Family Of Iron-based Superconductors Research Articles

Anisotropy of the dc conductivity due to orbital-selective spin fluctuations in the nematic phase of iron superconductors

We study the dc conductivity of iron-based superconductors within the orbital-selective spin fluctuation scenario. Within this approach, the anisotropy of spin fluctuations below the spin-nematic transition at T$_S$ is also responsible for the orbital ordering, induced by nematic self-energy corrections to the quasiparticle dispersion. As a consequence, the anisotropy of the dc conductivity below T$_S$ is determined not only by the anisotropy of the scattering rates as expected within a spin-nematic scenario, but also by the modification of the Fermi velocity due to the orbital reconstruction. More interestingly, it turns out that these two effects contribute to the dc-conductivity anisotropy with opposite signs. By using realistic band-structure parameters we compute the conductivity anisotropy for both 122 and FeSe compounds, discussing the possible origin of the different dc-conductivity anisotropy observed experimentally in these two families of iron-based superconductors.

Multigap nodeless superconductivity in CsCa2Fe4As4F2 probed by heat transport

Recently, a new family of iron-based superconductors called 12442 was discovered and the muon spin relaxation ($\mu$SR) measurements on KCa$_2$Fe$_4$As$_4$F$_2$ and CsCa$_2$Fe$_4$As$_4$F$_2$ polycrystals, two members of the family, indicated that both have a nodal superconducting gap structure with $s+d$ pairing symmetry. Here we report the ultralow-temperature thermal conductivity measurements on CsCa$_2$Fe$_4$As$_4$F$_2$ single crystals ($T_c$ = 29.3 K). A negligible residual linear term $\kappa_0/T$ in zero field and the field dependence of $\kappa_0/T$ suggest multiple nodeless superconducting gaps in CsCa$_2$Fe$_4$As$_4$F$_2$. This gap structure is similar to CaKFe$_4$As$_4$ and moderately doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$, but contrasts to the nodal gap structure indicated by the $\mu$SR measurements on CsCa$_2$Fe$_4$As$_4$F$_2$ polycrystals.

Spectral Evidence for Emergent Order in Ba_{1-x}Na_{x}Fe_{2}As_{2}.

We report an angle-resolved photoemission spectroscopy study of the iron-based superconductor family, Ba_{1-x}Na_{x}Fe_{2}As_{2}. This system harbors the recently discovered double-Q magnetic order appearing in a reentrant C_{4} phase deep within the underdoped regime of the phase diagram that is otherwise dominated by the coupled nematic phase and collinear antiferromagnetic order. From a detailed temperature-dependence study, we identify the electronic response to the nematic phase in an orbital-dependent band shift that strictly follows the rotational symmetry of the lattice and disappears when the system restores C_{4} symmetry in the low temperature phase. In addition, we report the observation of a distinct electronic reconstruction that cannot be explained by the known electronic orders in the system.

Trends in pressure-induced layer-selective half-collapsed tetragonal phases in the iron-based superconductor family AeAFe4As4

By performing pressure simulations within density functional theory for the family of iron-based superconductors $Ae{}A$Fe$_4$As$_4$ with $Ae$ = Ca, Sr, Ba and $A$ = K, Rb, Cs we predict in these systems the appearance of two consecutive half-collapsed tetragonal transitions at pressures $P_{c_1}$ and $P_{c_2}$, which have a different character in terms of their effect on the electronic structure. We find that, similarly to previous studies for CaKFe$_4$As$_4$, spin-vortex magnetic fluctuations on the Fe sublattice play a key role for an accurate structure prediction in these materials at zero pressure. We identify clear trends of critical pressures and discuss the relevance of the collapsed phases in connection to magnetism and superconductivity. Finally, the intriguing cases of EuRbFe$_4$As$_4$ and EuCsFe$_4$As$_4$, where Eu magnetism coexists with superconductivity, are discussed as well in the context of half-collapsed phases.

Time-reversal symmetry-breaking nematic superconductivity in FeSe

FeSe is a unique member of the family of iron-based superconductors, not only because of the high values of $T_{c}$ in FeSe monolayer, but also because in bulk FeSe superconductivity emerges inside a nematic phase without competing with long-range magnetic order. Near $T_{c}$, superconducting order necessarily has $s+d$ symmetry, because nematic order couples linearly the $s$-wave and $d$-wave harmonics of the superconducting order parameter. Here we argue that the near-degeneracy between $s$-wave and $d$-wave pairing instabilities in FeSe, combined with the sign-change of the nematic order parameter between hole and electron pockets, allows the superconducting order to break time-reversal symmetry at a temperature $T^{*}<T_{c}$. The transition from an $s+d$ state to an $s+\mathrm{e}^{i\alpha}d$ state should give rise to a peak in the specific heat and to the emergence of a soft collective mode that can be potentially detected by Raman spectroscopy.

