arXiv subjects

Hongliang Shi

Publications and source records attributed to Hongliang Shi.

18 recordsLinked to original sources

arXiv2025

Theoretical investigations of the origin of persistent luminescence in spinel oxides MgGa2O4 and MgAl2O4

MgGa2O4 and MgAl2O4 have attracted significant interest due to their unique intrinsic persistent luminescence, offering promising potential for various applications. In this paper, from the perspective of defect physics, we systemically investigate the origin of persistent luminescence phenomena in pristine MgGa2O4 and MgAl2O4, employing accurate hybrid functional calculations. Our results show that vacancies and antisite defects involving the two cations are the dominant point defects in both materials. Our calculated optical excitation and emission peaks associated with the MgGa defect agree well with the experimentally observed blue luminescence peak at about 2.9 eV in MgGa2O4. In MgAl2O4, the intradefect optical transition within the VO-MgAl donor-acceptor defect complex is identified as a likely origin for the observed 2.7 eV emission peak. Furthermore, the calculated radiative recombination coefficients of MgGa and VO-MgAl are significantly higher than their nonradiative counterparts, supporting their roles as efficient luminescent centers. Our results regarding the optical processes of oxygen vacancy VO in MgGa2O4 and MgAl2O4 are also in good agreement with experimental results. Based on the calculated defect thermodynamic transition levels, the intrinsic persistent luminescence in MgGa2O4 and MgAl2O4 may be attributed to electron traps, GaMg and VO, in the former and a hole trap, MgAl, in the latter. Donor-acceptor defect complexes (VO+VMg and VO+VGa) are also found to serve as effective carrier trapping centers in MgGa2O4. The calculated trap depths are also consistent with thermoluminescence spectroscopy measurements.

cond-mat.mtrl-sci↗

arXiv2021

Multi-Resolution Electron Spectrometer Array for Future Free-Electron Laser Experiments

We report the design of an angular array of electron Time-of-Flight (eToF) spectrometers intended for non-invasive spectral, temporal, and polarization characterization of single shots of high-repetition rate, quasi-continuous, short-wavelength Free-Electron Lasers (FELs) such as the LCLS-II at SLAC. This array also enables angle-resolved, high-resolution eToF spectroscopy to address a variety of scientific questions of ultrafast and nonlinear light--matter interaction at FELs. The presented device is specifically designed for the Time-resolved atomic, Molecular and Optical science end station (TMO) at LCLS-II. In its final version, it can comprise of up to 20 eToF spectrometers aligned to collect electrons from the interaction point defined by the intersection of the incoming FEL radiation and a gaseous target. There are 16 such spectrometers forming a circular equiangular array in the plane normal to x-ray propagation and 4 spectrometers at 54.7$^\circ$ angle relative to the principle linear x-ray polarization axis. The spectrometers are capable of independent and minimally chromatic electrostatic lensing and retardation in order to enable simultaneous angle-resolved photo-electron and Auger electron spectroscopy with high energy resolution. They are designed to ensure energy resolution of 0.25 eV across an energy window of up to 75 eV which can be individually centered via the adjustable retardation to cover ranges of electron kinetic energies relevant to soft x-ray methods, 0--2 keV. The full spectrometer array will enable non-invasive and online spectral-polarimetry measurements, polarization-sensitive attoclock spectroscopy for characterizing the full time--energy structure of even SASE or seeded LCLS-II pulses, and also supports emerging trends in molecular frame spectroscopy measurements.

physics.ins-det↗

arXiv2019

Impact of Metal ns2 Lone Pair on Luminescence Efficiency in Low-Dimensional Halide Perovskites

