Free Atom Values Research Articles

Complete reversal of the atomic unquenched orbital moment by a single electron

The orbital angular moment of magnetic atoms adsorbed on surfaces is often quenched as a result of an anisotropic crystal field. Due to spin-orbit coupling, what remains of the orbital moment typically delineates the orientation of the electron spin. These two effects limit the scope of information processing based on these atoms to essentially only one magnetic degree of freedom: the spin. In this work, we gain independent access to both the spin and orbital degrees of freedom of a single atom, inciting and probing excitations of each moment. By coordinating a single Fe atom atop the nitrogen site of the Cu2N lattice, we realize a single-atom system with a large zero-field splitting—the largest reported for Fe atoms on surfaces—and an unquenched uniaxial orbital moment that closely approaches the free-atom value. We demonstrate a full reversal of the orbital moment through a single-electron tunneling event between the tip and Fe atom, a process that is mediated by a charged virtual state and leaves the spin unchanged. These results, which we corroborate using density functional theory and first-principles multiplet calculations, demonstrate independent control over the spin and orbital degrees of freedom in a single-atom system.

Density functional theory study of transition metals doped B80 fullerene

Density functional theory calculations have been carried out to investigate 3d, Pd and Pt transition metal (TM) atoms exohedrally and endohedrally doped B 80 fullerene. We find that the most preferred doping site of the TM atom gradually moves from the outer surface ( TM = Sc ), to the inner surface ( TM = Ti and V ) and the center ( TM = Cr , Mn , Fe and Zn ), then to the outer surface ( TM = Co , Ni , Cu , Pd , and Pt ) again with the TM atom varying from Sc to Pt . From the formation energy calculations, we find that doping TM atom can further improve the stability of B 80 fullerene. The magnetic moments of doped V , Cr , Mn , Fe , Co and Ni atoms are reduced from their free-atom values and other TM atoms are completely quenched. Charge transfer and hybridization between 4s and 3d states of TM and 2s and 2p states of B were observed. The energy gaps of TM @ B 80 are usually smaller than that of the pure B 80. Endohedrally doped B 80 fullerene with two Mn and two Fe atoms were also considered, respectively. It is found that the antiferromagnetic (AFM) state is more energetically favorable than the ferromagnetic (FM) state for Mn 2- and Fe 2@ B 80. The Mn and Fe atoms carry the residual magnetic moments of ~ 3 μB and 2 μB in the AFM states.

Electronic structure of a cylindrically confined hydrogen atom by the B-splines method: energy levels and dipole polarizability

The non-relativistic static and dynamic dipole polarizabilities of a hydrogen atom experiencing a cylindrical confinement are investigated. Two methods based on B-splines are used for computations of energies and wavefunctions. The first method is a variational-based method while the second one proceeds by a fit of the non-separable Coulomb potential in the product form. The computed energies compare very well with previous computations. They converge, as well as the dipole polarizability, to the exact unconfined free atom values with increasing basis size. The fit approach is found to be advantageous, as it helps to reduce the computational time without loss of accuracy.

Rydberg–Ritz analysis and quantum defects for Rb and Cs atoms on helium nanodroplets

A Rydberg–Ritz approach is used for the interpretation of Rb–He and Cs–He Rydberg states and Rydberg series. Variations of the quantum defects within a Rydberg series give insight into the interaction between the alkali atom’s valence electron and the superfluid helium droplet. A screening of the valence electron from the alkali atom core by the helium droplet is observed for high Rydberg states. For states with lower principal quantum number, the effect decreases and the quantum defects are found to lie closer to free atom values, indicating an increased probability for the electron to be found inside the alkali atom core. An investigation of the spin–orbit splitting of the Cs–He nP(2Π) components reveals that the splitting of the lowest 2Π states is more atom-like [Hund’s case (c) coupling] than at higher n states [Hund’s case (a) coupling]. In addition, we report a detailed study of the droplet size dependence of Ak–He Rydberg series on the example of the Rb–He D(Δ) series. Higher Rydberg states of this series are strongly redshifted, which is also related to the screening effect.

