Kinetics of the polarization transfer in spatially disordered spin systems: Depolarization of impurity beta-active nuclei

We performed theoretical and experimental studies of the delocalization of the nuclear polarization of impurity beta-active nuclei ⁸Li in the spatially disordered system of ⁶Li nuclei in LiF single crystals. The process is controlled by the dipole-dipole interaction of nuclear magnetic moments with other nuclei (⁷Li, ¹⁹F) in the crystal. It is effective in a wide range of magnetic fields H ₀ = 150–3000 G as a result of a unique proximity of the g-factors of ⁸Li and ⁶Li nuclei: (g(⁸Li) − g(⁶Li))/g(⁶Li) = 0.0057. The kinetics of the ⁸Li depolarization is measured for an ⁶Li concentration of 0.15%–10.06% and in the field H ₀ of 200, 692 and 1210 G. A satisfactory explanation of the results is obtained on the basis of a numerical-analytical simulation of the process.     

Random walks in disordered systems and their investigations by means of magnetic resonance and relaxation of polarized beta-active nuclei

The experimental investigations of the polarization transport in the disordered spin system⁸Li −⁶Li in LiF crystal are considered. This process is initiated by one originally polarized beta-active⁸Li nucleus produced in a nuclear reaction. The autocorrelation function Foo(t) for the ran dom-walks-in-disordered-system problem has been measured over the range Foo(t)≥0.1 Foo(0) by analysing the β-emission anisotropy. The influence of the resonance process at double larmor frequency and of the thermal translational motion of⁸Li and⁶Li spins is studied. Results are in a good agre ement with the theory.     

Investigation of the external-magnetic-field dependence of polarization transfer in a 8Li-6Li disordered nuclear system

Random walks in disorderedmedia are studied by considering the example of the delocalization of nuclear polarization in an ensemble of ⁸Li beta-active nuclei produced in (n, γ) reactions induced by polarized thermal neutrons in LiF single crystals. The process is initiated by magnetic dipole-dipole interactions. It consists in polarization transfer from a primarily polarized ⁸Li nucleus to the closest (stable) ⁶Li nuclei and in subsequent polarizationmigration over 6Li nuclei, the return of the polarization in question to the ⁸Li nucleus being possible. In order to obtain information about the polarization of ⁸Li nuclei, the asymmetry of beta radiation from them was measured with respect to the external magnetic field H ₀. The process was studied until it reached the diffusion asymptotic regime. The concentration of ⁶Li was varied between 3% and 10%, while the field strength H ₀ was changed from 200 to 1200 G. It is shown that microscopic theory is in satisfactory agreement with the experimental results obtained in the way outlined above. A comparison with the investigation of the mathematically allied process of the Förster electricdipole transfer of localized excitons is performed.     

Spin exchange polarization and measurement of the hyperfine splitting of short-lived8Li

Free⁸Li (T _1/2=0.84 sec) atoms were polarized by spin exchange with optically pumped rubidium vapour. The⁸Li nuclei were produced by the reaction⁷Li(d, p)⁸Li in a thin LiF target. They recoiled into helium which was used as a buffer gas for optical pumping as well. Nuclear polarization up to 2% was detected by β-decay asymmetry. Rf transitions between hfs levels of the atomic ground state yielded the hfs separation ΔW=382.543(7) MHz. The nuclear spin assignmentI=2 was confirmed.     

NMR study of 6Li in LiNbO3

An experimental NMR study of the ⁶Li isotope in single crystals of lithium niobate has been performed, along with a computer simulation of ⁶Li NMR spectra for a crystal of congruent composition, containing defects in the cation sublattice. It is found that the mean value of the principal component of the electric field gradient tensor at the ⁶Li nuclei is 1.48 times larger than at the ⁷Li nuclei. It is surmised that there is a substantial difference in the character of the mobility of the ⁶Li and ⁷Li nuclei in the LiO₆ octahedra at room temperature. Fiz. Tverd. Tela (St. Petersburg) 40, 122–125 (January 1998)     

