The hole-doped ground state of a recently introduced extended "t-U-J" two-orbital Hubbard model for the Fe-based superconductors is studied via exact diagonalization methods on small clusters. Similarly as in the previously studied case of electron doping, A. Nicholson et al., Phys. Rev. Lett. 106 21702 (2011), upon hole doping it is observed that there are several competing pairing symmetries including A_{1g}, B_{1g}, and B_{2g}. However, contrary to the electron-doped case, the ground state of the hole-doped state has pseudocrystal momentum k=(π,π) in the unfolded Brillouin zone. In the two Fe-atom per unit cell representation, this indicates that the ground state involves anti-bonding, rather than bonding, combinations of the orbitals of the two Fe atoms in the unit-cell. The lowest state with k=(0,0) has only a slightly higher energy. These results indicate that this simple two-orbital model may be useful to capture some subtle aspects of the hole-doped pnictides since calculations for the five-orbital model have unveiled a hole pocket centered at M (k=(π,π)) in the unfolded Brillouin zone.
Josephson junctions between a $FeAs$-based superconductor with antiphase s-wave pairing and a conventional s-wave superconductor are studied. The translational invariance in a planar junction between a single crystal pnictide and an aluminum metal greatly enhances the relative weight of electron pockets in the pnictide to the critical current. In a wide doping region of the pnictide, a planar and a point contact junctions have opposite phases, which can be used to design a tri-junction ring with $π$ phase to probe the antiphase pairing.
We compare the one-loop functional renormalization group results for the cuprates and the iron pnictides. Interestingly a coherent picture suggesting that antiferromagnetic correlation causes pairing for both materials emerges.
We derive the effective action for superconducting fluctuations in a four-band model for pnictides, discussing the emergence of a single critical mode out of a dominant interband pairing mechanism. We then apply our model to calculate the paraconductivity in two-dimensional and layered three-dimensional systems, and compare our results with recent resistivity measurements in SmFeAsOF
Considering model Hamiltonians that respect the symmetry properties of the pnictides, it is argued that pairing interactions that couple electrons at different orbitals with an orbital-dependent pairing strength inevitably lead to interband pairing matrix elements, at least in some regions of the Brillouin zone. Such interband pairing has not been considered of relevance in multiorbital systems in previous investigations. It is also observed that if, instead, a purely intraband pairing interaction is postulated, this requires that the pairing operator has the form $Δ^+(k)=f(k) \sum_α d^+_{k,α,…
Using an RPA approximation, we have calculated the strengths of the singlet and triplet pairing interactions which arise from the exchange of spin and orbital fluctuations for a 2-orbital model of the Fe-pnictide superconductors. When the system is doped with F, the electron pockets become dominant and we find that the strongest pairing occurs in the singlet d-wave pairing and the triplet p-wave pairing channels, which compete closely. The pairing structure in the singlet d-wave channel corresponds to a superposition of near neighbor intra-orbital singlets with a minus sign phase difference be…
We introduce and study an extended "t-U-J" two-orbital model for the pnictides that includes Heisenberg terms deduced from the strong coupling expansion. Including these J terms explicitly allows us to enhance the strength of the (pi, 0)-(0, pi) spin order which favors the presence of tightly bound pairing states even in the small clusters that are here exactly diagonalized. The A1g and B2g pairing symmetries are found to compete in the realistic spin-ordered and metallic regime. The dynamical pairing susceptibility additionally unveils low-lying B1g states, suggesting that small changes in pa…
We present a polarization resolved study of the low energy band structure in the optimally doped iron pnictide superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ (T$_c$=37K) using angle resolved photoemission spectroscopy. Polarization-contrasted measurements are used to identify and trace all three low energy hole-like bands predicted by local density approximation (LDA) calculations. The photoemitted electrons reveal an inconsistency with LDA-predicted symmetries along the $Γ$-X high symmetry momentum axis, due to unexpectedly strong rotational anisotropy in electron kinetics. We evaluate many-b…
One of the major puzzles regarding unconventional superconductivity is how some of the most interesting superconductors are related to an insulating phase that lies in close proximity. Here we report scanning tunneling microscopy studies of the local electronic structure of Cu doped NaFeAs across the superconductor to insulator transition. We find that in the highly insulating regime the electronic spectrum develops an energy gap with diminishing density of state at the Fermi level. The overall lineshape and strong spatial variations of the spectra are strikingly similar to that of lightly dop…