Electronic and Magnetic Properties of Manganese and Iron Atoms Decorated With BO2 Superhalogens

Using density functional theory based calculations, we have systematically studied the equilibrium geometries, relative stabilities, and electronic and magnetic properties of Fe and Mn atoms interacting with a varying number of BO(2) moieties. These clusters are found to exhibit hyperhalogen behavior with electron affinities as high as 6.9 eV once the number of BO(2) moieties exceed the nominal valences of these transition metals toms, namely 2 for both Fe and Mn. In all cases the transition metal atoms retain a sizable spin magnetic moment, even exceeding their free atom values at certain compositions. We also note that when more than two BO(2) moieties are bound to neutral Fe and Mn atoms, they tend to dimerize. In the case of negative ions, this process occurs at n ≥ 3, thus leading to different neutral and anionic ground state geometries. The effect of these structural changes in the interpretation of photoelectron spectroscopy experiments is discussed.

On the ‘Atomic’ Polarizabilities in Small Sin Clusters and the Dielectric Constant of Bulk Silicon

Applying the classical discrete reaction field (DRF) approach, which includes a treatment for the solution of the many-body polarization in complex systems, we calculated the mean atomic polarizability for a Si atom from the known molecular polarizability of Si-3. With only this parameter (6.16 angstrom(3), i.e., close to the free atom value), and the geometries as input, the effective atomic mean polarizabilities and their averages ((n) = (n)/n) for the series Si-4-Si-10 were calculated and found to be in excellent agreement with theoretical and experimental values. These (n) are larger than the bulk value of 3.7 angstrom(3). We used the same input parameter for (by hand) constructed model systems up to n = 4950 with various geometries. For the larger clusters with the diamond lattice, we obtained the bulk value, implying that we predicted the dielectric constant of silicon almost from first principles. However, even the largest system is still too small for considering it as a real dielectric. In other lattices (primitive and face centered cubic), the (n) are significantly smaller than 3.7 angstrom(3), which we attribute to the tighter packing in these lattices in comparison with that of the diamond structure. The behavior in all these systems can be easily understood by accounting properly for the local fields and for damping the interaction between induced dipoles. We show that there is no need for additional (e.g., charge transfer) parameters.

Magnetic anisotropy of Fe and Co adatoms and Fe clusters magnetically decoupled fromNi3Al(111)by an alumina bilayer

The magnetic properties of individual Fe and Co atoms as well as of small Fe clusters adsorbed on two atomic layers of Al2O3 grown on Ni3Al(111) have been investigated by x-ray absorption spectroscopy and x-ray magnetic circular dichroism. We find ratios of the orbital over the effective spin magnetic moments of r=0.53 +/- 0.09 for Fe and r=0.91 +/- 0.06 for Co, which are both very close to the free-atom values of r=0.5 and 1, respectively. The magnetization curves acquired at the Fe and Ni edges demonstrate a distinctly different magnetization reversal of the transition-metal nanostructures and the substrate excluding magnetic dipolar or exchange coupling through the alumina film. Our data reveal an out-of-plane easy magnetization axis for Fe and Co originating from the spatially anisotropic hybridization of the 3d states of the adatom, with the 2p states of the oxygen terminated Al2O3 surface. We conclude that the alumina film effectively decouples the magnetic adatoms from the underlying metal substrate while providing a crystal-field environment giving rise to high magnetic anisotropy.

Electron momentum distribution and Compton profiles of europium

The electron momentum distribution and Compton profiles of the rare earth element Eu was measured using HPGe photon detector. The target atoms were excited by means of 59.54 keV gamma rays from Am-241 radioactive source of strength 300 mCi. Elemental foil of uniform aerial density, and purity better than 99.9% was used as target. The data were recorded and analyzed using a 4 K multichannel analyzer. These data duly corrected for instrumental resolution effects, sample absorption, energy dependence of the differential Compton cross-section and double scattering, and compared with theoretical renormalized free atom (RFA) values.

