In-Gap States and Band-Like Transport in Memristive Devices

Point defects such as oxygen vacancies cause emergent phenomena such as resistive switching in transition-metal oxides, but their influence on the electron-transport properties is far from being understood. Here, we employ direct mapping of the electronic structure of a memristive device by spectromicroscopy. We find that oxygen vacancies result in in-gap states that we use as input for single-band transport simulations. Because the in-gap states are situated below the Fermi level, they do not contribute to the current directly but impact the shape of the conduction band. Accordingly, we can describe our devices with band-like transport and tunneling across the Schottky barrier at the interface.

Understanding the Coexistence of Two Bipolar Resistive Switching Modes with Opposite Polarity in Pt/TiO2/Ti/Pt Nanosized ReRAM Devices

Redox-type resistive random access memories based on transition-metal oxides are studied as adjustable two-terminal devices for integrated network applications beyond von Neumann computing. The prevailing, so-called, counter-eight-wise (c8w) polarity of the switching hysteresis in filamentary-type valence change mechanism devices originates from a temperature- and field-controlled drift-diffusion process of mobile ions, predominantly oxygen vacancies in the switching oxide. Recently, a bipolar resistive switching (BRS) process with opposite polarity, so-called, eight-wise (8w) switching, has been reported that, especially for TiO₂ cells, is still not completely understood. Here, we report on nanosized (<0.01 μm²) asymmetric memristive cells from 3 to 6 nm thick TiO₂ films by atomic layer deposition, which reveal a coexistence of c8w and 8w switching in the same cell. As important characteristics for the studied Pt/TiO₂/Ti/Pt devices, the resistance states of both modes are nonvolatile and share one common state; i.e., the high-resistance state of the c8w mode equals the low-resistance state of the 8w-mode. A transition between the opposite hysteresis loops is possible by voltage control. Specifically, 8w BRS in the TiO₂ cells is a self-limited low-energy nonvolatile switching process. Additionally, the 8w reset process enables the programming of multilevel high-resistance states. Combining the experimental results with data from simulation studies allows to propose a model, which explains 8w BRS by an oxygen transfer process across the Pt/TiO₂ Schottky interface at the position of the c8w filament. Therefore, the coexistence of c8w and 8w BRS in the nanoscale asymmetric Pt/TiO₂/Ti/Pt cells is understood from a competition between drift/diffusion of oxygen vacancies in the oxide layer and an oxygen exchange reaction across the Pt/TiO₂ interface.

Characterization of the diffusion coefficient of blood

PURPOSE

To characterize the diffusion coefficient of human blood for accurate results in intravoxel incoherent motion imaging.

METHODS

Diffusion-weighted MRI of blood samples from 10 healthy volunteers was acquired with a single-shot echo-planar-imaging sequence at body temperature. Effects of gradient profile (monopolar or flow-compensated), diffusion time (40-100 ms), and echo time (60-200 ms) were investigated.

RESULTS

Although measured apparent diffusion coefficients of blood were larger for flow-compensated than for monopolar gradients, no dependence of the apparent diffusion coefficient on the diffusion time was found. Large differences between individual samples were observed, with results ranging from 1.26 to 1.66 µm2 /ms for flow-compensated and 0.94 to 1.52 µm2 /ms for monopolar gradients. Statistical analysis indicates correlations of the flow-compensated apparent diffusion coefficient with hematocrit (P = 0.007) and hemoglobin (P = 0.017), but not with mean corpuscular volume (P = 0.64). Results of Monte-Carlo simulations support the experimental observations.

CONCLUSIONS

Measured blood apparent diffusion coefficient values depend on hematocrit/hemoglobin concentration and applied gradient profile due to non-Gaussian diffusion. Because in vivo measurement is delicate, an estimation based on blood count results could be an alternative. For intravoxel incoherent motion modeling, the use of a blood self-diffusion constant Db  = 1.54 ± 0.12 µm2 /ms for flow-compensated and Db  = 1.30 ± 0.18 µm2 /ms for monopolar encoding is suggested. Magn Reson Med 79:2752-2758, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Model-based sphere localization (MBSL) in x-ray projections

