Transverse Nuclear Magnetization Research Articles

Features of a Low-Temperature Charge Density Wave in the Monoclinic Phase of NbS3 Manifested in the NMR and in Transport Properties

The relaxation of the transverse nuclear magnetization in the monoclinic phase of NbS3 has been studied by the 93Nb nuclear magnetic resonance method near the temperature TP2 = 150 K, at which a low-temperature charge density wave is formed. It has been shown that the critical slowing down of one of the vibrational modes of the lattice, which is quite slow even above TP2, occurs slightly below TP2. The transition at TP2 occurs not only in low-resistance samples, as thought previously, but also in high-resistance ones, and involves Nb atoms in the bulk of a sample. The transport properties of high-resistance samples, namely, the smearing of the depinning threshold for the charge density wave below TP2, confirm that the phase transition in them occurs at TP2. It has been concluded that the distortion of the lattice at TP2 is not due to the Peierls mechanism and can be attributed to the Keldysh–Kopaev transition. Another possible mechanism is the fluctuation distortion of the lattice above TP2 that prevents the sliding of the charge density wave.

Enhanced transverse nuclear magnetization of an atomic co-magnetometer at the optimal oscillating magnetic field

The nuclear magnetization produced by hyperpolarized noble-gas molecules of co-magnetometers presents excellent potential in ultrahigh accuracy measurement. Here we demonstrate an optimization method of the oscillating magnetic field to enhance the transverse nuclear magnetization for atomic co-magnetometers operated in the nuclear magnetic resonance (NMR) regime. Under normal operating conditions, the absolute magnetic field produced by transverse nuclear magnetization is studied based on the nuclear spin dynamics and measured by the in-situ magnetometer experimentally. The different oscillating magnetic fields are applied to obtain the maximum transverse nuclear magnetization in various operating temperatures. Combined with the measurements, we reveal the relationship between the transverse nuclear magnetization and the relaxation time, which is consistent with our method. The optimization method makes selecting the optimal operating point for each atomic co-magnetometer quick and accurate to enhance transverse nuclear magnetization. In addition, operating at the optimal oscillating magnetic field can effectively reduce the bias instability of the atomic co-magnetometer.

Enhanced Transverse Nuclear Magnetization of an Atomic Co-Magnetometer at the Optimal Oscillating Magnetic Field

Enhanced Transverse Nuclear Magnetization of an Atomic Co-Magnetometer at the Optimal Oscillating Magnetic Field

Peculiarities of NMR relaxation in micellar gels of Pluronic F-127

Based on the 1H relaxation of transverse nuclear magnetization of triblock-copolymer Pluronic F-127 in D2O, we proposed a model of the associated pluronic structure in which the polyethylene oxide of molecules in neighboring micelles are intertwined in regions of overlapping micellar coronas, while the polypropylene oxide cores of the micelles play a role of nodes in the 3D network.

Application of spin-exchange relaxation-free magnetometry to the Cosmic Axion Spin Precession Experiment

The Cosmic Axion Spin Precession Experiment (CASPEr) seeks to measure oscillating torques on nuclear spins caused by axion or axion-like-particle (ALP) dark matter via nuclear magnetic resonance (NMR) techniques. A sample spin-polarized along a leading magnetic field experiences a resonance when the Larmor frequency matches the axion/ALP Compton frequency, generating precessing transverse nuclear magnetization. Here we demonstrate a Spin-Exchange Relaxation-Free (SERF) magnetometer with sensitivity ≈1fT∕Hz and an effective sensing volume of 0.1 cm3 that may be useful for NMR detection in CASPEr. A potential drawback of SERF-magnetometer-based NMR detection is the SERF’s limited dynamic range. Use of a magnetic flux transformer to suppress the leading magnetic field is considered as a potential method to expand the SERF’s dynamic range in order to probe higher axion/ALP Compton frequencies.

