Spin phase

Chitoshi Yasuda, Satoru Akiyama

Spin–phonon interaction effects in a spin-1/2 antiferromagnetic Heisenberg chain coupled to phonons are investigated in systems with variable crystal structures. By performing a unitary transformation to a Hamiltonian of the system, we derive effective Hamiltonians expanded in powers of spin–phonon and spin–spin couplings. Ground-state phase transitions between a spin-liquid phase and a spin-gapped phase are investigated by a level spectroscopy analysis of numerical diagonalization data of effective systems. Our results depend on whether the ground state of the Heisenberg chain is in the spin-liquid phase or the spin-gapped phase. In the system where the ground state is in the spin-liquid phase, the spin–phonon interaction causes the usual phase transition to the spin-gapped phase. On the other hand, in the system where the ground state is in the spin-gapped phase, the spin–phonon interaction causes the novel phase transition to the spin-liquid phase for some geometrical structures. The phase transition from the spin-gapped phase to the spin-liquid phase is observed by changing a parameter to control the change in a geometrical structure.

Toshiya Hikihara, Takahiro Misawa, Tsutomu Momoi

We study frustrated spin-1/2 dimer systems in two dimensions with a bilayer structure, where spins are ferromagnetically coupled in dimers. Our model includes frustrated two-spin exchange interactions as well as four-spin interactions. We pay particular attention to the spin nematic phase, which does not exhibit any magnetic (spin-dipole) order but has a spin-quadrupolar long-range order. Employing a perturbation calculation, a mean-field approximation, and a numerical many-variable variational Monte Carlo method, we determine ground-state phase diagrams on various two-dimensional lattices. It is found that the model exhibits the spin nematic phase with ferroquadrupolar order in a wide parameter region, in addition to conventional magnetically ordered phases. In particular, it is shown that even when the four-spin interactions are absent, frustrated two-spin exchange interactions can realize the spin nematic phase as a result of strong interdimer correlations. It is also found that the phase transitions between the spin nematic phase and antiferromagnetic phases can be continuous. Furthermore, we present some exact arguments that various phases including the spin nematic phase and the vector chiral (p-type nematic) phase emerge from an SU(4) symmetric point in the model by the addition of appropriate perturbative interactions. The spin nematic phase generated from the SU(4) point is connected with the spin nematic phase found numerically in the system with only two-spin interactions.

Martin Heni, Matthias Freyberger, Wolfgang P. Schleich

Ji-Guo Wang, Yueqing Li, Yu-Fei Dong

We consider the SU(3) spin–orbit coupled spin-1 Bose–Einstein condensates in a two-dimensional harmonic trap. The competition between the SU(3) spin–orbit coupling and the spin-exchange interaction results in a rich variety of lattice configurations. The ground-state phase diagram spanned by the isotropic SU(3) spin–orbit coupling and the spin–spin interaction is presented. Five ground-state phases can be identified on the phase diagram, including the plane wave phase, the stripe phase, the kagome lattice phase, the stripe-honeycomb lattice phase, and the honeycomb hexagonal lattice phase. The system undergoes a sequence of phase transitions from the rectangular lattice phase to the honeycomb hexagonal lattice phase, and to the triangular lattice phase in spin-1 Bose–Einstein condensates with anisotrpic SU(3) spin–orbit coupling.

Miguel Orszag