Compton scattering study of electron momentum distribution in lithium fluoride using 662keV gamma radiations

Abstract Here we report the first ever 137 Cs Compton spectroscopy study of lithium fluoride. The spherical average Compton profiles of lithium fluoride are deduced from Compton scattering measurements on poly crystalline sample at gamma ray energy of 662 keV. To compare the experimental data, we have computed the spherical average Compton profiles using self-consistent Hartree–Fock wave functions employed on linear combination of atomic orbital (HF-LCAO) approximation. The directional Compton profiles and their anisotropic effects are also calculated using the same HF-LCAO approximation. The experimental spherical average profiles are found to be in good agreement with the corresponding HF-LCAO calculations and in qualitative agreement with Hartree–Fock free atom values. The present experimental isotropic and calculated directional profiles are also compared with the available experimental isotropic and directional Compton profiles using 59.54 and 159 keV γ-rays.

Tight-binding theory of manganese and iron oxides

The electronic structure is found to be understandable in terms of the free-atom term values and universal interorbital coupling parameters, since self-consistent tight-binding calculations indicate that the Coulomb shifts of the $d$-state energies are small. The special-points averages over the bands are seen to be equivalent to the treatment of local octahedral clusters, so that all properties are given by algebraic formulas in terms of these parameters. The cohesive energy per manganese for MnO, ${\text{Mn}}_{2}{\text{O}}_{3}$, and ${\text{MnO}}_{2}$, in which manganese exists in the valence states ${\text{Mn}}^{2+}$, ${\text{Mn}}^{3+}$, and ${\text{Mn}}^{4+}$, is very nearly the same and dominated by the transfer of manganese $s$ electrons to the oxygen $p$ states. There are small corrections, 1.37 eV per Mn in all cases, from the couplings of the minority-spin states. Transferring one majority-spin electron from an upper cluster state to a nonbonding oxygen state adds 1.67 eV to the cohesion for ${\text{Mn}}_{2}{\text{O}}_{3}$ and the two transfers add twice that for ${\text{MnO}}_{2}$. The electronic and magnetic properties are consistent with this description and appear to be understandable in terms of the same parameters.

Joint structure refinement of x-ray and neutron diffraction data on disordered materials:application to liquid water

X-ray diffraction data on liquids and disordered solids often provide useful complementarystructural information to neutron diffraction data. Interpretation of the x-ray diffractionpattern, which is produced by scattering from the atomic electrons rather than from theatomic nuclei as in the case of neutron diffraction, is, however, complicated by theQ-dependent electronic form factors, which cause the x-ray diffraction signal to decline rapidly with increasingQ,where Q is the wave vector change in the diffraction experiment. The problem is particularlyimportant in cases such as water where there is a significant molecular polarization causedby charge transfer within the molecule. This means that the electron form factorsapplicable to the molecule in the condensed environment often deviate from their free atomvalues. The technique of empirical potential structure refinement (EPSR) is used hereto focus on the problem of forming a single atomistic structural model whichis simultaneously consistent with both x-ray and neutron diffraction data. Thecase of liquid water is treated explicitly. It is found that x-ray data for water doindeed provide a powerful constraint on possible structural models, but that theQ-range of the different x-ray data sets (maximumQ rangesfrom 10.8 to ∼17.0 Å−1 for different x-ray experiments), combined with variations between different datasets, means that it is not possible to rigorously define the precise position andheight of the first peak in the OO radial distribution function. Equally, it is foundthat two different neutron datasets on water, although measured to a maximumQ of atleast 30 Å−1, give rise to further small uncertainties in the position of the hydrogenbond peaks. One general conclusion from the combined use of neutron andx-ray data is that many of the classical water potentials may have a corewhich is too repulsive at short distances. This produces too sharp a peak inr-space at too short a distance. A softer core potential is proposed here.

Compton profile of polycrystalline sodium chloride and sodium fluoride

We present here the Compton profile (CP) of polycrystalline sodium chloride and sodium fluoride. Our results consists of spherical average Compton profile based on measurements and calculation of spherical average Compton profile, directional Compton profile and their anisotropic effect using self-consistent Hartree–Fock wave functions employed on the linear combination of atomic orbital (HF–LCAO) approximation. The experimental results are compared with the HF–LCAO spherical average Compton profile and with tabulated Hartree–Fock free atom results. For both compounds the experimental results are found to be in good agreement with the HF–LCAO results and in qualitative agreement with Hartree–Fock free atom values.

