arXiv subjects

Hongjun Xiang

Publications and source records attributed to Hongjun Xiang.

At least 109 records · Page 6Linked to original sources

arXiv2016

Two Dimensional Phosphorus Oxides as Energy and Information Materials

Two-dimensional (2D) black phosphorus (i.e., phosphorene) has become a rising star in electronics. Recently, 2D phosphorus oxides with higher stability have been synthesized. In this work, we systematically explore the structures and properties of 2D phosphorus oxides on the basis of global optimization approach and first-principles calculations. We find that the structural features of 2D phosphorus oxides PxOy vary with the oxygen concentration. When the oxygen content is poor, the most stable 2D PxOy can be obtained by adsorbing O atoms on phosphorene. However, when the oxygen concentration becomes rich, stable structures are no longer based on phosphorene and will contain P-O-P motifs. For the 2D P4O4, we find that it has a direct band gap (about 2.24 eV), good optical absorption, and high stability in water, suggesting that it may be good candidate for photochemical water splitting application. Interestingly, 2D P2O3 adopt two possible stable ferroelectric structures (P2O3-I and P2O3-II) as the lowest energy configurations within a given layer thickness. The electric polarizations of P2O3-I and P2O3-II are perpendicular and parallel to the lateral plane, respectively. We propose that 2D P2O3 could be used in a novel nanoscale multiple-state memory device. Our work suggests that 2D phosphorus oxides may be excellent functional materials.

cond-mat.mtrl-sci↗

arXiv2016

Structure and Composition of the 200 K-Superconducting Phase of H2S under Ultrahigh Pressure: The Perovskite (SH-)(H3S+)

H2S is converted under ultrahigh pressure (> 110 GPa) to a metallic phase that becomes superconducting with a record Tc of 200 K. It has been proposed that the superconducting phase is body-centered cubic H3S ( Im3m , a = 3.089 Å) resulting from a decomposition reaction 3H2S --> 2H3S + S. The analogy of H2S and H2O leads us to a very different conclusion. The well-known dissociation of water into H3O+ and OH- increases by orders of magnitude under pressure. An equivalent behavior of H2S is anticipated under pressure with the dissociation, 2H2S --> H3S+ + SH- forming a perovskite structure (SH-)(H3S+), which consists of corner-sharing SH6 octahedra with SH- at each A-site (i.e., the center of each S8 cube). Our DFT calculations show that the perovskite (SH-)(H3S+) is thermodynamically more stable than the Im3m structure of H3S, and suggest that the A-site H atoms are most likely fluxional even at Tc.

cond-mat.supr-con↗

arXiv2016

Spin orientations of the spin-half Ir4+ ions in Sr3NiIrO6, Sr2IrO4 and Na2IrO3: Density functional, perturbation theory and Madelung potential analyses

The spins of the low-spin Ir4+ (S = 1/2, d5) ions at the octahedral sites of the oxides Sr3NiIrO6, Sr2IrO4 and Na2IrO3 exhibit preferred orientations with respect to their IrO6 octahedra. We evaluated the magnetic anisotropies of these S = 1/2 ions on the basis of DFT calculations including spin-orbit coupling (SOC), and probed their origin by performing perturbation theory analyses with SOC as perturbation within the LS coupling scheme. The observed spin orientations of Sr3NiIrO6 and Sr2IrO4 are correctly predicted by DFT calculations, and are accounted for by the perturbation theory analysis. As for the spin orientation of Na2IrO3, both experimental studies and DFT calculations have not been unequivocal. Our analysis reveals that the Ir4+ spin orientation of Na2IrO3 should have nonzero components along the c- and a-axes directions. The spin orientations determined by DFT calculations are sensitive to the accuracy of the crystal structures employed, which is explained by perturbation theory analyses when interactions between adjacent Ir4+ ions are taken into consideration. There are indications implying that the 5d electrons of Na2IrO3 are less strongly localized compared with those of Sr3NiIrO6 and Sr2IrO4. This implication was confirmed by showing that the Madelung potentials of the Ir4+ ions are less negative in Na2IrO3 than in Sr3NiIrO6, Sr2IrO4. Most transition-metal S = 1/2 ions do have magnetic anisotropies because the SOC induces interactions among their crystal-field split d-states, and the associated mixing of the states modifies only the orbital parts of the states. This finding cannot be mimicked by a spin Hamiltonian because this model Hamiltonian lacks the orbital degree of freedom, thereby leading to the spin-half syndrome. The spin-orbital entanglement for the 5d spin-half ions Ir4+ is not as strong as has been assumed lately.

