Low-temperature susceptibility of the noncentrosymmetric superconductor CePt3Si

We report ac susceptibility measurements of polycrystalline CePt(3)Si down to 60 mK and in applied fields up to 9 T. In a zero applied field, a full Meissner state emerges at temperatures T/T(c) < 0.3, where T(c) = 0.65 K is the onset transition temperature. Though transport measurements show a relatively high upper critical field B(c2) approximately 4-5 T, the low-temperature susceptibility chi(') is quite fragile to the applied field, with chi(') diminishing rapidly in fields of a few kG. Interestingly, the field dependence of chi(') is well described by the power law 4pichi(') + 1 = (B/B(c))(1/2), where B(c) is the field at which the onset of resistance is observed in transport measurements.     

Electric field measurements and computational modeling at ultrahigh-field MRI

While magnetic resonance images essentially contain a map of the both circularly polarized components of the RF transverse magnetic fields (B(1) field), the thermal heat and electromagnetic power deposition is generated by the associated electric fields. Measurement of electric field distributions/intensities across a sample yields an indirect indication of possible cause of heating within the sample and potentially enables the detection of "hot spots," which can be present within inhomogeneous radiofrequency (RF) fields, such as the case with magnetic resonance imaging at high field strength. As a result, establishing a valid technique for direct measurements of the electric field and its correlation, obtained using computational electromagnetics, is essential in assessing (1) the safety of the RF coil designs and (2) the validity of the calculations. In this work, a probe was built and used to measure the transverse electric field (E(1) field) distributions within an empty 8 T (tuned to 340 MHz) RF head coil and within a saline water phantom loaded in the same coil. The measured E(1) field distributions were favorably compared to the distributions obtained utilizing a finite difference time domain in-house package.     

Low field CIDNP of amino acids and proteins: characterization of transient radicals and NMR sensitivity enhancement

A new method for measuring and exploiting the magnetic field dependence of chemically induced dynamic nuclear polarization (CIDNP) is described. A solution of an amino acid or protein together with a flavin photosensitizer is irradiated with laser light at a position in the bore of a superconducting NMR magnet where the field is between 0.1 T and 7.0 T. The polarized sample is then transferred by rapid injection into an NMR tube at the centre of the magnet (at 9.4 T), where the spectrum is recorded. The observed ¹H CIDNP field dependence of tyrosine agrees well with the diffusion model of the radical pair mechanism. The field dependence of histidine, tryptophan and methionine CIDNP allows the g values of the transient radicals responsible for the polarization to be determined. Experiments in which amino acids compete for the photoexcited flavin indicate that methionine residues could be used as probes of surface accessibility, especially if the polarization is generated in low fields (～ 0.7 T) and detected in high fields (≥ 9.4 T). Possible extensions of the technique to study protein folding and the structures of partially denatured states of proteins are discussed.     

Artifacts in T1 rho-weighted imaging: compensation for B(1) and B(0) field imperfections

The origin of spin locking image artifacts in the presence of B(0) and B(1) magnetic field imperfections is shown theoretically using the Bloch equations and experimentally at low (omega(1) < Delta omega(0)), intermediate (omega(1) approximately Delta omega(0)) and high (omega(1) > Delta omega(0)) spin locking field strengths. At low spin locking fields, the magnetization is shown to oscillate about an effective field in the rotating frame causing signature banding artifacts in the image. At high spin lock fields, the effect of the resonance offset Deltao mega(0) is quenched, but imperfections in the flip angle cause oscillations about the omega(1) field. A new pulse sequence is presented that consists of an integrated spin echo and spin lock experiment followed by magnetization storage along the -z-axis. It is shown that this sequence almost entirely eliminates banding artifacts from both types of field inhomogeneities at all spin locking field strengths. The sequence was used to obtain artifact free images of agarose in inhomogeneous B(0) and B(1) fields, off-resonance spins in fat and in vivo human brain images at 3 T. The new pulse sequence can be used to probe very low frequency (0-400 Hz) dynamic and static interactions in tissues without contaminating B(0) and B(1) field artifacts.     

Electromagnetic fields that remain totally polarized under propagation

It is remarked that, in general, a totally polarized field becomes partially polarized at the output of an optical system, even though the field propagates through non-polarizing devices. Two kinds of fields are shown in the present work, which maintain the totally-polarized character at the output of any (deterministic) non-polarizing first-order optical system. This property is satisfied by uniformly totally polarized beams and also by those fields whose electromagnetic degree of coherence [T. Setälä, J. Tervo, A.T. Friberg, Opt. Lett. 29 (2004) 328.] equals 1.     