Ergodic theorem for an impurity spin subsystem in a paramagnet

A model for verifying and developing the fundamental ideas underlying the ergodic hypothesis is proposed. The model describes the dynamics of the spin subsystem formed by impurity charges with spin I and a small g factor in a crystal immersed in a strong constant external magnetic field under conditions where the spin system of the nuclei in the crystal is isolated from the other degrees of freedom. The additive integral of motion is the projection of the total spin of the subsystem onto the external field. Attention is focused mainly on the case of I=1/2. It is shown that the ergodic hypothesis holds if the correlation radius is finite in the initial state and that the ergodic hypothesis is violated if the initial state is sharply localized or has global correlation. The nonergodicity of the ⁸Li− ⁶Li spin subsystem, which is a convenient object for experimental investigations of spin dynamics, is revealed. An estimate is obtained for the time for transition from a sharply localized disturbance of the canonical distribution to a quasiequilibrium state. Zh. éksp. Teor. Fiz. 116, 1398–1418 (October 1999)     

Paramagnetic frenkel defects in irradiated alkali-haloid crystals

In the EPR spectra of γ-irradiated NaF,⁶LiF, and LiF crystals with natural content of isotopes (independent of the impurity composition), the hyperfine structure (HFS) is observed against the background of a broad band. Absorption saturation in the system of defects responsible for the HFS and the broadband occurs at widely different power levels of microwave radiation, and broad band suppression takes place at registration in quadrature. The experimentally measured intensity distribution and the number of EPR lines in the⁶LiF crystal correlate with the calculated data when the spin interaction of an unpaired electron with 14 equaivalent fluorine nuclei is taken into account. A model of major radiation-induced paramagnetic defects in the form of Frenkel pairs, in which one component (the negatively charged quasi-molecule consisting of two halogen atoms) can be responsible for the HFS and the other component (F-center) for the broad band in the EPR spectrum, is proposed.     

EPR of radiation defects in LiBaF 3 crystals

EPR spectra of LiBaF 3 crystals have been investigated after X-irradiation at RT. A spectrum consisting of approximately 35 nearly equidistant EPR lines has a strong angular dependence on the line intensities. The spectrum is caused by a hyperfine interaction (hfs) of a spin S =1/2 with neighbouring groups of nuclei. The observed large number of hfs lines required Li nuclei being in the first shell and fluorine nuclei in the more distant second shell. We analysed the spectrum in the F -centre model, taking reduced hfs values of the F -centre in LiF and found qualitative explanation of the number of hfs lines. The angular dependence of the line intensities could be explained by an anisotropy of the g -tensor with its main axis along the [1 v 0 v 0] axis of the crystal.     

Magnetic effects in the oxidation of silicon

The mass spectrometry study has indicated that the magnetic field accelerates the oxidation of the surface of silicon crystals. The oxidation rate also depends on the nuclear spin of silicon: the oxidation rate of atoms with magnetic nuclei (²⁹Si) is almost twice as high as that of atoms with spinless, unmagnetized nuclei (²⁸Si and ³⁰Si). Both effects—magnetic field and magnetic isotope—reliably prove that the oxidation of silicon is a spin-selective reaction involving radicals and radical pairs as intermediate paramagnetic particles. A spin-selective magnetic sensitive oxidation mechanism is discussed.     

Suppression of band crossing in the neutron-rich nuclei 172, 173Yb due to the absence of a static pair field

High-spin states in the neutron-rich nuclei ^172,173Yb have been populated in a ¹⁷⁰Er(⁷Li,(p,d,t)xn) incomplete-fusion reaction and the emitted γ-radiation was detected with the GASP array. The signature partners of the 7/2⁺[633] rotational band of the odd-N ¹⁷³Yb isotope have been newly established and were observed up to spin values of (45/2⁺) and (43/2⁺), respectively. The ground-state band of the even-even nucleus ¹⁷²Yb has been observed up to a spin value of (22⁺). No band crossings were found in these bands. To explain this observation, it is proposed that the static pair field is absent, considering that the neutron odd-even mass differences reach for these nuclei very small values and that the band crossing is absent in cranked shell modell calculations without pairing. The results indicate, however, that strong dynamic correlations are still present.     

Pairing correlations and soft dipole excitations in nuclei on the neutron-drip line

Paring correlations and soft dipole excitations in weakly bound nuclei on the edge of neutrondrip line are studied by using a three-body model. A density-dependent contact interaction is employed to calculate the ground state of halo nuclei ⁶He and ¹¹Li, as well as a skin nucleus ²⁴O. Dipole excitations in these nuclei are also studied within the same model. We point out that the dineutron-type correlation plays a dominant role in the halo nuclei ⁶He and ¹¹Li having the coupled spin of the two neutrons S = 0, while the correlation similar to the BCS type is important in ²⁴O. Contributions of the spin S = 1 and S = 0 configurations are separately discussed in the low-energy dipole excitations. The calculated results are compared with recent experimental data of ⁶He and ¹¹Li. The text was submitted by the authors in English.     

