Neutron thermometry on polarized silver nuclei at sub-microkelvin spin temperatures

We have utilized both neutron transmission and neutron diffraction to measure the spin polarization in a single crystal of ¹⁰⁹Ag, whereby the nuclear spin temperature could be obtained. Transmission of a polarized neutron beam provides an accurate and convenient primary thermometer. The feasibility of unpolarized neutron transmission was demonstrated as well. The diffraction measurements were strongly influenced by extinction, whence this method had to be calibrated against another thermometer. In this way we obtained information about extinction in the crystal.     

μ + Knight shift and spin relaxation in indium single crystalKnight shift and spin relaxation in indium single crystal

μ ⁺ SR measurements have been performed in a single crystal indium sample between 12 K and 300 K with a stroboscopic μSR spectrometer. The muonic Knight shiftK _μ and the muonic depolarization rate σ were obtained for various angles θ between the tetragonal crystallinec-axis and the direction of the external field. The isotropic part ofK _μ is only weakly temperature dependent and is consistent with the estimated Pauli spin susceptibility value. At a temperature of 12 K the angular dependence ofM ₂ (the second moment of the field distribution at the muon, obtained from the measured σ(θ) values) allows a clear determination of the muon location — the symmetric tetrahedral site. The observed anisotropicK _μ cannot be explained by the dipoles at the In atoms responsible for the bulk magnetic susceptibility but probably originates from an anisotropic Pauli spin susceptibility.     

Cross relaxation and spin thermodynamics in some diluted copper tutton salts

The dynamics of spin systems in some diluted copper Tutton salts is investigated with the method of pulsed ENDOR. This method is suited particularly to study the fast time-dependent behaviour of the spin temperatures. The dynamics in copper Tutton salts is explained with the introduction of four electron Zeeman systems, one combined electron dipole- dipole and proton Zeeman system and the lattice each with a temperature of its own. The cross-relaxation mechanism is a contact between the electron Zeeman systems and the electron dipole-dipole system. Its rate τ⁻¹_cr can be measured directly. Moreover information is gained about the contact between the proton Zeeman system and the electron dipole-dipole system. The experiments demonstrate that the model of a spherical diffusion barrier, that is often used in the theory of the relaxation of the proton magnetization by paramagnetic impurities, is not valid in our samples.     

Contribution to the theory of spin relaxation at finite temperatures in the odd-filling quantum Hall effect regime

Spin relaxation in a two-dimensional electron gas (2D EG) is treated as the establishment of equilibrium in a gas of spin excitons as a result of processes that change the number of spin excitons. Coalescence is the dominant channel above a temperature of the order of 1 K. The coalescence of excitons can occurr as a result of spin-orbit and Coulomb interactions in the 2D EG. The rate of coalescence falls exponentially at low temperatures. The relaxation time is calculated, and the critical temperature below which the main annihilation process becomes that due to the exciton-phonon interaction is determined. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 531–536 (25 October 1999)     

The effect of spin relaxation on ENDOR spectra recorded at high magnetic fields and low temperatures

A simple theoretical model that describes the pulsed Davies electron-nuclear double resonance (ENDOR) experiment for an electron spin S = (1/2) coupled to a nuclear spin I = (1/2) was developed to account for unusual W-band (95 GHz) ENDOR effects observed at low temperatures. This model takes into account the thermal polarization along with all internal relaxation processes in a four-level system represented by the electron- and nuclear-spin relaxation times T(1e) and T(1n), respectively, and the cross-relaxation time, T(1x). It is shown that under conditions of sufficiently high thermal spin polarization, nuclei can exhibit asymmetric ENDOR spectra in two cases: the first when t(mix) > T(1e) and T(1n), T(1x) > T(1e), where ENDOR signals from the alpha manifold are negative and those of the beta manifold positive, and the second when the cross- and/or nuclear-relaxation times are longer than the repetition time (t(mix) < T(1e) < t(R) and T(1n), T(1x) > t(R)). In that case the polarization of the ENDOR signals becomes opposite to the previous case, the lines in the alpha manifolds are positive, and those of the beta manifold are negative. This case is more likely to be encountered experimentally because it does not require a very long mixing time and is a consequence of the saturation of the nuclear transitions. Using this model the experimental t(mix) and t(R) dependencies of the W-band (1)H ENDOR amplitudes of [Cu(imidazole)(4)]Cl(2) were reproduced and the values of T(1e) and T(1x) > T(1e) were determined. The presence of asymmetry in the ENDOR spectrum is useful as it directly provides the sign of the hyperfine coupling. The presented model allows the experimentalist to adjust experimental parameters, such as t(mix) and t(R), in order to optimize the desired appearance of the spectrum.     

Spin relaxation in metals at very low temperatures

A general theory of spin relaxation in metals is developed from a statistical mechanical point of view. The theory is valid for all temperature domain and the multiple-time characteristics of the relaxation process are completely determined: The relaxation times are strongly dependent on the temperature and magnetic field. At very low temperatures, behaviours of the relaxation times are quite different from the usual ones showing a saturation effect. Temperature varations of the relaxation times for I ? 1 (I the magnitude of spin) are qualitatively different from those for $\text{I =}\text{1}\text{2}$. Namely, in the former case, the largest relaxation time has a maximum as a function of inverse temperature.     

