Charge trapping by iodine ions in photorefractive Sn2P2S6 crystals.

Electron paramagnetic resonance (EPR) is used to establish the role of iodine as an electron trap in tin hypothiodiphosphate (Sn2P2S6) crystals. Iodine ions are unintentionally incorporated when the crystals are grown by the chemical-vapor-transport method with SnI4 as the transport agent. The Sn2P2S6 crystals consist of Sn2+ ions and (P2S6)4- anionic groups. During growth, an iodine ion replaces a phosphorus in a few of the anionic groups, thus forming (IPS6)4- molecular ions. Following an exposure at low temperature to 633 nm laser light, these (IPS6)4- ions trap an electron and convert to EPR-active (IPS6)5- groups with S = 1/2. A concentration near 1.1 × 1017 cm-3 is produced. The EPR spectrum from the (IPS6)5- ions has well-resolved structure resulting from large hyperfine interactions with the 127I and 31P nuclei. Analysis of the angular dependence of the spectrum gives principal values of 1.9795, 2.0123, and 2.0581 for the g matrix, 232 MHz, 263 MHz, and 663 MHz for the 127I hyperfine matrix, and 1507 MHz, 1803 MHz, and 1997 MHz for the 31P hyperfine matrix. Results from quantum-chemistry modeling (unrestricted Hartree-Fock/second-order Møller-Plesset perturbation theory) support the (IPS6)5- assignment for the EPR spectrum. The transient two-beam coupling gain can be improved in these photorefractive Sn2P2S6 crystals by better controlling the point defects that trap charge.

Self-trapped holes (small polarons) in ferroelectric KH2PO4 crystals.

Density functional theory is used to establish the ground-state structure of the self-trapped hole (STH) in KH2PO4 crystals. The STHs in this nonlinear optical material are free small polarons, a fundamental intrinsic point defect. They are produced with ionizing radiation in the low-temperature orthorhombic structure of KH2PO4 and are only stable (i.e. long-lived) below approximately 70 K. A large 129-atom cluster, K19H40P14O56, is constructed to model the STH. The ωB97XD functional with the 6-31+G* basis set is used and geometry optimization is performed. Our results show that two of the oxygen ions in a PO4 unit relax toward each other and equally share the hole. These two oxygen ions do not initially have close hydrogen neighbors. This equal sharing of the hole is related to the presence of isolated, slightly distorted, PO4 units and is significantly different from the small-polaron behavior often observed in other oxide crystals where the hole is localized on only one oxygen ion. The computational results provide a detailed description of the lattice relaxation occurring during formation of the STH. Characteristic spectral features of this defect are a larger hyperfine interaction with one 31P nucleus and equal, but smaller, hyperfine interactions with two 1H nuclei. The computed values for these isotropic and anisotropic hyperfine coupling constants are in excellent agreement with results obtained from electron paramagnetic resonance experiments.

Intrinsic small polarons (Sn³âº ions) in photorefractive Sn2P2S6 crystals.

Unique holelike small polarons are produced at divalent cation sites by optical excitation at low temperature in single crystals of Sn2P2S6, a monoclinic ferroelectric and photorefractive material. Electron paramagnetic resonance (EPR) is used to observe these self-trapped holes. During an illumination near 25 K with either 442 or 633 nm laser light, photoexcited holes become localized at Sn(2+) (5s(2)) ions and form paramagnetic Sn(3+) (5s(1)) ions. The Sn(3+) ions are thermally stable below 50 K. The principal values of the g matrix are 2.0031, 2.0176, and 2.0273 and the principal values of the (119)Sn hyperfine matrix are 12.828, 12.886, and 13.060 GHz. The large interaction with the (119)Sn (and (117)Sn) nucleus results in a highly asymmetric hyperfine pattern in the EPR spectrum. Weaker hyperfine interactions with two neighboring Sn ions are also observed.

Broadband terahertz pulse emission from ZnGeP2.

