Observation of atomic spin precessions of isolated fluorine ions on recoil through weak transverse magnetic fields

A new experimental method for studying the hyperfine interactions of multi-electron configurations in isolated ions is illustrated for fluorine using the long-lived 197 keV¹⁹F(5/2⁺) state as probe. The observed atomic precessions were found to vary linearly with the applied weak magnetic field integrated over the flight path of the recoiling ions. The data are well accounted for, assuming configurations from the coupling of 2s and 2p electrons in the relevant ionic charge states.     

Atomic spin precessions of fluorine ions recoiling through vacuum in weak transverse magnetic fields

A new experimental method for unveiling the strong hyperfine interactions of multi-electron ion configurations is described. It consists in observing atomic spin precessions of nuclear-excited ions recoiling through vacuum in weak external transverse magnetic fields. Due to the strong hyperfine coupling the nuclear orientation follows the atomic precession, resulting in a rotation of the angular correlation of decayγ-rays. The method is illustrated using the long-lived 197keV¹⁹F (5/2⁺) state as probe. The observed large precessions as well as the strong attenuations of theγ-angular correlations are well accounted for by considering all possible configurations resulting from the coupling ofL-shell electrons in the relevant ionic charge states.     

Many-body spin Berry phases emerging from the pi-flux state: competition between antiferromagnetism and the valence-bond-solid state

We uncover new topology-related features of the pi-flux saddle-point solution of the D=2+1 Heisenberg antiferromagnet. We note that symmetries of the spinons sustain a built-in competition between antiferromagnetic (AFM) and valence-bond-solid (VBS) orders, the two tendencies central to recent developments on quantum criticality. An effective theory containing an analogue of the Wess-Zumino-Witten term is derived, which generates quantum phases related to AFM monopoles with VBS cores, and reproduces Haldane's hedgehog Berry phases. The theory readily generalizes to pi-flux states for all D.     

Pseudogap formation in copper oxide superconductors with inclusion of spin and charge fluctuations

The influence of spin and charge fluctuations on the pseudogap formation in cuprate superconductors has been studied using the diagram technique for Hubbard operators. It has been shown that the joint inclusion of the spin and charge fluctuations leads to the formation of “shadow” bands with a strong modulation of the spectral intensity and to a decrease in the density of electronic states at the Fermi level.     

Measurement of the charge state distribution of field evaporated ions originating from alloys

The charge state distribution of field evaporated ions from the pure metals Mo, W, Au, Pt and from alloys of these elements has been determined as a function of the electric field strength. The change of the mean charge of these elements when desorbed from an alloy with a different evaporation field can be explained by the model of post ionization. The general validity of this model has also been tested for the change of the mean charge of iridium ions evaporated in the presence of adsorbed hydrogen.     

Pressure dependence of the spectral lines of a high power, high pressure atomic fluorine laser pumped by a charge transfer from He2

Various transitions of a pressure tunable atomic fluorine laser have been observed over a rather wide spectral range, 634.8–755.2 nm, by exciting the atmospheric pressure of helium containing 0.2% F₂ in a preionized Blumlein discharge device with a charging voltage of 10–15 kV. The intensity of two quartet transitions, 739.9 and 755.2 nm, increases with pressure, whereas that of the other quartet and doublet lines, 634.8, 641.4, 712.8 and 731.1 nm, decreases with increasing atmospheric fill-pressure. The pumping mechanisms at high pressures appear to be different from those at relatively low pressures.     

Velocity and atomic number dependence of the transient magnetic field in iron

Thev- andZ-dependence of the transient magnetic field in iron has been investigated for light ions. The present work on transient fields for²⁰Ne and²⁴Mg at initial velocities up tov _i=8v _o (v _o=c/137) confirms the linearv-dependence of these fields. From the existing and present data a marked atomic shell effect has been found in theZ-dependence forZ26. This dependence can be described by a simple expression if the fields are assumed to be due to polarized electrons in s-shells. This shell effect can be understood qualitatively within the framework of an atomic model.     

Ground state valency and spin configuration of the Ni ions in nickelates

The ab initio self-interaction-corrected local-spin-density approximation is used to study the electronic structure of both stoichiometric and nonstoichiometric nickelates. From total energy considerations it emerges that, in their ground state, both LiNiO2 and NaNiO2 are insulators, with the Ni ion in the Ni3+ low-spin state (t(2g)(6)e(g)(1)) configuration. It is established that a substitution of a number of Li/Na atoms by divalent impurities drives an equivalent number of Ni ions in the NiO2 layers from the Jahn-Teller (JT)-active trivalent low-spin state to the JT-inactive divalent state. We describe how the observed considerable differences between LiNiO2 and NaNiO2 can be explained through the creation of Ni2+ impurities in LiNiO2. The indications are that the random distribution of the Ni2+ impurities might be responsible for the destruction of the long-range orbital ordering in LiNiO2.     

Momentum dependence of the spin and charge excitations in the two dimensional Hubbard model

The low energy spin and charge excitations in the 2D Hubbard model near half filling are analyzed. The RPA spectra derived from inhomoheneous mean field textures are analyzed. Spin excitations show a commensurate peak at half filling, incommensurate peaks near half filling, and a broad background typical of a dilute Fermi liquid away from half filling. Charge excitations, near half filling, are localized near (0,0), and they occupy a small portion of the Brillouin Zone, in a way consistent with the existence of a small density of carriers, and a small Fermi surface. At higher hole densities, they fill the entire BZ, and can be understood in terms of a conventional Fermi liquid picture. The results are consistent with the observed features of the high-T_c superconductors.     

