Complex Structures of Different CaFe2As2 Samples

Published

February 18, 2014

Author(s)

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

Abstract

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 T_s=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 T_N=T_S=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 T_N = 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.

Citation

Scientific Reports

Volume

4

Pub Type

Journals

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Keywords

superconductivity, neutron, magnetism

Citation

Saparov, B. , Cantoni, C. , Pan, M. , Hogan, T. , Ratcliff, W. , Wilson, S. , Fritsch, K. , Gaulin, B. and Sefat, A. (2014), Complex Structures of Different CaFe₂As₂ Samples, Scientific Reports, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=915043 (Accessed May 18, 2024)