Search for Li solar axions using resonant absorption in LiF crystal: Final results

The resonance excitation of the ⁷Li nuclei in a LiF crystal with mass of 553 g by hypothetical axions emitted in the deexcitation of the ⁷Li nuclei in the Sun was searched for deep underground at the Gran Sasso National Laboratories (LNGS) of INFN (3600 m w.e.). The data collected with a low background HP Ge detector 244 cm³ during 4044h have allowed us to set the limit on the axion mass: _ma<8.6 keV$m_a<8.6\text{keV}$ (90% C.L.) which is the best one for the ⁷Li solar axions to-date.     

Cross-relaxation processes of polarizedβ-active nuclei in various crystalline solids

Cross-processes of neutron activated, polarized nucleiI with surrounding stable host nucleiS are observed in diverse solids, using an in-beam NMR technique where the polarizedI nuclei are produced by capture of polarized thermal neutrons. The polarization is determined by measuring the 0°–180° asymmetry of the-decay radiation of these nuclei. The following cross-processes are studied:⁸Li–⁶Li in^natLi metal;⁸Li–⁷Li in⁷Li metal;⁸Li–⁶Li in^natLiF (single crystal);⁸Li–⁹³Nb in⁷LiNbO₃ (single crystal);²⁰F–¹⁹F in MgF₂ (single crystal). Two applications are presented: a) Determination of the temperature dependence of the electrical quadrupole coupling constant of⁹³Nb in LiNbO₃; b) Observation of radiation induced point defects in MgF₂ and their thermal annealing. All measurements are performed without any radiofrequency irradiation.     

γ-Ray emission from oriented nuclei in a multiaxis nuclear spin system: 166mHo165Ho

A new method is proposed to measure multiaxis nuclear spin structures using γ-ray emission from oriented radioactive nuclei. This method, which depends only on angular momentum theory, is presented for helical spin structures. The atomic magnetic structure can also be obtained when the nuclear magnetism is produced by hyperfine interaction. Measurements on ^166mHo in a single crystal of ¹⁶⁵Ho metal show that the spin axes form a single cone with a half-angle of 80.4±0.4°.     

Quadrupole relaxation enhancement--application to molecular crystals

A general theory of field dependent spin-lattice relaxation for nuclei of the spin quantum number 1/2 (1H, 19F, 13C) caused by dipole-dipole interactions with neighboring quadrupolar nuclei (nuclei possessing a quadrupolar moment) is presented. The theory is valid for arbitrary motional conditions and should be treated as a quadrupolar counterpart of the paramagnetic relaxation enhancement theory. When the energy level splitting of the dipolar spin (I=1/2) matches one of the transition frequencies of the quadrupolar nuclei one can observe a local enhancement of the dipolar spin relaxation (referred to as "quadrupolar peaks"). To see such effects the dynamics modulating the spin interactions has to be relatively slow. This brings the system beyond the validity range of perturbation approaches and requires the stochastic Liouville equation to be applied. The presented theory describes the quadrupolar relaxation enhancement (QRE) for an arbitrary spin quantum number of the quadrupolar nuclei and includes the asymmetry of the quadrupolar coupling. It has been applied to interpret the shape of magnetization curves (amplitude of 1H magnetization versus magnetic field) for the molecular crystal [C3N2H5]6[Bi4Br18] ([C3N2H5]-imidazolium). The magnetization curves show several dips (local minima) attributed to 1H-14N quadrupolar relaxation enhancement effects. In addition, as a limiting case a perturbation approach to QRE has been presented and its validity conditions have been discussed.     