Temperature dependence of the hyperfine fields for fluorine in ferromagnetic iron,nickel and gadolinium

The temperature dependence of the magnetic hyperfine fields at the sites of F nuclei implanted into ferromagnetic Fe, Ni and Gd has been studied in the temperature range from 77 K to 670 K. A pulsed proton beam was used to observe the time-differential precession of the 5/2⁺ state in¹⁹F. Deviations from the bulk magnetization were found for Fe and Ni. The damping of the two observed fields in Ni was interpreted in terms of a field distribution caused by an induced radiation damage. The occupation sites for F and possible mechanisms of the anomalous temperature dependence are discussed.     

One-dimensional atomic transport by clusters of self-interstitial atoms in iron and copper

Atomic-scale computer simulation has been used to study the thermally activated atomic transport of self-interstitial atoms (SIAs) in the form of planar clusters in pure Cu and f-Fe. There is strong evidence that such clusters are commonly formed in metals during irradiation with high-energy particles and play an important role in accumulation and spatial distribution of surviving defects. An extensive study of the mobility of SIA clusters containing two to 331 interstitials has been carried out using the molecular dynamics simulation technique for the temperature range from 180 to 1200 K. The results obtained show that clusters larger than three to four SIAs are one-dimensionally mobile in both Cu and Fe. Large clusters of more than 100 SIAs in Cu and 300 SIAs in Fe have significantly reduced mobility. The problem of describing one-dimensional (1D) motion in three-dimensional space is discussed. An attempt is made to describe the mobility of SIA clusters within the approximation of 1D diffusion. For clusters in both metals the effective migration energy of 1D diffusion as estimated via the jump frequency of the cluster centre of mass is found to be independent of the number of SIAs in the clusters, although the cluster jump frequency decreases with increasing cluster size. Mechanisms of 1D mobility of interstitial clusters are discussed.     

Electron spin relaxation of copper(II) ions in diamagnetic crystals

The electron spin-lattice and spin-spin phase relaxation measurements of Cu²⁺ ions in various crystals are reviewed and discussed. Examples of the Debye temperature determination from a wide temperature range measurements of the spin-lattice relaxation time T₁ are shown. An influence of the Jahn-Teller dynamics on T₁ is presented. The phase relaxation described by the phase memory time T_M is affected by temperature due to the spin packet width modulation by molecular motions. The T_M is anisotropic in crystals and can be different for different hyperfine lines of an EPR spectrum.     

Exchange interaction of S state ions with charge carriers in SnTe

ESR investigations of Mn, Eu and Gd in highly degenerate SnTe - a p-type semiconductor with metallic behaviour - prove the existence of an exchange interaction between localized magnetic moments and charge carriers. The ratio of the exchange parameters J_Mn and J_Eu between charge carriers and Mn or Eu ions, respectively, amounts to |J_Mn/J_Eu| = 8 ± 1.5. The consequence this result has on long range spin-spin interaction via charge carriers as well as on hyperfine interaction is discussed.     

Electron spin relaxation studies of impurity iron ions in ferroelectric lithium niobate

The nuclear hyperfine structure of dilute impurity iron ions in ferroelectric lithium niobate is investigated via⁵⁷Fe Mössbauer effect (ME). The ME spectra exhibit typical slow electron spin relaxation effects associated with Fe³⁺ ions in the⁶S_5/2 state, which are analysed using spin-Hamiltonian formalism. For Fe³⁺ ions, the principal axis of EFG tensor is found to be parallel to the crystallographic c-axis. The strong external magnetic field of 47 KOe is used to study magnetic and crystal field effects on the ME hyperfine structure.     

Projectile charge and mass dependence of large angle K-shell ionization probabilities on copper

The dependence on the scattering angle of the K-shell ionization probability has been measured with 0.5 MeV H⁺, 1 MeV D⁺ and 2 MeV He⁺ on copper targets. The experimental values are compared to SCA calculations corrected for binding, retardation and relativistic effects. The angular dependences are in good agreement with the theoretical predictions if the rapid variation of the nuclear relative velocity at the distance of closet approach is correctly taken into account (“hair-spin curve” effect).     

Dependence of hydrogen released on the charge state of incident ions

The behavior of polymers under heavy ion bombardment is of great interest. In the present study, hydrogen released from polypropylene (PP) and polyethylene terephthalate (PET) was investigated as a function of charge state (11⁺, 14⁺, and 25⁺) for 130 MeV ¹⁰⁷Ag ions. It was found that hydrogen released from the polymers varies as α q ⁿ , where n was found to be 2.98 and 1.94 for PP and PET, respectively, when compared with the value of ～3.0 reported in the literature for different polymers and ion combinations. Radii of the damaged zones or ion track formed were deduced from the slope of the hydrogen released versus ion fluence curves. This radius was also found to depend upon the charge state of the incident ion. It varies as β q ^m , where m is 1.25 and 0.741 for PP and PET, respectively.