We analyze antiferromagnetism and superconductivity in novel F e−based superconductors within the itinerant model of small electron and hole pockets near (0, 0) and (π, π). We argue that the effective interactions in both channels logarithmically flow towards the same values at low energies, i.e., antiferromagnetism and superconductivity must be treated on equal footings. The magnetic instability comes first for equal sizes of the two pockets, but looses to superconductivity upon doping. The superconducting gap has no nodes, but changes sign between the two Fermi surfaces (extended s-wave symmetry). We argue that the T dependencies of the spin susceptibility and NMR relaxation rate for such state are exponential only at very low T , and can be well fitted by power-laws over a wide T range below Tc.

A fundamental and unconventional characteristic of superconductivity in iron-based materials is that it occurs in the vicinity of two other instabilities. In addition to a tendency towards magnetic order, these Fe-based systems have a propensity for nematic ordering: a lowering of the rotational symmetry while time-reversal invariance is preserved. Setting the stage for superconductivity, it is heavily debated whether the nematic symmetry breaking is driven by lattice, orbital or spin degrees of freedom. Here, we report a very clear splitting of NMR resonance lines in FeSe at Tnem = 91 K, far above the superconducting Tc of 9.3 K. The splitting occurs for magnetic fields perpendicular to the Fe planes and has the temperature dependence of a Landau-type order parameter. Spin-lattice relaxation rates are not affected at Tnem, which unequivocally establishes orbital degrees of freedom as driving the nematic order. We demonstrate that superconductivity competes with the emerging nematicity.

Transition metal oxides with a perovskite-type structure constitute a large group of compounds with interesting properties. Among them are materials such as the prototypical ferroelectric system BaTiO(3), colossal magnetoresistance manganites and the high-T(c) superconductors. Hundreds of these compounds are magnetic, and hundreds of others are ferroelectric, but these properties very seldom coexist. Compounds with an interdependence of magnetism and ferroelectricity could be very useful: they would open up a plethora of new applications, such as switching of magnetic memory elements by electric fields. Here, we report on a possible way to avoid this incompatibility, and show that in charge-ordered and orbitally ordered perovskites it is possible to make use of the coupling between magnetic and charge ordering to obtain ferroelectric magnets. In particular, in manganites that are less than half doped there is a type of charge ordering that is intermediate between site-centred and bond-centred. Such a state breaks inversion symmetry and is predicted to be magnetic and ferroelectric.

We predict a p-wave Cooper pairing of the spin-polarized fermions in a binary fermion-boson mixture due to the exchange of density fluctuations of the bosonic medium. We then examine the dependence of the Cooper paring temperature on the parameters of the system. We finally estimate the effect of combining the boson-induced interaction with other pairing mechanisms, e.g the Kohn-Luttinger one, and find that the critical temperature of p-wave Cooper pairing can be realistic for experiment.

We extend previous calculations of the nonanalytic terms in the spin susceptibility s ͑T͒ and the specific heat C͑T͒ to systems in a magnetic field. Without a field, s ͑T͒ and C͑T͒ / T are linear in T in two dimensions ͑2D͒, while in 3D, s ͑T͒ ϰ T 2 and C͑T͒ / T ϰ T 2 ln T. We show that in a magnetic field, the linear in T terms in 2D become scaling functions of B H / T. We present explicit expressions for these functions and show that at high fields B H ӷ T, s ͑T , H͒ scales as ͉H͉. We also show that in 3D, s ͑T , H͒ becomes nonanalytic in a field and at high fields scales as H 2 ln͉H͉.

The discovery of superconductivity (SC) with a transition temperature, T c , up to 65 K in single-layer FeSe (bulk T c = 8 K) films grown on SrTiO 3 substrates has attracted special attention to Fe-based thin films. The high T c is a consequence of the combined effect of electron transfer from the oxygen-vacant substrate to the FeSe thin film and lattice tensile strain. Here we demonstrate the realization of SC in the parent compound BaFe 2 As 2 (no bulk T c ) just by tensile lattice strain without charge doping. We investigate the interplay between strain and SC in epitaxial BaFe 2 As 2 thin films on Fe-buffered MgAl 2 O 4 single crystalline substrates. The strong interfacial bonding between Fe and the FeAs sublattice increases the Fe-Fe distance due to the lattice misfit which leads to a suppression of the antiferromagnetic spin density wave and induces SC with bulk-T c ≈ 10 K. These results highlight the role of structural changes in controlling the phase diagram of Fe-based superconductors.

