Low Energy Excitations in La1.2Sr1.8Mn2O7 investigated by ellipsometry

The low-energy excitations of the bilayered manganite La1.2 Sr1.8 Mn2 O7 have been explored by spectral ellipsometry from two faces of a single crystal over the range from 0.006 to 0.6 eV. This compound is a paramagnetic insulator at ambient temperature, with a transition to a ferromagnetic metal below a Curie temperature (Tc) of 125 K. Both the ab -plane and c -axis temperature-dependent conductivities have been determined. Essentially no temperature-dependent behavior is observed above Tc although below Tc both the phonon and electronic contributions are strongly temperature sensitive. The highest-frequency phonons, especially those involving Mn-O bond stretching, split and show frequency changes consistent with structural results in the literature, and furthermore there is clear evidence of an increase in electron-phonon coupling at and below Tc. We interpret the temperature-dependent electronic spectral contribution in the light of recent calculations that indicate that a mixed phase exists in the doped manganites below Tc, with coexisting regions of an itinerant large-polaron phase and a localized small-polaron phase. © 2005 The American Physical Society

Annealing effects on the optical conductivity of single crystal (formula presented) (formula presented)

Optical reflectance spectra were measured in the temperature range 70–295 K, and in the energy range of 0.006–6 eV for single crystals of (formula presented) before and after annealing. The conductivity spectrum of the unannealed (formula presented) in its low-temperature metallic phase features a Drude-like peak, the spectral weight of which is dramatically increased by annealing the sample. Annealing also repairs a negative anomaly in the conductivity, which is thought to be associated with a surface layer damaged by polishing. A secondary ion mass spectrometry measurement shows, however, that the surface valence becomes depth dependent upon annealing. On the basis that the skin depth is far greater than the extent of annealing damages in the low-energy spectral region, analysis of the temperature dependence of the effective number of carriers (formula presented) below 0.5 eV is presented. © 2001 The American Physical Society

AC loss measurements in HTS coil assemblies with hybrid coil structures

This is the Accepted Manuscript version of an article accepted for publication in 'Superconductor Science and Technology". IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.1088/0953-2048/29/9/095011

Erratum: Annealing effects on the optical conductivity of single crystalLa1−xCaxMnO3,(x=0.1,0.265) [Phys. Rev. B64, 035106 (2001)]

No description supplie

Indications of a ferromagnetic quantum critical point in SmN 1-δ

We investigate the previously observed superconductivity in ferromagnetic SmN in the context of the breakdown of order between two magnetic phases. Nitrogen vacancy doped SmN 1-δ is a semiconductor which lies in the intermediary between ferromagnetic SmN and anti-ferromagnetic Sm. Optical data reported here corroborate the prediction that electrical transport is mediated by Sm 4f defect states, and electrical transport measurements characterise the metal-insulator transition over the doping range. Our measurements show that the superconducting state in nitrogen vacancy doped SmN 1-δ is the most robust near the breakdown of magnetic order, and indicate the location of a quantum critical point. Furthermore we provide additional evidence that the superconducting state is formed from majority spin electrons and thus of unconventional S = 1 type

AC Loss Measurements in a Hybrid REBCO/BSCCO Coil Assembly

Wire cost and ac loss are paramount issues for the future uptake of high-temperature superconductor (HTS) ac machines. In particular, new approaches which reduce the total manufacturing cost of HTS coils are critical to the future commercial uptake of HTS technology. Here, we present the results of ac loss measurements of a hybrid HTS coil assembly comprising end windings (EWs) wound with BSCCO wire and a central winding (CW) wound with REBCO wire. As a cost mitigation measure, we have chosen to use REBCO wire which possesses a relatively low self-field Ic at 77 K, equating to approximately half that of the BSCCO wire. Transport ac loss measurements were made on the total coil assembly at both 77 K and 65 K, using a recently developed lockin amplifier approach. We show the ac losses in the assembly at these temperatures scale with the overall transport critical current threshold of the hybrid coil assembly. The results are then compared with a reference coil assembly having similar geometrical dimensions, and in which both the EWs and CW were wound with identical BSCCO wire. Surprisingly, we observe that the ac loss of the hybrid coil is lower than that of the BSCCO coil, despite the CW of the hybrid coil employing wire a lower overall average Ic. We attribute this effect to the lower ac loss of coated conductor REBCO wire under parallel field, which is the dominant field orientation in the CW region. Based on this observation, we conclude that the hybrid assembly design is a good design strategy for HTS coil assemblies, enabling both wire cost and ac loss to be balanced. © 2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works

Development of a brushless HTS exciter for a 10 kW HTS synchronous generator

HTS synchronous generators, in which the rotor coils are wound from high-Tc superconducting wire, are exciting attention due to their potential to deliver very high torque and power densities. However, injection of the large DC currents required by the HTS rotor coils presents a technical challenge. In this paper we discuss the development of a brushless HTS exciter which operates across the cryostat wall to inject a superconducting DC current into the rotor coil circuit. This approach fundamentally alters the thermal load upon the cryogenic system by removing the need for thermally inefficient normal-conducting current leads. We report results from an experimental laboratory device and show that it operates as a constant voltage source with an effective internal resistance. We then discuss the design of a prototype HTS-PM exciter based on our experimental device, and describe its integration with a demonstration HTS generator. This 200 RPM, 10 kW synchronous generator comprises eight double pancake HTS rotor coils which are operated at 30 K, and are energised to 1.5 T field through the injection of 85 A per pole. We show how this excitation can be achieved using an HTS-PM exciter consisting of 12 stator poles of 12 mm YBCO coated-conductor wire and an external permanent magnet rotor. We demonstrate that such an exciter can excite the rotor windings of this generator without forming a thermal-bridge across the cryostat wall. Finally, we provide estimates of the thermal load imposed by our prototype HTS-PM exciter on the rotor cryostat. We show that duty cycle operation of the device ensures that this heat load can be minimised, and that it is substantially lower than that of equivalently-rated conventional current leads

Using optical spectroscopy to probe the impact of atomic disorder on the Heusler alloy Co2MnGa

The exceptional electronic and spintronic properties of magnetic Heusler alloys, which include half-metals and Weyl semimetals, are strongly sensitive to deviations from the ideal atomic structure. To ensure that these materials have been produced with the desired properties, it is necessary to determine both the structural ordering and the electronic structure, which can be challenging. Here, we present the results of a far-infrared-to-visible optical spectroscopy study of films of room-temperature ferromagnetic Weyl semimetal Co2MnGa. Combined with a determination of the level of ordering from x-ray diffraction, we have investigated near Fermi energy valence and conduction band intra- and interband transitions and their dependence on the atomic order. Motivated by band structure calculations, we have modeled our optical spectra with two Drude terms and two Lorentz oscillators, where the latter are assigned to interband transitions. The scattering rate of the itinerant carriers, determined from the width of the Drude term, increases threefold with increasing disorder, while the carrier density to effective mass ratio is unchanged. Based on our band structure and the joint density of states calculations, we have assigned the oscillator that dominates the interband spectral region near 1 eV to transitions across the minority spin gap along the Γ-X direction. It is found that the energy of this transition is strongly sensitive to the degree of order and decreases rapidly with increasing disorder as states fill a decreasing minority spin gap. Our results demonstrate optical spectroscopy is a sensitive way to fingerprint structural order in the technologically relevant near Fermi level electronic states in Heusler alloys