Nonlocal Spectrum Research Articles

Control of Andreev Bound States Using Superconducting Phase Texture.

Andreev bound states with opposite phase-inversion asymmetries are observed in local tunneling spectra at the two ends of a superconductor-semiconductor-superconductor planar Josephson junction in the presence of a perpendicular magnetic field, while the nonlocal spectra remain phase symmetric. Spectral signatures agree with a theoretical model, yielding a physical picture in which phase textures in superconducting leads localize and control the position of Andreev bound states in the junction, demonstrating a simple means of controlling the position and size of Andreev states within a planar junction.

Nonlocal Timoshenko simply supported beam: Second spectrum and modes

AbstractWe investigate the spectrum of frequencies of a nonlocal simply supported Timoshenko beam. When the mass matrix term is nonsingular, we can find the amplitudes of free vibrations as solutions of a second‐order matrix differential equation. These solutions are given in terms of a fundamental basis involving an impulsive matrix response and its derivative. This latter is given in closed‐form, involving a scalar function, its derivatives and coupling matrices. Simply supported boundary conditions have a nonclassical nature, but the frequencies are natural due to the influence of the nonlocal parameter. From the characteristic equation, we can find two branches of nonlocal frequencies referred to as the first and second spectrum and a critical frequency. This latter is the same cut‐off frequency for waves solutions of the nonlocal beam without boundary conditions. The nonlocal spectra are bounded, in contrast with the unbounded spectra of a local beam. For classifying eigenvalues as simple or double, the scalar wave function is useful. Simple eigenvalues occur when only one of the basic roots of the nonlocal characteristic polynomial are harmonically spaced, and double when both are harmonic and different. The eigenmodes have been found for both branches of the spectrum, and they are always oscillatory. At the critical frequency, the shape modes are determined by a limit process with the scalar function, resulting in occurrence the one pure‐shear mode. For a singular mass matrix, modes will exist for certain values of the nonlocal parameter but they are not unique.

Initial investigation of glucose metabolism in mouse brain using enriched 17 O-glucose and dynamic 17 O-MRS.

In this initial work, the in vivo degradation of 17 O-labeled glucose was studied during cellular glycolysis. To monitor cellular glucose metabolism, direct 17 O-magnetic resonance spectroscopy (MRS) was used in the mouse brain at 9.4T. Non-localized spectra were acquired with a custom-built transmit/receive (Tx/Rx) two-turn surface coil and a free induction decay (FID) sequence with a short TR of 5.4ms. The dynamics of labeled oxygen in the anomeric 1-OH and 6-CH2 OH groups was detected using a Hankel-Lanczos singular value decomposition (HLSVD) algorithm for water suppression. Time-resolved 17 O-MRS (temporal resolution, 42/10.5s) was performed in 10 anesthetized (1.25% isoflurane) mice after injection of a 2.2M solution containing 2.5mg/g body weight of differently labeled 17 O-glucose dissolved in 0.9% physiological saline. From a pharmacokinetic model fit of the H217 O concentration-time course, a mean apparent cerebral metabolic rate of 17 O-labeled glucose in mouse brain of CMRGlc =0.07±0.02μmol/g/min was extracted, which is of the same order of magnitude as a literature value of 0.26±0.06μmol/g/min reported by 18 F-fluorodeoxyglucose (18 F-FDG) positron emission tomography (PET). In addition, we studied the chemical exchange kinetics of aqueous solutions of 17 O-labeled glucose at the C1 and C6 positions with dynamic 17 O-MRS. In conclusion, the results of the exchange and in vivo experiments demonstrate that the C6-17 OH label in the 6-CH2 OH group is transformed only glycolytically by the enzyme enolase into the metabolic end-product H217 O, whereas C1-17 OH ends up in water via direct hydrolysis as well as glycolysis. Therefore, dynamic 17 O-MRS of highly labeled 17 O-glucose could provide a valuable non-radioactive alternative to FDG PET in order to investigate glucose metabolism.

Weak turbulence of Kelvin waves in superfluid He

The physics of small-scale quantum turbulence in superfluids is essentially based on knowledge of the energy spectrum of Kelvin waves, Ek. Here we derive a new type of kinetic equation for Kelvin waves on quantized vortex filaments with random large-scale curvature which describes a step-by-step energy cascade over scales resulting from five-wave interactions. This approach replaces the earlier six-wave theory, which has recently been shown to be inconsistent owing to nonlocalization Solving the four-wave kinetic equation, we found a new local spectrum with a universal (curvature-independent) exponent, Ek∝k−5∕3, which must replace the nonlocal spectrum of the six-wave theory, Ek∝k−7∕5 in any future theory, e.g., when determining the quantum turbulence decay rate, found by Kosik and Svistunov under an incorrect assumption of locality of energy transfer in six-wave interactions.

In vivo lactate signal enhancement using binomial spectral‐selective pulses in selective MQ coherence (SS‐SelMQC) spectroscopy

Tumor vasculature and tissue oxygen pressure can influence tumor growth, metastases, and patient survival. Elevated levels of lactate may be observed during the process of aggressive tumor development accompanied by angiogenesis (the evolution of the microenvironment). The noninvasive MR detection of lactate in tumor tissues as a potential biomarker is difficult due to the presence of co-resonating lipids that are present at high concentrations. Methods were previously reported for lactate editing using the SELective Multiple Quantum Coherence (SelMQC) method. Here we report a sequence "SS-SelMQC," Spectral-Selective SelMQC, which is a modified version of SelMQC using binomial pulses. Binomial pulses were employed in this editing sequence for frequency excitation or inversion of selective lactate resonances. Lactate detection has been demonstrated using SS-SelMQC, both in vitro (30 mM lactate/H(2)O doped with 25 microM Gd-DTPA) and in vivo (Dunning R3337-AT prostate tumors), and compared to similar measurements made with SelMQC. Lactate areas were measured from nonlocalized spectra, one-dimensional (1D) localized spectra, and two-dimensional chemical shift images (CSI) of the localized slice. In data from whole phantoms, the modified pulse sequence yielded enhancement of the lactate signal of 2.4 +/- 0.40 times compared to SelMQC. Similar in vivo lactate signal enhancement of 2.3 +/- 0.24 times was observed in 1D slice-localized experiment.

