NMR study of re-entrant antiferromagnetic property in a high-T_c pnictide LaFeAsO_(1-x)H_x, and the related topics

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Date/Time:Thursday, 28 Mar 2013 - Thursday, 28 Mar 2013
Location:PHYSICS Hall Room 5
Channel:Condensed Matter Physics
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Naoki Fujiwara Graduate School of Human & Environmental Studies, Kyoto University, Japan

My presentation consists of two parts: in the first part, I will show exotic phenomena observed from NMR measurements in an electron-doped 1111 system LaFeAsO1-xHx (x ≥ 0.4). In the second part, I will review phase diagrams of the 1111 systems focusing on electronic states at the boundary between the antiferromagnetic (AF) and superconducting (SC) phases.
LaFeAsO1-xHx is recently known for having double SC domes upon H doping [1]. In this system, an excess amount of H up to x=0.6 can be replaced with O2- and proton acts as H1- as well as F1-. The double-dome structure itself is rare, because further carrier doping, in general, breaks a SC phase and leads to a Fermi liquid state. On heavy carrier doping, we found revival of spin fluctuations together with enhancement of in-plane anisotropy. The former suggests AF ordering and the latter shows orbital-weight differentiation at each Fe site, being indicative of orbital ordering. I will mention the phase diagram obtained from NMR measurements.
Next, I focus on the 1111 series, LaFeAsO1-xFx and CaFe1-xCoxAsF. The latter exhibits an intermediate interlayer distance and phase diagram between LaFeAsO1-xFx and Ba(Fe1-xCox)2As2 [2, 3]. The AF and SC states coexist in CaFe1-xCoxAsF around the boundary between AF and SC phases, whereas the phase segregation occurs in LaFeAsO1-xFx [4]. I will review phase diagrams of the 1111 series systematically in the viewpoint of uniform or uniaxial compression.

[1] S. Iimura et al., Nature communications, 3 943 (2012)
[2] T. Nakano et al., Phys. Rev. B 83 (2011) 180508(R)
[3] S. Tsutumi et al., Phys. Rev. B 86 (2012) 060515(R)
[4] N. Fujiwara et al., Phys. Rev. B 85 (2012) 094501