Seminar Laboratorije za primenu racunara u nauci
U cetvrtak 11. septembra 2008. godine u 13:00 sati,
u citaonici biblioteke Instituta za fiziku, Pregrevica br. 118, Zemun
Priv-Doz. Dr. Axel Pelster
University of Duisburg-Essen, Germany
Critical Temperature of Dipolar Interacting Bose Gases
Ultracold atomic quantum gases are many-body systems where a wide variety of macroscopic quantum phenomena is observable. For the original Bose-Einstein condensates (BECs) of alkali atoms, it has been sufficient to describe the dominant two-particle interaction by a short-range and isotropic contact potential. Its strength is described by the s-wave scattering length as which is varied by using a so-called Feshbach resonance over a broad range. Recently, a BEC has also been realized in a dipolar quantum gas of 52Cr atoms where the magnetic dipole moments are around six times larger than those of alkali atoms. Therefore, the physical properties of such a chromium BEC also depend on a long-range and anisotropic magnetic dipole-dipole interaction. Other many-body systems with dipolar interactions are, for instance, Rydberg atoms or atomic condensates where a strong electric field induces electric dipole moments of the order of 10^(−2) Debye. Permanent dipole moments in heteronuclear molecules are much larger with typical values of 1 Debye, so their dipolar effects could be a few hundred times stronger than those of chromium atoms. Such a gas of ultracold heteronuclear molecules is produced either by sophisticated cooling and trapping techniques or by photoassociation. The dipolar interaction strength can be tuned for induced dipole moments by varying the field strength and for permanent dipoles by using rotating magnetic fields. Combining this rotation technique with Feshbach resonances, will allow in the near future experiments where the interaction varies from purely contact to purely dipolar.
Here we investigate how the critical temperature of a dipolar BEC depends on the dipole-dipole interaction. Consider an atomic gas trapped in a cylinder-symmetric harmonic potential whose dipole moments m have the angle with the z-axis. In particular, we are interested in the two extreme configurations I and II where the symmetry axis of the dipole moments is parallel (alpha=0) and perpendicular (alpha=pi/2) to the symmetry axis of the harmonic trap, respectively. The figure below illustrates both cases where the atomic dipole moments lead to a residual attractive and repulsive interaction, respectively. At first we show that the corresponding critical temperature in configuration I and II is shifted above and below the value of a pure contact interaction. Subsequently we suggest to determine the difference of the critical temperatures in both configurations I and II to cancel out the influence of the isotropic contact interaction. Furthermore, we shall estimate which experimental parameters allow us to enhance this signal.