Droplets and Supersolids in Dipolar Bose-Einstein Condensates

Abstract

Ultracold quantum gases have emerged as a powerful platform for exploring complex quantum many-body physics due to their tunability and precise control. Engineering interatomic potentials via external fields leads to a wealth of exciting phenomena in ultracold gases. In particular, dipole-dipole interaction, characterized by its long-range and anisotropic nature, introduces intriguing physics. Notably, it leads to the formation of quantum droplets: self-bound liquid-like states stabilized by quantum fluctuations, and the fascinating supersolid phase, which simultaneously exhibits superfluidity and crystalline order. We have investigated a unique scenario in cold atom physics, where a single atom possesses both the electric and magnetic dipole moments, leading to the prediction of doubly dipolar Bose-Einstein condensates (DDBEC). The additional control pa- rameters of relative strength and orientation between the two dipole moments pave the pathway to realize a novel pancake quantum droplet, contrary to the typical cigar droplets in dipolar condensates. Thermal fluctuations critically affect the properties of quantum gases. In this thesis, we explore the characteristics of a doubly dipolar Bose gas at finite temper- atures, using quantum Monte Carlo techniques. Our investigation demonstrates the highly anisotropic superfluid behavior of a pancake droplet and the shift in critical temperature for condensation due to the doubly dipolar interactions. The interplay between doubly dipolar interactions, quantum stabilization, and external confinement results in a rich ground-state physics of supersolids and inco- herent droplet arrays in doubly dipolar condensates. Our research under a beyond- mean-field framework reveals the supersolid-supersolid transitions and the realiza- tion of a novel pancake array of supersolids. We also observe the formation of a single density-modulated droplet. Finally, we have investigated the exciting possibility of creating a supersolid in dipolar Bose gases, without relying on the quantum stabilization. Rotating a dipolar condensate perpendicular to its dipole polarization direction, we observe the formation of mean-field dilute supersolids stabilized by the emergence of quantized vortex lines.