We review the properties of Ni-based superconductors which contain Ni2X2 (X=As, P, Bi, Si, Ge, B) planes, a common structural element found also in the recently discovered FeAs superconductors. Strong evidence for the fully gapped nature of the superconducting state has come from field dependent thermal conductivity results on BaNi2As2. Coupled with the lack of magnetism, the majority of evidence suggests that the Ni-based compounds are conventional electron-phonon mediated superconductors. However, the increase in Tc in LaNiAsO with doping is anomalous, and mimics the behavior in LaFeAsO. Furthermore, comparisons of the properties of Ni- and Fe-based systems show many similarities, particularly with regards to structure-property relationships. This suggests a deeper connection between the physics of the FeAs superconductors and the related Ni-based systems which deserves further investigation.
The ThCr2Si2-type arsenide-selenide phase KFe2AsSe is proposed as a parent system for a new "intermediate" group of Fe-based superconducting materials bridging the known families of Fe-pnictides (such as BaFe2As2) and Fe-chalcogenides (such as KxFe2-ySe2) superconductors. The characterization of the proposed charge-balanced phase by means of FLAPW-GGA approach covers the crystal structure, As/Se atomic ordering, stability, electronic bands, Fermi surface, and density of electronic states.
Impurity scattering is found to lead to quasi-one dimensional nanoscale modulation of the local density of states in the iron pnictides and chalcogenides. This `quasiparticle interference' feature is remarkably similar across a wide variety of pnictide and chalcogenide phases, suggesting a common origin. We show that a unified understanding of the experiments can be obtained by simply invoking a four-fold symmetry breaking $d_{xz}-d_{yz}$ orbital splitting, of a magnitude already suggested by the experiments. This can explain the one-dimensional characteristics in the local density of states o…
This review discusses and compares electronic spectra of new iron-based high-temperature superconductors (HTSC), as well as a number of chemically similar compounds. Particular attention is payed to iron chalcogenide K_{1-x}Fe_{2-y}Se_2, which is isostructural to BaFe_2As_2 (122) pnictide. It is shown, that Fermi surfaces of K_{1-x}Fe_{2-y}Se_2 are essentially different from those for pnictides. Using LDA+DMFT and LDA'+DMFT calculations we show, that electronic correlations in K_{1-x}Fe_{2-y}Se_2 influence the electronic structure much more significantly, than in the most studied 122 system. W…
Superconductivity in the iron pnictides and chalcogenides arises at the border of antiferromagnetism, which raises the question of the role of quantum criticality. In this topical review, we describe the theoretical work that led to the prediction for a magnetic quantum critical point arising out of a competition between electronic localization and itinerancy, and the proposal for accessing it by using isoelectronic P substitution for As in the undoped iron pnictides. We go on to compile the emerging experimental evidence in support of the existence of such a quantum critical point in isoelect…
Superconductivity develops in metals upon the formation of a coherent macroscopic quantum state of electron pairs. Iron pnictides and chalcogenides are materials that have high superconducting transition temperatures. In this Review, we describe the advances in the field that have led to higher superconducting transition temperatures in iron-based superconductors and the wide range of materials that form them. We summarize both the essential aspects of the normal state and the mechanism for superconductivity. We emphasize the degree of electron-electron correlations and their manifestation in …
Superconducting condensation energy $U_0^{int}$ has been determined by integrating the electronic entropy in various iron pnictide/chalcogenide superconducting systems. It is found that $U_0^{int}\propto T_c^n$ with $n$ = 3 to 4, which is in sharp contrast to the simple BCS prediction $U_0^{BCS}=1/2N_FΔ_s^2$ with $N_F$ the quasiparticle density of states at the Fermi energy, $Δ_s$ the superconducting gap. A similar correlation holds if we compute the condensation energy through $U_0^{cal}=3γ_n^{eff}Δ_s^2/4π^2k_B^2$ with $γ_n^{eff}$ the effective normal state electronic specific heat coefficien…
In this paper we summarize the effects of magnetic and nonmagnetic impurities on the spin dynamics in Febased superconductors and their parent compounds. The effects of chemical substitution, vacancies, and disorder on the suppression or stabilization of superconductivity and spin-density-wave phases are reviewed in the context of recent neutron-spectroscopy measurements of spin excitations. We also present new results on the structure of magnetic fluctuations in BaFe2As2 single crystals doped with Mn local moments and discuss them in relationship to the previously reported (pi, pi) branch of …
