Spin Relaxation Properties Research Articles

MRI profiles over very wide concentration ranges: Application to swelling of a bentonite clay

In MRI investigation of soils, clays, and rocks, mainly mobile water is detected, similarly to that in biological and medical samples. However, the spin relaxation properties of water in these materials and/or low water concentration may make it difficult to use standard MRI approaches. Despite these limitations, one can combine MRI techniques developed for solid and liquid states and use independent information on relaxation properties of water, interacting with the material of interest, to obtain true images of both water and material content. We present procedures for obtaining such true density maps and demonstrate their use for studying the swelling of bentonite clay by water. A constant time imaging protocol provides 1D mapping of the clay distribution in regions with clay concentration above 10 vol%. T 1 relaxation time imaging is employed to monitor the clay content down to 10 −3 vol%. Data provided by those two approaches are in good agreement in the overlapping range of concentrations. Covering five orders of magnitude of clay concentration, swelling of sodium-exchanged bentonite clays from pre-compacted pellets into a gel phase is followed in detail.

Hole spin relaxation inp-type GaAs quantum wires investigated by numerically solving fully microscopic kinetic spin Bloch equations

We investigate the spin relaxation of $p$-type GaAs quantum wires by numerically solving the fully microscopic kinetic spin Bloch equations. We find that the quantum-wire size influences the spin-relaxation time effectively by modulating the energy spectrum and the heavy-hole--light-hole mixing of wire states. The effects of quantum-wire size, temperature, hole density, and initial polarization are investigated in detail. We show that, depending on the situation, the spin-relaxation time can either increase or decrease with hole density. Due to the different subband structure and effects arising from spin-orbit coupling, many spin-relaxation properties are quite different from those of holes in the bulk or in quantum wells, and the intersubband scattering makes a marked contribution to the spin relaxation.

Electrical detection of spin pumping: dc voltage generated by ferromagnetic resonance at ferromagnet/nonmagnet contact

We describe electrical detection of spin pumping in metallic nanostructures. In the spin pumping effect, a precessing ferromagnet attached to a normal metal acts as a pump of spin-polarized current, giving rise to a spin accumulation. The resulting spin accumulation induces a backflow of spin current into the ferromagnet and generates a dc voltage due to the spin dependent conductivities of the ferromagnet. The magnitude of such voltage is proportional to the spin-relaxation properties of the normal metal. By using platinum as a contact material we observe, in agreement with theory, that the voltage is significantly reduced as compared to the case when aluminum was used. Furthermore, the effects of rectification between the circulating rf currents and the magnetization precession of the ferromagnet are examined. Most significantly, we show that using an improved layout device geometry, these effects can be minimized.

The Direct DIVAM Experiment: A Spin Dynamics Analysis

Domain selection in polymer NMR is limited to experiments specifically suited to each structural domain owing to its particular spin dynamics and relaxation properties. The DIVAM experiment can be tuned to select for signal from the domain of interest, making it possible to obtain signals specific to different domains using only one experiment. An early description of this sequence explains this tunability using a simple one-spin-relaxation model, thereby limiting the selection mechanism to incoherent processes and thus ignoring the coherent terms such as chemical shift anisotropy (CSA), dipolar coupling and offset terms. Experiments have shown that when the DIVAM sequence is applied directly to the nucleus of interest, referred to as direct DIVAM (DD), transient behavior is observed in the signal intensity on the sample spinning time scale. This indicates that the coherent terms are involved in the selection process; the exact role of these terms is explored in this work. SIMPSON simulations illustrate that the CSA and offset terms can play a dominant role in domain selection; however, the dipole term was relatively ineffective and required large values before substantial selection was predicted. Using a one-spin-relaxation model, which now includes a chemical shift evolution term, an analytical expression for the signal intensity was provided as a function of interpulse delay (tau), excitation angle (theta), relaxation time (T2), and offset frequency (Deltanu). These indicate that the selection behavior varies substantially with differing time scales and excitation angles. For small angles and long delay times DD behaves primarily as a relaxation filter, whereas for larger angles and short delay times the coherent terms take over dominated by the CSA interactions. The DD sequence can therefore be set to select on the basis of the transverse relaxation rate or the strength of the CSA interaction, depending on the excitation angle used.

