G Q Zhang

Origins of both insulator–metal transition and colossal magnetoresistance in doped manganese perovskites

Based on experiments of both transport and paramagnetic resonance for (La1−xYx)2/3Ca1/3MnO3, the ground state above Tc and possible origins of both insulator–metal transition and colossal magnetoresistance near Tc are discussed. Modeling the system as a network of junctions, each consisting of a paramagnetic region sandwiched between two ferromagnetic domains, a phenomenological expression is proposed for resistance as a function of temperature and magnetic field. We show that the observed transport and magnetotransport phenomena can be quantitatively explained by the present model for the whole temperature range studied.

Comparison of absorbed fractions of electrons and photons using three kinds of computational phantoms of rat

Rats have been widely used in radiopharmaceutics researches, making it reasonable to develop fine structured computational models for rat internal dose investigation. In this work, we developed three existing types of computational phantoms based on the same data set of rat, simulated photon and electron particles transport using Monte Carlo method and calculated absorbed fractions for organs. We further compared the dose discrepancies of these three types of phantoms in internal dosimetry and found the stylized phantom might cause an underestimation of the self-absorbed S-factors. The developed phantoms will contribute to the application of computational phantoms in dosimetry studies.

Grain boundaries and low-field transport properties in colossal magnetoresistance materials

The grain-boundary structure and the temperature dependence of resistivity were investigated for (1-x)LCMO + xYSZ, where LCMO and YSZ represent La2/3Ca1/3MnO3 and yttria-stabilized zirconia, respectively. It is shown that the YSZ doped samples for x 2%. The metal-insulator transition temperature (Tp) decreases for x 2% as x is increased. LFMR increases with x for x 2%. The experimental observations are discussed on the basis of scanning electron microscopy (SEM) analysis, which reveals YSZ appearing at the grain boundaries of LCMO for x 2%.

Effect of annealing temperature on electrical transport in La2/3Ca1/3MnO3

Abstract The electrical conductivity of La2/3Ca1/3MnO3 samples prepared using a sol–gel method is shown to be strongly dependent on the annealing temperature (hence the grain size). This is studied using a random resistor-network model in which ferromagnetic metallic particles with the number density p randomly fill the space of sample. The Monte Carlo simulation for the temperature dependence of resistivity shows results in excellent agreement with the experimental data measured in the larger and smaller grain size samples. From this simulation, we find that p is largely decreased for the whole temperature range below the Curie temperature as the grain size decreases. On the basis of temperature dependence of p obtained in the larger and smaller grain size samples, the effect of grain-size on the electrical transport is discussed and a possible explanation is also presented for the grain-size-dependent magnetoresistance behavior.

Development of a Rat Computational Phantom Using Boundary Representation Method for Monte Carlo Simulation in Radiological Imaging

Computational models for small experimental animals are important in medical imaging and radiation dosimetry researches. The boundary representation method by use of nonuniform rational B-splines (NURBS) is adopted in this paper to develop a rat computational phantom based on the previously obtained segmentation data for cryosectional color photographic images of an adult male Sprague-Dawley rat. Continuous two-dimensional contours for a total of 14 major structures were outlined from the downscaled anatomical atlas, corresponding to a voxel size of 0.2 times 0.2 times 0.2 mm3 and stacked to reconstruct the three-dimensional (3-D) shapes. The NURBS model was then appropriately fitted through the surface of each organ. Monte Carlo simulation of cone beam X-ray computed tomography was performed focusing on the thorax region to demonstrate the usefulness of the computational phantom in radiological imaging. The integrated whole-body geometry is presented with smooth internal and exterior boundaries. Organ centroid coordinates and volume information are tabulated for future comparison purposes. The rat phantom can be used in 3-D dose calculation and other computational applications as well.

Electrical transport and low-field magnetoresistance in the series of mixed polycrystals (1-m)La2/3Ca1/3MnO3 + mLa2/3Sr1/3MnO3

The perovskites of La2/3Ca1/3MnO3 (LCMO) and La2/3Sr1/3MnO3 (LSMO) were synthesized by the sol-gel method. The two perovskites were mixed into polycrystalline materials, (1-m)LCMO + mLSMO, where m is the relative weight ratio of LSMO. The compound with m~0.3-0.5 has the lowest semiconductor-metal transition temperature Tp and the largest low-T resistivity. Moving away from this ratio, Tp increases while the low-T resistivity decreases with increasing or decreasing m. Measurements of the magnetoresistance (MR) show that the mixed polycrystals exhibit a sizable MR effect in low magnetic fields which increases upon cooling and reaches its largest value for T?0?K. It is also interesting to observe the almost T-independent MR effect over a wide temperature window below 300?K for the mixed materials with smaller m.