Characterization and Suppression Techniques for Degree of Radiation Damping in Inversion Recovery Measurements

Radiation damping is a phenomenon in which transverse nuclear magnetization couples with the current in a coil used in nuclear magnetic resonance experiments. This results in an additional magnetic field that increases the relaxation pathway for the magnetization, which then relaxes back to equilibrium more quickly. Radiation damping has been shown to affect longitudinal relaxation time (T1) measurement in inversion recovery experiments. In this work, we demonstrate that the extent of radiation damping depends upon the T1 of the sample. Radiation damping difference spectroscopy is used to characterize the severity of radiation damping, while gradient inversion recovery is used for radiation damping suppression in T1 measurements. At field strength of 9.4 T, for the radiation damping characteristic time (Trd) of 50 ms, these investigations show non-negligible radiation damping effects for T1 values greater than Trd, with severe distortions for T1 longer than about 150 ms, showing reasonable agreement with the predicted Trd. We also report a discrepancy between published expressions for the characteristic radiation damping time.

1H NMR signal broadening in spectra of alkane molecules adsorbed on MFI-type zeolites

The anisotropic behavior of C 1–C 6 alkane molecules adsorbed in MFI zeolite was studied by 1H nuclear magnetic resonance (NMR) using single-pulse excitation, Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence, Hahn echo (HE) pulse sequence, and magic-angle spinning. The molecular order parameter was obtained by both static 2H NMR spectroscopy and molecular simulations. This yields an order parameter in the range of 0.28–0.42 for linear alkanes in MFI zeolite, whereas the parameter equals zero for FAU zeolite with a cubic symmetry. Thus, in the case of a zeolite with a non-cubic symmetry like MFI, the mobility of the molecules in one crystallite cannot fully average the dipolar interaction. As a consequence, transverse nuclear magnetization as revealed in the echo attenuation notably deviates from a mono-exponential decay. This information is of particular relevance for the performance of pulsed field gradient (PFG) NMR diffusion experiments, since the occurrence of non-exponential magnetization attenuation could be taken as an indication of the existence of different molecules or of molecules in different states of mobility.

Multiexponential T2‐relaxation analysis in cerebrally damaged rats in the absence and presence of a gadolinium contrast agent

An analysis of the multiexponential relaxation of transverse nuclear magnetization with and without a gadolinium-based paramagnetic contrast agent in spontaneously hypertensive stroke-prone rats (SHR-SP) and in the rat model of ischemia induced by middle cerebral artery occlusion is described. From the multiexponential relaxation, the presence of two T(2) relaxation times in the range of 0.03-0.5 s, T(2A) (shortest) and T(2B) (longest), with very different relative weights (respectively, A and B), is evidenced. In our models of cerebral damage, the changes in A and B were more evident than those in T(2A) and T(2B). The two T(2) values were interpreted as belonging to water molecules in two different compartments; therefore, the difference between the damaged and normal regions revealed by means of standard T(2)-weighted images is suggested to be due to a different water distribution in the two compartments, rather than different T(2)'s. The T(2) relaxation in the SHR-SP stroke model is analyzed for the first time using a multiexponential method. The power of a detailed analysis of MRI relaxation times is confirmed by the correspondence between the revealed changes in T(2A), T(2B), A and B, and the known T(2)W and DWI results about blood-brain barrier functionality.

Spin relaxation of quadrupole nuclei in paramagnetic and magnetically ordered insulators

The longitudinal and transverse spin relaxation through a (generally anisotropic) electron-nucleus interaction in paramagnetic and magnetically ordered insulators is theoretically studied for nuclei with a resolved quadrupole structure. Expressions are derived for the relaxation rates of both the transverse nuclear magnetization components when individual transitions are excited in the quadrupole structure and the total longitudinal nuclear magnetization component. These expressions are reduced to a form that contains the Fourier transforms of the time correlation functions only for the electron spins. Given the specific form of these correlation functions corresponding to different phase states of the electron spins and different origins of their fluctuations, the temperature dependences of the nuclear relaxation rates are ascertained in various cases, including those for dipole and isotropic hyperfine interactions. Calculations are performed for arbitrary electron and half-integer nuclear spins by taking into account the possible quadrupole splitting of the NMR spectrum without any restriction on the smallness of the ratio ℏω s/kBT (ωs is the resonance frequency of the electron spins). The derived expressions are compared with available experimental data on the longitudinal and transverse nuclear relaxation in colossal-magnetoresistance lanthanum manganites in the part of their phase diagram where the corresponding samples are either paramagnetic or magnetically ordered insulators and near the points of transition to an ordered state. Interpretations alternative to the existing ones are offered.