β-NMR and nuclear spin diffusion in a spatially disordered subsystem

The results of theoretical and experimental investigations of delocalization of nuclear polarization in spatially disordered media are presented. The experiment is based on the measurement of a depolarization of β-active ⁸Li impurity nuclei (β-nuclei) in the spatially disordered subsystem of ⁸Li-⁶Li nuclei in LiF single crystals. The process is determined by the dipole-dipole interaction in this subsystem and by its dipole interactions with nuclei of the ⁷Li¹⁹F matrix. It is effective in a wide range of external magnetic fields H ₀ = 200–3000 G owing to proximity of g-factors. The kinetics of a depolarization of β-nuclei at the ⁶Li concentration c = 0.15–10.06% in fields H ₀ = 200 and 1210 G is investigated. A satisfactory explanation of the results is based on the numerical-analytical simulation of the process.     

Studies of light halo nuclei by the isotope shift method

Recent advances in atomic theory make possible the determination of nuclear charge radii for light isotopes from their isotope shift relative to other known isotopes. This method provides a unique measurement tool for the halo nuclei such as ⁶He and ¹¹Li. The theoretical techniques are reviewed and applied to recent measurements performed at Argonne, GSI, and TRIUMF.     

Single crystal surfaces probed by polarized nuclei

Nuclear magnetic resonance has developed into a very powerful technique to study the structure and dynamics of atomic and molecular systems, both in liquid and solid phase. However the investigation of single crystal surfaces with “conventional” NMR methods is essentially impossible due to the small sample size of less than 10¹⁵ sites on a cm². To overcome this for the important class of alkali adsorbates on metals and semiconductors, two methods are presented. Common to both is the preparation of a highly nuclear spin polarized atomic beam of ⁶Li in the one case and ⁸Li in the other. The latter isotope is radioactive and undergoes a \beta‐decay with a halflife of 0.84 s. Li adsorbed on the close packed Ru(001) surface is investigated. The T{in1} relaxation rate is the main observable and is used to deduce the local electronic density of states (LDOS(E_F,r=0)) and the Li diffusion barriers at low and high adsorbate coverage. The second experiment uses ⁶Li as an adsorbate, also studied on Ru(001). The nuclear polarization is measured by beam foil spectroscopy. A novel particle detected (photon counting) Fourier‐Transform NMR technique is demonstrated, by observing the time dependent flux of circularly polarized light emitted behind the foil after a 90^\circ‐pulse has been employed at the surface. Development and prospects of the latter technique are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dynamical polarization of radioactive nuclei

In order to polarize radioactive nuclei implanted in a highly polarized protonic target, it is proposed to use methods for the dynamical orientation of nuclei. The angular distribution of photons that originate from the cascade beta-gamma decay of the ²²Na(3⁺) state in a strong magnetic field is calculated. It is shown that, if the populations of Zeeman magnetic sublevels obey the Boltzmann distribution, the angular distribution of emitted photons is independent of the sign of spin temperature; at the same time, the tensor polarization of quadrupole nuclei occurring in the intrinsic field of a crystal causes a strong dependence of the anisotropy on the sign of spin temperature. A rich potential of a dynamical orientation for studying the magnetic structure of rare nuclei and the dynamics of their spin-spin interactions in dielectric host materials is demonstrated. Physical and technological advantages and disadvantages of the method in the on-line regime of the implantation of heavy nuclei are discussed.     

Anomalous Phases in Cavity-Free 240 GHz Pulsed ENDOR Spectra of 1.44 GeV Xe-Irradiated LiF

Paramagnetic centers generated by swift heavy ion irradiation of LiF crystals could be identified as electrons trapped at regular anion vacancy sites (F centers). Well-resolved electron-nuclear double resonance (ENDOR) spectra resulting from the hyperfine interaction with ⁷Li and ¹⁹F nuclei located in six different shells could be recorded. In order to preserve the millimeter-sized crystals, a cavity-free setup was used for the ENDOR experiments at an electronic Larmor frequency of 240 GHz. Apparently even under conditions of extremely high local energy loss in the ion track, the local density of persistent F centers is still sufficiently low to prevent distortions of the ionic crystal. The spread of hyperfine coupling constants was less than 5 %. Neither in electron paramagnetic resonance (EPR) nor in ENDOR spectra there was evidence for different types of paramagnetic centers. When performing ENDOR by applying the radiofrequency pulse directly after the 3-pulse Mims-type microwave sequence, an anomalous ENDOR effect was observed. The observed “positive” and “negative” ENDOR response can be attributed to efficient hole and anti-hole formation in the inhomogeneously broadened EPR spectrum and can be used to determine the sign of hyperfine coupling constants.