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[MaterialsPropertySeminar]
From CaFe2As2 to CaKFe4As4: physical properties and effect of pressure
Date : Monday, August 28th, 2017 11:00 - 12:00 Place : Seminar Room 5 (A615), 6th Floor, ISSP Lecturer : Dr. Sergey L. Bud'ko Affiliation : Ames Laboratory US DOE, and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA Committee Chair : Yoshiya UWATOKO (63330)
e-mail: uwatoko@issp.u-tokyo.ac.jp

An overview of the synthesis and anisotropic thermodynamic and transport properties of single-crystalline, single-phase CaKFe4As4 will be presented. [1] The samples were grown out of a high-temperature, quaternary melt. Temperature-dependent measurements of x-ray diffraction, electrical and thermal transport, magnetization, specific heat, and 57Fe Mossbauer spectroscopy measurements, combined with field-dependent measurements of electrical resistivity and field and pressure-dependent measurements of magnetization indicate that CaKFe4As4 is an ordered, stoichiometric, Fe-based superconductor with a superconducting critical temperature, Tc = 35 K. Other than superconductivity, there is no indication of any other phase transition for 1.8K < T < 300 K. All of these thermodynamic and transport data reveal striking similarities to those found for optimally or slightly overdoped (Ba1-xKx)Fe2As2, suggesting that stoichiometric CaKFe4As4 is intrinsically close to what is referred to as “optimally-doped” on a generalized phase diagram for Fe-based superconductors. Additionally, pressure response of CaKFe4As4 will be discussed. [2] With increasing pressure, both resistivity and magnetization data show that the bulk superconducting transition of CaKFe4As4 is suppressed and then disappears at P > 4 GPa. High pressure x-ray data clearly indicate a phase transition to a collapsed tetragonal phase in CaKFe4As4 under pressure that coincides with the abrupt loss of bulk superconductivity near 4 GPa. Band structure calculations also find a sudden transition to a collapsed tetragonal state near 4 GPa, as As-As bonding takes place across the Ca-layer. Bonding across the K-layer only occurs for P ≥ 12 GPa. These findings demonstrate a new type of collapsed tetragonal phase in CaKFe4As4: a half-collapsed-tetragonal phase.
There data will be compared in detailed with the literature data for CaFe2As2. [3]

1. W. R. Meier, et al., Phys. Rev. B 94, 064501 (2016).
2. Udhara S. Kaluarachchi et al., arXiv:1702.06622.
3. P. C. Canfield et al., Physica C 469, 404 (2009).


(Published on: Thursday August 24th, 2017)