Properties of 2D and Quasi-2D Dipolar Bosons with Non-zero Tilt Angles at T=0

Abstract

Recent experimental advances in creating stable dipolar bosonic systems, including polar molecules with large electric dipole moments, have led to vigorous theoretical activities. Recent reporting of observation of roton feature in dipolar erbium has provided added impetus to theoretical and experimental work. Here we discuss our mean-field theory work on 2D and quasi-2D dipolar bosons with dipoles oriented at an angle to the direction perpendicular to the confining 2D plane, i.e. for {\it non-zero tilt angles}. Using Bogoliubov-de Gennes equations, we present results on a number of T=0 properties of both 2D and quasi-2D systems, such as excitation spectra, structure functions, sound velocities, quantum depletion, etc. We explore instabilities at varying tilt angle, density and dipolar coupling. We map out phase diagrams as a function of tilt angle, dipole strength and density. We find the development of maxon-roton behavior leading to roton instabilities at large densities for small tilt angles, and at low densities for large tilt angles. The behavior is anisotropic in k-space; accordingly the roton instabilities occur first in the $k_y$ direction, suggestive of inhomogeneity and stripe phase, with density mode becoming soft in the $y$-direction. Beyond a critical tilt angle, at any density, the dipolar system collapses owing to a phonon instability. We discuss similarities and differences between the properties of 2D and quasi-2D dipolar systems at non-zero tilt angles.