The recently discovered Fe-pnictide and chalcogenide superconductors display low-temperature properties suggesting superconducting gap structures which appear to vary substantially from family to ...family, and even within families as a function of doping or pressure. We propose that this apparent nonuniversality can actually be understood by considering the predictions of spin fluctuation theory and accounting for the peculiar electronic structure of these systems, coupled with the likely 'sign-changing s-wave' (s sub(+ or -)) symmetry. We review theoretical aspects, materials properties and experimental evidence relevant to this suggestion, and discuss which further measurements would be useful to settle these issues.
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.
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).
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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.
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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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.)
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The recent discovery of superconductivity at 190 K in highly compressed H sub(2)S is spectacular not only because it sets a record high critical temperature, but because it does so in a material that ...appears to be, and we argue here that it is, a conventional strong-coupling BCS superconductor. Intriguingly, superconductivity in the observed pressure and temperature range was predicted theoretically in a similar compound, H sub(3)S. Several important questions about this remarkable result, however, are left unanswered: (1) Does the stoichiometry of the superconducting compound differ from the nominal composition, and could it be the predicted H sub(3)S compound? (2) Is the physical origin of the anomalously high critical temperature related only to the high H phonon frequencies, or does strong electron-ion coupling play a role? We show that at experimentally relevant pressures H sub(2)S is unstable, decomposing into H sub(3)S and S, and that H sub(3)S has a record high T sub(c) due to its covalent bonds driven metallic, which make this compound rather similar to MgB sub(2), but unlike most other good conventional superconductors.
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The recently discovered Fe-based superconductor (FeBS) LaFe2As2 seems to break away from an established pattern that doping an FeBS beyond 0.2e/Fe destroys superconductivity. LaFe2As2 has an apparent ...doping of 0.5e, yet superconducts at 12.1 K. Its Fermi surface bears no visual resemblance with the canonical FeBS fermiology. It also exhibits two phases, none magnetic and only one superconducting. We show that the difference between them nonetheless has a magnetic origin, the one featuring disordered moments, and the other locally nonmagnetic. We find that La there assumes an unusual valence of +2.6 to +2.7, so that the effective doping is reduced to 0.30−0.35e. A closer look reveals the same key elements: hole Fermi surfaces near Γ−Z and electron ones near the X−P lines, with the corresponding peak in susceptibility, and a strong tendency to stripe magnetism. The physics of LaFe2As2 is thus more similar to the FeBS paradigm than hitherto appreciated.
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While the phase diagrams of the one- and multiorbital Hubbard model have been well studied, the physics of real Mott insulators is often much richer, material dependent, and poorly understood. In the ...prototype Mott insulator V2O3, chemical pressure was initially believed to explain why the paramagnetic-metal to antiferromagnetic-insulator transition temperature is lowered by Ti doping while Cr doping strengthens correlations, eventually rendering the high-temperature phase paramagnetic insulating. However, this scenario has been recently shown both experimentally and theoretically to be untenable. Based on full structural optimization, we demonstrate via the charge self-consistent combination of density functional theory and dynamical mean-field theory that changes in the V2O3 phase diagram are driven by defect-induced local symmetry breakings resulting from dramatically different couplings of Cr and Ti dopants to the host system. This finding emphasizes the high sensitivity of the Mott metal-insulator transition to the local environment and the importance of accurately accounting for the one-electron Hamiltonian, since correlations crucially respond to it.
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