The effect of energy concentration of earthquake ground motions on the nonlinear response of RC structures

Usually different nonlinear time responses due to earthquake ground motion are distinguished by non-localized spectra, such as the response or power spectra. However, these spectra are often not able to explain the large discrepancy among structural responses caused by different earthquake records. The local spectrum, obtained by the wavelet transform, shows the energy distribution in the time-frequency domain, and helps to understand the very different structural responses. By changing the energy distribution in time of several earthquake records, the effect of energy concentration on the structural nonlinear response is demonstrated. This paper proposes the use of the characteristic peak ground acceleration, which is the peak of the signal constructed by only a few special wavelet components of an earthquake record, to quantify the difference between earthquake records, since this measure indicates the magnitude of the energy concentrated around the fundamental period of a structure.

On the nature of incompressible magnetohydrodynamic turbulence

A novel model of incompressible magnetohydrodynamic turbulence in the presence of a strong external magnetic field is proposed for the explanation of recent numerical results. According to the proposed model, in the presence of the strong external magnetic field, incompressible magnetohydrodynamic turbulence becomes nonlocal in the sense that low-frequency modes cause decorrelation of interacting high-frequency modes from the inertial interval. It is shown that the obtained nonlocal spectrum of the inertial range of incompressible magnetohydrodynamic turbulence represents an anisotropic analogue of Kraichnan’s nonlocal spectrum of hydrodynamic turbulence. Based on the analysis performed in the framework of the weak-coupling approximation, which represents one of the equivalent formulations of the direct interaction approximation, it is shown that incompressible magnetohydrodynamic turbulence could be both local and nonlocal, and therefore anisotropic analogues of both the Kolmogorov and Kraichnan spectra are realizable in incompressible magnetohydrodynamic turbulence.

Non-local MHD turbulence

We consider an example of strongly non-local interaction in incompressible magnetohydrodynamic (MHD) turbulence which corresponds to the case where the Alfvén waves travelling in the opposite directions have essentially different characteristic wavelengths. We use two approaches to the dynamics of turbulent Alfvénic wavepackets: the first is a geometrical WKB theory [Phys. Lett. A 165 (1992) 330] and the second one is a three-wave kinetic equation derived for weakly turbulent waves [J. Plasma Phys., in press]. We show that these theories have a common limit of weak turbulence with scale separation in which they both predict the same Fokker–Planck equation for the wave power spectrum. In both cases the packet wavenumbers (and therefore the Lagrangian field-line separations) are allowed to experience order 1 changes. The WKB theory developed here formalises an intuitive geometrical argument of Goldreich and Sridhar [ApJ 485 (1997) 680] and allows one to see where such an intuition leads to a wrong conclusion about the inapplicability of the three-wave kinetic equation for order 1 wavepacket distortions. We show that the exponent of the constant flux non-local spectrum matches the value previously found for local turbulence at the boundary of the locality interval. The relationship between the WKB theory and the weak turbulence theory found in this paper for an ensemble of Alfvén waves seems to be general for three-wave systems.

Quantitative lactate-specific MR imaging and 1H spectroscopy of skeletal muscle at macroscopic and microscopic resolutions using a zero-quantum/double-quantum coherence filter and SLIM/GSLIM localization.

Quantitative lactate imaging and spectroscopy were performed on phantoms and on electrically stimulated, excised frog skeletal muscle at macroscopic and microscopic resolutions. Lactate selectivity was achieved by use of a zero-quantum/double-quantum coherence (ZQC/DQC) lactate filter, which suppressed all signals besides lactate, including water and lipid, to below noise level. Three-dimensional lactate data sets were acquired in 1-3 h; one of these spatial dimensions was frequency-encoded and the other two were phase-encoded. High-resolution images were reconstructed using the spectral localization by imaging (SLIM) and generalized SLIM (GSLIM) techniques. Lactate quantitation was achieved by employing an external lactate concentration standard and was verified by comparison to quantitative STEAM-localized and nonlocalized spectra that used total creatine as an internal concentration reference. Additionally, quantitatively accurate behavior of the SLIM and GSLIM techniques as applied to data sets of low signal-to-noise ratio and to macroscopically heterogeneous objects was verified using simulations and real muscle lactate data sets with known heterogeneity.

Proton-decoupled 19F spectroscopy of 5-FU catabolites in human liver.

An RF network and a dual-tuned surface coil are described for obtaining proton-decoupled, NOE enhanced 19F spectra from a whole body clinical imager operating at 1.5 Tesia. The network removes 19F frequency noise from the decoupler transmitter, and prevents preamplifier saturation from high-level decoupling signals. Proton decoupling of 19F spectra was optimized using a sample of urine containing 5-fluorouracil (5-FU) and its catabolite fluoro-beta-alanine (FBAL). Proton-decoupled 19F spectroscopy in vivo is demonstrated by obtaining both nonlocalized spectra and spectra localized with three-dimensional chemical shift imaging from the liver of patients undergoing 5-FU chemotherapy.