Recently, An electron-doped 12442-type iron-based superconductor BaTh$_2$Fe$_4$As$_4$(N$_{0.7}$O$_{0.3}$)$_2$ has been successfully synthesized with high-temperature solid-state reactions on basis of a structural design. The inter-block-layer charge transfer between the constituent units of "BaFe$_2$As$_2$" and "ThFeAsN$_{0.7}$O$_{0.3}$" was found to be essential to stabilize the target compound. Dominant electron-type conduction and bulk superconducting transition at ~22 K were demonstrated.
Using a variational Monte Carlo method, we investigate the nematic state in iron-base superconductors based on a three-band Hubbard model. Our results demonstrate that the nematic state, formed by introducing an anisotropic hopping order into the projected wave function, can arise in the underdoped regime when a realistic off-site Coulomb interaction $V$ is considered. {\color {red} We demonstrate that the off-site Coulomb interaction $V$, which is neglected so far in the analysis of iron-base superconductors, make a dominant contribution to the stabilization of nematic state. We calculate the…
Magnetism is widely considered to be a key ingredient of unconventional superconductivity. In contrast to cuprate high-temperature superconductors, antiferromagnetism in Fe-based superconductors (FeSCs) is characterized by a pair of magnetic propagation vectors. Consequently, three different types of magnetic order are possible. Of theses, only stripe-type spin-density wave (SSDW) and spin-charge-density wave (SCDW) orders have been observed. A realization of the proposed spin-vortex crystal (SVC) order is noticeably absent. We report a magnetic phase consistent with the hedgehog variation of …
In the iron-pnictide material CeFeAsO not only the Fe moments, but also the local 4f moments of the Ce order antiferromagnetically at low temperatures. We elucidate on the peculiar role of the Ce on the emergence of superconductivity. While application of pressure suppresses the iron SDW ordering temperature monotonously up to 4 GPa, the Ce-4f magnetism is stabilized, until both types of magnetic orders disappear abruptly and a narrow SC dome develops. With further increasing pressure characteristics of a Kondo-lattice system become more and more apparent in the electrical resistivity. This su…
We briefly review the recently constructed two orbital microscopic model for iron-based superconductors based on $S_4$ symmetry (PRX 2 021009(2012)). With this faithful representation of the kinematics of the tri-layer FeAs or FeSe structure, the model provides answers and physical pictures to fundamental questions related to the robustness of superconductivity and pairing symmetry, unifies different families of iron-based superconductors, casts new insight into the connections to the other high $T_c$ superconductors, cuprates, and reveals why an s-wave pairing can be stabilized by repulsive i…
In this work, we study a biquadratic Heisenberg model with coupled orbital degree of freedom using Monte Carlo simulation in order to investigate the phase transitions in iron-based superconductors. The antiferro-quadrupolar state, which may be related to the magnetism of FeSe [Phys. Rev. Lett. 115, 116401 (2015)], is stabilized by the anisotropic biquadratic interaction induced by a ferro-orbital-ordered state. It is revealed that the orbital and nematic transitions occur at the same temperature for all the cases, supporting the mechanism of the orbital-driven nematicity as revealed in most r…
The intricate interplay of interactions and Fermiology can give rise to a close competition between nodeless (e.g. s-wave) and nodal (e.g. d-wave) order in electronically driven unconventional superconductors. We analyze how such a scenario is affected by a Zeeman magnetic field $H_{\text{Z}}$ and temperature $T$. In the neighborhood of a zero temperature first order critical point separating a nodal from a nodeless phase, the phase boundary at low $H_{\text{Z}}$ and/or low $T$ has a universal line shape cubic in $H_{\text{Z}}$ or $T$, such that the nodal state is stabilized at the expense of …
Recent experimental developments in the iron pnictides have unambiguously demonstrated the existence of in-plane electronic anisotropy in the absence of the long-range magnetic order. Such anisotropy can arise from orbital ordering, which is described by an energy splitting between the two otherwise degenerate $d_{xz}$ and $d_{yz}$ orbitals. By including this phenomenological orbital order into a five-orbital Hubbard model, we obtain the mean-field solutions where the magnetic order is determined self-consistently. Despite sensitivity of the resulting states to the input parameters, we find th…
The 1144 iron arsenide (e.g. CaKFe4As4) has recently been discovered and inspired a tide of search for superconductors. Such far, the discovered compounds are confined to iron arsenides (ABFe4As4), where A and B are either alkali metals or alkaline earth elements. In this work, we propose two directions in searching 1144 structures: (i) using tri-valence cations for A; (ii) substituting the transition metal, e.g. replacing Fe by Co. Following the two directions, we employ density functional theory to study stability and electronic structures of 1144 pnictides of various tri-valence cations (La…
