Session S10: Fe-based Superconductors -- Nematicity II

Interplay between nematicity and superconductivity in iron-based superconductors

The iron-based high-Tc superconductors exhibit several remarkable features, including the multi-orbital character and the ubiquity of the nematic phase. One consequence of the multi-orbital Fermi surface is that the spin-fluctuation mediated pairing interactions are sensitive to the orbital spectral weight at the Fermi surface, leading to several different possible gap structures, such as nodeless s±, nodal s±, and d-wave. Focused on the orbital order induced in the nematic phase, I will discuss how the nematic order can manipulate the properties of SC. Our calculation shows that not only T_c is enhanced, but more importantly, the gap structure becomes a mixture of nearly degenerate s and d-wave states by increasing the external strain. This mixture of s and d wave pairing channels has been recently found in the superconducting phase of the bulk FeSe, when SC occurs deeply insider the nematic phase.   

Probing nematic fluctuations in iron-based superconductors with pair distribution function analysis

The origin and implications of nematicity in iron-based superconductors remain among the most pressing questions in the field. Recent efforts to address this topic have focused not only on the nematic phase itself, but also on the nematic fluctuations that exist outside the region of static nematicity. Pair distribution function (PDF) analysis of x-ray and neutron total scattering data is a proven method of studying local, short-range structural correlations that deviate from the average structure, such as the orthorhombic distortions associated with nematic fluctuations in the high-temperature tetragonal phase of iron-based superconductors. Focusing primarily on the representative hole-doped system (Sr,Na)Fe₂As₂, we present PDF analysis that reveals a remarkably large region of nanometer-scale local orthorhombic distortions in temperature-composition space, reaching up to approximately 500 K for the parent compound and extending to doping levels beyond the C₂ dome near optimal superconductivity. These results offer a rich and detailed view of nematic fluctuations in iron-based superconductors and should be helpful in guiding future experimental and theoretical work.   

Spin correlations and B2g nematicity in extremely hole doped iron pnictides

There is growing experimental evidence for B_2g nematic correlations in the extremely hole doped iron pnictide compounds AFe₂As₂ (A=K,Rb,Cs). In this work, we construct a general Ginzburg-Landau theory for the nematic transitions driven by incommensurate spin correlations. We find various nematic orders depending on the nature of the incommensurate spin correlations, and demonstrate a unified description of the nematicity in both the parent and doped iron pnictides. For the extremely hole doped iron pnictides, our analysis provides a mechanism for a B_2g nematic order developing from the pertinent incommensurate spin fluctuations. We further study an extended bilinear-biquadratic Heisenberg model, which provides additional evidence for the proposed mechanism.   

Doping-dependent nematic susceptibility and strain-tunable Tc in FeTe1-xSex

In comparison to the end member FeSe, the role of nematicity in the FeTe_1-xSe_x family is less explored. Using the elastoresistivity technique, we measured the nematic susceptibility of FeTe_1-xSe_x over a wide range of doping, and revealed the presence of strong B_2g nematic fluctuations despite the absence of static nematic order. We also studied the effects of anisotropic strain on the superconducting transition temperature, providing insight into the interplay between nematic fluctuations and the superconductivity.   

Anisotropy in Superconducting CsCa2Fe4As4F2 Single Crystals

CsCa₂Fe₄As₄F₂ is a typical example of 12442-type iron-based superconductors. The novel material superconducts at 29.5 K without extrinsic chemical doping. It contains double Fe₂As₂ layers that are separated by insulating Ca₂F₂ layers. The structural feature may lead to the weak interlayer coupling and strong anisotropy. In this work we successfully synthesized high quality CsCa₂Fe₄As₄F₂ single crystals with CsAs flux. Measurements of X-ray diffraction, anisotropic magnetotransport and magnetization were carried out to characterize the single crystals. The anisotropic superconducting upper H_c2 (T), lower critical field H_c1 (T), the anisotropy parameter γ (T) and other related parameters were obtained. The results indicate that CsCa₂Fe₄As₄F₂ is one of most anisotropic iron-based superconductors.   

Unusual nematic behavior of heavily hole-doped AFe2As2

We investigate via density functional theory calculations the recently reported unusual nematic behavior of heavily hole-doped pnictides AFe₂As₂ (A = Rb, Cs) [1,2]. In contrast to the B_2g nematic order of the parent 122 compounds, characterized by unequal Fe-Fe bonds, in the former systems nematic order is observed in the B_1g channel, characterized by unequal Fe-As-Fe bonds. We attribute this behavior to the evolution of the magnetic ground state along the compound series Sr_1-xRb_xFe₂As₂, from single stripes for small x to double stripes for large x. Our simulations using the reduced Stoner theory show that fluctuations of Fe moments are essential for the stability of the double-stripe configuration. We discuss the relationship between these magnetic orderings and the nature of nematicity in terms of vestigial orders [3].

References
1. X. Liu et al., arXiv:1803.07304 (2018).
2. S. Onari and H. Kontani, arXiv:1809.08017 (2018).
3. V. Borisov et al. In preparation.   

