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Magnetic correlations in superconducting LiFeAs were studied by elastic and by inelastic neutron scattering experiments. There is no indication for static magnetic ordering but inelastic correlations appear at the incommensurate wave vector (0.5 ± δ, 0.5 ∓ δ, 0) with δ ∼0.07 slightly shifted from the commensurate ordering observed in other FeAs-based compounds. The incommensurate magnetic excitations respond to the opening of the superconducting gap by a transfer of spectral weight.PACS numbers: 74.25. Ha,74.25.Jb,78.70.Nx,75.10.Lp Superconductivity in the FeAs-based materials [1] appears to be closely related to magnetism as the superconducting state emerges out of an antiferromagnetic phase by doping [1][2][3][4] or by application of pressure [5]. The only FeAs-based exception to this behavior has been found in LiFeAs, which is an ambient-pressure superconductor with a high T C of ∼17 K without any doping [6][7][8]. LiFeAs exhibits the same FeAs layers as the other materials but FeAs 4 tetrahedrons are quite distorted [8] suggesting a different occupation of orbital bands. Indeed ARPES studies on LiFeAs find an electronic band structure different from that in LaOFeAs or BaFe 2 As 2 type compounds [9]. The Fermi-surface nesting, which is proposed to drive the spin-density wave (SDW) order in the other FeAs parent compounds, is absent in LiFeAs [9] suggesting that this magnetic instability is less relevant. The main cause for the suppression of the nesting consists in the hole pocket around the zone center which is shallow in LiFeAs [10]. In consequence, there is more density of states near the Fermi level which might favor a ferromagnetic instability. Using a three-band model Brydon et al.[10] find this ferromagnetic instability to dominate and discuss the implication for the superconducting order parameter proposing LiFeAs to be a spin-triplet superconductor with odd symmetry. However, other theoretical analyzes of the electronic band-structure still find an antiferromagnetic instability which more closely resembles those observed in the other FeAs-based materials [11].Inelastic neutron scattering (INS) experiments revealed magnetic order and magnetic excitations in many FeAs-based families [2,[12][13][14]. Strong magnetic correlations persist far beyond the ordered state, and, most importantly, the opening of the superconducting gap results in a pronounced redistribution of spectral weight [13][14][15], which is frequently interpreted in terms of a resonance mode. Recently a powder INS experiment on superconducting LiFeAs reported magnetic excitations to be rather similar to those observed in the previously studied materials [16] but with a spin gap even in the normal-conducting phase. Magnetic excitations observed in a recent single-crystal INS study on nonsuperconducting Li deficient Li 1−x FeAs (x∼0.06) were described by spin-waves associated with commensurate antiferromagnetism, again with a large temperature independent spin gap of 13 meV [17]. We have performed INS experiments on superconducting sing...

Platelet-like single crystals of the Ca(Fe 1-x Co x ) 2 As 2 series having lateral dimensions up to 15 mm and thickness up to 0.5 mm were obtained from the high temperature solution growth technique using Sn flux. Upon Co doping, the c-axis of the tetragonal unit cell decreases, while the a-axis shows a less significant variation. Pristine CaFe 2 As 2 shows a combined spin-density-wave and structural transition near T = 166 K which gradually shifts to lower temperatures and splits with increasing Co-doping. Both transitions terminate abruptly at a critical Co-concentration of x c = 0.075. For x ≥ 0.05, superconductivity appears at low temperatures with a maximum transition temperature T C of around 20 K. The superconducting volume fraction increases with Co concentration up to x = 0.09 followed by a gradual decrease with further increase of the doping level. The electronic phase diagram of Ca(Fe 1-x Co x ) 2 As 2 (0 ≤ x ≤ 0.2) series is constructed from the magnetization and electric resistivity data. We show that the low-temperature superconducting properties of Co-doped CaFe 2 As 2 differ considerably from those of BaFe 2 As 2 reported previously. These differences seem to be related to the extreme pressure sensitivity of CaFe 2 As 2 relative to its Ba counterpart.

A crucial step in revealing the nature of unconventional superconductivity is to investigate the symmetry of the superconducting order parameter. Scanning tunneling spectroscopy has proven a powerful technique to probe this symmetry by measuring the quasiparticle interference (QPI) which sensitively depends on the superconducting pairing mechanism. A particularly well suited material to apply this technique is the stoichiometric superconductor LiFeAs as it features clean, charge neutral cleaved surfaces without surface states and a relatively high Tc ∼ 18 K. Our data reveal that in LiFeAs the quasiparticle scattering is governed by a van-Hove singularity at the center of the Brillouin zone which is in stark contrast with other pnictide superconductors where nesting is crucial for both scattering and s±-superconductivity. Indeed, within a minimal model and using the most elementary order parameters, calculations of the QPI suggest a dominating role of the holelike bands for the quasiparticle scattering. Our theoretical findings do not support the elementary singlet pairing symmetries s++, s±, and d-wave. This brings to mind that the superconducting pairing mechanism in LiFeAs is based on an unusual pairing symmetry such as an elementary pwave (which provides optimal agreement between the experimental data and QPI simulations) or a more complex order parameter (e.g. s + id-wave symmetry).The discovery of iron-based superconductors [1] has generated enormous research activities to reveal the nature of superconductivity in these materials. In particular, s ± -pairing, i.e. an s-wave order parameter with alternating sign between almost perfectly nested hole and electron pockets has been suggested to be prevailing for the entire class of iron-based superconductors [2][3][4]. In this regard, the material LiFeAs is of particular interest since experiments have proven an absence of nesting [5] and theoretical works yield contradictory results, i.e., both s ± -wave singlet as well as p-wave triplet pairing has been suggested [6,7].A powerful method to probe the symmetry of the superconducting order parameter is to map out the spatial dependence of the local density of states (DOS) by scanning tunneling spectroscopy (STS). In such experiments, the relation DOS(E) ∝ dI/dV (V bias ) with tunneling voltage V and current I at energy E and bias voltage V bias (E = eV bias ) is exploited. The spatial dependence of the DOS often arises from an impurity scattering of the conduction electrons. In this case the incident and scattered quasiparticle waves interfere and give rise to Friedel-like oscillations in the local density of states (LDOS). Such quasiparticle interference (QPI) effects have first been observed by STS experiments on normal state metal surfaces [8][9][10]. A convenient way to extract the dominating scattering vectors q from a spatially resolved image of the QPI pattern is by analysis of its Fourier transformed image [9,10]. This so-called spectroscopic-imaging scanning tunneling microscopy (SI-STM) has proven to be...