[en] We employed a modified Bridgman and self-flux methods to grow single crystals of A(Fe, Co, Ni)₂As₂ (A=Ba,Sr) and (A_1-xK_x)Fe₂As₂(A=Ba,Eu) series, respectively. Using these techniques large single-crystals with dimensions up to 25 x 10 mm² and thickness up to 1 mm were grown. Susceptibility and resistivity of these crystals were studied. Narrow superconducting transition widths (∼0.5 K) and large residual resistivity ratios (∼ 7) indicate the high-quality of our single crystals. Here we present details of our crystal growth methods, structural characterizations and physical properties.

[en] We have performed inelastic neutron scattering (INS) studies on two undoped compounds: superconducting paramagnetic LiFeAs and non-superconducting antiferromagnetic (AF) BaFe_2As_2. INS using LiFeAs single crystals revealed clear evidence for AF fluctuations at an incommensurate Q-vector, which vanish at low energies. The excitations clearly responds to the opening of the superconducting gap by a redistribution of spectral weight. Applying more sophisticated methods like polarized neutron scattering with longitudinal polarization analysis to BaFe_2As_2 single crystals revealed anisotropic fluctuations, where the out-of-plane mode lies lower than the in-plane mode. This finding is astonishing as the static moment aligns along the FeAs layers. The fit of a J_1-J_2 model using the coupling constants of SrFe_2As_2 and taking into account the instrumental resolution yielded the single-ion anisotropies responsible for both gaps. In comparison to theoretical values we find that the order of the gaps is reversed and that the spin-orbit coupling is underestimated in the calculations.

[en] In this work we present the results of the bulk magnetization measurements in a superconducting state of single crystals of Ba_0.65Na_0.35Fe₂As₂. The isothermal magnetic field (H ∥ c axis) dependent magnetization (M) loops exhibit a second peak (SP) or ‘fishtail effect’, as well as remarkable flux jumps at low temperatures. The critical current density J_c obtained from the M(H) loops is rather high, of the order of 10⁶ A cm⁻². The analysis of the temperature- and field-dependent J_c implies that high J_c is mainly due to collective (weak) pinning of vortices by dense microscopic point defects with some contribution from a strong pinning mechanism. Pronounced magnetic instabilities in terms of flux jumps depend strongly on temperature as well as on the field sweep rate. The field for the first flux jump as calculated from an adiabatic model, however, is much lower than the experimentally observed values, and this enhanced stability is attributed to a flux creep phenomenon. The analysis of field-dependent magnetic relaxation data additionally supports a collective pinning model. The data further suggest that SP in M(H) is likely related to the crossover in creep dynamics from an elastic to a plastic mechanism. We have constructed the vortex phase diagram on the field–temperature plane. (paper)

[en] We report on thermodynamic properties of single crystals A(Fe_1-xCo_x)₂As₂ (A=Ba,Ca,K) combining magnetization, specific heat and resistivity investigations. In comparison to a similar phase diagram for Co-doped Ba122 and Ca122, which exhibits split quasicontinuous magnetic and structural transitions upon Co-doping, the parent compound KFe₂As₂ lacks such structural/magnetic transitions completely. Furthermore, its superconducting transition temperature has been determined to be around 3.8 K, while Ca122 and Ba122 systems only become superconducting upon doping. In addition to these doping-dependent studies, thorough investigations on the magnetic phase diagram have been performed on individual compounds of all three systems. The Werthamer-Helfand-Hohenberg model was used to determine the upper critical field for different directions H parallel c and H parallel (ab).

[en] Measurements of magneto-resistivity and magnetic susceptibility were performed on single crystals of superconducting Ba(Fe_0.9Co_0.1)₂As₂ close to the conditions of optimal doping. The high quality of the investigated samples allows us to reveal dynamic scaling behaviour associated with a vortex–glass phase transition in the limit of a weak degree of quenched disorder. Accordingly, the dissipative component of the ac susceptibility is reproduced well within the framework of Havriliak–Negami relaxation, assuming a critical power-law divergence for the characteristic correlation time τ of the vortex dynamics. Remarkably, the random disorder introduced by the Fe_1−xCo_x chemical substitution is found to act on the vortices as a much weaker quenched disorder than previously reported for cuprate superconductors such as Y_1−xPr_xBa₂Cu₃O_7−δ. (paper)

