Collective excitations and screening in two-dimensional tilted nodal-line semimetals

Topological nodal-line semimetals are characterized by symmetry-protected one-dimensional band-touching lines or loops, which give rise to their peculiar Fermi surfaces at low energies. Furthermore, if time-reversal or inversion symmetry breaking tilts the bands, anisotropic Fermi surfaces hosting electron and hole carriers simultaneously can also appear. We analytically investigate the linear density-density response function of a two-dimensional tilted nodal-line semimetal in the intrinsic and doped regimes. Despite the anisotropic electronic bands, the polarizability remains isotropic in our model system. We find that the plasmon dispersion in the long wavelength limit exhibits a standard behavior that is proportional to the square root of the wave vector, characteristic of two-dimensional electron liquids. Tilting tends to enhance the plasmon frequency, and the Drude weight does not depend on the carrier density at low doping levels. In these regimes, unlike the intrinsic and highly-doped ones, the static polarizability has two distinct singularities at finite wave vectors. This results in beat patterns in the Friedel oscillations.Comment: 8 pages, 7 figures, submitte

Density and pseudo-spin rotons in a bilayer of soft-core bosons

We study the dynamics of a bilayer system of bosons with repulsive soft-core Rydberg-dressed interactions within the mean-field Bogoliubov-de Gennes approximation. We find roton minima in both symmetric and asymmetric collective density modes of the symmetric bilayer. Depending on the density of bosons in each layer and the spacing between two layers, the homogeneous superfluid phase becomes unstable in either (or both) of these two channels, leading to density and pseudo-spin-density wave instabilities in the system. Breaking the symmetry between two layers, either with a finite counterflow or a density imbalance renormalizes the dispersion of collective modes and makes the system more susceptible to density-wave instability.Comment: 7 pages, 10 figures, submitte

Theory of correlations in strongly interacting fluids of two-dimensional dipolar bosons

Ground-state properties of a two-dimensional fluid of bosons with repulsive dipole-dipole interactions are studied by means of the Euler-Lagrange hypernetted-chain approximation. We present a self-consistent semi-analytical theory of the pair distribution function $g(r)$ and ground-state energy of this system. Our approach is based on the solution of a zero-energy scattering Schr\"{o}dinger equation for the "pair amplitude" $\sqrt{g(r)}$ with an effective potential from Jastrow-Feenberg correlations. We find excellent agreement with quantum Monte Carlo results over a wide range of coupling strength, nearly up to the critical coupling for the liquid-to-crystal quantum phase transition. We also calculate the one-body density matrix and related quantities, such as the momentum distribution function and the condensate fraction.Comment: 8 pages, 8 figures, submitte