Lattice Relaxation Rate Research Articles

Spatially resolved magnetic resonance studies of swift heavy ion induced defects and radiolysis products in LiF crystals

Spatially resolved NMR (19F and 7Li) spin–lattice relaxation rates were measured on LiF single crystals irradiated with swift heavy ions of about 10 MeV per nucleon corresponding to several tens of micrometer range. Beyond the ion penetration depth, where the relaxation rate is small enough, also the relaxation dispersion was studied by a spatially resolved mechanical field cycling technique. Experiments were performed on crystals exposed to a variety of beam parameters (ion species, projectile energies and fluences) as well as for varying thermal annealing conditions. In addition to NMR, Q-band EPR was applied to study radiolysis products as produced by annealing. The most important findings are a dose dependent defect region within the ion range and an enhanced relaxation rate beyond the ion range due to characteristic X-ray emission (in case of heavy projectile ions) or nuclear reactions (in case of light projectile ions). Moreover, under certain irradiation conditions molecular fluorine F2 is produced, and thermal annealing involves the formation of metallic lithium, F-center clusters and impurity aggregates.

Vison-Majorana complex zero-energy resonance in the Kitaev spin liquid

We study the effect of site dilution in Kitaev's model. We derive an analytical solution of the dynamical spin correlation functions for arbitrary configurations of $Z_2$ fluxes. By incorporating this solution into classical Monte Carlo scheme, we address how a site vacancy affects the experimental observables, such as the static spin susceptibility and the spin lattice relaxation rate, $1/T_1$. As a result, we found an enhancement of dynamical magnetic response in the vicinity of vacancy, which leads to Friedel-like oscillation in local $1/T_1$, in contrast to limited influences on the static susceptibility. Furthermore, we found a sharp zero-energy peak in the magnetic excitation spectrum, which is attributed to the Vison & Majorana zero mode trapped near the site vacancy. This zero mode can be interpreted as fractionalized spin hole into an Ising triplet with differentiated magnetic axes, which leads to the characteristic temperature and field-orientational dependence of $1/T_1$.

Proton mobility in Ruddlesden–Popper phase H2La2Ti3O10 studied by 1H NMR

To study protons localization in H1.83K0.17La2Ti3O10·0.17H2O and their motional characteristics, complementary Nuclear Magnetic Resonance (NMR) techniques have been applied. 1H Magic Angle Spinning NMR evidences the presence of different proton containing species. By analyzing the temperature dependence of the 1H MAS NMR spectrum we attribute the observed lines to interlayer H+ in regular sites (isolated and in water rich environment), water protons and protons from various defects. The temperature behaviors of the spectral lines intensities and widths point out that intercalated water molecules are involved in translational motion that is confirmed by spin lattice relaxation rate (R1) and spin-lattice relaxation rate in rotating frame (R1ρ) measurements. It has been shown that for a correct determination of the proton motional parameters the Kohlrausch-Williams-Watts correlation function must be used. Its application results in the following parameters of proton motion in the interlayer space of H1.83K0.17La2Ti3O10·0.17H2O: Ea = 0.194(2) eV, β = 0.28(1), τ0=6.2(1)×10−10 s.

Magnetic properties of the itinerant A- type antiferromagnet CaCo2P2 studied by Co59 and P31 nuclear magnetic resonance

$^{59}$Co and $^{31}$P nuclear magnetic resonance (NMR) measurements in external magnetic and zero magnetic fields have been performed to investigate the magnetic properties of the A-type antiferromagnetic (AFM) CaCo$_2$P$_2$. NMR data, especially, the nuclear spin lattice relaxation rates 1/$T_1$ exhibiting a clear peak, provide clear evidence for the AFM transition at a N\'eel temperature of $T_{\rm N}\sim$110~K. The magnetic fluctuations in the paramagnetic state were found to be three-dimensional ferromagnetic, suggesting ferromagnetic interaction between Co spins in the ${\it ab}$ plane characterize the spin correlations in the paramagnetic state. In the AFM state below $T_{\rm N}$, we have observed $^{59}$Co and $^{31}$P NMR signals under zero magnetic field. From $^{59}$Co NMR data, the ordered magnetic moments of Co are found to be in $ab$ plane and are estimated to be 0.35 $\mu_{\rm B}$ at 4.2 K. Furthermore, the external field dependence of $^{59}$Co NMR spectrum in the AFM state suggests a very weak magnetic anisotropy of the Co ions and also provides microscopic evidence of canting the Co ordered moments along the external magnetic field directions. The magnetic state of the Co ions in CaCo$_2$P$_2$ is well explained by the local moment picture in the AFM state, although the system is metallic as seen by $1/T_1T$ = constant behavior.