We use inelastic neutron scattering to show that the spin waves in the iron chalcogenide Fe$_{1.05}$Te display novel dispersion clearly different from those in the related iron pnictide systems. By fitting the spin waves to a Heisenberg Hamiltonian, we extract magnetic exchange couplings that are dramatically different from both predictions by density functional calculations and measurements on the iron pnictide CaFe$_2$As$_2$. While the nearest-neighbor exchange couplings in CaFe$_2$As$_2$ and Fe$_{1.05}$Te are quite different, their next-nearest-neighbor exchange couplings are similar. These…
The indirect RKKY interaction in iron pnictide and chalcogenide metals is calculated for a simplified four bands Fermi surface (FS) model. We investigate the specific multi-band features and show that distinct length scales of the RKKY oscillations appear. For the regular lattice of the local moments, the generalized RKKY interaction is defined in momentum space. We consider its momentum dependence in paramagnetic and spin density wave (SDW) phases, discuss its implications for the possible type of magnetic order and compare it to the results obtained from more realistic tight-binding type Fer…
Over the past few years Fe chalcogenides (FeSe/Te) have advanced to the forefront of Fe-based superconductors (FeBS) research. The most intriguing results thus far are for intercalated and monolayer FeSe, however experimental studies are still inconclusive. Yet, bulk FeSe itself remains an unusual case when compared with pnictogen-based FeBS, and may hold clues to understanding the more exotic FeSe-derivatives. The FeSe phase diagram is unlike the pnictides: the orthorhombic distortion, which is likely to be of a "spin-nematic" nature in numerous pnictides, is not accompanied by magnetic order…
By applying density functional theory calculations to iron chalcogenides, we find that magnetic order in Fe$_{1+y}$Te and magnetic instability at $(π,π)$ in K$_y$Fe$_2$Se$_2$ are controlled by interstitial and interlayer cations, respectively. While in Fe$_{1+y}$Te, magnetic phase transitions occur among collinear, exotic bicollinear and plaquette-ordered antiferronmagnetic states when the height of interstitial irons measured from iron plane or the concentration of interstitial irons is varied, the magnetic instability at $(π,π)$ which is believed to be responsible for the Cooper pairing in i…
We use point contact spectroscopy to probe $\rm{AEFe_2As_2}$ ($\rm{AE=Ca, Sr, Ba}$) and $\rm{Fe_{1+y}Te}$. For $\rm{AE=Sr, Ba}$ we detect orbital fluctuations above $T_S$ while for AE=Ca these fluctuations start below $T_S$. Co doping preserves the orbital fluctuations while K doping suppresses it. The fluctuations are only seen at those dopings and temperatures where an in-plane resistive anisotropy is known to exist. We predict an in-plane resistive anisotropy of $\rm{Fe_{1+y}Te}$ above $T_S$. Our data are examined in light of the recent work by W.-C. Lee and P. Phillips (arXiv:1110.5917v2).…
We present a brief review of the present day situation with studies of high-temperature superconductivity in iron pnictides and chalcogenides. Recent discovery of superconductivity with T_c > 30 K in A_xFe_{2-x/2}Se_2 (A=K,Cs,Tl,...) represents the major new step in the development of new concepts in the physics of Fe - based high-temperature superconductors. We compare LDA and ARPES data on the band structure and Fermi surfaces of novel superconductors and those of the previously studied FeAs superconductors, especially isostructural 122 - superconductors like BaFe_2As_2. It appears that elec…
The present article reviews recent experimental investigations of spin dynamics in iron-based superconductors and their parent compounds by means of inelastic neutron scattering. It mainly focuses on the most contemporary developments in this field, pertaining to the observations of magnetic resonant modes in new superconductors, spin anisotropy of low-energy magnetic fluctuations that has now been observed in a wide range of chemical compositions and doping levels, as well as their momentum-space anisotropy incurred by the spin-nematic order. The implications of these new findings for our und…
Undoped iron superconductors accommodate $n=6$ electrons in five d-orbitals. Experimental and theoretical evidence shows that the strength of correlations increases with hole-doping, as the electronic filling approaches half-filling with $n=5$ electrons. This evidence delineates a scenario in which the parent compound of iron superconductors is the half-filled system, in analogy to cuprate superconductors. In cuprates the superconductivity can be induced upon electron or hole doping. In this work we propose to search for high-Tc superconductivity and strong correlations in chromium pnictides a…
Structural and physical properties of layered chalcogenide superconductors are summarized. In particular, we review the remarkable properties of the Fe-chalcogenide superconductors, FeSe and FeTe-based materials. Furthermore, we introduce the recently-discovered new BiS2-based layered superconductors and discuss its prospects.
