arXiv subjects

Robert G. Buckley

Publications and source records attributed to Robert G. Buckley.

5 recordsLinked to original sources

arXiv2026

Strain-induced suppression of thermochromism in divalent cobalt molybdate thin films

Thermochromic oxides provide a platform for coupling lattice, electronic, and magnetic degrees of freedom, with divalent cobalt molybdate $CoMoO_4$ as a prototypical example. Despite extensive studies on powders, its thin-film behaviour - critical for device applications - remains largely unexplored. Here, we present a combined experimental and theoretical investigation of $CoMoO_4$ thin films, including the first THz-VIS-UV spectra of $β- CoMoO_4$ in thin-film form. In contrast to bulk powders, which undergo a first order $β\rightarrow α$ structural transition near 230K, the thin films retain the high-temperature $β-$phase across the entire temperature range. We show that microstrain fundamentally reshapes the phase landscape, suppressing thermochromism and stabilizing the $β-$phase. The optical spectra reveal pronounced phonon and electronic anomalies, including a softening of a low-energy, cation-dominated phonon (42cm$^{-1}$) upon cooling, in contrast to conventional mode-hardening. This behaviour indicates incipient atomic displacements analogous to those driving the bulk $β\rightarrow α$ transition, despite the absence of a structural phase change, and saturates between 200 and 150K. Temperature-dependent X-ray diffraction confirms persistent β-phase symmetry with increasing microstrain, consistent with strain-induced frustration of the first-order transition. At higher energies (0.12-3.7eV), the optical response exhibits a blue-shift of Co$^{2+}$crystal-field transitions and charge-transfer excitations, indicating a strain-enhanced ligand field. Supported by structural refinements and theoretical calculations, we identify crystal field strengthening as the key mechanism stabilizing the $β$-phase. These results establish strain as a thermodynamic lever to control phase stability and functional properties in thermochromic oxides.

cond-mat.mtrl-sci↗

arXiv2021

Rotation, Embedding and Topology for the Szekeres Geometry

Recent work on the Szekeres inhomogeneous cosmological models uncovered a surprising rotation effect. Hellaby showed that the angular $(θ, ϕ)$ coordinates do not have a constant orientation, while Buckley and Schlegel provided explicit expressions for the rate of rotation from shell to shell, as well as the rate of tilt when the 3-space is embedded in a flat 4-d Euclidean space. We here investigate some properties of this embedding, for the quasi-spherical recollapsing case, and use it to show that the two sets of results are in complete agreement. We also show how to construct Szekeres models that are closed in the 'radial' direction, and hence have a 'natural' embedded torus topology. Several explicit models illustrate the embedding as well as the shell rotation and tilt effects.

gr-qc↗

arXiv2020

Physical geometry of the quasispherical Szekeres models

The quasispherical Szekeres metric is an exact solution to Einstein's equations describing an inhomogeneous and anisotropic cosmology. Though its governing equations are well-known, there are subtle, often-overlooked details in how the model's functions relate to its physical layout, including the shapes and relative positions of structures. We present an illustrated overview of the quasispherical Szekeres models and show exactly how the model functions relate to the physical shape and distribution of matter. In particular, we describe a shell rotation effect that has not previously been fully understood. We show how this effect relates to other known properties, and lay out some mathematical tools useful for constructing models and picturing them accurately.

gr-qc↗

arXiv2019

Exploring Disorder in the Spin Gapless Semiconductor Mn$_2$CoAl

Since the prediction of spin-gapless semiconducting behaviour in the Heusler compound Mn$_2$CoAl, evidence of spin-gapless behaviour in thin films has typically been inferred from magnetotransport measurements. The spin gapless state is however fragile, and further, band structure calculations indicate that even a small amount of atomic disorder may destroy it. To explore the impact of disorder on the properties of Mn$_2$CoAl, we have undertaken an experimental study of the structural, magnetotransport and optical properties from the far infrared to the UV, on DC magnetron sputtered Mn$_2$CoAl thin films. A very short mean free path, of the order of a lattice spacing, is extracted from the DC transport data. A room temperature resistivity of 200 $μ$$Ω$cm along with a small and negative temperature coefficient of resistance between 4 and 400 K was measured. We note that parameters of this magnitude are often observed in disordered metals. We find this behaviour is well described by a weak localisation model, a result that is supported by a large Drude contribution to the optical response, where a high scattering rate is derived, which is equal to the value derived from the DC conductivity and Hall effect data. We also note the strong similarities between the magnetotransport behaviour reported for Mn$_2$CoAl films in the literature, including ours. We conclude that, based on comparisons between the experimental data, and recent band structure calculations that explicitly include disorder, as-prepared Mn$_2$CoAl films are best described as a disordered metal, rather than a spin gapless semiconductor.

cond-mat.mtrl-sci↗

arXiv2019

CMB dipoles and other low-order multipoles in the quasispherical Szekeres model

Several authors have previously shown that Gpc-scale void based on the spherically symmetric LTB model can provide a good fit to certain cosmological data, including the SNIa data, but it is only consistent with the observed CMB dipole if we are located very close to the center, in violation of the Copernican principle. In this work we investigate the more general quasispherical Szekeres model, which does not include spherical symmetry, in order to determine whether this option may be less constricting. We find that the observer is still constrained to a small region, but it is not as geometrically ``special'' as the center of an LTB void. Furthermore, whereas the quadrupole and octupole near the center of an LTB void are necessarily small, certain Szekeres models can include a significant quadrupole while still being consistent with the observed dipole, hinting that Szekeres models may be able give an explanation for the observed quadrupole/octupole anomalies.

astro-ph.CO↗