Proton dynamics in tetramethylammonium cadmium chloride (CH3)4NCdCl3 single crystal by using 1H NMR measurements

Tetramethylammonium (TMA) cadmium chloride (TMCC), (CH3)4NCdCl3, has four phases with first-order transition temperatures at 104 and 118 K. To investigate the proton dynamics, we performed 1H nuclear magnetic resonance spectroscopy on a single TMCC crystal and measured the spin–lattice relaxation rate 1/T1 and rotating frame relaxation rate 1/T1ρ in the range of 65−300 K at 206.4 MHz under ∼4.8 T. 1/T1 displays an abrupt increase at 111.5 K and 1/T1ρ exhibits a sudden jump between 110 and 120 K. The discontinuities of both 1/T1 and 1/T1ρ across the phase transitions are unique in TMCC and have not been observed in other TMA-based compounds. Above 118 K, the temperature dependence of both T1 and T1ρ is linear, which indicates Arrhenius behavior. Below 118 K, T1 has a single minimum, and both T1 and T1ρ are linear in the low temperature range. This behavior at 206.4 MHz is very different from the double minima observed in T1 at 14.7 MHz for powder TMCC samples and in both T1 and T1ρ for other TMA-based compounds. By fitting two different regimes to the Bloembergen-Purcell-Pound curves, we extract both Ea and τ0 above and below 118 K. By carefully analyzing the T1 and T1ρ data, we ascertain that the coexistence of CH3 and TMA motions displays the double minima, unlike the T1 and T1ρ data at 206.4 MHz. Therefore, we conclude that the only active proton dynamics of TMCC originate from the CH3 rotation, and there is no reorientation of TMA ions, unlike other TMA-based compounds. Both T1 and T1ρ data confirm that the proton dynamics of the CH3 groups suddenly switch from rapid rotation above 118 K to slow rotation below 118 K, with no TMA ion dynamics in TMCC.

63,65Cu NQR Spectra and Spin–Lattice Relaxation in Thermoelectric CuAlO2

The 63,65Cu nuclear quadrupole resonance spectra and spin–lattice relaxation rate (1/T1) have been measured in the semiconductor compound CuAlO2. The value of the nuclei quadrupole interaction constant QCC = 56.24(6) MHz (T = 298 K) has been obtained. The broad maximum has been found in the temperature dependence of 1/T1 in the low-temperature region (below 276 K). This maximum can be associated with the presence of energy levels in the forbidden band. The activation energy has been estimated in CuAlO2 [EA = 45(2) meV], assuming the activation character of the mobility of holes.

14N Nuclear Magnetic Resonance and Relaxation in the Paramagnetic Region of Uranium Mononitride

The spin susceptibility of a polycrystalline sample of uranium mononitride UN is studied by measuring the 14N NMR line shift, spin–lattice relaxation rates of the nuclear spin, and static magnetic susceptibility in the temperature region of 1.5TN < T < 7TN A joint analysis of the results obtained has revealed the temperature dependence of the characteristic energy of spin fluctuations of the uranium 5f electrons: Γnmr(T) ∝ T0.54(4) close to the dependence Γ(T) ∝ T0.5 characteristic of concentrated Kondo systems above the coherent state formation temperature.

A proton T1-nuclear magnetic resonance dispersion study of water motion in snowflakes and hexagonal ice

ABSTRACTSnowflakes and ordinary hexagonal ice were studied measuring water proton spin–lattice relaxation rate -nuclear magnetic resonance dispersion (NMRD) profiles at proton Larmor frequencies ranging from 1 to 30 MHz and at different temperatures ranging from C to C. The spin–spin relaxation rate 1/ was determined at a single Larmor frequency of MHz. The high-field wing of the proton -NMRD profile was characterised by two parameters: a correlation time which described the dipole–dipole spectral density, and the relaxation rate at low fields which was determined from . The correlation time depended on the dynamic model used. A rotation diffusion model yield approximatively s at C to about s at C, whereas for a more realistic six-site discrete exchange model, the correlation times decreased slightly to about 80% for the same temperature interval. Proton dipole–dipole interactions were divided into intramolecular and intermolecular contributions where the intermolecular contribution was about 0.4–0.8 × the intramolecular contribution. It was not possible to discriminate between the dynamic models or to detect ice/water interface effects by comparing the NMRD data from snowflakes with ordinary hexagonal ice data.

