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Lattice and electronic structures of Y2C and MgONa. (a), (c) Band structures. (b), (d) Lattice structures and partial electron density isosurfaces of Y2C and MgONa. The monolayer electride contains three atom layers, with two metal atoms in the outermost layers. The electron density isosurfaces show that electronic states around Fermi level (EF) are mainly on the surface.
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Two-dimensional (2D) superconductors are always a research hotspot in superconductivity, due to the huge importance of 2D superconductors in quantum phenomena transitions. And the natural metallic 2D electride with exceptional physical properties is one member of candidate materials for 2D superconductors in the Bardeen-Cooper-Schrieffer theory. Th...
Citations
... [24] However, such effects cannot be maintained at ambient pressure, which severely limits their applications in superconducting quantum interference devices, [25] single-electron superconductor quantum dot devices, [26,27] etc. In contrast, 2D electrides could be intrinsic superconductors without external pressure; however, they usually suffer from low c, e.g., 0.9 K for Y2C, [28] 3.4 K for MgONa, [28] 4.7 K for Ca2N, [29] and 3.4 K for Ba2N. [30] Therefore, it is highly desirable to explore 2D superconducting electrides with relatively highc and further uncover the microscopic mechanisms of superconductivity in low-dimensional electrides. ...
... [24] However, such effects cannot be maintained at ambient pressure, which severely limits their applications in superconducting quantum interference devices, [25] single-electron superconductor quantum dot devices, [26,27] etc. In contrast, 2D electrides could be intrinsic superconductors without external pressure; however, they usually suffer from low c, e.g., 0.9 K for Y2C, [28] 3.4 K for MgONa, [28] 4.7 K for Ca2N, [29] and 3.4 K for Ba2N. [30] Therefore, it is highly desirable to explore 2D superconducting electrides with relatively highc and further uncover the microscopic mechanisms of superconductivity in low-dimensional electrides. ...
... By substituting Al with Mg (one electron less than Al), the resulting 1H-MgH2 monolayer shows absence of the interstitial electrons in ELF (Fig. S4) and is dynamically unstable (Fig. S5). All these results demonstrate that 1H-AlH2 monolayer is a 0D electride in formula of [AlH2] + − , similar to [Y2C] 1.8+ 1.8 − , [45] [MgONa] + − , [28] and [Ba2N] + − [30] electrides. We also investigated the key factors for stabilization of 1H-AlH2 electride using homologous element substitution (Ga, In, Tl) for Al. ...
Electrides, which confine “excess anionic electrons” in subnanometer-sized cavities of the lattice, are exotic ionic crystals. We propose a non-stoichiometric strategy to realize intrinsic two-dimensional (2D) superconducting electride. AlH2 monolayer, which is structurally identical to 1H-MoS 2 , possesses zero-dimensionally confined anionic electrons in the interstitial sites of Al triangles, corresponding to a chemical formula of [AlH 2 ] ⁺ e ⁻ . The interaction between interstitial anionic electrons (IAEs) and host cation lattice mainly accounts for stabilization of 1H-AlH 2 electride. Impressively, 1H-AlH 2 monolayer is an intrinsic BCS superconductor with T c = 38 K, which is the direct consequence of strong coupling of the H-dominated high electronic states with Al acoustic branch vibrations and mid-frequency H-derived phonon softening modes caused by Kohn anomalies. Under tensile strain, IAEs transform into itinerant electrons, favoring the formation of stable Cooper pairs; therefore, T c reaches up to 53 K at a biaxial fracture strain of 5%. Our findings provide valuable insights into the correlation between non-stoichiometric electrides and superconducting microscopic mechanisms at the 2D limit.
... Zeng et al. studied the electron-phonon interaction of Ca 2 N monolayer theoretically and suggested that it is a Bardeen-Cooper-Schrieffer superconductor with a superconducting T c of 4.7 K [37]. Besides Ca 2 N, some other 2D electride superconductors Y 2 C and MgONa have also been proposed with the respective T c s of 0.9 and 3.4 K [40]. However, very few 2D electrides have been reported to be superconductors so far, and the predicted superconducting transition temperatures of them are relatively low. ...
