First-Principles Studies of Structural Evolutions in Cathode Materials LiMO_(2)(M=Co,Mn,Ni)

We explore the structural evolutions of stoichiometric LiMO_(2)using the first-principles calculations combined with the cluster expansion method.We automatically obtain the ground state structures of the stoichiometric LiMO_(2)by just considering the cation orderings in the quasi rock-salt structures and the following structural relaxations due to both the atomic size mismatches and the Jahn–Teller distortions.We point out that,on the one hand,the cation orderings are mainly determined by the nearest,the second nearest,and the third nearest cation interactions and can be obtained from the‘phase diagram’we have built using the relative strengths of effective cluster interaction(ECI).On the other hand,the structural relaxations are dominated by the crystal field splitting(CFS)energies,i.e.,structures with larger CFS energies are more stable.By calculating the ECIs and CFS energies for various structures of LiMO_(2),we clearly show how ECI and CFS play roles in determining the structural evolution mechanism of these systems.

Ferroelectricity in Charge-Ordering Crystals with Centrosymmetric Lattices

The switchability between the two ferroelectric(FE)states of an FE material makes FEs widely used in memories and other electronic devices.However,for conventional FEs,its FE switching only occurs between the two FE states whose spatial inversion symmetry is broken.The search for FE materials is therefore subject to certain limitations.We propose a new type of FEs whose FE states still contain spatial inversion centers.The change in polarization of this new type of FEs originates from electronic transfer between two centrosymmetric FE states under an external electric field.Taking BaBiO_(3) as an example,we show that charge-ordering systems can be a typical representative of this new type of FEs.Moreover,unlike traditional ferroelectrics,the change in polarization in this new type of FEs is quantum in nature with the direction dependent on the specific FE transition path.Our work therefore not only extends the concept of FEs but may also open up a new way to find multiferroics.

Frustrated Magnetic Interactions and Quenched Spin Fluctuations in CrAs

The discovery of pressure-induced superconductivity in helimagnets(CrAs,MnP)has attracted considerable interest in understanding the relationship between complex magnetism and unconventional superconductivity.However,the nature of the magnetism and magnetic interactions that drive the unusual double-helical magnetic order in these materials remains unclear.Here,we report neutron scattering measurements of magnetic excitations in CrAs single crystals at ambient pressure.Our experiments reveal well defined spin wave excitations up to about 150 meV with a pseudogap below 7 meV,which can be effectively described by the Heisenberg model with nearest neighbor exchange interactions.Most surprisingly,the spin excitations are largely quenched above the Néel temperature,in contrast to cuprates and iron pnictides where the spectral weight is mostly preserved in the paramagnetic state.Our results suggest that the helimagnetic order is driven by strongly frustrated exchange interactions,and that CrAs is at the verge of itinerant and correlation-induced localized states,which is therefore highly pressure-tunable and favorable for superconductivity.

Tunable spin textures in polar antiferromagnetic hybrid organic–inorganic perovskites by electric and magnetic fields

The hybrid organic–inorganic perovskites(HOIPs)have attracted much attention for their potential applications as novel optoelectronic devices.Remarkably,the Rashba band splitting,together with specific spin orientations in k-space(i.e.,spin texture),has been found to be relevant for the optoelectronic performances.In this work,by using first-principles calculations and symmetry analysis,we study the electric polarization,magnetism.

Field-tuned quantum effects in a triangular-lattice Ising magnet

We report thermodynamic and neutron scattering measurements of the triangular-lattice quantum Ising magnet TmMgGaO_(4)in longitudinal magnetic fields.Our experiments reveal a quasi-plateau state induced by quantum fluctuations.This state exhibits an unconventional non-monotonic field and temperature dependence of the magnetic order and excitation gap.In the high field regime where the quantum fluctuations are largely suppressed,we observed a disordered state with coherent magnon-like excitations despite the suppression of the spin excitation intensity.Through detailed semi-classical calculations,we are able to understand these behaviors quantitatively from the subtle competition between quantum fluctuations and frustrated Ising interactions.

A dynamic stiffness-based framework for harmonic input estimation and response reconstruction considering damage

In structural health monitoring(SHM),the measurement is point-wise but structures are continuous.Thus,input estimation has become a hot research subject with which the full-field structural response can be calculated with a finite element model(FEM).This paper proposes a framework based on the dynamic stiffness theory,to estimate harmonic input,reconstruct responses,and to localize damages from seriously deficient measurements.To begin,Fourier transform converts the dynamic equilibrium equation to an equivalent static one in the frequency domain,which is underdetermined since the dimension of measurement vector is far less than the FEM-node number.The principal component analysis has been adopted to“compress”the under-determined equation,and formed an over-determined equation to estimate the unknown input.Then,inverse Fourier transform converts the estimated input in the frequency domain to the time domain.Applying this to the FEM can reconstruct the target responses.If a structure is damaged,the estimated nodal force can localize the damage.To improve the damage-detection accuracy,a multi-measurement-based indicator has been proposed.Numerical simulations have validated that the proposed framework can capably estimate input and reconstruct multi-types of full-field responses,and the damage indicator can localize minor damages even with the existence of noise.