Nematicity in the heavily hole-doped iron pnictide superconductors remains controversial. Sizeable nematic fluctuations and even nematic orders far from a magnetic instability were declared in RbFe$_2$As$_2$ and its sister compounds. Here we report a systematic elastoresistance study of series of isovalent- and electron-doped KFe$_2$As$_2$ crystals. We found divergent elastoresistance upon cooling for all the crystals along their [110] direction. The amplitude of elastoresistivity diverges if K is substituted with larger ions or if the system is driven towards a Lifshitz transition. However, w…
A two-orbital model for Fe-pnictide superconductors is investigated using computational techniques on two-dimensional square clusters. The hopping amplitudes are derived from orbital overlap integrals, or by band structure fits, and the spin frustrating effect of the plaquette-diagonal Fe-Fe hopping is remarked. A spin 'striped' state is stable in a broad range of couplings in the undoped regime, in agreement with neutron scattering. Adding two electrons to the undoped ground state of a small cluster, the dominant pairing operators are found. Depending on parameters, two pairing operators were…
Possibility of unconventional pairing due to Coulomb interaction in iron-pnictide superconductors is studied by applying a perturbative approach to realistic 2- and 5-band Hubbard models. The linearized Eliashberg equation is solved by expanding the effective pairing interaction perturbatively up to third order in the on-site Coulomb integrals. The numerical results for the 5-band model suggest that the eigenvalues of the Eliashberg equation are sufficiently large to explain the actual high Tc for realistic values of Coulomb interaction and the most probable pairing state is spin-singlet s-wav…
Orbital polarization and electronic correlation are two essential aspects in understanding the normal state and superconducting properties of multiorbital FeAs-based superconductors. In this Letter, we present a systematical study on the orbital polarization of iron pnictides from weak to strong Coulomb correlations within the KRSB approach. The magnetic phase diagram of the two-orbital model for LaFeAsO clearly shows that the striped antiferromagnetic metallic phase with considerable orbital polarization exists over a wide doping range. The reversal of the orbital polarization occurs in the i…
Weak-coupling approaches to the pairing problem in the iron pnictide superconductors have predicted a wide variety of superconducting ground states. We argue here that this is due both to the inadequacy of certain approximations to the effective low-energy band structure, and to the natural near-degeneracy of different pairing channels in superconductors with many distinct Fermi surface sheets. In particular, we review attempts to construct two-orbital effective band models, the argument for their fundamental inconsistency with the symmetry of these materials, and the comparison of the dynamic…
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 nature of the pairing state in iron-based superconductors is the subject of much debate. Here we argue that in one material, the stoichiometric iron pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state, characterized by symmetry-imposed vertical line nodes in the superconducting gap. This evidence is reviewed, with a focus on thermal conductivity and the strong impact of impurity scattering on the critical temperature Tc. We then compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the pairing symmetry is s-wave and the Tc is ten times higher. The t…
We investigate the two-orbital Hubbard model, which reproduces the electron and hole Fermi surfaces in the iron pnictide superconductors, in the presence of the Jahn-Teller electron-phonon coupling by using the dynamical mean-field theory. When the intra- and inter-orbital Coulomb interactions, $U$ and $U'$, increase with $U=U'$, both the local spin and orbital susceptibilities, $χ_{s}$ and $χ_{o}$, increase with $χ_{s}=χ_{o}$ because of the spin-orbital symmetry. Due to the Hund's rule coupling $J$, $χ_{s}$ is enhanced and dominates over $χ_{o}$ resulting in the repulsive local pairing intera…
We examine the nature of Josephson junction between two identical Fe-pnictides with anti-phase s-wave pairing. pi-phase shift is found if the junction barrier is thick and the two Fe-pnictides are oriented in certain directions relative to the interface. Our theory provides a possible explanation for the observed half integer flux quantum transitions in a niobium/polycrystal NdFeAsO loop, and attributes the pi-phase shift to intergrain junctions of Fe-pnictides.
We study the symmetry and strength of the superconducting pairing in a two-orbital t-J1-J2-K model for iron pnictides using the salve boson mean-field theory. We show that the nearest-neighbor biquadratic interaction -K (S_i * S_j)^2 inflences the superconducting pairing phase diagram by promoting the d_{x2-y2} B_{1g} and the s_{x^2+y^2} A_{1g} channels. The resulting phase diagram consists of several competing pairing channels, including an isotropic s_{\pm} A_{1g} channel, an anisotropic d_{x2-y2} B_{1g} channel, and two s+id pairing channels. We have investigated the evolution of supercondu…
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 propose a trilayer $π$-junction that takes advantage of the unconventional $s_{x^2 y^2}=\cos k_x \cos k_y$ pairing symmetry which changes sign between electron and hole Fermi pockets in the iron pnictides. In addition, we also present theoretical results for Andreev bound states in thin superconductor-normal metal (or insulator) iron-pnictide junctions. The presence of nontrivial in-gap states, which uniquely appear in this unconventional pairing state, is a distinct feature in comparison to other singlet pairing states.