Infrared study of the iron-based superconductors

Iron-based superconductors are discovered by Japanese scientists Hosono, which have layered structures and characterized by multi-orbital nature. Although a large amount of investigations have been done with different experimental techniques on various families of iron-based superconductors, the most important issue concerning the superconducting mechanism remains controversial, and much more efforts need to be done in order to resolve this issue. Infrared spectroscopy is an important technique to study the electron excitations near the Fermi surface and lattice dynamics in condensed matter, it has played an important role in the early studies of superconductivity in determining the superconducting gaps and electron-phonon couplings. In this paper, we first make a brief introduction to the experimental techniques of infrared spectroscopy, and then we review some recent progress in the investigation of iron-based superconductors with infrared optical spectroscopy. The content includes: (1) superconducting gap in iron-based superconductors; (2) quantum critical and non-Fermi-liquid behavior in (Ba,K)Fe2As2; (3) orbital-selective physics in KFe2As2; (4) lattice dynamics, and (5) nematicity in iron-based superconductors. At last, we summarize what can be further explored by infrared spectroscopy in understanding of superconducting mechanism in iron-based superconductors.

Three-dimensional topological critical Dirac semimetal in AMgBi(A= K, Rb, Cs)

We predicted that AMgBi (A=K,Rb Cs), which have the same lattice structures as the 111 family of iron-based superconductors (Na/LiFeAs), are symmetry-protected Dirac semimetals located near the boundary of type-I and type-II Dirac semimetal phases. Doping Rb or Cs into KMgBi can drive the transition between the two phases. The materials can also be turned into Weyl semimetals and topological insulators by explicitly or spontaneously breaking time-reversal symmetry and C$_4$ lattice symmetry respectively.

Quasi-two-dimensional behavior of 112-type iron-based superconductors

Fluctuation magnetoconductivity and magnetization above the superconducting transition temperature (Tc) are measured on the recently discovered 112 family of iron-based superconductors (IBS), Ca$_{1-x}$La$_x$Fe$_{1-y}$Ni$_y$As$_2$, which presents an extra As-As chain spacer-layer. The analysis in terms of a generalization of the Lawrence-Doniach (LD) approach to finite applied magnetic fields indicates that these compounds are among the most anisotropic IBS ($\gamma$ up to ~30), and provides a compelling evidence of a quasi-two-dimensional behavior for doping levels near the optimal one.

Interplay of magnetism and superconductivity in the compressed Fe-ladder compound BaFe2Se3

High pressure resistance, susceptibility, and Fe $K\ensuremath{\beta}$ x-ray emission spectroscopy measurements were performed on Fe-ladder compound ${\mathrm{BaFe}}_{2}{\mathrm{Se}}_{3}$. Pressure-induced superconductivity was observed which is similar to the previously reported superconductivity in the ${\mathrm{BaFe}}_{2}{\mathrm{S}}_{3}$ samples. The slope of local magnetic moment versus pressure shows an anomaly across the insulator-metal transition pressure in the ${\mathrm{BaFe}}_{2}{\mathrm{Se}}_{3}$ samples. The local magnetic moment is continuously decreasing with increasing pressure, and the superconductivity appears only when the local magnetic moment value is comparable to the one in the iron-pnictide superconductors. Our results indicate that the compressed ${\mathrm{BaFe}}_{2}C{h}_{3}$ ($Ch=\mathrm{S}$, Se) is a new family of iron-based superconductors. Despite the crystal structures completely different from the known iron-based superconducting materials, the magnetism in this Fe-ladder material plays a critical role in superconductivity. This behavior is similar to the other members of iron-based superconducting materials.

Stabilizing the spin vortex crystal phase in two-dimensional iron-based superconductors

We present an investigation of the magnetic structure for iron-based superconductors (FeSCs) when inversion symmetry is broken, such as in substrate-supported monolayers or in the presence of a $c$-axis electric field. We perform group-, mean-field-, and density-functional-theoretic analyses on a model system of monolayer iron selenide (FeSe) on a strontium titanate $[{\mathrm{SrTiO}}_{3}$(001)] substrate. Our group- and mean-field-theoretic calculations are more generally applicable to thin films of the rest of the 11 (e.g., FeSe) family of iron-based superconductors, as well as to thin films of the 111 (e.g., LiFeAs) and 1111 (e.g., LaOFeAs) families, as these all belong to the same space group. We find that in systems with a collinear antiferromagnetic phase in bulk, when inversion symmetry is broken, the transition is instead into a ``spin vortex crystal'' phase and that a further phase transition can occur at a lower temperature in some circumstances. The spin vortex crystal is a ${C}_{4}$-symmetric magnetic phase which is related to this parent ${C}_{2}$-symmetric collinear antiferromagnetic (stripe) phase which is ubiquitous among the iron-based superconductors.

