Spontaneous demagnetization of a dipolar spinor Bose gas at ultra-low magnetic field

Quantum degenerate Bose gases with an internal degree of freedom, known as spinor condensates, are natural candidates to study the interplay between magnetism and superfluidity. In the spinor condensates made of alkali atoms studied so far, the spinor properties are set by contact interactions, while magnetization is dynamically frozen, due to small magnetic dipole-dipole interactions. Here, we study the spinor properties of S=3 $^{52}$Cr atoms, in which relatively strong dipole-dipole interactions allow changes in magnetization. We observe a phase transition between a ferromagnetic phase and an unpolarized phase when the magnetic field is quenched to an extremely low value, below which interactions overwhelm the linear Zeeman effect. The BEC magnetization changes due to magnetic dipole-dipole interactions that set the dynamics. Our work opens up the experimental study of quantum magnetism with free magnetization using ultra-cold atoms.Comment: 6 pages, 4 figures, 2 appendice

Excitations d un condensat de Bose-Einstein dipolaire

Nos expériences ont été consacrées à l étude d un condensat de chrome, élément possédant un fort moment magnétique. Elles nous ont permis de mettre en évidence l in uence des interactions dipolaires sur les propriétés hydrodynamiques et magnétiques d un condensat. Ces interactions sont, contrairement aux interactions de contact, anisotropes et à longue portée. Nous avons étudié un mode d oscillations collectives, de type quadrupolaire, du condensat de chrome. Nous avons observé que la fréquence de ce mode dépend légèrement de l orientation des dipôles, xée par le champ magnétique. De plus, nous avons utilisé la di usion de Bragg pour sonder le spectre des excitations de Bogoliubov du condensat. Nous avons caractérisé l e et des interactions dipolaires sur les énergies d excitation. Dans le régime phononique, nous en avons déduit une anisotropie de la vitesse du son, qui dépend de la direction de propagation de l onde sonore relativement à l axe de polarisation des dipôles. Nous avons également étudié les collisions inélastiques dues à l interaction dipôle-dipôle, appelées relaxations dipolaires. Nous avons observé, d une part, que le taux de collision est directement lié aux corrélations de paires dans le condensat, et d autre part, que ce taux dépend fortement de la dimensionnalité du système. À très bas champ magnétique, nous avons mis en évidence une dynamique spinorielle, due aux interactions dipolaires, qui se traduit par une démagnétisation spontanée du condensat. Ces expériences ont été facilitées par la mise au point d une cavité optique passive ultra-stable , utilisée pour asservir nos nouveaux lasers.Abstract : In our experiments, we have studied a chromium Bose-Einstein condensate. Because of the strong magnetic moment of chromium, these experiments enabled us to characterize the in uence of dipolar interactions on the hydrodynamic and magnetic properties of a condensate. These interactions are anisotropic and long range, as opposed to contact interactions. We have studied a quadrupole-like collective excitation mode of the chromium condensate. We have observed that the frequency of this mode is dependent on the orientations of the atomic dipoles, given by the direction of the external magnetic eld. Moreover, we have used Bragg spectroscopy in order to measure the shift caused by dipolar interactions on the Bogoliubov excitations energies. In the phononic regime, our results reveal the anisotropy of the velocity of sound, which depends on the direction of the sound wave with respect to the axis of the dipoles. We have also studied dipolar relaxation, which is a type of inelastic collision caused by dipolar interactions. We have observed that the collision rate is directly related to pair correlations in the condensate. Furthermore, we have shown that this rate is strongly dependent on the dimensionality of the system. At very low magnetic elds, dipolar interactions are responsible for a spontaneous demagnetization of the condensate, which we have also observed. Our experiments were made easier by the building of a passive optical cavity, which was used to e ectively stabilize our new lasers.PARIS13-BU Sciences (930792102) / SudocSudocFranceF