Buckled Honeycomb Lattice Physics

• Buckled honeycomb lattices are characterized by two vertically displaced triangular sublattices that enable tunable inversion symmetry breaking and enhanced spin–orbit coupling.
• They facilitate emergent phenomena such as Rashba splitting, quantum anomalous Hall effects, and topological superconductivity through precise control of strain and electric fields.
• Material realizations in oxide heterostructures, group-IV/–V monolayers, and 2D intermetallics demonstrate high-temperature magnetism, correlated electronic order, and advanced spintronic applications.

Buckled honeycomb lattices comprise two interpenetrating triangular sublattices displaced vertically, resulting in non-coplanar coordination central to many emergent phenomena in electronic, magnetic, and phononic systems. Their realization in oxide heterostructures, group-IV/–V monolayers, as well as 2D intermetallics has enabled detailed studies of symmetry-breaking, topological band structures, correlated magnetism, and nematic order. The buckled geometry introduces a natural symmetry “knob” for tuning inversion breaking, enhancing spin–orbit coupling (SOC), and enabling sublattice-selective fields—key ingredients underpinning Rashba effects, quantum anomalous Hall (QAH) physics, and boundary-obstructed topological superconductivity.

1. Lattice Geometry, Symmetry, and Structural Parameterization

Buckled honeycomb lattices are defined by two sublattices displaced along the out-of-plane direction. In oxide (111) superlattices such as (SrHfO₃)₂/(LaAlO₃)₄(111) or (LaXO₃)₂/(LaAlO₃)₄ (X: 3d transition metals), the cation layers stack as alternating planes, yielding a vertical sublattice separation $d_z \simeq$ 0.15–0.57$a_0$; in 2D group-IV monolayers (silicene, germanene, blue phosphorene), buckling heights range from ~0.45 Å (Si, Ge) to ~1 Å (Sb) (Ono, 2021, Liu et al., 2019). The point group symmetry is typically lowered from D₆ₕ (planar) to D₃d or lower, depending on buckling and octahedral rotations.

Buckling breaks inversion symmetry if the two sublattices are inequivalent or when structural distortions remove the inversion center (e.g., P1 phase in SrHfO₃ heterostructures (Köksal, 4 Aug 2025)). This symmetry lowering is critical: inversion-preserving (centrosymmetric) phases such as P321 maintain spin degeneracy, while non-centrosymmetric phases enable Rashba coupling and valley contrasts.

2. Electronic Structure, Spin–Orbit Coupling, and Topological Phase Transitions

Buckled honeycomb lattices are archetypes for Dirac bands, with the additional presence of substantial SOC ($\lambda_\mathrm{SO}$) and sublattice staggering ($\Delta$) due to vertical displacement (Fig. 1 and Eqs. (1,3) in (Köksal, 4 Aug 2025), Eq. (1) in (Yan et al., 2015)). The effective low-energy Hamiltonian is generically: $$H(\mathbf{k}) = \hbar v_F (\tau_z k_x \sigma_x + k_y \sigma_y) + \tau_z s_z \lambda_{\rm SO} \sigma_z + \Delta \sigma_z$$ with $\tau_z$ (valley), $s_z$ (spin), $\sigma_i$ (sublattice Pauli matrices), and $\Delta$ proportional to the symmetry-breaking potential ($\sim eE_z d$).

SOC is enhanced by buckling-induced hybridization between $\pi$ and $\sigma$ orbitals (Yan et al., 2015). Strain further tunes $\lambda_{\rm SO}$ through modifications of the buckling angle: compressive biaxial strain ($\varepsilon<0$) increases $\lambda_{\rm SO}$, while tension reduces it. This directly controls the gap and the critical field $E_c$ for topological transitions, with $$E_c(\varepsilon) = \lambda_{\rm SO}(\varepsilon) / l(\varepsilon)$$ (Yan et al., 2015). The band topology, characterized by $Z_2$ or Chern invariants, undergoes transitions when the Dirac mass $$m_{\tau_z s_z} = \tau_z s_z \lambda_{\rm SO} - l(\varepsilon) E_z$$ changes sign (Doennig et al., 2015).

In Mott-insulating buckled honeycomb oxides, the combination of large $U$, strong SOC, and staggered $\Delta$ enables topological antiferromagnetic Chern insulator (AFCI) phases (Hafez-Torbati et al., 6 Nov 2025). The quantum phase transition is controlled by gap closings at $K$, $K'$, with gap criteria: $\Delta + \alpha m \pm 3\sqrt{3}\lambda = 0$ where $m$ is the AF mean field.

3. Buckled Honeycomb Magnetism: Mott Physics, High-TN Orders, and Chern Phases

In buckled honeycomb transition-metal systems, the geometry fundamentally reorganizes exchange paths and anisotropy:

• In Sr₃CaOs₂O₉, Os ions form a highly buckled honeycomb layer (buckling amplitude ~0.17 Å), with Os–O–Os bond angles of 171–177° (Thakur et al., 2022). Strong nearest-neighbor AF exchange ($J_1 \sim +8.5$ meV), sizable interlayer coupling ($J_c \sim +4.4$ meV), and robust Neel order at $T_N = 385$ K result—demonstrating that strong SOC ($\lambda \sim 30$ meV) and buckling lift $T_N$ to or above room temperature.
• In Ba₂NiTeO₆, the buckled lattice promotes stripe magnetic order, stabilized by the competition between nearest and third-neighbor interactions and enhanced single-ion anisotropy ($D \sim 1.1$ meV), resulting in a low-temperature gap $\Delta \sim 2$ meV (Asai et al., 2017).
• Canted AF or noncollinear order is realized in Co₄Ta₂O₉, where buckled honeycomb (Co1) and flat (Co2) layers alternate along $c$, facilitating Dzyaloshinskii–Moriya interactions and strong magnetoelectric coupling (Choi et al., 2020).

