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Register a new userSpin nematic order in antiferromagnetic spinor condensates
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Large spin systems can exhibit unconventional types of magnetic ordering different from the ferromagnetic or Neel-like antiferromagnetic order commonly found in spin 1/2 systems. Spin-nematic phases, for instance, do not break time-reversal invariance and their magnetic order parameter is characterized by a second rank tensor with the symmetry of an ellipsoid. Here we show direct experimental evidence for spin-nematic ordering in a spin-1 Bose-Einstein condensate of sodium atoms with antiferromagnetic interactions. In a mean field description this order is enforced by locking the relative phase between spin components. We reveal this mechanism by studying the spin noise after a spin rotation, which is shown to contain information hidden when looking only at averages. The method should be applicable to high spin systems in order to reveal complex magnetic phases.
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We investigate, both experimentally and theoretically, the quench dynamics of antiferromagnetic spinor Bose-Einstein condensates in the vicinity of a zero temperature quantum phase transition at zero quadratic Zeeman shift q. Both the rate of instability and the associated finite wavevector of the unstable modes - show good agreement with predictions based upon numerical solutions to the Bogoliubov de-Gennes equations. A key feature of this work is inclusion of magnetic field inhomogeneities that smooth the phase transition. Once these were removed, we observed a dramatic sharpening of the transition point, which could then be resolved within a quadratic Zeeman shift of only 1-2 Hz. Our results point to the use of dynamics, rather than equilibrium quantities for high precision measurements of phase transitions in quantum gases.
We have experimentally investigated the quench dynamics of antiferromagnetic spinor Bose-Einstein condensates in the vicinity of a zero temperature quantum phase transition at zero quadratic Zeeman shift $q$. The rate of instability shows good agreement with predictions based upon solutions to the Bogoliubov de-Gennes equations. A key feature of this work was removal of magnetic field inhomogeneities, resulting in a steep change in behavior near the transition point. The quadratic Zeeman shift at the transition point was resolved to 250 milliHertz uncertainty, equivalent to an energy resolution of $k_B times 12$ picoKelvin. To our knowledge, this is the first demonstration of sub-Hz precision measurement of a phase transition in quantum gases. Our results point to the use of dynamics, rather than equilibrium studies for high precision measurements of phase transitions in quantum gases.
We create and study persistent currents in a toroidal two-component Bose gas, consisting of $^{87}$Rb atoms in two different spin states. For a large spin-population imbalance we observe supercurrents persisting for over two minutes. However we find that the supercurrent is unstable for spin polarisation below a well defined critical value. We also investigate the role of phase coherence between the two spin components and show that only the magnitude of the spin-polarisation vector, rather than its orientation in spin space, is relevant for supercurrent stability.
The fragmentation of spin-orbit coupled spin-1 Bose gas with a weak interaction in external harmonic trap is explored by both exact diagonalization and mean-field theory. This fragmentation tendency, which originates from the total angular momentum conservation, is affected obviously by the spin-orbit coupling strength and the spin-dependent interaction. Strong spin-orbit interaction raises the inverse participation ratio, which describes the number of significantly occupied single-particle states. As the spin-dependent interaction changes from anti-ferromagnetic to ferromagnetic, the peak values in the inverse participation ratio become lower. Without the confinement of the appointed total angular momentum, the condensate chooses a zero or finite total angular momentum ground state, which is determined by both the interaction and the spin-orbit coupling strength.
We develop a symmetry classification scheme to find ground states of pseudo spin-1/2, spin-1, and spin-2 spin-orbit coupled spinor Bose-Einstein condensates, and show that as the SO(2) symmetry of simultaneous spin and space rotations is broken into discrete cyclic groups, various types of lattice structures emerge in the absence of a lattice potential, examples include two different kagaome lattices for pseudo spin-1/2 condensates and a nematic vortex lattice in which uniaxial and biaxial spin textures align alternatively for spin-2 condensates. For the pseudo spin-1/2 system, although mean-field states always break time-reversal symmetry, there exists a time-reversal invariant many-body ground state, which is fragmented and expected to be observed in a micro-condensate.
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