工作一：Appl. Phys. Lett. 124, 043101 (2024)

Doping induced multiferroicity and quantum anomalous Hall effect in α-In2Se3 thin films

α-In2Se3薄膜中，摻雜誘導(dǎo)的多鐵性和量子反常霍爾效應(yīng)

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Abstract:?In flatband materials, the strong Coulomb interaction between electrons can lead to exotic physical phenomena. Recently, α-In₂Se₃?thin films were found to possess ferroelectricity and flat bands. In this work, using first-principles calculations, we find that for the monolayer, there is a Weyl point at Γ in the flatband, where the inclusion of the spin–orbit coupling opens a gap. Shifting the Fermi level into the spin–orbit gap gives rise to nontrivial band topology, which is preserved for the bilayer regardless of the interlayer polarization couplings. We further calculate the Chern number and edge states for both the monolayer and bilayer, for which the results suggest that they become quantum anomalous Hall insulators under appropriate dopings. Moreover, we find that the doping-induced magnetism for the In2Se3 bilayer is strongly dependent on the interlayer polarization coupling. Therefore, doping the flat bands in In2Se3 bilayer can also yield multiferroicity, where the magnetism is electrically tunable as the system transforms between different polarization states.?Our study, thus, reveals that multiferroicity and nontrivial band topology can be unified into one material for designing multifunctional electronic devices.

摘要：在平帶材料中，電子之間的強(qiáng)庫(kù)侖相互作用會(huì)導(dǎo)致奇異的物理現(xiàn)象。最近，發(fā)現(xiàn)α-In2Se3薄膜具有鐵電性和平帶。在這項(xiàng)工作中，使用第一原理計(jì)算，我們發(fā)現(xiàn)對(duì)于單層，在平帶中的Γ處存在一個(gè)Weyl點(diǎn)，考慮自旋軌道耦合后打開(kāi)了一個(gè)能隙。將費(fèi)米能級(jí)轉(zhuǎn)移到自旋軌道間隙中會(huì)產(chǎn)生非平凡的能帶拓?fù)洌瑹o(wú)論層間極化耦合如何，雙層都會(huì)保留這種拓?fù)洹?/span>我們進(jìn)一步計(jì)算了單層和雙層的陳數(shù)和邊緣態(tài)，結(jié)果表明它們?cè)谶m當(dāng)?shù)膿诫s下成為量子反常霍爾絕緣體。此外，我們發(fā)現(xiàn)In2Se3雙層中摻雜誘導(dǎo)的磁性強(qiáng)烈依賴于層間極化耦合。因此，在In2Se3雙層中對(duì)平帶進(jìn)行摻雜也可以產(chǎn)生多鐵性，當(dāng)系統(tǒng)在不同極化狀態(tài)之間轉(zhuǎn)換時(shí)，磁性是電可調(diào)的。因此，我們的研究表明，多鐵性和非平凡的能帶拓?fù)淇梢越y(tǒng)一在一種材料，用于設(shè)計(jì)多功能電子器件。

圖1. 單層In2Se3的幾何結(jié)構(gòu)和電子結(jié)構(gòu)。(a, b)?單層In2Se3的兩個(gè)極化態(tài)，分別稱為FE1和FE2。P表示極化，箭頭表示其方向。(c, d)?分別為無(wú)自旋軌道耦合和有自旋軌道耦合的能帶結(jié)構(gòu)。(e, f)?費(fèi)米能級(jí)分別在EF1和EF2的單層In2Se3的WCC和邊緣態(tài)。

圖2. 摻雜對(duì)單層In2Se3的幾何結(jié)構(gòu)和電子結(jié)構(gòu)的影響。(a)摻雜誘導(dǎo)的磁化強(qiáng)度隨摻雜大小的變化。(b)?Se2原子的位移隨摻雜大小的變化。(c)和(d)分別為沒(méi)有SOC和有SOC時(shí)，單層In2Se3在0.5 h/f.u.時(shí)的能帶結(jié)構(gòu)。(e)和(f)分別為計(jì)算出的WCC和邊緣態(tài)。

圖3. 無(wú)和有SOC的三種極化態(tài)的能帶結(jié)構(gòu)。圖層中的極化值用箭頭表示。插圖描述了極化狀態(tài)，分別命名為C1、C2和C3。(a)和(b)、(c)和(d)，以及(e)和(f)分別是 C1、C2和C3。

圖4. 空穴摻雜的In2Se3雙層中的極化相關(guān)鐵磁性。(a)磁化強(qiáng)度隨空穴摻雜的變化。(b)C1、C2和C3極化構(gòu)型中原子的磁化強(qiáng)度分布。箭頭表示各層中的極化。(c)摻雜0.3 h/f.u的In2Se3雙層極化態(tài)轉(zhuǎn)變的動(dòng)力學(xué)途徑。(d) C1 WCC。