Antiferromagnetic Chern insulators are theoretically accessible in buckled honeycomb Mott systems under perpendicular electric fields: the staggered potential $\Delta$ drives a transition from trivial AF Mott to topological AFCI as determined by the sign change of the Dirac mass at $K$/$K'$ (Hafez-Torbati et al., 6 Nov 2025). For $\lambda \sim 30$ meV, $t \sim 0.3$–$0.5$ eV, and achievable $E_z$, the quantization temperature for the Hall conductance, $T_q \sim 0.92\lambda$, can reach room temperature, as in Sr₃CaOs₂O₉.

4. Rashba Splitting, Imaginative Hopping, and Spin–Charge Conversion

The absence of inversion symmetry (e.g., $P1$ phase in SrHfO₃-based superlattices) allows for Rashba-type spin splitting near the $M$ and $K$ points, realized through imaginary components of second-nearest-neighbor (t₂) inter-orbital hoppings and on-site SOC ($\lambda$) (Köksal, 4 Aug 2025). This is captured by a Wannier-based tight-binding model, where enhanced imaginary $t_2$ in noncentrosymmetric phases produces a Rashba coefficient $\alpha_R \approx 0.34$ eV·Å and Rashba energy $E_R = 29$ meV, comparable to leading oxide systems.

Berry curvature, directly tied to these imaginary hoppings, peaks near $\Gamma$ and creates “hot spots” essential for spin–charge conversion. These effects are symmetry-controlled: by switching between $P321$ (inversion symmetric) and $P1$, the Rashba effect can be switched on/off, enabling functionality for oxide-based spintronics and field-effect devices (Köksal, 4 Aug 2025).

5. Topological Flat Bands, Quantum Anomalous Hall, and Superconductivity

Buckled honeycomb lattices manipulated via “one-side saturation” or sublattice-selective potentials realize flat bands and promote robust correlation-driven ferromagnetism. In one-side-saturated germanene, the unsaturated sublattice supports narrow midgap bands; even modest $U$ triggers flat-band ferromagnetism, while small $U$ maintains nontrivial QAH phases with Chern numbers $C_n = -1$, and for special commensurate vacancy fractions, $C_n = +2$ (Huang et al., 2014).

Buckled honeycombs with magnetic order (e.g., AF or FM) subjected to magnetic fields can display arbitrarily high spin-Chern numbers, a regime inaccessible in conventional QH systems (Ezawa, 2013). Control of sublattice asymmetry, exchange fields, and SOC enables isolation of spin-polarized “fan” Landau levels, resulting in quantized charge and spin-Hall conductivities.

Buckled honeycomb lattices also support boundary-obstructed topological superconductivity (BOTS): with $f$-wave spin-triplet pairing and nonzero sublattice potential, the system hosts Kramers-pair Majorana corner states governed by the Kitaev-chain boundary reduction (Ghadimi et al., 2023).

6. Phononic Topology, Mechanical Response, and Rotational Invariance

The buckled geometry substantially modifies mechanical and phononic properties:

• In group-V monolayers (blue phosphorene, arsenene, antimonene), the critical strain for mechanical failure balances bond stretch and rotation; under tensile strain, the buckling height $h$ decreases, and failure occurs preferentially through elastic instability or, for zigzag loading, by phonon softening in the flexural (ZA) mode (Liu et al., 2019).
• Topological phonon phases are predicted in the abstract force-constant space of the buckled honeycomb lattice, with up to nine topological phases indicated by Wilson-loop windings (Gutierrez-Amigo et al., 2022). However, real BHL materials typically occupy the trivial phase due to rapid decay of force-constants with neighbor distance. Engineering of topological phonons may rely on strain, alloying, or metamaterial construction.
• Rotational invariance in spin-elastic models is crucial for describing rippled-to-buckled transitions in membranes: coupling internal degrees of freedom to curvature (rather than gradients) results in thermally driven, rotationally invariant buckling transitions observed in heated graphene (García-Valladares et al., 2022).

7. Material Realization and Applications

The interplay of buckling, inversion symmetry breaking, SOC, and electronically correlated ground states makes buckled honeycomb systems uniquely tunable for topological spintronics, high-temperature quantum anomalous Hall insulators, oxide-based spin–charge interconversion, and phononic devices:

Material System	Role of Buckling	Key Emergent Phenomena
SrHfO₃/(LaAlO₃)(111) hetero.	Inversion symmetry control	Electric-field-tunable Rashba SOC, Berry curvature “hot spots” (Köksal, 4 Aug 2025)
Sr₃CaOs₂O₉ (triple perovskite)	Robust AF + large SOC	Room-temperature AF Chern insulator, high $T_q$ (Thakur et al., 2022, Hafez-Torbati et al., 6 Nov 2025)
Silicene/germanene monolayers	Sublattice control	Gate-tunable QSH, QAH, high spin-Chern, and BOTS phases (Huang et al., 2014, Yan et al., 2015, Ghadimi et al., 2023)
Group-V BHL monolayers	Buckling/strain	Mechanical robustness, failure mechanics, phononic topology (Liu et al., 2019, Gutierrez-Amigo et al., 2022)

Tunability via electric field, strain, and symmetry breaking allows dynamic “on–off” control of topological, Rashba, and magnetic phenomena, positioning buckled honeycomb lattices at the forefront of topological material and device research.