Based on first-principles calculations, we show that chemically active metal ns2 lone pairs play an important role in exciton relaxation and dissociation in low-dimensional halide perovskites. We studied excited-state properties of several recently discovered luminescent all-inorganic and hybrid organic-inorganic zero-dimensional (0D) Sn and Pb halides. The results show that, despite the similarity in ground-state electronic structure between Sn and Pb halide perovskites, the chemically more active Sn2+ lone pair leads to stronger excited-state structural distortion and larger Stokes shift in Sn halides. The enhanced Stokes shift hinders excitation energy transport, which reduces energy loss to defects and increases the photoluminescence quantum efficiency (PLQE). The presence of the ns2 metal cations in the 0D halide perovskites also promotes the exciton dissociation into electron and hole polarons especially in all-inorganic compounds, in which the coupling between metal-halide clusters is significant.

cond-mat.mtrl-sci↗

arXiv2015

Magnetism in Na-filled Fe-based skutterudites

The interplay of superconductivity and magnetism is a subject of ongoing interest, stimulated most recently by the discovery of Fe-based superconductivity and the recognition that spin-fluctuations near a magnetic quantum critical point may provide an explanation for the superconductivity and the order parameter. Here we investigate magnetism in the Na filled Fe-based skutterudites using first principles calculations. NaFe4Sb12 is a known ferromagnet near a quantum critical point. We find a ferromagnetic metallic state for this compound driven by a Stoner type instability, consistent with prior work. In accord with prior work, the magnetization is overestimated, as expected for a material near an itinerant ferromagnetic quantum critical point. NaFe4P12 also shows a ferromagnetic instability at the density functional level, but this instability is much weaker than that of NaFe4Sb12, possibly placing it on the paramagnetic side of the quantum critical point. NaFe4As12 shows intermediate behavior. We also present results for skutterudite FeSb3, which is a metastable phase that has been reported in thin film form.

cond-mat.str-el↗

arXiv2015

Ba_{2}TeO as an optoelectronic material: First-principles study

The band structure, optical and defects properties of Ba_{2}TeO are systematically investigated using density functional theory with a view to understanding its potential as an optoelectronic or trans- parent conducting material. Ba_{2}TeO crystallizes with tetragonal structure (space group P4/nmm) and with a 2.93 eV optical band gap 1 . We find relatively modest band masses for both electrons and holes suggesting applications. Optical properties show a infrared-red absorption when doped. This could potentially be useful for combining wavelength filtering and transparent conducting functions. Furthermore, our defect calculations show that Ba_{2}TeO is intrinsically p-type conducting under Ba-poor condition. However, the spontaneous formation of the donor defects may constrain the p-type transport properties and would need to be addressed to enable applications.

cond-mat.mtrl-sci↗

arXiv2014

Connecting thermoelectric performance and topological-insulator behavior: Bi$_2$Te$_3$ and Bi$_{2}$Te$_{2}$Se from first principles

Thermoelectric performance is of interest for numerous applications such as waste heat recovery and solid state energy conversion, and will be seen to be closely connected to topological insulator behavior. In this context we here report first principles transport and defect calculations for Bi$_{2}$Te$_{2}$Se in relation to Bi$_{2}$Te$_{3}$. The two compounds are found to contain remarkably different electronic structures in spite of being isostructural and isoelectronic. We discuss these results in terms of the topological insulator characteristics of these compounds.

cond-mat.mtrl-sci↗

arXiv2013

Quasiparticle band structures and optical properties of strained monolayer MoS2 and WS2

The quasiparticle (QP) band structures of both strainless and strained monolayer MoS$_{2}$ are investigated using more accurate many body perturbation \emph{GW} theory and maximally localized Wannier functions (MLWFs) approach. By solving the Bethe-Salpeter equation (BSE) including excitonic effects on top of the partially self-consistent \emph{GW$_{0}$} (sc\emph{GW$_{0}$}) calculation, the predicted optical gap magnitude is in a good agreement with available experimental data. With increasing strain, the exciton binding energy is nearly unchanged, while optical gap is reduced significantly. The sc\emph{GW$_{0}$} and BSE calculations are also performed on monolayer WS$_{2}$, similar characteristics are predicted and WS$_{2}$ possesses the lightest effective mass at the same strain among monolayers Mo(S,Se) and W(S,Se). Our results also show that the electron effective mass decreases as the tensile strain increases, resulting in an enhanced carrier mobility. The present calculation results suggest a viable route to tune the electronic properties of monolayer transition-metal dichalcogenides (TMDs) using strain engineering for potential applications in high performance electronic devices.

