Locations of implanted 19F* atoms in Si,Ge and diamond studied through their nuclear quadrupole interactions

The Hartree-Fock cluster procedure was used to obtain the associated electronic distributions for ¹⁹F^* (I = 5/2, E ^x = 197 KeV excited nuclear state of the ¹⁹F atom) at possible sites in crystalline Si, Ge and diamond and to calculate nuclear quadrupole coupling constants v _Q and the asymmetry parameter η of the electric field gradient at the modelled sites. Lattice relaxation effects have been incorporated by employing a geometry optimization method to obtain minimum energy configurations for the clusters modelling each site. The intrabond (IB), antibonding (AB) and substitutional (S) sites in the bulk and the atop site on the surface were studied. From a comparison with v _Q and η values observed in time differential perturbed angular distribution (TDPAD) measurements we were able to identify the high frequency component in Si and Ge with ¹⁹F^* at the intrabond site. In diamond two high frequency components are observed. These are identified with ¹⁹F^* at intrabond and substitutional sites. For the low frequency site in Si and Ge the assignment is made to ¹⁹F^* implants at dangling bonds in the bulk resulting from implantation damage. In diamond none of the sites studied could provide lower frequency nuclear quadrupole parameters close to the observed ones. This revised version was published online in August 2006 with corrections to the Cover Date.     

Explanation of irregular trend in19F* (I=5/2) nuclear quadrupole interaction in CF4, SiF4, and GeF4

A theoretical investigation of the electric field gradient at excited nuclei¹⁹F^*(I=5/2) of the tetrafluorides CF₄, SiF₄, and GeF₄, has been carried out using the Self-Consistent Charge Extended Hückel procedure to obtain the electronic wave functions. The values of ¦e²qQ¦ that have been obtained are 50.3, 30.4, and 36.3 MHz for CF₄, SiF₄, and GeF₄, respectively, agreeing with the trend in recent experimental data. Physical reasons are presented for the trend, which is irregular as compared to expectations from Townes-Dailey Theory which would have predicted a continuus decrease in going from CF₄ to GeF₄.     

第一性原理计算研究金属Nb和间隙氢原子的相互作用

应用第一性原理计算方法, 研究了H在金属Nb体心立方晶格中的间隙占位情况, 并讨论了占位能和间隙大小的关系. 分析了H在间隙位和Nb金属晶格的相互作用, 并讨论了相互作用对电子结构的影响. 结果表明: 除了间隙大小直接影响溶解能的大小之外, H的1s电子和Nb的3d电子有比较强的成键作用, 也是导致H在Nb晶格中溶解能较低的一个重要原因. 估算了500 ℃ 下H在Nb晶格中的扩散系数大约为7.8× 10^-9 m²/s, 和实验结果基本符合.     

First principles cluster investigation of electronic structure and hyperfine interaction for nitrogen in diamond

The self-consistent field Unrestricted Hartree-Fock cluster procedure has been used to study the location, electronic structure and hyperfine properties of nitrogen impurity in diamond. From the analysis of the potential energy curve for nitrogen along the <111> axis, it was found that nitrogen is located at a position 0.3A away from the substitutional site towards the plane formed by the three nearest neighbour carbon atoms. The calculated values of the magnetic hyperfine constants and nuclear quadrupole coupling constants for¹⁴N agree very well with values obtained from electron paramagnetic resonance and electronnuclear double resonance measurements.     

金属Fe与间隙H原子相互作用的密度泛函研究

采用基于密度泛函理论的第一性原理方法, 研究了不同摩尔比下H在α-Fe和γ-Fe晶格中的间隙占位情况, 计算了稳态晶体的总能量、结合能、溶解热、电子态密度、电荷差分密度和电荷布居, 分析了间隙H原子和Fe晶格之间的相互作用, 讨论了H溶解对α-Fe和γ -Fe晶体电子结构的影响. 结果表明: H溶解引起α-Fe和γ-Fe晶体点阵晶格畸变, 体积膨胀率随着溶氢量的增加而增加. 从能量角度分析发现, H优先占据α-Fe的四面体间隙位, 而在γ -Fe中优先 占据八面体间隙位. 态密度、电荷差分密度以及电荷布居分析发现, 间隙H原子与Fe晶格的相互作用仅由H的1s轨道电子和Fe的4s轨道电子所贡献, 二者作用力相对较弱, 这是造成H在Fe晶格中固溶度较低的主要原因之一. 关键词： 金属Fe 间隙H原子 第一性原理 溶解热     

The electric hyperfine interaction in the F2-Molecule and in some tri- and tetrafluorides

The quadrupole coupling constants in the fluorine molecule and in the group V trifluorides and the group IV tetrafluorides have been measured. In the case of fluorine the value of the resulting efg is in good agreement with recent theoretical calculations; the other results are discussed in the framework of the Townes and Dai1ey theory.     

First principles study of nuclear quadrupole interactions in the molecular solid BF3 and the nature of binding between the molecules

The LPCTrap facility is coupled to the low-energy beam line LIRAT of the SPIRAL source at GANIL (France). The facility comprises an RFQ trap for beam preparation and a transparent Paul trap for in-trap decay studies. The system has been tested for several ion species. The Paul trap has been fully characterized for ⁶Li⁺ and ²³Na⁺ ions. This characterization together with GEANT4 simulations of the in-trap decay setup (Paul trap and detection system) has permitted to predict the effect of the size of the ion cloud on the decay study of ⁶He⁺.     

PAD investigation of19F in fullerites

Perturbed angular distribution experiments on¹⁹F implanted into fullerites revealed a large fraction of fluorine atoms which are not subject to static quadrupole interactions in contrast to other allotropic forms of carbon.     

