Novel superconducting characteristics and unusual normal-state properties in iron-based pnictide superconductors: 57FeNMR and 75AsNQR/NMR studies in REFeAsO1−y (RE = La,Pr, Nd) and Ba0.6K0.4Fe2As2

We discuss the novel superconducting characteristics and unusual normal-state properties of iron (Fe)-based pnictide superconductors REFeAsO_1?y (RE = La, Pr, Nd) and Ba_0.6K_0.4Fe₂As₂ (T_c = 38 K) by means of ⁵⁷FeNMR and ⁷⁵AsNQR/NMR. In the superconducting state of LaFeAsO_0.7 (T_c = 28 K), the spin component of the ⁵⁷Fe-Knight shift decreases to almost zero at low temperatures, which provide firm evidence of the superconducting state formed by spin-singlet Cooper pairing. The nuclear spin–lattice relaxation rates (1/T₁) in LaFeAsO_0.7 and Ba_0.6K_0.4Fe₂As₂ exhibit a T³-like dependence without a coherence peak just below T_c, indicating that an unconventional superconducting state is commonly realized in these Fe-based pnictide compounds. All these events below T_c are consistently argued in terms of an extended s_±-wave pairing with a sign reversal of the order parameter among Fermi surfaces. In the normal state, 1/T₁T decreases remarkably upon cooling for both the Fe and As sites of LaFeAsO_0.7. In contrast, it gradually increases upon cooling in Ba_0.6K_0.4Fe₂As₂. Despite the similarity between the superconducting properties of these compounds, a crucial difference was observed in their normal-state properties depending on whether electrons or holes are doped into the FeAs layers. These results may provide some hint to address a possible mechanism of Fe-based pnictide superconductors.     

Ni2X2 (X = pnictide,chalcogenide, or B) based superconductors

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 T_c 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.     

High pressure studies on Fe-pnictide superconductors

A review of high pressure studies on Fe-pnictide superconductors is given. The pressure effects on the magnetic and superconducting transitions are discussed for different classes of doped and undoped FeAs-compounds: ROFeAs (R = rare-earth), AeFe₂As₂ (Ae = Ca, Sr, Ba), and AFeAs (A = Li, Na). Pressure tends to decrease the magnetic transition temperature in the undoped or only slightly doped compounds. The superconducting T_c increases with low pressure for underdoped FeAs-pnictides, remains approximately constant for optimal doping, and decreases linearly in the overdoped range. The undoped LaOFeAs and AeFe₂As₂ become superconducting under pressure although non-hydrostatic pressure condition seems to play a role in CaFe₂As₂. The superconductivity in the (undoped) AFeAs is explained as a chemical pressure effect due to the volume contraction caused by the small ionic size of the A-elements. The binary FeSe shows the largest pressure coefficient of T_c in the Se-deficient superconducting phase.     

Non-magnetic impurity effect on the optimally carrier doped superconductor BaFe1.87Co0.13As2 prepared at ambient pressure

We report properties of the Zn-substituted polycrystalline superconductors BaFe_1.87?xZn_xCo_0.13As₂ (x = 0, 0.04, 0.06 and 0.08) prepared at ambient pressure condition. Electrical conductivity and magnetization measurements revealed that the superconductivity is suppressed by at most 2% substitution of Zn, which shows typical behavior of an anisotropically gaped superconductivity rather than an isotropically gaped superconductivity. With increasing the amount of Zn, weak change in superconductivity was observed. It is likely impossible to obtain BaFe_1.87?xZn_xCo_0.13As₂ crystals with x > 0.04 at ambient pressure condition. On the basis of the structural and the physical properties of the present system, we discuss a possible mechanism of the superconducting pairing.     

Comparison of the local structures of Ca0.82La0.18FeAs2 and Ba0.64K0.36Fe2As2 pnictide superconductors using atomic pair distribution function analysis

A comparative local structure study of pnictide superconductors Ca_0.82La_0.18FeAs₂ (112-type, T_c∼ 40 K) and Ba_0.64K_0.36Fe₂As₂ (122-type, T_c∼ 37 K), using room temperature x-ray total scattering measurements is reported. The Fe–As superconducting active layer is found to be globally similar in both the systems consisting of edge-sharing FeAs_4/4 tetrahedra as in all the iron-pnictide superconductors discovered so far. Although optimally superconducting, the active layer in these compounds is found to sustain a large local inhomogeneity. These results thus imply that a nanoscopic manipulation of the Fe–As active layer, rather than its isotropic structural tuning, is the key parameter to control the superconducting properties of the iron-based systems.     

