磁阱中超冷玻色气体临界行为的观测

Observation of critical behavior of ultra-cold Bose gas in a magnetic trap

超冷玻色气体为研究量子临界现象提供了一个非常干净的实验系统.弱相互作用下的三维玻色气体的临界行为与~4He发生超流相变时的临界行为类似,都属于三维XY型普适类.从正常流体到超流的量子相变过程中,系统会经历一个从无序相到长程有序相的转变;而在相变点附近,系统参量会表现出一些奇点的特征.本文从实验上观测到了静磁阱中超冷^(87)Rb玻色气体在凝聚体相变温度T_c附近的临界行为.原子气体从静磁阱中释放,经过30 ms的自由飞行后,通过吸收成像得到原子气体的动量分布;然后从中扣除热原子气体的动量分布,提取出空间上处于临界区域内的原子气体动量分布,并对不同温度下的动量分布半高宽进行统计.统计结果显示:在非常接近相变温度T_c时,动量分布的半高宽突然减小,表现出十分明显的奇点行为.

Quantum criticality emerges when the collective fluctuations of matter undergo a continuous phase transition at zero temperature and has been a research focus in conventional condensed-matter physics over the past several decades.In the quantum critical regime,the exotic and universal properties are expected.These properties are independent of the microscopic details of the system,but depend only on a few general properties of the system,such as its dimensionality and the symmetry of the order parameter.The research of quantum criticality can not only help us to understand quantum phase transitions,but also provide a novel route to new material design and discovery.Ultracold bosonic gases have provided a clean system for studying the quantum critical phenomena.The critical behavior of a weakly interacting three-dimensional（3D） Bose gas should be identical to that of 4 He at the superfluid transition,which belongs to the 3D XY universality class.From the normal fluid to the superfluid,the system undergoes a phase transition from completely disorder to long-range order,while in the vicinity of the phase transition point,the system parameters will show some singularity characteristics.In this paper,we observe the critical behavior of ^（87）Rb Bose gas in a quadrupole-Ioffe configuration（QUIC） trap near the phase transition temperature Tc.A novel singularity behavior of the full width at half maximum of momentum distribution（FWHMMD） of atomic gas is discovered in the experiment.Prior to our experiment,we prepare a sample with 7.8×10~5 ^（87）Rb atoms in the 5S_（1/2） ｜F=2,m_F=2state.Then the sample is held in a QUIC trap for a presetting period of time to control the temperature of atom sample precisely.During the holding time,the sample is heated up due to background gas collisions or fluctuations of the trap potential.In our experiment,the heating rate is deduced to be 0.348±0.078 nK/ms from the absorption image.For a bosonic gas in a harmonic trap,critical gas can only cover a finite-size region due to a spatially varying density.We define the finite-size region as a critical region determined by the Ginzburg criterion.Then the FWHMMDs of atomic gas in the critical region are measured for different temperatures near the critical point.To this aim,we first extract the momentum distribution of atomic gas from the absorption image of the atomic clouds released from the QIUC trap after free expansion.Thus momentum distribution of atomic gas in the critical region can be extracted from the absorption image by subtracting the momentum distribution of thermal gas outside the critical region.According to the statistical results of the FWHMMD at different temperatures,we find that the FWHMMD suddenly reduces,thus revealing a very notable singularity behavior when the temperature is very close to the phase transition temperature T_c.