First‐principles calculations of Mössbauer hyperfine parameters for solids and large molecules

Electronic structure calculations based on density functional theory were performed for solids and large molecules. The solids were represented by clusters of 60–100 atoms embedded in the potential of the external crystal. Magnetic moments and Mössbauer hyperfine parameters were derived.     

First principles study of phosphorus and boron substitutional defects in Si-XII

We present a first principles study of boron and phosphorus substitutional defects in Si-XII. Recent results from nanoindentation experiments reveal that the Si-XII phase is semiconducting and has the interesting property that it can be doped n- and p-type at room temperature without an annealing step. Using the hybrid functional of Heyd, Scuseria and Ernzerhof (HSE), we examine the formation energies of the B and P defects at the two distinct atomic sites in Si-XII to find on which site the substitutional defects are more easily accommodated. We also estimate the thermodynamic transition levels of each defect in its relevant charge states. The hybrid calculations also give an independent prediction that Si-XII is semiconducting, in agreement with recent experimental data.     

V+注入锐钛矿TiO2第一性原理研究

用金属离子注入方法在锐钛矿TiO2薄膜中掺杂了V+,采用全势线性缀加平面波方法计算了锐钛矿TiO2及V+掺杂TiO2超原胞的电子结构,通过紫外-可见吸收光谱测试方法检测了注入不同剂量的V+对TiO2薄膜吸收光谱的影响.理论计算和实验结果表明,锐钛矿TiO2薄膜注入V+后,带隙宽度变小,吸收光谱发生红移,并且TiO2的带隙宽度随着注入V+剂量的增加而减小.     

First principles calculations of shock compressed fluid helium

The properties of hot dense helium at megabar pressures are studied with two first principles computer simulation techniques: path integral Monte Carlo simulation and density functional molecular dynamics. The simulations predict that the compressibility of helium is substantially increased by electronic excitations that are present in the hot fluid at thermodynamic equilibrium. A maximum compression ratio of 5.24(4)-fold the initial density was predicted for 360 GPa and 150,000 K. This result distinguishes helium from deuterium, for which simulations predicted a maximum compression ratio of 4.3(1). Hugoniot curves for statically precompressed samples are also discussed.     

First principles calculations of armchair graphene nanoribbons interacting with Cu atoms

We have investigated the electronic properties of bare, H-terminated, Cu-terminated and Cu-doped armchair graphene nanoribbons (AGNRs) using ab-initio approach. We found that H-termination enhances the stability and band gap whereas H extraction introduces dangling bands and lowers the band gap making bare ribbons indirect band gap semiconductors. The calculations revealed that strong hybridization between Cu atoms and AGNRs, lessen the band gap for Cu-terminated ribbons and gives rise to metallicity in Cu-doped AGNRs irrespective of their widths. Formation energy of considered ribbons yield that H-terminated AGNRs with lowest formation energy are most energetically favored, next are one edge Cu-terminated ribbons followed by bare ones whereas both edges Cu-doped ribbons are least energetically plausible. We predict that presence of Cu atoms in GNRs, significantly alter the band gap and can be used in band gap engineering of nanoribbons.     

固相硝基甲烷相变的第一性原理计算

研究极端条件下固相分子晶体含能材料的相变机理,对于人们认识固相含能材料的爆轰反应有着重要的意义.采用基于校正密度泛函理论的第一性原理方法研究固相硝基甲烷在静水压下的行为.分析晶格参数a,b和c轴随压强的变化,发现在1 GPa到12 GPa时晶格参数出现不连续的变化,表明体系发生相变.在相变时最大的二面角从155.3?增加到177.5?,二面角的增加限制CH3官能团自由旋转,使得C-N和C-H键的键长发生变化.在相变之前,体系主要存在由C-H···O组成的分子间的氢键,而在相变之后存在分子内的H···O和分子间C-H···O组成的氢键.此外通过对硝基甲烷体系的电子结构进行计算,发现相变会影响带隙随压强的变化,而且还会影响费米能级附近的态密度结构.     

First principles calculations of cobalt doped zigzag graphene nanoribbons

We have investigated the stability and electronic properties of Co-doped zigzag graphene nanoribbons (ZGNR) by employing first principles calculations based on density functional theory. The results show that Co impurities settled in antiferromagnetic ground state which is ～2 meV favourable than ferromagnetic state. The formation energy indicates spontaneous formation of one-edge and centre doped structures, however, one-edge doping is found to be the most energetically favourable configuration. A charge transfer takes place from C to Co atoms which shows the formation of chemical bonding between C and Co. Binding energy also confirms the strong bonding of dopant Co impurity with C. The calculations show that band structures of all the ZGNR is substantially modified due to CoC charge transfer and the characteristic edge states of ZGNR are completely lost. Co-doping induces site independent enhanced metallicity irrespective of the ribbon widths. The broken degeneracy of electronic states in one-edge and centre doped ZGNR is important for spintronic applications.     

First principles calculations of the low temperature electrical resistivity of potassium

The results of first principles calculations of the low temperature (2 K ?T?20 K) phonon-limited electrical resistivity of potassium are presented for both the Bloch limit (possible phonon drag effects are completely ignored) and the phonon drag limit (phonon-phonon collisions, which tend to suppress phonon drag, are neglected). In the former case the agreement with experiment is very good; in the latter, the calculated results are much too low. These results are at serious variance with the conclusions of Kaveh and Wiser and reopen the question of whether or not there is any evidence for important phonon drag effects in the low temperature electrical resistivity of potassium.     

