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Starting from a microscopic itinerant model, we derive and analyze the effective low-energy model for collective magnetic excitations in the iron pnictides. We show that the stripe magnetic order is generally preempted by an Ising-nematic order which breaks C4 lattice symmetry but preserves O(3) spin-rotational symmetry. This leads to a rich phase diagram as function of doping, pressure, and elastic moduli, displaying split magnetic and nematic tri-critical points. The nematic transition may instantly bring the system to the verge of a magnetic transition, or it may occur first, being followed by a magnetic transition at a lower temperature. In the latter case, the preemptive nematic transition is accompanied by either a jump or a rapid increase of the magnetic correlation length, triggering a pseudogap behavior associated with magnetic precursors. Furthermore, due to the distinct orbital character of each Fermi pocket, the nematic transition also induces orbital order. We compare our results to various experiments, showing that they correctly address the changes in the character of the magneto-structural transition across the phase diagrams of different compounds, as well as the relationship between the orthorhombic and magnetic order parameters.

The last few years have seen an explosion of interest in hydrodynamic effects in interacting electron systems in ultra-pure materials. In this paper we briefly review the recent advances, both theoretical and experimental, in the hydrodynamic approach to electronic transport in graphene, focusing on viscous phenomena, Coulomb drag, non-local transport measurements, and possibilities for observing nonlinear effects.Hydrodynamics describes a great variety of phenomena around (and inside) us, including, e.g., the flow of water in rivers, seas, and oceans, atmospheric phenomena, aircraft motion, the flow of petroleum in pipelines, or the blood flow through blood vessels in humans and animals. It has been realized a long time ago that the flow of electrons in a conductor should, under certain circumstances, also obey the laws of hydrodynamics. In particular, Gurzhi 1,2 predicted that electrons can exhibit a Poiseuille-type flow 3,4 analogous to that of liquids in pipes. This should result in an initial power-law decrease of resistivity with the transverse cross-section of a sample and temperature, leading to a pronounced minimum. It has turned out, however, that an experimental realization of such a regime in a metal or a semiconductor is a highly non-trivial task. Three decades have passed before de Jong and Molenkamp 5 observed the Gurzhi effect, and even in that work the magnitude of the effect did not exceed 20%.Why is it so difficult to implement electron hydrodynamics in a laboratory experiment? In contrast to molecules of a conventional liquid, electrons move in the environment formed by the crystal lattice. Therefore, the electrons experience not only collisions among themselves, but also scatter off thermally excited lattice vibrations -phonons -as well as various lattice imperfections (impurities). The hydrodynamic regime is realized when the frequency of electron-electron collisions is much larger than the rates of both, electron-phonon and electron-impurity scattering. These two requirements limit the temperature window for the hydrodynamic flow both from above and from below, and may even be in a conflict with each other. In a typical solid, the elastic impurity scattering dominates electronic transport at low temperatures, whereas at high temperatures the leading mechanism is the electron-phonon scattering. Thus, the requirement of the electron-electron scattering being the fastest process -which is the key condition for the hydrodynamics -may only be satisfied, if at all, in an intermediate temperature range. It turns out that this regime is not well developed in conventional conductors, with the possible exception of the ultrahigh-mobility GaAs quantum wells 6-8 exhibiting negative magnetoresistance 9 and ultra-pure palladium cobaltate 10 . The experimental discovery of graphene 11 has given a new boost to the research in the field of quantum transport. In particular, it has been realized that, among other remarkable properties, graphene is an excellent material for the realization of hydrodynamic flo...

A hallmark of the phase diagrams of quantum materials is the existence of multiple electronic ordered states, which, in many cases, are not independent competing phases, but instead display a complex intertwinement. In this review, we focus on a particular realization of intertwined orders: a primary phase characterized by a multi-component order parameter and a fluctuation-driven vestigial phase characterized by a composite order parameter. This concept has been widely employed to elucidate nematicity in iron-based and cuprate superconductors. Here we present a group-theoretical framework that extends this notion to a variety of phases, providing a classification of vestigial orders of unconventional superconductors and density-waves. Electronic states with scalar and vector chiral order, spin-nematic order, Ising-nematic order, time-reversal symmetry-breaking order, and algebraic vestigial order emerge from one underlying principle. The formalism provides a framework to understand the complexity of quantum materials based on symmetry, largely without resorting to microscopic models. arXiv:1804.00818v1 [cond-mat.str-el]

Pinpointing the microscopic mechanism for superconductivity has proven to be one of the most outstanding challenges in the physics of correlated quantum matter. Thus far, the most direct evidence for an electronic pairing mechanism is the observation of a new symmetry of the order parameter, as done in the cuprate high-temperature superconductors. Alternatively, global, topological invariants allow for a sharp discrimination between states of matter that cannot be transformed into each other adiabatically. Here we propose an unconventional pairing state for the electron fluid in two-dimensional oxide interfaces and establish a direct link to the emergence of non-trivial topological invariants. Topological signatures, in particular Majorana edge states, can then be used to detect the microscopic origin of superconductivity. In addition, we show that also the density wave states that compete with superconductivity sensitively depend on the nature of the pairing interaction.