cond-mat.str-el↗

arXiv2015

Prediction of spin orientations in terms of HOMO-LUMO interactions using spin-orbit coupling as perturbation

The preferred spin orientation of a magnetic ion can be predicted on the basis of density functional theory (DFT) calculations including electron correlation and spin-orbit coupling (SOC). However, most chemists and physicists are unaware of how the observed and/or calculated spin orientations are related to the local electronic structures of the magnetic ions. The objective of this article is to provide a conceptual framework of thinking about and predicting the preferred spin orientation of a magnetic ion by examining the relationship between the spin orientation and the local electronic structure of the ion. In general, a magnetic ion (i.e., an ion possessing unpaired spins) in a solid or a molecule is surrounded with main-group ligand atoms to form a polyhedron, and the d-states of the polyhedron are split because the antibonding interactions of the metal d-orbitals with the p orbitals of the surrounding ligands depend on the symmetries of the orbitals involved. The magnetic ion of the polyhedron has a certain preferred spin direction because its split d-states interact among themselves under SOC and because the energy lowering associated with the SOC-induced interactions depends on spin orientation. The preferred spin direction can be readily predicted on the basis of perturbation theory, in which the SOC is taken as perturbation and the split d-states as unperturbed states, by inspecting the magnetic quantum numbers of its d-orbitals present in the HOMO and LUMO of the polyhedron. Experimentally, the determination of the preferred spin orientations of magnetic ions requires a sophisticated level of experiments. Theoretically, it requires an elaborate level of DFT electronic structure calculations. We show that the outcomes of such experimental measurements and theoretical calculations can be predicted by a simple perturbation theory analysis.

cond-mat.str-el↗

arXiv2015

Giant biquadratic interaction induced magnetic anisotropy in the iron-based superconductor AxFe2-ySe2

The emergence of the electron-pocket only iron-based superconductor AxFe2-ySe2 (A = alkali metal) challenges the Fermi-surface nesting picture established in iron-pnictides. It was widely believed that magnetism is correlated with the superconductivity in AxFe2-ySe2. Unfortunately, the highly anisotropic exchange parameters and the disagreement between theoretical calculations and experimental results triggered a fierce debate on the nature of magnetism in AxFe2-ySe2. Here we find that the strong magnetic anisotropy is from the anisotropic biquadratic interaction. In order to accurately obtain the magnetic interaction parameters, we propose a universal method, which does not need including other high energy configurations as did in conventional energy mapping method. We show that our model successfully captures the magnetic interactions in AxFe2-ySe2 and correctly predicts the spin wave spectrum, in quantitative agreement with the experimental observation. These results suggest that the local moment picture, including the biquadratic term, can describe accurately the magnetic properties and spin excitations in AxFe2-ySe2, which sheds new light on the future study of the high-Tc iron-based superconductors.

cond-mat.supr-con↗

arXiv2015

Structural Evolution and Optoelectronic Applications of Multilayer Silicene

Despite the recent progress on two-dimensional multilayer materials (2DMM) with weak interlayer interactions, the investigation on 2DMM with strong interlayer interactions is far from its sufficiency. Here we report on first-principles calculations that clarify the structural evolution and optoelectronic properties of such a 2DMM, multilayer silicene. With our newly developed global optimization algorithm, we discover the existence of rich dynamically stable multilayer silicene phases, the stability of which is closely related to the extent of sp3 hybridization that can be evaluated by the average bonds and effective bond angles. The stable Si(111) surface structures are obtained when the silicene thickness gets up to four, showing the critical thickness for the structural evolution. We also find that the multilayer silicene with pi-bonded surfaces present outstanding optoelectronic properties for the solar cells and optical fiber communications due to the incorporation of sp2-type bonds in the sp3-type bonds dominated system. This study is helpful to complete the picture of structure and related property evolution of 2DMM with strong interlayer interactions.