In several members of the pnictide materials, spin density wave order coexists with superconductivity over a range of dopings and temperature. In this paper we show that odd-frequency superconductivity emerges on the edges of pnictides in such a coexistence phase. In particular, the breaking of spin-rotation symmetry by SDW and translation symmetry by the edge can lead to the development of odd-frequency spin-triplet Cooper pairing on edges of superconducting pnictide samples. In this case, the odd-frequency pairing has even parity components, which are immune to disorder. Our results show tha…
Doping dependence of the spin fluctuations and the electron correlations in the effective five-band Hubbard model for iron pnictides is investigated using the fluctuation-exchange approximation. For a moderate hole doping, we find a dominant low-energy spin excitation at Q=(π,0), which becomes critical at low temperature. The low-energy spin excitations in the heavily hole-doped region are characterized by weak Q dependence. The electron doping leads to an appearance of a pseudogap in spin excitation spectrum. Correspondingly, the NMR-1/T1 relaxation rate is strongly enhanced on the hole-doped…
We constructed an effective tight-binding model with five Cr $3d$ orbitals for LaOCrAs according to first-principles calculations. Basing on this model, we investigated possible superconductivity induced by correlations in doped LaOCrAs using the functional renormalization group (FRG). We find that there are two domes of superconductivity in electron-doped LaOCrAs. With increasing electron doping, the ground state of the system evolves from G-type antiferromagnetism in the parent compound to an incipient $s_\pm$-wave superconducting phase dominated by electron bands derived from the $d_{3z^2-r…
We have observed the Josephson effect in junctions formed between single crystals of SrFe1.74Co0.26As2 and Ba0.23K0.77Fe2As2. I-V curves showed resistively-shunted junction characteristics, and the ac Josephson effect was observed under microwave irradiation. By applying an in-plane magnetic field, the critical current is completely modulated and shows a relatively symmetric diffraction pattern, consistent with the intermediate junction limit. The observation of the Josephson effect in the p-n bicrystal structure not only has significant implications for designing phase-sensitive junctions to …
The discovery of the new class of pnictide superconductors has engendered a controversy about their pairing symmetry, with proposals ranging from an extended s-wave or "s$_{\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.
In most magnetically-ordered iron pnictides, the magnetic moments lie in the FeAs planes, parallel to the modulation direction of the spin stripes. However, recent experiments in hole-doped iron pnictides have observed a reorientation of the magnetic moments from in-plane to out-of-plane. Interestingly, this reorientation is accompanied by a change in the magnetic ground state from a stripe antiferromagnet to a tetragonal non-uniform magnetic configuration. Motivated by these recent observations, here we investigate the origin of the spin anisotropy in iron pnictides using an itinerant microsc…
The nature of the normal state and the mechanism of superconductivity in two families of high-temperature superconductors, cuprates and pnictides, remain a matter of intense discussions. According to band-structure calculations, confirmed by ARPES data, the electronic structure of pnictides differs considerably from that of cuprates. However, in spite of these differences, it looks like in both cases there exists some general and fairly "coarse" mechanism independent of fine details of the band structure and responsible for superconductive pairing in these materials. Here, we suggest a qualita…
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…
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 …
We studied two BaFe2-xNixAs2 (Ni-doped Ba-122) single crystals at two dfferent doping levels (underdoped and optimally doped) using an optical spectroscopic technique. The underdoped sample shows a magnetic phase transition around 80 K. We analyze the data with a Drude-Lorentz model with two Drude components (D1 and D2). It is known that the narrow D1 component originates from electron carriers in the electron-pockets and the broad D2 mode is from hole carriers in the hole-pockets. While the plasma frequencies of both Drude components and the static scattering rate of the broad D2 component sh…
