Direct Evidence for Dominant Bond-directional Interactions in a Honeycomb Lattice Iridate Na2IrO3 (1504.03618v1)

Abstract: Heisenberg interactions are ubiquitous in magnetic materials and have been prevailing in modeling and designing quantum magnets. Bond-directional interactions offer a novel alternative to Heisenberg exchange and provide the building blocks of the Kitaev model, which has a quantum spin liquid (QSL) as its exact ground state. Honeycomb iridates, A2IrO3 (A=Na,Li), offer potential realizations of the Kitaev model, and their reported magnetic behaviors may be interpreted within the Kitaev framework. However, the extent of their relevance to the Kitaev model remains unclear, as evidence for bond-directional interactions remains indirect or conjectural. Here, we present direct evidence for dominant bond-directional interactions in antiferromagnetic Na2IrO3 and show that they lead to strong magnetic frustration. Diffuse magnetic x-ray scattering reveals broken spin-rotational symmetry even above Neel temperature, with the three spin components exhibiting nano-scale correlations along distinct crystallographic directions. This spin-space and real-space entanglement directly manifests the bond-directional interactions, provides the missing link to Kitaev physics in honeycomb iridates, and establishes a new design strategy toward frustrated magnetism.

• The paper demonstrates that anisotropic bond-directional interactions govern magnetic behavior in Na2IrO3, challenging the classical Heisenberg model.
• It employs diffuse magnetic x-ray scattering above 12–15 K to reveal nano-scale, spin-component-resolved correlations consistent with Kitaev physics.
• The results imply that strong spin-orbit coupling in the honeycomb lattice could pave the way for novel quantum spin liquid states.

Direct Evidence for Dominant Bond-Directional Interactions in a Honeycomb Lattice Iridate Na$_2$IrO$_3$

The paper of magnetism in transition metal oxides has undergone significant refinement with the exploration of bond-directional interactions, deviating from the classical Heisenberg model. The paper "Direct Evidence for Dominant Bond-directional Interactions in a Honeycomb Lattice Iridate Na$_2$IrO$_3$" provides a compelling examination of these interactions within Na$_2$IrO$_3$, a potential realization of the Kitaev model. The findings are pivotal in corroborating the presence of anisotropic exchange terms and their dominance over isotropic interactions in this compound, providing a vital link to Kitaev physics.

The iridate Na$_2$IrO$_3$ features a network of Ir$^{4+}$ ions forming a quasi-two-dimensional honeycomb lattice. Due to strong spin-orbit coupling, pseudospin-1/2 moments emerge, constrained by the edge-shared octahedral bonding, often leading to the suppression of isotropic magnetic interactions. Instead, bond-directional interactions prevail, aligning with the predictions of the Kitaev model. Such interactions are vital for proposing new phases of matter, notably the quantum spin liquid (QSL) state.

Significantly, the authors employ diffuse magnetic x-ray scattering to uncover the nature of the spin interactions above the Néel temperature (T$_{\textrm N}$=12–15 K). They report on the observation of broken spin-rotational symmetry, with nanometer-scale correlations persisting in distinct crystallographic directions. This is a critical indicator of bond-directional interactions, reinforcing their presence beyond theoretical postulations. The spin-component-resolved correlations reveal a nontrivial spin-space entanglement; these nano-scale patterns distinctly support the bond-dependent interactions akin to those described by the Kitaev model.

The zig-zag magnetic ordering observed in Na$_2$IrO$_3$, reminiscent of the Kitaev-Heisenberg framework, faces suppression due to energy-level degeneracies and the exertion of competing magnetic interactions. Despite this, measurements show that these zig-zag correlations extend well beyond T$_{\textrm N}$, emphasizing significant frustration. This characterizes the fabric of its quantum spin liquids’ potential, as indicated by a substantial frustration parameter — the ratio of the Weiss temperature to T$_{\textrm N}$. The findings suggest that even though the low-temperature physics of Na$_2$IrO$_3$ might not align perfectly with a pure Kitaev model, substantial anisotropic interactions inch it closer than other studied systems.

From a practical standpoint, these insights underline the potential for synthesizing novel quantum materials where bond-directional interactions are pivotal. This understanding could propel design principles for harnessing Kitaev quantum spin liquids, thereby advancing technologies reliant on robust, unconventional magnetic states, such as quantum computing frameworks utilizing Majorana fermions.

In summary, the research substantially advances our grasp of magnetic phenomena in honeycomb iridates through systematic evidence of bond-directional interactions. Future explorations might focus on fine-tuning the balance of interaction terms to further stabilize the sought-after QSL phase. This will necessitate precision in experimental techniques and collaboration between theoretical and computational efforts to characterize such complex interactions accurately. Through this paper and further research, the Kitaev materials landscape continues to expand, offering enriching prospects in the exploration of condensed matter systems.