Superconductivity and magnetism in iron sulfides intercalated by metal hydroxides.

Inspired by naturally occurring sulfide minerals, we present a new family of iron-based superconductors. A metastable form of FeS known as the mineral mackinawite forms two-dimensional sheets that can be readily intercalated by various cationic guest species. Under hydrothermal conditions using alkali metal hydroxides, we prepare three different cation and metal hydroxide-intercalated FeS phases including (Li1-x Fe x OH)FeS, [(Na1-x Fe x )(OH)2]FeS, and K x Fe2-y S2. Upon successful intercalation of the FeS layer, the superconducting critical temperature Tc of mackinawite is enhanced from 5 K to 8 K for the (Li1-x Fe x OH) δ+ intercalate. Layered heterostructures of [(Na1-x Fe x )(OH)2]FeS resemble the natural mineral tochilinite, which contains an iron square lattice interleaved with a hexagonal hydroxide lattice. Whilst heterostructured [(Na1-x Fe x )(OH)2]FeS displays long-range magnetic ordering near 15 K, K x Fe2-y S2 displays short range antiferromagnetism.

Angular dependent torque measurements on CaFe0.88Co0.12AsF

Out-of-plane angular dependent torque measurements were performed on CaFe0.88Co0.12AsF (Ca1 1 1 1) single crystals. In the normal state, the torque data shows angular dependence and H2 magnetic field dependence, as a result of paramagnetism. In the mixed state, the torque signal is a combination of the vortex torque and paramagnetic torque, and the former allows the determination of the anisotropy parameter γ. At T = 11.5 K, γ (11.5 K ≃ 0.5 Tc) = 19.1, which is similar to the result of SmFeAsO0.8F0.2, at . So the 11 1 1 is more anisotropic compared to 11 and 122 families of iron-based superconductors. This may suggest that the electronic coupling between layers in 1 1 1 1 is less effective than in 11 and 122 families.

Discerning the nematic connection

Superconductivity The phase diagram of any given family of iron-based superconductors is complicated: Superconductivity competes with antiferromagnetism, with a structural transition often thrown in for good measure. Transport experiments have shown that in one of these families, Ba(Fe1- x Cox)2As2, a rotational electronic asymmetry, dubbed nematicity, drives the structural transition. Kuo et al. detected nematic fluctuations in five Fe-based superconductor families in the vicinity of optimal chemical doping: the doping that maximizes the superconducting transition temperature. Thus, nematicity may play a role in the mechanism of superconductivity in these compounds. Science , this issue p. [958][1] [1]: /lookup/doi/10.1126/science.aab0103

Effects of Coulomb interactions on the superconducting gaps in iron-based superconductors

Recent angle-resolved photoemission spectroscopy measurements of Co-doped LiFeAs report a large and robust superconducting gap on the $\Gamma$-centered hole band that lies 8 meV below the Fermi level. We show that, unlike a conventional superconductor described by BCS theory, a multiband system with strong interband Coulomb interactions can explain these observations. We model LiFeAs with a five-band model in which the shallow hole band is coupled with the other bands by only Coulomb interactions. Using Eliashberg theory, we find reasonable interaction parameters that reproduce the $T_c$ and all five gaps of LiFeAs. The energy independence of the Coulomb interactions then ensures the robustness of the gap induced on the shallow band. Furthermore, due to the repulsive nature of the Coulomb interactions, the gap changes sign between the shallow band and the other hole pockets, corresponding to an unconventional $s_{\pm}$ gap symmetry. Unlike other families of iron-based superconductors, the gap symmetry of LiFeAs has not been ascertained experimentally. The experimental implications of this sign-changing state are discussed.

Robustness of s-wave pairing symmetry in iron-based superconductors and its implications for fundamentals of magnetically driven high-temperature superconductivity

Based on the assumption that the superconducting state belongs to a single irreducible representation of lattice symmetry, we propose that the pairing symmetry in all measured iron-based superconductors is generally consistent with the A1g s-wave. Robust s-wave pairing throughout the different families of iron-based superconductors at different doping regions signals two fundamental principles behind high-Tc superconducting mechanisms: (i) the correspondence principle: the short-range magnetic-exchange interactions and the Fermi surfaces act collaboratively to achieve high-Tc superconductivity and determine pairing symmetries; (ii) the magnetic-selection pairing rule: superconductivity is only induced by the magnetic-exchange couplings from the super-exchange mechanism through cation-anion-cation chemical bonding. These principles explain why unconventional high-Tc superconductivity appears to be such a rare but robust phenomena, with its strict requirements regarding the electronic environment. The results will help us to identify new electronic structures that can support high-Tc superconductivity.