Effects of barrier-induced nuclear spin magnetization inhomogeneities on diffusion-attenuated MR signal.

The spatial distribution of the transverse nuclear spin magnetization, appearing in a single compartment with impermeable boundaries in a Stejskal-Tanner gradient pulse MR experiment, is analyzed in detail. At short diffusion times the presence of diffusion-restrictive barriers (membranes) reduces effective diffusivity near the membranes and leads to an inhomogeneous spin magnetization distribution (the edge-enhancement effect). In this case, the signal reveals a quasi-two-compartment behavior and can be empirically modeled remarkably well by a biexponential function. The current results provide a framework for interpreting experimental MR data on various phenomena, including water diffusion in giant axons, metabolite diffusion in the brain, and hyperpolarized gas diffusion in lung airways.

Protonic conductivity in KH2PO4 family studied by NMR

Protonic conductivity in the KH2PO4 family of hydrogen-bonded insulators has been studied via the proton NMR lineshape and the decay of the NMR transverse nuclear spin magnetization of 87Rb nuclei in a spatially inhomogeneous electric field gradient. A large difference in the time scales of the decays of Hahn spin echo and Carr–Purcell echo train was observed. The Hahn echo decay curves show an exponential decay with the exponent proportional to the cube of time that is characteristic for diffusive motions. The proton NMR spectrum demonstrates the presence of mobile protons which produce a motionally narrowed absorption line above 220 K. The same effects were observed in several pure ferroelectric and antiferroelectric members of the KH2PO4 family as well as in their mixtures – the proton glasses.

Self-Diffusion in IcosahedralAl72.4Pd20.5Mn7.1and Phason Percolation at Low Temperatures Studied byA27lNMR

Aluminum self-diffusion in a monodomain icosahedral quasicrystal ${\mathrm{Al}}_{72.4}{\mathrm{Pd}}_{20.5}{\mathrm{Mn}}_{7.1}$ has been studied at temperatures between 4 and 400 K by ${}^{27}\mathrm{Al}$ NMR measurements of the decay of transverse nuclear spin magnetization in a spatially inhomogeneous electric field gradient. Temperature dependence of the renormalized diffusion constant can be explained in the framework of the Kalugin-Katz phason-assisted diffusion model where the atomic jumps occur as a result of the presence of matching-rules violation defects. The observed self-diffusion process resembles a percolation transition with the percolation threshold at 72 K.

Theory of the SCOTCH experiment for reactions involving radical pairs

SCOTCH (spin coherence transfer in chemical reactions) is a 2D NMR experiment in which cross peaks connect the chemical shifts of nuclei in starting compound and reaction product. If a reaction proceeds via a radical pair, the intensity of the cross peaks may be reduced due to dephasing processes occurring in the paramagnetic intermediate. This reduction is different for transverse and longitudinal nuclear spin magnetization. First, an expression for the amount of transferred magnetization and thus for the 2D cross peak intensity is derived in terms of the strength of the electron-nuclear hyperfine interaction and the lifetime of the radical pair; the influence of electron and nuclear spin relaxation is discussed as well. Then, the nuclear spin magnetization that is transferred to product is expressed as function of the reencounter probability and of the expectation value of finding the radical pair in a singlet electronic state. Two models that describe the reencounter probability are considered: an exponential model and a diffusion model. The resulting theoretical expressions are compared with experimental results of the SCOTCH experiment performed on the photochemical reaction of methyl tert-butyl ketone with tetrachloromethane.

Fluxon thermal motion detected by nuclear spin echo decay measurements: 89Y NMR in YBa2Cu3O7.

A new NMR approach was devised to investigate thermal motion of vortices based on the observation of dephasing effects of the transverse nuclear magnetization below ${\mathit{T}}_{\mathit{c}}$. The effect was observed in $^{89}\mathrm{Y}$ spin-spin relaxation time ${\mathit{T}}_{2}$ measurements of ${\mathrm{TBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ for two crystal orientations with respect to the applied magnetic field. The data were fitted with a simple model of diffusion of $^{89}\mathrm{Y}$ nuclei with respect to the local magnetic field gradient generated by the moving vortices. Small values of a thermally activated effective diffusion constant of the vortices' motion well below ${\mathit{T}}_{\mathit{c}}$ with reasonable activation energies were derived.