Phase separation and insulator-metal behaviour in highly Ba2+-doped La1-xBaxMnO3 compounds

Investigations of structural and transport features of La1-xBaxMnO3 with x = 1/3 and 2/3 are performed. The x = 2/3 sample is shown to be phase separated into a mixture of La2/3Ba1/3MnO3 and BaMnO3 with the same volume fractions, ~50%. In this two-phase system, La2/3Ba1/3MnO3 regions are connected in a percolative manner, so the electrical transport is dominated by flow along these percolative paths. Using the recently proposed random resistor network based on electronic phase separation between ferromagnetic metallic and paramagnetic insulating domains, we show that the model can yield results in quantitative agreement with the resistance versus temperature dependence measured for the x = 1/3 and 2/3 samples by using the metallic number density as a fitting parameter. This approach suggests a simple quantitative picture that can be used to explain the insulator-metal behaviour in La1-xBaxMnO3 with higher x.

Monte Carlo simulation of colossal magnetoresistance in doped manganese perovskites

Resistance as a function of temperature and applied magnetic field for doped manganese perovskites is simulated on the basis of a random-resistor-network model. The network, consisting of ferromagnetic metallic particles with number density p and paramagnetic insulating particles with number density 1-p, is generated through the Monte Carlo method. Approximating p by the reduced magnetization (m) determined from the mean-field theory, we show that the simulation can yield the main features of the colossal magnetoresistance in doped manganese perovskites. Comparisons between simulated and experimental data are also presented for (La1-xYx)2/3Ca1/3MnO3 (x = 0.2). The excellent agreement between the simulations and experiments gives strong support to the present approach.

Bowtie filtration for dedicated cone beam CT of the head and neck: a simulation study

OBJECTIVE To investigate the influence of bowtie filtration on dedicated cone beam CT (CBCT) of the head and neck. METHODS A validated hybrid simulation technique was used to model a commercial CBCT system with offset scanning geometry, 90 kV tube potential and 145×75 mm imaging field of view. Three bowtie filters were formulated to produce uniform flux intensity in the projection image of cylindrical objects of diameter 14, 16 and 18 cm. The influence of these simulated filters was compared with the original flat filtration in terms of the output radiation field, the dose delivered to the object, the scatter distribution in projections and the quality of the reconstructed image. RESULTS Compared against flat filtration, dose reduction for the bowtie case, examined as a function of radial distance within a 16-cm-diameter water cylinder, varied from 8.7% at the centre to 53.8% at the periphery. Scatter reduction, quantified using scatter-to-primary ratio in projection images, was up to 37.6% for a 14-cm-diameter cylindrical contrast phantom. Using the supplied routine image reconstruction, bowtie filtration resulted in comparable noise appearance, contrast resolution and artefact pattern for computational anatomical phantoms, with <5% difference in contrast-to-noise ratio. CONCLUSION Bowtie filtration can effectively reduce the dose and scatter in CBCT of the head and neck. For better image quality, corresponding modification to the image pre-processing and reconstruction is needed. ADVANCES IN KNOWLEDGE The hybrid simulation approach can usefully explore the impact of proposed system component and design changes.

Effect of A-site deficiency on electrical transport properties of YSZ doped manganites

Abstract The electrical transport properties of the composite samples: A-site stoichiometry (1−x)La2/3Ca1/3MnO3+xYSZ and A-site deficiency (1−x)(La2/3Ca1/3)0.95MnO3+xYSZ with different YSZ percentage x were investigated, respectively. The similar metal–insulator transition at different temperature TP was observed in the two types of composite samples: With increasing of the YSZ percentage, the zero-field resistivity of the composite samples increases and TP shifts to the low temperature for the range of x 2%. For the same YSZ percentage x, TP of (1−x)La2/3Ca1/3MnO3+xYSZ is lower than that of (1−x)(La2/3Ca1/3)0.95MnO3+xYSZ. The different effects between the two type composites were also observed in the nonlinear current–voltage (I–V) behaviors. The results suggest that A-site deficiency and YSZ dopant can be used to adjust purposely the metal–insulator transition temperature and resistivity of composites.