The detection of spherical markers in x-ray projections is an important task in a variety of applications, e.g. geometric calibration and detector distortion correction. Therein, the projection of the sphere center on the detector is of particular interest as the used spherical beads are no ideal point-like objects. Only few methods have been proposed to estimate this respective position on the detector with sufficient accuracy and surrogate positions, e.g. the center of gravity, are used, impairing the results of subsequent algorithms. We propose to estimate the projection of the sphere center on the detector using a simulation-based method matching an artificial projection to the actual measurement. The proposed algorithm intrinsically corrects for all polychromatic effects included in the measurement and absent in the simulation by a polynomial which is estimated simultaneously. Furthermore, neither the acquisition geometry nor any object properties besides the fact that the object is of spherical shape need to be known to find the center of the bead. It is shown by simulations that the algorithm estimates the center projection with an error of less than [Formula: see text] of the detector pixel size in case of realistic noise levels and that the method is robust to the sphere material, sphere size, and acquisition parameters. A comparison to three reference methods using simulations and measurements indicates that the proposed method is an order of magnitude more accurate compared to these algorithms. The proposed method is an accurate algorithm to estimate the center of spherical markers in CT projections in the presence of polychromatic effects and noise.

Ultrasound measurements at the proximal phalanges in healthy women and patients with hip fractures

Measurements of bone mineral density (BMD) are useful for the assessment of fracture risk in osteoporosis. First prospective studies showed that quantitative ultrasound as measured at the calcaneus also predicts future hip fracture risk, independently of BMD and as accurately as BMD. The aim of this study was to compile a reference population for a new ultrasound device that determines amplitude-dependent speed of sound (AD-SOS) through the proximal phalanges of the hand and to prove its ability to distinguish between health volunteers and osteoporotic patients. In a case-control study we examined 139 healthy women aged 21-94 years and a group of 24 female patients aged 69-94 years with recent hip fractures. In the healthy reference population additional BMD measurements were performed with dual-energy X-ray absorptiometry (DXA) and quantitative ultrasound measurements at the calcaneus were carried out. In vivo precision of AD-SOS measurements through the phalanges was 0.52% CV. Simple regression analyses showed a negative correlation with age (r = -0.73, p < 0.001); modest significant correlations with BMD of the lumbar spine (r = 0.36, p < 0.001) and BMD of the femoral neck (r = 0.37, p = 0.002) as measured with DXA were shown. The comparison with another ultrasound device measuring SOS and broadband ultrasound attenuation (BUA) through the calcaneus showed correlation with SOS (r = 0.50, p < 0.001); no significant correlation was found with BUA measurements. Furthermore a dependency of AD-SOS values in anthropometric factors such as body mass index (r = 0.37, p < 0.001), height (r = 0.40, p < 0.001) and weight (r = 0.23, p < 0.05) was shown. First study results on 24 clinically diagnosed osteoporotic patients, defined as patients with recent (< 1 week) pertrochanteric or femoral neck fractures, showed a good separation between age- and sex-matched controls and osteoporotic patients (Z = -2.0 SD). Receiver operating characteristic (ROC) curves showed an area under the fitted curve of 0.83 +/- 0.06. These results are powerful for a device measuring AD-SOS through the proximal phalanges of the hand, and further prospective studies have proven the capability of phalangeal ultrasound in fracture risk assessment.

Ultrasound velocity of the tibia in normal German women and hip fracture patients

One of the latest developments in quantitative ultrasound (QUS) is the measurement of the speed of sound (SOS) of cortical bone of the midtibia. To determine the diagnostic validity of this method we measured 150 healthy women aged 22-94 years. Additionally, we report on first results of patients with hip fracture. Precision in vivo of the tibial QUS expressed as the percentage coefficient of variation (CV) was 0.39% for the first day and 0.45% after repositioning the second day (mean CV = 0.42%). No significant dependency of tibial SOS was found with weight, height, and body mass index in pre- and postmenopausal women. There was a significant decline of SOS with age in postmenopausal women (SOS = 4225 - 5.3 age, r = -0.46, P < 0. 001), whereas premenopausal women showed no decline (SOS = 3906 + 1. 3 age, r = 0.13, ns) Mean SOS values of premenopausal women were significantly higher than those of postmenopausal women (3960 +/- 78.7 m/second and 3898 +/- 120 m/second, respectively, P < 0.001). Postmenopausal women on estrogen substitution had significantly higher mean tibial SOS values than age-comparable postmenopausal women without estrogen substitution (3980 +/- 99 m/second and 3869 +/- 100 m/second, respectively, P < 0.001). Significant difference between age-matched healthy women, n = 11, and hip fracture patients, n = 13, expressed as z-score of -1.4 SD was found. In conclusion, tibial QUS declines with age and detects higher values in premenopausal women and postmenopausal women on estrogen substitution and lower values in hip fracture patients. Further prospective studies are needed to clarify its role in fracture risk assessment.