The exploration of superconductivity in low-dimensional materials has attracted intensive attention for decades. Based on first-principles electronic structure calculations, we have systematically investigated the electronic and superconducting properties of the two-dimensional electride Ba2N in the monolayer limit. Our results show that monolayer Ba2N has a low work function of 3.0 eV and a predicted superconducting transition temperature (Tc) of 3.4 K. The superconductivity can be further improved with the tensile strain, which results from the increase of density of states at the Fermi level as well as the enhanced coupling between inner-layer electrons and phonons. Remarkably, at the 4% tensile strain, the acoustic branches have noticeable softening at the K point of the Brillouin zone and the superconducting Tc can reach 10.8 K. The effect of lattice strain on the electron transfer from the superficial region to the inner-layer region of monolayer Ba2N may also apply to other electride materials and influence their physical properties.
... If μ* is set as 0.05, the T c of 36.9 K for MgB 2 with a λ of 0.71 is obtained, which is in good agreement with the experimental T c value (40 K) and λ value (~ 0.75) [95] . Thus, μ* = 0.05 was used to estimate T c for the 2D CuB monolayer. ...
To achieve specific applications, it is always desirable to design new materials with peculiar topological properties. Herein, based on a D2h B2Cu6H6 molecule with the unique chemical bonding of planar pentacoordinate boron (ppB) as a building block, we constructed an infinite CuB monolayer by linking B2Cu6 subunits in an orthorhombic lattice. The planarity of the CuB sheet is attributed to the multicenter bonds and electron donation-back donation, as revealed by chemical bonding analysis. As a global minimum confirmed by the particle swarm optimization method, the CuB monolayer is expected to be highly stable, as indicated by its rather high cohesive energy, absence of soft phonon modes, and good resistance to high temperature, and thus is highly feasible for experimental realization. Remarkably, this CuB monolayer is metallic and predicted to be superconducting with an estimated critical temperature (Tc) of 4.6 K, and the critical temperature could be further enhanced by tensile strains (to 21 K at atmospheric pressure).
... Among a few superconducting candidates, previous electrides are reported to exhibit low superconducting transition temperature c , such as [Ca 24 Al 28 O 64 ] 4+ ( − ) 4 (C12A7: − ) − , [14] Mn 5 Si 3 -type Nb 5 Ir 3 , [15] 2 C and MgONa. [16] Further exploration of new superconducting electrides can be helpful for deep understanding their formation mechanisms and superconducting behavior. ...
... Among a few superconducting candidates, previous electrides are reported to exhibit low superconducting transition temperature c , such as [Ca 24 [14] Mn 5 Si 3 -type Nb 5 Ir 3 , [15] 2 C and MgONa. [16] Further exploration of new superconducting electrides can be helpful for deep understanding their formation mechanisms and superconducting behavior. ...
Due to their unique structure properties, most of the electrides that possess extra electrons locating in interstitial regions as anions are insulators. Metallic and superconducting electrides are very rare under ambient conditions. We systematically search possible compounds in Ca–S systems stabilized under various pressures up to 200 GPa, and investigate their crystal structures and properties using first-principles calculations. We predict a series of novel stoichiometries in Ca–S systems as potential superconductors, including P 2 1 / m Ca 3 S, P 4mbm Ca 3 S, Pnma Ca 2 S, Cmcm Ca 2 S, Fddd CaS 2 , Immm CaS 3 and C 2/ c CaS 4 . The P 4 mbm Ca 3 S phase exhibits a maximum T c value of ∼20 K. It is interesting to notice that the P 2 1 / m Ca 3 S and Pnma Ca 2 S stabilized at 60 and 50 GPa behave as superconducting electrides with critical temperatures T c of 7.04 K and 0.26 K, respectively. More importantly, our results demonstrate that P 2 1 / m Ca 3 S and Pnma Ca 2 S are dynamically stable at 5 GPa and 0 GPa, respectively, indicating a high possibility to be quenched to ambient condition or synthesized using the large volume press.
