Biomedical subjects

P A Montano

Publications and source records attributed to P A Montano.

4 recordsLinked to original sources

PubMed2004

Temperature-dependent orbital degree of freedom of a bilayer manganite by magnetic compton scattering.

We have measured temperature-dependent magnetic Compton profiles (MCPs) from a single crystal of La1.2Sr1.8Mn2O7. The MCPs, which involved the scattering of circularly polarized x rays, are in general related to the momentum density of all the unpaired spins in the system. Nevertheless, we show that when the x-ray scattering vector lies along the [110] direction, the number of magnetic electrons of a specific symmetry, i.e., d electrons of x(2)-y(2) symmetry, yield a distinct signature in the MCP, allowing us to monitor substantial changes in the occupancy of the dx(2)(-y(2)) states over the investigated temperature range of 5-200 K. This study indicates that magnetic Compton scattering can provide a powerful window on the properties of specific magnetic electrons in complex materials.

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PubMed2002

Momentum-resolved charge excitations in a prototype one-dimensional mott insulator.

We report momentum-resolved charge excitations in a one-dimensional (1D) Mott insulator studied using high resolution inelastic x-ray scattering over the entire Brillouin zone for the first time. Excitations at the insulating gap edge are found to be highly dispersive (momentum dependent) compared to excitations observed in two-dimensional Mott insulators. The observed dispersion in 1D cuprates ( SrCuO2 and Sr2CuO3) is consistent with charge excitations involving holons which is unique to spin-1/2 quantum chain systems. These results point to the potential utility of momentum-resolved inelastic x-ray scattering in providing valuable information about electronic structure of strongly correlated insulators.

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PubMed2001

Local structure of guest molecules in gas hydrates--a model study of Kr and Xe clathrates.

Gas hydrates constitute a class of solids in which small molecules occupy cavities inside an ice-like structure. There is enormous scientific and technological interest in understanding the structure, stability and formation mechanism of clathrates. We developed and constructed a variable temperature high-pressure cell for x-rays measurements, which allows in situ studies of clathrate formation or decomposition. We used XAFS and Diffraction techniques to study the evolution of the structure during formation and decomposition. We studied two clathrates structures, structure I (Xe) and structure II (Kr). We were able to identify the local structure around the guest atom. We identified the rare gas-water complexes that act as precursor to the formation of the crystalline phases. We observed the transformation of the clathrate from structure II to structure I when Xe is added to Kr clathrates.

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PubMed1998

CdZnTe array detectors for synchrotron radiation applications.

An X-ray linear-array detector was fabricated using high-pressure Bridgman-grown CdZnTe. The detector area was 175 x 800 microm and the pitch size was 250 microm. The measured dark current for the test 16-element detector was as low as 0.1 pA at 800 V cm(-1) with excellent uniformity. Energy spectra were measured using a 57Co radiation source. Both a small-pixel effect and charge sharing were observed. For the arrays, an average 5.8% full width at half-maximum (FWHM) at the 122 keV photopeak was obtained with a standard deviation of 0.2%. A large-area detector (1 x 1 cm) of the same material before fabrication exhibited a low-energy tail at the photopeak, which limits the photopeak FWHM to 8%, typically due to hole trapping. At energies below 60 keV, charge sharing between elements was observed. The charge sharing was greatly reduced by providing a path to ground for unwanted charges. A prototype readout electronic system for an eight-channel array detector was developed. A readout system intended for a multielement solid-state detector system was also used. The array detector will be used for high-energy diffraction and Compton scattering measurements at the Advanced Photon Source.

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