Evidence for static magnetism in the vortex cores of ortho-II YBa2Cu3O6.50

Evidence for static alternating magnetic fields in the vortex cores of underdoped YBa2Cu3O6+x is reported. Muon spin rotation measurements of the internal magnetic field distribution of the vortex state of YBa2Cu3O6.50 in applied fields of H = 1 T and H = 4 T reveal a feature in the high-field tail of the field distribution which is not present in optimally doped YBa2Cu3O6.95 and which fits well to a model with static magnetic fields in the vortex cores. The magnitude of the fields is estimated to be 18(2) G and decreases above T = 10 K. We discuss possible origins of the additional vortex core magnetism within the context of existing theories.     

Superconducting fluctuations and the pseudogap in the slightly overdoped high- T(c) superconductor TlSr2CaCu2O6.8: high magnetic field NMR studies

From measurements of the 63Cu Knight shift ( K) and the nuclear spin-lattice relaxation rate ( 1/T1) under magnetic fields from zero up to 28 T in the slightly overdoped high- T(c) superconductor TlSr2CaCu2O6.8 ( T(c) = 68 K), we find that the pseudogap behavior, i.e., the reductions of 1/T1T and K above T(c) from the values expected from the normal state at high T, is strongly field dependent and follows a scaling relation. We show that this scaling is consistent with the effects of the Cooper pair density fluctuations. The present finding contrasts sharply with the pseudogap property reported previously in the underdoped regime where no field effect was seen up to 23.2 T. The implications are discussed.     

Vortex glass transition and quantum vortex liquid at low temperature in a thick a- MoxSi1-x film

We present measurements of ac complex resistivity, as well as dc resistivity, for a thick amorphous MoxSi1-x film at low temperatures ( T>0.04 K) in various constant fields B. We find that the vortex glass transition (VGT) persists down to T approximately 0.04Tc0 up to B approximately 0.9Bc2(0), where Tc0 and Bc2(0) are the mean-field transition temperature and upper critical field at T = 0, respectively. In the limit T-->0, the VGT line Bg(T) extrapolates to a field below Bc2(0), while the dc resistivity rho(T) tends to the finite nonzero value in fields just above Bg(0). These results indicate the presence of a metallic quantum vortex liquid at T = 0 in the regime Bg(0)相似文献   

A compact high-speed mechanical sample shuttle for field-dependent high-resolution solution NMR

Analysis of NMR relaxation data has provided significant insight on molecular dynamic, leading to a more comprehensive understanding of macromolecular functions. However, traditional methodology allows relaxation measurements performed only at a few fixed high fields, thus severely restricting their potential for extracting more complete dynamic information. Here we report the design and performance of a compact high-speed servo-mechanical shuttle assembly adapted to a commercial 600 MHz high-field superconducting magnet. The assembly is capable of shuttling the sample in a regular NMR tube from the center of the magnet to the top (fringe field ～0.01 T) in 100 ms with no loss of sensitivity other than that due to intrinsic relaxation. The shuttle device can be installed by a single experienced user in 30 min. Excellent 2D-(15)N-HSQC spectra of (u-(13)C, (15)N)-ubiquitin with relaxation at low fields (3.77 T) and detection at 14.1T were obtained to illustrate its utility in R(1) measurements of macromolecules at low fields. Field-dependent (13)C-R(1) data of (3,3,3-d)-alanine at various field strengths were determined and analyzed to assess CSA and (1)H-(13)C dipolar contributions to the carboxyl (13)C-R(1).     

Efficiency of paramagnetic relaxation enhancement in off-resonance rotating frame

Transferring from laboratory frame to off-resonance rotating frame for the (1)H spin can compensate the relaxivity loss for paramagnetic agents at the magnetic field strength higher than 3 Tesla and enhance water relaxation rate constant significantly. A comprehensive theory for calculating the relaxation rate constants in the off-resonance rotating frame is described. This theory considers the contributions from both inner shell and outer shell water. The derived relaxation rate constants and relaxation enhancement efficiency as a function of the magnetic field strength and the effective field parameters are directly correlated to the structures, dynamics and environments of paramagnetic agents. To validate the theoretical predictions, we have measured the relaxation enhancement efficiency for a series of macromolecule conjugated gadolinium chelates at 9.4 Tesla. The experimental results confirmed the theoretical predictions. The theory also predicts the relaxation enhancement for T(2)-type paramagnetic agents at high magnetic fields. Promising fields of applications include situations where T(1)- or T(2)-type paramagnetic agents are used for labeling molecular/cellular events.     

