductors

Abstract

In the iron pnictides and chalcogenides, multiple orbitals participate in the superconducting state, enabling different gap structures to be realized in distinct materials. Here we argue that the spectral weights of these orbitals can in principle be controlled by a tetragonal symmetry-breaking uniaxial strain, due to the enhanced nematic susceptibility of many iron-based superconductors. By investigating multi-orbital microscopic models in the presence of orbital order, we show that not only Tc can be enhanced, but pairs of accidental gap nodes can be annihilated and created in the Fermi surface by an increasing external strain. We explain our results as a mixture of nearly-degenerate superconducting states promoted by strain, and show that the annihilation and creation of nodes can be detected experimentally via anisotropic penetration depth measurements. Our results provide a promising framework to externally control the superconducting properties of iron-based materials.A distinguishing feature of iron pnictides and chalcogenides [1,2] [13][14][15]. This diversity of behaviors opens up the interesting possibility of manipulating the superconducting ground state by tuning the appropriate external parameters. While this can be achieved empirically by mixing different types of doping, such as Ba(Fe 1−x Co x ) 2 (As 1−y P y ) 2 [16], control of the SC state requires understanding the mechanisms responsible for this non-universality of the gap structure.Theoretically, spin fluctuations have been widely proposed to cause pairing in iron pnictides and chalcogenides [17]. In this model, the non-universal behavior of the gap structure stems from the multi-orbital character of these materials that arises due to the 3d 6 configuration of Fe [18]. In fact, first-principle calculations [22] and ARPES experiments [19,20] reveal that the disconnected pockets that form the Fermi surface of most pnictides contain significant spectral weight from the d xz , d yz , and d xy orbitals (see Fig. 1(a) [33,34], is a distinguishing feature of the iron-based materials, since in most superconductors one SC state usually has a much lower energy than all the other ones.Therefore, in this framework, the properties of the SC state of the iron pnictides could be manipulated if the orbital content of their Fermi surface could be tuned. In this paper, we propose that this can be achieved via application of uniaxial strain ∂ i u i , where u denotes the displacement vector. Experimentally, many optimallydoped iron-based superconductors display a large nematic susceptibility χ nem [35][36][37][38][39][40][41][42][43], implying that even a small uniaxial strain [44][45][46][47] can trigger a nematic state with sizable anisotropies in the lattice and, more interestingly, in the magnetic and orbital degrees of freedom [48]. While previous works investigated how superconductivity is affected by the nematic-induced anisotropy in the magnetic spectrum [49], little is known about the impact of the induced anisotropy in the electronic spectrum. Indeed,...

Structure of the pairing gap from orbital nematic fluctuations

“…3(b). It is interesting that the enhancement of the gap modulation in the nematic phase is also obtained in the spin fluctuation mechanism [38]. On the other hand, FS3 and FS4 consist of essentially a single orbital component and thus the modulation of k F remains very weak.…”

“…An important future issue is to clarify the condition of which mechanism, spin fluctuations or orbital nematic fluctuations, is dominant over the other or whether both mechanisms should be considered on an equal footing in general. Although these two mechanisms rely on different physics, interestingly they share some aspects of SC: (i) the pairing gap with s-wave symmetry [4][5][6], (ii) the presence of a d x 2 −y 2 -wave solution nearly degenerate to the leading instability in the tetragonal phase [5,31], (iii) the weak modulation of the pairing gap on the hole FSs in the tetragonal phase [34][35][36], and (iv) its enhancement in the nematic phase [38]. The positions of gap minima/maxima obtained in the spin fluctuation mechanism [38], however, vary from the present work.…”

Probing the pairing symmetry in the over-doped Fe-based superconductorBa0.35Rb0.65Fe2As2as a function of hydrostatic pressure

“…Recently, we have shown unambiguous evidence for the appearance of SC nodes in optimally-doped Ba 1−x Rb x Fe 2 As 2 upon applied pressure, consistent with a change from a nodeless s +− -wave state to a d-wave state 39 . Interestingly, the theoretical calculations [41][42][43][44][45][46][47] as well as Raman experiments 48,49 revealed a sub-dominant d-wave state close in energy to the dominant s +− state. It seems that pressure affects this intricate balance, and tip the balance in favor of the d-wave state.…”

Enhanced Manifold of States Achieved in Heterostructures of Iron Selenide and Boron-Doped Graphene

“…Enforcing the superstructure renders the softer system exposed to a stress-strain field owing to boundary conditions set by the stiffer subsystem. The impact of stress on multiorbital superconductivity in general and in the iron based superconductors in particular has been discussed previously [24]. Rather than being detrimental to the superconductivity it was shown how the phenomenon could even be even promoted by stress.…”