... Additionally, all five compounds are metallic with electronic density of states at Fermi level dominated by Y atoms The referenced structures of Y and S are taken from Refs. [18,21,[42][43][44][45]. Panel (c) shows the phase diagrams of all phases as functions of pressure. ...
... Interestingly, ELF results show that Y 3 S 2 is an electride with dumbbell-like electrons localizing in the interstices of two neighboring SY 6 layers [ Fig. 3(d)]. This kind of layer-structured magnetic electride is also found in Y 2 C [45]. At the transformation to the P4/mbm structure, the ferromagnetism of Y 3 S 2 disappears deduced from the same profiles of spin-up and spin-down DOSs [ Fig. 3(b)]. ...
Yttrium sulfides are found to exhibit rich phase diagram and diverse properties under pressures. Here, we systematically investigate the stability of binary Y-S system up to 50 GPa using first-principles swarm-intelligence structure search. We identify seven hitherto unknown stoichiometries (Y7S6, Y6S5, Y7S8, Y6S7, Y5S6, Y3S2, and YS3) that are energetically stable with respect to the decomposition into elemental components. All phases exhibit metallic nature, of which YS3 shows potential superconductivity with estimated Tc of 18.5 K at 50 GPa. The Tc in YS3 is comparable to the 17 K superconductivity in elements yttrium (89.3 GPa) and sulfur (160 GPa), but could be achieved at much lower pressure. The high superconductivity is attributed to the high electronic density of states from S atoms at Fermi level. Additionally, Y3S2 is predicted to be a layer-structured magnetic electride with magnetic moment of 0.5 μB per formula unit at 6 GPa, which transforms to a three-dimensional phase with weak superconductivity around 17 GPa.
... External pressure is a very powerful tool to enhance the superconducting properties 17 and achieve novel energetically favorable electride compounds. 13,15,18 Recently, a 0D pressure-induced stable Li 6 P electride has been identified as a superconductor with a predicted superconducting transition temperature T c of 39.3 K at 270 GPa external pressure by theoretical calculation, 19 which is the highest among the already known electrides such as C12A7:e − , 20,21 Nb 5 Ir 3, 22 and Y 2 C. 23 It is worth noting that the currently known superconducting electrides are mostly 0D electrides with localized intrinsic excess electrons. 6 Since 1D electron confinement can enhance the thermal stability of electrides, 14,15 further extending the superconducting electrides to the 1D case may help to achieve high superconducting transition temperature at lower external pressure. ...
Inorganic electrides have gained remarkable attentions for their intrinsic physical properties derived from loosely bound anionic electrons. Herein, using ab initio evolutionary structure search, we found that the formulation of Ca and Si with the stoichiometric ratio of 3:1 can be stabilized under mildly external pressure, where the hexagonal P63/mmc phase is the most stable structure under a wide pressure range from 13.5 to 104 GPa. Based on the analysis of the electrostatic difference potential as an identifier of electrides, together with the electronic structure and electron localization function results, we have identified the P63/mmc Ca3Si as an one dimensional (1D) electride, whose chemical formula could be expressed as [Ca3Si]2+:2e−. Interestingly, the electron mobility and the electron-phonon interaction strength of P63/mmc Ca3Si electride present the strong electronic anisotropy, illustrating the 1D electron confinement nature. Moreover, due to the strong electron-phonon coupling between interstitial electrons and phonons from Ca atoms, the P63/mmc-Ca3Si exhibits superconductivity with a predicted superconducting transition temperature Tc of about 17.6 K at 100 GPa, which is the highest among the already known 1D electrides. Our works provide new insight into new thermodynamically stable related alkaline-earth based electrides and their potential for high performance in electronics and catalytic applications.