cond-mat.mtrl-sci↗

arXiv2010

Structural, electronic, and thermodynamic properties of UN: Systematic density functional calculations

A systematic first-principle study is performed to calculate the lattice parameters, electronic structure, and thermodynamic properties of UN using the local-density approximation (LDA)+\emph{U} and the generalized gradient approximation (GGA)+\emph{U} formalisms. To properly describe the strong correlation in the U $5f$ electrons, we optimized the \emph{U} parameter in calculating the total energy, lattice parameters, and bulk modulus at the nonmagnetic (NM), ferromagnetic (FM), and antiferromagnetic (AFM) configurations. Our results show that by choosing the Hubbard \emph{U} around 2 eV within the GGA+\emph{U} approach, it is promising to correctly and consistently describe the above mentioned properties of UN. The localization behavior of 5$f$ electrons is found to be stronger than that of UC and our electronic analysis indicates that the effective charge of UN can be represented as U$^{1.71+}$N$^{1.71-}$. As for the thermodynamic study, the phonon dispersion illustrates the stability of UN and we further predict the lattice vibration energy, thermal expansion, and specific heat by utilizing the quasiharmonic approximation. Our calculated specific heat is well consistent with experiments.

cond-mat.mtrl-sci↗

arXiv2010

Magnetic coupling properties of rare-earth metals (Gd, Nd) doped ZnO: first-principles calculations

The electronic structure and magnetic coupling properties of rare-earth metals (Gd, Nd) doped ZnO have been investigated using first-principles methods. We show that the magnetic coupling between Gd or Nd ions in the nearest neighbor sites is ferromagnetic. The stability of the ferromagnetic coupling between Gd ions can be enhanced by appropriate electron doping into ZnO:Gd system and the room-temperature ferromagnetism can be achieved. However, for ZnO:Nd system, the ferromagnetism between Nd ions can be enhanced by appropriate holes doping into the sample. The room-temperature ferromagnetism can also be achieved in the \emph{n}-conducting ZnO:Nd sample. Our calculated results are in good agreement with the conclusions of the recent experiments. The effect of native defects (V$_{\rm{Zn}}$, V$_{\rm{O}}$) on the ferromagnetism is also discussed.

cond-mat.mtrl-sci↗

arXiv2010

Anomalous optical and electronic properties of dense sodium

Based on ab initio density-functional-theory using generalized gradient approximation, we systematically study the optical and electronic properties of the insulating dense sodium phase (Na-hp4) reported recently [Ma \textit{et al.}, Nature \textbf{458}, 182 (2009)]. The structure is found optically anisotropic and transparent to visible light, which can be well interpreted using its electronic band structure and angular moment decomposed density of states. Through the bader analysis of Na-hp4 at different pressures, we conclude that ionicity exists in the structure and becomes stronger with increasing pressure. In addition, the absorption spectra in the energy range from 1.4 to 2.4 eV are compared with recent experimental results and found good agreement. It is found that the deep-lying valence electrons participate in the interband transition.

cond-mat.mtrl-sci↗

arXiv2010

Optical properties of CeO2 using screened hybrid functional and GW+U methods

The optical spectra of CeO2 have been systematically investigated using three first-principles computational approaches for comparison, namely, the Heyd-Scuseria-Ernzerhof (HSE) screened hybrid functional, HSE+\emph{U}, and \emph{GW}+\emph{U}. Our results show that by using the HSE+\emph{U} method, the calculated electronic structures are in good agreement with experimental spectra and the resulting imaginary part of the optical dielectric function spectrum well reproduces the main features exhibited in experimental observations. The important adsorption spectrum and energy loss function also accord well with the experimental results.

