We report on the observation of half-quantum vortices (HQVs) in the easy-plane polar phase of an antiferromagnetic spinor Bose-Einstein condensate. Using in situ magnetization-sensitive imaging, we observe that pairs of HQVs with opposite core magnetization are generated when singly charged quantum vortices are injected into the condensate. The dynamics of HQV pair formation is characterized by measuring the temporal evolutions of the pair separation distance and the core magnetization, which reveals the short-range nature of the repulsive interactions between the HQVs. We find that spin fluctuations arising from thermal population of axial magnon excitations do not significantly affect the HQV pair formation dynamics. Our results demonstrate the instability of a singly charged vortex in the antiferromagnetic spinor condensate. In a scalar superfluid, the supercurrent circulation around quantum vortices is quantized in units of h/m due to U (1) gauge symmetry [1], where h is the Planck constant and m is particle mass. However, when a superfluid possesses an internal spin degree of freedom, there is an intriguing possibility for the superfluid to host quantum vortices of a fractional circulation of h/m. The superfluid phase interwinds with the spin orientation and a new relation is imposed on the supercurrent circulation in connection with spin texture [2]. Fractional quantum vortices are of particular interest in two-dimensional (2D) superfluidity. In the absence of long-range order in two dimensions [3], the superfluid phase transition in 2D is associated with vortex-antivortex pairing as described by the Berezinskii-Kosterlitz-Thouless (BKT) theory [4,5]. Hence fractional quantum vortices, introduced as new point defects, represent an interesting opportunity to explore for exotic superfluid phases, possibly beyond the BKT physics.Quantum vortices having h/2m circulation, so-called half-quantum vortices (HQVs) have been experimentally observed in spinor superfluid systems such as excitonpolariton condensates [6][7][8] and triplet superconductors [9]. In previous cold atom experiments, HQV states were created with an optical method in two-component Bose-Einstein condensates (BECs) [10], where the two components are not symmetric in terms of interactions. Recently, a spin-1 BEC with antiferromagnetic interactions has been considered with great interest because HQVs are topologically allowed in the polar phase of the system [11][12][13]. Theoretical studies predicted an anomalous superfluid density jump at the phase transition in two dimensions [14][15][16] as well as a new superfluid state that has completely broken spin ordering [17,18].In this Letter, we report on the observation of HQVs in the easy-plane polar phase of an antiferromagnetic spinor BEC of 23 Na atoms. Using magnetization-sensitive imaging, we observe that pairs of HQVs with opposite core magnetization are generated when singly charged quantum vortices are injected into the condensate. The temporal evolutions of the pair separation distance and t...

We report the observation of spin domain walls bounded by half-quantum vortices (HQVs) in a spin-1 Bose-Einstein condensate with antiferromagnetic interactions. A spinor condensate is initially prepared in the easy-plane polar phase, and then, suddenly quenched into the easy-axis polar phase. Domain walls are created via the spontaneous Z2 symmetry breaking in the phase transition and the walls dynamically split into composite defects due to snake instability. The end points of the defects are identified as HQVs for the polar order parameter and the mass supercurrent in their proximity is demonstrated using Bragg scattering. In a strong quench regime, we observe that singly charged quantum vortices are formed with the relaxation of free wall-vortex composite defects. Our results demonstrate a nucleation mechanism for composite defects via phase transition dynamics.Topological defects in a continuous ordered system are a splendid manifestation of symmetry breaking, with their fundamental types, such as walls, strings, and monopoles, inevitably determined by the topology of the order parameter space. However, if there is a hierarchy of energy (length) scales with different symmetries, composite defects, such as domain walls bounded by strings and strings terminated by monopoles may exist in the system [1]. In cosmology, it has been noted that such composite defects can be nucleated through successive phase transitions with different symmetry breaking in grand unification theories; furthermore, composite defect formation has been proposed as a possible mechanism for galaxy formation [1,2] and baryogenesis [3] in the early Universe.Spinful superfluid systems with multiple symmetry breaking provide an experimental platform for studying the physics of composite defects and, thus, to examine the cosmological scenario. In superfluid 3 He-B, it has been observed that a spin-mass vortex, on which a planar soliton terminates, can survive after phase transitions by being pinned on the vortex lattice [4,5] or nafen [6]. Composite defects have also been theoretically studied in the atomic Bose-Einstein condensate (BEC) system. Vortex confinement with a domain wall was predicted to occur in a two-component BEC under coherent intercomponent coupling [7]. In particular, for a spin-1 Bose gas with antiferromagnetic interactions, half-quantum vortices (HQVs) joined by a spin domain wall were anticipated to be responsible for the emergence of an exotic 2D superfluid phase with spin-singlet pair correlations [8,9].In this Letter, we report the experimental observation of wall-vortex composite defects in a quasi-2D antiferromagnetic spin-1 BEC. The composite defects are nucleated via a two-step instability mechanism in quantum quench dynamics from the easy-plane polar (EPP) phase into the easy-axis polar (EAP) phase. In the first step, * yishin@snu.ac.kr spontaneous Z 2 symmetry breaking causes domain wall formation, the core of which is occupied by the EPP phase. In the second step, the snake instability splits the domain walls into ...

We have found that one of the details on the spin-dependent phase-contrast imaging method is incorrectly presented in the text. The frequency of the probe light was detuned by −20 MHz from the 3S 1=2 jF ¼ 1i → 3P 3=2 jF 0 ¼ 2i transition, not the 3S 1=2 jF ¼ 1i → 3P 1=2 jF 0 ¼ 2i transition. In the abstract and the text, the gapless magnon excitations associated with the observed spin fluctuations are described as transverse, but they should be referred to as axial magnons [1]. We observed no population of the m z ¼ 0 component in the antiferromagnetic phase. This nomenclatural correction dose not affect the results and conclusion of the Letter.

We investigate the phase transition dynamics of a quasi-2D antiferromagnetic spin-1 Bose-Einstein condensate from the easy-axis polar phase to the easy-plane polar phase, which is initiated by suddenly changing the sign of the quadratic Zeeman energy q. We observe the emergence and decay of spin turbulence and the formation of half-quantum vortices (HQVs) in the quenched condensate. The characteristic time and length scales of the turbulence generation dynamics are proportional to |q| −1/2 as inherited from the dynamic instability of the initial state. In the evolution of the spin turbulence, spin wave excitations develop from large to small length scales, suggesting a direct energy cascade, and the spin population for the axial polar domains exhibit a nonexponential decay. The final equilibrated condensate contains HQVs, and the number is found to increase and saturate with increasing |q|. Our results demonstrate the time-space scaling properties of the phase transition dynamics near the critical point and the peculiarities of the spin turbulence state of the antiferromagnetic spinor condensate.