Soliton dynamics and nuclear spin-lattice relaxation in incommensurately modulated crystals

The nuclear spin-lattice relaxation rate in incommensurate systems is analysed for the the so-called soliton limit which often can be applied near the transition to a subsequent commensurate phase. Previous calculations are corrected by taking into account adequately the eigenfunctions of the relevant fluctuations. In contrast to previous conclusions, it is shown that for nuclei in the discommensurations the spin-lattice relaxation rate is expected to increase on approaching the phase transition. The theoretical predictions are confirmed by experimental data obtained from NMR measurements on the prototype incommensurate systems Rb₂ZnCl₄ and BCCD. b]References     

Spin diffusion and nuclear spin-lattice relaxation in irradiated solids: a multiple-pulse NQR study

We present a detailed theoretical and experimental NQR multiple-pulse spin-locking study of spin-lattice relaxation and spin diffusion processes in the presence of paramagnetic impurities in solids. The relaxation function of the nuclear spin system at the beginning of the relaxation process is given by exp , where T_1ρ is spin-lattice relaxation time in rotating frame and =d/6, d is the sample dimensionality. Then the relaxation proceeds asymptotically to an exponential function of time, which was attributed to the spin-diffusion regime. Using the experimental data obtained from the analysis of those two relaxation regimes in γ-irradiated powdered NaClO₃, spin diffusion coefficient has been determined and the radius of the diffusion barrier has been estimated.     

Muon spin-lattice relaxation and muonium diffusion in ice

Longitudinal muon spin relaxation is measured in ice, using samples with and without enrichment in H₂ ¹⁷O, with a view to studying the mobility of the muonium fraction. A conventional analysis of the data, on the assumption that relaxation of the diamagnetic fraction is negligible, suggests that more than one mechanism of muonium relaxation is at work. A Bayesian analysis warns that separation of the diamagnetic and paramagnetic signals may not be so straightforward.     

Rouse dynamics of colloids bound to polymer networks

We present experimental evidence of a transition in the short-time Brownian motion of colloids from diffusive to subdiffusive, Rouse-like. This transition is seen for particles that are bound, through physical adsorption, to transient polymer networks. The characteristic Rouse scaling of the mean square particle displacement with radical t, found in the experiments, is rationalized using an analytical bead-spring model of a large particle anchored to a set of polymer chains.     

Mechanism of 3He-mediated nuclear spin-lattice relaxation at surfaces

We measured the nuclear spin-lattice relaxation time T₁, of several surface-bound nuclei, ¹H, ¹⁹F, ¹¹B, ¹³C, ²⁹Si, and ²H, immersed in liquid ³He over the temperature range 0.01 K ⩽ T < 1 K. The Larmor frequencies of these nuclei in a 3.39 T field extended from 22 to 144 MHz. All T₁ values were temperature-independent and ranged from a few seconds to several hours, depending on the particular nucleus and the surface geometry of the sample. The results indicate that the coupled relaxation of surface spins is a phenomenon occurring in all solids immersed in ³He and thus provides a general mechanism for obtaining high nuclear polarization in solids, that the relaxation is controlled by direct dipole-dipole interactions between the surface spins and ³He in the first surface layer, that the ³He motion dynamics do not change appreciably from one surface to another, and that measurements of T₁ may thus be useful for determining the structure of surfaces.     

Random distribution of paramagnetic centers in the nuclear spin-lattice relaxation

A numerical solution is presented for the differential equation governing the nuclear spin-lattice relaxation via paramagnetic centers for spherically symmetric spin-diffusion constantD and direct relaxation transition probabilityC in the one-paramagnetic-center approximation. An interpolation function is given which reproduces the computedT ₁ values within ±5% for both the rapid diffusion and diffusion-limited cases. The introduction of a random distribution of the paramagnetic centers over the sample causes the relaxation to occur as exp(?at ^h ) with 0.5<h<1.0 forβ=(C/D)^1/4<R _av=the average radius of the influence spheres. This differs from the experimentally observed exp(?a ₁ t) behaviour. However, the random distribution seems to explain the difference between the theoretically calculated fluorine spin-diffusion in CaF₂ and that extracted from experimental NMR data by using the uniform-distribution model.     

Critical behavior of a ferroelectric studied by nuclear spin-lattice relaxation

Nuclear spin-lattice relaxation measurements have been performed on ¹¹B nuclei in colemanite CaB₃O₄(OH)₃·H₂O in the ferroelectric and paraelectric phases. Results obtained with a temperature accuracy of 0.01° C show a logarithmic behavior of T₁^?1 in the very vicinity of T_c.     

