Effect of chemical pressure on spin density wave and superconductivity in undoped and 15% F-doped La1-yYyFeAsO compounds (original) (raw)
Related papers
Physical Review B, 2009
We present a study concerning the partial substitution of yttrium at the lanthanum site of the undoped LaFeAsO and superconducting LaFeAsO0.85F0.15 compounds. We prepared samples with a nominal yttrium content up to 70% producing simultaneous shrinkage of both the a- and c-lattice parameters by 1.8% and 1.7%, respectively. The chemical pressure provided by the partial substitution with this smaller ion size causes a lowering of the spin density wave temperature in the undoped compounds, as well as an increase of the superconducting transition temperatures in the doped ones. The 15% fluorine-doped samples reach a maximum critical temperature of 40.2 K for the 50% yttrium substitution. Comparison with literature data indicates that chemical pressure cannot be the only mechanism which tunes drastically both TSDW and Tc in 1111 compounds. Our data suggest that structural disorder induced by the partial substitution in the La site or by doping could play an important role as well.
Impact of concomitant Y and Mn substitution on superconductivity in La1−yYyFe1−xMnxAsO0.89F0.11
Physical Review B
We discuss the impact of concomitant substitution of Fe by Mn and La by Y in optimally F-doped LaFeAsO 0.89 F 0.11. Mn has a known poisoning effect on superconductivity which is particularly strong in the La1111 system, where 0.2% of Mn were reported to completely suppress superconductivity. Through isovalent substitution of La by the much smaller Y we are able to inflict chemical pressure on the structure, which we show is stabilizing the superconducting state, resulting in a drastically larger amount of Mn needed to completely quench superconductivity. Interestingly, we find that the lattice parameter c changes significantly even for small amounts of Mn substitution within a series, which is unexpected taking only the differences between ionic radii into account. We discuss our findings in the light of electron localization caused by small amounts of paramagnetic Mn impurities in La 1−y Y y Fe 1−x Mn x AsO 0.89 F 0.11 also indicated by resistivity and Mößbauer measurements.
Crystallographic phase transition and high- T c superconductivity in LaFeAsO:F
Superconductor Science & Technology, 2008
Undoped LaFeAsO, parent compound of the newly found high-Tc superconductor, exhibits a sharp decrease in the temperature-dependent resistivity at ~160 K. The anomaly can be suppressed by F doping and the superconductivity appears correspondingly, suggesting a close associate of the anomaly with the superconductivity. We examined the crystal structures, magnetic properties and superconductivity of undoped (normal conductor) and 14 at.% F-doped LaFeAsO (Tc = 20 K) by synchrotron X-ray diffraction, DC magnetic measurements, and ab initio calculations to demonstrate that the anomaly is associated with a phase transition from tetragonal (P4/nmm) to orthorhombic (Cmma) phases at ~160 K as well as an antiferromagnetic transition at ~140 K. These transitions can be explained by spin configuration-dependent potential energy surfaces derived from the ab initio calculations. The suppression of the transitions is ascribed to interrelated effects of geometric and electronic structural changes due to doping by F- ions.
Physical Review B, 2015
Mn-doped La 1−y Y y FeAsO 0.89 F 0.11 superconductors 19 F NMR measurements in optimally electron-doped La1−yYyFe1−xMnxAsO0.89F0.11 superconductors are presented. In these materials the effect of Mn doping on the superconducting phase is studied for two series of compounds (y = 0 and y = 0.2) where the chemical pressure is varied by substituting La with Y. In the y = 0.2 series superconductivity is suppressed for Mn contents an order of magnitude larger than for the y = 0 series. For both series a peak in the 19 F NMR nuclear spin-lattice relaxation rate 1/T1 emerges upon Mn doping and gets significantly enhanced on approaching the quantum phase transition between the superconducting and magnetic phases. 19 F NMR linewidth measurements show that for similar Mn contents magnetic correlations are more pronounced in the y = 0 series, at variance with what one would expect for Q = (π/a, 0) spin correlations. These observations suggest that Mn doping tends to reduce fluctuations at Q = (π/a, 0) and to enhance other low-frequency modes. The effect of this transfer of spectral weight on the superconducting pairing is discussed along with the charge localization induced by Mn.
