Laser-assisted field evaporation is studied in a large number of compounds, including amorphous and crystalline phase change materials employing atom probe tomography. This study reveals significant differences in field evaporation between amorphous and crystalline phase change materials. High probabilities for multiple events with more than a single ion detected per laser pulse are only found for crystalline phase change materials. The specifics of this unusual field evaporation are unlike any other mechanism shown previously to lead to high probabilities of multiple events. On the contrary, amorphous phase change materials as well as other covalently bonded compounds and metals possess much lower probabilities for multiple events. Hence, laser-assisted field evaporation in amorphous and crystalline phase change materials reveals striking differences in bond rupture. This is indicative for pronounced differences in bonding. These findings imply that the bonding mechanism in crystalline phase change materials differs substantially from conventional bonding mechanisms such as metallic, ionic, and covalent bonding. Instead, the data reported here confirm a recently developed conjecture, namely that metavalent bonding is a novel bonding mechanism besides those mentioned previously.

To date, slow Set operation speed and high Reset operation power remain to be important limitations for substituting dynamic random access memory by phase change memory. Here, we demonstrate phase change memory cell based on Ti0.4Sb2Te3 alloy, showing one order of magnitude faster Set operation speed and as low as one-fifth Reset operation power, compared with Ge2Sb2Te5-based phase change memory cell at the same size. The enhancements may be rooted in the common presence of titanium-centred octahedral motifs in both amorphous and crystalline Ti0.4Sb2Te3 phases. The essentially unchanged local structures around the titanium atoms may be responsible for the significantly improved performance, as these structures could act as nucleation centres to facilitate a swift, low-energy order-disorder transition for the rest of the Sb-centred octahedrons. Our study may provide an alternative to the development of high-speed, low-power dynamic random access memory-like phase change memory technology.

Key words: OEF; BOLD; qBOLD; brain metabolism; brain hemodynamics; fMRIWhile the dynamic properties of blood oxygenation level dependent (BOLD) contrast in MRI during functional activation have received much consideration, very little attention has been paid to the nature of the BOLD contrast during the resting or baseline level of neuronal activity in the brain. Because "resting brain" is responsible for approximately 20% of total human body oxygen consumption (1,2), understanding brain functioning in the baseline state is important for understanding brain performance in health and disease. One of the important parameters defining oxygen consumption is oxygen extraction fraction (OEF) -the percent of the oxygen removed from the blood by tissue during its passage through the capillary network. Previously, Raichle et al. (2,3) used this parameter to characterize the baseline state of the normal human brain. Such a characterization is germane because OEF maps of normal human subjects, resting quietly with their eyes closed, demonstrate remarkable uniformity (2,4) despite substantial regional variations of cerebral blood flow and the cerebral metabolic rate of oxygen consumption (2,3). This uniformity of the OEF in the absence of specific goal-directed activities supports the hypothesis that an established equilibrium exists between the local metabolic requirements necessary to sustain a long term modal level of neural activity and the level of blood flow in a particular region.Thus far most quantitative imaging studies mapping tissue OEF were conducted using oxygen-15 based positron emission tomography (PET) imaging techniques (5). The advent of BOLD MR imaging initiated by Ogawa et al. (6) opened new opportunities to noninvasively study brain hemodynamics. BOLD approach capitalizes on the fact that deoxygenated blood has different magnetic susceptibility as compared to oxygenated blood (7), which in turn has magnetic susceptibility similar to the tissue (6). Due to this effect, the deoxyhemoglobin containing part of the blood vessel network in the brain creates mesoscopic field inhomogeneities in the surrounding tissue leading to more rapid MRI signal decay than from standard T2 decay alone. Because these field inhomogeneities are tissue specific, measuring the MRI signal decay rate may provide information on the tissue structure and functioning. Previously this lab has developed a theoretical model of BOLD contrast that analytically connects the BOLD signal to hemodynamic parameters such as the deoxyhemoglobincontaining blood volume (DBV), deoxyhemoglobin concentration, and OEF (8). A subsequent publication (9) quantitatively validated important features of the model in phantom studies and developed a theoretical background and experimental method (based on the Gradient Echo Sampling of Spin Echo (GESSE) sequence) that allows the separation of mesoscopic field inhomogeneity effects from both macroscopic and microscopic inhomogeneities. Such separation allows one to take full advantage of the mesoscopic, tissue...

The most common MR-based approach to noninvasively measure brain temperature relies on the linear relationship between the 1 H MR resonance frequency of tissue water and the tissue's temperature. Herein we provide the most accurate in vivo assessment existing thus far of such a relationship. It was derived by acquiring in vivo MR spectra from a rat brain using a high field (11.74 Tesla [T]) MRI scanner and a single-voxel MR spectroscopy technique based on a LASER pulse sequence. Data were analyzed using three different methods to estimate the 1 H resonance frequencies of water and the metabolites NAA, Cho, and Cr, which are used as temperature-independent internal (frequency) references. Standard modeling of frequency-domain data as composed of resonances characterized by Lorentzian line shapes gave the tightest resonance-frequency versus temperature correlation. An analysis of the uncertainty in temperature estimation has shown that the major limiting factor is an error in estimating the metabolite frequency. Proton ( 1 H) MRS thermometry provides a unique noninvasive method to monitor changes in brain temperature or to quantify absolute brain temperature. The MR frequency of water protons depends on temperature, and it changes with a coefficient of approximately Ϫ0.01 ppm/°C (1). It has been shown by means of magnetic resonance imaging (2) and spectroscopy that this approach can be used for in vivo noninvasive temperature mapping (3) and tracking of changes in tissue temperature during functional activation (4). The possibility to quantify absolute brain temperature by monitoring the difference between the water resonance frequency and a temperature independent metabolite resonance frequency (internal reference) has also been demonstrated (5,6).The 1 H resonance of the N-acetyl-aspartate (NAA) methyl group is most often used to provide a temperature independent internal reference frequency for in vivo temperature quantification in brain tissue due to the relatively high concentration of NAA as compared to other metabolites (5-11). However, because of the small temperature/ water-chemical-shift correlation coefficient and low NAA signal amplitude (NAA concentration in the normal brain tissue is approximately 10,000 times smaller than the water concentration) the accuracy of the method is limited. Two major factors define this accuracy: (i) the predetermined temperature versus 1 H MR water frequency correlation (ϳ Ϫ0.01 ppm/°C) and (ii) the accuracy in estimating the internal reference (metabolite) 1 H resonance frequency. Existing literature data suggest there is substantial room for improvement in obtaining a more accurate, internally referenced, calibration curve. This topic is the principal focus of this study. MATERIALS AND METHODS Animal PreparationMale Sprague Dawley rats (n ϭ 3) weighing ϳ300 -400 g were initially anesthetized with a ketamine/xylazine combination anesthesia (72.9 mg/kg and 10.4 mg/kg, respectively) and given time to stabilize. The animals were then restrained in a prone position within a...