Screening and collective modes in gapped bilayer graphene

We study the static and dynamic screening of gapped bilayer graphene. Unlike previous works we use the 4-band model instead of the simplified 2-band model. We find that there are important qualitative differences between the dielectric screening function obtained using the 2-band and that obtained using the 4-band model. In particular within the 4-band model in the presence of a band-gap the static screening exhibits Kohn anomalies that are absent within the 2-band model. Moreover, using the 4-band model, we are able to examine the effect of trigonal warping (absent in the 2-band model) on the screening properties of bilayer graphene. We also find that the plasmon modes have qualitatively different character in the 4-band model compared to 2-band results.Comment: 4 pages, 5 figures. Published versio

Astrophysical Consequences of a Neutrinophilic Two-Higgs-Doublet Model

In a recently proposed neutrinophilic two-Higgs doublet model, the low-energy (sub-MeV) effective theory consists of a real scalar with a vev of O(0.1) eV and three Dirac neutrinos. Other models could lead to the same low energy theory. In this Brief Report, we study constraints on the parameter space of the model, including vacuum stability, unitarity, perturbativity and the effects on the invisible Z width. Interestingly, we find that all neutrinos become massless at temperatures above approximately 1000 K, but can find no phenomenological effects of this finding. The most direct test of the model is that it predicts that in a galactic supernova, the energy distributions of the electron, muon and tau neutrinos will be Fermi-Dirac with identical temperatures, unlike the conventional distributions.Comment: 5 pages, 1 figure. Minor revisions. Version to be published in Phys. Rev.

Pair symmetry conversion in driven multiband superconductors

It was recently shown that odd-frequency superconducting pair amplitudes can be induced in conventional superconductors subjected to a spatially-nonuniform time-dependent drive. It has also been shown that, in the presence of interband scattering, multiband superconductors will possess bulk odd-frequency superconducting pair amplitudes. In this work we build on these previous results to demonstrate that by subjecting a multiband superconductor with interband scattering to a time-dependent drive even-frequency pair amplitudes can be converted to odd-frequency pair amplitudes and vice versa. We will discuss the physical conditions under which these pair symmetry conversions can be achieved and possible experimental signatures of their presence.Comment: 14 pages, 5 figure

Odd-frequency pairing and Kerr effect in the heavy-fermion superconductor UPt3_3

We study the emergence of odd-frequency superconducting pairing in UPt$_3$. Starting from a tight-binding model accounting for the nonsymmorphic crystal symmetry of UPt$_3$ and assuming an order parameter in the $E_{2u}$ representation, we demonstrate that odd-frequency pairing arises very generally, as soon as inter-sublattice hopping or spin-orbit coupling is present. We also show that in the low temperature superconducting $B$ phase, the presence of a chiral order parameter together with spin-orbit coupling, leads to additional odd-frequency pair amplitudes not present in the $A$ or $C$ phases. Furthermore, we show that a finite Kerr rotation in the $B$ phase is only present if odd-$\omega$ pairing also exists.Comment: 8 pages, 2 figure

Odd-frequency Superconductivity in Driven Systems

We show that Berezinskii's classification of the symmetries of Cooper pair amplitudes holds for driven systems even in the absence of translation invariance. We then consider a model Hamiltonian for a superconductor coupled to an external driving potential and, treating the driving potential as a perturbation, we investigate the corrections to the anomalous Green's function, density of states, and spectral function. We find that in the presence of an external drive the anomalous Green's function develops terms that are odd in frequency and that the same mechanism responsible for these odd-frequency terms generates additional features in the density of states and spectral function.Comment: 10 pages, 3 figure

Odd-frequency superconductivity induced by non-magnetic impurities

A growing body of literature suggests that odd-frequency superconducting pair amplitudes can be generated in normal metal-superconductor junctions. The emergence of odd-frequency pairing in these systems is often attributed to the breaking of translation invariance. In this work, we study the pair symmetry of a one-dimensional $s$-wave superconductor in the presence of a single non-magnetic impurity and demonstrate that translation symmetry breaking is not sufficient for inducing odd-frequency pairing. We consider three kinds of impurities: a local perturbation of the chemical potential, an impurity possessing a quantum energy level, and a local perturbation of the superconducting gap. Surprisingly, we find local perturbations of the chemical potential do not induce any odd-frequency pairing, despite the fact that they break translation invariance. Moreover, although odd-frequency can be induced by both the quantum impurity and the perturbation of the gap, we find these odd-frequency amplitudes emerge from entirely different kinds of scattering processes. The quantum impurity generates odd-frequency pairs by allowing one of the quasiparticles belonging to an equal-time Cooper pair to tunnel onto the impurity state and then back to the superconductor, giving rise to odd-frequency amplitudes with a temporal broadening inversely proportional to the energy level of the impurity. In contrast to this, the perturbation of the gap leads to odd-frequency pairing by "gluing-together" normal state quasiparticles from different points in space and time, leading to odd-frequency amplitudes which are very localized in the time domain.Comment: 7 pages, 1 figur