Here we summarize the physical properties of the newly discovered Fe-chalcogenide superconductors. The Fe-chalcogenide superconductors attract us as the simplest Fe-based superconductors. Furthermore, Fe chalcogenides show a huge pressure effect on their superconducting properties. The origin of the high transition temperature was discussed with both the change in crystal structure and magnetism. The progress on the thin-film and superconducting-wire fabrications are also described.
We report Fe K beta x-ray emission spectroscopy study of local magnetic moments in various iron based superconductors in their paramagnetic phases. Local magnetic moments are found in all samples studied: PrFeAsO, Ba(Fe,Co)2As2, LiFeAs, Fe1+x(Te,Se), and A2Fe4Se5 (A=K, Rb, and Cs). The moment size varies significantly across different families. Specifically, all iron pnictides samples have local moments of about 1 $μ_B$/Fe, while FeTe and K2Fe4Se5 families have much larger local moments of ~2$μ_B$/Fe, ~3.3$μ_B$/Fe, respectively. In addition, we find that neither carrier doping nor temperature …
Deviations of low-energy electronic structure of iron-based superconductors from density functional theory predictions have been parametrized in terms of band- and orbital-dependent mass renormalizations and energy shifts. The former have typically been described in terms of a local self-energy within the framework of dynamical mean field theory, while the latter appears to require non-local effects due to interband scattering. By calculating the renormalized bandstructure in both random phase approximation (RPA) and the two-particle self-consistent approximation (TPSC), we show that correlati…
We analyze the interplay between superconductivity and the formation of bound pairs of fermions (BCS-BEC crossover) in 2D models of interacting fermions with small Fermi energy EF and weak attractive interaction, which extends to energies well above EF. The 2D case is special because one has to distinguish between bound state formation and superconductivity already at weak coupling. We briefly review the situation in the one-band model and then consider two different two-band models: the one with a hole and an electron band, and the one with two electron bands. In each case we obtain the bound…
Superconductivity in the iron pnictides and chalcogenides is closely connected to a bad-metal normal state and a nearby antiferromagnetic order. Therefore, considerable attention has been focused on the role of electron correlations and spin dynamics. In this article, we summarize some key experiments that quite directly imply strong electron correlations in these materials, and discuss aspects of the recent theoretical studies on these issues. In particular, we outline a $w$-expansion, which treats the correlation effects using the Mott transition as the reference point. For the parent system…
We review the current status of point contact spectroscopy on the iron based superconductors, focusing on their normal state. Point contact spectroscopy is generally used to study superconductors via Andreev reflection, but in recent years it has also proved to be a useful bulk probe of strongly correlated electron systems. Point contact spectroscopy picks up a conductance enhancement in the normal state, above the structural phase transition, of certain iron based compounds. These include Co doped $\rm{BaFe_2As_2}$, $\rm{SrFe_2As_2}$, $\rm{Fe_{1+y}Te}$ and F doped $\rm{SmFeAsO}$ and $\rm{LaFe…
The iron chalcogenide Fe1+y(Te1-xSex) is structurally the simplest of the Fe-based superconductors. Although the Fermi surface is similar to iron pnictides, the parent compound Fe1+yTe exhibits antiferromagnetic order with in-plane magnetic wave-vector (pi, 0). This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave-vector (pi, pi) that connects hole and electron parts of the Fermi surface. Despite these differences, both the pnictide and chalcogenide Fe-superconductors exhibit superconducting spin resonances around (pi, pi), suggesting a common symm…
