Spontaneous Formation of a Superconducting and Antiferromagnetic Hybrid State in SrFe₂As₂ under High Pressure

Competition between magnetism and superconductivity is ubiquitous in unconventional superconductors such as cuprates, heavy fermions, and the recently discovered family of iron pnictide, indicating close relation between the magnetic interaction and the pairing mechanism. Unlike in cuprates, where the superconductivity is obtained by carrier doping (chemical substitution), some of the antiferromagnetic (AF) iron pnictide parent compounds become superconducting also by applying pressure. This enables us to investigate the phase diagram precisely, without being disturbed by disorder.


We have performed ⁷⁵As NMR experiments in SrFe₂As₂ under high pressure [1] using the newly developed space-efficient opposed-anvil pressure cell [2]. Figure 1 shows the temperature (T) dependence of 1/(T₁T) (the nuclear relaxation rate (1/T₁) divided by T) at various pressure values. For the pressure up to 4.2 GPa, 1/(T₁T) increases with lowering temperature towards the Neel temperature, where the simultaneous structural and antiferromagnetic (AF) transition takes place. At 5.4 GPa, however, the paramagnetic (PM) NMR signal persists down to the lowest temperature. The sudden drop of 1/(T₁T) marks the superconducting (SC) transition as confirmed by the Knight shift and the ac-susceptibility measurements. The phase diagram shown in the inset of Fig. 1 indicates that the SC phase, which coexists with the AF phase as described below, is limited to a narrow pressure range near 6 GPa. Such a fragile SC state as opposed to more stable SC states obtained by carrier doping may be related to the compensated nature of the electron and hole Fermi surfaces.




Fig. 1. The main panel shows the temperature dependence of 1/(T₁T) in the paramagnetic (PM) phase for the magnetic field perpendicular (colored in dark blue to sky blue) or parallel (colored in orange to green) to the c-axis. For the pressure up to 4.2 GPa, 1/(T₁T) increases with lowering temperature down to the Neel temperature, where the first-order transition from the tetragonal paramagnetic (PM) phase into the orthorhombic antiferromagnetic (AF) phase takes place. At 5.4 GPa, superconducting (SC) state appears below 30 K causing the rapid drop of 1/(T₁T). The finite value of 1/(T₁T) at the lowest temperature indicates a substantial residual density of states in the SC phase. Neither SC nor AF phase was observed at 7.2 GPa. The inset show the phase diagram obtained from the present study (open symbol) and the results in ref.[3] (crosses).

The NMR spectra at low temperatures provide microscopic evidence for the coexistence of the SC and AF phases. For the field along the c-axis, the NMR spectrum consists of a set of quadrupole split three resonance lines from paramagnetic SC domains and a pair of such sets split by the hyperfine field from the stripe-type AF domains (not shown). Figure 2 shows the variation of the NMR spectra with temperature for the field in the ab-plane. In this case, the spectrum from the AF domains does not split but largely shifts by the hyperfine field. The spectrum at 4.2 K provides unambiguous evidence that the tetragonal SC phase coexists with the orthorhombic AF phase. We should emphasize that there is a single phase transition near 30K, below which the entire sample goes into the SC+AF coexisting phase. Thus the coexistence of two different orders should not be due to extrinsic inhomogeneity. The SC and AF domains are likely to form a nano-scale hybrid structure, which has not been seen in any other class of materials.




Fig. 2. The main panel shows the ⁷⁵As NMR spectra in the paramagnetic normal phase (30 K), and in the SC+AF coexisting phase (26 K and 4.2 K). The spectrum at 26 K, slightly lower than the SC+AF transition temperature, is significantly broadened due likely to incommensurate spin structure. The variation of the spectra at 4.2 K with the field direction in the ab-plane provides microscopic evidence for the coexistence of the tetragonal SC phase and the twinned orthorhombic AF phase. The color plot in the inset shows the temperature variation of the spectral intensity of the quadrupole split central line.


References
[1] K. Kitagawa, N. Katayama, H. Gotou, T. Yagi, K. Ohgushi, T. Matsumoto, Y. Uwatoko, and M. Takigawa, Phys. Rev. Lett. 103 (2009) 257002.
[2] K. Kitagawa, H. Gotou, T. Yagi, A. Yamada, T. Matsumoto, Y. Uwatoko, and M. Takigawa, J. Phys. Soc. Jpn. 79 (2010) 024001.
[3] K. Matsubayashi, N. Katayama, K. Ohgushi, A. Yamada, K. Munakata, T. Matsumoto, and Y. Uwatoko, J. Phys. Soc. Jpn. 78 (2009) 073706.