Optical rectification is demonstrated in (110)-cut ZnGeP(2) (ZGP) providing broadband terahertz (THz) generation. The source is compared to both GaP and GaAs over a wavelength range of 1150 nm to 1600 nm and peak-intensity range of 0.5 GW/cm(2) to 40 GW/cm(2). ZGP peak-to-peak field amplitude is larger than in the other materials due to either lower nonlinear absorption or larger second-order nonlinearity. This material is well suited for broadband THz generation across a wide range of infrared excitation wavelengths.

Surgical technique for minimally invasive fibula fracture fixation.

This paper describes a minimally invasive percutaneous technique for reduction and fixation of distal fibula fractures using plate osteosynthesis. We believe this technique benefits patients with poor quality soft tissue envelopes. So far a total of 25 patients have undergone percutaneous fixation, 22 females and 3 males. At no stage yet has a minimally invasive procedure been abandoned intra-operatively in favour of conversion to an open procedure. The mean age was 61.6 years (range 25-80 years). The mean time to surgery was 2.00 days (range 0-5) and mean time to discharge was 4.20 days (range 1-9). At a minimum of over 1 year's follow-up all fractures have healed, with no delayed unions or complications so far experienced.

Effect of Co doping on the structural, optical and magnetic properties of ZnO nanoparticles.

We report the results of a detailed investigation of sol-gel-synthesized nanoscale Zn(1-x)Co(x)O powders processed at 350 °C with 0≤x≤0.12 to understand how the structural, morphological, optical and magnetic properties of ZnO are modified by Co doping, in addition to searching for the theoretically predicted ferromagnetism. With x increasing to 0.03, both lattice parameters a and c of the hexagonal ZnO decreased, suggesting substitutional doping of Co at the tetrahedral Zn(2+) sites. For x>0.03, these trends reversed and the lattice showed a gradual expansion as x approached 0.12, probably due to additional interstitial incorporation of Co. Raman spectroscopy measurements showed a rapid change in the ZnO peak positions for x>0.03, suggesting significant disorder and changes in the ZnO structure, in support of additional interstitial Co doping possibility. Combined x-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance spectroscopy, photoluminescence spectroscopy and diffuse reflectance spectroscopy showed clear evidence for tetrahedrally coordinated high-spin Co(2+) ions occupying the lattice sites of ZnO host system, which became saturated for x>0.03. Magnetic measurements showed a paramagnetic behaviour in Zn(1-x)Co(x)O with increasing antiferromagnetic interactions as x increased to 0.10. Surprisingly, a weak ferromagnetic behaviour was observed for the sample with x = 0.12 with a characteristic hysteresis loop showing a coercivity H(c)∼350 Oe, 25% remanence M(r), a low saturation magnetization M(s)∼0.04 emu g(-1) and with a Curie temperature T(c)∼540 K. The XPS data collected from Zn(1-x)Co(x)O samples showed a gradual increase in the oxygen concentration, changing the oxygen-deficient undoped ZnO to an excess oxygen state for x = 0.12. This indicates that such high Co concentrations and appropriate oxygen stoichiometry may be needed to achieve adequate ferromagnetic exchange coupling between the incorporated Co(2+) ions.

Photoluminescence of n-type CdGeAs2.

Samples of n-type CdGeAs2 were produced by intentional doping with indium, selenium, or tellurium impurities. A near-edge photoluminescence (PL) band from heavily In-doped CdGeAs2 samples shifts to higher energy and becomes broader with increasing electron concentration. The observed shifts in peak energies are compared to predictions for donor-acceptor pair and free-to-bound (electron-acceptor) recombinations including band filling, band tailing, and band gap shrinkage effects due to the high doping levels. For n>2 × 1018 cm-3, the free-to-bound PL transition related to a shallow 120 meV acceptor level is dominant. A lower energy PL band due to deep acceptors and normally seen for p-type samples is the only emission observed from less n-type samples (n∼1016-1017 cm-3) doped with indium, selenium, or tellurium impurities. Transitions involving the deep acceptor level are not present in the PL for heavily In-doped CdGeAs2 crystals, which suggests that the deep acceptor may be a Cd vacancy.