Tracking a hysteretic and disorder-broadened phase transition via the electromagnon response in improper ferroelectrics

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 antiferromagnetic/ferroelectric phase transition in Cu1-xZnxO (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

Enhanced polarisation control and extreme electric fields: advances in terahertz spectroscopy applied to anisotropic materials and magnetic phase transitions

This thesis reports advances in terahertz time-domain spectroscopy, relating to the development of new techniques and components that enhance the experimentalist's control over the terahertz polarisation state produced by photoconductive emitters, and in utilising the dynamic magnetoelectric response at THz frequencies, in the form of electromagnons, to probe material properties at a transition between two magnetically ordered phases. Two methods of controlling the terahertz polarisation state are reported: The first method is based upon mechanical rotation of an interdigitated photoconductive emitter, and is implemented in a rotatable-polarisation terahertz time-domain spectrometer, the calibration of which is demonstrated to produce a highly uniform polarisation state at all angles. This method is then demonstrated experimentally to identify the orientations of the normal modes of propagation in the plane of birefringent samples, to extract the full complex refractive index along these directions, and to investigate the optical selection rules of an absorbing material. The second method presents a new photoconductive emitter design, based upon separate interdigitated pixel elements for the generation of the horizontally and vertically polarised components of the terahertz pulses, that permits rotation of the polarisation state solely by electrical control. The design, fabrication and experimental verification of the device is reported, demonstrating polarisation control on timescales orders of magnitude faster than those achievable in mechanical rotation methods. A method of using the electromagnon response in Cu

Revisiting the exercise heart rate-music tempo preference relationship

In the present study, we investigated a hypothesized quartic relationship (meaning three inflection points) between exercise heart rate (HR) and preferred music tempo. Initial theoretical predictions suggested a positive linear relationship (Iwanaga, 1995a, 1995b); however, recent experimental work has shown that as exercise HR increases, step changes and plateaus that punctuate the profile of music tempo preference may occur (Karageorghis, Jones, & Stuart, 2008). Tempi bands consisted of slow (95–100 bpm), medium (115–120 bpm), fast (135–140 bpm), and very fast (155–160 bpm) music. Twenty-eight active undergraduate students cycled at exercise intensities representing 40, 50, 60, 70, 80, and 90% of their maximal HR reserve while their music preference was assessed using a 10-point scale. The Exercise Intensity x Music Tempo interaction was significant, F(6.16, 160.05) = 7.08, p < .001, ηp 2 =.21, as was the test for both cubic and quartic trajectories in the exercise HR–preferred-music-tempo relationship (p < .001). Whereas slow tempo music was not preferred at any exercise intensity, preference for fast tempo increased, relative to medium and very fast tempo music, as exercise intensity increased. The implications for the prescription of music in exercise and physical activity contexts are discussed

Colossal terahertz magnetoresistance at room temperature in epitaxial La0.7Sr0.3MnO3 nanocomposites and single-phase thin films

Colossal magnetoresistance (CMR) is demonstrated at terahertz (THz) frequencies by using terahertz time-domain magnetospectroscopy to examine vertically-aligned nanocomposites (VANs) and planar thin films of La_0.7Sr_0.3MnO_3. At the Curie temperature (room temperature) the THz conductivity of the VAN was dramatically enhanced by over 2 orders of magnitude under the application of a magnetic field, with a non-Drude THz conductivity that increased with frequency. The dc CMR of the VAN is controlled by extrinsic magnetotransport mechanisms such as spin-polarized tunneling between nano-grains. In contrast, we find that THz CMR is dominated by intrinsic, intragrain transport: the mean free path was smaller than the nanocolumn size, and the planar thin-film exhibited similar THz CMR to the VAN. Surprisingly, the observed colossal THz magnetoresistance suggests that the magnetoresistance can be large for ac motion on nanometre length scales, even when the magnetoresistance is negligible on the macroscopic length scales probed by dc transport. This suggests that colossal magnetoresistance at THz frequencies may find use in nanoelectronics and in THz optical components controlled by magnetic fields. The VAN can be scaled in thickness while retaining a high structural quality, and offers a larger THz CMR at room temperature than the planar film

The political economy of progressive fiscal contracts in Africa and Latin America