Some of the iron pnictides show coexisting superconductivity and spin-density-wave order. We study the superconducting pairing instability in the spin-density-wave phase. Assuming that the pairing interaction is due to spin fluctuations, we calculate the effective pairing interactions in the singlet and triplet channels by summing the bubble and ladder diagrams taking the reconstructed band structure into account. The leading pairing instabilities and the corresponding superconducting gap structures are then obtained from the superconducting gap equation. We illustrate this approach for a mini…
The longitudinal in-plane magnetoresistance (LMR) has been measured in different Ba(Fe_(1-x)Co_x)2As2 single crystals and in LiFeAs. For all these compounds, we find a negative LMR in the paramagnetic phase whose magnitude increases as H^2. We show that this negative LMR can be readily explained in terms of suppression of the spin fluctuations by the magnetic field. In the Co-doped samples, the absolute value of the LMR coefficient is found to decrease with doping content in the paramagnetic phase. The analysis of its T dependence in an itinerant nearly antiferromagnetic Fermi liquid model evi…
The recent discovery of iron-based superconductors challenges the existing paradigm of high-temperature superconductivity. Owing to their unusual multi-orbital band structure, magnetism, and electron correlation, theories propose a unique sign reversed s-wave pairing state, with the order parameter changing sign between the electron and hole Fermi pockets. However, because of the complex Fermi surface topology and material related issues, the predicted sign reversal remains unconfirmed. Here we report a novel phase-sensitive technique for probing unconventional pairing symmetry in the polycrys…
Motivated by the bad metal behavior of the iron pnictides, we study a multi-orbital $\mathrm{t-J_1-J_2}$ model and investigate possible singlet superconducting pairings. Magnetic frustration by itself leads to a large degeneracy in the pairing states. The kinetic energy breaks this into a quasi-degeneracy among a reduced set of pairing states. For small electron and hole Fermi pockets, an $A_{1g}$ state dominates over the phase diagram but a $B_{1g}$ state has close-by energy. In addition to the nodeless $A_{1g}$ $s_{x^2y^2}$ channel, the nodal $A_{1g}$ $s_{x^2+y^2}$ and $B_{1g}$ $d_{x^2-y^2}$…
The high-transition-temperature (high-Tc) superconductivity discovered recently in iron pnictides is analyzed within a perturbation theory. Specifically, the probable pairing symmetry, the doping dependence of the transition temperature and the pairing mechanism are studied by solving the Eliashberg equation for multi-band (2- and 5-band) Hubbard models with realistic electronic structures. The effective pairing interaction is expanded perturbatively in the on-site Coulomb integrals up to third order. Our perturbative weak-coupling approach shows that sufficiently large eigenvalues of the Elia…
In this paper, we investigate analytically the experimental observations of structural collapse and Tc enhancement in the rare earth-doped 122-type iron-based pnictide superconductors [S. R. Saha et.al. Phys. Rev. B 85, 024525 (2012), arXiv:1105.4798 (2011); B. Lv et al. PNAS 108, 15705 (2011).]. Based on the real-space effective c-axis lattice constant theory of superconductivity [X. Q. Huang, arXiv:1001.5067], it is shown that the abrupt c-axis reduction of the superconductors is due to the structural phase transition (an ultra-short-range symmetry breaking) of the charge stripe lattice. The…
We examine the possibility that the superconductivity in the newly discovered FeAs materials may be caused by the Coulomb interaction between d-electrons of the iron atoms. We find that when the Hund's rule ferromagnetic interaction is strong enough, the leading pairing instability is in spin-triplet p-wave channel in the weak coupling limit. The resulting superconducting gap has nodal lines on the 3D Fermi surfaces. The k dependent hybridization of several orbitals around a Fermi pocket is the key for the appearance of the spin-triplet p-wave pairing.