PAC study of dynamic hyperfine interactions at 111In-doped Sc2O3 semiconductor and comparison with ab initio calculations

Time-Differential $\upgamma$ - $\upgamma$ Perturbed-Angular-Correlation (PAC) measurements were performed in ¹¹¹In-difussed Sc₂O₃ polycrystals in order to characterize the electric-field-gradient tensor at ¹¹¹Cd nuclei located at the two non-equivalent cation sites of the host lattice. The experimental data were compared with ab initio calculations performed using the Full-Potential Augmented Plane Wave plus local orbital (FP-APW+lo) method in the framework of the Density Functional Theory. The PAC experiments were carried out in air in the temperature range 10–900 K. The spectra present a strong damping below 650 K. This damping is associated with dynamic hyperfine interactions that were analyzed with the perturbation factor proposed by Bäverstam et al.. A model based in the population of impurity levels that are introduced by the Cd probes (supported by FP-APW+lo results) is proposed in order to explain the origin of the observed dynamic interactions.     

First principles calculations of Sr-based fluoroperovskite compounds under applied pressure

The structural, electronic and thermoelectric properties of SrXF₃ (X?=?Li, Na, K, Rb) compounds are performed using first principle calculations. The mBJ-GGA method has been considered to obtain accurate band gaps. The present compounds are found to be thermodynamically stable under 0?GPa and 10?GPa. This stability has been determined using the standard enthalypy of formation. The band structures of the compounds display direct band-gap (Γ-Γ). The band gap has slightly increased for almost studied compounds under 10?GPa. The Boltzmann transport calculations are used to calculate and explain the thermoelectric properties as a function of temperature within the range 20–1500?K. The majority charge carriers of SrXF₃ compounds are holes rather than electrons. Under 10 GP pressure the SrLiF₃ compound is shifted from n-type to p-type doping, whereas SrKF₃ and SrRbF₃ are shifted from p-type to n-type. SrNaF₃ has p-type doping character under 0?GPa and 10?GPa. The Seebeck coeffiecient is found to decrease, whereas σ/τ and S² σ/τ increase for higher temperature. According to the figure of merit and the high S² σ/τ values for SrXF₃, promising thermoelectric applications are expected for the present compounds.     

First principles theory of hyperfine interactions in spinels: ZnAl2O4

The nuclear quadrupole interactions of²⁷Al and⁶⁷Zn, both at the B-site in the spinel ZnAl₂O₄ have been studied using the Hartree-Fock cluster procedure including the influence of the ions outside the cluster. The theoretical value –3.85 MHz ofe ² qQ for²⁷Al is in very good a agreement with the experimental value of 3.68 MHz (sign undetermined). For⁶⁷Zn at the Al site the theoretical value is –8.26 MHz in reasonable agreement with the experimental value of –11.34 MHz indicating that lattice distortion effects associated with Zn as an impurity at the Al site are relatively small.     

First principles calculations of relationship between the Cu surface states and relaxations

In this paper the relationship between the surface relaxations and the electron density distributions of surface states of Cu(100), Cu(110), and Cu(111) surfaces is obtained by first-principles calculations. The calculations indicate that relaxations mainly occur in the layers at which the surface states electrons are localized, and the magnitudes of the multilayer relaxations correspond to the difference of electron density of surface states between adjacent layers. The larger the interlayer relaxation is, the larger the difference of electron density of surface states between two layers is.     

The relaxation of a reconstructed metallic surface and its influence on the EFG acting on a substitutional PAC probe

The structure of the 1×2 reconstructed Au(110) surface, calculated using equivalent crystal theory, is studied. We perform point-charge calculations to estimate the electric field gradient (EFG) acting on a substitutional¹¹¹In probe deposited in that surface. The relaxation breaks the local symmetry producing an asymmetric EFG but the calculated asymmetry is lower than the experimental one. Comparing the computed orientations of the principal axes of the EFG to the experimental values we conclude that In probes prefer to substitute gold into the high coordinated sites at the bottom of the channels.     

Pb-Sn合金及Pb的热力学性质的第一性原理计算

本文基于第一性原理的密度泛函理论(DFT)和密度泛函微扰理论(DFPT)方法,利用虚晶近似的计算方法研究了Pb-Sn合金的晶格结构、电子能带、声子能带及热力学性质,并用晶格能量差可与达到熔化温度时的振动能量相当的固-液相变机理研究了熔化温度,同时与所计算Pb的所有结果进行了对比.     

V+注入锐钛矿TiO2第一性原理研究

用金属离子注入方法在锐钛矿TiO₂薄膜中掺杂了V⁺，采用全势线性缀加平面波方法计算了锐钛矿TiO₂及V⁺掺杂TiO₂超原胞的电子结构，通过紫外-可见吸收光谱测试方法检测了注入不同剂量的V⁺对TiO₂薄膜吸收光谱的影响.理论计算和实验结果表明，锐钛矿TiO₂薄注入V⁺后，带隙宽度变小，吸收光谱发生红移，并且TiO 关键词： +注入')" href="#">V⁺注入 2')" href="#">TiO₂ 全势线性缀加平面波方法 能带结构     

第一性原理计算MgB2薄膜拉伸对超导电性的影响

应用全势线性响应线性糕模轨道方法计算MgB2的电子能带结构、声子谱及电声子耦合常数,并讨论MgB2的超导电性.通过比较MgB2薄膜双轴拉伸前后超导电性的变化可以看出,随着a轴晶格常数增大和c轴晶格常数减小,声子谱中硼的E2g声子频率显著下降,使得电声子耦合强度λ和声子对数平均频率ωln增强,提高了MgB2薄膜的超导转变温度.理论计算结果与文献中的实验测量结果相符.