High Relaxation Rate Research Articles

Effect of different magnetic nanoparticle coatings on the efficiency of stem cell labeling

Maghemite nanoparticles with various coatings were prepared by the coprecipitation method and characterized by transmission electron microscopy, dynamic light scattering and IR in terms of morphology, size, polydispersity and surface coating. The labeling efficiency and the viability of both rat and human mesenchymal stem cells labeled with Endorem ®, poly( l-lysine) (PLL)-modified Endorem ®, uncoated γ-Fe 2O 3, d-mannose-, PLL- or poly( N,N-dimethylacrylamide) (PDMAAm)-coated γ-Fe 2O 3 nanoparticles were compared. Coated γ-Fe 2O 3 nanoparticles labeled cells better than did Endorem ®. High relaxation rates and in vitro magnetic resonance imaging of cells labeled with coated nanoparticles showed clearly visible contrast compared with unlabeled cells or cells labeled with Endorem ®.

On-chip green silica upconversion microlaser

We demonstrate an erbium-doped silica toroidal microcavity upconversion laser on a silicon chip lasing in the visible spectral range (510-580 nm). The microcavity is pumped at 1458 nm by a tapered optical fiber coupled to the cavity and the lasing threshold is 690 muW. Lasing is observed at room temperature despite the high nonradiative relaxation rates of Er in pure silica that usually precludes upconversion lasing from higher excited states. This is attributed to the very high circulating pump power in the high-Q microcavity (Q>10(7)).

Dynamics of Multiferroic Domain Wall in Spin-Cycloidal FerroelectricDyMnO3

We report the dielectric dispersion of the giant magnetocapacitance (GMC) in multiferroic DyMnO3 over a wide frequency range. The GMC is found to be attributable not to the softened electromagnon but to the electric-field-driven motion of multiferroic domain wall (DW). In contrast to conventional ferroelectric DWs, the present multiferroic DW motion holds an extremely high relaxation rate of approximately 10;{7} s;{-1} even at low temperatures. This mobile nature as well as the model simulation suggests that the multiferroic DW is not atomically thin as in ferroelectrics but thick, reflecting its magnetic origin.

Toward the first study of chemical reaction dynamics of Mu with vibrational-state-selected reactants in the gas phase: The reaction by stimulated Raman pumping

Abstract Stimulated Raman pumping (SRP) is used to produce H 2 in its first vibrational state, in order to measure, for the first time, the Mu + H 2 * ( v = 1 ) → MuH + H reaction rate at room temperature, as a prototypical example of new directions in gas-phase muonium chemistry, utilizing the pulsed muon beam and a new dedicated laser system at the RIKEN/RAL Laboratory. Reported here is a preliminary result but the final results are expected to provide definitive new tests of reaction rate theory on the highly accurate H 3 potential energy surface. The major difficulty in this experiment, compared to the standard SRP process, is to ensure a homogeneous excitation over a volume of several cm 3 and of sufficient intensity to ensure a measurable Mu relaxation rate. The techniques used to accomplish this are described. The experiment utilizes the 2nd harmonic output of a Nd:YAG laser (532 nm) with pulse energies up to 500 mJ at a repetition rate of 25 Hz. Different optical setups have been constructed and tested in order to optimize the number of laser-pumped H 2 molecules and their overlap with the stopping profile of the muon beam in the reaction cell (total volume ∼ 100 × 40 × 4 mm 3 ). The first result of this experiment gives a measured relaxation rate due to laser excitation of λ * = 0.085 ± 0.051 μ s - 1 , consistent with theory but limited by both low statistics and particularly a high background relaxation rate.

