Professor Stephen Blundell

Abstract:

LiOsO3 is the first example of a new class of material called a ferroelectric metal. We performed zero-field and longitudinal-field μSR, along with a combination of electronic structure and dipole field calculations, to determine the magneticground state of LiOsO3. We find that the sample contains both static Li nuclear moments and dynamic Os electronic moments. Below ≈0.7 K, the fluctuations of the Os moments slow down, though remain dynamic down to 0.08 K. We expect this could result in a frozen-out, disordered ground state at even lower temperatures.

Abstract:

We report the results of a muon-spin rotation ($\mu$SR) experiment to determine the superconducting ground state of the iron-based superconductor $CsCa_2Fe_4As_4F_2$ with $T_{\rm c} \approx 28.3\,K$. This compound is related to the fully-gapped superconductor $CaCsFe_4As_4$, but here the Ca-containing spacer layer is replaced with one containing $Ca_2F_2$. The temperature evolution of the penetration depth strongly suggests the presence of line nodes and is best modelled by a system consisting of both an $s-$ and a $d-$wave gap. We also find a potentially magnetic phase which appears below $\approx 10\,K$ but does not appear to compete with the superconductivity. This compound contains the largest alkali atom in this family of superconductors and our results yield a value for the in-plane penetration depth of $\lambda_{ab}(T=0)=423(5)\,nm$.

Abstract:

The anion-ordered oxide-hydride SrVO2H is an antiferromagnetic insulator due to strong correlations between vanadium d electrons. In an attempt to hole-dope SrVO2H into a metallic state, a strategy of first preparing SrV1-xTixO3 phases and then converting them to the corresponding SrV1-xTixO2H phases via reaction with CaH2 was followed. This revealed that the solid solution between SrVO3 and SrTiO3 is only stable at high temperature. In addition, reactions between SrV0.95Ti0.05O3 and CaH2 were observed to yield SrV0.95Ti0.05O1.5H1.5 not SrV0.95Ti0.05O2H. This dramatic change in reactivity for a very modest change in initial chemical composition is attributed to an electronic destabilization of SrVO2H on titanium substitution. Density functional theory calculations indicate that the presence of an anion-ordered, tetragonal SrMO2H phase is uniquely associated with a d2 electron count and that titanium substitution leads to an electronic destabilization of SrV1-xTixO2H phases, which, ultimately, drives further reaction of SrV1-xTixO2H to SrV1-xTixO1.5H1.5. The observed sensitivity of the reaction products to the chemical composition of initial phases highlights some of the difficulties associated with electronically doping metastable materials prepared by topochemical reactions.