Water diffusion-exchange effect on the paramagnetic relaxation enhancement in off-resonance rotating frame

The off-resonance rotating frame technique based on the spin relaxation properties of off-resonance T 1 ρ can significantly increase the sensitivity of detecting paramagnetic labeling at high magnetic fields by MRI. However, the in vivo detectable dimension for labeled cell clusters/tissues in T 1 ρ -weighted images is limited by the water diffusion-exchange between mesoscopic scale compartments. An experimental investigation of the effect of water diffusion-exchange between compartments on the paramagnetic relaxation enhancement of paramagnetic agent compartment is presented for in vitro/ in vivo models. In these models, the size of paramagnetic agent compartment is comparable to the mean diffusion displacement of water molecules during the long RF pulses that are used to generate the off-resonance rotating frame. The three main objectives of this study were: (1) to qualitatively correlate the effect of water diffusion-exchange with the RF parameters of the long pulse and the rates of water diffusion, (2) to explore the effect of water diffusion-exchange on the paramagnetic relaxation enhancement in vitro, and (3) to demonstrate the paramagnetic relaxation enhancement in vivo. The in vitro models include the water permeable dialysis tubes or water permeable hollow fibers embedded in cross-linked proteins gels. The MWCO of the dialysis tubes was chosen from 0.1 to 15 kDa to control the water diffusion rate. Thin hollow fibers were chosen to provide sub-millimeter scale compartments for the paramagnetic agents. The in vivo model utilized the rat cerebral vasculatures as a paramagnetic agent compartment, and intravascular agents (Gd-DTPA) 30–BSA were administrated into the compartment via bolus injections. Both in vitro and in vivo results demonstrate that the paramagnetic relaxation enhancement is predominant in the T 1 ρ -weighted image in the presence of water diffusion-exchange. The T 1 ρ contrast has substantially higher sensitivity than the conventional T 1 contrast in detecting paramagnetic agents, especially at low paramagnetic agent volumetric fractions, low paramagnetic agent concentrations, and low RF amplitudes. Short pulse duration, short pulse recycle delay and efficient paramagnetic relaxation can reduce the influence of water diffusion-exchange on the paramagnetic enhancement. This study paves the way for the design of off-resonance rotating experiments to detect labeled cell clusters/tissue compartments in vivo at a sub-millimeter scale.

Optical Measurement and Control of Spin Diffusion inn-Doped GaAs Quantum Wells

Transient spin gratings are used to study spin diffusion in lightly n-doped GaAs quantum wells at low temperatures. The spin grating is shown to form in the excess electrons from doping, providing spin relaxation and transport properties of the carriers most relevant to spintronic applications. We demonstrate that spin diffusion of the these carriers is accelerated by increasing the density or energy of the optically excited carriers. These results can be used to better understand and even control spin transport in experiments that optically excite spin-polarized carriers.

A resonant spin lifetime transistor

We present a device concept for a spintronic transistor based on the spin relaxation properties a two-dimensional electron gas (2DEG). The device design is very similar to that of the Datta and Das spin transistor. However, our proposed device works in the diffusive regime rather than in the ballistic regime. This eases lithographical and processing requirements. The switching action is achieved through the biasing of a gate contact, which controls the lifetime of spins injected into the 2DEG from a ferromagnetic emitter, thus allowing the traveling spins to be either aligned with a ferromagnetic collector or randomizing them before collection. The device configuration can easily be turned into a memory and a readout head for magnetically stored information.

Rb-Xe spin relaxation in dilute Xe mixtures

We present measurements of spin-relaxation of Rb atoms in He(98%)-N2(1%)-Xe(1%) mixtures similar to those used in magnetic resonance imaging polarizers. The pressure dependence allows us to separate out contributions from Rb-Xe van der Waals molecules and binary collisions. For the first time, we observe the predicted increase in the molecular contribution at high pressure. Our data suggest that the deduced molecular breakup rate has a strong temperature dependence. The realization of magnetic resonance imaging of human subjects using hyperpolarized noble gases @1# has stimulated the intense development of Xe-Rb spin-exchange optical pumping @2# in high pressure buffer gases @3‐5#. The need to minimize the mismatch between the Rb absorption linewidth and the linewidth of inexpensive, high power but broadband diode laser sources requires pressure broadening the Rb resonance lines with many atmospheres of He gas, chosen for its weak spin-relaxation properties @6#. Despite the widespread use of high pressure He for Xe-Rb spin exchange, there exist no systematic studies of spin relaxation and spin exchange rates under such conditions, as most of the pioneering work in Xe spin-relaxation and spin-exchange either had no buffer gas @7# or used N2 buffer gas @8#. Two recent experiments report relaxation rates measured at a single pressure and temperature @4,5#. In this paper we report measurements of spin-relaxation rates at 80 and 150 °C, in

Spin injection and relaxation properties of anisotropic superconductors

We report the spin injection and relaxation properties of the anisotropic YBa 2Cu 3O y(YBCO) superconductor by injection of spin-polarized quasiparticles using a cobalt ferromagnetic injector. The observed current gain depends on the thickness of the Au interlayer and is directly related to the nonequilibrium magnetization due to spin relaxation effects.