Nuclear orientation and nuclear spin-lattice relaxation in insulating magnetically ordered crystals

The mechanism of nuclear spin-lattice relaxation in insulating magnetically ordered crystals at low temperatures is discussed. In 3-dimensional systems the relaxation time T₁ is long, but in the lower dimensional systems it can be quite short making them possible candidates for hosts in on-line experiments. An experiment in 2-dimensional⁵⁴Mn−Mn (COOCH₃)₂·4H₂O is discussed with particular reference to the relatively short value of T₁ in low applied fields. A preliminary experiment in the 1-dimensional system⁵⁴Mn−(CH₃)₄NMnCl₃ (TMMC) is also described. Pulsed NMRON measurements in⁵⁴Mn−MnCl₂·4H₂O are outlined and the advant-systems with fast relaxation emphasized.     

Magnons and solitons in CsNiF3 investigated by nuclear spin-lattice relaxation

By nuclear spin-lattice relaxation of¹³³Cs we studied linear and nonlinear magnetic excitations in the one-dimensional easy-plane ferromagnet CsNiF₃. The measurements were performed in the temperature range from 3 to 20 K and with external magnetic fields from 3 to 55 kOe applied perpendicular to the chain direction. Universal dependence on was obtained forT/T ₁. A quantitative interpretation of theT ₁ data in terms of two-magnon and soliton contributions could be achieved by considering renormalization effects resulting from magnon-magnon and soliton-magnon interaction as well as from quantum corrections. The instability of solitons due to out-of-plane fluctuations expected in high magnetic fields is discussed.     

Temperature dependence of nuclear spin-lattice relaxation in the heavy-fermion superconducting state

Some features of the experimental data on nuclear spin relaxation time T₁ in the heavy-fermion superconducting state can be explained by taking into account the effect of the electron Zeeman energy. It is found that at intermediate temperatures the usual quasiparticle spin-flip scattering dominates, while at very low temperatures a new process, pair creation (annihilation), dominates and gives T^-1₁ ∝ T.     

The183Re ground state dipole moment and nuclear spin-lattice relaxation of Re in Fe

The hyperfine interaction of the system¹⁸³Re(70d)Fe has been investigated with the NMR/ON technique. With the hyperfine field valueB _hf(ReFe)=–76.0(1.5) T the ground state magnetic moment was determined as: (5/2⁺,¹⁸³Re)=+3.12(6) _N. The field dependent nuclear spin-lattice relaxation time has been measured. The result for the high-field relaxation rateR _exp=1.65(5)·10^–15 T ²s K^–1 is explained in terms of indirect spin-wave interaction.     

The “corset effect” of spin-lattice relaxation in polymer melts confined in nanoporous media

Linear polyethylene oxides with molecular weightsM _w of 1665 and 10170 confined in pores with variable diameters in a solid methacrylate matrix were studied by proton field-cycling nuclear magnetic resonance relaxometry. The pore diameter was varied in the range of 9–57 nm. In all cases, the spin-lattice relaxation time shows a frequency dependence close toT ₁∞ v^3/4 in the range ofv=3·10^?1-2·10¹ MHz as predicted by the tube-reptation model. This protonT ₁ dispersion essentially reproduces that found in a previous deuteron study (R. Kimmich, R.-O. Seitter, U. Beginn, M. Möller, N. Fatkullin: Chem. Phys. Lett. 307, 147, 1999). As a feature particularly characteristic for reptation, this finding suggests that reptation is the dominating chain dynamics mechanism under pore confinement in the corresponding time range. The absolute values of the spin-lattice relaxation times indicate that the diameter of the effective tubes in which reptation occurs is much smaller than the pore diameters on the time scale of spin-lattice relaxation experimens. An estimation leads to a valued ^*～0.5 nm. The impenetrability of the solid pore walls, the uncrossability of polymer chains (“excluded volume”) and the low value of the compressibility in polymer melts create the “corset effect” which reduces the lateral motions of polymer chains to a microscopic scale of only a few tenths of a nanometer.     

Mechanism of nuclear spin-lattice relaxation and its field dependence for ultraslow atomic motion

The contribution of ultraslow self-diffusion of polycrystalline benzene molecules to the spin-lattice relaxation of protons is studied as a function of effective magnetic field H ₂ in a doubly rotating frame (DRF). Proton relaxation time T _1ρρ is measured by direct recording of NMR in a rotating frame (RF). The effective fields have a “magic” orientation corresponding to angles arccos(1/√3) in the RF and π/2 in the DRF so that the secular part of the dipole-dipole interactions of protons is suppressed in two orders of perturbation theory, while the nonsecular part becomes predominant. It is found that the diffusion contribution of benzene molecules to proton relaxation time T _1ρρ is a linear function of the square of field H ₂ and exhibits all peculiarities typical of the model of strong collisions generalized to only fluctuating nonsecular dipole interactions in fields exceeding the local field. This means that the model can also be employed in the given conditions. It is shown that perfect agreement with such a dependence can also be obtained in the model of weak collisions if we take into account the concept of the locally effective quantization field, whose magnitude and direction are controlled by the vector sum of field H ₂, and the nonsecular local field perpendicular to it.