Static and Intermittent Hyperfine Coupling for the Muoniated Radical in Tellurium

The hyperfine constant for muonium defect centres in elemental tellurium, measured spectroscopically at low temperature, corresponds to 7% of the free-atom value. The centres are tentatively identified as the diatomic species TeMu, analogous to the hydroxyl radical, OH. Their spectrum disappears as the centres ionize around 80 K but the muon spin response indicates that the same centres form and reionise, rapidly and repeatedly, above 200 K. The capture and loss of charge carriers provide a model for the electrical activity of hydrogen impurity in this low-gap semiconductor.

Ferrimagnetism in Mn 7 cluster

A synergistic approach consisting of Stern–Gerlach magnetic deflection experiments and first principles calculations provides the first conclusive evidence of ferrimagnetism in Mn clusters containing as few as seven atoms. The calculated magnetic moment of 0.71 μ B/atom of a Mn 7 cluster is in perfect agreement with the experimental value of 0.72 ± 0.42 μ B/atom. This low value, which is a factor of seven smaller than its free atom value, is shown to result not from the reduction of the individual atomic moments, but from their ferrimagnetic coupling. The large uncertainty in the experimental measurement is explained by the existence of nearly degenerate isomers.

Atomic structure, binding energy, and magnetic properties of iron atoms supported on a polyaromatic hydrocarbon

The atomic structure, energetics, and properties of gas-phase cluster complexes containing coronene (C24H12) molecule and up to two iron atoms are studied for the first time using density functional theory and generalized gradient approximation for exchange and correlation. The geometries of the neutral and cationic iron–coronene complexes are optimized without symmetry constraint and by examining the possibility that iron atoms could occupy various sites via individual π or bridging interactions. In both neutral and cationic complexes a single Fe atom is found to preferentially occupy the on-top site above the outer ring, while two Fe atoms dimerize and reside on the top of center of the outer rings. The binding energy of neutral Fe2–coronene defined with respect to dissociation into coronene and Fe2 is larger than that of Fe–coronene while reverse is true for the corresponding cations. Although the ionization potentials of these complexes are not very sensitive to the number of adsorbed Fe atoms, they are significantly reduced from those of the Fe atom or the coronene molecule. The photodecomposition of cationic (Fen–coronene)+ complexes proceeds through the ejection of either coronene+ or (Fe–coronene)+ cations while in the case of neutral Fe2–coronene, the ejection of Fe2 is energetically preferred. The coupling between the Fe atoms remains ferromagnetic although the magnetic moment/atom is reduced from the free-atom value. The results compare well with recent mass ion intensity and photofragmentation experiments.

RelativeKx-ray intensity studies of the valence electronic structure of3dtransition metals

$K\ensuremath{\beta}\ensuremath{-}\mathrm{to}\ensuremath{-}K\ensuremath{\alpha}$ x-ray intensity ratios have been measured for all the $3d$ transition metals from titanium to copper, and the valence electronic structures of these metals have been determined by comparing the measured $K\ensuremath{\beta}\ensuremath{-}\mathrm{to}\ensuremath{-}K\ensuremath{\alpha}$ x-ray intensity ratios with the results of our multiconfiguration Dirac-Fock calculation. Our experimentally determined valence electronic structures for all the metals except V, Cr, and Mn are found to agree reasonably well with the results of augmented plane-wave (APW) band structure calculations. Our results for the valence electronic structure of V and Mn are found to be closer to the free atom values whereas the valence electronic structure of Cr is found to lie in between the valence electronic structure predicted by Compton profile studies and the one given by APW band structure calculations. The electron occupancies of $3d$ and $(4s,4p)$ states of V, Mn, and Cu are similar to free atom occupancies whereas for the other metals we find rearrangement of electrons between $3d$ and $(4s,4p)$ states of the metal with the transfer of electrons from $3d$ to $(4s,4p)$ states for Cr and from $(4s,4p)$ to $3d$ states in the case of Ti, Fe, Co, and Ni.

