2008 Progress Report on Brain Research

Highlights new research on various disorders, nervous system injuries, neuroethics, neuroimmunology, pain, sense and body function, stem cells and neurogenesis, and thought and memory. Includes essays on arts and cognition and on deep brain stimulation

Non-Fermi-liquid behavior in nearly ferromagnetic metallic SrIrO3 single crystals

We report transport and thermodynamic properties of single-crystal SrIrO3 as a function of temperature T and applied magnetic field H. We find that SrIrO3 is a non-Fermi-liquid metal near a ferromagnetic instability, as characterized by the following properties: (1) small ordered moment but no evidence for long-range order down to 1.7 K; (2) strongly enhanced magnetic susceptibility that diverges as T or T1/2 at low temperatures, depending on the applied field; (3) heat capacity C(T,H) ~ -Tlog T that is readily amplified by low applied fields; (4) a strikingly large Wilson ratio at T< 4K; and (5) a T3/2-dependence of electrical resistivity over the range 1.7 < T < 120 K. A phase diagram based on the data implies SrIrO3 is a rare example of a stoichiometric oxide compound that exhibits non-Fermi-liquid behavior near a quantum critical point (T = 0 and H = 0.23 T)

Global tectonic studies: Hotspots and anomalous topography

Volcanic activity on Earth and its secular variations are compared with that on other terrestrial planets. Activity at divergent, transform, and convergent plate margins is described with particular emphasis on hot spots and flood basalts. The timing and causing of uplifting above 500 meters, which in not associated with either plate boundaries or the normal nonplate margin edges of continents is considered with particular focus on the Guyana Highlands in southern Venezuela and western British Guiana, and the Brazilian Highlands in the central, eastern, and southern parts of the country. The mode and mechanism of plateau uplifting and the re-elevation of old mountain belts and subsidence of intra-continental basins are also discussed

Coexisting charge and magnetic orders in the dimer-chain iridate Ba5AlIr2O11

We have synthesized and studied single-crystal Ba5AlIr2O11 that features dimer chains of two inequivalent octahedra occupied by tetravalent and pentavalent ions, respectively. Ba5AlIr2O11 is a Mott insulator that undergoes a subtle structural phase transition near 210 K and a magnetic transition at 4.5 K; the latter transition is surprisingly resistant to applied magnetic fields up to 12 T, but sensitive to modest applied pressure. All results indicate that the phase transition at 210 K signals an enhanced charge order that induces electrical dipoles and strong dielectric response near 210 K. It is clear that the strong covalency and spin-orbit interaction (SOI) suppress double exchange in Ir dimers and stabilize a novel magnetic state. The behavior of Ba5AlIr2O11 therefore provides unique insights into the physics of SOI along with strong covalency in competition with double exchange interactions of comparable strength.Comment: 6 figures, 20 pages. arXiv admin note: text overlap with arXiv:1505.0087

Pressure-Induced Insulating State in Ba1-xRExIrO3 (RE = Gd, Eu) Single Crystals

BaIrO3 is a novel insulator with coexistent weak ferromagnetism, charge and spin density wave. Dilute RE doping for Ba induces a metallic state, whereas application of modest pressure readily restores an insulating state characterized by a three-order-of-magnitude increase of resistivity. Since pressure generally increases orbital overlap and broadens energy bands, a pressure-induced insulating state is not commonplace. The profoundly dissimilar responses of the ground state to light doping and low hydrostatic pressures signal an unusual, delicate interplay between structural and electronic degrees of freedom in BaIrO3

Tuning Jeff = 1/2 Insulating State via Electron Doping and Pressure in Double-Layered Iridate Sr3Ir2O7

Sr3Ir2O7 exhibits a novel Jeff=1/2 insulating state that features a splitting between Jeff=1/2 and 3/2 bands due to spin-orbit interaction. We report a metal-insulator transition in Sr3Ir2O7 via either dilute electron doping (La3+ for Sr2+) or application of high pressure up to 35 GPa. Our study of single-crystal Sr3Ir2O7 and (Sr1-xLax)3Ir2O7 reveals that application of high hydrostatic pressure P leads to a drastic reduction in the electrical resistivity by as much as six orders of magnitude at a critical pressure, PC = 13.2 GPa, manifesting a closing of the gap; but further increasing P up to 35 GPa produces no fully metallic state at low temperatures, possibly as a consequence of localization due to a narrow distribution of bonding angles {\theta}. In contrast, slight doping of La3+ ions for Sr2+ ions in Sr3Ir2O7 readily induces a robust metallic state in the resistivity at low temperatures; the magnetic ordering temperature is significantly suppressed but remains finite for (Sr0.95La0.05)3Ir2O7 where the metallic state occurs. The results are discussed along with comparisons drawn with Sr2IrO4, a prototype of the Jeff = 1/2 insulator.Comment: five figure