Quantum magnetism and quantum phase transitions

Abstract:

We report high-resolution inelastic neutron scattering measurements of the excitation spectrum in large single crystals of the spin-1/2 triangular-lattice Ising-like antiferromagnet ${\mathrm{Na}}_2\mathrm{BaCo}{\left({\mathrm{PO}}_4\right)}_2$ in magnetic fields applied transverse to the Ising axis. In the high-field polarized phase above a critical field $B_{\mathrm{C}}$ , we observe sharp magnons, as expected in the case of no exchange disorder. Through simultaneous fits to the dispersions including data in a polarizing field along the Ising axis, we obtain an excellent match to an Ising-like XXZ Hamiltonian and rule out previously proposed Kitaev exchanges. In the intermediate-field phase below $B_{\mathrm{C}}$ , we observe three dispersive modes, out of which only the lowest energy one is sharp and the others are broad and overlap with continuum scattering. We propose that the broadening effects are due to magnon decays into two-magnon excitations and confirm that such processes are kinematically allowed. The continuum scattering becomes progressively stronger upon lowering the field and, at 0.25 T and zero field, it dominates the entire spectrum with no clear evidence for even broadened magnon modes. We discuss the relevance of the continuous manifold of mean-field degenerate ground states of the refined Hamiltonian for capturing the observed spectrum in zero field, and compare the data with the one- and two-magnon spectrum averaged over this manifold. We also propose a model of the interlayer couplings to explain the observed finite interlayer magnetic propagation vector of the zero-field magnetic order; this requires the breaking of the mirror symmetry in the nominal $P\overline{3}m1$ space group and through refinement of x-ray diffraction data on an untwinned single crystal, we indeed confirm a rotation of the ${\mathrm{CoO}}_6$ octahedra around the $c$ axis, which lowers the symmetry to $P\overline{3}$ .

Abstract:

Zr6CoAl2-type Zr6RuTe2 is found to show bulk superconductivity below the superconducting transition temperature Tc = 1.1 K, according to the electrical resistivity, magnetization, and heat capacity measurements using synthesized polycrystalline samples. This Tc exceeds that of Zr6MTe2 compounds in which M is other transition metals, indicating that M = Ru is favorable for superconductivity in Zr6CoAl2-type Zr6MX2.

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.