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Register a new userSpin fluctuations away from (pi,0) in the superconducting phase of molecular-intercalated FeSe
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Magnetic fluctuations in the molecular-intercalated FeSe superconductor Li{x}(ND2){y}(ND3){1-y}Fe2Se2 (Tc = 43K) have been measured by inelastic neutron scattering from a powder sample. The strongest magnetic scattering is observed at a wave vector Q ~ 1.4 A^{-1}, which is not consistent with the (pi,0) nesting wave vector that characterizes magnetic fluctuations in several other iron-based superconductors, but is close to the (pi, pi/2) position found for A{x}Fe{2-y}Se2 systems. At the energies probed (~ 5kB Tc), the magnetic scattering increases in intensity with decreasing temperature below Tc, consistent with the superconductivity-induced magnetic resonance found in other iron-based superconductors.
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We have performed powder inelastic neutron scattering measurements on the unconventional superconductor $beta$-FeSe ($T_{rm c} simeq 8,mathrm{K}$). The spectra reveal highly dispersive paramagnetic fluctuations emerging from the square-lattice wave vector $(pi,0)$ extending beyond 80 meV in energy. Measurements as a function of temperature at an energy of $sim 13,mathrm{meV}$ did not show any variation from $T_{rm c}$ to $104,mathrm{K}$. The results show that FeSe is close to an instability towards $(pi,0)$ antiferromagnetism characteristic of the parent phases of the high-$T_{rm c}$ iron arsenide superconductors, and that the iron paramagnetic moment is neither affected by the orthorhombic-to-tetragonal structural transition at $T_{rm s} simeq 90,mathrm{K}$ nor does it undergo a change in spin state over the temperature range studied.
We study the effect of combining spin fluctuations and forward scattering electron-phonon ({eph}) coupling on the superconductivity in the FeSe/SrTiO$_3$ system modeled by a phenomenological two-band Hubbard model with long-range {eph} interactions. We treat the electron and phonon degrees of freedom on an equal footing using a emph{fully} self-consistent FLEX plus Migdal-Eliashberg calculation, which includes a self-consistent determination of the spin fluctuation spectrum. Based on FeSe monolayers, we focus on the case where one of the bands lies below the Fermi level (i.e. incipient), and demonstrate that the combined interactions can enhance or suppress $T_c$, depending on their relative strength. For a suitable choice of parameters, the spin-fluctuation mechanism yields a $T_c approx 46.8$ K incipient $s_pm$ superconductor, consistent with surface-doped FeSe thin films. A forward-focused {eph} interaction further enhances the $T_c$, as observed in monolayer FeSe on SrTiO$_3$.
Recent nuclear magnetic resonance studies [A. Pustogow {it et al.}, arXiv:1904.00047] have challenged the prevalent chiral triplet pairing scenario proposed for Sr$_2$RuO$_4$. To provide guidance from microscopic theory as to which other pair states might be compatible with the new data, we perform a detailed theoretical study of spin-fluctuation mediated pairing for this compound. We map out the phase diagram as a function of spin-orbit coupling, interaction parameters, and band-structure properties over physically reasonable ranges, comparing when possible with photoemission and inelastic neutron scattering data information. We find that even-parity pseudospin singlet solutions dominate large regions of the phase diagram, but in certain regimes spin-orbit coupling favors a near-nodal odd-parity triplet superconducting state, which is either helical or chiral depending on the proximity of the $gamma$ band to the van Hove points. A surprising near-degeneracy of the nodal $s^prime$- and $d_{x^2-y^2}$-wave solutions leads to the possibility of a near-nodal time-reversal symmetry broken $s^prime+id_{x^2-y^2}$ pair state. Predictions for the temperature dependence of the Knight shift for fields in and out of plane are presented for all states.
When exposed to high magnetic fields, certain materials manifest an exotic superconducting (SC) phase that attracts considerable attention. A proposed explanation of the origin of the high-field phase is the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state. This state is characterized by inhomogeneous superconductivity, where the Cooper pairs have finite center-of-mass momenta. Recently, the high-field phase has been observed in FeSe, and it was deemed to originate from the FFLO state. Here, we synthesized FeSe single crystals with different levels of disorders. The level of disorder is expressed by the ratio of the mean free path to the coherence length and ranges between 35 and 1.2. The upper critical field $B_{rm{c}2}$ was systematically studied over a wide range of temperatures, which went as low as $sim$ 0.5 K, and magnetic fields, which went up to $sim$ 38 T along the $c$ axis and in the $ab$ plane. In the high-field region parallel to the $ab$ plane, an unusual SC phase was confirmed in all the crystals, and the phase was found to be robust to disorders. This result suggests that the high-filed SC state in FeSe may not be a FFLO state, which should be sensitive to disorders.
Elucidating the microscopic origin of nematic order in iron-based superconducting materials is important because the interactions that drive nematic order may also mediate the Cooper pairing. Nematic order breaks fourfold rotational symmetry in the iron plane, which is believed to be driven by either orbital or spin degrees of freedom. However, as the nematic phase often develops at a temperature just above or coincides with a stripe magnetic phase transition, experimentally determining the dominant driving force of nematic order is difficult. Here, we use neutron scattering to study structurally the simplest iron-based superconductor FeSe, which displays a nematic (orthorhombic) phase transition at $T_s=90$ K, but does not order antiferromagnetically. Our data reveal substantial stripe spin fluctuations, which are coupled with orthorhombicity and are enhanced abruptly on cooling to below $T_s$. Moreover, a sharp spin resonance develops in the superconducting state, whose energy (~4 meV) is consistent with an electron boson coupling mode revealed by scanning tunneling spectroscopy, thereby suggesting a spin fluctuation-mediated sign-changing pairing symmetry. By normalizing the dynamic susceptibility into absolute units, we show that the magnetic spectral weight in FeSe is comparable to that of the iron arsenides. Our findings support recent theoretical proposals that both nematicity and superconductivity are driven by spin fluctuations.
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