Shuhui Lv

New horizon for high performance Mg-based biomaterial with uniform degradation behavior: Formation of stacking faults.

Designing the new microstructure is an effective way to accelerate the biomedical application of magnesium (Mg) alloys. In this study, a novel Mg–8Er–1Zn alloy with profuse nano-spaced basal plane stacking faults (SFs) was prepared by combined processes of direct-chill semi-continuous casting, heat-treatment and hot-extrusion. The formation of SFs made the alloy possess outstanding comprehensive performance as the biodegradable implant material. The ultimate tensile strength (UTS: 318 MPa), tensile yield strength (TYS: 207 MPa) and elongation (21%) of the alloy with SFs were superior to those of most reported degradable Mg-based alloys. This new alloy showed acceptable biotoxicity and degradation rate (0.34 mm/year), and the latter could be further slowed down through optimizing the microstructure. Most amazing of all, the uniquely uniform in vitro/vivo corrosion behavior was obtained due to the formation of SFs. Accordingly we proposed an original corrosion mechanism for the novel Mg alloy with SFs. The present study opens a new horizon for developing new Mg-based biomaterials with highly desirable performances.

Two-dimensional electron gas at the Ti-diffused BiFeO3/SrTiO3 interface

Oxide heterostructures with the broken translational symmetry often trigger a two-dimensional quantum confinement and associated unique electronic properties that cannot be observed in bulk constituents. Particular interest is devoted to the formation of two-dimensional electron gas (2DEG) at heterointerfaces between two insulators, which offers a fertile ground for fabricating advanced electronic devices. Here, we combine atomic force microscopy, transmission electron microscopy, and atomistic first-principles calculations to demonstrate that the (100) BiFeO3/SrTiO3 interface takes on a metallic nature and a 2DEG is generated at this interface. Our findings also reveal that the electronic reconstruction due to the polar discontinuity and the variation in valence state of Ti arising from diffusion of Ti cations in SrTiO3 to Fe sites of BiFeO3 are critical to the formation of 2DEG at the heterointerface.

Atomic and electronic structure of the SrNbO3/SrNbO3.4 interface

We have determined the atomic-scale structure of the SrNbO3/SrNbO3.4 interface and related it to the electronic structure. Experimentally, transmission electron microscopy observations reveal that SrNbO3 and SrNbO3.4 show orientation relationships [ 1¯10]SrNbO3 // [010]SrNbO3.4 and (110)SrNbO3 // (001)SrNbO3.4, and that their interface is coherent and atomically abrupt. Theoretically, this interface is found to be strongly anisotropic in electronic structure, and takes on quasi-one-dimensional nature. We also find that the interface impacts greatly the electron occupation of Nb d orbitals, particularly dz2 orbital. The combined study represents a relevant advance in atomically bridging structures to properties of heterointerfaces.

Mechanism of A-B intersite charge transfer and negative thermal expansion in A-site-ordered perovskite LaCu3Fe4O12

Temperature induced intermetallic charge transfer and negative thermal expansion in compounds hold promise for many applications. Here, we report, by the first-principles calculations, the mechanism behind these effects and the associated electrical and magnetic properties of an A-site-ordered perovskite LaCu3Fe4O12. We find that the sensitive expansion of Cu-O bonds to temperature can trigger a transformation from Cu3+ to Cu2+, which imposes a covalent state transition of B-site Fe from +3 to +3.75. The resultant shrinkage of the Fe-O bonds is demonstrated to play a pivotal role in the volume contraction of the oxide at high temperatures.

Quantum transport through the system of parallel quantum dots with Majorana bound states

We study the tunneling transport properties through a system of parallel quantum dots which are coupled to Majorana bound states (MBSs). The conductance and spectral function are computed using the retarded Greens function method based on the equation of motion. The conductance of the system is 2e2/h at zero Fermi energy and is robust against the coupling between the MBSs and the quantum dots. The dependence of the Fermi energy on the spectral function is different for the first dot (dot1) than for the second dot (dot2) with fixed dot2-MBSs coupling. The influence of the Majorana bound states on the spectral function was studied for the series and parallel configurations of the system. It was found that when the configuration is in series, the Majorana bound states play an important role, resulting in a spectral function with three peaks. However, the spectral function shows two peaks when the system is in a parallel configuration. The zero Fermi energy spectral function is always 1/2 not only in series ...