Pulsed EPR and ENDOR investigation of hydrogen atoms in silsesquioxane cages

High sensitivity and spectral resolution provided by pulsed electron paramagnetic resonance and electron nuclear double resonance techniques at high Larmor frequencies open the way for a study of atoms in chemical traps. As an example we studied deuteron atoms encased in silsesquioxane cages to probe the cage symmetry as function of temperature. An analysis of the temperature dependence showed that the system undergoes a structural phase transition near 100 K. At this temperature the character of distortion of the ideal cubic symmetry changes from oblate to prolate (or vice versa). With quantum chemical methods, a model for cage escape of the encased atom could be derived. The calculated escape barrier of 0.9 eV is close to the experimental value derived by thermal release experiments. Although the encased deuterium atom exhibits an isotropic hyperfine coupling constant nearly identical with the free atom value, a spin population analysis revealed that approximately 10% of the spin density is transferred to the cage. We therefore conclude that confinement of the hydrogen atom leads to a compression of its wave function compensating the decrease of spin density. In this respect the system falls somewhat short of the properties of an ideal cage, being defined by well decoupled atomic and molecular wave functions.

Shallow donor muonium states in II-VI semiconductor compounds

Experimental data on shallow donor muonium states in the $\mathrm{II}\mathrm{\ensuremath{-}}\mathrm{VI}$ semiconductor compounds CdS, CdSe, CdTe, and ZnO are presented. These are characterized by very weak hyperfine interactions amounting to approximately ${10}^{\ensuremath{-}4}$ of the free-atom value, and by donor levels with binding energies comparable to those calculated on the hydrogenic model of the shallow centers. The data are discussed in terms of a model generalizing the knowledge of muonium and hydrogen states in tetrahedrally bonded semiconductors. Within this model the shallow muonium state is attributed to muonium bound to the anion of the $\mathrm{II}\mathrm{\ensuremath{-}}\mathrm{VI}$ compound and located at the antibonding site or close to it in the bond direction.

Unique magnetic signature of transition metal atoms supported on benzene

The magnetic moments of neutral Sc, V, and Ti atoms supported on a benzene molecule are found to be substantially enhanced over their free-atom value while those of neutral Mn, Fe, and Co atoms experience a similar reduction. The magnetic moment of the neutral Cr atom, the highest (6 μ B) in the 3d-transition metal series, remains unchanged and that of neutral Ni is completely quenched. These results, based on density functional theory and the generalized gradient approximation, are shown to be a simple consequence of the Pauli exclusion principle. Similar calculations on positively and negatively charged 3d-metal–benzene complexes are carried out.

Equilibrium geometries, electronic structure and magnetic properties of small manganese clusters

The equilibrium geometries, electronic structure and magnetic properties of small Mn clusters consisting of up to five atoms have been calculated self-consistently using first principles molecular orbital theory. The electron-electron interaction has been accounted for using the local spin density and generalized gradient approximation to the density functional theory. The atomic orbitals forming the molecular orbital have been represented separately by Gaussian and numerical basis sets. Two different computer codes (Gaussian 94 and DMOL) were used to check the numerical consistency of our calculations. is found to be a weakly bound van der Waals molecule and its binding energy depends sensitively on the choice of basis set as well as the form of the exchange-correlation potential. The binding energies are less sensitive to these approximations in larger clusters. The binding improves with cluster size, but remains significantly lower than those in other transition metal clusters. The equilibrium geometries are fairly compact and symmetric although other isomers with distorted geometries and with nearly the same energy as that of the ground state do exist for . The clusters also exhibit a variety of low-lying spin multiplicities, but the ground state spin configuration is ferromagnetic with a magnetic moment of . This not only contrasts with its bulk behaviour which is antiferromagnetic, but also differs from the behaviour in other transition-metal clusters where the magnetic moments/atom are always less than the free-atom value. The results are compared with available experiments on matrix isolated Mn clusters.