Stabilization and modulation of the topological magnetic phase with a Z2Z_2-vortex lattice in the Kitaev-Heisenberg honeycomb model: The key role of the third-nearest-neighbor interaction

The topologically nontrivial magnetic phase with a $Z_2$-vortex (Z$_2$V) lattice is investigated by simulation in the Kitaev-Heisenberg honeycomb model expanded by considering the second- and third-nearest-neighbor Heisenberg interactions ($J_{H2}$ and $J_{H3}$). On the parameter region of the Z$_2$V phase, a gradual modulation of vortex density is observed, together with a transition from single-Z$_2$V to triple-Z$_2$V state driven by the variation of frustration. Additionally, $Z_2$ vortices are arranged in different manners on the whole honeycomb structure for these two types of Z$_2$V states. Moreover, topologically equivalent states are revealed to exist in single-Z$_2$V dominant and triple-Z$_2$V dominant styles on different parameter points, which can be controlled to switch between each other without energy consumption. It is worth noting that $J_{H3}$ plays a key role in expanding the Z$_2$V phase, and also in stabilizing the single-Z$_2$V state.Comment: 14 pages, 6 figure

Quantum-Confinement-Induced Magnetism in LaNiO3_3-LaMnO3_3 Superlattices

The emergence of magnetic reconstructions at the interfaces of oxide heterostructures are often explained via subtle modifications in the electronic densities, exchange couplings, or strain. Here an additional possible route for induced magnetism is studied in the context of the (LaNiO$_3$)$_n$/(LaMnO$_3$)$_n$ superlattices using a hybrid tight-binding model. In the LaNiO$_3$ region, the induced magnetizations decouple from the intensity of charge leakage from Mn to Ni, but originate from the spin-filtered quantum confinement present in these nanostructures. In general, the induced magnetization is the largest for the (111)-stacking and the weakest for the (001)-stacking superlattices, results compatible with the exchange bias effects reported by Gibert et al. Nat. Mater. 11, 195 (2012).Comment: 8 pages, 8 figure

Challenges in Band Alignment between Semiconducting Materials: A Case of Rutile and Anatase TiO2_2

This topical review focuses on the recently active debate on the band alignment between two polymorphs of TiO$_2$, rutile and anatase. A summary is given to the popular methods for measurement and calculation of band alignment between materials. We point out, through examination of recently experimental and theoretical reports, that the outstanding discrepancy in the band alignment between two TiO$_2$ phases is attributed to factors that influence band alignment rather than needs a definite answer of which band alignment is right. According to an important factor, the presence of an interface, a new classification of band alignment is proposed as the coupled and intrinsic band alignments. This classification indeed reveals that the rutile/anatase interface can qualitatively change the type of their band alignment. However, further systematic information of the interface and other factors that influence band alignment will be needed to understand changes in energy bands of materials better. The results obtained from discussion of the band alignment between rutile and anatase may also work for the band alignment between other semiconductors.Comment: 21 pages, 9 figures. A topical revie

Full Control of Magnetism in Manganite Bilayer by Ferroelectric Polarization

An oxide heterostructure made of manganite bilayers and ferroelectric perovskites is predicted to lead to the full control of magnetism when switching the ferroelectric polarizations. By using asymmetric polar interfaces in the superlattices, more electrons occupy the Mn layer at the $n$-type interface side than at the $p$-type side. This charge disproportionation can be enhanced or suppressed by the ferroelectric polarization. Quantum model and density functional theory calculations reach the same conclusion: a ferromagnetic-ferrimagnetic phase transition with maximal change $>90$ of the total magnetization can be achieved by switching the polarization's direction. This function is robust and provides full control of the magnetization's magnitude, not only its direction, via electrical methods.Comment: 9 pages, 7 figure

Orientation-dependent magnetism and orbital structure of strained YTiO3_3 films on LaAlO3_3 substrates

The strain tuned magnetism of YTiO$_3$ film grown on the LaAlO$_3$ ($110$) substrate is studied by the method of the first principles, and compared with that of the ($001$)-oriented one. The obtained magnetism is totally different, which is ferromagnetic for the film on the ($110$) substrate but A-type antiferromagnetic on the ($001$) one. This orientation-dependent magnetism is attributed to the subtle orbital ordering of YTiO$_3$ film. The $d_{xz}$/$d_{yz}$-type orbital ordering is predominant for the ($001$) one, but for the ($110$) case, the $d_{xy}$ orbital is mostly occupied plus a few contribution from the $d_{xz}$/$d_{yz}$ orbital. Moreover, the lattice mismatch is modest for the ($110$) case but more serious for the ($001$) one, which is also responsible for this contrasting magnetism.Comment: 4 pages, 4 figures; Proceeding of the 59th Annual Magnetism and Magnetic Materials (MMM) Conferenc

