Comparison of Pr-doped Ca 122 and Ca 112 Pnictides by Low-field Microwave Absorption Spectroscopy
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Comparison of Pr-doped Ca 122 and Ca 112 Pnictides by Low-field Microwave Absorption Spectroscopy Austin R. Howard,1 Jonathan D. Yuen,1 Bing Lv,2 Myron Salamon,1,3 Ching-Wu Chu,2 Anvar A. Zakhidov1 1 The University of Texas at Dallas, Richardson, TX 75080, U.S.A. 2 Texas Center for Superconductivity, University of Houston, Houston, TX 77004 3 MPA-CMMS, Los Alamos National Laboratory, Los Alamos, NM 87545 ABSTRACT Praseodymium doped CaFe2As2 (122 structure) and CaFeAs2 (112 structure) are characterized by modulated Low Magnetic Field Microwave Absorption (LFMA) spectroscopy. In both (Pr,Ca)122 and (Pr,Ca)112 structures, a strong hysteretic LFMA is found, with a TcH of ~30 K and ~26 K, respectively. However, in (Pr,Ca)122, measurements also show an unusual Narrow Peak (NP) LFMA signal appearing at higher temperatures, above the lower TcH superconducting state until a TcNP of 49 K. We associate this NP LFMA with interfacial superconductivity, which has been found previously by highly anisotropic magnetization measurements. Furthermore, the absence of NP in (Pr,Ca)112 correlates with the absence of an interfacial phase. These results give useful information about the microwave signature of interfacial superconductivity present in the (Pr,Ca)122 system, and may form a roadmap towards a stabilized high temperature superconducting phase in pnictides. INTRODUCTION Among the recently discovered class of doped pnictide superconductors, the so-called 122 systems (MFe2As2 with M an element of valency 1 or 2 [1]) have attracted significant interest [1-5]. In particular, superconductivity in the studied Ca122 pnictide has been found to be significantly different from a seemingly similar system, Ca112 [6,7]. The maximum Tc in reported 122 systems is only 38 K [1,5]; however, a recent independent observation of a high temperature phase appearing between Tc1 = 21 K and Tc2 = 49 K in Pr-doped Ca122 [2] opens a new perspective to search for even higher critical temperatures. The exceedingly small volume fraction, ease of suppression by magnetic field, absence of the characteristic specific heat anomaly of superconductivity [2], and the high magnetic anisotropy [8] imply that the high temperature phase is formed from interfaces present in the crystal. This “interfacial superconductivity” may in the future be improved or stabilized [9], leading the way for high temperature superconducting (HTSC) pnictides. In this paper, we report results of modulated low magnetic field microwave absorption (LFMA), a powerful technique in examining the microscopic superconducting behavior of each system. The (Pr,Ca)122 system exhibits an unusual signal, with a very narrow absorption peak (NP) centered on zero field, between TcNP = 49 K and a transition to a hysteretic signal below around TcH = 30 K. This type of signal is not observed in the (Pr,Ca)112 pnictides. EXPERIMENTAL DETAILS (Pr,Ca)122 and (Pr,Ca)112 single crystals were grown from the self-flux method, as described
previously by Lv, et al [2]. For LFMA measurements, the 122 crystals were aff
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