Magnetic Frustration

Spins often prefer to anti-align with their neighbors in antiferromagnetic correlation. Materials with triangle lattices exhibit energetic degeneracy among the possible rearrangements of anti-aligned spins, which is denoted geometric frustration that is associated with strongly depressed transitions to magnetic order. Honeycomb iridates and ruthenates, pyrochlore systems, and double-perovskite iridates all feature triangular lattices as primary building blocks of their structures. Another frustration mechanism evolves from the Kitaev’s exact solution of a spin-liquid model on a honeycomb lattice with strong spin-orbit interactions. The protracted search for a Kitaev spin liquid has recently focused on the honeycomb itidates Na2IrO3 and Li2IrO3. A newer kind of quantum liquid has been identified in the magnetic insulator Ba4Ir3O10, where Ir3O12 trimers form an unfrustrated square lattice.

Imaging the formation and surface phase separation of the CE phase

The CE phase is an extraordinary phase exhibiting the simultaneous spin, charge, and orbital ordering due to strong electron correlation. It is an ideal platform to investigate the role of the multiple orderings in the phase transitions and discover emergent properties. Here, we use a cryogenic high-field magnetic force microscope to image the phase transitions and properties of the CE phase in a Pr0.5Ca0.5MnO3 thin film. In a high magnetic field, we observed a clear suppression of magnetic susceptibility at the charge-ordering insulator transition temperature (TCOI), whereas, at the Néel temperature (TN), no significant change is observed. This observation favors the scenario of strong antiferromagnetic correlation developed below TCOI but raises questions about the Zener polaron paramagnetic phase picture. Besides, we discoverd a phase-separated surface state in the CE phase regime. Ferromagnetic phase domains residing at the surface already exist in zero magnetic field and show ultra-high magnetic anisotropy. Our results provide microscopic insights into the unconventional spin- and charge-ordering transitions and revealed essential attributes of the CE phase, highlighting unusual behaviors when multiple electronic orderings are involved.

Superconductivity, Antiferromagnetism, and Kinetic Correlation in Strongly Correlated Electron Systems

We investigate the ground state of two-dimensional Hubbard model on the basis of the variational Monte Carlo method. We use wave functions that include kinetic correlation and doublon-holon correlation beyond the Gutzwiller ansatz. It is still not clear whether the Hubbard model accounts for high-temperature superconductivity. The antiferromagnetic correlation plays a key role in the study of pairing mechanism because the superconductive phase exists usually close to the antiferromagnetic phase. We investigate the stability of the antiferromagnetic state when holes are doped as a function of the Coulomb repulsionU. We show that the antiferromagnetic correlation is suppressed asUis increased exceeding the bandwidth. High-temperature superconductivity is possible in this region with enhanced antiferromagnetic spin fluctuation and pairing interaction.

QUANTUM MONTE CARLO STUDY OF MAGNETIC CORRELATION IN GRAPHENE NANORIBBONS AND QUANTUM DOTS

To realize the application of spintronics, possible magnetism in graphene-based material is an important issue to be addressed. At the tight banding level of armchair graphene nanoribbons, there are two flat bands in the band structure, two Van Hove singularities in the density of states, and the introducing of the next-nearest-neighbor hopping term cause high asymmetry in them, which plays a key role in the behavior of magnetic correlation. We further our studies within determinant quantum Monte Carlo simulation to treat the electron–electron interaction. It is found that the armchair graphene nanoribbons show carrier mediated magnetic correlation. In the armchair graphene nanoribbons, the antiferromagnetic correlation dominates around half filling, while the ferromagnetic correlation dominates as electron filling is lower than 0.8. Moreover, the ferromagnetic correlation is strengthened markedly as the next-nearest-neighbor hopping energy increases. The resultant manipulation of ferromagnetism in graphene-based material may facilitate the development of spintronics.