Electrically detected electron spin resonance in a high-mobility silicon quantum well

The resistivity change due to electron spin resonance (ESR) absorption is investigated in a high-mobility two-dimensional electron system formed in a Si/SiGe heterostructure. Results for a specific Landau level configuration demonstrate that the primary cause of the ESR signal is a reduction of the spin polarization, not the effect of electron heating. The longitudinal spin relaxation time T1 is obtained to be of the order of 1 ms in an in-plane magnetic field of 3.55 T. The suppression of the effect of the Rashba fields due to high-frequency spin precession explains the very long T1.     

Microimaging at 14 tesla using GESEPI for removal of magnetic susceptibility artifacts in T(2)(*)-weighted image contrast.

In magnetic resonance imaging (MRI), T(2)(*)-weighted contrast is significantly enhanced by extremely high magnetic field strength, offering broad potential applications. However, the T(2)(*)-weighted image contrast distortion and signal loss artifact arising from discontinuities of magnetic susceptibility within and around the sample are also increased, limiting utilization of high field systems for T(2)(*)-weighted contrast applications. Due to the B(0) dependence of the contrast distortions and signal losses, and the heterogeneity of magnetic susceptibility in biological samples, magnetic susceptibility artifacts worsen dramatically for in vivo microimaging at higher fields. Practical applications of T(2)(*)-sensitive techniques enhanced by higher magnetic fields are therefore challenged. This report shows that magnetic susceptibility artifacts dominate T(2)(*)-weighted image contrast at 14 T, and demonstrates that the GESEPI (gradient echo slice excitation profile imaging) technique effectively reduces or eliminates these artifacts at long TE in the highest field (14 T) currently available for (1)H imaging.     

Field-induced freezing of a quantum spin liquid on the kagome lattice

We report (17)O NMR measurements in the S=1/2 (Cu(2+)) kagome antiferromagnet Herbertsmithite ZnCu(3)(OH)(6)Cl(2) down to 45 mK in magnetic fields ranging from 2 to 12 T. While Herbertsmithite displays a gapless spin-liquid behavior in zero field, we uncover an instability toward a spin-solid phase at sub-Kelvin temperature induced by an applied magnetic field. The latter phase shows largely suppressed moments ?0.1 μ(B) and gapped excitations. The H-T phase diagram suggests the existence of a quantum critical point at the small but finite magnetic field μ(0)H(c)=1.55(25) T. We discuss this finding in light of the perturbative Dzyaloshinskii-Moriya interaction which was theoretically proposed to sustain a quantum critical regime for the quantum kagome Heisenberg antiferromagnet model.     

Gradient-induced longitudinal relaxation of hyperpolarized noble gases in the fringe fields of superconducting magnets used for magnetic resonance

When hyperpolarized noble gases are brought into the bore of a superconducting magnet for magnetic resonance imaging (MRI) or spectroscopy studies, the gases must pass through substantial field gradients, which can cause rapid longitudinal relaxation. In this communication, we present a means of calculating this spatially dependent relaxation rate in the fringe field of typical magnets. We then compare these predictions to experimental measurements of (3)He relaxation at various positions near a medium-bore 2-T small animal MRI system. The calculated and measured relaxation rates on the central axis of the magnet agree well and show a maximum (3)He relaxation rate of 3.83×10(-3) s(-1) (T(1)=4.4 min) at a distance of 47 cm from the magnet isocenter. We also show that if this magnet were self-shielded, its minimum T(1) would drop to 1.2 min. In contrast, a typical self-shielded 1.5-T clinical MRI scanner will induce a minimum on-axis T(1) of 12 min. Additionally, we show that the cylindrically symmetric fields of these magnets enable gradient-induced relaxation to be calculated using only knowledge of the on-axis longitudinal field, which can either be measured directly or calculated from a simple field model. Thus, while most MRI magnets employ complex and proprietary current configurations, we show that their fringe fields and the resulting gradient-induced relaxation are well approximated by simple solenoid models. Finally, our modeling also demonstrates that relaxation rates can increase by nearly an order of magnitude at radial distances equivalent to the solenoid radius.     