We theoretically propose a method to identify $\pm s$-wave order parameter in recently discovered Fe-pnictide superconductors. Our idea uses the Riedel anomaly in ac-Josephson current through an SI($\pm$ S) (single-band s-wave superconductor/insulator/$\pm$s-wave two-band superconductor) junction. We show that the Riedel peak effect leads to vanishing ac-Josephson current at some values of biased voltage. This phenomenon does not occur in the case when the $\pm s$-wave superconductor is replaced by a conventional s-wave one, so that the observation of this vanishing Josephson current would be …
Iron-chalcogenide compounds with FeSe(Te, S) layers did not attract much attention until the discovery of high-Tc superconductivity (SC) in the iron-pnictide compounds at the begining of 2008. Compared with FeAs-based superconductors, iron-chalcogenide superconductors have aroused enormous enthusiasm to study the relationship between SC and magnetisms with several distinct features, such as different antiferromagnetic ground states with relatively large moments in the parents, indicating possibly different superconducting mechanisms, the existence of the excess Fe atoms or Fe vacancies in the …
We performed model calculations of Raman responses for multiband 2D superconductors. The multiband effects of screening in the A_{1g} symmetry channel were investigated analytically and numerically for a band structure model mimicing ARPES data on iron-pnictide materials. An acceptable agreement between our model calculations and recent experimental data is demonstrated by modification of the band structure parameters.
By applying an exact unitary transformation to a two-band hamiltonian which also includes the effects due to large pnictogen polarizabilities, we show that an attractive spin-mediated Hubbard term appears in the $d_{xz}$, $d_{yz}$ nearest-neighbour channel. This pairing mechanism implies a singlet superconducting order parameter in iron pnictides.
The iron pnictide and chalcogenide compounds are a subject of intensive investigations due to their high temperature superconductivity.\cite{a-LaFeAsO} They all share the same structure, but there is significant variation in their physical properties, such as magnetic ordered moments, effective masses, superconducting gaps and T$_c$. Many theoretical techniques have been applied to individual compounds but no consistent description of the trends is available \cite{np-review}. We carry out a comparative theoretical study of a large number of iron-based compounds in both their magnetic and param…
PbO-type Fe chalcogenide has been drawing much attention as the Fe-based superconductor with the most simple crystal structure. Whereas FeSe is an intrinsic superconductor, FeTe, while having a structure analogous to FeSe, exhibits antiferromagnetic ordering. The relationship between antiferromagnetism and superconductivity provides key information to understand better superconductivity in Fe chalcogenides. Furthermore the significant pressure effect on Tc in the Fe-chalcogenide superconductors, which is likely to be correlated with the crystal structure and/or magnetism, is important in eluci…
We explain that the study of impurity effect in iron pnictides are correctly calculated based on the T-matrix approximation, contrary to the Comment by Bang. The replacement T with T-I proposed by Bang breaks the perturbation theory and is therefore erroneous.
The magnetic moment in the parent phase of the iron-pnictide superconductors varies with composition even when the nominal charge of iron is unchanged. We propose the spin-lattice coupling due to the magneto-volume effect as the primary origin of this effect, and formulate a Landau theory to describe the dependence of the moment to the Fe-As layer separation. We then compare the superconductive critical temperature of doped iron pnictides to the local moment predicted by the theory, and suggest that the spin-phonon coupling may play a role in the superconductivity of this compound.
One of the outstanding problems in Iron pnictide research is the unambiguous detection of its pairing symmetry. The most probable candidates are the two-band s$++$ and sign reversed s$\pm$ wave pairing. In this work, the Andreev conductance and shot noise are used as a probe for the pairing symmetry of Iron pnictide superconductors. Clear differences emerge in both the zero bias differential conductance and the shot noise in the tunneling limit for the two cases enabling an effective distinction between the two.
