Excitations and Stripe Phase Formation in a 2D Dipolar Bose Gas with Tilted Polarization

TL;DR

This paper investigates how tilting the polarization in a 2D dipolar Bose gas induces anisotropic excitations and leads to a quantum phase transition from a gas to a stripe phase, revealing the role of polarization angle in phase stability.

Contribution

It introduces a combined theoretical approach to analyze the ground state, excitations, and phase transition in a tilted dipolar Bose gas, highlighting the emergence of stripe order.

Findings

Density instability occurs at high polarization angles.

Roton energy decreases and vanishes near the phase transition.

Stripe phase exhibits long-range order in the strongly interacting direction.

Abstract

We present calculations of the ground state and excitations of an anisotropic dipolar Bose gas in two dimensions, realized by a non-perpendicular polarization with respect to the system plane. For sufficiently high density an increase of the polarization angle leads to a density instability of the gas phase in the direction where the anisotropic interaction is strongest. Using a dynamic many-body theory, we calculate the dynamic structure function in the gas phase which shows the anisotropic dispersion of the excitations. We find that the energy of roton excitations in the strongly interacting direction decreases with increasing polarization angle and almost vanishes close to the instability. Exact path integral ground state Monte Carlo simulations show that this instability is indeed a quantum phase transition to a stripe phase, characterized by long-range order in the strongly interacting direction.