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

Technical Summary: Enhanced Electronic Correlations and Altermagnetic Ground State of Two-Dimensional CsCr3Sb5 Monolayers

Problem Statement
Layered kagome metals, particularly the vanadium-based family $AV_3Sb_5$ ($A$ = K, Rb, Cs), are known for emergent properties like unconventional superconductivity and charge density waves (CDW), driven by the interplay of Coulomb interactions and van Hove singularities (vHSs) near the Fermi level ($E_F$). However, in bulk $AV_3Sb_5$, flat bands are often distant from $E_F$, and the materials exhibit weak electronic correlations without intrinsic magnetism. The chromium-based kagome metal $CsCr_3Sb_5$ presents a more promising candidate, exhibiting an incipient flat band near $E_F$ and an altermagnetic ground state below ~55 K. Despite these features, bulk $CsCr_3Sb_5$ suffers from a limitation where vHSs and Dirac cones remain far from $E_F$, hindering the exploration of related electron transport properties. Furthermore, the energetic positions of these features in the bulk material are rigid and resistant to tuning. While two-dimensional (2D) materials offer a pathway to modify electronic structures via dimensional reduction and strain, the specific electronic structures, tunability, and magnetic ground states of 2D $CsCr_3Sb_5$ and its derivative $Cr_3Sb_5$ monolayers have remained unexplored.

Methodology
The authors employed first-principles calculations based on Density Functional Theory (DFT) using the Vienna Ab initio Simulation Package (VASP). The study utilized the Perdew-Burke-Ernzerhof for solids (PBEsol) functional with van der Waals corrections (DFT-D3). The researchers modeled:

1. Structural Models: Freestanding 2D $Cr_3Sb_5$ monolayers and six possible configurations of $CsCr_3Sb_5$ monolayers (constructed within a $2 \times 2$ supercell by removing half the Cs atoms to preserve stoichiometry).
2. Strain Engineering: Biaxial strain (both compressive and tensile) was applied to the lattice constants to manipulate electronic structures.
3. Magnetic Analysis: Various magnetic configurations, including ferromagnetic, antiferromagnetic, and altermagnetic (specifically the "failed AF-SOD" spin density wave) states, were evaluated to determine the ground state.
4. Dimensional Reduction: The study compared the electronic structures of bulk $CsCr_3Sb_5$ with the derived 2D monolayers to analyze the effects of confinement and the removal of interlayer interactions.

Key Contributions and Results

• Dimensional Reduction and Electronic Structure:

  • Upon reducing bulk $CsCr_3Sb_5$ to a 2D monolayer, the electronic bands undergo a pronounced upward shift due to enhanced effective Coulomb repulsion.
  • In the $Cr_3Sb_5$ monolayer (Cs-free), dimensional reduction brings both flat bands and vHSs into close proximity with the Fermi level simultaneously. This configuration signifies significantly enhanced electronic correlations compared to the bulk.
  • In the B-type $CsCr_3Sb_5$ monolayer (the lowest energy configuration among Cs-containing variants), the lattice distorts into a rectangular geometry with $3 \times 1$ translational symmetry, causing Brillouin zone folding. This rearranges vHSs to approximately 400 meV below $E_F$ and extends flat bands across the $\Gamma \leftrightarrow K \leftrightarrow M$ path.

• Strain Tunability:

  • The electronic structures of both monolayers are highly susceptible to strain.
  • Tensile Strain: Drives the incipient flat bands and vHSs of both monolayers closer to the Fermi level. In $Cr_3Sb_5$, this enhances electronic correlations. In B-type $CsCr_3Sb_5$, a 6% tensile strain shifts the incipient flat band to the immediate vicinity of $E_F$, while a 5% strain induces a long-range flat band dominated by $d_{z^2}$ orbitals extending across the Brillouin zone.
  • Compressive Strain: Induces inverse shifts. In $Cr_3Sb_5$, compressive strain (e.g., -2% to -6%) tunes short flat bands derived from $d_{xz}/d_{yz}$ and $d_{z^2}$ orbitals toward $E_F$, creating sharp peaks in the density of states (DOS).
  • The study demonstrates that strain engineering can selectively tune specific orbital-derived flat bands toward or away from $E_F$, offering a mechanism to manipulate emergent quantum states.

• Altermagnetic Ground State:

  • The study confirms that the altermagnetic ground state, characterized by compensated antiferromagnetic spin polarization and momentum-dependent band splitting, persists in both 2D monolayers.
  • This state is stabilized by the retained mirror symmetry between the two sublattices (connected by symmetry operations such as $\{M_{100} | 0, 1/2, 0\}$ or $\{M_{100} | 1/2, 0, 0\}$).
  • Even when considering spin density wave (SDW) modulation, the altermagnetic configuration remains the lowest energy state for both $Cr_3Sb_5$ and B-type $CsCr_3Sb_5$ monolayers.

Significance
The paper claims that these findings advance the understanding of 2D kagome materials by demonstrating that dimensional reduction and strain engineering can effectively modulate electronic correlations and magnetic states. Specifically:

1. Enhanced Correlations: The simultaneous proximity of flat bands and vHSs to $E_F$ in the 2D limit suggests a platform for exploring unconventional quantum phenomena such as charge density waves and the anomalous Hall effect, which are difficult to access in the bulk.
2. Strain Control: The high degree of tunability via strain provides a viable pathway to manipulate the incipient flat bands and vHSs, potentially enabling the control of emergent quantum states.
3. 2D Altermagnetism: The retention of the altermagnetic ground state in these 2D systems identifies them as new candidates for realizing 2D altermagnetism, which holds potential for future spintronic device design due to their non-relativistic spin and crystal rotation symmetry properties.

The authors conclude that while 2D $CsCr_3Sb_5$ monolayers are theoretically predicted to be realizable via mechanical exfoliation (similar to $CsV_3Sb_5$), the simultaneous presence of tunable flat bands, vHSs, and altermagnetism makes them a promising system for future experimental and theoretical investigation into unconventional transport properties and altermagnetism.