Field-cycling NMR relaxometry: the benefit of constructing master curves

ABSTRACTThe currently available field-cycling NMR relaxometers still suffer from a rather narrow frequency range; they facilitate measurements of the 1H spin–lattice relaxation rate in the frequency of 10 kHz–30 MHz. This limit may be overcome by constructing master curves via exploiting frequency-temperature superposition, the latter being an intrinsic feature of the collective dynamics in many soft matter systems. It states that the shape of the motional time-correlation function is essentially temperature independent over a large temperature interval. As will be demonstrated, master curves may be built in the susceptibility or the spectral density representation of spin–lattice relaxation data. As a result, the effective frequency range is expanded up to ten decades, and the applied shift factors provide the temperature dependence of the corresponding correlation time. Three examples are presented: cyano adamantane in its plastically crystalline phase, the liquid glycerol, and the melt of poly(ethylene propylene). Advantages and limitations of the approach are discussed.

Mode Analysis of NMR Relaxation in a Polymer Melt

Spin–lattice relaxation rates of unentangled poly(dimethylsiloxane) (PDMS) melts of different M measured by field-cycling (FC) 1H NMR are analyzed using a new approach. By fitting the time domain mode distribution of the Rouse model to the experimental data, interpolation of the latter is achieved and a mode separation is performed. The evolution of the Rouse relaxation spectrum with increasing M is studied. From the model parameters, the diffusion coefficient D, the Rouse time τR, and the statistical length are calculated, all in good agreement with literature values and the Rouse model. The new approach allows for a more accurate calculation of the segmental mean square displacement, removes previous discrepancies between FC and field-gradient NMR and verifies the relevant relaxation theory. Furthermore, for the first time, the analysis provides the radius of gyration RG, the values of which are in agreement with literature values obtained by small-angle neutron scattering. The M-dependence RG ∝ M0.53±0...

75As-NQR Investigation of the Relationship between the Instability of Metal–Insulator Transition and Superconductivity in Ru1−xRhxAs

We performed 75As nuclear quadrupole resonance (NQR) measurements on single-crystalline Ru1−xRhxAs to study the relationship between the superconductivity observed in a limited Rh-substitution region and the critical fluctuations seen in the nuclear spin–lattice relaxation rate at the metal–insulator transition of RuAs. The critical fluctuation at TMI1 ∼ 255 K for RuAs is suppressed by the Rh substitution, and superconductivity with the maximum transition temperature is realized in the region where critical fluctuations are weakened significantly. The nuclear spin–lattice relaxation rate exhibits a Hebel–Slichter peak together with typical metallic behavior, demonstrating that superconductivity with a conventional s-wave character is realized without remarkable development of magnetic or electric fluctuations in Ru1−xRhxAs.

Anisotropy of the electron spin–lattice relaxation. PO32− radical in glycinium phosphite gly·H3PO3 crystal

Electron spin–lattice relaxation has been measured for PO32− radical in glycinium phosphite gly·H3PO3 crystal and its deuterated analogue in temperature range 40–300 K. Angular dependence of the relaxation rate was measured in three crystal planes at room temperature. The Debye cut-off temperature has been calculated as ΘD = 97 K, which indicates that the temperature dependence of 1/T1 is governed by radical local vibrations localized in optical phonon modes range. Theories of possible 1/T1 angular dependence are reviewed. The anisotropy of spin–lattice relaxation rate in our crystal is assumed to be a result of local magnetic field fluctuations due to electron–proton dipolar coupling. Theoretical evaluation of 1/T1 were performed for coupling with protons at distances up to 0.46 nm. Not perfect agreement was found with motion correlation time τc = 2·10−9 s.

Spatiotemporal mapping of oxygen in a microbially-impacted packed bed using 19F Nuclear magnetic resonance oximetry

19F magnetic resonance has been used in the medical field for quantifying oxygenation in blood, tissues, and tumors. The 19F NMR oximetry technique exploits the affinity of molecular oxygen for liquid fluorocarbon phases, and the resulting linear dependence of 19F spin–lattice relaxation rate R1 on local oxygen concentration. Bacterial biofilms, aggregates of bacteria encased in a self-secreted matrix of extracellular polymers, are important in environmental, industrial, and clinical settings and oxygen gradients represent a critical determinant of biofilm function. However, measurement of oxygen distribution in biofilms and biofouled porous media is difficult. Here the ability of 19F NMR oximetry to accurately track oxygen profile development in microbial impacted packed bed systems without impacting oxygen transport is demonstrated. Time-stable and inert fluorocarbon containing particles are designed which act as oxygen reporters in porous media systems. Particles are generated by emulsifying and entrapping perfluorooctylbromide (PFOB) into alginate gel, resulting in oxygen-sensing alginate beads that are then used as the solid matrix of the packed bed. 19F oxygenation maps, when combined with 1H velocity maps, allow for insight into the interplay between fluid dynamics and oxygen transport phenomena in these complex biofouled systems. Spatial maps of oxygen consumption rate constants are calculated. The growth characteristics of two bacteria, a non-biofilm forming Escherichia coli and Staphylococcus epidermidis, a strong biofilm-former, are used to demonstrate the novel data provided by the method.