Radical Anions of Mono- and Bis-fulleropyrrolidines: An EPR Study

The radical anions of three C60 derivatives (N-methyl-3,4-fulleropyrrolidine, N-methyl-2-(3,6,9-trioxadecyl)3,4-fulleropyrrolidine, and bis-N-methyl-3,4-fulleropyrrolidine are respectively MFP - , tegMFP - , and bisMFP - ) have been investigated by continuous wave (cw) and pulsed EPR in methyltetrahydrofuran and tetrahydrofuran solutions. The EPR spectra for MFP - and tegMFP - show a 1:1:1 triplet, due to the hyperfine coupling with the pyrrolidine nitrogen. The EPR spectrum of bisMFP - shows a superimposition of two signals, each due to a 1:2:3:2:1 quintet. Each quintet is due to the hyperfine coupling with the two pyrrolidine nitrogen atoms. The two signals are attributed to two different conformers of the radical. A direct transfer of spin density from the C60 sphere to the nitrogen atoms is discussed as arising from the bent conformation of the pyrrolidine rings. All the spectra show several weaker satellite lines, due to the radical anions containing a 13 C atom. From their analysis a C 2V symmetry for the spin distribution is obtained in the case of MFP - and tegMFP - . The EPR line widths of these latter radicals at room temperature are larger than those expected on the basis of spin relaxation due to rotational diffusion in solution, and the line widths decrease upon decreasing the temperature. Pulsed EPR measurements of T1 in liquid and frozen solutions show that the EPR line widths are determined by the anomalously short lifetimes of the spin states. The temperature dependence of T1 obeys to the Arrhenius law, giving an activation energy of 7 kJ/mol. The latter results are compared with those obtained in the past for the lone C60 . They are attributed to the closeness of the electronic levels, due to the weak perturbation of the pyrrolidine ring on the symmetrical electronic distribution of C 60. On the other hand, the spin relaxation properties of bisMFP - are shown those expected for a nonsymmetrical radical. The comparison of cw- and pulsed-EPR spectra of MFP - in frozen solutions of MeTHF and THF indicates the presence of aggregates of interacting radicals in THF.

Fourier transform EPR study of N@C 60 in solution

Using pulsed EPR techniques, the electron spin relaxation properties of nitrogen atoms encapsulated in C 60 have been studied. Because of the high symmetry of the cage, most of the conventional relaxation mechanisms otherwise affecting electron spins in solution are absent, resulting in an exceptional homogeneous ERP linewidth of only 2.5 kHz. Apparently, solvent collision-induced deformations of the carbon shell which modulate the zero-field-splitting sensed by the quartet spin state of N@C 60 are the dominant spin relaxation mechanism.

Spin relaxation and magnetic properties of benzo-1,2,3-trithiolium radical cations

Magnetic and dynamics properties of paramagnetic centers in various trithiolium cations were characterized by static magnetic susceptibility measurements and EPR spectroscopy. Magnetically correlated units consisting of at least pairs of spins with ferromagnetic exchange coupling were found in some trithioles. The rates of intramolecular spin diffusion and intermolecular spin hopping were estimated separately. The rates and the anisotropy (v/v=20–300) of spin dynamics were shown to depend on the molecular structure.