Search for an effect of superconductivity on the phonons in Ba(Fe1−xCox)2As2

Inelastic neutron scattering was used to search for an influence of superconductivity on the phonons in optimally doped and in slightly overdoped Ba(Fe_1?xCo_x)₂As₂, x = 0.06 and x = 0.10. The study focused on phonons with energies close to the superconducting gap energy 2Δ because it is well known that such phonons will respond most strongly to the opening of the gap. We were able to obtain high quality data but nevertheless, we could not detect any influence of superconductivity on the phonons, neither on the linewidths nor on the frequencies. Our results imply that any coupling of low energy phonons to the electrons has to be very small, much smaller than observed in conventional superconductors with a high T_c. Our results are in line with the low coupling strength predicted by density functional theory for the investigated phonon branches.     

Surface states and tunneling spectroscopy of high-Tcsuperconductors

Recent studies of high- T_csuperconductors have clarified new aspects of tunneling spectroscopy. The unconventional pairing states, i.e. d-wave symmetry in these materials have been established through various measurements. Differently from isotropic s-wave superconductors, d-wave pairing states have an internal phase of the pair potential. The internal phase modifies the surface states due to the interference effect of the quasiparticles. Along these lines, a novel formula of tunneling spectroscopy has been presented that fully takes into account of the anisotropy of the pair potential. The most essential difference of this formula from conventional ones is that it suggests the phase-sensitive capability of tunneling spectroscopy. The formula suggests that the symmetry of the pair potential is determined by the orientational dependence measurements of tunneling spectroscopy. Along these lines, several experiments have been performed on high-T_c superconductors. The observation of the zero-bias conductance peaks (ZBCP) on Y Ba₂Cu₃O_7 − δstrongly suggests the d_{x² − y²}-wave pairing states of hole-doped high-T_c superconductors. On the other hand, the absence of ZBCP on (electron-doped)Nd_1.85Ce_0.15CuO_4 − δindicates that the pair potential of this material is a nodeless state. In this paper, recent developments of tunneling spectroscopy for anisotropic superconductors are reviewed both on theoretical and experimental aspects.     

Neutron studies of the iron-based family of high TC magnetic superconductors

We review neutron scattering investigations of the crystal structures, magnetic structures, and spin dynamics of the iron-based RFe(As, P)(O, F) (R = La, Ce, Pr, Nd), (Ba,Sr,Ca)Fe₂As₂, and Fe_1+x(Te–Se) systems. On cooling from room temperature all the undoped materials exhibit universal behavior, where a tetragonal-to-orthorhombic/monoclinic structural transition occurs, below which the systems become antiferromagnets. For the first two classes of materials the magnetic structure within the a–b plane consists of chains of parallel Fe spins that are coupled antiferromagnetically in the orthogonal direction, with an ordered moment typically less than one Bohr magneton. Hence these are itinerant electron magnets, with a spin structure that is consistent with Fermi-surface nesting and a very energetic spin wave bandwidth ～0.2 eV. With doping, the structural and magnetic transitions are suppressed in favor of superconductivity, with superconducting transition temperatures up to ≈55 K. Magnetic correlations are observed in the superconducting regime, with a magnetic resonance that follows the superconducting order parameter just like the cuprates. The rare earth moments order antiferromagnetically at low T like ‘conventional’ magnetic superconductors, while the Ce crystal field linewidths are affected when superconductivity sets in. The application of pressure in CaFe₂As₂ transforms the system from a magnetically ordered orthorhombic material to a ‘collapsed’ non-magnetic tetragonal system. Tetragonal Fe_1+xTe transforms to a low T monoclinic structure at small x that changes to orthorhombic at larger x, which is accompanied by a crossover from commensurate to incommensurate magnetic order. Se doping suppresses the magnetic order, while incommensurate magnetic correlations are observed in the superconducting regime.     

BaT2As2 single crystals (T = Fe,Co, Ni) and superconductivity upon Co-doping

The crystal structure and physical properties of BaFe₂As₂, BaCo₂As₂, and BaNi₂As₂ single crystals are surveyed. BaFe₂As₂ gives a magnetic and structural transition at T_N = 132(1) K, BaCo₂As₂ is a paramagnetic metal, while BaNi₂As₂ has a structural phase transition at T₀ = 131 K, followed by superconductivity below T_c = 0.69 K. The bulk superconductivity in Co-doped BaFe₂As₂ below T_c = 22 K is demonstrated by resistivity, magnetic susceptibility, and specific heat data. In contrast to the cuprates, the Fe-based system appears to tolerate considerable disorder in the transition metal layers. First principles calculations for BaFe_1.84Co_0.16As₂ indicate the inter-band scattering due to Co is weak.     