Conductivity relaxation in NiTiO3 at high temperatures

NiTiO₃ ceramics were prepared via the traditional solid-state reaction route. The dielectric properties of NiTiO₃ ceramics have been systematically investigated in the temperature range from room temperature to 1073 K NiTiO₃ ceramics exhibit intrinsic dielectric response in the temperature range below 400 K. Two relaxations were observed in the temperature range higher than 400 K. The relaxation activation energy is 0.95 eV and 1.17 eV for the low- and high-temperature relaxations, respectively. Our results strongly indicate that the two relaxations are related to conductivity relaxation associated with the singly and doubly ionized oxygen vacancies.     

Conductivity relaxation in industrial refractories at high temperatures

Results are given for the potential distribution and fall in current for alumina, firebrick, and chrome-magnesite. Conductivity relaxation is explained via the ion-electron mechanism and thick depleted barrier layers. The data confirm theoretical calculations.     

Long-range order at low temperatures in dipolar spin ice

It has recently been suggested that long-range magnetic dipolar interactions are responsible for spin ice behavior in the Ising pyrochlore magnets Dy2Ti2O7 and Ho2Ti2O7. We report here numerical results on the low temperature properties of the dipolar spin ice model, obtained via a new loop algorithm which greatly improves the dynamics at low temperature. We recover the previously reported missing entropy in this model, and find a first order transition to a long-range ordered phase with zero total magnetization at very low temperature. We discuss the relevance of these results to Dy2Ti2O7 and Ho2Ti2O7.     

Cross relaxation studied by β NMR

The dipole-dipole interaction between polarized β active probes implanted into a metal and the surrounding host nuclei may lead to a transfer of polarization from the probe to the host. As energy must be conserved in this cross-relaxation (CR) process, resonance-like dips show up at certainB values, if the polarization of the probes is measured as a function of magnetic inductionB. The probe isotope¹²B has been studied in several metallic stoppers. For¹²B in the fec crystals Al, Cu the three possible cubic implantation sites could easily be discriminated by means of the CR technique. The temperature dependence of the dips yielded information on the diffusion of the probes. In the case of¹²B, in Cu above 400 K a further CR structure was found due to trapping of the probe at another site. In the bcc metal VCR spectra belonging to two different¹²B trapping sites could be separately registered using a special radiofrequency technique. Both, static and dynamical properties of the CR could be quantitatively explained by theory.     

Anomalous muonium relaxation rates in quartz at high temperatures

Muonium spin relaxation measurements of the \alpha‐, \beta‐, and \gamma‐tridymite phases of quartz have been carried out over a temperature range from 300 to 1250 K. Anomalous relaxation rate increases are observed which may result either from resonance coupling between the other impurity ions and defects in the lattice and the diffusing muonium, or may result from phonon interactions with the muonium quadrupole moment. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nuclear spin-lattice relaxation of thallium at low temperatures

The nuclear spin-lattice relaxation time in thallium metal has been measured from 0.4 to 75 mK in magnetic fields from 18 to 1000 mT. The value of the Korringa constant к = T₁T is 4.37(8) ms K. It is independent of field and temperature over the ranges investigated and differs significantly the previously measured value.     

Surface-enhanced absorption by self-organized silver films with aciniform-like nanoaggregates at elevated temperatures

Morphological evolution of silver nanocomposite films prepared by the wet colloidal route and surface-enhanced phenomena on aggregate nanostructures evolved during annealing were investigated. Dramatic changes in morphologies of particles and pores incurred by rearragement, coarsening, premelting, and dewetting of the silver clusters at different concentrations (i.e., mass thicknesses). At a higher mass thickness, the morphological transitions from self-organized nanoaggregates with aciniform pattern at 300 °C to elongated and coarsened particles with circular holes at 400 °C to island clusters at 500 °C occurred in the films. The peculiar absorption with a much redder and broader surface plasmon feature, which gone far beyond the theoretical prediction, induced by the formation of aciniform nanoaggregates embedded in the porous polymer matrix at a critical mass thickness of 9.6 nm during partial degradation of the PVP polymer and rearrangement of silver clusters at 300 °C. The surface-enhanced absorption was dramatically reduced by the elemination of the aggregate nanostructures and the spontaneous formation of the silver nanoisland film at the dewetting temperature of 500 °C.     

Nuclear magnetic relaxation without spin diffusion in polymers at interfaces

We consider theoretically the possibility of solid state NMR experiments with frozen linear polymer chains at interfaces. Three different cases are studied, namely, when the macromolecules are grafted on the surface, when they are adsorbed, and when they are very strongly adsorbed from a melt that is subsequently washed by a good solvent. The latter case is somewhat intermediate between the two former ones. For each case, we consider the relaxation when paramagnetic centers are located on the surface. We show that the shape of the relaxation curves depends critically on the monomer concentration profile, and exhibits characteristic power-law variations.