Dynamic phase diagram derived from large deformation non-linear mechano-optical behavior of polyethylene naphthalate nanocomposites

The effect of nanoparticles on the mechano-optical behavior of PEN was investigated using a real time true stress–true strain birefringence measuring system [Toki S, Valladares D, Sen TZ, Cakmak M. ANTEC Proc 2001:1830; Koike Y, Cakmak M. Polymer 2003;44:4249]. The large deformation stress–optical behavior revealed that there are at most three distinct regimes that may be augmented by an initial additional glassy component at lower temperatures and/or high rates. The presence of nanoparticles did not change the birefringence development of PEN before strain induced crystallization. However, after strain induced crystallization occurred in PEN, the birefringence increase was found to be diminished due primarily to the decrease in the amount of crystallinity and amorphous chain orientation with the addition of nanoparticles. The final structure and deformation behavior of the nanocomposites have been mapped out in a dynamic phase diagram as a function of stretching temperature, rate and nanoparticle content. This dynamic phase diagram shows that the material undergoes three critical structural transitions: (i) nematic–crystalline transition wherein the material stretched below a certain temperature does not undergo orientation-induced crystallization but develops a highly ordered nematic state. (ii) A kinematic transition wherein the material transforms from a ‘structured liquid’ to a ‘true liquid’ state exhibited by the disappearance of the initial glassy component as the material becomes devoid of the inherent structure due to segmental correlations. (iii) While unfilled PEN remains amorphous at high temperatures due to high relaxation rates combined with decreased thermal crystallizability, the presence of nanoplatelets was found to facilitate strain induced crystallization even at such high temperatures primarily due to the decrease in orientation relaxation in their presence.

Spin–orbit dependence on carrier momentum in (1 1 0) GaAs quantum wells

Surface acoustic waves (SAW) are employed to transport optically generated spin ensembles over distances exceeding 60 μ m in (1 1 0) GaAs quantum wells (QW). The dependence of the spin–orbit (SO) coupling on carrier momentum is investigated by using SAWs to transport spins with well-defined velocity along different directions in the QW plane. For transport along the [0 0 1] direction, the high relaxation rates for the in-plane spin component lead to fast spin decoherence under a magnetic field. For the [ 1 ¯ 1 0 ] direction, in contrast, a non-zero average value of the SO internal magnetic field retards the longitudinal spin relaxation.

Effect of Glycation on the Structure and Dynamics of DNA: A Critical Spectroscopic Approach

Glycated DNA is considered to be a pathogenic factor for diabetes mellitus. Here we present a novel and preliminary study on normal and glycated (with fructose and glucose-6-phosphate as reducing sugars) human placenta DNA using agarose gel electrophoresis and photon correlation spectroscopy. The former is used to find structural alterations, while the latter is exploited to observe differences in the dynamics between normal (i.e., pure) and glycated DNA molecules. For scattering angles up to 90 degrees , we obtained a quasi-single-exponential relaxation process for the pure DNA, whereas at higher scattering angles the relaxation of pure DNA becomes broader with a stretching parameter beta approximately 0.6 at 130 degrees. Interestingly, for both the glycated DNAs stretched relaxation profiles and higher relaxation rates (Omega) are observed for all scattering angles. Moreover, a separate and very fast relaxation (e.g., relaxation time tau approximately 2 micros at 90 degrees ) can be noticed for both the glycated DNAs at all the studied scattering angles. Thus, the dramatic changes in the relaxation parameters (Omega, tau, and beta) of the glycated DNA show at the molecular level, for the first time, that the structure and dynamics of DNA are strongly affected by glycation. Implications of the results are discussed.

Simultaneous imaging and R2* mapping using a radial multi‐gradient‐echo (rMGE) sequence

To demonstrate a rapid MR technique that combines imaging and R2* mapping based on a single radial multi-gradient-echo (rMGE) data set. The technique provides a fast method for online monitoring of the administration of (super-)paramagnetic contrast agents as well as image-guided drug delivery. Data are acquired using an rMGE sequence, resulting in interleaved undersampled radial k-spaces representing different echo times (TEs). These data sets are reconstructed separately, yielding a series of images with different TEs used for pixelwise R2* mapping. A fast numerical algorithm implemented on a real-time reconstruction platform provides online estimation of the relaxation rate R2*. Simultaneously the images are summed for the computation of a high-resolution image. Convenient high-resolution R2* maps of phantoms and the liver of a healthy volunteer were obtained. In addition to stable intrinsic baseline maps, the proposed technique provides particularly accurate results for the high relaxation rates observed during the presence of (super-)paramagnetic contrast agents. Assuming that the change in R2* is proportional to the concentration of the agent, the technique offers a rough estimate for dynamic dosage. The simultaneous online display of morphological and parametric information permits convenient, quantitative surveillance of contrast-agent administration.

