Inverse Iron Isotope Effect on the Transition Temperature of the(Ba,K)Fe2As2Superconductor

Abstract

Abstract:We report that (Ba,K)Fe 2 As 2 superconductor (a transition temperature, T c ~ 38 K) has inverse iron isotope coefficient α Fe = -0.18(3) (where T c ~ M -αFe and M is the iron isotope mass), i.e. the sample containing the larger iron isotope mass depicts higher T c . Systematic inverse shifts in T c were clearly observed between the samples using three types of Fe-isotopes ( Polycrystalline samples of (Ba,K)Fe 2 As 2 were prepared by high-pressure synthesis method [14]. The sample synthesized from a stoichiometric starting composition of (Ba 1-x K x )Fe 2 As 2 includes a FeAs impurity phase probably because K -4-and Ba exclude from the samples during heat treatment. This difficulty was overcome by using the 50% excess of Ba and K for the starting composition i.e.(Ba 0.5 K 0.5 ) 1.5 Fe 2 As 2 . Precursors of BaAs, KAs and As powder were mixed homogeneously and divided into two equal weights, then mixed with the two different iron isotopes in order to rule out K concentration difference between the two samples which can be a factor to change T c . The materials were ground by an agate mortar in a glove box. The mixed powders were pressed into pellets. Two pellets were put into a BN crucible as shown in the inset of Fig.1; to rule out difference in synthesis temperature and pressure, which also can affect in a variation in the T c between the samples. The samples were simultaneously heated at about 900°C under a pressure of about 1 GPa for 1 h. 54 Fe and n Fe were tested for sample preparation to ensure systematic behavior of T c .We obtained 7 sets of samples (S1-S7) synthesized in the same conditions except for the iron isotope mass.Powder X-ray diffraction (XRD) patterns of the samples were measured by using CuK a radiation. The dc magnetic susceptibility was measured by using a SQUID magnetometer (Quantum Design MPMS) under a magnetic field of 5 Oe. The resistivity of the two isotope containing samples was measured simultaneously by a four-probe method (Quantum Design PPMS).All the samples are an almost single-phase and difference in lattice parameters between the two samples synthesized simultaneously was small (negligible). The XRD patterns of the sample set S2 synthesized using 54 Fe and 57 Fe isotopic powders, S2( 54 Fe) and S2( 57 Fe) are represented in Fig. 1 is almost constant against x, which is suitable for verification of the isotope effect. We also confirmed the homogeneity of the samples by using scanning electron microscope with energy dispersive x-ray spectroscopy (SEM-EDX) analysis, and we detected the x = 0.40(2) throughout the pellet. (ΔT c(χ) ) and α Fe of the sample sets S1-S7 are listed in Table 1.In Fig. 3 the temperature dependent normalized resistivity of the samples S2( 57 Fe) and S2( 54 Fe) are shown. It should be noted that the normal state in the normalized resistivity plot is almost overlapped each other except the region of T c, assuring the same quality of the samples. T c(ρ) is determined by the definition as shown in the inset of Fig. 3. This result also clearly sho...

Intrinsic and structural isotope effects in iron-based superconductors

“…As such, the independent on T c value of α int Fe would suggest α str Fe ∼ −0.4 for SmFeAsO 1−x studied by Shirage et al 17 To conclude, the currently available Fe isotope effect data on the superconducting transition temperature T c for various Fe-based HTS were reanalyzed by separating the measured Fe-IE exponent α Fe into a structural and an intrinsic (unrelated to the structural changes) component. Accounting for the empirical relation between T c and the anion atom height h An 21 we have demonstrated that the structural contribution to the Fe-IE exponent is negative for Ba 0.6 K 0.4 Fe 2 As 2 and SmFeAsO 1−x studied by Shirage et al, 16,17 positive for FeSe 1−x , 18 and close to 0 for SmFeAsO 0.85 F 0.15 and Ba 0.6 K 0.4 Fe 2 As 2 measured by Liu et al 15 By taking such corrections into account we infer that the value of the genuine Fe-IE exponent is close to α int Fe ∼ 0.35 − 0.4 for compounds belonging to at least three different families of Fe-based HTS. We are convinced that the analysis presented in our paper helps in clarifying the existing controversy on the isotope effect in Fe-based superconductors.…”

“…However, a zero, within the experimental accuracy, Fe isotope shift of the c-axis lattice constant for Ba 0.6 K 0.4 Fe 2 As 2 as reported by Liu et al 15 is a clear indication that no structural effect is present for this particular set of the samples. Consequently, the negative isotope effect exponent α Fe ≃ −0.18 obtained for nominally identically doped Ba 0.6 K 0.4 Fe 2 As 2 by Shirage et al 16 stems from summing both effects, i.e., −0.18(α Fe ) = 0.35(α int Fe ) − 0.52(α str Fe ), see Eq.…”

“…Especially, in the case of Ba 0.6 K 0.4 Fe 2 As 2 , nominally identical samples were isotope replaced with one exhibiting a positive 15 and the other a negative isotope exponent. 16 Note, that the sign reversed isotope exponent seen by Shirage et al 16,17 was attributed to multiband superconductivity with different pairing channels, namely a phononic one and an antiferromagnetic (AF) fluctuation dominated one. 19 On the other hand a multiband model cannot exhibit any sign reversed isotope exponent even if solely AF fluctuations were the pairing glue.…”

“…Currently we are aware of four papers reporting, however, rather contradictory results. [15][16][17][18] Liu et al 15 and Khasanov et al 18 and SmFeAsO 1−y , 17 respectively. These controversial results are unlikely to stem from different pairing mechanisms to be realized in different Fe-based superconductors.…”

Gauge Fields, Massless Modes and Topology of Gauge Fields in Multi-Band Superconductors

“…The phase-difference mode would yield new phenomena [6][7][8][9][10] and new excitation modes in multi-gapped superconductors. The negative isotope effect in Fe pnictides is an example of the multi-band effect [11,12]. The existence of fractionally quantized flux vortices is very significant and attractive.…”