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Phase transitions are generally a many-body phenomenon, and in order to access the full range of interesting physics of phase transitions, one needs interactions between the microscopic constituents. In this thesis, the phase transitions of atomic systems with interparticle dipole-dipole interaction controlled by laser fields are studied.
In the first half of the thesis, a system with externally polarized dipole molecules at half-filling moving along a one-dimensional zigzag chain is studied, including groundstate phase diagrams. The dipoles are oriented in-plane. Together with the geometry of the chain, this gives rise to a bond-alternating nearest-neighbor interaction due to simultaneous attractive and repulsive interactions. By tuning the ratio between the nearest-neighbor interaction and hopping, various phases can be accessed by controlling the polarization angle. In the ultrastrong coupling limit, the system simplifies to a frustrated extended axial Ising model. For the small coupling limit, a qualitative discussion of the ordering behavior using effective field theory arguments is provided. We show that when the chain angle is small, the system mostly exhibits a phase transition from the gappless phase into the gapped phase, whereas a large chain angle would drive the system into a dimerized phase, where the hopping strength is closely related to the orientation of the dimerized pairs of the molecules.
In the latter part of the thesis, the interatomic correlations of a semiclassical driven dissipative Dicke model are studied. By numerically examining the genuine multiparticle entanglement of the reduced systems of various particle numbers, we show that the entanglement is built up at the transition point, even when the system makes transitions into a highly mixed state. This suggests that the phase transition is of quantum nature. Additionally, the quantum discord of the system is computed. By the use of the full permutation invariance of the system, we show that the numerical complexity in computing quantum discord is significantly reduced. The result indicates that when the dissipation becomes dominant, the system is not entangled but possesses large quantum discord. |
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