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Highlights Three-gap superconductivity with possible momentum dependence (anisotropy) in LiFeAs. Temperature dependences of the superconducting gaps indicate a moderate interband coupling. Superconducting energy parameters scale with T c under minor lithium deficiency.

Highlights • T -dependence of local distortions in BaFe 2 As 2 by X-ray PDF and EXAFS methods. • PDF (EXAFS) data exhibit (do not exhibit) anomaly at the structure transition. • Data supporting the local orthorhombicity at 300 K (in the tetragonal phase) for BaFe 2 As 2. • Arguments on the origin(s) of the 4-fold symmetry-breaking in the macroscopically tetragonal phase. Abstract X-ray pair distribution function (PDF) and Extended X-ray absorption fine structure (EXAFS) measurements were carried out on two Fe-based systems BaFe 2 As 2 and LiFeAs to study mainly on the local structures of their FeAs planes. For the former system exhibiting the tetragonal (space group I4/mmm)-orthorhombic (Fmmm) structural phase transition at temperature T S ∼140 K, we show the T dependent characteristics of the local distortion from room temperature to the low temperature Fmmm phase, which support the existence of another local orthorhombic phase (I2mm) [Cantoni et al.: Adv. Mater. 27 (2015) 2715] at 300 K. We show the magnitudes of the local Fe atom shifts from their average Fe sites in these phases as the important information on what roles the lattice distortion has in the arguments on various physical properties of FeAs planes. For LiFeAs with the superconducting transition at ∼18 K but no structural transition, a small distortion has been found below 200 K. [ABSTRACT FROM AUTHOR]

First-principles methods combined with homogeneous deformation methods are used to investigate the second- and third- order elastic constants of LiFeAs and NaFeAs with tetragonal structure. The effective second-order elastic constants are obtained from the second- and third- order elastic constants for LiFeAs and NaFeAs. Pugh's modulus ratio, Poisson's ratio, Vickers hardness, elastic anisotropy and Debye temperature under high pressure are further investigated. The ductility increases and the hardness decreases with pressure, and the ductility of NaFeAs is better than that of LiFeAs and the hardness of NaFeAs is always lower than that of LiFeAs. For LiFeAs and NaFeAs, the shear anisotropy factor A { 100 } along the {100} plane decreases quickly and the shear anisotropy factor A { 001 } along the {001} plane increases with increasing pressure. [ABSTRACT FROM AUTHOR]

The onset of superconductivity at the transition temperature is marked by the onset of order, which is characterized by an energy gap. Most models of the iron-based superconductors find a sign-changing (s±) order parameter [1–6], with the physical implication that pairing is driven by spin fluctuations. Recent work, however, has indicated that LiFeAs has a simple isotropic order parameter [7–9] and spin fluctuations are not necessary [7,10], contrary to the models [1–6]. The strength of the spin fluctuations has been controversial [11,12], meaning that the mechanism of superconductivity cannot as yet be determined. We report the momentum dependence of the superconducting energy gap, where we find an anisotropy that rules out coupling through spin fluctuations and the sign change. The results instead suggest that orbital fluctuations assisted by phonons [13,14] are the best explanation for superconductivity.

Measurements of DC magnetization for single crystal specimens of LiFeAs have been performed under pressure using liquid argon and NaCl as pressure transmitting media (PTM) to generate hydrostatic and nearly uniaxial pressure along c -axis, respectively. It has been found that T c linearly decreases under pressure with pressure coefficient dT c / dP ∼ − 1.6 − 1.7 K/GPa, and then shows a pressure independent behavior with T c ∼ 8 K above 5 GPa. These behaviors are observed independently of whether we select liquid argon or NaCl as PTM. This suggests that c -axis lattice constant is not an important factor to determine T c in LiFeAs, in contrast to K x Fe 2 − y Se 2 and ( NH 3 ) y A x Fe 2 Se 2 (A=alkali metal). [ABSTRACT FROM AUTHOR]

A series of single crystal with the doping level were grown. The resistivity measurement was conducted. The results show that the 10% Cu doping completely suppresses superconducting transition temperature and when the doping level increases to 16%, it presents a semiconducting behavior in the region of 2 K to 300 K. [ABSTRACT FROM AUTHOR]

Abstract: The flux-flow resistivity and the electronic state inside the vortex core in single crystals of LiFe(As, P) were investigated by microwave-surface impedance. The flux-flow resistivity of these materials increase linearly with the magnetic field, and it is consistent with an isotropic gap nature of materials. However, the gradient is larger than theoretical predictions for superconductors with isotropic gap structure. It is probably the consequence of multiple-bands nature of these materials. We also found that the materials have the moderately clean core, and the mean free path of the quasi-particle inside the vortex core is much shorter than that outside the core, and rather comparable to the core radius. We believe that this is a novel and a universal feature of all superconductors. [Copyright &y& Elsevier]

