arXiv subjects
Publications and source records attributed to Stephen R. Julian.
The field of hydride superconductivity has recently been mired in a controversy that might divert attention from the question of central importance: do hydrides support genuine superconductivity or not? We examine some key papers from the field, and conclude that hydride superconductivity is real.
We have measured magnetic torque on a $T_N=7$ K single crystal of $α$-RuCl$_3$, as a function of the field angle in the $ab$-plane, focusing on temperatures between 2 and 20 K and fields from 0 to 9 T. We find a number of features, many of which can be classified by their angular periodicity. The sample shows an oscillation with a period of 180$^\circ$ (i.e.\ two-fold periodicity) and within the magnetically ordered zigzag phase there is a 60$^\circ$ period (i.e.\ six-fold) sawtooth pattern, which can be explained by reorientation of the zigzag domains as the crystal rotates in the applied field. We argue that the six-fold sawtooth and the two-fold sinusoidal signals arise from distinct regions of the crystal. Suppressing the zigzag order with an applied field above $\sim8$ T at low temperature, a six-fold {\sl sinusoidal} signal remains, suggesting that there is fluctuating zigzag order in the putative field-induced quantum spin liquid state. Finally, in testing theoretical results which predict a torque response with divergent slope across C$_2$-preserving $b$-axes (B1-axis), we find no features like that predicted for Ising topological order. Instead we find features at low temperatures and fields just above the zigzag phase across the non-C$_2$-preserving $b$-axes (B2-axes). Interpretation of this feature is complicated by the development of other similar signatures nearby at slightly lower fields, and by clear enhancement with thermal cycling. Additionally, we contrast the torque response of $T_N \sim$ 7 K and 14 K samples.
We probe the relationship between magnetism and "non-metallic" transport in the magnetically frustrated metal FeCrAs using the low-field Hall effect. We find that the low-field Hall coefficient is dependent on crystal orientation, and exhibits two temperature-induced sign reversals when the current is within the $ab$-plane. We suggest that these observations are due to scattering from magnetic fluctuations that become momentum-dependent near the onset of long-range magnetic order. It seems, however, that this additional spin-fluctuation scattering rides on top of the non-metallic resistivity, and Hund's metal interorbital correlations remain a possible source of the non-metallic behaviour.
We show that the pressure-temperature phase diagram of the Mott insulator Ca$_{2}$RuO$_{4}$ features a metal-insulator transition at 0.5GPa: at 300K from paramagnetic insulator to paramagnetic quasi-two-dimensional metal; at $T \leq$ 12K from antiferromagnetic insulator to ferromagnetic, highly anisotropic, three-dimensional metal. % We compare the metallic state to that of the structurally related p-wave superconductor Sr$_{2}$RuO$_{4}$, and discuss the importance of structural distortions, which are expected to couple strongly to pressure.