Control of Impurity Phase Segregation in a PdCrO2_2/CuCrO2_2 Heterostructure

PdCrO$_2$ films are synthesized on CuCrO$_2$ buffer layers on Al$_2$O$_3$ substrates. This synthesis is accompanied by impurity phase segregation, which hampers the synthesis of high quality PdCrO$_2$ films. The potential causes of impurity phase segregation were studied by using a combination of experiments and ab initio calculations. X-ray diffraction and scanning transmission electron microscopy experiments revealed impurity phases of Cu$_x$Pd$_{1-x}$ alloy and chromium oxides, Cr$_2$O$_3$ and Cr$_3$O$_4$, in PdCrO$_2$. Calculations determined that oxygen deficiency can cause the impurity phase segregation. Therefore, preventing oxygen release from delafossites could suppress the impurity phase segregation. The amounts of Cr$_2$O$_3$ and Cr$_3$O$_4$ depend differently on temperature and oxygen partial pressure. A reasonable theory-based explanation for this experimental observation is provided

Effects of paramagnetic fluctuations on the thermochemistry of MnO (100) surfaces in the oxygen evolution reaction

We investigated the effects of paramagnetic (PM) fluctuations on the thermochemistry of the MnO(100) surface in the oxygen evolution reaction (OER) using the "noncollinear magnetic sampling method \textit{plus} $U$" (NCMSM$+U$). Various physical properties, such as the electronic structure, free energy, and charge occupation, of the MnO (100) surface in the PM state with several OER intermediates, were reckoned and compared to those in the antiferromagnetic (AFM) state. We found that PM fluctuation enhances charge transfer from a surface Mn ion to each of the intermediates and strengthens the chemical bond between them, while not altering the overall features, such as the rate determining step and resting state, in reaction pathways. The enhanced charge transfer can be attributed to the delocalized nature of valence bands observed in the PM surface. In addition, it was observed that chemical-bond enhancement depends on the intermediates, resulting in significant deviations in reaction energy barriers. Our study suggests that PM fluctuations play a significant role in the thermochemistry of chemical reactions occurring on correlated oxide surfaces.Comment: Maintext: 15 pages, 3 figures 2 tables; SI: 3 pages, 2 figure

Spatial symmetry constraint of charge-ordered kagome superconductor CsV3_3Sb5_5

Elucidating the symmetry of intertwined orders in exotic superconductors is at the quantum frontier. Recent surface sensitive studies of the topological kagome superconductor CsV$_3$Sb$_5$ discovered a cascade 4a$_0$ superlattice below the charge density wave (CDW) ordering temperature, which can be related to the pair density modulations in the superconducting state. If the 4a$_0$ phase is a bulk and intrinsic property of the kagome lattice, this would form a striking analogy to the stripe order and pair density wave discovered in the cuprate high-temperature superconductors, and the cascade ordering found in twisted bilayer graphene. High-resolution X-ray diffraction has recently been established as an ultra-sensitive probe for bulk translational symmetry-breaking orders, even for short-range orders at the diffusive limit. Here, combining high-resolution X-ray diffraction, scanning tunneling microscopy and scanning transmission electron microscopy, we demonstrate that the 4a$_0$ superstructure emerges uniquely on the surface and hence exclude the 4a$_0$ phase as the origin of any bulk transport or spectroscopic anomaly. Crucially, we show that our detected 2$\times$2$\times$2 CDW order breaks the bulk rotational symmetry to C2, which can be the driver for the bulk nematic orders and nematic surface superlattices including the 4a$_0$ phase. Our high-resolution data impose decisive spatial symmetry constraints on emergent electronic orders in the kagome superconductor CsV$_3$Sb$_5$

Correlated oxide Dirac semimetal in the extreme quantum limit

Quantum materials (QMs) with strong correlation and nontrivial topology are indispensable to next-generation information and computing technologies. Exploitation of topological band structure is an ideal starting point to realize correlated topological QMs. Here, we report that strain-induced symmetry modification in correlated oxide SrNbO3 thin films creates an emerging topological band structure. Dirac electrons in strained SrNbO3 films reveal ultrahigh mobility (mu(max) approximate to 100,000 cm(2)/Vs), exceptionally small effective mass (m* similar to 0.04m(e)), and nonzero Berry phase. Strained SrNbO3 films reach the extreme quantum limit, exhibiting a sign of fractional occupation of Landau levels and giant mass enhancement. Our results suggest that symmetry-modified SrNbO3 is a rare example of correlated oxide Dirac semimetals, in which strong correlation of Dirac electrons leads to the realization of a novel correlated topological QM

Preferential growth of boron layer in magnesium diboride (MgB2) by Mg diffusion method

Growth mechanism and grain boundary (GB) contact of polycrystalline MgB2 fabricated by Mg diffusion method are studied by STEM and EELS analyses. In contrast to the previous reports based on the computational calculation, preferential growth of (001) boron (B) layer and the B-B contact at MgB2 GBs are confirmed by annular dark field (ADF) -STEM image and the combined EELS analyses. The effect of B-B contact at the GB on the superconductivity is further evaluated using First principles calculation. Superior GB linkage of the supercurrent flow via GB B-B contact is expected from the calculated density of states at Fermi level. B-terminated growth mechanism in Mg diffusion method and the effect of GB connectivity via B-B and Mg-Mg contacts are discussed. Finally, we suggest a model of GB linkage of supercurrent flow via B-B contact in polycrystalline MgB2

Unveiling origin of additional capacity of SnO2 anode in lithium-ion batteries by realistic ex situ TEM analysis

The SnO2 material has been considered as a promising lithium -ion battery anode candidate, and recently, the importance has been increased due to its high performance in sodium -ion batteries. Remarkably, the SnO2 lithium -ion battery anode usually shows extra specific capacity that greatly exceeds the theoretical value. Partial reversibility of conversion reaction has been commonly considered to contribute the extra capacity, however, this has not clearly solved due to the indirect experimental evidences. Here, a realistic ex situ transmission electron microscopy (TEM) analysis technique was developed to reveal the origin of the extra capacity. We demonstrate that reactions of Li20 phase contribute to the extra capacity and the reverse conversion reaction of SnO2 hardly occurs in the real battery system. This work provides significant implications for establishing an accurate electrochemical reaction mechanism of SnO2 lithium -ion battery anode, which may lead to inspiration on enhancing performance of the SnO2 anode in lithium- and sodium -ion batteries as well. Furthermore, the robust ex situ TEM experimental approach we have introduced is extensively applicable to analyses of various battery electrode materials.