Papers by Author: Grant V. M. Williams

Abstract: A portable fibre optic dosimeter has been developed that uses transparent fluroperovskite dosimeters at the end of a multimode polymer fibre. It uses two stimulation sources that take advantage of the bimodal trap distribution so that the dose rate and cumulative dose can be monitored. Photoluminescence (PL), optically stimulated luminescence (OSL) and radioluminescence (RL) measurements were made on nearly tissue equivalent NaMgF₃:Eu²⁺, NaMgF₃:Mn²⁺, and high sensitivity RbMgF₃:Eu²⁺ and CsCdF₃:Mn²⁺. Initial dose rate and dose measurements were made using our fibre dosimeter during and after exposure to 60 keV gamma rays.

Abstract: Bragg gratings were inscribed in poled host-guest polymer thin films containing anelectro-optically active chromophore. Probing the grating in reflection, with a transparent con-ducting electrode on one side of the film and a gold-coated electrode on the other, a modulatingelectric field was observed to induce a modulation in the grating efficiency consistent with thatexpected from the measured electro-optic coefficient.

Abstract: We observe a large magneto-resistance in very thin Ba₂FeMoO₆ films of ~-12% at 10K, which is not significantly changed when thin NiFe/Ba₂FeMoO₆ bi-layers are made. This suggests that the magneto-resistance in both cases is dominated by inter-grain tunnelling in the Ba₂FeMoO₆ thin film. There is an anomalous peak in the resistance at ~300K in the bi-layer film that can not be easily explained. However, we know that the temperature where this peak occurs is close to the Ba₂FeMoO₆ Curie temperature.

Abstract: We report a high electo-optic coefficient (r₃₃) of 175 pm/V in guest-host polymer films made by mixing amorphous polycarbonate with the chromophore, PYR-3 that was synthesized by our team. We have previously shown that PYR-3 has a large 2^nd order nonlinear optical figure of merit. The measured r₃₃ is five times larger than that of conventional inorganic materials such as lithium niobate. The decay of r₃₃ with time has been studied for more than 1000 hours at room temperature and ambient atmosphere, and we find that the r₃₃ value gradually reduced to a stable value of 34 pm/V after 40 days.

Abstract: We report the results from magnetotransport measurements on polycrystalline Sr_2-xLa_xFeMoO₆ samples at magnetic fields of up to 8 T. We observe a normal as well as an anomalous Hall effect. We find that there is a small decrease in the normal Hall coefficient for La doped samples.

Abstract: We have fabricated surface magnetic iron nanoclusters using low energy Fe⁺ implantation and electron beam annealing. We find that changing the fluence has a significant effect on the nanocluster growth, structural and magnetic properties. Low fluences lead to small nanoclusters and superparamagnetism, while high fluences result in larger chain-like nanoclusters that have high remnant magnetizations and a significantly reduced saturation field. Our results show that the nanostructure and the magnetic properties can be tuned by varying the Fe⁺ fluence, which means that a reliable method can be used to make surface nanoclusters for a variety of applications (e.g. large magnetoresistance sensors with no hysteresis).

Abstract: NMR spin shift data of La1:85Sr0:15CuO4, HgBa2CuO4+ at ambient pressure, andof YBa2Cu4O8 at pressure up to 63 kbar are discussed that fail the traditional single-electronicuid picture used for the discussion of NMR data. Instead the results point to a commonexplanation for all three systems in terms of two electronic components. One of them hasa temperature-dependent magnetic susceptibility while the second component, as well as itscoupling to the rst component, are constant above the superconducting transition temperatureTc. All susceptibilities vanish below Tc.

Abstract: In this paper we report the results from characterization and optical measurements on samarium doped KMgF₃ nanoparticles. Photoluminescence (PL) is observed that can be attributed to Sm²⁺ and Sm³⁺. The Sm²⁺ temperature dependent PL lifetime data can be interpreted in terms of back transfer from the ⁵D₀ level to the lowest 5d level followed by non-radiative decay from the 5d level.

Abstract: Raman measurements were made on polycrystalline Bi2􀀀xPbxSr1:6Ln0:4CuO6􀀀 for Ln=fLa,Nd, Sm, Eug with various x and doping states. Our measurements suggest a signi cant Bicontribution to the 118cm􀀀1 mode but not to the 70cm􀀀1 mode which instead shifts to lowerfrequencies with increasing number of CuO2 layers in the Bi-based family. Shifts in the O(2)Srapical oxygen mode can be attributed to unit cell contraction within experimental error.