Microsecond motions probed by near-rotary-resonance R1\u03c115N MAS NMR experiments: the model case of protein overall-rocking in crystals

Solid-state near-rotary-resonance measurements of the spin–lattice relaxation rate in the rotating frame (R1ρ) is a powerful NMR technique for studying molecular dynamics in the microsecond time scale. The small difference between the spin-lock (SL) and magic-angle-spinning (MAS) frequencies allows sampling very slow motions, at the same time it brings up some methodological challenges. In this work, several issues affecting correct measurements and analysis of 15N R1ρ data are considered in detail. Among them are signal amplitude as a function of the difference between SL and MAS frequencies, “dead time” in the initial part of the relaxation decay caused by transient spin-dynamic oscillations, measurements under HORROR condition and proper treatment of the multi-exponential relaxation decays. The multiple 15N R1ρ measurements at different SL fields and temperatures have been conducted in 1D mode (i.e. without site-specific resolution) for a set of four different microcrystalline protein samples (GB1, SH3, MPD-ubiquitin and cubic-PEG-ubiquitin) to study the overall protein rocking in a crystal. While the amplitude of this motion varies very significantly, its correlation time for all four sample is practically the same, 30–50 μs. The amplitude of the rocking motion correlates with the packing density of a protein crystal. It has been suggested that the rocking motion is not diffusive but likely a jump-like dynamic process.

Anisotropic Magnetic Fluctuations in Ferromagnetic Superconductor UGe2: 73Ge-NQR Study at Ambient Pressure

We report a 73Ge nuclear quadrupole resonance reinvestigation of the ferromagnetic (FM) superconductor UGe2 at ambient pressure. Careful measurement and analysis of the paramagnetic (PM) spectrum taking into account the calculated electric field gradient (EFG) resulted in modification of the previously reported site assignments for three inequivalent Ge sites. The crossover anomaly at Tx in the FM state is not clearly seen in the temperature dependence of the static internal field at the Ge site, whereas it appears in the nuclear spin–lattice relaxation rate, 1/T1. A remarkable site dependence of 1/T1 is observed among the inequivalent Ge sites, where the directions of the principal axes of the EFG are different. This result reveals that the magnetic fluctuation of UGe2 is highly anisotropic, which is consistent with the presence of Ising magnetic fluctuations in the PM state.

Sb121,123nuclear quadrupole resonance as a microscopic probe in the Te-doped correlated semimetalFeSb2: Emergence of electronic Griffith phase, magnetism, and metallic behavior

$^{121,123}Sb$ nuclear quadrupole resonance (NQR) was applied to $Fe(Sb_{1-x}Te_x)_2$ in the low doping regime (\emph{x = 0, 0.01} and \emph{0.05}) as a microscopic zero field probe to study the evolution of \emph{3d} magnetism and the emergence of metallic behavior. Whereas the NQR spectra itself reflects the degree of local disorder via the width of the individual NQR lines, the spin lattice relaxation rate (SLRR) $1/T_1(T)$ probes the fluctuations at the $Sb$ - site. The fluctuations originate either from conduction electrons or from magnetic moments. In contrast to the semi metal $FeSb_2$ with a clear signature of the charge and spin gap formation in $1/T_1(T)T ( \sim exp/ (\Delta k_BT) ) $, the 1\% $Te$ doped system exhibits almost metallic conductivity and a almost filled gap. A weak divergence of the SLRR coefficient $1/T_1(T)T \sim T^{-n} \sim T^{-0.2}$ points towards the presence of electronic correlations towards low temperatures wheras the \textit{5\%} $Te$ doped sample exhibits a much larger divergence in the SLRR coefficient showing $1/T_1(T)T \sim T^{-0.72} $. According to the specific heat divergence a power law with $n\ =\ 2\ m\ =\ 0.56$ is expected for the SLRR. Furthermore $Te$-doped $FeSb_2$ as a disordered paramagnetic metal might be a platform for the electronic Griffith phase scenario. NQR evidences a substantial asymmetric broadening of the $^{121,123}Sb$ NQR spectrum for the \emph{5\%} sample. This has purely electronic origin in agreement with the electronic Griffith phase and stems probably from an enhanced $Sb$-$Te$ bond polarization and electronic density shift towards the $Te$ atom inside $Sb$-$Te$ dumbbell.