Motivation How can poorer developing countries escape from the vicious circle in which, because the state is fragile, those countries cannot raise sufficient public revenue to be able to finance development, leading to the persistence of poverty and state fragility? We explore a new approach to this problem, which we call progressive fiscal contracts, in which government earmarks the proceeds of particular taxes to be spent on forms of expenditure which will have widespread benefits for lower income groups, such as health, education and social protection. Taxpayers are thus offered a contractual relationship with government (better‐targeted delivery of public services in return for tax payments) in place of a coercive one (simply being ordered to pay taxes, with nothing being offered in exchange). We seek to examine whether this kind of contractual approach offers a way forward for developing countries. Purpose Across five countries (Bolivia, Ecuador, Venezuela, Ghana and Zambia) between 2000 and 2015, we seek to find out whether tax yields have improved following the introduction of progressive fiscal contracts, whether conflict and poverty have declined, and whether there have been countervailing costs in terms of reduced efficiency. We also examine the experience of two countries (Brazil and Chile) where there is no formal ear‐marking but government has encouraged the public to think of particular taxes as being associated with particular forms of expenditure. Approach and methods We assess the impact of changes in tax yields, welfare indicators and conflict indicators by means of panel‐data regressions, tabular comparisons and, in Bolivia, qualitative interviews. Changes in efficiency are assessed through examination of changes in tax structure. Findings Across all of the countries surveyed, the introduction of progressive fiscal contracts has been associated with a reduction in headcount poverty between 2000 and 2015, and in Bolivia our qualitative evidence suggests that the relationship can be seen as a causal one. In three cases out of five (Ghana, Bolivia and Ecuador) tax yields have increased, and in two (Ecuador and Bolivia) there was a significant reduction in political violence. In the Latin American cases examined, but not the African ones, there was a shift from royalty‐based taxation to income‐based taxation of natural resources, suggesting the likelihood of an improvement in efficiency over the period in those countries only. In these cases, the stereotypical view that progressive fiscal contracts improve equity at the expense of efficiency is contradicted. Policy implications (or conclusions) ‘Progressive fiscal contracts’, which originated as a device for making tax payments more palatable by offering social benefits in return, show promise as an innovative strategy for boosting tax ratios, reducing political violence and reducing poverty, which deserves further exploration

Unified View of Scaling Laws for River Networks

Scaling laws that describe the structure of river networks are shown to follow from three simple assumptions. These assumptions are: (1) river networks are structurally self-similar, (2) single channels are self-affine, and (3) overland flow into channels occurs over a characteristic distance (drainage density is uniform). We obtain a complete set of scaling relations connecting the exponents of these scaling laws and find that only two of these exponents are independent. We further demonstrate that the two predominant descriptions of network structure (Tokunaga's law and Horton's laws) are equivalent in the case of landscapes with uniform drainage density. The results are tested with data from both real landscapes and a special class of random networks.Comment: 14 pages, 9 figures, 4 tables (converted to Revtex4, PRE ref added

Geometry of River Networks I: Scaling, Fluctuations, and Deviations

This article is the first in a series of three papers investigating the detailed geometry of river networks. Large-scale river networks mark an important class of two-dimensional branching networks, being not only of intrinsic interest but also a pervasive natural phenomenon. In the description of river network structure, scaling laws are uniformly observed. Reported values of scaling exponents vary suggesting that no unique set of scaling exponents exists. To improve this current understanding of scaling in river networks and to provide a fuller description of branching network structure, we report here a theoretical and empirical study of fluctuations about and deviations from scaling. We examine data for continent-scale river networks such as the Mississippi and the Amazon and draw inspiration from a simple model of directed, random networks. We center our investigations on the scaling of the length of sub-basin's dominant stream with its area, a characterization of basin shape known as Hack's law. We generalize this relationship to a joint probability density and show that fluctuations about scaling are substantial. We find strong deviations from scaling at small scales which can be explained by the existence of linear network structure. At intermediate scales, we find slow drifts in exponent values indicating that scaling is only approximately obeyed and that universality remains indeterminate. At large scales, we observe a breakdown in scaling due to decreasing sample space and correlations with overall basin shape. The extent of approximate scaling is significantly restricted by these deviations and will not be improved by increases in network resolution.Comment: 16 pages, 13 figures, Revtex4, submitted to PR

Observation of eight-photon entanglement

Using ultra-bright sources of pure-state entangled photons from parametric down conversion, an eight-photon interferometer and post-selection detection, we demonstrate the ability to experimentally manipulate eight individual photons and report the creation of an eight-photon Schr\"odinger cat state with an observed fidelity of $0.708 \pm 0.016$.Comment: 6 pages, 4 figure