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First-principles investigation of the electronic states at perovskite and pyrite hetero-interfaces

機(jī)譯：鈣鈦礦和黃鐵礦異質(zhì)界面電子態(tài)的第一性原理研究

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摘要

Oxide heterostructures are attracting huge interest in recent years due to thespecial functionalities of quasi two-dimensional quantum gases. In this thesis, theelectronic states at the interface between perovskite oxides and pyrite compoundshave been studied by first-principles calculations based on density functional theory.Optimization of the atomic positions are taken into account, which is considered veryimportant at interfaces, as observed in the case of LaAlO3/SrTiO3.The creation of metallic states at the interfaces thus is explained in terms ofcharge transfer between the transition metal and oxygen atoms near the interface.It is observed that with typical thicknesses of at least 10-12 °A the gases still extendconsiderably in the third dimension, which essentially determines the magnitude ofquantum mechanical effects. To overcome this problem, we propose incorporation ofhighly electronegative cations (such as Ag) in the oxides. A fundamental interest isalso the thermodynamic stability of the interfaces due to the possibility of atomicintermixing in the interface region. Therefore, different cation intermixed configurationsare taken into account for the interfaces aiming at the energetically stablestate.The effect of O vacancies is also discussed for both polar and non-polar heterostructures.The interface metallicity is enhanced for the polar system with thecreation of O vacancies, while the clean interface at the non-polar heterostructureexhibits an insulating state and becomes metallic in presence of O vacancy. The Ovacancy formation energies are calculated and explained in terms of the increasingelectronegativity and effective volume of A the side cation.Along with these, the electronic and magnetic properties of an interface betweenthe ferromagnetic metal CoS2 and the non-magnetic semiconductor FeS2 is investigated.We find that this contact shows a metallic character. The CoS2 stays quasihalf metallic at the interface, while the FeS2 becomes metallic. At the interface,ferromagnetic ordering is found to be energetically favorable as compared to antiferromagneticordering. Furthermore, tensile strain is shown to strongly enhancethe spin polarization so that a virtually half-metallic interface can be achieved, forcomparably moderate strain.Our detailed study is aimed at complementing experiments on various oxide interfacesand obtaining a general picture how factors like cations, anions, their atomicweights and elecronegativities, O vacancies, lattice mismatch, lattice relaxation, magnetismetc play a combined role in device design.
機(jī)譯：由于準(zhǔn)二維量子氣體的特殊功能，氧化物異質(zhì)結(jié)構(gòu)近年來(lái)引起了極大的興趣。本文基于密度泛函理論通過(guò)第一性原理計(jì)算研究了鈣鈦礦氧化物與黃鐵礦化合物界面的電子態(tài)，并考慮了原子位置的優(yōu)化，認(rèn)為在界面處非常重要。通過(guò)過(guò)渡金屬和界面附近的氧原子之間的電荷轉(zhuǎn)移來(lái)解釋界面處金屬態(tài)的產(chǎn)生，觀察到典型厚度至少為10-12°A的氣體仍會(huì)大量擴(kuò)展在三維中，它基本上決定了量子力學(xué)效應(yīng)的大小。為了克服這個(gè)問(wèn)題，我們建議在氧化物中摻入高負(fù)電性陽(yáng)離子（例如Ag）。由于在界面區(qū)域中原子相互混合的可能性，界面的熱力學(xué)穩(wěn)定性也是基本的興趣。因此，針對(duì)界面的能量穩(wěn)定狀態(tài)，應(yīng)考慮不同的陽(yáng)離子混合構(gòu)型。還討論了O空位對(duì)極性和非極性異質(zhì)結(jié)構(gòu)的影響。非極性異質(zhì)結(jié)構(gòu)處的清潔界面表現(xiàn)出絕緣狀態(tài)，并在存在O空位的情況下變?yōu)榻饘賾B(tài)。計(jì)算并解釋了空位形成能，并根據(jù)A的側(cè)陽(yáng)離子的電負(fù)性和有效體積的增加進(jìn)行了解釋。該觸點(diǎn)顯示出金屬質(zhì)感。 CoS2在界面處保持準(zhǔn)金屬狀態(tài)，而FeS2變?yōu)榻饘贍顟B(tài)。在界面處，發(fā)現(xiàn)鐵磁排序與反鐵磁排序相比在能量上是有利的。此外，拉伸應(yīng)變可顯著增強(qiáng)自旋極化，因此可實(shí)現(xiàn)幾乎半金屬的界面，具有相當(dāng)中等的應(yīng)變。我們的詳細(xì)研究旨在補(bǔ)充各種氧化物界面的實(shí)驗(yàn)，并大致了解陽(yáng)離子，陰離子，它們的原子量和電子勢(shì)能，O空位，晶格失配，晶格弛豫，磁性等在器件設(shè)計(jì)中共同發(fā)揮作用。

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  • 作者

    Nazir Safdar;

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  • 年度 2012
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