Classification of magnons in rotated ferromagnetic Heisenberg model and their competing responses in transverse fields

Article

关键词: QUANTUM SPIN-GLASSES;    PHASE-TRANSITIONS;    ISING-MODELS;    BIPARTITE;    DYNAMICS;    SYSTEMS;    ROTORS;   

DOI  :  10.1103/PhysRevB.94.024409

来源: SCIE

【 摘 要 】

In this paper, we study the rotated ferromagnetic Heisenberg model (RFHM) in two different transverse fields, h(x) and h(z), which can be intuitively visualized as studying spin-orbit coupling (SOC) effects in two-dimensional (2D) Ising or anisotropic XY model in a transverse field. At a special SOC class, it was found in our previous work [Phys. Rev. A 92, 043609 (2015)] that the RFHM at a zero field owns an exact spin-orbit coupled ground state called the Y-x state. It supports not only the commensurate magnons (called C-C-0 and C-C-pi), but also the incommensurate magnons (called C-IC). These magnons are nonrelativistic, not embedded in the exact ground state, so need to be thermally excited or generated by various external probes. Their dramatic response under a longitudinal h(y) field was recently worked out by Sun et al. [arXiv:1502.05338]. Here we find they respond very differently under the two transverse fields. Any h(x)(h(z)) introduces quantum fluctuations to the ground state and changes the collinear Y-x state to a canted coplanar YX-x (YZ-x) state. The C-C-0, C-C-pi, and C-IC magnons become relativistic and sneak into the quantum ground state. We determine the competing boundaries among the C-C-0, C-C-pi, and C-IC magnons, especially the detailed dispersions of the C-IC magnons inside the canted phases, which can be mapped out by the transverse spin structure factors. As h(x)(h(z)) increases further, the C-C-0 magnons always win the competition and emerge as the seeds to drive a transition from the YX-x (or YZ-x) to the ferromagnetic along the X (orZ) direction called the X-FM (or Z-FM) phase. We show that the transition is in the 3D Ising universality class and it becomes the 3D XY transition at the two Abelian points. We evaluate these magnons' contributions to magnetization and specific heat at low temperatures which can be measured by various established experimental techniques. The nature of the finite-temperature transitions are also studied. Some analogies with quantum fluctuations generated multiple vortices and multiple landscapes in quantum spin glass are mentioned. The implications to cold-atom systems and some 4d and 5d materials with strong SOC are briefly discussed.

【 授权许可】

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