Inelastic neutron scattering studies of the spin and lattice dynamics in iron arsenide compounds

Although neutrons do not couple directly to the superconducting order parameter, they have nevertheless played an important role in advancing our understanding of the pairing mechanism and the symmetry of the superconducting energy gap in the iron arsenide compounds. Measurements of the spin and lattice dynamics have been performed on non-superconducting ‘parent’ compounds based on the LaFeAsO (‘1111’) and BaFe₂As₂ (‘122’) crystal structures, and on electron and hole-doped superconducting compounds, using both polycrystalline and single crystal samples. Neutron measurements of the phonon density-of-state, subsequently supported by single crystal inelastic X-ray scattering, are in good agreement with ab initio calculations, provided the magnetism of the iron atoms is taken into account. However, when combined with estimates of the electron–phonon coupling, the predicted superconducting transition temperatures are less than 1 K, making a conventional phononic mechanism for superconductivity highly unlikely. Measurements of the spin dynamics within the spin density wave phase of the parent compounds show evidence of strongly dispersive spin waves with exchange interactions consistent with the observed magnetic order and a large anisotropy gap. Antiferromagnetic fluctuations persist in the normal phase of the superconducting compounds, but they are more diffuse. Below T_c, there is evidence in three ‘122’ compounds that these fluctuations condense into a resonant spin excitation at the antiferromagnetic wavevector with an energy that scales with T_c. Such resonances have been observed in the high-T_c copper oxides and a number of heavy fermion superconductors, where they are considered to be evidence of d-wave symmetry. In the iron arsenides, they also provide evidence of unconventional superconductivity, but a comparison with ARPES and other measurements, which indicate that the gaps are isotropic, suggests that the symmetry is more likely to be extended-s_± wave in character.     

Inelastic neutron scattering on iron-based superconductor BaFe2(As,P)2

Inelastic neutron scattering has been performed on powder sample of an iron-based superconductor BaFe₂(As_0.65P_0.35)₂ with superconducting transition temperature (T_c) = 30 K, whose superconducting (SC) order parameter is expected to have line node. In the normal state, constant-E scan of dynamical structure factor, S(Q, E), exhibits a peak structure centered at momentum transfer Q ～ 1.20 Å^?1, corresponding to antiferromagnetic wave vector. Below T_c, the redistribution of the magnetic spectral weight takes place, resulting in the formation of a peak at E ～ 12 meV and a gap below 6 meV. The enhanced magnetic peak structure is ascribed to the spin resonance mode, evidencing sign change in the SC order parameter similar to other iron-based high-T_c superconductors. It suggests that fully-gapped s_± symmetry dominates in this superconductor, which gives rise to high-T_c (=30 K) despite the nodal symmetry.     

Renormalization group analysis of competing orders and the pairing symmetry in Fe-based superconductors

We analyze antiferromagnetism and superconductivity in novel Fe-based superconductors within the weak-coupling, itinerant model of electron and hole pockets near (0, 0) and (π, π) in the folded Brillouin zone. We discuss the interaction Hamiltonian, the nesting, the RG flow of the couplings at energies above and below the Fermi energy, and the interplay between SDW magnetism, superconductivity and charge orbital order. We argue that SDW antiferromagnetism wins at zero doping but looses to superconductivity upon doping. We show that the most likely symmetry of the superconducting gap is A_1g in the folded zone. This gap has no nodes on the Fermi surface but changes sign between hole and electron pockets. We also argue that at weak coupling, this pairing predominantly comes not from spin fluctuation exchange but from a direct pair hopping between hole and electron pockets.     

Effect of the distortion of FeX4 (X = P,As) tetrahedron for the electronic structure of iron-pnictide system

We have performed an ab initio band structure calculation for the new high-T_c related iron-pnictide compounds LaFeXO (X = P, As), BaFe₂As₂, CaFe₂As₂ and LiFeAs (X = P, As). We found that LaFeXO and CaFe₂As₂ have many similarities in their band structures, which is expected by an ionic model. We found that the degree of distortion of FeAs₄ tetrahedra in LaFeAsO considerably changes the slope of the density of states near the Fermi level, and this result may explain why REFeAsO (RE = Nd, Sm, …) have higher T_c than LaFeAsO when electrons are doped. For all the above compounds, the density of states at the Fermi level decreases when X atoms approaches to the Fe–Fe plane, which means that the hybridization between Fe and X orbitals considerably expands the Fe d-bands.     

