Pressure-induced superconductivity in lightly doped RFeAsO1-xFx (R=Sm and Nd)

It is important to establish temperature (T) – pressure (P) phase diagrams of iron-pnictide superconductors, where superconducting, antiferromagnetic (AFM) and nematic states coexist or compete, to throw further light on the superconducting mechanism. Indeed, superconducting phase in FeSe coexists with nematic phase for 0 ≦ P ≦ 1.5 GPa and with dome-shaped AFM phase for 1.5 ≦ P ≦ 6 GPa but highly enhanced above 6 GPa [1]. In RFeAsO1-xFx (R=La, rare earth), the T-P phase diagram however remains solved, although some studies were conducted on LaFeAsO1-xFx [2-4]. RFeAsO1-xFx is superconducting with Tc=26-52 K due to an optimal electron doping by substituting O by F, while non-doped RFeAsO is not superconducting but exhibit magnetic and structural phase transitions at 130-140 K and at 150-160 K, respectively. To study the relationship between superconducting and other phases in the phase diagram, we need to prepare lightly doped specimens so that the superconductivity is induced only above a critical pressure Pc, since the superconductivity and other phases are not expected to coexist in enough doped superconducting specimens. Thus, to discover pressure induced superconductivity in RFeAsO is the first step to establish the T-P phase diagram of RFeAsO. In the present work, we have performed the measurements of electrical resistivity and dc magnetization under pressure for lightly doped RFeAsO1-xFx with R=Sm and Nd using high density polycrystalline specimens obtained by pulse current sintering and report the pressure induced superconductivity. It is found that our specimen with R=Sm for x=0.09 shows superconductivity with Tc=6 K above ～3.8 GPa but that with R=Nd for x=0.08 shows superconductivity with Tc=6 K above ～1.0 GPa. superconducting volume fraction estimated from the amplitude of diamagnetic response for both materials is less than 5 %, suggesting that the superconductivity is filamentary one.
[1] J. P. Sun et al., Nat. Commun. 7, 12146 (2016).
[2] H. Okada et al., J. Phys. Soc. Jpn. 77, 113712 (2008).
[3] C.A. McElroy et al., Phys. Rev. B 90, 125134 (2014).
[4] K. Miyoshi et al., Physica B 536, 827 (2018).