D. Shao, P. Yotprayoonsak, V. Saunajoki, M. Ahlskog, J. Virtanen, V. Kangas, A. Volodin, C. Van Haesendonck, M. Burdanova, C. D. W. Mosley and J. Lloyd-Hughes
Nanotechnology 29 145203 (Feb 2018) [ pdf ] [ ref ]

We have examined the conductive properties of a carbon nanotube (CNT) based thin film, which were prepared via dispersion in water by non-covalent functionalization of the nanotubes with xylan, a type of hemicellulose. Measurements of low temperature conductivity, Kelvin Probe Force Microscopy, and high frequency (THz) conductivity elucidated the intra-tube and inter-tube charge transport processes in this material. The measurements show excellent conductive properties of the as prepared thin films, with bulk conductivity up to 2000 S/cm. The transport results demonstrate that the hemicellulose does not seriously interfere with the inter-tube conductance.

C. D. W. Mosley, D. Prabhakaran and J. Lloyd-Hughes
J. Phys. D: Applied Physics 51 084002 (Jan 2018) [ pdf ][ ref ]

We demonstrate that electromagnons can be used to directly probe the nature of a phase transition between magnetically ordered phases in an improper ferroelectric. The antiferromagnetic/paraelectric to antiferromagnet/ferroelectric phase transition in Cu_1-xZn_xO (x = 0, 0.05) alloys was tracked via the electromagnon response using terahertz time-domain spectroscopy, on heating and cooling through the phase transition. The transition was found to exhibit thermal hysteresis, confirming its first-order nature, and to broaden under the influence of spin-disorder upon Zn substitution. The energy of the electromagnon increases upon alloying, as a result of the non-magnetic ions modifying the magnetic interactions that give rise to the multiferroic phase and electromagnons. We describe our findings in the context of recent theoretical work that examined improper ferroelectricity and electromagnons in CuO from phenomenological and first-principles approaches.

C. D. W. Mosley, M. Failla, D. Prabhakaran and J. Lloyd-Hughes
Scientific Reports 7:12337 (Sept 2017) [ pdf ][ ref ]

We introduce a polarization-resolved terahertz time-domain spectrometer with a broadband (0.3-2.5THz), rotatable THz polarization state, and which exhibits minimal change in the electric field amplitude and polarization state upon rotation. This was achieved by rotating an interdigitated photoconductive emitter, and by detecting the orthogonal components of the generated THz pulse via electro-optic sampling. The high precision (<0.1°) and accuracy (<1.0°) of this approach is beneficial for the study of anisotropic materials without rotating the sample, which can be impractical, for instance for samples held in a cryostat. The versatility of this method was demonstrated by studying the anisotropic THz optical properties of uniaxial and biaxial oxide crystals. For uniaxial ZnO and LaAlO₃, which have minimal THz absorption across the measurement bandwidth, the orientations of the eigenmodes of propagation were conveniently identified as the orientation angles that produced a transmitted THz pulse with zero ellipticity, and the birefringence was quantified. In CuO, a multiferroic with improper ferroelectricity, the anisotropic THz absorption created by an electromagnon was investigated, mapping its selection rule precisely. For this biaxial crystal, which has phonon and electromagnon absorption, the polarization eigenvectors exhibited chromatic dispersion, as a result of the monoclinic crystal structure and the frequency-dependent complex refractive index.

J. Lloyd-Hughes, C. D. W. Mosley, S. P. P. Jones, M. R. Lees, A. Chen, Q. X. Jia, E. M. Choi and J. L. MacManus-Driscoll
Nano Lett. 17:2506 (Mar 2017) [ pdf ][ ref ]

We show that colossal magnetoresistance persists up to THz frequencies, in manganite nanocomposites and thin films. At the metal-insulator transition the THz conductivity of the nanocolumn film was dramatically enhanced by the application of a magnetic field, creating a non-Drude conductivity that increased with frequency. Surprisingly, the observed colossal THz magnetoresistance is large for ac motion on nanometre length scales, even when the dc magnetoresistance on macroscopic length scales is negligible.