Highly strain-relaxed ultrathin SiGe-on-insulator structure by Ge condensation process combined with H+ irradiation and postannealing

Strain-relaxation process of SiGe-on-insulator (SGOI) structures in the oxidation induced Ge condensation method has been investigated as a function of the SiGe thickness. Complete relaxation was obtained for thick SGOI layers (>100nm). However, the relaxation rates abruptly decreased with decreasing SiGe thickness below 50nm, i.e., the relaxation rate of 30% at 30nm SiGe thickness. In order to improve this phenomenon, a method combined with H+ irradiation with a medium dose (5×1015cm−2) and postannealing (1200°C) has been developed. This successfully achieved the high relaxation rate (70%) in the ultrathin SGOI (30nm).

μSR study of spin fluctuations in RMn 4Al 8 compounds ( [formula omitted], Dy, Ho, Er)

The compounds RMn 4Al 8 are characterized by very weak magnetic interactions between the rare earth (R) sites and the Mn sites. The Mn sites form one-dimensional chains and it likely that this inhibits moment formation and leads to low frequency spin fluctuations. ZF-μSR studies show similar behaviour in all four compounds with R = Tb , Dy, Ho and Er. The relaxation spectrum has two components: the slow relaxation rate peaks below the temperature where the ZFC susceptibility has a cusp, associated with ordering of the Mn moments. The second component has a high relaxation rate that increases rapidly at low temperatures. We tentatively attribute this to the slowing down of the rare earth spin dynamics.

Photoinduced Electron Transfer in Solid C60 Donor/Acceptor Complexes Studied by Light-Induced Electron-Spin Resonance

ABSTRACT Light-induced ESR (LESR) spectroscopy was used to study photoinduced electron transfer (PIET) in the crystals of the C60 donor/acceptor complexes. High-field LESR signals attributed to (g = 1.9984−1.9992 and ΔH = 3−6 G at 100 K) were observed in fullerene complexes with Leuco Crystal Violet (LCV·C60·C6H5Cl, 1), N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPDA·C60, 2), and bianthrone ((BA)2C60, 3), whereas a low-field LESR signal in 2 was attributed to TMPDA•+. Microwave power studies reveal two independent spins in 2 and show high relaxation rates for photoinduced spins on . The LESR signals consist of a major long-lived “persistent” component and a minor “prompt” one, which disappears immediately after light is off. The maximal intensity of the LESR signals was observed for the excitation of 1–3 below 2 eV. This range of photon energy corresponds to direct intermolecular charge transfer from donor to C60 molecules. PIET is temperature activated with Ea = 23, 43 and 49 meV for 1, 2, and 3, respectively. Photoinduced spin susceptibility attains the maximum at 30 K in 1 and decreases with temperature down to 4 K indicating a possible antiferromagnetic interaction of spins, while in 2 it increases with the temperature decrease down to 4 K.

Vortex dynamics in heavy fermion CeCoIn5

Local magnetic measurements utilizing a micro-Hall probe were performed on a superconducting heavy fermion CeCoIn5 single crystal. We show that the critical current follows a power law as predicted by Ginzburg–Landau theory. This behavior is found to be universal in different heavy fermion and high-Tc superconducting materials. Furthermore, we report on remanent magnetization relaxation showing a high relaxation rate with approximately linear temperature dependence. Although qualitatively similar to another undoped heavy fermion, UBe13, this relaxation rate is significantly higher, providing evidence that CeCoIn5 can be grown in the clean limit.