cond-mat.mes-hall↗

arXiv2015

Intrinsic Instability of the Hybrid Halide Perovskite Semiconductor CH3NH3PbI3

The organic-inorganic hybrid perovskite CH3NH3PbI3 has attracted significant interest for its high performance in converting solar light into electrical power with an efficiency exceeding 20%. Unfortunately, chemical stability is one major challenge in the development of the CH3NH3PbI3 solar cells. It was commonly assumed that moisture or oxygen in the environment causes the poor stability of hybrid halide perovskites, however, here we show from the first-principles calculations that the room-temperature tetragonal phase of CH3NH3PbI3 is thermodynamically unstable with respect to the phase separation into CH3NH3I + PbI2, i.e., the disproportionation is exothermic, independent of the humidity or oxygen in the atmosphere. When the structure is distorted to the low-temperature orthorhombic phase, the energetic cost of separation increases, but remains small. Contributions from vibrational and configurational entropy at room temperature have been considered, but the instability of CH3NH3PbI3 is unchanged. When I is replaced by Br or Cl, Pb by Sn, or the organic cation CH3NH3 by inorganic Cs, the perovskites become more stable and do not phase-separate spontaneously. Our study highlights that the poor chemical stability is intrinsic to CH3NH3PbI3 and suggests that element-substitution may solve the chemical stability problem in hybrid halide perovskite solar cells.

cond-mat.mtrl-sci↗

arXiv2015

Room Temperature Quantum Spin Hall Insulators with a Buckled Square Lattice

Two-dimensional (2D) topological insulators (TIs), also known as quantum spin Hall (QSH) insulators, are excellent candidates for coherent spin transport related applications because the edge states of 2D TIs are robust against nonmagnetic impurities since the only available backscattering channel is forbidden. Currently, most known 2D TIs are based on a hexagonal (specifically, honeycomb) lattice. Here, we propose that there exists the quantum spin Hall effect (QSHE) in a buckled square lattice. Through performing global structure optimization, we predict a new three-layer quasi-2D (Q2D) structure which has the lowest energy among all structures with the thickness less than 6.0 Å for the BiF system. It is identified to be a Q2D TI with a large band gap (0.69 eV). The electronic states of the Q2D BiF system near the Fermi level are mainly contributed by the middle Bi square lattice, which are sandwiched by two inert BiF2 layers. This is beneficial since the interaction between a substrate and the Q2D material may not change the topological properties of the system, as we demonstrate in the case of the NaF substrate. Finally, we come up with a new tight-binding model for a two-orbital system with the buckled square lattice to explain the low-energy physics of the Q2D BiF material. Our study not only predicts a QSH insulator for realistic room temperature applications, but also provides a new lattice system for engineering topological states such as quantum anomalous Hall effect.

cond-mat.mtrl-sci↗

arXiv2015

Oxygen Vacancy Induced Flat Phonon Mode at FeSe /SrTiO3 interface

A high-frequency optical phonon mode of SrTiO3 (STO) was found to assist the high-temperature superconductivity observed recently at the interface between monolayer FeSe and STO substrate. However, the origin of this mode is not clear. Through first-principles calculations, we find that there is a novel polar phonon mode on the surface layers of the STO substrate, which does not exist in the STO crystals. The oxygen vacancies near the FeSe/STO interface drives the dispersion of this phonon mode to be flat and lowers its energy, whereas the charge transfer between STO substrate and FeSe monolayer further reduces its energy to 81 meV. This energy is in good agreement with the experimental value fitted by Lee et al. for the phonon mode responsible for the observed replica band separations and the increased superconducting gap. The oxygen-vacancy-induced flat and polar phonon mode provides clues for understanding the origin of high Tc superconductivity at the FeSe/STO interface.

cond-mat.supr-con↗

arXiv2015

Prediction of Silicon-Based Layered Structures for Optoelectronic Applications

A method based on the particle swarm optimization (PSO) algorithm is presented to design quasi-two-dimensional (Q2D) materials. With this development, various single-layer and bi-layer materials in C, Si, Ge, Sn, and Pb were predicted. A new Si bi-layer structure is found to have a much-favored energy than the previously widely accepted configuration. Both single-layer and bi-layer Si materials have small band gaps, limiting their usages in optoelectronic applications. Hydrogenation has therefore been used to tune the electronic and optical properties of Si layers. We discover two hydrogenated materials of layered Si8H2 and Si6H2 possessing quasi-direct band gaps of 0.75 eV and 1.59 eV, respectively. Their potential applications for light emitting diode and photovoltaics are proposed and discussed. Our study opened up the possibility of hydrogenated Si layered materials as next-generation optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2014