... Additionally, all five compounds are metallic with electronic density of states at Fermi level dominated by Y atoms The referenced structures of Y and S are taken from Refs. [18,21,[42][43][44][45]. Panel (c) shows the phase diagrams of all phases as functions of pressure. ...
... Interestingly, ELF results show that Y 3 S 2 is an electride with dumbbell-like electrons localizing in the interstices of two neighboring SY 6 layers [ Fig. 3(d)]. This kind of layer-structured magnetic electride is also found in Y 2 C [45]. At the transformation to the P4/mbm structure, the ferromagnetism of Y 3 S 2 disappears deduced from the same profiles of spin-up and spin-down DOSs [ Fig. 3(b)]. ...
The search of two-dimensional superconductors has attracted increasing interest because of the potential application in constructing nanoscale superconducting devices. Through swarm-intelligence based CALYPSO method and the first-principles calculations, we identify a monolayer structure for yttrium sulfide (t-YS), which is energetically and dynamically stable. The application of biaxial strain turns t-YS to be a Bardeen–Cooper–Schrieffer superconductor, which mainly originates from the softening of in-plane modes of Y atoms. The superconducting critical temperature increases monotonously with strain, which reaches 6 K at a maximum strain of 10%. Calculations show that the doping at 0.3 holes per unit cell could further enhance the superconductivity to 7.3 K. Simulations propose a candidate substrate with ~8.3% lattice mismatch to obtain superconductive t-YS in experiments. The findings will enrich two-dimensional superconductors and stimulate immediate experimental interest.
... The interstitial electrons in Ca 24 Al 28 O 64 accommodate loosely in the cages of unit structures, rising up an uncommon conductivity [35]. Additionally, Mn 5 Si 3 -type Nb 5 Ir 3 [15] and two-dimensional Y 2 C [37] have been also found to hold both electride states and superconductivity. Additionally, as stated above, it is well known that pressure induces the formation of electrides. ...
Electrides are unique compounds where most of the electrons reside at interstitial regions of the crystal behaving as anions, which strongly determines its physical properties. Interestingly, the magnitude and distribution of interstitial electrons can be effectively modified either by modulating its chemical composition or external conditions (e.g., pressure). Most of the electrides under high pressure are nonmetallic, and superconducting electrides are very rare. Here we report that a pressure-induced stable Li6P electride, identified by first-principles swarm structure calculations, becomes a superconductor with a predicted superconducting transition temperature Tc of 39.3 K, which is the highest among the already known electrides. The interstitial electrons in Li6P, with dumbbell-like connected electride states, play a dominant role in the superconducting transition. Other Li-rich phosphides, Li5P, Li11P2, Li15P2, and Li8P, are also predicted to be superconducting electrides, but with a lower Tc. Superconductivity in all these compounds can be attributed to a combination of a weak electronegativity of phosphorus (P) with a strong electropositivity of lithium (Li), and opens up the interest to explore high-temperature superconductivity in similar binary compounds.
Electrides constitute a unique class of materials that can be developed as conventional superconductors with diverse dimensional superconductivity. However, the transition temperatures (Tc) of electride superconductors are generally low and promoting their Tc usually requires extremely high external pressures that are formidable for practical applications. Here, based on the first-principles calculations, we proposed that the recently reported electride, Be2N, can exhibit a two-dimensional (2D) superconductivity, which has a Tc of 10.3 K that is the highest Tc ever found for bulk electride superconductors at ambient pressure. More interestingly, we found that the high Tc of Be2N is mainly attributed to the large average phonon frequency, rather than the strong electron-phonon coupling, which can be further understood by the small atomic weight of Be atoms and the strong Be-N bonds. Moreover, compared to most conventional superconductors, we identified an unusual dependence of the superconductivity of Be2N on external pressures, originating from a unique charge transfer from its cationic framework to its anionic electron cloud. Our studies provide a deeper understanding of the superconductivity of 2D electrides and suggest a feasible way for the development of high-temperature electride superconductors at ambient pressure.