We demonstrate that the inclusion of van der Waals dispersive interaction sensibly improves the prediction of lattice constants by density functional theory in iron-chalcogenides (FeCh) superconductor compounds, namely FeSe and FeTe. We show how generalized gradient approximation (GGA) for the exchange correlation potential overestimates the out-of-plane lattice constants in both compounds when compared with experiments. In addition, GGA predicts a too weak bonding between the neutral FeCh layers, with a sensible underestimation of the bulk modulus. van der Waals corrected simulations complete…
A phenomenological theory of tunneling spectroscopy for Fe-pnictide superconductors is developed by taking into consideration of asymmetric interface scattering between particle and holes. It is shown that in consistent with anti-phase $s^{\pm}$-wave pairing, appreciable zero-energy surface bound states exist on the [100] surface of Fe-pnictide superconductors. However, in contrast to the [110] bound states in $d$-wave cuprate superconductors, these bound states arise as a result of non-conservation of momentum perpendicular to the interface for tunneling electrons and the $s^{\pm}$ pairing, a…
The discovery of the new class of pnictide superconductors has engendered a controversy about their pairing symmetry, with proposals ranging from an extended s-wave or "s$_{\pm}$" symmetry to nodal or nodeless d-wave symmetry to still more exotic order parameters such as p-wave. In this paper, building on the earlier, similar work performed for the cuprates, we propose several phase-sensitive Josephson interferometry experiments, each of which may allow resolution of the issue.
Transverse-field muon-spin rotation measurements performed on two samples of LiFeAs demonstrate that the superfluid stiffness of the superconducting condensate in relation to its superconducting transition temperature is enhanced compared to other pnictide superconductors. Evidence is seen for a field-induced magnetic state in a sample with a significantly suppressed superconducting transition temperature. The results in this system highlight the role of direct Fe-Fe interactions in frustrating pairing mediated by antiferromagnetic fluctuations and suggest that, in common with other pnictide s…
A linear temperature dependence of the electrical resistivity as T -> 0 is the hallmark of quantum criticality in heavy-fermion metals and the archetypal normal-state property of high-Tc superconductors, yet in both cases it remains unexplained. We report a linear resistivity on the border of spin-density-wave order in the organic superconductor (TMTSF)2X (X = PF6, ClO4), whose strength scales with the superconducting temperature Tc. This scaling, also present in the pnictide superconductors, reveals an intimate connection between linear-T scattering and pairing, shown by renormalization group…
The response of the worldwide scientific community to the discovery in 2008 of superconductivity at Tc = 26 K in the Fe-based compound LaFeAsO_{1-x}F_x has been very enthusiastic. In short order, other Fe-based superconductors with the same or related crystal structures were discovered with Tc up to 56 K. Many experiments were carried out and theories formulated to try to understand the basic properties of these new materials and the mechanism for Tc. In this selective critical review of the experimental literature, we distill some of this extensive body of work, and discuss relationships betw…
Nuclear magnetic relaxation rate 1/T_1 in iron-pnictide superconductors is calculated using the gap function obtained in a microscopic calculation. Based on the obtained results, we discuss the issues such as the rapid decrease of 1/T_1 just below the transition temperature and the difference between nodeless and nodal s-wave gap functions. We also investigate the effect of Coulomb interaction on 1/T_1 in the random phase approximation and show its importance in interpreting the experimental results.
In the cuprate and iron-pnictide systems, valence changes induce high-temperature superconductivity while the local structural chemistry and local spin order both independently generate the attractive interactions responsible for the high transition temperature. We argue that together they favor d-wave singlet superconductivity in the cuprates but s-wave singlet in the pnictides. This difference arises from the existence of a large on-site repulsion between carriers in the cuprates largely absent in the pnictides. Fluorine is responsible for raising Tc significantly in some pnictides and in th…
We present the results of a study of the vortex lattice (VL) of the nickel chalcogenide superconductor TlNi2Se2, using small angle neutron scattering. This superconductor has the same crystal symmetry as the iron arsenide materials. Previous work points to it being a two-gap superconductor, with an unknown pairing mechanism. No structural transitions in the vortex lattice are seen in the phase diagram, arguing against d-wave gap symmetry. Empirical fits of the temperature-dependence of the form factor and penetration depth rule out a simple s-wave model, supporting the presence of nodes in the…
The latest discovery of a new iron-chalcogenide superconductor A$_x$Fe$_{2-y}$Se$_2$(A=K, Cs, Rb, Tl and etc.) has attracted much attention due to a number of its unique characteristics, such as the possible insulating state of the parent compound, the existence of Fe-vacancy and its ordering, a new form of magnetic structure and its interplay with superconductivity, and the peculiar electronic structures that are distinct from other Fe-based superconductors. In this paper, we present a brief review on the structural, magnetic and electronic properties of this new superconductor, with an empha…
We have discovered first homologous series of iron pnictide oxide superconductors (Fe2As2)(Can+1(Sc,Ti)nOy) [n = 3,4,5]. These compounds have extremely thick blocking layers up to quintuple perovskite oxide layers sandwiched by the Fe2As2 layers. These samples exhibited bulk superconductivity with relatively high Tc up to 42 K. The relationship between Tc and the iron-plane interlayer distance suggested that superconductivity due to the mono Fe2As2 layer is substantially 40 K-class.
