arXiv subjects

Sian E. Dutton

Publications and source records attributed to Sian E. Dutton.

3 recordsLinked to original sources

arXiv2022

Free-spin dominated magnetocaloric effect in dense Gd$^{3+}$ double perovskites

Frustrated lanthanide oxides with dense magnetic lattices are of fundamental interest for their potential in cryogenic refrigeration due to a large ground state entropy and suppressed ordering temperatures, but can often be limited by short-range correlations. Here, we present examples of frustrated fcc oxides, Ba$_2$GdSbO$_6$ and Sr$_2$GdSbO$_6$ and the new site-disordered analog Ca$_2$GdSbO$_6$ ([CaGd]$_A$[CaSb]$_B$O$_6$), in which the magnetocaloric effect is influenced by minimal superexchange ($J_1 \sim 10$ mK). We report on the crystal structures using powder x-ray diffraction and the bulk magnetic properties through low-field susceptibility and isothermal magnetization measurements. The Gd compounds exhibit a magnetic entropy change of up to -15.8 J/K/mol$_\textrm{Gd}$ in a field of 7 T at 2 K, a 20 % excess compared to the value of -13.0 J/K/mol$_\textrm{Gd}$ for a standard in magnetic refrigeration, Gd$_3$Ga$_5$O$_{12}$. Heat capacity measurements indicate a lack of magnetic ordering down to 0.4 K for Ba$_2$GdSbO$_6$ and Sr$_2$GdSbO$_6$, suggesting cooling down through the liquid 4-He regime. A mean-field model is used to elucidate the role of primarily free spin behavior in the magnetocaloric performance of these compounds in comparison to other top-performing Gd-based oxides. The chemical flexibility of the double perovskites raises the possibility of further enhancement of the magnetocaloric effect in the Gd$^{3+}$ fcc lattices.

cond-mat.str-el↗

arXiv2016

Electronic and magnetic properties of superconducting LnO$_{1-x}$F$_{x}$BiS$_{2}$ (Ln = La, Ce, Pr, and Nd) from first principles

A density functional theory study of the BiS$_{2}$ superconductors containing rare-earths: \textit{Ln}O$_{1-x}$F$_{x}$BiS$_{2}$ (\textit{Ln} = La, Ce, Pr, and Nd) is presented. We find that CeO$_{0.5}$F$_{0.5}$BiS$_{2}$ has competing ferromagnetic and weak antiferromagnetic tendencies, the first one corresponding to experimental results. We show that PrO$_{0.5}$F$_{0.5}$BiS$_{2}$ has a strong tendency for magnetic order, which can be ferromagnetic or antiferromagnetic depending on subtle differences in 4$f$ orbital occupations. We demonstrate that NdO$_{0.5}$F$_{0.5}$BiS$_{2}$ has a stable magnetic ground state with weak tendency to order. Finally, we show that the change of rare earth does not affect the Fermi surface, and predict that CeOBiS$_{2}$ should display a pressure induced phase transition to a metallic, if not superconducting, phase under pressure.

cond-mat.supr-con↗

arXiv2014

Effects of stoichiometric doping in superconducting Bi-O-S compounds

Newly discovered Bi-O-S compounds remain an enigma in attempts to understand their electronic properties. A recent study of Bi$_{4}$O$_{4}$S$_{3}$ has shown it to be a mixture of two phases, Bi$_{2}$OS$_{2}$ and Bi$_{3}$O$_{2}$S$_{3}$, the latter being superconducting [W. A. Phelan et al., J. Am. Chem. Soc. 135, 5372 (2013)]. Using density functional theory, we explore the electronic structure of both the phases and the effect of the introduction of stacking faults. Our results demonstrate that the S$_{2}$ layers dope the bismuth-sulphur bands. The bands at the Fermi level are of clear two-dimensional character. One band manifold is confined to the two adjacent, square-lattice bismuth-sulphur planes, a second manifold is confined to the square lattice of sulphur dimers. We show that the introduction of defects in the stacking does not influence the electronic structure. Finally, we also show that spin-orbit coupling does not have any significant effect on the states close to the Fermi level at the energy scale considered.

cond-mat.supr-con↗