Direct observation of spin–orbit coupling in iron-based superconductors

Spin-orbit coupling (SOC) is a fundamental interaction in solids which can induce a broad spectrum of unusual physical properties from topologically non-trivial insulating states to unconventional pairing in superconductors. In iron-based superconductors (IBS) its role has so far been considered insignificant with the models based on spin- or orbital fluctuations pairing being the most advanced in the field. Using angle-resolved photoemission spectroscopy we directly observe a sizeable spin-orbit splitting in all main families of IBS. We demonstrate that its impact on the low-energy electronic structure and details of the Fermi surface topology is much stronger than that of possible nematic ordering. Intriguingly, the largest pairing gap is always supported exactly by SOC-induced Fermi surfaces.

Vortex pinning properties in Fe-chalcogenides

Among the families of iron-based superconductors, the 11-family is one of the most attractive for high field applications at low temperatures. Optimization of the fabrication processes for bulk, crystalline and/or thin film samples is the first step in producing wires and/or tapes for practical high power conductors. Here we present the results of a comparative study of pinning properties in iron-chalcogenides, investigating the flux pinning mechanisms in optimized Fe(SeTex) and FeSe samples by current–voltage characterization, magneto-resistance and magnetization measurements. In particular, from Arrhenius plots in magnetic fields up to 9 T, the activation energy is derived as a function of the magnetic field, whereas the activation energy as a function of temperature, is derived from relaxation magnetization curves. The high pinning energies, high upper critical field versus temperature slopes near critical temperatures, and highly isotropic pinning properties make iron-chalcogenide superconductors a technological material which could be a real competitor to cuprate high temperature superconductors for high field applications.

Ab initio Studies of Magnetism in the Iron Chalcogenides FeTe and FeSe

The iron chalcogenides FeTe and FeSe belong to the family of iron-based superconductors. We study the magnetism in these compounds in the normal state using the ab initio downfolding scheme developed for strongly correlated electron systems. In deriving ab initio low-energy effective models, we employ the constrained GW method to eliminate the double counting of electron correlations originating from the exchange correlations already taken into account in the density functional theory. By solving the derived ab initio effective models, we reveal that the elimination of the double counting is important in reproducing the bicollinear antiferromagnetic order in FeTe, as is observed in experiments. We also show that the elimination of the double counting induces a unique degeneracy of several magnetic orders in FeSe, which may explain the absence of the magnetic ordering. We discuss the relationship between the degeneracy and the recently found puzzling phenomena in FeSe as well as the magnetic ordering found under pressure.

Ab initiodownfolding study of the iron-based ladder superconductorBaFe2S3

Motivated by the recent discovery of superconductivity in the iron-based ladder compound ${\mathrm{BaFe}}_{2}{\mathrm{S}}_{3}$ under high pressure, we derive low-energy effective Hamiltonians from first principles. We show that the complex band structure around the Fermi level is represented only by the Fe $3{d}_{xz}$ (mixed with $3{d}_{xy}$) and $3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals. The characteristic band degeneracy allows us to construct a four-band model with the band unfolding approach. We also estimate the interaction parameters and show that the system is more correlated than the 1111 family of iron-based superconductors. Provided the superconductivity is mediated by spin fluctuations, the $3{d}_{xz}$-like band plays an essential role, and the gap function changes its sign between the Fermi surface around the $\mathrm{\ensuremath{\Gamma}}$ point and that around the Brillouin-zone boundary.

Interaction-induced singular Fermi surface in a high-temperature oxypnictide superconductor.

In the family of iron-based superconductors, LaFeAsO-type materials possess the simplest electronic structure due to their pronounced two-dimensionality. And yet they host superconductivity with the highest transition temperature Tc ≈ 55K. Early theoretical predictions of their electronic structure revealed multiple large circular portions of the Fermi surface with a very good geometrical overlap (nesting), believed to enhance the pairing interaction and thus superconductivity. The prevalence of such large circular features in the Fermi surface has since been associated with many other iron-based compounds and has grown to be generally accepted in the field. In this work we show that a prototypical compound of the 1111-type, SmFe0.92Co0.08AsO , is at odds with this description and possesses a distinctly different Fermi surface, which consists of two singular constructs formed by the edges of several bands, pulled to the Fermi level from the depths of the theoretically predicted band structure by strong electronic interactions. Such singularities dramatically affect the low-energy electronic properties of the material, including superconductivity. We further argue that occurrence of these singularities correlates with the maximum superconducting transition temperature attainable in each material class over the entire family of iron-based superconductors.