Antiferromagnetic ground state with pair-checkboard order in FeSe

Monolayer FeSe thin film grown on SrTiO_{3}(001) (STO) shows the sign of T_{c}> 77 K, which is higher than the T_{c}-record of 56 K for the bulk FeAs-based superconductors. However, little is known about the magnetic ground state of FeSe, which should be closely related to its unusual superconductivity. Previous studies presume the collinear stripe antiferromagnetic (AFM) state as the ground state of FeSe, same to that in FeAs superconductors. Here we find a novel magnetic order named "pair-checkboard AFM" as the magnetic ground state of tetragonal FeSe. The novel pair-checkboard order results from the interplay between the nearest, the next-nearest and the unnegligible next-next-nearest neighbor magnetic exchange couplings of Fe atoms. The monolayer FeSe in pair-checkbord order shows an unexpected insulating behavior with a Dirac-cone-like band structure related to the specific orbital order of d_{xz} and d_{yz} characters of Fe atoms, which could explain recently observed insulator-superconductor transition. The present results cast new insights on the magnetic ordering in FeSe monolayer and its derived superconductors.

cond-mat.supr-con↗

arXiv2014

Understanding the Magnetic Puzzles of Double Perovskites A2FeOsO6 (A=Ca, Sr)

Double perovskites Sr2FeOsO6 and Ca2FeOsO6 show puzzling magnetic properties, the former a low-temperature antiferromagnet while the later a high-temperature insulating ferrimagnet. Here, in order to understand the underlying mechanism, we have investigated the frustrated magnetism of A2FeOsO6 by employing density functional theory and maximally-localized Wannier functions. We find that lattice distortion enhances the antiferromagnetic nearest-neighboring Fe-O-Os interaction but weakens the antiferromagnetic interactions through the Os-O-O-Os and Fe-O-Os-O-Fe paths, which is responsible for the magnetic transition from the low-temperature antiferromagnetism to the high-temperature ferrimagnetism with the decrease of the radius of the A2+ ions. We also discuss the 5d3-3d5 superexchange and propose such superexchange is intrinsically antiferromagnetic instead of the expected ferromagnetic. Our work illustrate that the magnetic frustration can be effectively relieved by lattice distortion, which provides another dimension to tune the complex magnetism in other 3d-5d (4d) double perovskites.

cond-mat.mtrl-sci↗

arXiv2014

Origin of Polar Distortion in LiNbO3-type "Ferroelectric" Metals: Role of A-site Instability and Short-Range Interactions

Since conduction electrons of a metal screen effectively the local electric dipole moments, it was widely believed that the ferroelectric-like distortion cannot occur in metals. Recently, metallic LiOsO3, was discovered to be the first clear-cut example of an Anderson-Blount "ferroelectric" metal, which at 140 K undergoes a ferroelectric-like structural transition similar to insulating LiNbO3. This is very surprising because the mechanisms for structural phase transitions are usually quite distinct in metals and insulators. Through performing first principles calculations, here we reveal that the local polar distortion in LiOsO3 is solely due to the instability of the A-site Li atom, in contrast to the LiNbO3 case where the second order Jahn-Teller effect of the B-site Nb ion also plays an additional role. More importantly, the "ferroelectric"-like long range order of the local polar distortion is found to be due to the predominantly ferroelectric short-range pair interactions between the local polar modes which are not screened by conduction electrons. Furthermore, we predict that LiNbO3-type MgReO3 is also a "ferroelectric" metal, but with a much higher structural transition temperature by 391 K than LiOsO3. Our work not only unravels the origin of FE-like distortion in LiNbO3-type "ferroelectric" metals, but also provides clue for designing other multi-functional "ferroelectric" metals.

