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Register a new userSuperconductivity of highly spin-polarized electrons in FeSe probed by $^{77}$Se NMR
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A number of recent experiments indicate that the iron-chalcogenide FeSe provides the long-sought possibility to study bulk superconductivity in the cross-over regime between the weakly coupled Bardeen--Cooper--Schrieffer (BCS) pairing and the strongly coupled Bose--Einstein condensation (BEC). We report on $^{77}$Se nuclear magnetic resonance experiments of FeSe, focused on the superconducting phase for strong magnetic fields applied along the $c$ axis, where a distinct state with large spin polarization was reported. We determine this high-field state as bulk superconducting with high spatial homogeneity of the low-energy spin fluctuations. Further, we find that the static spin susceptibility becomes unusually small at temperatures approaching the superconducting state, despite the presence of pronounced spin fluctuations. Taken together, our results clearly indicate that FeSe indeed features an unusual field-induced superconducting state of a highly spin-polarized Fermi liquid in the BCS-BEC crossover regime.
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We conducted $^{77}$Se-nuclear magnetic resonance studies of the iron-based superconductor FeSe in magnetic fields of 0.6 to 19 T to investigate the superconducting and normal-state properties. The nuclear spin-lattice relaxation rate divided by the temperature $(T_1T)^{-1}$ increases below the structural transition temperature $T_mathrm{s}$ but starts to be suppressed below $T^*$, well above the superconducting transition temperature $T_mathrm{c}(H)$, resulting in a broad maximum of $(T_1T)^{-1}$ at $T_mathrm{p}(H)$. This is similar to the pseudogap behavior in optimally doped cuprate superconductors. Because $T^*$ and $T_mathrm{p}(H)$ decrease in the same manner as $T_mathrm{c}(H)$ with increasing $H$, the pseudogap behavior in FeSe is ascribed to superconducting fluctuations, which presumably originate from the theoretically predicted preformed pair above $T_mathrm{c}(H)$.
The 12%-S doped FeSe system has a high Tc of 30 K at a pressure of 3.0 GPa. We have successfully investigated its microscopic properties for the first time via $^{77}$Se-NMR measurements under pressure. The antiferromagnetic (AFM) fluctuations at the optimal pressure (~3 GPa) exhibited unexpected suppression compared with the AFM fluctuations at ambient pressure, even though the optimal pressure is close to the phase boundary of the AFM phase induced at the high-pressure region. In addition, we revealed that the SC phase at an applied field of 6.02 T exhibited a remarkable double-dome structure in the pressure-temperature phase diagram, unlike the SC phase at zero field.
The recent study of $^{77}$Se nuclear magnetic resonance (NMR) in a $beta$-FeSe single crystal proposed that ferro-orbital order breaks the $90^circ$ $C_4$ rotational symmetry, driving nematic ordering. Here, we report an NMR study of the impact of small strains generated by gluing on nematic state and spin fluctuations. We observe that the local strains strongly affect the nematic transition, considerably enhancing its onset temperature. On the contrary, no effect on low-energy spin fluctuations was found. Furthermore we investigate the interplay of the nematic phase and superconductivity. Our study demonstrates that the twinned nematic domains respond unequivalently to superconductivity, evidencing the twofold $C_2$ symmetry of superconductivity in this material. The obtained results are well understood in terms of the proposed ferro-orbital order.
We report $^{57}$Fe-NMR studies on the oxygen-deficient iron (Fe)-based oxypnictide superconductor LaFeAsO$_{0.7}$ ($T_{c}=$ 28 K) enriched by $^{57}$Fe isotope. In the superconducting state, the spin component of $^{57}$Fe-Knight shift $^{57}K$ decreases almost to zero at low temperatures and the nuclear spin-lattice relaxation rate $^{57}(1/T_{1})$ exhibits a $T^{3}$-like dependence without the coherence peak just below $T_{c}$, which give firm evidence of the unconventional superconducting state formed by spin-singlet Cooper pairing. All these events below $T_c$ are consistently argued in terms of the extended s$_{pm}$-wave pairing with a sign reversal of the order parameter among Fermi surfaces. In the normal state, we found the remarkable decrease of $1/T_1T$ upon cooling for both the Fe and As sites, which originates from the decrease of low-energy spectral weight of spin fluctuations over whole ${bm q}$ space upon cooling below room temperature. Such behavior has never been observed for other strongly correlated superconductors where an antiferromagnetic interaction plays a vital role in mediating the Cooper pairing.
The layered quasi-one-dimensional molecular superconductor (TMTSF)$_2$PF$_6$ is a very exotic material with a superconducting order parameter whose ground state symmetry has remained ill-defined. Here we present a pulsed NMR Knight shift (K) study of $^{77}$Se measured simultaneously with transport in pressurized (TMTSF)$_2$PF$_6$. The Knight shift is linearly dependent on the electron spin susceptibility $chi_s$, and is therefore a direct measure of the spin polarization in the superconducting state. For a singlet superconductor, the spin contribution to the Knight shift, K$_s$, falls rapidly on cooling through the transition. The present experiments indicate no observable change in K between the metallic and superconducting states, and thus strongly support the hypothesis of triplet p-wave superconductivity in (TMTSF)$_2$PF$_6$.
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