Quasiparticle properties in the superconducting state are masked by the superfluid and are not directly accessible to infrared spectroscopy. We show how one can use a Kramers–Kronig transformation to separate the quasiparticle from superfluid response and extract intrinsic quasiparticle properties in the superconducting state. We also address the issue of a narrow quasiparticle peak observed in...

We establish a fundamental relationship between the averaged local density of states and the extinction mean free path of waves propagating in random media. From the principle of causality and the Kramers-Kronig relations, we show that both quantities are connected by dispersion relations and are constrained by a frequency sum rule. The results should be helpful in the analysis of wave transpor...

We study Hamiltonians with point interactions in spaces of vectorvalued functions. Using some information from the theory of quantum graphs we describe a class of the operators which can be reduced to the direct sum of several one-dimensional problems. This class includes many “model” interactions, in particular, the so-called δ, δp, δ s, δ , and the Kirchhoff couplings. We apply this observati...

Electronic energy spectra «~q! of antiferromagnetically ordered media may display nonreciprocity; that is, the energies corresponding to Bloch states with wave numbers q and 2q may be different. In this paper a simple Kronig-Penney model, which includes a staggered microscopic magnetic and electric fields of the proper symmetry, is employed to estimate the magnitude of nonreciprocity effects in...

The Huttner−Barnett model is extended to a magnetodielectric medium by adding a new matter field to this model. The eigenoperators for the coupled system are calculated and electromagnetic field is written in terms of these operators. The electric and magnetic susceptibility of the medium are explicitly derived and shown to satisfy the Kramers−Kronig relations. It is shown that the results obta...

We study the bound states of a Kronig Penney potential for a nonlinear one-dimensional Schrodinger equation. This potential consists of a large, but not necessarily infinite, number of equidistant δ-function wells. We show that the ground state can be highly degenerate. Under certain conditions furthermore, even the bound state that would be normally the highest can have almost the same energy ...

We analyze the existence and stability of nonlinear localized waves described by the Kronig-Penney model with a nonlinear impurity. We study the properties of such waves in a homogeneous medium, and then analyze new effects introduced by periodicity of the medium parameters. In particular, we demonstrate the existence of a novel type of stable nonlinear band-gap localized states, and also revea...