Two-dimensional inverse Laplace transform NMR: altered relaxation times allow detection of exchange correlation

A pulse sequence for a two-dimensional inverse Laplace transform NMR experiment is proposed and demonstrated. The experiment is analogous to the two-dimensional Fourier transform protocol called EXSY, but detects exchange by monitoring alterations in the transverse relaxation time rather than the NMR frequency. The sequence may be useful for measurement of exchange and diffusion of water in vivo and for detecting slow exchange phenomena in glassy polymers. The application of the Fourier transform (FT) to nuclear magnetic resonance spectroscopy' and its extension to multiple dimensions2 has radically altered modern chemistry and medical science; NMR imaging3 and the determination of the structure of proteins in solution4 are two important advances that would be either impossible or orders-of-magnitude more difficult without the FT. The crucial feature of the transformation is the increase in availablespectroscopic resolution. If two signals havedifferent precession frequencies VI and vs as a result of some coherent interaction, the time evolution of the total transverse nuclear magnetization M(r) will be the sum of the time evolution of these two species (M1(t) + Ms(t)), and at every point in time it will be dependent on VI and us. However, the FT will still show resolved signals in frequency space, thus discriminating the contributions of the two species.

Two-dimensional spatially selective spin inversion and spin-echo refocusing with a single nuclear magnetic resonance pulse

A new class of nuclear magnetic resonance (NMR) pulses that provides simultaneous spatially selective inversion of nuclear spins in two dimensions following a single pulse application is described and demonstrated. The two-dimensional selective pulses consist of a single square- or amplitude-modulated π rf pulse applied in the presence of an amplitude-modulated magnetic field gradient that reorients through the two dimensions during the rf pulse. For example, square and Gaussian rf pulses produce sharply peaked sombrero-, egg-carton-, and stalagmite-shaped profiles of spin inversion in the xz plane when applied in the presence of a gradient that rotates or describes a figure eight in the xz plane. The theoretical profiles, computed by numerical integration of the Bloch equation, are in good agreement with experimental results obtained by incorporating the pulses into a conventional NMR imaging sequence. The pulses are directly applicable to restricted field-of-view high-resolution imaging for the amelioration of aliasing signal artifacts, and when combined with one-dimensional localized phosphorus (31P) chemical shift spectroscopy techniques that employ surface detection coils, should permit complete three-dimensionally localized 31P NMR spectroscopy. The π pulses provide similar two-dimensional spatial selectivity of the transverse nuclear magnetization when used for refocusing Hahn spin echoes.

Correction of the expression for the form of the nuclear-induction signal in linear magnetic systems

A method of modifying the expression for the drop in transverse nuclear magnetization is considered; the method is based on taking account of the linearity of the system of nuclear moments in a weak magnetic field. It is shown that, in the presence of a broad spectrum of correlation times, a multicomponent structure of the drop may arise.

Low density relaxation of the transverse nuclear magnetization in molecular gases

A time-domain analysis of the relaxation of the transverse nuclear magnetization in low density gases is presented. Attention is focussed upon the transition region where the phenomenological Bloch equation breaks down and non-exponential behaviour sets in. The analysis is based upon a perturbation parameter which makes it possible to obtain analytical results in an extremely, simple way and to a good degree of approximation. It corroborates the results recently obtained in the frequency-domain analysis of the same problem.

Effect of slow molecular motion on damping of transverse nuclear magnetization in amorphous polymers

The NMR pulse method was used to study the variation of the form of damping of transverse magnetization (DTM) in polybutadiene, polyisobutylene and natural rubber in the range of glass temperature of polymer up to 200° and in polyethyleneglycol at 75°, according to molecular weight. To describe the relations derived a simple model was introduced, in which relaxation was caused by random local fields, these fields having two components—a rapid and a slow fluctuation component. It is assumed furthermore, that fluctuations are Gaussian random Markoff processes. The ratios derived give a satisfactory description of all experimental curves of DTM.

The spin echo method as a means of determining molecular weight and molecular weight distribution

The relationship between nuclear spin-spin relaxation times and temperature, molecular weight and viscosity was investigated for a number of polymers. The results show that it is theoretically possible to determine M on the basis of viscosity and relaxation time measurements, while MWD can be determined from the mode of examination of transverse nuclear magnetization vectors.