The onset of superconductivity at the transition temperature is marked by the onset of order, which is characterized by an energy gap. Most models of the iron-based superconductors find a sign-changing (s±) order parameter [1-6], with the physical implication that pairing is driven by spin fluctuations. Recent work, however, has indicated that LiFeAs has a simple isotropic order parameter [7-9] and spin fluctuations are not necessary [7,10], contrary to the models [1-6]. The strength of the spin fluctuations has been controversial [11,12], meaning that the mechanism of superconductivity cannot as yet be determined. We report the momentum dependence of the superconducting energy gap, where we find an anisotropy that rules out coupling through spin fluctuations and the sign change. The results instead suggest that orbital fluctuations assisted by phonons [13,14] are the best explanation for superconductivity. [ABSTRACT FROM AUTHOR]

Abstract: We study microwave surface impedance measurements of LiFeAs, LiFe(As,P) and FeSe1− x Te x single crystals. The in-plane penetration depths of LiFeAs and LiFe(As,P) depend on temperature exponentially at low temperatures, which is a strong indication that these two materials have the nodeless superconducting gap. The temperature dependence of the superfluid density indicates that LiFeAs and LiFe(As,P) are multi-gap superconductors with at least two isotropic gaps. In FeSe1− x Te x , on the other hand, the quadratic temperature dependence of the in-plane penetration depth appears, which indicates the existence of some sort of quasiparticle excitations. In addtion, the real part of the microwave conductivity exhibits a large enhancement below T c in all three materials, which results from the increase of the quasiparticle relaxation time, τ, below T c. [Copyright &y& Elsevier]

The “111” type Li x FeAs ( x ranges from 0.8 to 1.2) with Cu2Sb type tetragonal structure was synthesized with T c 18 K. The isostructure NaFeAs was further studied, which shows superconductivity with T c up to 26 K. The effect of pressure on superconductivity was investigated. [ABSTRACT FROM AUTHOR]

Abstract: The newest homologous series of superconducting Fe-pnictides, LiFeAs (Li111) and NaFeAs (Na111) have been synthesized and investigated. Both crystallize with the layered tetragonal anti-PbFCl-type structure in P4/nmm space group. Polycrystalline samples and single crystals of Li111 and Na111 display superconducting transitions at ∼18K and 12–25K, respectively. No magnetic order has been found in either compound, although a weak magnetic background is clearly in evidence. The origin of the carriers and the stoichiometric compositions of Li111 and Na111 were explored. [Copyright &y& Elsevier]

The changes in elastic modulus and sound velocity of LaOFeP and LiFeAs at their transition temperatures were calculated using various thermodynamic data. Step discontinuities of 59 ppm in the bulk modulus, 101 ppm in Young modulus and 15 ppm in longitudinal sound velocity were estimated for LaOFeP at the superconducting transition. For LiFeAs, the calculated step discontinuities were 25 ppm in the bulk modulus, 38 ppm in Young modulus and 7 ppm in longitudinal sound velocity. [ABSTRACT FROM AUTHOR]

The “111” type of iron arsenide superconductors LiFeAs and NaFeAs were synthesized and the superconducting transition temperature was about 18 K and 26 K, respectively. The temperature dependence of the resistivity for these samples was measured under different magnetic fields. By taking the criterion of 50% of the normal state resistivity ρn, the upper critical field Hc2 values at zero temperature were estimated to be about 43.4 T for LiFeAs and 35.7 T for NaFeAs. [ABSTRACT FROM AUTHOR]

LiFeAs single crystal has been grown with superconducting transition temperature T comparable to that of polycrystals. A magnetic transition is found at about 160 K, which suggests the correlation of superconductivity with spin wave density. [ABSTRACT FROM AUTHOR]

Highlights: [•] High quality LiFe1-xCuxAs (x=0∼0.07) single crystals were synthesized using self-flux method. [•] The evolution of lattice parameter c was studied in the LiFe1-xCuxAs system. [•] Tc of LiFeAs was suppressed linearly with a rate of 1.9 K per 1% Cu-doped.. [ABSTRACT FROM AUTHOR]

Abstract: Superconductivity is observed in AFe2As2 (A=Cs, Rb, K) at temperatures below 4K. The metastable compound NaFe2As2 displays superconductivity with a high . These compounds are considered to be strongly hole-overdoped. The stoichiometric LiFeAs is superconducting below 18K. The origin of superconductivity in LiFeAs is critically investigated and we show that Li deficiency cannot explain the experimental results. We propose that superconducting LiFeAs is stoichiometric and it is equivalent to the high-pressure state of other undoped iron pnictides. [ABSTRACT FROM AUTHOR]