Quantum magnetism and quantum phase transitions

Abstract:

We report thermoelectric properties of sintered samples of undoped, W-doped, and Sb-doped ReSTe crystallized in a cubic MoSBr-type structure. All samples exhibited p-type thermoelectric properties. ReSTe and Re0.993W0.007STe exhibited the largest dimensionless figure of merit ZT, reaching 0.4 at 660 K. This high performance is attributed to a large power factor owing to the degenerate semiconducting state realized by the strong spin-orbit coupling and low lattice thermal conductivity of the sintered samples. Furthermore, electronic band dispersion of ReSTe is almost flat at the bottom of the conduction band, suggesting that n-type ReSTe is expected to exhibit much higher performance than p-type ReSTe.

Abstract:

Through a combination of crystal symmetry analysis and density functional theory calculations we unveil a possible microscopic origin of the unexpected insulating behavior reported in the honeycomb Kitaev material Cu₂⁢IrO₃. Our study suggests that this material hosts an instability towards charge ordering of the Ir ions, with alternating magnetic Ir⁴⁺ and nonmagnetic Ir³⁺ ions arranged on the honeycomb lattice. In this case, the next-nearest-neighbor interactions that couple magnetic Ir⁴⁺ ions form an enlarged triangular lattice, instead of the expected honeycomb lattice. The magnetic Cu²⁺ ions located at the center of the iridium honeycomb voids also form a triangular lattice, and additionally contribute to the magnetization of the system. Together, the interpenetrated Ir⁴⁺ and Cu²⁺ triangular lattices present a novel type of honeycomb Kitaev lattice composed of two types of magnetic ion.

Abstract:

We report the synthesis and electronic properties of polycrystalline samples of Zr6MBi2 (M = Ru and Fe) crystallizing in the hexagonal Zr6CoAl2-type structure. Based on their electrical resistivity, magnetization, and heat capacity data, Zr6RuBi2 and Zr6FeBi2 are found to exhibit bulk superconductivity below Tc = 4.9 and 1.4 K, respectively. Although Zr6RuBi2 is most likely a conventional superconductor, the considerably higher Tc for M = Ru compared with that for M = Fe differs from the trend in Tc for Zr6CoAl2-type superconductors reported to date. The superconductivity of an amorphous solid-solution phase, which may hinder elucidation of the superconducting properties of Zr6MBi2, is also discussed.