[en] We investigated the magnetic spin-1 perovskite Ba${}_3$NiNb${}_2$O${}_9$ by means of complex ac susceptibility measurements at extreme sample conditions. Ba${}_3$NiNb${}_2$O${}_9$ with cubic perovskite structure (Pm-3m) has a random occupation of Nb(66 %)/Ni(33 %) at the center of the cubic perovskite unit cell. Different from the isostoichiometric sister compound, Ba${}_3$NiNb${}_2$O${}_9$ with P-3m1 structure which shows both uud-spin configuration and multiferroicity, the magnetic properties of the investigated system have not been studied below 2 K yet. For our single crystals, we observe a spin freezing transition at around 0.7 K. Furthermore, the peak of ${\mathrm{\chi <!--\; ??\; -->}}^{\mathrm{\prime <!--\; ???\; -->}}$ is suppressed by applying an external dc field of 200 mT and ${\mathrm{\chi <!--\; ??\; -->}}^{\mathrm{\prime <!--\; ???\; -->}\mathrm{\prime <!--\; ???\; -->}}$ shows a sudden onset near the freezing temperature.

[en] I(V) characteristics and their first derivatives of ScS and ScN-type (S is superconductor, c is constriction, N is normal metal) point contacts (PCs) based on Ba_1_-_xNa_xFe_2As_2 (x = 0.25 and 0.35) were studied. ScS-type PCs with S = Nb,Ta, and Pb show Josephson-like resistively shunted I(V) curves with microwave induced Shapiro steps which satisfy relation 2eV =ℎ/ω. The I_cR_N product (I_c is critical current, R_N is normal state PC resistance) in these PCs is found to be up to 1.2 mV. All this data with the observed dependence of the I_c on the microwave power of ScS PCs with Pb counter electrode indicates the presence of the singlet s-wave type pairing in Ba_1_-_xNa_xFe_2As_2. From the dV/dI(V) curves of ScN-type PCs demonstrating Andreev-reflection like features, the superconducting gap δ ratio 2δ/k_BTc = 3.6 +- 1 for the compound with x = 0.35 was evaluated. Analysis of these dV/dI(V) at high biases V, that is well above δ, testifies transition to the thermal regime in PCs with a voltage

[en] Single crystals of the new unconventional superconductor KFe_2As_2 were grown using two different fluxes, i.e KAs and FeAs flux. The superconducting transition temperature and the superconducting volume fraction were found to be same in all crystals independent of the flux. However, the normal state susceptibility is changed significantly. On the other hand we investigated, the influence of various types of doping in KFe_2As_2. Specifically, we investigate the substitution of K by Na (yielding K_1_-_xNa_xFe_2As_2) and the substitution of Fe by other transition metals like Co, Cr, Rh (yielding K(Fe_1_-_xTM_x)_2As_2). We will present a systematic study of the crystal growth, characterization and superconducting properties of these newly synthesized superconductors.

[en] Single crystals of Ba_1-xNa_xFe₂As₂ with x = 0, 0.25, 0.35, 0.4 were grown using a self-flux high temperature solution growth technique. The superconducting and normal state properties were studied by temperature dependent magnetic susceptibility, electrical resistivity and specific heat revealing that the magnetic and structural transition is rapidly suppressed upon Na-substitution at the Ba-site in BaFe₂As₂, giving rise to superconductivity. A superconducting transition as high as 34 K is reached for a Na-content of x = 0.4. The positive Hall coefficient confirms that the substitution of Ba by Na results in hole-doping similarly to the substitution of Ba by K. Angle resolved photoemission spectroscopy was performed on all Ba_1-xNa_xFe₂As₂ crystals. The Fermi surface of hole-doped Ba_1-xNa_xFe₂As₂ is to high extent the same as the Fermi surface found for the K-doped sister compounds, suggesting a similar impact of the substitution of Ba by either K or Na on the electronic band dispersion at the Fermi level.

[en] The interplay of high- and low energy (LE) mass renormalizations with band shifts as seen in the positions of van Hove singularities (VHS) in the normal state of the strongly correlated and most hole-overdoped AFe${}_2$As${}_2$ (A122), A = K,Rb,Cs Fe pnictides is discussed phenomenologically from ARPES data and band structure (GGA) with full spin-orbit coupling and microscopically applying the GW-self-energy. The increase of the Sommerfeld coefficient $\mathrm{\gamma <!--\; ??\; -->}$ from K122 to Cs122 is ascribed to an enhanced coupling to LE bosons in the vicinity of a quantum critical point to an yet unknown incommensurate phase different from the tetragonal Mott phase. We find no sizable increase of correlation for Cs122 in contrast to scenarios proposed in the literature. The ARPES positions of VHS as compared with GGA and GW results point to slightly weaker correlations in accord with the low-T susceptibility data and the reduced Wilson ratio $\mathrm{\chi <!--\; ??\; -->}$(0)/$\mathrm{\gamma <!--\; ??\; -->}$. Other quasi-2D systems with generic VHS and their el-el interactions are briefly discussed.