Superconductivity and critical current density in LaFeAsO1−xFx compounds
Thin Solid Films, 2010
Here, we report our studies on the crystal structures, morphologies, and superconductivity in LaFeAsO 1 − x F x compounds which were fabricated by solid state reaction. The crystal structures were refined using Rietveld refinement. Superconducting properties, such as critical temperature, T c , critical current density, J c , and upper critical field, H c2 , were determined using magneto-transport and magnetic measurements over a wide range of temperature below and above T c , and in magnetic fields up to 14 T. A peak effect in the J c versus field is observed at T b 15 K. Upper critical field in the ab and c directions is H ab c2 = 122.8 T and H c c2 = 38.6 T, respectively.
Scientific Reports, 2017
The effect of high pressure (up to 8 GPa) on normal and superconducting state properties of PrFeAsO 0.6 F 0.12 , an 1111-type iron based superconductor close to optimal doped region, has been investigated by measuring the temperature dependence of resistivity. Initially, the superconducting transition temperature (T c) is observed to increase slowly by about 1 K as pressure (P) increases from 0 to 1.3 GPa. With further increase in pressure above 1.3 GPa, T c decreases at the rate of ~1.5 K/GPa. The normal-state resistivity decreases monotonically up to 8 GPa. We have also measured the pressure dependence of magnetization (M) on the same piece of PrFeAsO 0.6 F 0.12 sample up to 1.1 GPa and observed T c as well as the size of the Meissner signal to increase with pressure in this low-pressure region. In contrast, for an over-doped PrFeAsO 0.6 F 0.14 sample, magnetization measurements up to 1.06 GPa show that both T c and the Meissner signal decrease with pressure. The present study clearly reveals two distinct regions in the dome-shaped (T c-P) phase diagram of PrFeAsO 0.6 F 0.12. The discovery of superconductivity in iron-based compound LaFeAsO 1−x F x with high transition temperature, T c ~ 26 K, has created a renewed interest in the field of superconductivity 1. By replacing La with rare-earth elements (Ln) of smaller ionic size such as Ce, Pr, Nd, Sm and Gd, a new family of high-T c superconductors emerges with transition temperature in the range of 26-55 K 2,3. This behavior is very different from that observed in cuprate superconductors where T c is found to be insensitive to ionic radius of the rare-earth ion. In pnictide compounds, the iron-pnictide (Fe-Pn) layer consisting of edge-sharing FePn 4 tetrahedron is responsible for the occurrence of superconductivity and a strong correlation between the crystal structure and superconductivity is observed 4-6. In LnFeAsO (1111-type) superconductors, T c reaches maximum when FeAs 4 forms a regular tetrahedron, i.e., the As-Fe-As bond angle is close to 109.5°4 ,6. It has been suggested that the pnictogen height h Pn measured from the Fe plane plays a crucial role for the T c enhancement 6. The pnictogen height is observed to increase and the FeAs 4 tetrahedron tends towards regular shape as Ln changes from La to Gd and it acts as a switch between high-T c state with nodeless paring and low-T c state with nodal paring 6. The effect of chemical pressure as well as external pressure on lattice structure and hence on superconductivity has been studied extensively to disentangle the relative contribution of various factors such as bond lengths, tetrahedral angle, and pnictogen height that affect superconductivity and provide valuable information to elucidate the mechanism of superconductivity in pnictides 4-12. Unlike chemical substitution, pressure is a continuously tunable thermodynamic parameter which can be used to understand the phase transition as well as charge conduction mechanism without introducing disorder in the system. There are several reports on the effect of pressure on superconductivity in pnictides 5,11-27. However, the effect of pressure on T c in LnFeAsO 1−x F x , in particular, in the low-pressure region, has not been unambiguously settled, possibly due to the sample quality and non-hydrostatic nature of the applied pressure. Also, the dependence of T c on pressure is quite sensitive to doping level. In the low-pressure region, T c shows an enhancement or suppression for 1111-type compound depending on the doping level and Ln ions 11-23. In pressure dependent ρ(T)