We analyze the effect of disorder on the weak-coupling instabilities of quadratic band crossing point (QBCP) in two-dimensional Fermi systems, which, in the clean limit, display interactiondriven topological insulating phases. In the framework of a renormalization group procedure, which treats fermionic interactions and disorder on the same footing, we test all possible instabilities and identify the corresponding ordered phases in the presence of disorder for both single-valley and two-valley QBCP systems. We find that disorder generally suppresses the critical temperature at which the interaction-driven topologically non-trivial order sets in. Strong disorder can also cause a topological phase transition into a topologically trivial insulating state.PACS numbers: 73.43. Nq, 71.55.Jv, 11.30.Qc The study of topological phases of matter is one of the most active research areas in contemporary condensed matter physics. The explanation of the quantum Hall effect in terms of the topological properties of the Landau levels [1,2] in the 1980's triggered an intense research effort in the theoretical prediction [3][4][5] and the experimental discovery [6,7] of a plethora of different topologically non-trivial quantum phases. In two-dimensional (2D) insulating systems only two distinct topological non-trivial phases can be realized according to the well-established classification of topological insulators and superconductors [8,9] : (i) the quantum anomalous Hall state (QAH) [3] with a time-reversal symmetry-broken ground state and topologically protected chiral edge states and (ii) the time-reversal invariant quantum spin Hall (QSH) state [4,5], which possesses helical edge states with counterpropagating electrons of opposite spins.In recent years, attention has gradually shifted from non-interacting topological states of matter towards interaction-driven topological phases:many-particle quantum ground-states in which chiral orbital currents or spin-orbit couplings are spontaneously generated by electronic correlations. These states of matter possess both conventional order, characterized by an order parameter and a broken symmetry, and protected edge states associated with a topological quantum number. Interactiondriven QAH and QSH phases were first conceived in the context of 2D honeycomb lattice Dirac fermions [10] assuming sufficiently strong electronic repulsions although more recent analytical and numerical works question the proposal for this particular model [11][12][13][14].On the contrary, it has been proposed that 2D systems with a quadratic band crossing point (QBCP) are unstable to electronic correlation because of the finite density of states at the Fermi level leading to the possibility of The relationship between fixed points QAH and QAH-II is provided in the inset figure of Fig. 2(a).weak-coupling interaction-driven topological insulating phases [15][16][17]. And, indeed, QAH and QSH phases generated by electronic repulsions occur both in the checkerboard lattice model [15,18], and in two-valley QB...

A wide variety of complex phases in quantum materials are driven by electron-electron interactions, which are magnified by regions in momentum space where the density of states is enhanced. A well known example occurs at van Hove singularities where the Fermi surface undergoes a topological transition. Here we show that higher order singularities, where multiple disconnected leafs of Fermi surface touch all at once, naturally occur at points of high symmetry in the Brillouin zone. Such multicritical singularities can lead to stronger divergences in the density of states than canonical van Hove singularities, and further boost the formation of complex quantum phases via interactions. As a concrete example, we demonstrate these theoretical ideas in the analysis of experimental data on Sr3Ru2O7 in the vicinity of the metamagnetic quantum critical point, resolving several previously puzzling aspects of the data.Introduction. The properties of unconventional density waves in quantum materials are generally connected to features of the electronic band structure. Characteristic wave vectors of emergent order parameters can for example often be related to nesting-type features of the underlying Fermi surface as discussed for e.g. iron pnictides 1 , organics 2 , and transition metal dichalcogenides 3 . Yet these nesting features in themselves usually cannot account for the observed thermodynamic stability of such correlated quantum phases. Intriguingly in a range of these materials the band structure hosts energetically close-by singularities in the density of states (DOS), which have been conjectured to be crucial ingredients stabilising the emergent phases.Singularities in the DOS occur naturally at Fermi surface (FS) topological Lifshitz transitions (LT). A prominent example is the van Hove singularity (vHs) formed at a saddle point in the energy-momentum dispersion (see fig. 1a). A two-dimensional (2D) vHs has a relatively weak logarithmic divergence in the DOS but is known to lead to a wealth of phenomena such as ferromagnetism driven by the Stoner mechanism (see eg. Ref. 4). An important yet under-appreciated point is that the thermodynamic stability of the emergent phases does not only depend on the magnitude but also shape of the singularity as a function of energy 4 (i.e. gradient and curvature). As a consequence stronger power law divergences can have a much more dramatic impact on the formation of complex ordered phases than the relatively weak vHs.Building on this insight, we explore a generalisation of these concepts to multicritical topological transitions where multiple disjoint parts of a FS merge. Such multicritical FS topological transitions naturally occur at points of high crystal symmetry, where the number n of FS components merging depends on the particular symmetry. In fig. 1a-c we illustrate the symmetries associated with the n = 2 (vHs), 3 and 4 cases in 2D. When the singularity occurs at an edge of a Brillouin zone there are generically two pieces (n = 2) of the Fermi surface that join at the sin...