cond-mat.str-el↗

arXiv2010

Structural, mechanical, thermodynamic, and electronic properties of thorium hydrides from first principles

We perform first-principles calculations of the structural, electronic, mechanical, and thermodynamic properties of thorium hydrides (ThH$_{2}$ and Th$_{4}$H$_{15}$) based on the density functional theory with generalized gradient approximation. The equilibrium geometries, the total and partial densities of states, charge density, elastic constants, elastic moduli, Poisson's ratio, and phonon dispersion curves for these materials are systematically investigated and analyzed in comparison with experiments and previous calculations. These results show that our calculated equilibrium structural parameters are well consistent with experiments. The Th$-$H bonds in all thorium hydrides exhibit weak covalent character, but the ionic properties for ThH$_{2}$ and Th$_{4}$H$_{15}$ are different due to their different hydrogen concentration. It is found that while in ThH$_{2}$ about 1.5 electrons transfer from each Th atom to H, in Th$_{4}$H$_{15}$ the charge transfer from each Th atom is around 2.1 electrons. Our calculated phonon spectrum for the stable body-centered tetragonal phase of ThH$_{2}$ accords well with experiments. In addition we show that ThH$_{2}$ in the fluorite phase is mechanically and dynamically unstable.

cond-mat.mtrl-sci↗

arXiv2009

First-principles study of $α$-Pu2O3

We systematically investigate the electronic structure, magnetic order, and valence states of $α$-Pu$_{2}$O$_{3}$ (\emph{C}-type) by using first-principles calculations. $α$-Pu$_{2}$O$_{3}$ can be constructed from PuO$_{2}$ by removing 25% oxygen atoms. Our results show that the Pu 5\emph{f} orbitals are further localized after removing ordered oxygen atoms. This phenomenon is demonstrated by the combined fact that (i) the volume per unit cell expands 7% and (ii) the corresponding magnetic moments and valence states for Pu ions increase and decrease, respectively. According to the density of states and charge density distribution analysis, PuO$_{2}$ is found to be more covalent than $α$-Pu$_{2}$O$_{3}$, which is also because of the more localization of 5\emph{f} orbitals in the latter. The calculated lattice constants, bulk modulus, and electronic structures for PuO$_{2}$ and $α$-Pu$_{2}$O$_{3}$ are consistent well with experimental observations.

cond-mat.str-el↗

arXiv2009

Band-gap bowing and p-type doping of (Zn, Mg, Be)O wide-gap semiconductor alloys: a first-principles study

Using a first-principles band-structure method and a special quasirandom structure (SQS) approach, we systematically calculate the band gap bowing parameters and \emph{p}-type doping properties of (Zn, Mg, Be)O related random ternary and quaternary alloys. We show that the bowing parameters for ZnBeO and MgBeO alloys are large and dependent on composition. This is due to the size difference and chemical mismatch between Be and Zn(Mg) atoms. We also demonstrate that adding a small amount of Be into MgO reduces the band gap indicating that the bowing parameter is larger than the band-gap difference. We select an ideal N atom with lower \emph{p} atomic energy level as dopant to perform \emph{p}-type doping of ZnBeO and ZnMgBeO alloys. For N doped in ZnBeO alloy, we show that the acceptor transition energies become shallower as the number of the nearest neighbor Be atoms increases. This is thought to be because of the reduction of \emph{p}-\emph{d} repulsion. The N$_{\rm{O}}$ acceptor transition energies are deep in the ZnMgBeO quaternary alloy lattice-matched to GaN substrate due to the lower valence band maximum. These decrease slightly as there are more nearest neighbor Mg atoms surrounding the N dopant. The important natural valence band alignment between ZnO, MgO, BeO, ZnBeO, and ZnMgBeO quaternary alloy is also investigated.

