상호작용하는 보스 기체를 냉각하면 기체는 특정 임계 온도 이하에서 U(1) 대칭성을 깨며 초유체 상전이를 일으킨다. 이때, 멀리 떨어진 영역 간의 인과율적 독립성으로 인해 기체 내부에는 위상학적 결점 (topological defect)이 자발적으로 생성될 수 있다. 생성된 결점의 밀도는 키블-주렉 기작 (Kibble-Zurek mechanism)에 따라 상전이 속도와 멱법칙(power-law) 관계를 갖는 것으로 이해되어 왔다. 그러나 최근 기체 냉각이 빠르게 진행될 때, 결점의 밀도가 더 이상 증가하지 않고 포화되는 현상이 여러 실험 연구에서 관측되었다. 양자기체연구실의 연구진은 극저온 루비듐 원자 기체 시료를 이용하여 이러한 결점 포화 현상의 동력학적 원인을 규명하는 연구를 수행하였다. 연구진은 기체 냉각 속도를 조절하며 자발 생성된 위상 결점의 개수 분포와 기체 내 응집체의 성장 곡선을 살펴보았다. 먼저 포화 영역에서 결점 수의 분포가 푸아송 분포를 따르는 것을 관측하였는데, 이를 통해 결점 포화가 결점 생성 이후 결점들 간의 재결합 과정에 의한 완화 효과가 아님을 확인하였다. 또한 응집체의 성장이 포화 영역에서는 온도 변화에 대해 상대적으로 지체되는 것을 관측함으로써 결점 포화가 임계 온도를 지난 직후 발생하는 기체의 초기 조대화 (coarsening)에 기인할 수 있음을 제안하였다. 이번 연구에서 연구진은 결점 포화 현상 이해를 위한 새로운 실험 증거를 제시하였을 뿐만 아니라, 특별히 결점 수의 통계 분포를 비평형 상전이 동역학 연구에 활용하는 새로운 연구 방향을 선보였다. 연구 논문은 Physical Review Letter 지에 게재되었고, Editors’ Suggestion으로 선정되었다.

참여 연구원: 구준홍, 임영훈, 신용일

Defect Saturation in a Rapidly Quenched Bose Gas 

We investigate the saturation of defect density in an atomic Bose gas rapidly cooled into a superfluid phase. The number of quantum vortices, which are spontaneously created in the quenched gas, exhibits a Poissonian distribution not only for a slow quench in the Kibble-Zurek (KZ) scaling regime but also for a fast quench, in which case the mean vortex number is saturated. This shows that the saturation is not caused by destructive vortex collisions, but by the early-time coarsening in an emerging condensate, which is further supported by the observation that the condensate growth lags the quenching in the saturation regime. Our results demonstrate that the defect saturation is an effect beyond the KZ mechanism, opening a path for studying critical phase transition dynamics using the defect number distribution.

URL: https://link.aps.org/doi/10.1103/PhysRevLett.127.115701