The combination of non-Hermitian physics and strong correlations can give rise to new effects in open quantum many-body systems with balanced gain and loss. We propose a generalized Anderson impurity model that includes non-Hermitian hopping terms between an embedded quantum dot and two wires. These non-Hermitian hopping terms respect a parity-time (PT ) symmetry. In the regime of a singly occupied localized state, we map the problem to a PT -symmetric Kondo model and study the effects of the interactions using a perturbative renormalization group approach. We find that the Kondo effect persists if the couplings are below a critical value that corresponds to an exceptional point of the non-Hermitian Kondo interaction. On the other hand, in the regime of spontaneously broken PT symmetry, the Kondo effect is suppressed and the low-energy properties are governed by a local-moment fixed point with vanishing conductance. arXiv:1806.03116v2 [cond-mat.str-el]

No abstract

We consider the superconducting state in a clean crystal with antiferromagnetic (AF) structure of localized magnetic moments taking into account the exchange interaction between magnetic moments and conduction electrons. We assume that the localized moments order at the Neel temperature T N due to the RKKY interaction predominantly. In such crystals, the periodic exchange field acting on conducting electrons results in the formation of an insulating gap on the Fermi surface for electrons moving in directions that depend on the orientation of the wave vector of AF ordering. We assume a scenario in which the Cooper pairing occurs in the open parts of the Fermi surface at the temperature T c . We show that at high amplitudes of exchange field h e > T c /μ B , the structure of superconducting state just below the temperature T c depends on the relation between T c and T N . At low ratio T c /T N , a nonuniform superconducting state, like the Fulde-Farrell-Larkin-Ovchinnikov high-field phase, should exist, while at bigger ratio superconducting order parameter is uniform. The nonuniform structure of superconducting state may be probed by tunneling measurements.

We consider the superconducting pairing induced by spin waves exchange in a ferromagnet with both conduction and localized electrons, the latter being described as spins. We use the microscopic Eliashberg theory to describe the pairing of conducting electrons and the RPA approach to treat the localized spins assuming an exchange coupling between the conducting electrons and spins. In the framework of non relativistic Hamiltonian twe found that he spin wave exchange results in equal spin electron pairing described by the two components of the order parameter, ∆ ↑ (both spins up) and ∆ ↓ (both spins down). Due to the conservation of total spin projection on the axis of the spontaneous ferromagnetic moment, the spin wave exchange at low temperatures includes an emission of magnons and an absorption of thermal magnons by the conduction electrons. The absorption and emission processes depend differently on the temperature, with the absorption being progressively suppressed as the temperature drops. As a result, the superconducting pairing exists only if the electron-spin wave exchange parameter g exceeds some critical value g c . At g > g c pairing vanishes if the temperature drops below the lowest point T cl or increases above the upper critical point T ch ≈ T m (the Curie temperature) where the spin waves cease to exist. This behavior inherent to the spin carrying glue is in an obvious disagreement with the results of conventional BCS approach which assumes that the effective electron-electron attraction is simply proportional to the static magnetic susceptibility.

In this work we investigate the effect produced by the BCS coupling in spinless fermions in one spatial dimension. Using bosonization techniques our initial model is rewritten in terms of a sine-Gordon field and a free massless scalar field. As a result the Cooper pair in our scenario is made up of soliton and antisoliton particles. We calculate the single particle Green's function, the pair correlation function and the optical conductivity associated with the physical fermions and we show how they differ from their conventional quasiparticle analogues. Finally, we compare our results with related experimental findings for high temperature superconductors and we display how they fit qualitatively well the related observed effects produced by the anti-nodal quasiparticles in those materials.