Interfacial defect complex at the MgO/SrTiO3 heterojunction and its electronic impact

Atomic defects exist in various heterojunctions and can remarkably affect their properties, yet it remains a challenging task to identify each individual defect. Here, by combining advanced transmission electron microscopy, spectroscopy and first-principles calculations, we obtain a structural and element-selective imaging of defects at an interface between MgO and SrTiO3. We demonstrate that even for structurally simple MgO and SrTiO3, their interface involves a defect complex comprising of substitutional Ti and Mg vacancies, which induce a marked shift in electronic states. The Ti atoms diffused into MgO exhibit a valence state of +4 and a certain degree of covalency to the surrounding oxygen. Such an ability to determine defects allows us to precisely alter heterojunctions so as to critically improve the device performances.

Topological Anderson insulator induced by inter-cell hopping disorder

We have studied in detail the influence of same-orbit and different-orbit hopping disorders in HgTe/CdTe quantum wells. Intriguingly, similar to the behavior of the on-site Anderson disorder, a phase transition from a topologically trivial phase to a topological phase is induced at a proper strength of the same-orbit hopping disorder. For different-orbit hopping disorder, however, the phase transition does not occur. The results have been analytically verified by using effective medium theory. A consistent conclusion can be obtained by comparing phase diagrams, conductance, and conductance fluctuations. In addition, the influence of Rashba spin-orbit interaction (RSOI) on the system has been studied for different types of disorder, and the RSOI shows different influence on topological phase at different disorders. The topological phase induced by same-orbit hopping disorder is more robust against the RSOI than that induced by on-site Anderson disorder. For different-orbit hopping disorder, no matter wheth...

Atomic and electronic structure of La2CoMnO6 on SrTiO3 and LaAlO3 substrates from first principles

Adhesion energies, atomic structures, electronic states, and bonding nature of the La2CoMnO6(001)/SrTiO3(001) and La2CoMnO6(001)/LaAlO3(001) interfaces are systematically investigated from first principles by taking into account strain effect, electron correlation effect, and polarity continuity. A total of sixteen candidate geometries are considered for each interface, and the fundamental impact of strain on interfacial atomic structures is found to be minor, but its electronic impact is significant. For the La2CoMnO6/SrTiO3 interface where La2CoMnO6 suffers tensile strain, the 3d states of Co and Mn overlap Fermi level, demonstrating a metallic nature for this interface. Electrons are found to be injected into the Ti of SrTiO3 at this interface and spread layers away from interface, inducing an ordering of the in-plane dxy orbital. On the other hand, the compression strained La2CoMnO6/LaAlO3 interface takes on a half-metallic nature with a large degree of hybridization of interfacial Mn 3d with O 2p at ...

Oxygen segregation at coherent grain boundaries of cubic boron nitride

Segregation of even a trace amount of impurities to grain boundaries (GBs) can often modify properties of polycrystalline materials. Here, we demonstrate, by a combined study of advanced transmission electron microscopy with atomistic first-principles calculations to two coherent Σ9 and Σ3 GBs of cubic boron nitride (BN), that the two GBs are inclined to trap oxygen, which induces notable electronic states at Fermi level in the forbidden band gap of bulk BN and lowers the GB adhesion energies significantly. Such GB weakening by oxygen segregation is attributed to the lessened charge transfer between grains and more ionic bonding nature at GB.

Atomic-scale structure and electronic property of the La2FeCrO6/SrTiO3 interface

Combining transmission electron microscopy with first-principles calculations, we investigate atomic-scale structure of a La2FeCrO6/SrTiO3 interface and related it to its electronic properties at the atomic scale with special focus on the strain effect. We find that the La2FeCrO6 film shows a clean and direct contact to the substrate and that an epitaxial, coherent and atomically sharp interface is formed between La2FeCrO6 and SrTiO3. By estimating the adhesion energies for 21 possible candidate interface geometries, we determine the most stable interface structure theoretically, in consistence with the experimental data. The strain is found to play an insignificant role in affecting interfacial atomic structures, yet impose a substantial electronic impact. The strain induces interfacial electronic states and is responsible for the covalent nature of the interfacial bonding. Moreover, the valence states of Fe and Cr are identified to be +3, and a ferrimagnetic coupling is revealed between Fe and Cr.