Point-contact Andreev-reflection spectroscopy in ReFeAsO1−xFx (Re = La,Sm): Possible evidence for two nodeless gaps

A deep understanding of the character of superconductivity in the recently discovered Fe-based oxypnictides ReFeAsO_1?xF_x (Re = rare-earth) necessarily requires the determination of the number of the gaps and their symmetry in k space, which are fundamental ingredients of any model for the pairing mechanism in these new superconductors. In the present paper, we show that point-contact Andreev-reflection spectroscopy experiments performed on LaFeAsO_1?xF_x (La-1111) polycrystals with T_c ～ 27 K and SmFeAsO_0.8F_0.2 (Sm-1111) polycrystals with T_c ～ 53 K gave differential conductance curves exhibiting two peaks at low bias and two additional structures (peaks or shoulders) at higher bias voltages, an experimental situation quite similar to that observed by the same technique in pure and doped MgB₂. The single-band Blonder–Tinkham–Klapwijk model is totally unable to properly fit the conductance curves, while the two-gap one accounts remarkably well for the shape of the whole experimental dI/dV vs. V curves. These results give direct evidence of two nodeless gaps in the superconducting state of ReFeAsO_1?xF_x (Re = La, Sm): a small gap, Δ₁, smaller than the BCS value (2Δ₁/k_BT_c ～ 2.2–3.2) and a much larger gap Δ₂ which gives a ratio 2Δ₂/k_BT_c ～ 6.5–9.0. In Sm-1111 both gaps close at the same temperature, very similar to the bulk T_c, and follow a BCS-like behaviour, while in La-1111 the situation is more complex, the temperature dependence of the gaps showing remarkable deviations from the BCS behaviour at T close to T_c.The normal-state conductance reproducibly shows an unusual, but different, shape in La-1111 and Sm-1111 with a depression or a hump at zero bias, respectively. These structures survive in the normal state up to T¹ ～ 140 K, close to the temperatures at which structural and magnetic transitions occur in the parent, undoped compound.     

A brief review on <Emphasis Type="Italic">μ</Emphasis>SR studies of unconventional Fe- and Cr-based superconductors

Muon spin relaxation/rotation (μSR) is a vital technique for probing the superconducting gap structure, pairing symmetry and time reversal symmetry breaking, enabling an understanding of the mechanisms behind the unconventional superconductivity of cuprates and Fe-based high-temperature superconductors, which remain a puzzle. Very recently double layered Fe-based super- conductors having quasi-2D crystal structures and Cr-based superconductors with a quasi-1D structure have drawn considerable attention. Here we present a brief review of the characteristics of a few selected Fe- and Cr-based superconducting materials and highlight some of the major outstanding problems, with an emphasis on the superconducting pairing symmetries of these materials. We focus on μSR studies of the newly discovered superconductors ACa₂Fe₄As₄F₂ (A = K, Rb, and Cs), ThFeAsN, and A₂Cr₃As₃ (A = K, Cs), which were used to determine the superconducting gap structures, the presence of spin fluctuations, and to search for time reversal symmetry breaking in the superconducting states. We also briefly discuss the results of μSR investigations of the superconductivity in hole and electron doped BaFe₂As₂.     

Point contact Andreev reflection spectroscopy of superconducting energy gaps in 122-type family of iron pnictides

A brief overview of the superconducting energy gap studies on 122-type family of iron pnictides is given. It seems that the situation in the hole doped Ba_1?xK_xFe₂As₂ is well resolved. Most of the measurements including the presented here point contact Andreev reflection spectra agree on existence of multiple nodeless gaps in the excitation spectrum of this multiband system. The gaps have basically two sizes – the small one with a strength up to the BCS weak coupling limit and the large one with a very strong coupling with 2Δ_L/kT_c > 6–8. In the electron doped Ba(Fe_1?xCo_x)₂As₂ the most of the experiments including our point contact measurements reveal in quite broadened spectra only a single gap with a strong coupling strength. The high precision ARPES measurements on this system identified two gaps but very close to each other, both showing a strong coupling with 2Δ/kT_c ～ 5 and 6, respectively.     

