We review the main ingredients for an unconventional pairing state in the ferropnictides, with particular emphasis on interband pairing due to magnetic fluctuations. Summarizing the key experimental ...prerequisites for such pairing, the electronic structure and nature of magnetic excitations, we discuss the properties of the
s
± state that emerges as a likely candidate pairing state for these materials and survey experimental evidence in favor of and against this novel state of matter.
Superconductivity, the resistance-free flow of electrical charges, is one of the most exotic phenomena in solid-state physics. Even though it was discovered almost a century ago, many questions ...remain unanswered, in particular those concerning the physics of high-temperature superconductivity. The recent discovery of iron-based superconductors was arguably the most important breakthrough in this field for more than two decades and may provide new avenues for understanding this high-temperature phenomenon. Here I present my view of the recent developments in this field that have led to the current understanding of this important new class of superconductor.
Over the past few years iron chalcogenides have been intensively studied as part of the wider family of iron-based superconductors, with many intriguing results reported so far on intercalated and ...monolayer FeSe. Nevertheless, bulk FeSe itself remains an unusual case when compared with pnictogen-based iron superconductors, and may hold clues to understanding the more exotic derivatives of the FeSe system. The FeSe phase diagram is distinct from the pnictides: the orthorhombic distortion, which is likely to be of a 'spin-nematic' nature in numerous pnictides, is not accompanied by magnetic order in FeSe, and the superconducting transition temperature Tc rises significantly with pressure before decreasing. Here we show that the magnetic interactions in FeSe, as opposed to most pnictides, demonstrate an unusual and unanticipated frustration, which suppresses magnetic (but not nematic) order, triggers ferro-orbital order in the nematic phase and can naturally explain the non-monotonic pressure dependence of the superconducting critical temperature Tc (P).
Recently, the most intensely studied objects in the electronic theory of solids have been strongly correlated systems and graphene. However, the fact that the Dirac bands in graphene are made up of ...sp(2) electrons, which are subject to neither strong Hubbard repulsion U nor strong Hund's rule coupling J, creates certain limitations in terms of novel, interaction-induced physics that could be derived from Dirac points. Here we propose GaCu3(OH)6Cl2 (Ga-substituted herbertsmithite) as a correlated Dirac-Kagome metal combining Dirac electrons, strong interactions and frustrated magnetic interactions. Using density functional theory, we calculate its crystallographic and electronic properties, and observe that it has symmetry-protected Dirac points at the Fermi level. Its many-body physics is diverse, with possible charge, magnetic and superconducting instabilities. Through a combination of various many-body methods we study possible symmetry-lowering phase transitions such as Mott-Hubbard, charge or magnetic ordering, and unconventional superconductivity, which in this compound assumes an f-wave symmetry.
When sulfur and silicon are incorporated in monolayer 2H-NbSe
the superconducting transition temperature, T
, has been found to vary non-monotonically. This was assumed to be a manifestation of ...fractal superconductivity. Using first-principles calculations, we show that the nonmonotonic dependence of T
is insufficient evidence for multifractality. A unifying aspect in our study are selenium vacancies in NbSe
, which are magnetic pair-breaking defects that we propose can be present in considerable concentrations in as-grown NbSe
. We show that sulfur and silicon can occupy the selenium sites and reduce the pair-breaking effect. Furthermore, when sulfur is incorporated in NbSe
, the density of states at the Fermi level and the proximity to magnetism in the alloy are both reduced compared to the parent compound. Based on our results, we propose an alternative explanation of the non-monotonic change in T
which does not require the conjecture of multifractality.
Recent observations A. Pustogow et al., Nature (London) 574, 72 (2019). of a drop of the 17O nuclear magnetic resonance (NMR) Knight shift in the superconducting state of Sr2RuO4 challenged the ...popular picture of a chiral odd-parity paired state in this compound. Here we use polarized neutron scattering (PNS) to show that there is a 34 ± 6 % drop in the magnetic susceptibility at the Ru site below the superconducting transition temperature. We measure at lower fields ... than a previous PNS study allowing the suppression to be observed. The PNS measurements show a smaller susceptibility suppression than NMR measurements performed at similar field and temperature. Our results rule out the chiral odd-parity ... state and are consistent with several recent proposals for the order parameter including even-parity B1g and odd-parity helical states.(ProQuest: ... denotes formulae omitted.)
Recently discovered superconductivity in YbC6 and CaC6, at temperatures substantially higher than previously known for intercalated graphites, raises several new questions. (1) Is the mechanism ...considerably different from that of previously known intercalated graphites? (2) If superconductivity is conventional, what are the relevant phonons? (3) Given the extreme similarity between YbC6 and CaC6, why are their critical temperatures so different? We address these questions on the basis of first-principles calculations and conclude that coupling with intercalant phonons is likely to be the main force for superconductivity in YbC6 and CaC6, but not in alkaline-intercalated compounds, and explain the difference in T(c) by the "isotope effect" due to the difference in Yb and Ca atomic masses.
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