Robert George Buckley

Thermochromic oxides provide a platform for coupling lattice, electronic, and magnetic degrees of freedom, with divalent cobalt molybdate (CoMoO ₄ ) as a prototypical example.

GdN is a ferromagnetic semiconductor which has seen increasing interest in the preceding decades particularly in the areas of spin- and superconducting- based electronics. Here we report a detailed computational and optical spectroscopy study of the electronic structure of stoichiometric and nitrogen vacancy doped GdN. Based on our calculations we provide the effective mass tensor for undoped GdN, and some indicative values for electron doped GdN. Such a property is valuable as it can affect device design, and can be measured experimentally to validate the existing computation results.

We report microsecond timescale switching between the superconducting and normal states of commercially manufactured high-Tc superconducting wires using optical radiation to heat the conductor above Tc. The achieved voltages and switching times have significant implications for a new class of thermal switches. The report contains experimental data for photo-induced microsecond scale voltage transients as well as numerical analysis of heat propagation through the material in response to optical radiation. The microbridges are etched into commercially manufactured coated conductors and submerged in liquid nitrogen. By varying the magnitude of the transport current, two different optical responses are identified. At low transport currents, short-lived voltage transients occur, transitioning to persistent latching once the current is increased above a threshold value. The microbridge, therefore, behaves as a fast opening and closing switch when carrying low currents or an optically latched thyristor at high currents. This is understood to occur due to critical current suppression as a result of heating from the optical pulse. Optically induced heating reduces the superconducting volume fraction, and Joule heating due to the transport current interaction with the high temperature superconducting occurs, which either causes short-lived or stable self-heating normal regions dependent on the magnitude of the transport current. This study is concluded with an estimate of the energy necessary to drive the microbridge into the resistive state. The observed behavior can potentially be utilized as a switching element in superconducting transformer rectifiers where high frequency switch operation is required.

High-temperature superconducting traction transformers (HTSTTs) have the merits of small size and lightweight in comparison with their conventional counterparts. This article reports the development progress of a 6.6 MVA HTSTT operating at 65 K, including the design, testing, and system cooling. The introduction of flux diverters and an optimized winding design realized a short-circuit impedance higher than 43% and AC loss less than 3 kW. The insulation structure was designed to pass insulation tests specified in standard in China GB/T 25120-2010. An open cooling system with reduced pressure was developed, which realized the efficiency of the 6.6 MVA HTSTT above 99%. Before assembling the prototype transformer, we conducted tests for critical current and dielectric performance of the HTS double pancake coils (DPCs) used in high-voltage (HV) and low-voltage (LV) windings to verify the current-carrying and insulation performances of each DPC. Finally, we measured the critical current and no-load loss of the HTSTT prototype at 77 K. Test results showed that the mass of the transformer is 33% less than conventional transformers. At 77 K, the critical current of the LV winding and HV winding is higher than 700 A and 50 A, respectively. Moreover, the HTSTT on a no-load test reached the test voltage of 25 000 V and loss of 6 kW. In the next step, we will continue to conduct experimental research, and verify the feasibility of the HTSTT on the train, and develop a circulating cooling system, all meeting the commercial requirements of the HTSTT.

Far infrared measurements of the absorption into the IR-active TO(Γ) mode are reported for a selection of the rare-earth mononitrides. The frequencies harden as anticipated by ≈15% as the lattice constant shrinks by ≈6% from SmN to LuN, though they are typically ∼25% softer than the LSDA+U prediction published a decade ago. The data are in much closer agreement with our computation based on more recent software for two in the series: GdN and LuN. The resonances show surprisingly heavy damping, diminishing from the lighter to heavier rare earth elements.

We report electrical transport and optical spectroscopy measurements on LuN thin films variously doped with nitrogen vacancies along with the computed band structures of stoichiometric and nitrogen vacancy doped LuN. LuN has been the subject of several recent computational studies; however, the most recent experimental studies regarding its electronic properties are already over four decades old. Here, we bridge the void between computation and experiment with a combined study of LuN focusing on its electronic properties. We find that LuN is a semiconductor with an optical bandgap of ∼1.7 eV. Its conductivity can be controlled by nitrogen vacancy doping, which results in defect states at the conduction band minimum and valence band maximum. These results not only provide information on LuN but also help underpin understanding of the electronic properties of the entire rare-earth nitride series.