Elastic properties and chemical bonding in ternary arsenide SrFe2As2 and quaternary oxyarsenide LaFeAsO – Basic phases for new 38–55 K superconductors from first principles

Here we report the first-principle FLAPW-GGA calculations of the elastic properties of two related layered phases, namely, the ternary arsenide SrFe₂As₂ and the quaternary oxyarsenide LaFeAsO – basic phases for the newly discovered “1 2 2” and “1 1 1 1” 38–55 K superconductors. The independent elastic constants (C_ij), bulk moduli, compressibility, and shear moduli are evaluated and discussed. The numerical estimates of the elastic parameters of the polycrystalline SrFe₂As₂ and LaFeAsO ceramics are performed for the first time. Additionally, the peculiarities of chemical bonding in these phases are discussed.     

Fluctuation induced magneto-conductivity studies in YBa2Cu3O7−δ + xBaZrO3 composite high-Tc superconductors

Fluctuations on the electrical conductivity of polycrystalline YBa₂Cu₃O_7?δ + xBaZrO₃ (x = 1.0, 2.5, 5.0 and 10.0 wt.%) superconductors were investigated from the resistivity vs. temperature data for zero field and 8 T (Tesla) external magnetic fields. Attempts have been made to identify the optimum inclusion of BaZrO₃ (BZO) in YBa₂Cu₃O_7?δ (YBCO) superconductors. The phase formation, texture and grain alignments were analyzed by XRD and SEM techniques. Then the effects of superconducting fluctuations on the electrical conductivity of granular composite superconductors were studied for zero field and 8 T external magnetic fields. Though inclusions of BZO sub-micron particles are not expected to influence superconducting order-parameter fluctuation (SCOPF) much, the transition from 2D to 3D of the order parameter in the mean-field region depends on the BZO content in the composites. It has been observed that BZO residing at the grain boundary of YBCO matrix influences the tailing region without having significant change in the mean-field critical temperature. In the present work, attention has been focused mostly in the experimental domain relatively above the T_c. It reveals that, 1 wt.% composite exhibits a better superconducting property in comparison with pure YBCO.     

High-pressure synthesis and physical properties of new iron (nickel)-based superconductors

We have utilized a high-pressure (HP) technique to synthesize a series of newly-discovered iron (nickel)-based superconductors. For the LnFeAsO-based superconductors (Ln = lanthanide), we show that the introduction of oxygen (O)-deficiency in the LnO layers, which is achievable only through HP process, is an effective way to dope electron carriers into the system, which results in yielding the superconducting transition temperature (T_c) comparable with those for F-substituted counterpart. The effect of O-deficiency, variation of Ln ions, and the external pressure on T_c are examined. All the experimental data indicate strong correlation between the crystal structure and the superconductivity of the oxypnictide superconductors. Upper critical field measurement on single crystalline sample of PrFeAsO_1?y shows the superconducting anisotropy of 5, which is smaller than cuprates. We also demonstrate that HP technique is applicable for the so-called ‘122’ systems, by showing the results on polycrystalline (Ca, Na)Fe₂As₂, (Ba, K)Fe₂As₂, as well as single crystal BaNi₂P₂ samples.     

Single crystals of LnFeAsO1−xFx (Ln = La,Pr, Nd,Sm, Gd) and Ba1−xRbxFe2As2: Growth,structure and superconducting properties

A review of our investigations on single crystals of LnFeAsO_1?xF_x (Ln = La, Pr, Nd, Sm, Gd) and Ba_1?xRb_xFe₂As₂ is presented. A high-pressure technique has been applied for the growth of LnFeAsO_1?xF_x crystals, while Ba_1?xRb_xFe₂As₂ crystals were grown using a quartz ampoule method. Single crystals were used for electrical transport, structure, magnetic torque and spectroscopic studies. Investigations of the crystal structure confirmed high structural perfection and show incomplete occupation of the (O, F) position in superconducting LnFeAsO_1?xF_x crystals. Resistivity measurements on LnFeAsO_1?xF_x crystals show a significant broadening of the transition in high magnetic fields, whereas the resistive transition in Ba_1?xRb_xFe₂As₂ simply shifts to lower temperature. The critical current density for both compounds is relatively high and exceeds 2 × 10⁹ A/m² at 15 K in 7 T. The anisotropy of magnetic penetration depth, measured on LnFeAsO_1?xF_x crystals by torque magnetometry is temperature dependent and apparently larger than the anisotropy of the upper critical field. Ba_1?xRb_xFe₂As₂ crystals are electronically significantly less anisotropic. Point-Contact Andreev-Reflection spectroscopy indicates the existence of two energy gaps in LnFeAsO_1?xF_x. Scanning Tunneling Spectroscopy reveals in addition to a superconducting gap, also some feature at high energy (～20 meV).