Anisotropies of electrical resistivity, upper critical field, London penetration depth and critical currents have been measured in single crystals of the optimally doped iron pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, $x$=0.074 and $T_c \sim$23 K. The normal state resistivity anisotropy was obtained by employing both the Montgomery technique and direct measurements on samples cut along principal crystallographic directions. The ratio $γ_ρ = ρ_c /ρ_a$ is about 4$\pm$1 just above $T_c$ and becomes half of that at room temperature. The anisotropy of the upper critical field, $γ_{H} =…
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…
Various recent experimental data and especially the large Fe-isotope effect point against unconventional pairings, since the large intra-band impurity scattering is strongly pair-breaking for them. The strength of the inter-band impurity scattering in some single crystals may be strong and probably beyond the Born scattering limit. In that case the proposed s(+-) pairing (hole(h)- and electron(el)-gaps are of opposite signs) is suppressed but possibly not completely destroyed. The data imply that the intra-band pairing in the h- and in the el-band, which are inevitably due to some nonmagnetic …
In Ba(Fe0.95Co0.05)2As2 all of the 75As NMR intensity at the paramagnetic resonance position vanishes abruptly below Tonset(SDW)=56 K, indicating that magnetic (spin density wave) order is present in all of the sample volume, despite bulk superconductivity below Tc=15 K. The two phases thus coexist homogeneously at the microscopic scale. In Ba0.6K0.4Fe2As2, on the other hand, the signal loss below Tonset(SDW)~75 K is not complete, revealing that magnetic order is bound to finite-size areas of the sample, while the remaining NMR signal shows a clear superconducting response below Tc=37 K. Thus,…
A leading contender for the pairing symmetry in the Fe-pnictide high temperature superconductors is extended s-wave $s_\pm$, a nodeless state in which the pairing changes sign between Fermi surfaces. Verifying such a pairing symmetry requires a special phase sensitive probe that is also momentum selective. We show that the sign structure of $s_\pm$ pairing leads to surface Andreev bound states at the sample edge. In the clean limit they only occur when the edge is along the nearest neighbor Fe-Fe bond, but not for a diagonal edge or a surface orthogonal to the c-axis. In contrast to d-wave 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 suggest that an inhomogeneous (non-zero momentum) paired phase can appear in the absence of an external magnetic field in the system with a predominant interband pairing and with separate Fermi-surface sheets. The Fermi wave vector mismatch which appears in such situation can be compensated by nonzero center-of-mass momentum of the Cooper pairs, what can lead to a spontaneous appearance of the Fulde-Ferrell type of superconducting state. The idea is examined using a tight-binding model which emulates the hole-like and the electron-like bands of iron based superconductor. The state can appea…
We use transport and magnetization measurements to study the effect of Cr-doping to the phase diagram of the electron-doped superconducting BaFe2-xNixAs2 iron pnictides. In principle, adding Cr to electron-doped BaFe2-xNixAs2 should be equivalent to the effect of hole-doping. However, we find that Cr doping suppresses superconductivity via impurity effect, while not affecting the normal state resistivity above 100 K. We establish the phase diagram of Cr-doped BaFe2-x-yNixCryAs2 iron pnictides, and demonstrate that Cr-doping near optimal superconductivity restore the long-range antiferromagneti…
We show that the competition between magnetism and superconductivity can be used to determine the pairing state in the iron arsenides. To this end we demonstrate that the itinerant antiferromagnetic phase (AFM) and the unconventional $s^{+-}$ sign-changing superconducting state (SC) are near the borderline of microscopic coexistence and macroscopic phase separation, explaining the experimentally observed competition of both ordered states. In contrast, conventional $s^{++}$ pairing is not able to coexist with magnetism. Expanding the microscopic free energy of the system with competing orders …
In copper-oxide and iron-based high temperature (high-$T_{\rm c}$) superconductors, many physical properties exhibit in-plane anisotropy, which is believed to be caused by a rotational symmetry-breaking nematic order, whose origin and its relationship to superconductivity remain elusive. In many iron-pnictides, a tetragonal-to-orthorhombic structural transition temperature $T_{\rm s}$ coincides with the magnetic transition temperature $T_{\rm N}$, making the orbital and spin degrees of freedom highly entangled. NaFeAs is a system where $T_{\rm s}$ = 54 K is well separated from $T_{\rm N}$ = 42…
One of the major themes in the physics of condensed matter is unconventional superconductivity in correlated electronic systems. The symmetry structure of Cooper pairs is thought to be the key to the understanding of the pairing mechanism of their superconductivity. In iron pnictide superconductors, the precise symmetry of the order parameter as well its evolution with doping remains highly controversial. There is no general consensus on the nature of pairing in iron-based superconductors leaving the perspectives ranging from $s^{++}$ wave, to $s^{\pm}$, and $d$ wave. Here we show that doping …
We study the orbital-dependent superconducting pairing in a five-orbital t-J1-J2 model for iron pnictides. Depending on the orbital selectivity of electron correlations and the orbital characters along the Fermi surface, the superconducting gap in an A_{1g} pairing state may exhibit anisotropy. This anisotropy varies with the degree of J1-J2 magnetic frustration. We have also calculated the dynamical spin susceptibility in the superconducting state. The frequency dependence of the susceptibility at the antiferromagnetic wavevector (π,0) shows a resonance, whose width is enhanced by the orbital…
Based on a phenomenological model with competing spin-density-wave (SDW) and extended $s-$wave superconductivity, the vortex states in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ are investigated by solving Bogoliubov-de Gennes equations. Our result for the optimally doped compound without induced SDW is in qualitative agreement with recent scanning tunneling microscopy experiment. We also propose that the main effect of the SDW on the vortex states is to reduce the intensity of the in-gap peak in the local density of states and transfer the spectral weight to form additional peaks outside the gap.
