Enhanced electronic correlations and altermagnetic ground state of two-dimensional CsCr3Sb5 monolayers

Abstract: Recently, layered corrected kagome metal CsCr3Sb5 have garnered significant attention attributed to its flat bands near the Fermi level (EF) and altermagnetic ground state [ Yi Liu et al., Nature 632, 1032 (2024)]. However, the van Hove singularities (vHSs) in bulk CsCr3Sb5 are far away from the EF, while an effective modulation of VHS toward the EF is essential for exploring intriguing electron transport properties. In this work, using first-principles calculations, we investigate electronic structures of two-dimensional (2D) Cr3Sb5 and CsCr3Sb5 monolayers which may be mechanically exfoliated from bulk materials. Notably, it is revealed that both flat bands and vHSs simultaneously reside in close proximity to the EF in Cr3Sb5 monolayer, signifying enhanced electronic correlations. Importantly, a tensile strain further shifts the incipient flat bands and vHSs of two monolayers simultaneously to the vicinity of the EF, suggesting strain tunable electronic correlations and concomitant quantum effects. Furthermore, altermagnetic ground state is also revealed due to retained mirror symmetry between two sublattices in these two monolayers. Thus, this work advances understanding and modulations of electronic properties of 2D CsCr3Sb5 monolayers, strengthening their great potential for exploring unconventional quantum phenomena and altermagnetism.

• The paper demonstrates that dimensional reduction in CsCr₃Sb₅ monolayers shifts flat bands and vHSs near the Fermi level, enhancing electronic correlations.
• It employs DFT with strain engineering to reveal tunable band structure modifications and orbital-dependent shifts.
• The work confirms a stable altermagnetic ground state, promising advances in spintronic devices and correlated quantum phenomena.

Enhanced Electronic Correlations and Altermagnetic Order in 2D CsCr₃Sb₅ Monolayers

Background and Motivation

Layered kagome metals have demonstrated a rich landscape of quantum phenomena driven by geometrically frustrated lattices, flat bands, Dirac cones, and van Hove singularities (vHSs). In particular, the AV₃Sb₅ (A = K, Rb, Cs) family has emerged as a paradigm for unconventional superconductivity, charge-density waves (CDWs), and topological states. However, these compounds lack intrinsic magnetism and exhibit weak electronic correlations due to flat bands and vHSs lying far from the Fermi level ($E_F$). Recent studies on CsCr₃Sb₅ highlighted the presence of an incipient flat band near $E_F$, a phase transition to an altermagnetic spin density wave (SDW) state at 55 K, and emergent antiferromagnetic order. Despite these advancements, the bulk form’s vHSs and Dirac features remain energetically distant from $E_F$, limiting the exploration of correlated transport and quantum effects.

This work addresses these gaps by leveraging two-dimensional (2D) monolayer architectures of CsCr₃Sb₅, exploiting the tunability of electronic structure via dimensional reduction and strain engineering. The study aims to elucidate the evolution of electronic correlations, magnetic ground states, and strain-driven quantum effects in exfoliable monolayers.

Methods

The electronic properties of 2D Cr₃Sb₅ and CsCr₃Sb₅ monolayers were computed via density functional theory (DFT) using VASP with the PBEsol exchange-correlation functional. Van der Waals corrections (DFT-D3) and a plane-wave cutoff of 450 eV ensured accurate geometry optimizations. Monolayer models were constructed based on successful exfoliation schemes, considering multiple Cs configurations within 2 × 2 supercells to preserve stoichiometry. Biaxial strain ($\varepsilon = (a-a_0)/a_0$) was systematically applied to probe modifications in band structure and magnetic ordering.

Electronic Structure Modulation by Dimensional Reduction

Dimensional reduction from bulk to monolayer induces pronounced band structure modifications. The Cr₃Sb₅ monolayer exhibits both an extended flat band and vHSs proximate to $E_F$, enhancing electronic correlations compared to bulk CsCr₃Sb₅. In bulk, the incipient flat band from Cr $d_{xz}/d_{yz}$ orbitals resides at +300 meV and vHSs at -280 meV relative to $E_F$, while dimensional reduction drives both features closer to the Fermi level. Brillouin zone folding in monolayers further repositions vHSs ($\sim$400 meV below $E_F$), resulting in rearranged flat bands across $M \rightarrow K \rightarrow T$.

The absence of the Cs capping layer in Cr₃Sb₅ monolayers intensifies Coulomb interactions, shifting incipient flat bands and vHSs even nearer to $E_F$0. Such tunability is not attainable in the bulk phase, underscoring the value of monolayer synthesis for correlation-driven physics.

Strain Engineering and Correlated Quantum States

Monolayer platforms allow for direct strain application, which robustly manipulates the electronic landscape. Tensile strain moves the incipient flat band and vHSs of both Cr₃Sb₅ and CsCr₃Sb₅ monolayers toward $E_F$1, increasing the density of states (DOS) and amplifying electronic correlations. Compression and tension independently control the orbital character of the flat bands: compressive strain drives $E_F$2 bands upward, inducing additional vHSs and short flat bands that align with $E_F$3 at –2% strain, while tensile strain narrows the $E_F$4-derived band and positions it at $E_F$5, producing prominent, extended flat bands across the Brillouin zone at 5–6% tension.

These correlated features facilitate selective tuning of the band structure, enabling investigations of respective effects from distinct orbital-derived flat bands. The study demonstrates that strain engineering is a potent mechanism for modulating unconventional charge order, anomalous Hall effect, and quantum transport phenomena.

Altermagnetic Ground State in 2D Monolayers

First-principles calculations confirm that both Cr₃Sb₅ and B-type CsCr₃Sb₅ monolayers preserve the altermagnetic ground state found in bulk, rooted in retained mirror symmetry operations between sublattices ($E_F$6 or $E_F$7). The energetic preference for altermagnetic order persists across all considered magnetic configurations. Upon SDW modulation, these monolayers achieve a space group reduction (B-type: Pba2), but symmetry operations vital for altermagnetism remain intact. Notably, strain tuning further sharpens DOS features and narrows specific $E_F$8-derived bands without destabilizing the altermagnetic order.

The coexistence of high electronic correlations and altermagnetism in 2D kagome monolayers presents attractive prospects for spintronic devices utilizing compensated antiferromagnetic spin polarization and momentum-dependent band splitting.

Implications and Future Directions

The identified simultaneous appearance of flat bands, vHSs, and Dirac cones near $E_F$9 in CsCr₃Sb₅ monolayers implies potential for coexisting quantum phenomena—CDW, anomalous Hall effects, and correlated transport. The demonstrated high tunability of these electronic features by strain engineering provides an experimental route for manipulating emergent phases. The retention of altermagnetic ground state in 2D compounds suggests robust applications in high-density and low-power memory architectures for spintronics.

On the theoretical front, enhanced correlations and symmetry-protected altermagnetic order in monolayers may facilitate studies of correlated topological phases, unconventional superconductivity, and quantum order transitions. Future developments may include experimental realization of strained monolayers, electrical control of quantum phases, and integration into device platforms.

Conclusion

This study provides a comprehensive analysis of electronic structure and magnetic order in 2D CsCr₃Sb₅ monolayers. Dimensional reduction and strain engineering concurrently position flat bands and vHSs near $E_F$0, accelerating electronic correlations and facilitating unconventional quantum effects. The persistence of altermagnetic ground state in monolayers substantiates their suitability for spintronic applications requiring symmetry-driven band splitting and compensated magnetic order. Overall, 2D CsCr₃Sb₅ represents a versatile framework for exploring and manipulating correlated electronic and magnetic phenomena in kagome systems (2606.19740).