Possible Microscopic Doping Mechanism in Tl-2201

X-ray absorption spectroscopy on oxygen-annealed, self-flux-grown single crystals of Tl-2201 suggests a microscopic doping mechanism whereby interstitial oxygens are attracted to copper substituted on the thallium site, contributing holes to both the planes and to these coppers, and typically promoting only one hole to the plane rather than two. These copper substituents would provide an intrinsic hole doping. The evidence for this is discussed, along with an alternative interpretation.Comment: 2 pages, 1 figure, submitted as conference proceedings for M2S-IX, Toky

Encapsulated Single Crystal Growth and Annealing of the High-Temperature Superconductor Tl-2201

Highly-perfect platelet single crystals of Tl_2Ba_2CuO_{6+d} (Tl-2201) were grown by a self-flux technique. A novel encapsulation scheme allowed the precursors to react prior to the sealing required to contain volatile thallium oxides, and permitted the removal of melt at the conclusion of growth, reproducibly producing high yields of clean crystals. The crystals were annealed under well-controlled oxygen partial pressures, then characterised. They have sharp superconducting transitions, narrow X-ray rocking curves and a low 4% substitution of thallium by copper, all evidence of their high perfection and homogeneity. The crystals are orthorhombic at most dopings, and a previously unreported commensurate superlattice distortion is observed.Comment: 8 pages, 5 figures, submitted to Journal of Crystal Growt

Spiral magnetism, spin flop, and pressure induced ferromagnetism in the negative charge transfer gap insulator Sr2FeO4

Iron IV oxides are strongly correlated materials with negative charge transfer energy negative Delta , and exhibit peculiar electronic and magnetic properties such as topological helical spin structures in themetallic cubic perovskite SrFeO3. Here, the spin structure of the layered negative Delta insulator Sr2FeO4 was studied by powder neutron diffraction in zero field and magnetic fields up to 6.5 T. Below TN 56K, Sr2FeO4 adopts an elliptical cycloidal spin structure with modulated magnetic moments between 1.9 and 3.5 amp; 956;B and a propagation vector k amp; 964;, amp; 964;, 0 with amp; 964; 0.137. With increasing magnetic field the spin structure undergoes a spin flop transition near 5 T. Synchrotron 57Fe Mössbauer spectroscopy reveals that the spin spiral transforms to a ferromagnetic structure at pressures between 5 and 8 GPa, just in the pressure range where a Raman active phonon nonintrinsic to the K2NiF4 type crystal structure vanishes. These results indicate an insulating ground state which is stabilized by a hidden structural distortion and differs from the charge disproportionation in other Fe IV oxide

Magnetic phase diagram of Sr3Fe2O7 x

Magnetometry, electrical transport, and neutron scattering measurements were performed on single crystals of the Fe4 containing perovskite related phase Sr3Fe2O7 x as a function of oxygen content. Although both the crystal structure and electron configuration of this compound are closely similar to those of well studied ruthenates and manganates, it exhibits very different physical properties. The fully oxygenated compound x 0 exhibits a charge disproportionation transition at TD 340 K, and an antiferromagnetic transition at TN 115 K. For temperatures T lt; TD, the material is a small gap insulator; the antiferromagnetic order is incommensurate, which implies competing exchange interactions between the Fe4 moments. The fully deoxygenated compound x 1 is highly insulating, and its Fe3 moments exhibit commensurate antiferromagnetic order below TN similar to 600 K. Compounds with intermediate delta exhibit different order with lower TN, likely as a consequence of frustrated exchange interactions between Fe3 and Fe4 sublattices. A previous proposal that the magnetic transition temperature reaches zero is not supporte

Hidden Charge Order in an Iron Oxide Square Lattice Compound

Since the discovery of charge disproportionation in the FeO2 square lattice compound Sr3Fe2O7 by Mössbauer spectroscopy more than fifty years ago, the spatial ordering pattern of the disproportionated charges has remained hidden to conventional diffraction probes, despite numerous x ray and neutron scattering studies. We have used neutron Larmor diffraction and Fe K edge resonant x ray scattering to demonstrate checkerboard charge order in the FeO2 planes that vanishes at a sharp second order phase transition upon heating above 332 K. Stacking disorder of the checkerboard pattern due to frustrated interlayer interactions broadens the corresponding superstructure reflections and greatly reduces their amplitude, thus explaining the difficulty of detecting them by conventional probes. We discuss the implications of these findings for research on hidden order in other material

Incommensurate and multiple q magnetic misfit order in the frustrated quantum spin ladder material antlerite Cu3SO4 OH 4

In frustrated magnetic systems, the competition amongst interactions can introduce extremely high degeneracy and prevent the system from readily selecting a unique ground state. In such cases, the magnetic order is often exquisitely sensitive to the balance among the interactions, allowing tuning among novel magnetically ordered phases. In antlerite, Cu3SO4 OH 4, Cu2 S 1 2 quantum spins populate three leg zigzag ladders in a highly frustrated quasi one dimensional structural motif. We demonstrate that at zero applied field, in addition to its recently reported low temperature phase of coupled ferromagnetic and antiferromagnetic spin chains, this mineral hosts an incommensurate helical cycloidal state, an idle spin state, and a multiple q phase which is the magnetic analog of misfit crystal structures. The antiferromagnetic order on the central leg is reentrant. The high tunability of the magnetism in antlerite makes it a particularly promising platform for pursuing exotic magnetic orde