Structure-driven intercalated architecture of septuple-atomic-layer $MA_2Z_4$ family with diverse properties from semiconductor to topological insulator to Ising superconductor (2008.02981v1)

Abstract: Motivated by the fact that septuple-atomic-layer MnBi$_2$Te$_4$ can be structurally viewed as the combination of double-atomic-layer MnTe intercalating into quintuple-atomic-layer Bi$_2$Te$_3$, we present a general approach of constructing twelve septuple-atomic-layer $\alpha_i$- and $\beta_i$-$MA_2Z_4$ monolayer family (\emph{i} = 1 to 6) by intercalating MoS$_2$-type $MZ$$_2$ monolayer into InSe-type A$_2$Z$_2$ monolayer. Besides reproducing the experimentally synthesized $\alpha_1$-MoSi$_2$N$_4$, $\alpha_1$-WSi$_2$N$_4$ and $\beta_5$-MnBi$_2$Te$_4$ monolayer materials, another 66 thermodynamically and dynamically stable $MA_2Z_4$ were predicted, which span a wide range of properties upon the number of valence electrons (VEC). $MA_2Z_4$ with the rules of 32 or 34 VEC are mostly semiconductors with direct or indirect band gap and, however, with 33 VEC are generally metal, half-metal ferromagnetism, or spin-gapless semiconductor upon whether or not an unpaired electron is spin polarized. Moreover, we propose $\alpha_2$-WSi$_2$P$_4$ for the spin-valley polarization, $\alpha_1$-TaSi$_2$N$_4$ for Ising superconductor and $\beta_2$-SrGa$_2$Se$_4$ for topological insulator.

Structure-driven Intercalated Architecture of $MA_2Z_4$ Monolayers

This paper presents a comprehensive paper on the structure-driven intercalated architecture of septuple-atomic-layer $MA_2Z_4$ monolayers, which offer a polymorphous range of properties spanning semiconducting, topological insulating, and superconducting behaviors. Notably, the $MA_2Z_4$ family demonstrated intriguing prospects from these diverse electronic characteristics, forming a subject ripe for further exploration in material science research.

Septuple-atomic-layer $MA_2Z_4$ Family and its Properties

The authors construct twelve distinguished septuple-atomic-layer $\alpha_i$ and $\beta_i$ $MA_2Z_4$ monolayer structures. This structural engineering is achieved by intercalating a MoS$_2$-type $MZ_2$ monolayer within an InSe-type $A_2Z_2$ monolayer, featuring potential variations denoted as $\alpha_i$ and $\beta_i$ (where i = 1 to 6). Such innovative architectural design widens the scope for fabricating thermodynamically and dynamically stable materials. It leads to 66 potential structures, aside from well-characterized materials such as $\alpha_1$-MoSi$_2$N$_4$, $\alpha_1$-WSi$_2$N$_4$, and $\beta_5$-MnBi$_2$Te$_4$.

The paper identifies distinct rules based on the number of valence electrons (VEC), playing a pivotal role in determining electronic properties. $MA_2Z_4$ materials with VECs of 32 or 34 typically exhibit semiconducting properties, while 33 VEC yields metal, half-metal ferromagnetism, or spin-gapless semiconductors. Furthermore, these materials possess additional traits such as spin-valley polarization in $\alpha_2$-WSi$_2$P$_4$, Ising superconductivity in $\alpha_1$-TaSi$_2$N$_4$, and topological insulation in $\beta_2$-SrGa$_2$Se$_4$.

Electronic and Phononic Insights

From detailed electronic band structure analysis, materials with 32 VEC predominantly show semiconductor behavior, confirmed by direct and indirect band gap calculations. Notable is the spin-valley coupling observed in $\alpha_2$-WSi$_2$P$_4$, which displays robust valley-dependent optical selectivity akin to 2H-WSe$_2$. The phononic studies endorse stability, illustrating viable phonon dispersions across these compositions.

Regarding superconductivity, $\alpha_1$-TaSi$_2$N$_4$ material showcases inspiring Ising superconductivity catalyzed by spin-orbit coupling (SOC) induced Zeeman-type spin splitting. Calculations hint at a superconductive transition temperature ($T_c$) approximately nearing 9.67 K.

Beyond Conventional Two-dimensional Materials

The research accentuates the methodological novelty, pushing material exploration boundaries beyond known structures, such as graphene or TMDCs. This approach suggests potential applications in future energy devices, nanoelectronics, and spintronics, pending further verification and analytical expansion.

The findings lay significant groundwork for advanced material synthesis expectations, steering new directions for two-dimensional materials enriched with unprecedented electronic properties. The promise of $MA_2Z_4$ monolayers to affect technological advancements is particularly pronounced given the diversity and adaptability owing to the structural manipulation explored within this research.

Implications and Future Exploration

The paper implies substantial potential for further enhancement of electronic devices, harnessing the cryptic interplay of spin-related phenomena, magnetic ordering, and electronic band manipulation due to the architectural fortitude of these materials. Anticipated evolutions within the realms of semiconductors and quantum materials prompt continued research to validate practical applications. Moreover, such research innately demands refinement in synthesis methodologies to visualize materialization prospects fully.

In conclusion, the intercalated architectural approach posited here heralds a frontier of expanded material characteristics providing an ample platform for innovation in material sciences, further paralleling ongoing theoretical and experimental engagements with novel two-dimensional materials.