cond-mat.mtrl-sci↗

arXiv2014

Most spin-1/2 transition-metal ions do have single ion anisotropy

The cause for the preferred spin orientation in magnetic systems containing spin-1/2 transition-metal ions was explored by studying the origin of the easy-plane anisotropy of the spin-1/2 Cu2+ ions in CuCl2.2H2O, LiCuVO4, CuCl2 and CuBr2 on the basis of density functional theory and magnetic dipole-dipole energy calculations as well as a perturbation theory treatment of the spin-orbit coupling. We find that the spin orientation observed for these spin-1/2 ions is not caused by their anisotropic spin exchange interactions, nor by their magnetic dipole-dipole interactions, but by the spin-orbit coupling associated with their crystal-field split d-states. Our study also predicts in-plane anisotropy for the Cu2+ ions of Bi2CuO4 and Li2CuO2. The results of our investigations dispel the mistaken belief that magnetic systems with spin-1/2 ions have no magnetic anisotropy induced by spin-orbit coupling.

cond-mat.str-el↗

arXiv2013

The interfacial effects on the spin density wave in FeSe/SrTiO3 thin film

Recently, the signs of both superconducting transition temperature (Tc) beyond 60 K and spin density wave (SDW) have been observed in FeSe thin film on SrTiO3 (STO) substrate, which suggests a strong interplay between superconductivity and magnetism. With the first-principles calculations, we find that the substrate-induced tensile strain tends to stabilize the SDW state in FeSe thin film by enhancing of the next-nearest-neighbor superexchange antiferromagnetic interaction bridged through Se atoms. On the other hand, we find that when there are oxygen vacancies in the substrate, the significant charge transfer from the substrate to the first FeSe layer would suppress the magnetic order there, and thus the high-temperature superconductivity could occur. In addition, the stability of the SDW is lowered when FeSe is on a defect-free STO substrate due to the redistribution of charges among the Fe 3d-orbitals. Our results provide a comprehensive microscopic explanation for the recent experimental findings, and build a foundation for the further exploration of the superconductivity and magnetism in this novel superconducting interface.

cond-mat.supr-con↗

arXiv2013

Unveiling the Origin of the Insulating Ferromagnetism in LaMnO3 Thin Film

By combining genetic algorithm optimizations, first-principles calculations and the double-exchange model studies, we have unveiled that the exotic insulating ferromagnetism in LaMnO3 thin film originates from the previously unreported G-type d_{3z^2-r^2}/d_{x^2-y^2} orbital ordering. An insulating gap opens as a result of both the orbital ordering and the strong electron-phonon coupling. Therefore, there exist two strain induced phase transitions in the LaMnO3 thin film, from the insulating A-type antiferromagnetic phase to the insulating ferromagnetic phase and then to the metallic ferromagnetic phase. These phase transitions may be exploited in tunneling magnetoresistance and tunneling electroresistance related devices.

cond-mat.str-el↗

arXiv2013

What Is Armchair-Zigzag Grain Boundary Structure in Graphene?

We have developed a new global optimization method for the determination of interface structure based on the differential evolution algorithm. Here, we applied this method to search for the ground state atomic structures of the grain boundary between the armchair and zigzag oriented graphene. We find two new grain boundary structures with considerably lower formation energy of about 1 eV/nm than that of the previously widely used structural models. These newly predicted structures show better mechanical property under external uniaxial strain, and distinguishable scanning tunneling microscope features, compared with the previous structural models. Our results provide important new information for the determination of grain boundary structures and henceforth the electronic properties of defected graphene.

cond-mat.mtrl-sci↗

arXiv2013

Prediction of (TiO2)x(Cu2O)y Alloys for Photoelectrochemical Water Splitting

The formation of (TiO2)x(Cu2O)y solid-solutions are investigated using a global optimization evolutionary algorithm. First-principles calculations based on density functional theory are then used to gain insight into the electronic properties of these alloys. We find that: (i) Ti and Cu in (TiO2)x(Cu2O)y alloys have similar local environments as in bulk TiO2 and Cu2O except for (TiO2)(Cu2O) which has some trigonal-planar Cu ions. (ii) The predicted optical band gaps are around 2.1 eV (590 nm), thus having much better performance for the absorption of visible light compared with both binary oxides. (iii) (TiO2)2(Cu2O) has the lowest formation energy amongst all studied alloys and the positions of its band edges are found to be suitable for solar-driven water splitting applications.

cond-mat.mtrl-sci↗