The signature of the nuclear quadrupole interaction in continuous phase transitions

A correlation between the largest component of the electric field gradient (EFG) tensor, V_zz, and its asymmetry parameter η is discovered in a series of compounds which exhibt phase transitions. It is shown that all the EFG tensor components depend linearly on a single control parameter. A convenient way to unravel this correlation from V_zz, η-data is presented.     

Temperature dependence of electrically induced perturbations of nuclear quadrupole interaction

The temperature dependence of electrically induced perturbations of the nuclear quadrupole interaction is experimentally found and is theoretically investigated. A good agreement between the theory and experiment is obtained through the use of the order-disorder ferroelectric phase transition model.     

19F nuclear magnetic resonance in CeF3

A ¹⁹F pulsed NMR investigation of single crystals of CeF₃ and CaF₂-doped CeF₃ has been undertaken in the temperature range 300–570K at a frequency of 14 and 60 MHz. It was found that the free-induction decay is non-linear and could be fit by a sum of exponentials using a nonlinear least square procedure to an appropriate functional form. The results of this analysis indicate that there are three distinct spin-spin relaxation times T₂, which is consistent with the existence of three inequivalent fluorine sublattices. At room temperature, fluorine ions on two of the sublattices were found to be in motion, with the fastest ion specified as the F3-type in the Oftedal structure.The temperature dependence of the spin-spin relaxation time T₂ for the fastest ions was found to undergo an inversion at 400 and 385K for the pure and doped samples, respectively. The inversion indicates an increase in the interchange rate of ions between two sublattices. Activation energies are 0.29 ± 0.02 eV in the low temperature region and 0.21 ± 0.01 eV in the high temperature region for the pure sample, with corresponding values of 0.24 ± 0.01 eV and 0.23 ± 0.02 eV for the doped sample.T₁ was approximately the same for both samples, and essentially temperature independent, indicating an exchange-dominated spin-lattice decay mechanism.     

First principles investigation of the spin transport properties in graphene-porphine-graphene nanojunctions

The spin-dependent transport properties of a porphine molecule linking with two zigzag-edged graphene nanoribbon (ZGNR) electrodes have been investigated by using the density of functional theory (DFT) combined with the non-equilibrium Green’s function (NEGF) method. The device shows clearly negative differential resistance (NDR), large tunnelling magnetoresistance (TMR) of 10³ magnitude and nearly 100% perfect spin filter efficiency (SFE) properties, respectively. What’s more, the projected density of states (PDOS), molecular projected self-consistent Hamiltonian (MPSH) eigenvalues and transmission eigenstates were carried out to discuss the transport properties of the ZGNR/Porphine/ZGNR nanojunction. These results suggest that the ZGNR/Porphine/ZGNR device is a promising candidate for multi-functional spintronic devices.     

Theory of14N nuclear quadrupole interaction in pyrrole,indole and carbazole

As a test of the accuracy of the Hartree-Fock Cluster procedure which is being increasingly used for the study of electronic structures and hyperfine properties of solid state systems, we have applied it to study the¹⁴N quadrupole coupling constants of the free molecules pyrrole, indole and carbazole, which have been measured by microwave spectroscopic techniques. Our results explain the experimental trends over the three molecules and provide very good quantitative agreement with experiment.     

A new approach to stroboscopic resonance measurements of the nuclear quadrupole interaction

A new stroboscopic resonance method for the measurement of the quadrupole interaction of long-lived nuclear isomers excited by pulsed beams with a fixed repetition frequency is demonstrated. The method utilizes the phenomenon of beats which occurs when a magnetic perturbation is introduced in the presence of a pure quadrupole interaction. The nuclear precession is thus controlled by the beat frequencies which can be varied by the external magnetic field which is oriented parallel to the symmetry axis of the crystal. Stroboscopic resonance can be achieved by varying thebeat frequency to be an integral multiple of the beam pulsing frequency. The method is demonstrated using the 398 keV level of⁶⁹Ge excited and recoil implanted into a single crystal of tin by means of a heavyion reaction.Supported in part by the National Science Foundation.     

Investigation of the nuclear quadrupole interaction in ZnO by the method of perturbed angular correlations of nuclear radiations

It has been demonstrated that the sign and magnitude of the nuclear quadrupole interaction in ZnO can be determined using perturbed angular correlations of nuclear radiations. Information on the magnitude of the nuclear quadrupole interaction for the 184-keV ⁶⁷Zn level can be obtained from the observation of the gamma-gamma perturbed correlation of the directions for the ⁶⁷Ga → ⁶⁷Zn decay, and information on the magnitude and sign of the nuclear quadrupole interaction can be derived from the experiment on the induced beta-gamma correlation of the directions for the ⁶⁷Cu $ \xrightarrow{{\beta ^ - }} $ \xrightarrow{{\beta ^ - }} ⁶⁷Zn decay. The results of measurements have been interpreted using the calculation of the electric field gradient in the ZnO crystal by the full-potential linearized augmented plane wave method with the generalized gradient approximation of the exchange-correlation potential.     

Temperature dependence of the nuclear quadrupole interaction and the Knight shift in rhenium metal

The temperature dependences of the static nuclear quadrupole interaction and the Knight shift in rhenium metal were investigated for 2K ≦ T ≦150 K by the method of Nuclear Acoustic Resonance (NAR). For the quadrupole interaction ¦e²qQ/h¦ an increase occurs from 255.2(5) MHz at 2 K to 257.2(8) MHz at 150 K in¹⁸⁷Re [269.5(7) to 271.5(10) in¹⁸⁵Re] whereas for the Knight shift a slight decrease from 1.14(4)% at 2K to 1.10(8)% at 150 K [1.16(6) to 1.12(10)] is found.