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工作二：Phys. Rev. B 109, 085432 (2024)

Ferroelectrically tunable topological phase transition in In2Se3 thin films

In2Se3薄膜中的鐵電可調(diào)拓?fù)湎嘧?/span>

Abstract:?Materials with ferroelectrically switchable topological properties are of interest for both fundamental physics and practical applications. Using first-principles calculations, we find that stacking ferroelectric?α-In2Se3 monolayers into a bilayer leads to polarization-dependent band structures, which yields polarization-dependent topological properties. Specifically, we find that the states with interlayer ferroelectric couplings are quantum spin Hall insulators, while those with antiferroelectric polarizations are normal insulators. We further find that In2Se3 trilayer and quadlayer exhibit nontrivial band topology as long as in the structure the ferroelectric In2Se3 bilayer is antiferroelectrically coupled to In2Se3 monolayers or other ferroelectric In2Se3 bilayer. Otherwise the system is topologically trivial. The reason is that near the Fermi level the band structure of the ferroelectric In2Se3 bilayer has to be maintained for the nontrivial band topology. This feature can be used to design?nontrivial band topology for the thicker films by a proper combination of the interlayer polarization couplings. The topological properties can be ferroelectrically tunable using the dipole locking effect. Our study reveals switchable band topology in a family of natural ferroelectrics, which provide a platform for designing new?functional devices.

摘要：具有鐵電可切換拓?fù)涮匦缘牟牧蠈?duì)于基礎(chǔ)物理和實(shí)際應(yīng)用都很感興趣。使用第一性原理計(jì)算，我們發(fā)現(xiàn)將鐵電α-In2Se3單層堆疊成雙層會(huì)產(chǎn)生極化相關(guān)的能帶結(jié)構(gòu)，從而產(chǎn)生極化相關(guān)的拓?fù)涮匦浴?/span>具體來(lái)說(shuō)，我們發(fā)現(xiàn)具有層間鐵電耦合的狀態(tài)是量子自旋霍爾絕緣體，而具有反鐵電極化的狀態(tài)是正常絕緣體。我們進(jìn)一步發(fā)現(xiàn)，只要在結(jié)構(gòu)中鐵電In2Se3雙層與In2Se3單層或其他In2Se3雙層具有反鐵電耦合，那么In2Se3三層和四層就表現(xiàn)出非平凡的帶拓?fù)洹７駝t，系統(tǒng)在拓?fù)渖鲜俏⒉蛔愕赖摹?/span>原因是在費(fèi)米能級(jí)附近，必須保持鐵電 In2Se3 雙層的能帶結(jié)構(gòu)才能實(shí)現(xiàn)非平凡的能帶拓?fù)洹Ｔ摴δ芸捎糜谕ㄟ^(guò)層間偏振耦合的適當(dāng)組合來(lái)設(shè)計(jì)較厚薄膜的非平凡能帶拓?fù)洹＠门紭O鎖定效應(yīng)可以對(duì)拓?fù)涮匦赃M(jìn)行鐵電調(diào)節(jié)。我們的研究揭示了一系列天然鐵電體中的可切換能帶拓?fù)洌@為設(shè)計(jì)新功能器件提供了平臺(tái)。

圖1.?In2Se3-1L的電子結(jié)構(gòu)。(a)具有不同極化態(tài)的In2Se3-1L的幾何結(jié)構(gòu)。極化現(xiàn)象用箭頭表示。具有向下極化的狀態(tài)表示為FE1，而具有向上極化的狀態(tài)則稱為FE2。(b) 具有自旋–軌道耦合的能帶結(jié)構(gòu)。(c)在Γ處的價(jià)帶和導(dǎo)帶的電荷密度分布，即VBΓ和CBΓ. (d) VBΓ和CBΓ的波函數(shù). (e) In2Se3-1L的WCC的演化。

圖2. In2Se3- 2L的能帶結(jié)構(gòu)的極化依賴性。(a)In2Se3-2L的極化態(tài)。每個(gè)結(jié)構(gòu)的總能量在結(jié)構(gòu)下面給出。在(a).中所示的狀態(tài)的(b)能帶結(jié)構(gòu)(c)價(jià)帶和導(dǎo)帶的波函數(shù)。

圖3. In2Se3-2L中FE的能帶拓?fù)浣Y(jié)構(gòu)，即構(gòu)型C3。(a)帶有SOC的能帶結(jié)構(gòu)。(b)WCC的演化。(c)鋸齒狀邊緣的邊緣態(tài)。

圖4. In2Se3-3L在四種狀態(tài)下具有SOC的能帶結(jié)構(gòu)。插圖顯示了等效的極化構(gòu)型。

圖5.?(a)-(d) In2Se3-3L在S1、S2、S3和S4極化態(tài)下WCC的演化。(e)-(h)它們各自對(duì)應(yīng)的一個(gè)鋸齒形邊緣的邊界態(tài)。

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