The bad metal behavior in the normal state of the iron-based superconductors suggests an intimate connection between the superconductivity and a proximity to a Mott transition. At the same time, there is strong evidence for the multi-orbital nature of the electronic excitations. It is then important to understand the orbital-dependent effects of electron correlations. In this paper we review the recent theoretical progresses on the metal-to-insulator transition in multiorbital models for the iron-based superconductors. These include studies of models that contain at least the 3d xy and 3d xz/y…
This study leads to the investigation of the non-equilibrium electron relaxation in the normal state of iron pnictides. Here we consider the relaxation of electrons due to their coupling with magnons and phonons in the metallic state of iron pnictides using the memory function approach. In the present model, electrons live at a higher temperature than that of the phonon and magnon baths, mimicking a non-equilibrium steady state situation. Further we analyze theoretically the generalized Drude scattering rate within the framework of Two Temperature Model and study the full frequency and tempera…
We focus on a broad class of tetragonal itinerant systems sharing a tendency towards the spontaneous formation of incommensurate magnetism with ordering wavevectors $\mathbf{Q}_{1,2}=(π-δ,0)/(0,π-δ)$ or $\mathbf{Q}_{1,2}=(π,δ)/(-δ,π)$. Employing a Landau approach, we obtain the generic magnetic phase diagram and identify the leading instabilities near the paramagnetic-magnetic transition. Nine distinct magnetic phases exist that either preserve or violate the assumed $C_4$-symmetry of the paramagnetic phase. These are single- and double-$\mathbf{Q}$ phases consisting of magnetic stripes, helic…
We report a high-pressure study of simultaneous low-temperature electrical resistivity and Hall effect measurements on high quality single-crystalline KFe2As2 using designer diamond anvil cell techniques with applied pressures up to 33 GPa. In the low pressure regime, we show that the superconducting transition temperature T_c finds a maximum onset value of 7 K near 2 GPa, in contrast to previous reports that find a minimum T_c and reversal of pressure dependence at this pressure. Upon applying higher pressures, this T_c is diminished until a sudden drastic enhancement occurs coincident with a…
Motivated by recent experimental progress on iron-based ladder compounds, we study the doped two-orbital Hubbard model for the two-leg ladder BaFe$_2$S$_3$. The model is constructed by using {\it ab initio} hopping parameters and the ground state properties are investigated using the density matrix renormalization group method. We show that the $(π,0)$ magnetic ordering at half-filling, with ferromagnetic rungs and antiferromagnetic legs, becomes incommensurate upon hole doping. Moreover, depending on the strength of the Hubbard $U$ coupling, other magnetic patterns, such as $(0,π)$, are also …
Electronic nematicity plays important role in iron-based superconductors. These materials have layered structure and theoretical description of their magnetic and nematic transitions has been well established in two-dimensional approximation, i.e., when the layers can be treated independently. However, the interaction between iron layers mediated by electron tunneling may cause non-trivial three-dimensional behavior. Starting from the simplest model for orbital nematic in a single layer, we investigate the influence of interlayer tunneling on bulk nematic order and possible preemptive state wh…
A time-resolved synchrotron X-ray total scattering study sheds light on the evolution of the different structural length scales involved during the intercalation of the layered iron-selenide host by organic molecular donors, aiming at the formation of the expanded lattice $Li_x(C_5H_5N)_yFe_{2-z}Se_2$ hybrid superconductor. The intercalates are found to crystallize in the tetragonal $ThCr_2Si_2$-type structure at the average level, however, with an enhanced interlayer iron-selenide spacing (d= 16.2 Å) that accommodates the heterocyclic molecular spacers. Quantitative atomic pair distribution f…
Applying an external pressure to indirectly electron-doped 122-type (Ba1-xLax)Fe2As2 epitaxial films enhances the superconducting critical temperature (Tc) up to 30.3 K. Different from the other family compounds, the Tc is enhanced not only in the under-doped region but also in the optimally doped and over-doped regions. Narrowing of the superconducting transition width and an increase in the carrier density take place simultaneously in the whole doping region, except at the heavily over-doped limit. This characteristic is unique to and observed only in (Ba1-xLax)Fe2As2, in which the La doping…
