Nuclear hyperfine interactions in spherical top molecules

The nuclear hyperfine Hamiltonian for XY₄ and XY₆ spherical top molecules in their ground electronic state is built up by a tensor method, and is given by a linear combination of invariant tensor products in the group ^L0(3) × G(G = T_d or 0_h); the case when an external magnetic field is applied is also considered. For the study of a given vibrational state, the appropriate restriction is easily determined; as examples were consider the ground vibronic state and a triply degenerate fundamental state.     

Nuclear magnetic relaxation by quadrupole interactions in non-spherical molecules

The nuclear magnetic relaxation times associated with quadrupole interactions are investigated by an analytical method for molecules that may be linear or symmetric rotators, or may be totally asymmetric. The molecules are subject to random thermal couples, and it is supposed that the consequent rotational motion is Brownian. The results are in agreement with those obtained otherwise by Hubbard for the special case of spherical molecules.     

Nuclear physics with hyperfine methods

In this talk the discussion of nuclear physics studied by hyperfine methods is limited to a few topics of high actuality:

Magnetic hyperfine interactions in GdAl3

We have studied the magnetic hyperfine interactions in GdAl₃ using¹⁵⁵Gd Mössbauer spectroscopy between the temperatures of 4K and 90K. Previous studies on GdAl₃ have shown that antiferromagnetic ordering occurs at 18K, and that a fit of the susceptibility to 1/(T-θ_p) yields a θ_p value of ?89K. The large ratio of θ_p to T_N is indicative of magnetic frustration between competing ferro-and antiferromagnetic interactions, which may be due to a combination of the oscillatory nature of the RKKY interaction and the geometry of the hexagonal lattice. Our studies show that the saturation magnetic hyperfine field at the Gd site is ?24.0 T, with the moments lying in the basal plane. The efg at the gadolinium site is 2.55(1)×10¹⁷V cm^?2 which is considerably larger than the value predicted by a point charge calculation. This difference may indicate that there is a conduction electron contribution. A helical magnetic structure has been calculated from RKKY theory.     

Muon hyperfine interactions in magnetic oxides

Muon spin rotation studies on magnetic oxides among which-Fe₂O₃ and Fe₃O₄ have shown that at temperatures below approximately 500 K the muons form a muon-oxygen bond, analogous to a hydrogen bond. Generally, the muon hyperfine interactions in magnetic oxides can be explained in terms of supertransfer (hyperfine) and dipole fields. Supertransfer fields result from covalent bonding effects. For the rare earth (R) orthoferrite series (RFeO₃) comparison is made with Mössbauer results on covalency effects in hyperfine interactions. Suggestions for next stages of experimentation in solid state research in oxides bySR are mentioned.     

Theoretical aspects of hyperfine interactions

This work reviews the basics of the theoretical determination of the hyperfine interactions. Starting from the Dirac equation the relevant expressions for the isomer shift, the hyperfine field and the electric field gradient are derived. In the second part examples from band structure calculations are given for Fe₄N as a prototypical system where all features of the pressure dependence of the hyperfine interactions can be demonstrated. This revised version was published online in July 2006 with corrections to the Cover Date.     

Beta decay and hyperfine interactions

Experimental searches for the second-class currents and the soft pion effects in the nuclear weak axial currents are discussed. Also discussed are the hyperfine interactions of light interstitial impurities in ferromagnetic Fe and Ni, as well as in fcc and bcc non-magnetic metals. For these spin-ray correlation experiments, it is emphasized that the technical developments and improvements of the nuclear spin control by NMR and the production of polarization in the short-lived nuclear states are vital necessities.     

Theory of hyperfine interactions in lithiumlike systems

The valence, exchange core polarization and correlation contributions to the magnetic hyperfine constants in the ground states of the lithiumlike ions Be⁺, B²⁺, C³⁺, N⁴⁺, O⁵⁺, F⁶⁺, Ne⁷⁺ and Bi⁸⁰⁺ are investigated using relativistic many-body perturbation theory. Radiative corrections have also been studied. Comparison is made with available experimental data and the trends in the various contributions are analyzed.     

Stroboscopic observation of quadrupole hyperfine interactions

A stroboscopic technique for the observation of quadrupole hyperfine interactions of isomeric nuclear states has been successfully developed. The inherent precision and resolution of this technique have been demonstrated by measuring the quadrupole hyperfine frequency for ${}^{69}\text{Ge(}\text{9}\text{2}{}^{\mathrm{+}}{}_1\text{, τ = 4.0μ)}$ in Zn metal at several temperatures; ω₀ = [19.67 ± 0.06] × 10⁶s^?1 (at 623 ± 3 K).     

HYSCORE analysis of nitrogen hyperfine interactions

A semiquantitative treatment of hyperfine sublevel correlation spectra of aS=1/2,I=1 spin system is presented. The derived closed analytical expressions allow the determination of components of the dipolar and quadrupolar coupling tensors. The method is applicable if nonsecular terms can be neglected and if canonical orientations of the hyperfine interactions can be selected via a dominantg matrix or hyperfine anisotropy.     

Collision hamiltonian for non-linear molecules

A model hamiltonian which describes the non-reactive collision of two non-linear molecules has been constructed. This hamiltonian gives insight into the character of the complex translational, vibrational and rotational motions. The main features of the isolated molecule case are essentially disturbed by the non-adiabatic interaction between the macroscopic molecular motions and the vibrations. The usual angular momentum expressions are found to be the asymptotic limits of the true angular momenta in the case when these non-adiabaticities are switched off.     

Temperature dependence of F-centre hyperfine interactions

We report the temperature dependence of F-centre Isotropic hyperfine energies for the first shells of KCl, KBr, NaCl, LiF and Lid, and the second shell of KCl. In all cases at least 30 ENDOR data points were taken between 1.3° and 300°K, and the Fermi contact energies were found to adhere closely to a curve of the form a(T) = A + B coth (C/T).The effects of pure lattice expansion may be removed with the aid of high pressure ENDOR data. The remaining temperature dependence can be explained in terms of a configuration co-ordinate model, with the F-electron interacting with a single effective mode of lattice vibration.     

The range of transferred hyperfine interactions in EuS

We report on a Mössbauer study of the transferred hyperfine fields at the ¹⁵¹Eu nuclei in the Eu_1?xSr_x series of mixed compounds. The totally transferred hyperfine field amount ?(5.3 ± 0.3) T corresponding to saturation values for the hyperfine field of ?(33.4 ± 0.2) T for x = 0 and ?(28.1 ± 0.2) T extrapolated for x = 1. Our results strongly support fairly long range magnetic interactions instead of the generally accepted model covering only nearest and next nearest neighbour interactions.     

Mixed hyperfine interactions in Fe-Mg-O: magnetic behaviour

The complex Mössbauer spectra exhibited by Fe_xO (x0.91) and (Fe_1–y Mg _y )_xO (y=0.15–0.85) powder samples at liquid helium temperature have been analysed by a Hamiltonian treatment to allow for the significant electric field gradients present at the Fe²⁺ defect sites. The magnetic behaviour of the defect clusters are considered in terms of antiferromagnetic couplings, consistent with the spin glass-like behaviour reported recently for magnesiowüstite.