Strong dipolar effects in a quantum ferrofluid (0706.1670v2)

Abstract: We report on the realization of a Chromium Bose-Einstein condensate (BEC) with strong dipolar interaction. By using a Feshbach resonance, we reduce the usual isotropic contact interaction, such that the anisotropic magnetic dipole-dipole interaction between 52Cr atoms becomes comparable in strength. This induces a change of the aspect ratio of the cloud, and, for strong dipolar interaction, the inversion of ellipticity during expansion - the usual "smoking gun" evidence for BEC - can even be suppressed. These effects are accounted for by taking into account the dipolar interaction in the superfluid hydrodynamic equations governing the dynamics of the gas, in the same way as classical ferrofluids can be described by including dipolar terms in the classical hydrodynamic equations. Our results are a first step in the exploration of the unique properties of quantum ferrofluids.

• The paper demonstrates that tuning the Feshbach resonance in a Chromium BEC drastically reduces contact interactions, allowing dipolar interactions to dominate.
• It quantifies the dipolar effect using the dimensionless ratio ε₍dd₎ and links key changes in the condensate’s aspect ratio to strong anisotropy.
• The findings open avenues for exploring novel dipolar quantum phases and advancing the control of quantum ferrofluids in engineered traps.

Analysis of Strong Dipolar Effects in a Quantum Ferrofluid

This paper addresses the critical investigation of symmetry-breaking interactions within a novel Bose-Einstein Condensate (BEC) characterized by strong dipolar interactions. By focusing on a BEC composed of Chromium atoms, the authors have methodically manipulated interactions to emphasize anisotropic magnetic dipole-dipole interactions (MDDI) relative to isotropic contact interactions. The employment of a Feshbach resonance to modulate the scattering length between Chromium atoms illustrates the intricate balance between these interactions.

Key Contributions

The paper provides a detailed exploration of the dynamics of a Chromium BEC under the influence of strong MDDI, facilitated by the tuning of a Feshbach resonance at approximately 589 G. This experimental setup enabled a drastic reduction in the contact scattering length to a point where dipolar interactions predominate. The authors quantify this interaction using the dimensionless ratio $\varepsilon_{dd}$, highlighting outcomes when $\varepsilon_{dd}$ approaches and surpasses unity—a threshold where the system is predicted to become unstable against dipolar collapse under classical hydrodynamic models.

Experimental Observations

Key quantitative outcomes include:

• Modification of Aspect Ratio: A significant finding is the observed change in the aspect ratio of the BEC cloud during expansion, with the inhibition of the usual inversion of ellipticity. This behavior is a definitive indicator of strong dipolar interactions dominating the dynamics.
• Dispersive Scattering Length: The paper characterizes the scattering length as a function of the applied magnetic field, revealing its dispersive nature around the Feshbach resonance. At reduced scattering lengths, enhanced inelastic losses were noted, consistent with the increased dipolar interaction strength.
• Dipolar Elongation: Assessments of BEC elongation in response to increased $\varepsilon_{dd}$ elucidate the profound influence of anisotropic interactions, contrasting with purely contact-based interaction dynamics.

Theoretical and Practical Implications

The paper's findings leverage the hydrodynamic formulation of the Gross-Pitaevskii equation incorporating both contact and dipole-dipole interactions, substantiating the experimental trends without an adjustable theoretical model. This work significantly contributes to the broader understanding of quantum ferrofluids, especially as a precursor to examining more complex dipolar quantum phases such as supersolids or checkerboard formations.

Future Directions

This investigation opens numerous avenues for further research. Future studies could explore structures within anisotropic traps, potentially observing phenomena such as roton-like excitation spectra or biconcave density distributions. Moreover, the stability of dipolar condensates in optical lattices, where $\varepsilon_{dd} > 1$, is a promising field for examining non-trivial excitation dynamics and possible novel ground states.

Additionally, the paper lays groundwork for manipulating other BECs of elements with notable magnetic dipole moments or synthesizing polar molecules using Feshbach resonances—each holding potential for advancements in quantum information processing and the development of new quantum materials.

In summary, the paper demonstrates substantial progress in enhancing our comprehension of dipolar interactions in quantum systems. The work's empirical fidelity and its implications for the paper of strongly correlated dipolar quantum gases are notable contributions to the field of atomic and optical physics.