arXiv subjects

Rebecca Nicholls

Publications and source records attributed to Rebecca Nicholls.

5 recordsLinked to original sources

arXiv2026

Synthesis and stability of high-$T_c$ LaH$_{10\pmδ}$ films at high pressures

High-pressure hydrides hold the record for the highest superconducting critical temperatures across all classes of superconductors. Currently lanthanum decahydride, LaH$_{10}$, exhibits the highest critical temperature among binaries, with $T_c \approx$ 250 K at pressures between 140-180 GPa. Here, we report the synthesis of LaH$_{10\pmδ}$ films in two DACs at pressures of 168 GPa and 176 GPa via in situ laser heating of elemental lanthanum films with ammonia borane (NH$_3$BH$_3$) as the hydrogen donor. The high-symmetry fcc lanthanum sublattice (space group $Fm\bar3m$) is resolved using synchrotron X-ray diffraction, with unit cell parameters in excellent agreement with previous studies on bulk samples. We provide confirmation of high-$T_c$ superconductivity in LaH$_{10\pmδ}$ with highest $T_c$ of 247 K at 176 GPa evidenced in electrical measurements. The characteristic suppression of superconductivity is observed in magnetic fields. Furthermore, combined diffraction and electrical measurements reveal remarkable temporal stability of both the crystal structure and the high-$T_c$ superconductivity over the full measurement period of about 300 days post laser heating. Our work establishes film precursors using physical vapour deposition (PVD) techniques as a practical route to hydride formation, opening a pathway toward the controlled synthesis of promising ternary hydrides and the integration of micro-fabricated device geometries in diamond anvil cells.

cond-mat.supr-con↗

arXiv2026

Local Structure of Epitaxial Single Crystal UO$_{2+x}$ Thin Films

The influence of oxygen stoichiometry on the uranium local environment is explored in epitaxial single crystal uranium oxide thin films grown by DC magnetron sputtering. Through post-growth annealing, the stoichiometry of as-grown UO$_{2}$ films are tuned over an approximate stoichiometry range of $0.07 \leq x \leq 0.20$, estimated with X-ray photoelectron spectroscopy measurements of the U$-4f$ and O$-1s$ peaks. The local structure of the thin films are then probed using extended X-ray absorption fine structure measurements at the U $L_{3}$ absorption edge. We observe both the evolution of the U local environment of as a function of oxidation in UO$_{2+x}$, and that the near stoichiometric UO$_{2}$ film replicates the local structure of bulk UO$_{2}$ material standards well. The series of stoichiometrically varied samples highlights the non-trivial transitional behaviour of the UO$_{2+x}$ oxygen sublattice with increasing oxygen content in this stoichiometric regime, while also demonstrating the efficacy of this thin film synthesis route for actinide studies beyond their established use as idealised surfaces, which could be readily adapted for further stoichiometrically tailored material studies and UO$_{2+x}$ device fabrication.

cond-mat.mtrl-sci↗

arXiv2025

Doping dependence of the low temperature planar carrier density in overdoped YBa$_2$Cu$_3$O$_{7-δ}$

Whether a quantum critical point (QCP) demarcates the end of the pseudogap (PG) regime in hole-doped cuprates at a singular doping level $p^* \approx 0.19$ remains an open question. A crucial part of this puzzle is how the carrier density predicted by electronic structure calculations is recovered for $p > p^*$. Here, we use magnetic fields up to 67 T to suppress superconductivity down to 50 K, allowing simultaneous measurement of the low-temperature Hall number $n_{\mathrm{H}}$ and the in-plane resistivity anisotropy $ρ_a/ρ_b$ in overdoped Y$_{1-x}$Ca$_x$Ba$_2$Cu$_3$O$_{7-δ}$ single crystals. We confirm a previous finding [Badoux et al., Nature 531, 210 (2016)] that $n_{\mathrm{H}}$(50 K) exhibits a sharp increase below $p^*$. Using the measured resistivity anisotropy, we extract the planar carrier density $n_{\mathrm{pl}} = n_{\mathrm{H}} (ρ_a/ρ_b)^{-1}$. The doping dependence of $n_{\mathrm{pl}}$(50 K) reveals two key findings: (i) at optimal doping, $n_{\mathrm{pl}} \approx p$, and (ii) the sharp rise in $n_{\mathrm{H}}(p)$ is softened such that the full Fermi volume ($n_{\mathrm{pl}} = 1 + p$) is only partially recovered at $p^*$. This result disfavors a conventional QCP scenario in which the PG endpoint corresponds to a reconstructed Fermi surface.

cond-mat.supr-con↗

arXiv2024

Polyepitaxial grain matching to study the oxidation of uranium dioxide

Although the principal physical behaviour of a material is inherently connected to its fundamental crystal structure, the behaviours observed in the real-world are often driven by the microstructure, which for many polycrystalline materials, equates to the size and shape of the constituent crystal grains. Here we highlight a cutting edge synthesis route to the controlled engineering of grain structures in thin films and the simplification of associated 3-dimensional problems to less complex 2D ones. This has been applied to the actinide ceramic, uranium dioxide, to replicate structures typical in nuclear fission fuel pellets, in order to investigate the oxidation and subsequent transformation of cubic UO$_{2}$ to orthorhombic U$_{3}$O$_{8}$. This article shows how this synthesis approach could be utilised to investigate a range of phenomena, affected by grain morphology, and highlights some unusual results in the oxidation behaviour of UO$_{2}$, regarding the phase transition to U$_{3}$O$_{8}$.

cond-mat.mtrl-sci↗

arXiv2022

Structure and Phase Transitions of Metastable Hexagonal Uranium Thin Films

We report a simple technique for the synthesis of uniaxially textured, metastable hexagonal close-packed-like uranium thin films with thicknesses between 175-2800 Å. The initial structure and texture of the layers have been studied via X-ray diffraction and reflectivity and the time-dependent transitions of the samples into various orientations of orthorhombic $α$-U have been mapped by similar techniques. The final crystallographic orientations of each system and the timescales on which the transitions occur are found to depend on the lattice parameters of the original layer. The absence of the $α$-U (001) orientation in the transition products suggests that the transitions in these layers are mediated by mechanisms other than the [110] transverse acoustic phonon mode previously suggested for the cubic $γ$-U(110) to hcp-U(00.1) to $α$-U(001) displacive phase transition. Alternative transition pathways are discussed.

cond-mat.mtrl-sci↗