Nematic fluctuations at nonzero q in FeSe vs. Ba(Fe1-xCox)2As2

FeSe and Ba(Fe1-xCox)2As2 (Ba122) have several shared phase transitions such as the tetragonal-to-orthorhombic structural transition associated with nematic electronic order as well as a superconducting transition. There are also important differences, including magnetic order found in the Ba122 but not the FeSe compounds. Our recent inelastic X-ray and neutron scattering measurements of TA phonons in these compounds focused on anomalies in their dispersion. These anomalies reflect nematic fluctuations at nonzero q. We obtained nematic correlation length as a function of temperature from fits to these anomalous dispersions and found that it is similar in FeSe and Ba(Fe0.96Co0.04)2As2: compounds whose structural transition temperatures are close. In addition, we measured TA phonon dispersions and linewidths in optimally-doped Ba122 in detail. I will discuss the implications of the observed phenomena, focusing on the relation between nematic correlation length and the structural transitions, magnetic order, and superconductivity.   

Nematic fluctuations in Ba(Fe1-xTMx)2As2 (TM = Cr, Mn, V and Cu): Why superconductivity cannot be achieved by these dopants

We have systematically studied nematic susceptibilities in nonsuperconducting Ba(Fe_1-xTM_x)₂As₂ (TM = Cr, Mn, V and Cu) single crystals by measuring the uniaxial pressure dependence of the resistivity along the Fe-As-Fe direction. The nematic susceptibilities in all samples show the Curie-Weiss-like behavior at high temperatures, where the nematic Curie constant A_n can be derived. In Cr, Mn and V doped samples, A_n decreases with the doping level, suggesting the suppression of nematic fluctuations by these dopants. In Ba(Fe_1-xCu_x)₂As₂, detailed neutron-diffraction measurements reveal that the collinear antiferromagnetic order persists up to x = 0.08 but becomes short-range above x > 0.04. Moreover, the nematic susceptibilities in the x >0.04 samples are significantly suppressed at low temperatures although A_n obtained by fitting the high-temperature data increases with increasing x. Compared with those systems that clearly exhibit superconductivity, such as Co, Ni, K or P doped samples, our results suggest that the reason that no superconductivity is found by doping Cr, Mn, V and Cu in BaFe₂As₂may be correlated with the suppression of nematic fluctuations.   

Effect of controlled point-like disorder induced by 2.5 MeV electron irradiation on nematic resistivity anisotropy of hole-doped (Ba,K)Fe2As2

In iron-based superconductors in-plane anisotropy of electrical resistivity in strain-detwinned samples strongly depends on residual resistivity [1]. The mechanism of the electronic transport responcible for sign change of in-plane resistivity anisotropy in hole-doped (Ba,K)Fe₂As₂ [2] attracts notable interest, since contributions from both elestic scattering due to impurities/defects and inelastic scattering on magnetic excitations and phonons can be anisotropic. We use irradiation with relativistic 2.5 MeV electrons at low temperatures to change residual resistivity and study the contribution of elastic scattering into anisotropic electronic transport.The modification of detwinning technique enabled measurements of the same samples before and after irradiation.
X Prefer Oral Session

[1] S. Ishida, et. al., Phys. Rev. B 84, 184514 (2011).
[2] E.C. Blomberg, et. al., Nature Comm. 4, 1914 (2013).   

Nematicity and Superconducting Enhancement in Ba1-xSrxNi2As2

The nematic phase, wherein electronic degrees of freedom drive a reduction in crystal rotational symmetry, is a common motif across a number of high temperature superconductor systems. In an effort to expand on the limited number of known nematic systems we report the evolution of a number of physical properties, including elastoresistance, in the Ba_1-xSr_xNi₂As₂ series, which shows a crossover between structural and electronically driven nematicity in the absence of long range magnetic order and unconventional superconducting pairing. In addition to establishing BaNi₂As₂ as a new and unique nematic system, we also observe dramatically enhanced superconducting Tc near the zero temperature nematic phase boundary in this series, pointing towards enhanced pairing due to strengthened nematic fluctuations.   

Nematic Glassy Behavior Probed by NMR under Strain in Iron Pnictide Superconductors

NMR studies of Ba(M_xFe_1-x)2As2 (M = Co, Cu) have uncovered
inhomogeneous glassy dynamics via stretched exponential relaxation and
wipeout of the NMR signal. These signatures emerge below approximately
100-150 K, depending on the doping level, and reach a maximum onset
temperature around the nematic quantum critical point. Here we
present NMR data under uniaxial strain in Ba(CoxFe1-x)2As2 (x=0.08) at
the nematic quantum critical point. The relaxation behavior reveals
inhomogeneous relaxation that is strongly temperature-dependent and
hysteretic with strain. The stretching exponent, which probes the
degree of inhomogeneity, remains unchanged under strain. These results
suggest that the inhomogeneous relaxation observed by NMR reflects a
nematic glass, in which large random strain fields are present and
frustrate long-range nematic order.