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Complex Structures of Different CaFe2As2 Samples



Bayrammurad Saparov, Claudia Cantoni, Minghu Pan, Thomas C. Hogan, William D. Ratcliff, Stephen D. Wilson, Katharina Fritsch, Bruce D. Gaulin, Athena S. Sefat


We use temperature-dependent X-ray and neutron diffraction, low-temperature scanning tunneling microscopy, and room-temperature scanning-transmission electron-microscopy, to investigate the interplay between magnetism and crystal structures in three CaFe2As2 phases. For the as-grown non-magnetic crystals, different crystalline domains with greatly varying lattice parameters are found, and three structural phases (namely orthorhombic, tetragonal, and collapsed tetragonal) coexist between Ts=95 K and 45 K manifesting a small broad peak in high capacity. We find that thermal annealing leads to strain relief through a large (1.3%) expansion of the c-parameter and a small (0.2%) contraction of the a-parameter, and to local atomic displacements up to 0.2 A at the atomic-level. Annealing at 350 °C results in the most homogenous crystals for which the antiferromagnetic and orthorhombic phase transitions occur at TN=TS=168(1) K, supported by a sharp heat capacity peak. Scanning tunneling spectroscopy below the transition temperature reveals a homogenous distribution of bosonic excitations, an indication of spin-fluctuations. In the 700 °C-annealed crystal, an intermediate structural strain regime takes place, with tetragonal and orthorhombic structural phases coexisting between 80 to 120 K, and TN = 118(4) K. For such complex phases of CaFe2As2, the origin of such strong shifts in the transition temperatures can be tied to structural parameters. Most importantly, with annealing, an increase in the Fe-As bond length leads to more localized Fe electrons and higher local magnetic moments on Fe ions. Synergistic contribution of other structural parameters, including a decrease in the Fe-Fe distance, and a dramatic increase of the cparameter, which enchances the Fermi surface nesting in CaFe2As2, are also discussed.
Scientific Reports


superconductivity, neutron, magnetism


Saparov, B. , Cantoni, C. , Pan, M. , Hogan, T. , Ratcliff, W. , Wilson, S. , Fritsch, K. , Gaulin, B. and Sefat, A. (2014), Complex Structures of Different CaFe<sub>2</sub>As<sub>2</sub> Samples, Scientific Reports, [online], (Accessed May 19, 2024)


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Created February 17, 2014, Updated October 12, 2021