Magnetic properties of geometrically frustrated Zn xLi 1− xV 2O 4

μSR spectroscopy on ZnV 2O 4 shows paramagnetism for T>40 K. For 12 K ⩽T⩽40 K , the paramagnetic state coexists with a dynamically correlated spin-glass-like state whose volume fraction increases as temperature is reduced, reaching 100% around 12 K. Cooling further leads to a highly damped Bessel-type oscillatory pattern which indicates an incommensurate spin-density-wave structure. The high transverse relaxation rate means large local spin disorder, probably preventing true long-range order, which would explain the absence of magnetic Bragg peaks in neutron diffraction. Furthermore, persistent slow spin fluctuations are observed. These features are indicators of frustration and it is concluded that the structural distortion around 50 K removes only some of the frustration. For Zn 0.2Li 0.8V 2O 4 no structural phase transition occurs and full frustration is expected. The μSR data show paramagnetic response above ∼12 K. The relaxation rate follows a critical law on approaching 12 K from above, as does the T 1 relaxation in NMR. Below 12 K a fast relaxing dynamic Gaussian Kubo–Toyabe signal appears, which is the signature of a dynamic disordered magnetic state. Spin freezing is not observed down to 2 K. The formation of this dynamic spin-glass-like state is consistent with a spin-glass transition temperature of ∼10 K deduced from specific heat and magnetisation measurements.

On the appearance of bile in clinical MR cholangiopancreatography.

To study the appearance of bile in clinical MR cholangiopancreatography (MRCP) with special reference to its chemical and physical properties. Gallbladder bile was collected during surgery from 38 patients and studied with respect to chemical constituents. The relaxation rates 1/T1 and 1/T2 of bile were also determined in vitro. In 16 of these 38 patients, abdominal imaging was performed using MRCP as well as T1-weighted GE sequences. For 9 of the 13 chemical parameters studied, a positive significant correlation with 1/T1 as well as 1/T2 was found. The median relaxation rates 1/T1 and 1/T2 were 0.76 and 1.48 s-1, respectively. The corresponding ranges were 0.38-3.13 s-1 and 0.70-5.75 s-1, respectively. On the MRCP images a few patients showed gallbladder of poor visibility due to low signal-to-noise ratio. This coincided with a high relaxation rate 1/T2 of bile. On the T1-weighted GE sequences a few patients showed hyperintense gallbladder relative to liver, coinciding with high relaxation rates 1/T1 of bile. Bile was found to show a large interindividual variation with respect to relaxation rates 1/T1 and 1/T2. The relaxation rates increased with increasing amounts of substances in the bile. For some patients (11%) MRCP imaging is unsuccessful due to high relaxation rate of bile.

On the appearance of bile in clinical MR cholangiopancreatography

To study the appearance of bile in clinical MR cholangiopancreatography (MRCP) with special reference to its chemical and physical properties. Gallbladder bile was collected during surgery from 38 patients and studied with respect to chemical constituents. The relaxation rates 1/T1 and 1/T2 of bile were also determined in vitro. In 16 of these 38 patients, abdominal imaging was performed using MRCP as well as T1-weighted GE sequences. For 9 of the 13 chemical parameters studied, a positive significant correlation with 1/T1 as well as 1/T2 was found. The median relaxation rates 1/T1 and 1/T2 were 0.76 and 1.48 s-1, respectively. The corresponding ranges were 0.38-3.13 s-1 and 0.70-5.75 s-1, respectively. On the MRCP images a few patients showed gallbladder of poor visibility due to low signal-to-noise ratio. This coincided with a high relaxation rate 1/T2 of bile. On the T1-weighted GE sequences a few patients showed hyperintense gallbladder relative to liver, coinciding with high relaxation rates 1/T1 of bile. Bile was found to show a large interindividual variation with respect to relaxation rates 1/T1 and 1/T2. The relaxation rates increased with increasing amounts of substances in the bile. For some patients (11%) MRCP imaging is unsuccessful due to high relaxation rate of bile.

Electron localization in(7×7)reconstructed and hydrogen-covered Si(111) surfaces as seen by NMR on adsorbed Li

The observation of ''Korringa-like nuclear-spin-lattice relaxation of 8 Li probe atoms, adsorbed at extremely low coverages on the Si(111)-(7×7) surface (10 - 4 ML and below) points to the existence of a correlated two-dimensional electron gas. The observed high relaxation rates as compared to adsorption of 8 Li on metals are in accordance with an enhanced electron localization in the adatom dangling bonds. For 8 Li adsorbed on semiconducting hydrogen covered Si(111) surfaces (now at coverages around 10 - 3 ML) ''Korringa-like spin-lattice relaxation is also observed, quite surprisingly. Independent of the preparation of the hydrogen coverage, i.e., in vacuum or wet chemically terminated, the relaxation rates are moreover all of the same size. This points to a narrow hand at the Fermi energy generated by the adsorbed Li itself. Quantitative ab initio all-electron density-functional calculations for Li coverage as low as 0.04 on the perfectly hydrogen-terminated Si(111) surface together with a qualitative reasoning following Mott's arguments on semiconductor-metal transitions support this view.