Reconstitution of iron-sulfur center FB results in complete restoration of NADP+ photoreduction in Hg-treated Photosystem I complexes from Synechococcus sp. PCC 6301

The FB iron-sulfur cluster is destroyed preferentially by treating Photosystem I complexes with HgCl2(Kojima Y, Niinomi Y, Tsuboi S, Hiyama T and Sakurai H (1987) Bot Mag 100: 243-53). When FB is 95% depleted but FAis quantitatively retained in cyanobacterial PS I complexes, the reduction potential of FA remains highly electronegative (Em=-530 mV, n=1), the EPR spectral and spin relaxation properties of FA and FXremain unchanged, but NADP(+) photoreduction rates decline from 552 to 72 μmol mg Chl(-1) h(-1).When FB is reconstituted with FeCl3, Na2S and β-mercaptoethanol, NADP(+)photoreduction rates recover to 528 μmol mg Chl(-1) h(-1). The correlation between the presence of FBand NADP(+) photoreduction provides direct experimental evidence that this iron-sulfur cluster is required for electron throughput from cytochromec 6 to flavodoxin or ferredoxin in Photosystem I.

Photoexcited EOSIN as an EPR spin probe

Photoinduced spin triplets usually exhibit high spin polarization, which exceeds by orders of magnitude the equilibrium spin polarization found at room temperature. Equivalently, one may speak of a cryogenic effective spin temperature. The spin relaxation properties however still reflect the dynamics of the molecule at room temperature. This allows investigation of room temperature dynamics with a sensitivity normally reached at cryogenic temperatures only. We investigate photoexcited EOSIN embedded in a polymer matrix. The parameters of the triplet spin Hamiltonian are determined from transient nutation EPR experiments at room temperature. The effective spin temperature is found to be 6 K.

EPR characterization of soluble fragments of succinate dehydrogenase from mutant strains of Bacillus subtilis

Succinate dehydrogenase is a membrane-bound metallo-flavo-enzyme containing a bi- (S-1), a tri- (S-3) and a tetranuclear (S-2) iron-sulfur cluster. The catalytic portion of the enzyme contains two distinct subunits designated Fp and Ip. Using concentrated extracts from mutant strains of Bacillus subtilis it was demonstrated, by using low temperature EPR, that cluster S-2 can be assembled in a soluble succinate dehydrogenase. In a mutant with a truncated Ip subunit which lacks 7 of the 11 conserved cysteine residues, cluster S-1 lacked the spin relaxation properties attributable to an adjacent cluster S-2. These data are consistent with a model where one or more cysteine residues from the middle set of 4 conserved cysteines in the Ip subunit are ligands to the tetranuclear cluster. Succinate dehydrogenase; Iron-sulfur cluster; Flavo-enzyme; Metallo-enzyme assembly

Low-temperature properties of nuclear spin relaxation in inorganic glasses

Abstract The temperature and frequency dependence of nuclear spin relaxation has been investigated for different nuclear probes in a large variety of inorganic glasses between 1.5 K and room temperature. Below about 200 K, the data can be interpreted consistently in the framework of thermally-activated excitations of disordered modes (two-level-systems; TLS). The analysis demonstrates the common properties of the TLS in all the glasses. In particular, low attempt frequencies of the TLS excitations were found indicating that a TLS arises probably from a group of atoms.

Consequences of resonant impurity scattering in anisotropic superconductors: Thermal and spin relaxation properties.

We present a systematic discussion of the effect of resonant impurity scattering on anisotropic model states of heavy-fermion superconductors. The impurity scattering is treated in the self-consistent T-matrix approximation including a renormalization of the frequency \ensuremath{\omega} and the quasiparticle energy ${\ensuremath{\xi}}_{k}$. Model states considered include the axial and polar states familiar from superfluid $^{3}\mathrm{He}$ as well as two states, termed hexial and hybrid, occurring in the group-theoretical classification of singlet states in hexagonal symmetry. We calculate the density of states, the critical temperature and the order parameter, the specific heat, the thermal conductivity, and the spin-lattice relaxation rate. Vertex corrections are included in the calculations of two-particle quantities. The observed properties of the prototype Fermi-liquid material ${\mathrm{UPt}}_{3}$ show many qualitative features in common with our model. However, it turns out to be difficult to identify any given state with certainty. An experimental test of our predictions on the behavior as a function of impurity concentration, particularly in the gapless regime at low temperatures, would allow for a more definitive characterization of the superconducting state.

Electron paramagnetic resonance studies of mammalian succinate dehydrogenase. Detection of the tetranuclear cluster S2.