cond-mat.mtrl-sci↗

arXiv2009

Optical properties of UO2 and PuO2

We perform first-principles calculations of electronic structure and optical properties for UO2 and PuO2 based on the density functional theory using the generalized gradient approximation (GGA)+\emph{U} scheme. The main features in orbital-resolved partial density of states for occupied \emph{f} and \emph{p} orbitals, unoccupied \emph{d} orbitals, and related gaps are well reproduced compared to experimental observations. Based on the satisfactory ground-state electronic structure calculations, the dynamical dielectric function and related optical spectra, i.e., the reflectivity, adsorption coefficient, energy-loss, and refractive index spectrum, are obtained. These results are consistent well with the attainable experiments.

cond-mat.str-el↗

arXiv2009

First-principles LDA+U and GGA+U study of neptunium dioxide

We have performed a systematic first-principles investigation to calculate the electronic structures, mechanical properties, and phonon dispersion curves of NpO$_{2}$. The local density approximation$+U$ and the generalized gradient approximation$+U$ formalisms have been used to account for the strong on-site Coulomb repulsion among the localized Np $5f$ electrons. By choosing the Hubbard \emph{U} parameter around 4 eV, the orbital occupancy characters of Np 5\emph{f} and O 2\emph{p} are in good agreement with recent experiments [J. Nucl. Mater. \textbf{389}, 470 (2009)]. Comparing with our previous study of ThO$_{2}$, we note that stronger covalency exists in NpO$_{2}$ due to the more localization behavior of 5\emph{f} electrons of Np in line with the localization-delocalization trend exhibited by the actinides series.

cond-mat.mtrl-sci↗

arXiv2009

First-principles study of ground state properties and high pressure behavior of ThO2

The mechanical properties, electronic structure and phonon dispersion of ground state ThO$_{2}$ as well as the structure behavior up to 240 GPa are studied by using first-principles density-functional theory. Our calculated elastic constants indicate that both the ground state fluorite structure and high pressure cotunnite structure of ThO$_{2}$ are mechanically stable. The bulk modulus, shear modulus, and Young's modulus of cotunnite ThO$_{2}$ are all smaller by approximately 25% compared with those of fluorite ThO$_{2}$. The Poisson's ratios of both structures are approximately equal to 0.3 and the hardness of fluorite ThO$_{2}$ is 27.33 GPa. The electronic structure and bonding nature of fluorite ThO$_{2}$ are fully analyzed, which show that the Th-O bond displays a mixed ionic/covalent character. The valence of Th and O ions in fluorite ThO$_{2}$ can be represented as Th$^{3.834+}$ and O$^{0.452-}$. The phase transition from the fluorite to cotunnite structure is calculated to be at the pressure of 26.5 GPa, consistent with recent experimental measurement by Idiri \emph{et al}. \cite{Idiri}. For the cotunnite phase it is further predicted that an isostructural transition takes place in the pressure region of 80 to 130 GPa.

cond-mat.mtrl-sci↗

arXiv2009

Mechanical and chemical bonding properties of ground state BeH$_2$

The crystal structure, mechanical properties and electronic structure of ground state BeH$_{2}$ are calculated employing the first-principles methods based on the density functional theory. Our calculated structural parameters at equilibrium volume are well consistent with experimental results. Elastic constants, which well obey the mechanical stability criteria, are firstly theoretically acquired. The bulk modulus \emph{B}, Shear modulus \emph{G}, Young's modulus \emph{E} and Poisson's ratio $\upsilon$ are deduced from the elastic constants. The bonding nature in BeH$_{2}$ is fully interpreted by combining characteristics in band structure, density of state, and charge distribution. The ionicity in the Be$-$H bond is mainly featured by charge transfer from Be 2\emph{s} to H 1\emph{s} atomic orbitals while its covalency is dominated by the hybridization of H 1\emph{s} and Be 2\emph{p} states. The valency in BeH$_{2}$ can be represented as Be$^{1.99+}$H$^{0.63-}$, which suggests that significant charge transfer process exists.

cond-mat.mtrl-sci↗