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Register a new userDichotomy between the hole and electrons behavior in the multiband FeSe probed by ultra high magnetic fields
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Magnetoresistivity r{ho}xx and Hall resistivity r{ho}xy in ultra high magnetic fields up to 88T are measured down to 0.15K to clarify the multiband electronic structure in high-quality single crystals of superconducting FeSe. At low temperatures and high fields we observe quantum oscillations in both resistivity and Hall effect, confirming the multiband Fermi surface with small volumes. We propose a novel and independent approach to identify the sign of corresponding cyclotron orbit in a compensated metal from magnetotransport measurements. The observed significant differences in the relative amplitudes of the quantum oscillations between the r{ho}xx and r{ho}xy components, together with the positive sign of the high-field r{ho}xy , reveal that the largest pocket should correspond to the hole band. The low-field magnetotransport data in the normal state suggest that, in addition to one hole and one almost compensated electron bands, the orthorhombic phase of FeSe exhibits an additional tiny electron pocket with a high mobility.
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The discovery of high-temperature (Tc) superconductivity in monolayer FeSe on SrTiO3 raised a fundamental question whether high Tc is commonly realized in monolayer iron-based superconductors. Tetragonal FeS is a key material to resolve this issue because bulk FeS is a superconductor with Tc comparable to that of isostructural FeSe. However, difficulty in synthesizing tetragonal monolayer FeS due to its metastable nature has hindered further investigations. Here we report elucidation of band structure of monolayer FeS on SrTiO3, enabled by a unique combination of in-situ topotactic reaction and molecular-beam epitaxy. Our angle-resolved photoemission spectroscopy on FeS and FeSe revealed marked similarities in the electronic structure, such as heavy electron doping and interfacial electron-phonon coupling, both of which have been regarded as possible sources of high Tc in FeSe. However, surprisingly, high-Tc superconductivity is absent in monolayer FeS. This is linked to the weak superconducting pairing in electron-doped multilayer FeS in which the interfacial effects are absent. Our results strongly suggest that the cross-interface electron-phonon coupling enhances Tc only when it cooperates with the pairing interaction inherent to the superconducting layer. This finding provides a key insight to explore new heterointerface high-Tc superconductors.
Interlayer tunneling resistivity is used to probe the low-energy density-of-states (DOS) depletion due to the pseudogap in the normal state of Bi$_2$Sr$_2$CaCu$_2$O$_{8+y}$. Measurements up to 60 T reveal that a field that restores DOS to its ungapped state shows strikingly different temperature and doping dependencies from the characteristic fields of the superconducting state. The pseudogap closing field and the pseudogap temperature $T^{star}$ evaluated independently are related through a simple Zeeman energy scaling. These findings indicate a predominant role of spins over the orbital effects in the formation of the pseudogap.
There is growing evidence that the superconducting semimetal FeSe ($T_csim8$ K) is in the crossover regime between weak-coupling Bardeen-Cooper-Schrieffer (BCS) and strong-coupling Bose-Einstein-condensate (BEC) limits. We report on longitudinal and transverse thermal conductivities, $kappa_{xx}$ and $kappa_{xy}$, respectively, in magnetic fields up to 20 T. The field dependences of $kappa_{xx}$ and $kappa_{xy}$ imply that a highly anisotropic small superconducting gap forms at the electron Fermi-surface pocket whereas a more isotropic and larger gap forms at the hole pocket. Below $sim1.0$ K, both $kappa_{xx}$ and $kappa_{xy}$ exhibit distinct anomalies (kinks) at the upper critical field $H_{c2}$ and at a field $H^*$ slightly below $H_{c2}$. The analysis of the thermal Hall angle ($kappa_{xy}/kappa_{xx}$) indicates a change of the quasiparticle scattering rate at $H^*$. These results provide strong support to the previous suggestion that above $H^*$ a distinct field-induced superconducting phase emerges with an unprecedented large spin imbalance.
We report a high-resolution laser-based angle-resolved photoemission spectroscopy (laser-ARPES) study of single crystals of FeSe, focusing on the temperature-dependence of the hole-like bands around the ${rm Gamma}$ point. As the system cools through the tetragonal-orthorhombic nematic structural transition at 90~K, the splitting of the $d_{xz}$/$d_{yz}$ bands is observed to increase by a magnitude of 13 meV. Moreover, the onset of a $sim$10 meV downward shift of the $d_{xy}$ band is also at 90~K. These measurements provide clarity on the nature, magnitude and temperature-dependence of the band shifts at the ${rm Gamma}$ point in the nematic phase of FeSe.
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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