Temperature−field phase diagram of extreme magnetoresistance

Proceedings of the National Academy of Sciences of the United States of America Volume 113, Issue 25, 21 June 2016, Pages E3475-E3481.The recent discovery of extreme magnetoresistance (XMR) in LaSb introduced lanthanum monopnictides as a new platform to study this effect in the absence of broken inversion symmetry or protected linear band crossing. In this work, we report XMR in LaBi. Through a comparative study of magnetotransport effects in LaBi and LaSb, we construct a temperature−field phase diagram with triangular shape that illustrates how a magnetic field tunes the electronic behavior in these materials. We show that the triangular phase diagram can be generalized to other topological semimetals with different crystal structures and different chemical compositions. By comparing our experimental results to band structure calculations, we suggest that XMR in LaBi and LaSb originates from a combination of compensated electron−hole pockets and a particular orbital texture on the electron pocket. Such orbital texture is likely to be a generic feature of various topological semimetals, giving rise to their small residual resistivity at zero field and subject to strong scattering induced by a magnetic field

Pressure Induced Quantum Phase Transitions in Metallics Oxides and Pnictides

Quantum phase transitions occur as a result of competing ground states. The focus of the present work is to understand quantum criticality and its consequences when the competition is between insulating and metallic ground states. Metal-insulator transitions are studied by means of electronic transport measurements and quantum critical points are approached by applying hydrostatic pressure in two different compounds namely Eu$_2$Ir$_2$O$_7$ and FeCrAs. The former is a ternary metal oxide and the latter is a ternary metal pnictide. A major component of this work was the development of the ultra-high pressure measurements by means of Anvil cells. A novel design is introduced which minimizes the alignment accessory components hence, making the cell more robust and easier to use. Eu$_2$Ir$_2$O$_7$ is a ternary metal oxide and a member of the pyrochlore iridate family. Resistivity measurements under pressure in moissanite anvil cells show the evolution of the ground state of the system from insulating to metallic. The quantum phase transition at $P_c\sim6$ GPa appears to be continuous. A remarkable correspondence is revealed between the effect of the hydrostatic pressure on Eu$_2$Ir$_2$O$_7$ and the effect of chemical pressure by changing the R size in the R$_2$Ir$_2$O$_7$ series. This suggests that in both cases the tuning parameter controls the $t_{2\textrm{g}}$ bandwidth of the iridium $5d$ electrons. Moreover, hydrostatic pressure unveils a curious cross-over from incoherent to conventional metallic behaviour at a $T^* >$ 150 K in the neighbourhood of $P_c$, suggesting a connection between the high and low temperature phases. The possibility of a topological semi-metallic ground state, predicted in recent theoretical studies, is explained. FeCrAs is a ternary metal pnictide with Fermi liquid specific heat and susceptibility behaviour but non-metallic non-Fermi liquid resistivity behaviour. Characteristic properties of the compound are explained and compared to those of superconducting pnictides. Antiferromagnetic (AFM) order sets in at $\sim125$ K with the magnetic moments residing on the Cr site. Pressure measurements are carried out in moissanite and diamond anvil cells in order to suppress the AFM order and resolve the underlying electronic transport properties. While AFM order is destroyed by pressure, the non-metallic non-Fermi liquid behaviour is shown to be robust against pressure.Ph

Tuning the electronic and the crystalline structure of LaBi by pressure: From extreme magnetoresistance to superconductivity

Physical Review B. Volume 95, Issue 1, 10 January 2017, Article number 014507.Extreme magnetoresistance (XMR) in topological semimetals is a recent discovery which attracts attention due to its robust appearance in a growing number of materials. To search for a relation between XMR and superconductivity, we study the effect of pressure on LaBi. By increasing pressure, we observe the disappearance of XMR followed by the appearance of superconductivity at P≈3.5 GPa. We find a region of coexistence between superconductivity and XMR in LaBi in contrast to other superconducting XMR materials. The suppression of XMR is correlated with increasing zero-field resistance instead of decreasing in-field resistance. At higher pressures, P≈11 GPa, we find a structural transition from the face-centered cubic lattice to a primitive tetragonal lattice, in agreement with theoretical predictions. The relationship between extreme magnetoresistance, superconductivity, and structural transition in LaBi is discussed. © 2017 American Physical Society

Trivalent Iridium Oxides: Layered Triangular Lattice Iridate K_0.75Na_0.25IrO₂ and Oxyhydroxide IrOOH

Solid oxides with transition-metal ions in unusual oxidation states attract enormous attention due to their electronic, magnetic, and catalytic properties. Yet, no crystalline oxide compounds based on purely trivalent iridium have been characterized to date. Here, we present the synthesis and thorough investigation of the properties of the compounds K_0.75Na_0.25IrO₂ and IrOOH, both containing trivalent iridium on a triangular lattice in layers of [IrO₂]⁻. K_0.75Na_0.25IrO₂ crystallizes in a P2-structure with the space group <i>P</i>6₃/<i>mmc</i>, while the crystal structure of IrOOH adopts the direct maximal subgroup <i>P</i>3̅<i>m</i>1. The trivalent state of the iridium ion is discussed with regards to the iridium–oxygen bond length from X-ray diffraction, the chemical composition, the electronic and magnetic behavior of the material, and X-ray photoemission spectroscopy. The discovery of a new valence state in ternary crystalline iridium oxides is not only of interest from a fundamental perspective, but also has far-reaching implications for such diverse fields as electrochromism, solid-state magnetism, and especially heterogeneous catalysis