Monte Carlo study of the ordering in a strongly frustrated liquid crystal.

We have performed Monte Carlo simulations to investigate the temperature dependence of the ordering of the side chains of the X-shaped liquid crystal molecules which are arranged in a hexagonal array. Each hexagon contains six side chains, one from each side of the hexagon. Each liquid crystal molecule has two, dissimilar, side chains, one that contains silicon and one that contains fluorine. Like chains attract each other more strongly than unlike chains and this drives an order-disorder transition. The system is frustrated because it is not possible to find a configuration in which all the hexagons are occupied by either all silicon or all fluorine chains. There are two phase transitions. If only pairwise interactions are included it is found that there is an interesting fluctuating phase between the disordered phase and the fully ordered ground state. This did not agree with the experiments where an intermediate phase was seen that had long range order on one of the three sublattices. Agreement was found when the calculations were modified to include attractive three-body interactions between the silicon chains.

Diffusive benefits of cylinders in front of a Schroeder diffuser.

A numerical investigation is performed into the diffusive effects of cylinders positioned in front of a Schroeder diffuser. A regular line of cylinders is shown to offer notable improvements to diffusion from a periodic Schroeder device, provided lateral cylinder spacing is incommensurable with the Schroeder period width. Further investigation considers angular dependence and low frequency results in greater detail, as well as the effects on narrowband and modulated Schroeder devices. An optimization procedure is subsequently performed to investigate the effects of an irregular cylinder arrangement, which provides further diffusive benefits.

Magnetoresistance of magnetically doped ZnO films.

Magnetoresistance measurements have been made at 5 K on doped ZnO thin films grown by pulsed laser deposition. ZnCoO, ZnCoAlO and ZnMnAlO samples have been investigated and compared to similar films containing no transition metal dopants. It is found that the Co-doped samples with a high carrier concentration have a small negative magnetoresistance, irrespective of their magnetic moment. On decreasing the carrier concentration, a positive contribution to the magnetoresistance appears and a further negative contribution. This second, negative contribution, which occurs at very low carrier densities, correlates with the onset of ferromagnetism due to bound magnetic polarons suggesting that the negative magnetoresistance results from the destruction of polarons by a magnetic field. An investigation of the anisotropic magnetoresistance showed that the orientation of the applied magnetic field, relative to the sample, had a large effect. The results for the ZnMnAlO samples showed less consistent trends.

Two magnetic regimes in doped ZnO corresponding to a dilute magnetic semiconductor and a dilute magnetic insulator.

Films of ZnO doped with magnetic ions Mn and Co and, in some cases, with Al have been fabricated with a very wide range of carrier densities. Ferromagnetic behavior is observed in both insulating and metallic films, but not when the carrier density is intermediate. Insulating films exhibit variable range hopping at low temperatures and are ferromagnetic at room temperature due to the interaction of the localized spins with static localized states. The magnetism is quenched when carriers in the localized states become mobile. In the metallic (degenerate semiconductor) range, robust ferromagnetism reappears together with very strong magneto-optic signals and room temperature anomalous Hall data. This demonstrates the polarization of the conduction bands and indicates that, when ZnO is doped into the metallic regime, it behaves as a genuine magnetic semiconductor.

Room-temperature magneto-optics of ferromagnetic transition-metal-doped ZnO thin films.

Magneto-optic studies of ZnO doped with transition metals Co, Mn, V, and Ti indicate a significant magnetic circular dichroism (MCD) at the ZnO band edge at room temperature, together with an associated dispersive Faraday rotation. Similar spectra occur for each dopant, which implies that the ferromagnetism is an intrinsic property of the bulk ZnO lattice. At 10 K, additional paramagnetic contributions to the MCD are observed, but above about 150 K, the magnitude of the MCD signal is dominated by the ferromagnetism and is almost temperature independent. The MCD at the ZnO band edge shows room temperature hysteretic behavior.

Ferromagnetism above room temperature in bulk and transparent thin films of Mn-doped ZnO.

The search for ferromagnetism above room temperature in dilute magnetic semiconductors has been intense in recent years. We report the first observations of ferromagnetism above room temperature for dilute (<4 at.%) Mn-doped ZnO. The Mn is found to carry an average magnetic moment of 0.16 mu(B) per ion. Our ab initio calculations find a valance state of Mn(2+) and that the magnetic moments are ordered ferromagnetically, consistent with the experimental findings. We have obtained room-temperature ferromagnetic ordering in bulk pellets, in transparent films 2-3 microm thick, and in the powder form of the same material. The unique feature of our sample preparation was the low-temperature processing. When standard high-temperature (T > 700 degrees C) methods were used, samples were found to exhibit clustering and were not ferromagnetic at room temperature. This capability to fabricate ferromagnetic Mn-doped ZnO semiconductors promises new spintronic devices as well as magneto-optic components.

The Fermi surface of [Formula: see text].

The heavy-fermion compound [Formula: see text] has been studied using the fully relativistic spin-polarized mean muffin-tin orbital method within the local density approximation. Two separate calculations, one where the f electron is treated as a valence electron and the other where it is treated as part of the core, have been performed and the Fermi surface is obtained. The angular-dependent de Haas - van Alphen (dHvA) frequencies are calculated in both cases and they are compared with the experimental dHvA frequencies. We also calculated the electron momentum densities and compared them with the electron - positron momentum densities measured from the two-dimensional angular correlation of electron - positron annihilation radiation. The spin polarization of the Fermi surface is analysed and we present a new interpretation of the experimental data of Harrison et al.