Low field RYDMR: effects of orthogonal static and oscillating magnetic fields on radical recombination reactions

Reactions involving spin correlated radical pairs as intermediates are known to be sensitive to applied static and/or oscillating magnetic fields. In the reaction yield detected magnetic resonance (RYDMR) technique, an electromagnetic field in resonance with the electron Zeeman splitting produced by a strong static field is used to perturb the singlet ? triplet interconversion of the radical pair and so to affect the yield of geminate recombination. New experiments are described in which weak radiofrequency fields (? 300μT) in the frequency range 1–80 MHz are applied to radical ion pairs derived from pyrene and 1,3-dicyanobenzene, in the presence of a weak (? 3.0 mT) static magnetic field. Such experiments test the viability of RYDMR in low fields, provide insight into the crossover region between the zero-field and high field cases, and may give information on the distribution of radical pair lifetimes.     

Magnetic field enhancement of heat transport in the 2D Heisenberg antiferromagnet K2V3O8

The thermal conductivity and heat capacity of single crystals of the spin 1/2 quasi-2D Heisenberg antiferromagnet K(2)V(3)O(8) have been measured from 1.9 to 300 K in magnetic fields from 0 to 8 T. The zero field thermal conductivity data are consistent with resonant scattering of phonons by magnons near the zone boundary. Application of a magnetic field greater than 1 T, however, produces a new magnetic ground state with substantial heat transport by long wavelength magnons.     

Field theory in two-time physics with N=1 supersymmetry

We construct N=1 supersymmetric (SUSY) field theory in 4+2 dimensions compatible with the theoretical framework of two-time (2T) physics and its gauge symmetries. The fields are arranged into 4+2 dimensional chiral and vector supermultiplets, and their interactions are uniquely fixed by SUSY and 2T physics gauge symmetries. In a particular gauge the 4+2 theory reduces to ordinary supersymmetric field theory in 3+1 dimensions without any Kaluza-Klein remnants, but with some additional constraints in 3+1 dimensions of interesting phenomenological relevance. This construction is another significant step in the development of 2T physics as a structure that stands above 1T physics.     

稳态强磁场的细胞生物学效应

随着科学技术的发展以及稳态强磁场在医疗诊断中的广泛应用,人们接触到1 T以上稳态强磁场的机会越来越多,稳态强磁场对人体健康的潜在影响也备受关注.虽然目前由于实验条件的限制,稳态强磁场对动物以及人体的研究报道依然有限,但是细胞作为生物体的基本单位,其研究相对较多.然而由于实验中磁场参数、细胞类型等各种因素的不同,使得稳态强磁场对细胞的影响在不同的研究中存在着差异.因此,本文不仅总结和分析了国内外1 T以上稳态强磁场细胞生物学效应的相关研究,包括细胞取向、增殖、微管和纺锤体等,而且对现有研究结果进行比较和概括,并对可能造成实验差异的因素进行分析,例如磁场强度和细胞类型等,从而为下一步研究稳态强磁场下的细胞生物学效应提供基础和依据.     

Thermal studies of the spin liquid state and analog to the He melting curve in a geometrically frustrated magnet

Gadolinium gallium garnet, Gd₃Ga₅O₁₂ (GGG) has an extraordinary low-temperature phase diagram. Although the Curie–Weiss temperature of GGG is −2 K, GGG shows no long-range order down to T0.4 K. At low temperatures GGG has a spin glass phase at low fields (0.1 T), a field-induced long-range ordered antiferromagnetic state at fields of between 0.7 and 1.3 T, and, at intermediate fields, an apparent spin-liquid state without long-range order. We have characterized the intermediate field (IF) state through heat capacity, thermal conductivity, and magnetocaloric measurements. Our results show a sharp high-field phase boundary of the thermal irreversibility of the spin glass phase of GGG implying that the intermediate field phase is distinct from the spin glass. The lower field boundary of the AFM phase is shown to have distinct minimum at T0.2 K, in analogy to the minimum in the melting curve of ⁴He. The existence of such a minimum is confirmed by measurements of the latent heat of the transition below that temperature.     

Diamagnetic shielding and Meissner effect in the high Tc superconductor Sr0.2La1.8CuO4

SQUID susceptibility measurements of the high T_c superconductor Sr_0.2La_1.8CuO₄, with a 40 K resistive onset, are reported for fields from 1 μT to 4.5 T (10 mG to 45 kG). Data show an average lower critical field of 30 mT at 5 K, but with nonlinearities appearing as low as 100 μT. Zero field cooling and subsequent application of a field shows a diamagnetic shielding effect reaching 100% in the lowest fields with a 37 K onset and 31 K midpoint. The observed Meissner effect in single phase material exceeds 40% but decreases with oxygen annealing even though T_c increases.