We argue that the newly discovered superconductivity in a nearly magnetic, Fe-based layered compound is unconventional and mediated by antiferromagnetic spin fluctuations, though different from the usual superexchange and specific to this compound. This resulting state is an example of extended s-wave pairing with a sign reversal of the order parameter between different Fermi surface sheets. The main role of doping in this scenario is to lower the density of states and suppress the pair-breaking ferromagnetic fluctuations.

The concept of a CDW induced by Fermi-surface nesting originated from the Peierls idea of electronic instabilities in purely 1D metals and is now often applied to charge ordering in real lowdimensional materials. The idea is that if Fermi surface contours coincide when shifted along the observed CDW wave vector, then the CDW is considered to be nesting-derived. We show that in most cases this procedure has no predictive power, since Fermi surfaces either do not nest at the right wave vector, or nest more strongly at the wrong vector. We argue that only a tiny fraction, if any, of the observed charge ordering phase transitions are true analogues of the Peierls instability because electronic instabilities are easily destroyed by even small deviations from perfect nesting conditions. Using prototypical CDW materials NbSe2, TaSe2, and CeTe3, we show that such conditions are hardly ever fulfilled, and that the CDW phases are actually structural phase transitions, driven by the concerted action of electronic and ionic subsystems, i.e., q-dependent electron-phonon coupling plays an indispensable part. We also show mathematically that the original Peierls construction is so fragile as to be unlikely to apply to real materials. We argue that no meaningful distinction between a CDW and an incommensurate lattice transition exists.

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Insulators occur in more than one guise, a recent finding was a class of topological insulators, which host a conducting surface juxtaposed with an insulating bulk. Here we report the observation of an unusual insulating state with an electrically insulating bulk that simultaneously yields bulk quantum oscillations with characteristics of an unconventional Fermi liquid. We present quantum oscillation measurements of magnetic torque in high purity single crystals of the Kondo insulator SmB 6 , which reveal quantum oscillation frequencies characteristic of a large three-dimensional conduction electron Fermi surface similar to the metallic rare earth hexaborides such as PrB 6 and LaB 6 . The quantum oscillation amplitude strongly increases at low temperatures, appearing strikingly at variance with conventional metallic behaviour.Kondo insulators, a class of materials positioned close to the border between insulating and metallic behaviour, provide fertile ground for unusual physics [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. This class of strongly correlated materials is thought to be characterised by a 1 arXiv:1507.01129v1 [cond-mat.str-el] 4 Jul 2015

The 2008 discovery of superconducting ferropnictides with T c~2 6K-56K introduced a new family of materials into the category of high T c superconductors. The ongoing project of understanding the superconducting mechanism and pairing symmetry has already revealed a complicated and often contradictory underlying picture of the structural and magnetic properties. There is an almost unprecedented sensitivity of the calculated magnetism and Fermi surface to structural details that prohibits correspondence with experiment. Furthermore, experimental probes of the order parameter symmetry are in surprisingly strong disagreement, even considering the relative immaturity of the field. Here we outline all of the various and seemingly contradictory evidences, both theoretical and experimental, and show that they can be rectified if the system is assumed to be highly magnetic with a spin density wave that is well-defined but with magnetic twin and anti-phase boundaries that are dynamic on the time-scale of experiments.Under this assumption, we find that our calculations can accurately reproduce even very fine details of the structure, and a natural explanation for the temperature separation of structural and magnetic transitions is provided. Thus, our theory restores agreement between experiment and theory in crucial areas, making further cooperative progress possible on both fronts. We believe that fluctuating magnetic domains will be an essential component of unravelling the interplay between magnetic interactions and superconductivity in these newest high T c superconductors.

We use highly accurate density functional calculations to study the band structure and Fermi surfaces of NbSe2. We calculate the real part of the non-interacting susceptibility, ℜχ0(q), which is the relevant quantity for a charge density wave (CDW) instability and the imaginary part,ℑχ0(q), which directly shows Fermi surface (FS) nesting. We show that there are very weak peaks in ℜχ0(q) near the CDW wave vector, but that no such peaks are visible in ℑχ0(q), definitively eliminating FS nesting as a factor in CDW formation. Because the peak in ℜχ0(q) is broad and shallow, it is unlikely to be the direct cause of the CDW instability. We briefly address the possibility that electronelectron interactions (local field effects) produce additional structure in the total (renormalized) susceptibility, and we discuss the role of electron-ion matrix elements.In 1964 V.L. Ginzburg proposed excitonic superconductivity in quasi-2D structures 1 composed of metal layers sandwiched between insulating layers. By that time, only two layered materials were known to be superconducting, 2,3 PdTe 2 and NbSe 2 . In the four decades since then, NbSe 2 and isostructural selenides have been intensively investigated (close to 1.5 thousand papers published to date). However, the main interest in this compound has shifted from the fact that it is a layered material (hundreds of layered superconductors are now known), to the existence of a nearly-commensurate charge density wave (CDW) 4 instability and it's possible interplay with the superconductivity that sets in at a lower temperature. We mainly forego discussion of the interesting issues surrounding the superconducting state, its origin, and its relationship to the CDW state, and instead concentrate on the mechanism behind the CDW transition itself.The first electronic structure calculations for NbSe 2 were presented by Mattheiss in 1973 5 . Using a non-selfconsistent potential he was able to produce a band structure with basic features in reasonable agreement with more recent self-consistent calculations 6,7,8 , but which showed only two bands crossing the Fermi energy (it is now known that there are three), and underestimated the energy depth of a saddle point at ∼ 1 2 ΓK. Fermi surfaces based on this band structure led to early suggestions that the CDW transition was driven by nesting 4 , an assumption that has carried through to the present time. The nearness of the saddle point to the Fermi energy (E F ) led Rice and Scott 9 to argue that CDW formation was driven, not by Fermi surface (FS) nesting in the conventional sense, but rather by saddle points lying within k B T CDW of E F and separated by the CDW wavevector, Q CDW = ( 1 3 , 0, 0). A significant amount of effort has been spent on resolving the 'controversy' between the nesting and saddle point theories for NbSe 2 and for related CDW compounds such as 2H-TaSe 2 , 1T-TaSe 2 , TaS 2 and others, but no specific feature that would give rise to an instability at Q CDW has been convincingly isolated. As early as 1978, Doran et al 10...

X-ray and neutron diffraction, Raman spectroscopy, complex impedance spectroscopy and electron microscopy were used to characterize the tetragonal vs. cubic phase stability in superionic conducting garnet-oxide electrolyte.

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