The rich variety of iron-based superconductors and their complex electronic structure lead to a wide range of possibilities for gap symmetry and pairing components. Here we solve in the 2-Fe Brillouin zone the full frequency-dependent linearized Eliashberg equations for LiFeAs with spin-fluctuation mediated pairing interactions. The magnetic excitations are calculated with the random phase approximation on a correlated electronic structure obtained with density functional theory and dynamical mean field theory. Correlations induce long-lived local moments with orbital-dependent dynamics. The i…
The effects of twin boundaries (TBs) on the complex interaction between magnetism and superconductivity in slightly electron-doped Ba(Ca)(FeAs)$_2$ superconductors are investigated. The spatial distributions of the magnetic, superconducting and charge density orders near two different types of TBs are calculated. We find that TBs corresponding to a 90$^\circ$ lattice rotation in the a-b plane enable magnetic domain walls to form with only a small effective Coulomb interaction between valance electrons, and that superconductivity is enhanced at such TBs. Contrastingly, we find that superconduct…
We use the functional renormalization group method to analyze the phase diagram of a 4-band model for the iron-pnictides subject to band interactions with certain A_{1g} momentum dependence. We determine the parameter regimes where an extended s-wave pairing instability with and without nodes emerges. On the electron-doped side, the parameter regime in which a nodal gap appears is found to be much narrower than recently predicted in arXiv:0903.5547. On the hole-doped side, the extended s-wave pairing never becomes nodal: above a critical strength of the intra-band repulsion, the system favors …
Point contact spectroscopy (PCS) is a technique which can reveal the size and symmetry of a superconducting gap ($Δ$) and is especially useful for new materials such as the iron-based superconductors. PCS is usually employed in conjunction with the extended Blonder-Tinkham-Klapwijk (BTK) model which is used to extract information such as the existence of nodes in $Δ$ from PCS data obtained on unconventional superconductors. The BTK model uses a dimensionless parameter $Z$ to quantify the barrier strength across a normal metal - superconductor junction. We have used a unique feature of PCS whic…
We investigate the electronic state and the superconductivity in the 5-orbital Hubbard model for iron pnictides by using the dynamical mean-field theory in conjunction with the Eliashberg equation. The renormalization factor exhibits significant orbital dependence resulting in the large change in the band dispersion as observed in recent ARPES experiments. The critical interactions towards the magnetic, orbital and superconducting instabilities are suppressed as compared with those from the random phase approximation (RPA) due to local correlation effects. Remarkably, the s++-pairing phase due…
We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the undoped and electron-doped iron pnictides BaFe2\_{-x}CoxAs2 (Ba122) (x=0, 0.14) and studied the Fermi surfaces (FSs) and band dispersions near the Fermi level. The FS sheets we observed are consistent with the shrinkage of the hole-like pockets around the Brillouin Zone (BZ) center and the expansion of the electron pockets around the BZ corner in the electron-doped compound as compared to the undoped parent compound. Band dispersions and FSs around the BZ center strongly depend on the photon energy, indicating …
I describe the issues of the competing orders in normal state of a parent and Ni-doped iron pnictide superconductor, SrF e 2 As 2 , using LMTO band structure calculation plus multi orbital dynamical mean-field theory. Strong, electronic correlations along with minimal antiferromagnetic instability create a partially gapped Fermi surface, in qualitative agreement with earlier experiments. A good semiquantitative similarity in both normal and ordered state with the earlier experimental transport data is observed along with suppression of magnetic ordering and explained within a correlated, mul- …
We establish in a combination of ab initio theory and experiments that the tunneling process in scanning tunneling microscopy/spectroscopy on the A-122 iron pnictide superconductors - in this case BaFe$_{2-x}$Co$_x$As$_2$ - involve a strong adatom filtering of the differential conductance from the near-EF Fe3d states, which in turn originates from the top-most sub-surface Fe layer of the crystal. The calculations show that the dominance of surface Ba-related tunneling pathways leaves fingerprints found in the experimental differential conductance data, including large particle-hole asymmetry a…