Strain-engineered A-type antiferromagnetic order in YTiO3_3: a first-principles calculation

The epitaxial strain effects on the magnetic ground state of YTiO$_3$ films grown on LaAlO$_3$ substrates have been studied using the first-principles density-functional theory. With the in-plane compressive strain induced by LaAlO$_3$ (001) substrate, A-type antiferromagnetic order emerges against the original ferromagnetic order. This phase transition from ferromagnet to A-type antiferromagnet in YTiO$_3$ film is robust since the energy gain is about 7.64 meV per formula unit despite the Hubbard interaction and modest lattice changes, even though the A-type antiferromagnetic order does not exist in any $R$TiO$_3$ bulks.Comment: 3 pages, 2 figures. Proceeding of the 12th Joint MMM/Intermag Conference. Accepted by JA

Unusual Ferroelectricity of Trans-Unitcell Ion-Displacement and Multiferroic Soliton in Sodium and Potassium Hydroxides

We show the first-principles evidence of a hitherto unreported type of ferroelectricity with ultra-long ion-displacement in sodium and potassium hydroxides. Even a small amount of proton vacancies can completely change the mode of proton-transfer from intra-unitcell to trans-unitcell, giving rise to multiferroic soliton with mobile magnetism and a tremendous polarization that can be orders of magnitude higher compared with most perovskite ferroelectrics. Their vertical polarizations of thin-film are robust against depolarizing field, rendering various designs of 2D ferroelectric field-transistors with non-destructive readout and ultra-high on/off ratio via sensing the switchable metallic/insulating state

Strain enhanced superconductivity of MoX2X_2 (XX=S or Se) bilayers with Na intercalation

Mo$X_2$ ($X$=S or Se) is a semiconductor family with two-dimensional structure. And a recent calculation predicted the superconductivity in electron doped MoS$_2$ monolayer. In this work, the electronic structure and lattice dynamics of Mo$X_2$ bilayers with monolayer Na intercalated, have been calculated. According to the electron-phonon interaction, it is predicted that these bilayers can be transformed from indirect-gap semiconductors to a superconductors by Na intercalation. More interestingly, the biaxial tensile strain can significantly enhance the superconducting temperature up to $\sim10$ K in Na-intercalated MoS$_2$. In addition, the phonon mean free path at room-temperature is also greatly improved in Na intercalated MoSe$_2$, which is advantaged for related applications.Comment: 6 pages, 5 figure

Protective layer enhanced the stability and superconductivity of tailored antimonene bilayer

For two-dimensional superconductors, the high stability in ambient conditions is critical for experiments and applications. Few-layer antimonene can be non-degradative over a couple of months, which is superior to the akin black phosphorus. Based on the anisotropic Migdal-Eliashberg theory and maximally-localised Wannier functions, this work predicts that electron-doping and Ca-intercalation can transform $\beta$-Sb bilayer from a semimetal to a superconductor. However, the stability of antimonene bilayer in air trends to be decreased due to the electron doping. To overcome this drawback, two kinds of protective layers (graphene and $h$-BN) are proposed to enhance the stability. Interestingly, the superconducting transition temperature will also be enhanced to $9.6$ K, making it a promising candidate as nanoscale superconductor.Comment: 7 pages, 11 figure

First-principles study of the relaxor ferroelectricity of Ba(Zr,Ti)O3_3

Ba(Zr,Ti)O$_3$ is a lead-free relaxor ferroelectric. Using the first-principles method, the ferroelectric dipole moments for pure BaTiO$_3$ and Ba(Zr,Ti)O$_3$ supercells have been studied. All possible ion configurations of BaZr$_{0.5}$Ti$_{0.5}$O$_3$ and BaZr$_{0.25}$Ti$_{0.75}$O$_3$ are constructed in a $2\times2\times2$ supercell. For the half-substituted case, divergence of ferroelectric properties has been found among these structures, which seriously depends on the arrangement of Ti and Zr ions. Thus our results provide a reasonable explanation to the relaxor behavior of Ba(Zr,Ti)O$_3$. In addition, a model based on the thermal statistics gives the averaged polarization for Ba(Zr,Ti)O$_3$, which depends on the temperature of synthesis. Our result is helpful to understand and tune the relaxor ferroelectricity of lead-free Ba(Zr,Ti)O$_3$.Comment: 15 pages, 2 figures, 2 table