Fe–As Bond Fluctuations in a Double-Well Potential in LaFeAsO

Abstract

While for a long time iron based superconductors have been investigated looking at the average crystalline structure there is now growing interest on the divergence of local from the average structure. Here we use advanced data analysis of EXAFS, a fast and local probe, which has key advantages compared to crystallographic measurements, in unveiling the dynamics of the local structure. The results show that the Fe-As pair oscillates in the double-well potential both in undoped and in cobalt doped LaFeAsO crystals.The parameters and the characteristics of the double-well potential are determined by curve fits of the polarized EXAFS data. Temperature dependent Fe-As pair distribution and tunneling frequency are derived from selected model of two potential wells. We observe the softening of the Fe-As oscillations at the tetragonal to orthorhombic structural phase transition at 150 K in LaFeAsO. A weak broad softening is observed while decreasing the temperature from 70 to 40 K in superconducting LaFe 0.89 Co 0.11 AsO which could be correlated with the onset of short range charge density wave order.

Local electronic structure rearrangements and strong anharmonicity in YH3 under pressures up to 180 GPa

“…However, at 10 K, the MSRD experiences a sharp jump from the harmonic curve by ~0.003 Å 2 , even exceeding the amplitude of thermal vibrations at 300 K. This points to an anomalous increase in the amplitude of thermal vibrations with a decrease in temperature. This low-temperature anomaly in the temperature dependence of MSRD indicates strong anharmonicity, similar to that observed earlier in high temperature superconducting cuprates 48 – 50 , BaBiO 3 -based superconducting oxides 51 , 52 , and Fe-based HTSC 53 , which was successfully explained by vibration of oxygen ions in a double-well potential.…”

Nonadiabatic coupling of the dynamical structure to the superconductivity in YSr 2 Cu 2.75 Mo 0.25 O 7.54 and Sr 2 CuO 3.3

“…A remaining factor is the dynamical structure of specific Cu-O pairs, the iqtps. The identification of iqtps as a ubiquitous factor in exotic superconductivity follows from their identification by EXAFS measurements in all classes of cuprates, La 2-x Ba/Sr x CuO 4+δ ( 42 , 47 , 51 – 53 ), YBa 2 Cu 3 O 7 ( 20 , 46 ), Bi- and Tl-based layered cuprates ( 23 , 24 , 54 , 55 ), and even related Bi- and Fe-based compounds ( 56 – 58 ). The basis for this superiority of EXAFS for revealing iqtps is unclear—perhaps it is simply easier than neutron pdf measurements—but it is empirically incontrovertible.…”

Constraints on the total coupling strengthto bosons in the iron based superconductors

“…A lattice instability might be thwarted there by additional barriers stabilizing the metastable glassy state. In this general context, the recent observation of strong Fe–As bond fluctuations in a double ‐well potential by EXAFS measurements on undoped and superconducting LaFeCoAsO single crystals is of interest. There, T ‐dependent tunneling frequencies at about 24.6 and 26.7 meV (6 and 6.5 THz) were reported at low ‐temperature.…”

“…Another option is the vicinity of a QCP of a novel SDW phase with magnetic stripes (discussed in the next section) or strong anharmonicity of the Fe–As bond phonons . The latter has been observed in electron overdoped BaFeCoAs 2 system with , above the ordinary spin ‐stripe phase.…”