Electric quadrupolar contribution to the nuclear spin-lattice relaxation of Ir in Fe.

We report on the first quantitative determination of the electric quadrupolar contribution to the nuclear spin-lattice relaxation in a transition metal. For 186Ir and 189Ir in Fe we have determined the magnetic and the electric quadrupolar part of the relaxation for magnetic fields between 0.01 and 2 T. The quadrupolar part gives information on the role of the orbital motion of the electrons for the relaxation process. Our results prove that the unexpected high relaxation rates in Fe and their magnetic field dependence are due to a nonorbital relaxation mechanism.

Stacking of metamorphic InAlAs/InGaAs heterostructures on GaAs substrate

In this work, two lattice mismatched heterostructures called stacked metamorphic high-electron mobility transistors have been grown showing the feasibility of the metamorphic concept for the vertical integration of structures having different lattice parameters. Molecular-beam epitaxy of linear graded buffer layers allows the relaxation of the mismatch strain by generation of misfit dislocations. X-ray reciprocal space mapping and cross sectional transmission electron microscopy confirm the interest of growing inverse step buffer layers to achieve high relaxation rates with good confinement of misfit dislocations. The quality of these stacked structures was confirmed by photoluminescence, Hall effect measurements, and the dc characteristics of the transistors.

Fast-field-cycling NMR: Applications and instrumentation

Magnetic field cycling in nuclear magnetic resonance (NMR) experiments has been used since the early days of NMR. Originally such time-dependent magnetic field experiments were motivated to study cross relaxation, spin system thermodynamics and indirect detection of quadrupolar resonance. The first apparatus used mechanical or pneumatic systems to shoot the sample between two magnets, the typical “flying time” being a few hundreds of milliseconds. As a natural evolution of the experimental technique and the need to extend its application to samples with higher relaxation rates, faster magnetic field switching devices were developed during the last years. Special electric networks combined with sophisticated air core magnets allowed one to switch magnetic fields between zero and fields of the order of 0.5 T in a few milliseconds. Today we refer to this new generation of instruments as “fast-field-cycling” devices. The technique has been successfully used during the last years to obtain information on the molecular dynamics and order in different materials, ranging from organic solids, metals, polymers, liquid crystals, porous media to biological systems. At present it is also turning to be an important tool for the design of contrast agents for magnetic resonance imaging. Fast field cycling was mainly oriented toT 1 relaxometry as a unique technique offering a dynamic window of several decades, ranging from few kilohertz to several megahertz. However, there exist less conventional applications of the technique that can also provide relevant information concerning molecular dynamics, structure and molecular order. In this article we will briefly deal with basic aspects of the technique, its evolution, present-day relevant applications and the last improvements concerning specialized instrumentation.

Mössbauer spectra of ferrofluids characterized using a many state relaxation model for superparamagnets

Using a many-state relaxation formalism and combining it with an accurate expression for the relaxation time of single-domain ferromagnetic particles and a log-normal size distribution we are able to fit Mössbauer spectra which span temperatures from where a pure hyperfine field distribution is present to where a single broad line occurs due to high relaxation rates. With this model, we have fitted the Mössbauer spectra of two Fe3O4 ferrofluids, from 12 to 180 K. Fits to the ferrofluid with a mean particle size of 4.5 nm yield an anisotropy energy (K) of 3.0±0.1×104 J/m3 with the onset of magnetic relaxation at 30±5 K while data for a mean particle size of 6.0 nm give K=2.4±0.2×104 J/m3 with magnetic relaxation starting at 55±5 K. These blocking temperatures agree with those extrapolated from frequency dependent χac data.