We pointed out that the main claim of Ref.[1] that the presence of the orbital degree of freedom is essential to describe the correct impurity effects for Iron-based superconductors is incorrect and demonstrated the equivalent results of Tc suppression due to impurities by both the band basis model and the orbital basis model.
There is increasing recognition that the multiorbital nature of the 3d electrons is important to the proper description of the electronic states in the normal state of the iron-based superconductors. Earlier studies of the pertinent multiorbital Hubbard models identified an orbital-selective Mott phase, which anchors the orbital-selective behavior seen in the overall phase diagram. An important characteristics of the models is that the orbitals are kinetically coupled -- i.e. hybridized -- to each other, which makes the orbital-selective Mott phase especially nontrivial. A U(1) slave-spin meth…
We examine the effect of a single, non-magnetic impurity in a multiband, extended s-wave superconductor allowing for anisotropy of the gaps on the Fermi surfaces. We derive analytic expressions for the Green's functions in the continuum and analyse the conditions for the existence of sharp impurity-induced resonant states. Underlying band structure is more relevant for the multiband than for single band case, and mismatch between the bands generically makes the formation of the impurity states less likely in the physical regime of parameters. We confirm these conclusions by numerically solving…
We review theoretical aspects of the studies on the recently discovered BiCh2 (Ch=chalcogen) based superconductors. We first focus on the electronic band structure, the Fermi surface and their correlation with the lattice structure. We then discuss the phonon features and the issue of the lattice instability. Finally, we survey the phonon-mediated as well as the unconventional pairing mechanisms that have been proposed so far.
Charge doping of iron-pnictide superconductors leads to collective pinning of flux vortices, whereas isovalent doping does not. Moreover, flux pinning in the charge-doped compounds is consistently described by the mean-free path fluctuations introduced by the dopant atoms, allowing for the extraction of the elastic quasiparticle scattering rate. The absence of scattering by dopant atoms in isovalently doped BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ is consistent with the observation of a linear temperature dependence of the low-temperature penetration depth in this material.
We study Josephson junctions between a multi-band iron-pnictide Ba1-xNaxFe2As2 and conventional s-wave superconductors Nb and Cu/Nb bilayer. We observe that junctions with a Cu interlayer exhibit much larger IcRn, despite a weaker proximity-induced superconductivity. This counterintuitive result is attributed to the difference in Fermi surface geometries of Nb and Cu, which leads to a selective one-band tunneling from Cu and a non-selective multi-band tunnelng from Nb. The latter leads to a mutual cancellation of supercurrents due to the sign-reversal s+- symmetry of the order parameter in the…
Muon-spin rotation experiments were performed on the pnictide high temperature superconductor SmFeAsO$_{1-x}$F$_{x}$ with x=0.18 and 0.3. We observed an unusual enhancement of slow spin fluctuations in the vicinity of the superconducting which suggests that the spin fluctuations contribute to the formation of an unconventional superconducting state. An estimate of the in-plane penetration depth $λ_{ab}(0)=190(5)$\thinspace nm was obtained, which confirms that the pnictide superconductors obey an Uemura-style relationship between $T_{\mathrm{c}}$ and $λ_{ab}(0)^{-2}$.
