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).     

Transport properties and the anisotropy of Ba1 − x K x Fe2As2 single crystals in normal and superconducting states

The transport and superconducting properties of Ba_{1 ? x} K _x Fe₂As₂ single crystals with T _c ≈ 31 K were studied. Both in-plane and out-of-plane resistivity was measured by a modified Montgomery method. The in-plane resistivity is almost the same for all studied samples, unlike the out-of-plane resistivity, which differs considerably. We have found that the resistivity anisotropy γ = ρ _c /ρ _ab is almost independent of temperature and lies in the range 10–30 for the studied samples. This indicates the extrinsic nature of high out-of-plane resistivity, which may be due to the presence of flat defects along Fe-As layers in the samples. This statement is supported by comparatively small effective mass anisotropy, obtained from the upper critical field measurements, and from the observation of the so-called “Friedel transition,” which indicates the existence of some disorder in the samples in the c-direction.     

Electrical and thermal properties of bulk superconductors Eu0.95Pr0.05Ba2(Cu1−xMx)3O7−δ (M = Fe,Ni, Zn and Mn)

Measurements of transition temperature, magneto-resistance, thermal conductivity, thermo-electric power and specific heat have been carried out on co-doped samples of Eu_0.95Pr_0.05Ba₂(Cu_1?xM_x)₃O_7?δ (M = Fe, Ni, Zn and Mn) compounds. It is found that all samples exhibit metallic behavior, except the co-doped sample of Fe that shows semiconducting behavior. It is seen that the upper-critical field H_c2 decreases with Pr-doping. However, an increase in H_c2 for dopants like Fe and Mn is observed. Thermal conductivity for the pristine sample of EuBa₂Cu₃O_7?δ (Eu-123) exhibits a pronounced hump below the superconducting transition temperature T_C. However, the peak height of the hump decreases with Pr-doping and such a feature is further suppressed in the co-doped samples. The negative sign of the measured thermo-electric power of Eu-123 indicates that the dominant carrier in this sample is electron-like, whereas it turns to hole-like for all of the doped samples. A jump in specific heat C_P is detected in the pure sample of Eu-123 at T_C, while only a change in slope in C_P is seen around the transition temperature in the Pr-doped sample.     

Coexistence of magnetism and superconductivity in R1.4Ce0.6RuSr2Cu2O10 − δ(R = Eu,Sm and Gd)

The superconducting R_1.4Ce_0.6RuSr₂Cu₂O_10 − δ(R = Sm, Eu and Gd) withT_c ∼ 28, 32 and 42 K are also magnetically ordered atT_N ∼ 220, 122 and 180 K, respectively, thus,T_N ⪢ T_c. This is in contrast to intermetallic magnetic superconductors (such as RNi₂B₂C) in whichT_c ⪢  T_N. Magnetic susceptibility and Mossbauer spectroscopy show that superconductivity is confined to the CuO₂planes, whereas magnetism is due to the Ru sublattice. Irreversibility phenomena and magnetic anomalies, observed at low magnetic fields originate from antisymmetric exchange coupling of the Dzyaloshinsky–Moria type, and from spin reorientation of the Ru moments. The shielding fraction is about 100%, supporting the conclusion that the materials consist of a single phase, manifesting both magnetism and superconductivity at once.     

Defect chemistry and oxygen transport of (La0.6Sr0.4 − xMx)0.99Co0.2Fe0.8O3 − δ, M = Ca (x = 0.05, 0.1), Ba (x = 0.1, 0.2), Sr: Part II: Oxygen transport

This paper is the second part of a two part series, where the effects of varying the A-site dopant on the defect chemistry and transport properties of the materials (La_0.6Sr_0.4 ? xM_x)_0.99Co_0.2Fe_0.8O_3 ? δ, M = Sr, Ca (x = 0.05, 0.1), Ba (x = 0.1, 0.2) (LSMFC) have been investigated. In part I, the findings on the defect chemistry were reported, while the oxygen transport properties are reported here in part II. In the investigated material series, the amount of divalent dopant has been kept constant, while Sr ions have been substituted with Ca ions (smaller ionic radius) or Ba ions (larger ionic radius). The size difference induces different strains into the crystal structure in each composition. The possibility of simple relationships between various crystal strain parameters and the transport properties were analyzed. Oxygen pump controlled permeation experiments and a surface sensitive electrolyte probe were used to extract the permeability and surface resistance, r_s. The highest permeability was found for (La_0.6Sr_0.3Ca_0.1)_0.99Co_0.2Fe_0.8O_3 ? δ. The apparent activation energy of the permeability was 78 kJ/mol. The inverse surface resistance, r_s^? 1, also had an activated behavior with an activation energy close to 180 kJ/mol for most of the materials. A reversible transition to an abnormally low r_s was found in (La_0.6Sr_0.3Ca_0.1)_0.99Co_0.2Fe_0.8O_3 ? δ at T > 1223 K.     

Electron paramagnetic resonance,electrical resistivity,and magnetization studies in the high Tc superconductors EuBa2Cu3O9−xandEuBa2 (Cu1−yMy)3O9−x (M = Cr,Mn, Fe,Co, Ni,ORZn)

We have measured electron paramagnetic resonance (EPR), electrical resistivity, and dc magnetic susceptibility from 2 K – 300 K for the high T_c oxide superconductor EuBa₂Cu₃O_9−x, either undoped or doped with 3d ions (Cr, Mn, Fe, Ni, Co, or Zn), which presumably substitute at the Cu sites. We have observed an EPR line at low temperatures (T ≤ 40 K), which exhibits an increase in intensity and decrease in field for resonance as the temperature is lowered. The EPR linewidth is also temperature dependent and exhibits a minimum at about 15 K. In some of the samples another EPR signal is observed over the entire temperature range studied, with properties that depend on sample preparation conditions. This signal is likely to be due to small amounts of an additional phase. The behavior and origin of these EPR signals are discussed. The variation of T_c with 3d ion concentration over the range (1 – 8%) is also presented.     

Initial suppression of Tc in optimally-doped Bi2Sr1.6RE0.4Cu1−xZnxO6+δ (RE = La,Pr, Nd,Sm, and Eu)

We have investigated the initial suppression of the superconducting transition temperature, T_c, by Zn substitution in Bi₂Sr_1.6RE_0.4CuO_6+δ (Bi(RE)-2201) with a single CuO₂ plane. We propose a phase diagram of Bi(RE)-2201 as a function of the degree of disorder outside the CuO₂ plane. Upon increasing the degree of disorder, while T_c decreased gradually, the pseudogap is significantly strengthened as a quadratic function of the degree of disorder. The initial suppression rate of T_c is well described by considering the enhancement of the pseudogap by the out-of-plane disorder.