Dimensionality Controlled Octahedral Symmetry-Mismatch and Functionalities in Epitaxial LaCoO₃/SrTiO₃ Heterostructures

Abstract

Epitaxial strain provides a powerful approach to manipulate physical properties of materials through rigid compression or extension of their chemical bonds via lattice-mismatch. Although symmetry-mismatch can lead to new physics by stabilizing novel interfacial structures, challenges in obtaining atomic-level structural information as well as lack of a suitable approach to separate it from the parasitical lattice-mismatch have limited the development of this field. Here, we present unambiguous experimental evidence that the symmetry-mismatch can be strongly controlled by dimensionality and significantly impact the collective electronic and magnetic functionalities in ultrathin perovskite LaCoO3/SrTiO3 heterojunctions. State-of-art diffraction and microscopy reveal that symmetry breaking dramatically modifies the interfacial structure of CoO6 octahedral building-blocks, resulting in expanded octahedron volume, reduced covalent screening, and stronger electron correlations. Such phenomena fundamentally alter the electronic and magnetic behaviors of LaCoO3 thin-films. We conclude that for epitaxial systems, correlation strength can be tuned by changing orbital hybridization, thus affecting the Coulomb repulsion, U, instead of by changing the band structure as the common paradigm in bulks. These results clarify the origin of magnetic ordering for epitaxial LaCoO3 and provide a route to manipulate electron correlation and magnetic functionality by orbital engineering at oxide heterojunctions.

Sodium‐Mediated Epitaxial Growth of 2D Ultrathin Sb2Se3 Flakes for Broadband Photodetection

“…The realization of lateral growth of 2D Sb 2 Se 3 along [010] requires to break this symmetry restriction, which is thermodynamically unfavorable. In analogy to classic lattice epitaxy in oxide or semiconductor heterostructures,34–36 in this work, we introduce a unique sodium‐mediated epitaxial growth strategy to promote 2D planar growth of Sb 2 Se 3 crystals, as schematically illustrated in Figure 1b. Briefly, high purity argon (Ar) is used as the carrier gas, the powder mixture of Sb 2 Se 3 and NaCl is employed as the precursor, and the freshly cleaved fluorophlogopite mica ([KMg 3 (AlSi 3 O 10 )F 2 ]) is chosen as the growth substrate, which is placed above on the powder mixture with the freshly cleaved side facing down (see Experimental Section for the details).…”

Structural, electronic, and magnetic properties of bulk and epitaxial LaCoO3 through diffusion Monte Carlo

“…Using lattice distortions, the balance between crystal field splitting and Hund's exchange can be manipulated to drive metal/insulator and ferromagnetic/antiferromagnetic (FM/AFM) transitions [1][2][3]. Lanthanum cobaltite (LCO, LaCoO 3 ) is an example of a material that becomes a ferromagnet under epitaxial strain [4][5][6][7][8][9]. Ferromagnetism in epitaxial LCO is particularly interesting, as the bulk material was thought to be nonmagnetic [10].…”

Competition of defect ordering and site disproportionation in strained LaCoO3 on SrTiO3 (001)

“…Experimental growth of LCO films on STO(001) is usually carried out at around 650-750 • C and 1-320 mTorr oxygen pressure, as summarized in Table II. Fuchs et al, 22 Freeland et al, 21 and Qiao et al 24 exposed the samples to a 200-750 Torr oxygen atmosphere after deposition; nevertheless, they observed a similar Curie temperature as Mehta et al, 20,23 Choi et al, 17 Biškup et al, 19 and Feng et al 25 The relatively high temperatures employed during the growth process legitimate our thermodynamic approach, which is strictly valid only in equilibrium. 42,68 The phase diagram in Fig.…”

“…This may offer an explanation why the 3×1 reconstruction of LCO films strained on STO(001) is sometimes not observed. 24,25 Both phases exhibit a low conductivity, in particular the semiconducting IS/IS/IS phase, but also the LS/IS/IS phase due to the insulating LS planes. Moreover, the coexistence of FM and insulating NM domains in LCO films may add to the resistivity of the samples.…”

“…[17][18][19][20] These stripes were shown to be related to a short/short/long modulation of the La-La distances along the [100] direction. 19 Moreover, an insulating 21 and ferromagnetic (FM) ground state emerges, with a Curie temperature of T C ≈ 80 K. 17,[19][20][21][22][23][24][25] Two distinct models have been put forward to explain these observations. The first model is based on an ordered arrangement of oxygen vacancies in every third Co (100) plane, 19 formally LaCoO 2.…”