Single impurity problem in iron-pnictide superconductors is investigated by solving Bogoliubov-de Gennes (BdG) equation in the five-orbital model, which enables us to distinguish s$_{+-}$ and s$_{++}$ superconducting states. We construct a five-orbital model suitable to BdG analysis. This model reproduces the results of random phase approximation in the uniform case. Using this model, we study the local density of states around a non-magnetic impurity and discuss the bound-state peak structure, which can be used for distinguishing s$_{+-}$ and s$_{++}$ states. A bound state with nearly zero-en…
Based on a phenomenological model with $s_{\pm}$ or s-wave pairing symmetry, the mixed-state effect on the low-energy spin dynamics in optimally-doped iron pnictide superconductors is studied by solving Bogoliubov-de Gennes equations. Our results of the spin susceptibility at $\mathbf{q}=\mathbf{Q}$ in the normal, superconducting and mixed states agree qualitatively with recent neutron scattering experiments. We also propose that the field-induced intensity change shows different behaviors between the $s_{\pm}$ and s-wave symmetries in both momentum and real space, thus it can be used to disti…
The puzzling nature of magnetic and lattice phase transitions of iron pnictides is investigated via a first-principles Wannier function analysis of representative parent compound LaOFeAs. A rare ferro-orbital ordering is found to give rise to the recently observed highly anisotropic magnetic coupling, and drive the phase transitions--without resorting to widely employed frustration or nesting picture. The revealed necessity of the additional orbital physics leads to a correlated electronic structure fundamentally distinct from that of the cuprates. In particular, the strong coupling to the mag…
In- and out-of-plane magnetic penetration depths were measured in three iron-based pnictide superconducting systems. All studied samples of both 122 systems show a robust power-law behavior, $λ(T) T^n$, with the sample-dependent exponent n=2-2.5, which is indicative of unconventional pairing. This scenario could be possible either through scattering in a $s_{\pm }$ state or due to nodes in the superconducting gap. In the Nd-1111 system, the interpretation of data may be obscured by the magnetism of rare-earth ions. The overall anisotropy of the pnictide superconductors is small. The 1111 syste…
We report investigation on Fe1+yTexSe1-x single crystals by using scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Both nonsuperconducting samples with excess iron and superconducting samples demonstrate nanoscale phase separation and chemical inhomogeneity of Te/Se content, which we attribute to a miscibility gap. The line scan EELS technique indicates ~20% or less fluctuation of Te concentration from the nominal compositions.
We discuss the type of pairing in the hexagonal pnictide superconductor SrPtAs, taking into account its multiband structure. The topological chiral $d$-wave state with time-reversal-symmetry breaking has been anticipated from the spontaneous magnetization observed by the muon-spin-relaxation experiment. We point out in this paper that the recent experimental reports on the nuclear-spin-lattice relaxation rate $T_1^{-1}$ and superfluid density $n_s(T)$, which seemingly support the conventional $s$-wave pairing, are also consistent with the chiral $d$-wave state. The compatibility of the gap and…
We studied double superconducting (SC) domes in LaFeAsO1-xHx by using 75As- and 1H-nuclear magnetic resonance techniques, and unexpectedly discovered that a new antiferromagnetic (AF) phase follows the double SC domes on further H doping, forming a symmetric alignment of AF and SC phases in the electronic phase diagram. We demonstrated that the new AF ordering originates from the nesting between electron pockets, unlike the nesting between electron and hole pockets as seen in the majority of undoped pnictides. The new AF ordering is derived from the features common to high-Tc pnictides: howeve…
We microscopically examine the intergrain Josephson current in iron-pnictide superconductors in order to solve the puzzle of why the intergrain current is much lower than the intragrain one. The theory predicts that the intergrain Josephson current is significantly reduced by the $\pm$s-wave symmetry when the incoherent tunneling becomes predominant and the density of states and the gap amplitude between two bands are identical. We find in such a situation that the temperature dependence of the intergrain Josephson current shows an anomalously flat curve over a wide temperature range. Finally,…
Electrical resistivity measurements of detwinned single crystals of the representative iron arsenide Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ reveal a dramatic in-plane anisotropy associated with the tetragonal-to-orthorhombic structural transition that precedes the onset of long-range antiferromagnetic order. These results indicate that the structural transition in this family of compounds is fundamentally electronic in origin.