Electron paramagnetic resonance studies of Complex II from the mitochondrial respiratory chain and soluble preparations of succinate dehydrogenase have, for the first time, identified a signal arising from a [4Fe-4S]1+ cluster, S2, in dithionite-reduced samples. Redox titrations, monitored by electron paramagnetic resonance spectroscopy demonstrate that this signal appears at the same midpoint potential as the enhancement of the spin relaxation properties of the [2Fe-2S]1+ center, S1, in both Complex II and reconstitutively active soluble enzyme. The results complement recent magnetic circular dichroism studies of succinate dehydrogenase (Johnson, M. K., Morningstar, J. E., Bennett, D. E., Ackrell, B. A. C., and Kearney, E. B. (1985) J. Biol. Chem. 260, 7368-7378) which assigned cluster S2 as a [4Fe-4S]2+,1+ center and provide evidence for spin interaction between the paramagnetic reduced forms of centers S1 and S2.

Studies on the stabilized ubisemiquinone species in the succinate-cytochrome c reductase segment of the intact mitochondrial membrane system.

1. Evidence is presented for the presence of a stable ubisemiquinone pair in the vicinity of iron-sulphur centre S-3, based on its thermodynamic and spin relaxation properties. 2. These semiquinones are coupled by dipolar interaction; quantitative analysis of the signals of the spin-coupled semiquinones (at pH 7.4) gives midpoint redox potentials E1 (oxidized to semiquinone state) and E2 (semiquinone to fully reduced state) of 140 and 80mV, respectively, for individual ubiquinones. 3. Values of pKS (pK of the semiquinone form) below 6.5 and pKR (pK of the fully reduced ubiquinone) of about 8.0 or above were estimated from the pH-dependence of the midpoint potentials of the spin coupled signals. Thus the ubisemiquinone associated with succinate dehydrogenase (designated as SQS) functions mostly in the anionic form of the physiological pH range. 4. Theonyltrifluoroacetone, a specific inhibitor of the succinate-ubiquinone reductase segment of the respiratory chain, destabilized the intermediate redox state; thus it quenches both the g = 2.00 signal and ubisemiquinone (SQS) and split signals from the spin coupled pair. This inhibitor has no significant effect on another bound ubisemiquinone species present in the cytochrome bc1 region (designated as SQC). 5. The possible function and location of these stabilized ubisemiquinone species were discussed in connection with Site-II energy transduction.

Determination by cadmium-113 nuclear magnetic resonance of the structural basis for metal ion dependent anticooperativity in alkaline phosphatase.

Cadmium-113 nuclear magnetic resonance (113Cd NMR) has been used to probe the binding characteristics of 113Cd2+ to the three classes of metal binding sites in Escherichia coli alkaline phosphatase to help elucidate the molecular origin of the metal ion dependent "half-sites" reactivity exhibited by this dimeric Zn2+ metalloenzyme [Otvos, J.D., Armitage, I.M., Chlebowski, J.F., & Coleman, J.E. (1979) J. Biol. Chem. 254, 4707-4713]. In the absence of phosphate, the first two 113Cd2+ ions added to the apodimer give rise to a single 113Cd resonance (169 ppm), indicating selective binding to the pair of symmetrically disposed A sites. Resonances arising from additional 113Cd2+ bound to the B and C sites cannot be observed; B- and/or C-site occupation also renders the A-site 113Cd resonance undetectable. Both these observations have been attributed to severe chemical exchange broadening in the A-, B-, and C-site 113Cd signals induced by an unknown modulation process(es). Interestingly, covalent phosphorylation of the active-site serine residues abolishes this exchange modulation, allowing three separate resonances to be detected and assigned to 113Cd2+ located at each of the three classes of metal binding sites in the enzyme. By varying the metal composition of the phosphorylated enzyme, we have characterized the correlations that exist between the chemical shifts ana intensities of these 113Cd resonances and the metal occupancies of the A, B, and C sites in the individual subunits. This information has allowed us to conclude that the half-sites phosphorylation of the Cd2 2+ enzyme is accompanied by a slow migration of half the Cd2+ originally located at the A sites to the B sites on the phosphorylated subunits. The driving force for this metal redistribution, which at equilibrium leaves half the subnits devoid of metal ion and thereby incapable of binding phosphate, is apparently the dramatic stabilization of the complex of Cd2+ with the B sites, which was demonstrated to occur in those subunits that become phosphorylated. From the kinetics of both phosphorylation and metal redistribution in Cd2 2+ enzyme, we suggest that population of the A and B sites in a subunit, rather than the A site alone, constitutes the minimum requirement for induction of catalytic function in alkaline phosphatase. The spin relaxation properties of the enzyme-bound 113Cd2+ ions are also briefly discussed.