Nuclear Magnetic Resonance Study of Anion and Cation Reorientational Dynamics in (NH4)2B12H12

Diammonium dodecahydro-closo-dodecaborate (NH4)2B12H12 is the ionic compound combining NH4+ cations and [B12H12]2– anions, both of which can exhibit high reorientational mobility. To study the dynamical properties of this material, we measured the proton NMR spectra and spin–lattice relaxation rates in (NH4)2B12H12 over the temperature range of 6–475 K. Two reorientational processes occurring at different frequency scales have been revealed. In the temperature range of 200–475 K, the proton spin–lattice relaxation data are governed by thermally activated reorientations of the icosahedral [B12H12]2– anions. This motional process is characterized by the activation energy of 486(8) meV, and the corresponding reorientational jump rate reaches ∼108 s–1 near 410 K. Below 100 K, the relaxation data are governed by the extremely fast process of NH4+ reorientations which are not “frozen out” at the NMR frequency scale down to 6 K. The experimental results in this range are described in terms of a gradual transition from the regime of low-temperature quantum dynamics (rotational tunneling of NH4 groups) to the regime of classical jump reorientations of NH4 groups with an activation energy of 26.5 meV. Our study offers physical insights into the rich dynamical behavior of (NH4)2B12H12 on an atomic level, providing a link between the microscopic and thermodynamic properties of this compound.

Interaction mechanism of olaparib binding to human serum albumin investigated with NMR relaxation data and computational methods.

The interaction mechanism between olaparib (OLA) and human serum albumin (HSA) has been investigated using experimental and computational techniques. An NMR relaxation approach based on the analysis of proton selective and non-selective spin–lattice relaxation rates at different temperatures can provide quantitative information about the affinity index and the thermodynamic equilibrium constant of the OLA–HSA system. The affinity index and the thermodynamic equilibrium constant decreased as temperature increased, indicating that the interactions between OLA and HSA could be weakened as temperature increased. Molecular docking and dynamics simulations revealed that OLA stably bound to subdomain II (site 1), and OLA could induce the conformational and micro-environmental changes in HSA. CD results suggested that α-helix content decreased after OLA was added, demonstrating that OLA affected the secondary structure of HSA.

Pseudogap Behavior of the Nuclear Spin–Lattice Relaxation Rate in FeSe Probed by 77Se-NMR

We conducted $^{77}$Se-nuclear magnetic resonance studies of the iron-based superconductor FeSe in magnetic fields of 0.6 to 19 T to investigate the superconducting and normal-state properties. The nuclear spin-lattice relaxation rate divided by the temperature $(T_1T)^{-1}$ increases below the structural transition temperature $T_\mathrm{s}$ but starts to be suppressed below $T^*$, well above the superconducting transition temperature $T_\mathrm{c}(H)$, resulting in a broad maximum of $(T_1T)^{-1}$ at $T_\mathrm{p}(H)$. This is similar to the pseudogap behavior in optimally doped cuprate superconductors. Because $T^*$ and $T_\mathrm{p}(H)$ decrease in the same manner as $T_\mathrm{c}(H)$ with increasing $H$, the pseudogap behavior in FeSe is ascribed to superconducting fluctuations, which presumably originate from the theoretically predicted preformed pair above $T_\mathrm{c}(H)$.

Heavy-atom effect on optically excited triplet state kinetics

In several fields of research, like e.g. photosensitization, photovoltaics, organic electroluminescent devices, dynamic nuclear polarization, or pulsed dipolar electron paramagnetic resonance spectroscopy, triplet state kinetics play an important role. It is therefore desirable to tailor the kinetics of photoexcited triplet states, e.g. by exploiting the intramolecular heavy-atom effect, and to determine the respective kinetic parameters. In this work, we set out to systematically investigate the photoexcited triplet state kinetics of a series of haloanthracenes by time-resolved electron paramagnetic resonance spectroscopy in combination with synchronized laser excitation. For this purpose, a procedure to simulate time traces by solving the differential equation system governing the triplet kinetics numerically is developed. This way, spin lattice relaxation rates and zero-field triplet life times are obtained concurrently by a global fit to experimental data measured at three different cryogenic temperatures.