We present the first copper iridium binary metal oxide with the chemical formula CuIrO. The material is synthesized from the parent compound NaIrO by a topotactic reaction where sodium is exchanged with copper under mild conditions. CuIrO has the same monoclinic space group (C2/c) as NaIrO with a layered honeycomb structure. The parent compound NaIrO is proposed to be relevant to the Kitaev spin liquid on the basis of having Ir with an effective spin of 1/2 on a honeycomb lattice. Remarkably, whereas NaIrO shows a long-range magnetic order at 15 K and fails to become a true spin liquid, CuIrO remains disordered until 2.7 K, at which point a short-range order develops. Rietveld analysis shows less distortions in the honeycomb structure of CuIrO with bond angles closer to 120° compared to NaIrO. Thus, the weak short-range magnetism combined with the nearly ideal honeycomb structure places CuIrO closer to a Kitaev spin liquid than its predecessors.

Anyonic excitations emerging from a Kitaev spin liquid can form a basis for quantum computers 1, 2 . Searching for such excitations motivated intense research on the honeycomb iridate materials 3-17 . However, access to a spin liquid ground state has been hindered by magnetic ordering 5 . Cu 2 IrO 3 is a new honeycomb iridate without thermodynamic signatures of a long-range order 18 . Here, we use muon spin relaxation to uncover the magnetic ground state of Cu 2 IrO 3 . We find a two-component depolarization with slow and fast relaxation rates corresponding to distinct regions with dynamic and static magnetism, respectively. X-ray absorption spectroscopy and first principles calculations identify a mixed copper valence as the origin of this behavior. Our results suggest that a minority of Cu 2+ ions nucleate regions of static magnetism whereas the majority of Cu + /Ir 4+ on the honeycomb lattice give rise to a Kitaev spin liquid.

Magnetic van der Waals (vdW) materials are the centerpiece of atomically thin devices with spintronic and optoelectronic functions. Exploring new chemistry paths to tune their magnetic and optical properties enables significant progress in fabricating heterostructures and ultracompact devices by mechanical exfoliation. The key parameter to sustain ferromagnetism in 2D is magnetic anisotropy-a tendency of spins to align in a certain crystallographic direction known as easy-axis. In layered materials, two limits of easy-axis are in-plane (XY) and out-of-plane (Ising). Light polarization and the helicity of topological states can couple to magnetic anisotropy with promising photoluminescence or spin-orbitronic functions. Here, a unique experiment is designed to control the easy-axis, the magnetic transition temperature, and the optical gap simultaneously in a series of CrCl Br crystals between CrCl with XY and CrBr with Ising anisotropy. The easy-axis is controlled between the two limits by varying spin-orbit coupling with the Br content in CrCl Br . The optical gap, magnetic transition temperature, and interlayer spacing are all tuned linearly with x. This is the first report of controlling exchange anisotropy in a layered crystal and the first unveiling of mixed halide chemistry as a powerful technique to produce functional materials for spintronic devices.

The family of binary Lanthanum monopnictides, LaBi and LaSb, have attracted a great deal of attention as they display an unusual extreme magnetoresistance (XMR) that is not well understood. Two classes of explanations have been raised for this: the presence of non-trivial topology, and the compensation between electron and hole densities. Here, by synthesizing a new member of the family, LaAs, and performing transport measurements, Angle Resolved Photoemission Spectroscopy (ARPES), and Density Functional Theory (DFT) calculations, we show that (a) LaAs retains all qualitative features characteristic of the XMR effect but with a siginificant reduction in magnitude compared to LaSb and LaBi, (b) the absence of a band inversion or a Dirac cone in LaAs indicates that topology is insignificant to XMR, (c) the equal number of electron and hole carriers indicates that compensation is necessary for XMR but does not explain its magnitude, and (d) the ratio of electron and hole mobilities is much different in LaAs compared to LaSb and LaBi. We argue that the compensation is responsible for the XMR profile and the mobility mismatch constrains the magnitude of XMR.

Wadsley−Roth phases accommodate variable cation charge by crystallographic shear planes delineating blocks of the parent ReO 3 structure. The homologous series TiNb x O 2+2.5x provides possible new anode materials for lithium-ion batteries. The thermodynamic stability of three of these shear phases was determined by high-temperature oxide melt solution calorimetry. TiNb 2 O 7 , TiNb 24 O 62 , and TiNb 5 O 14.5 (often called Ti 2 Nb 10 O 29 ) all have positive enthalpies of formation from binary oxides (TiO 2 and Nb 2 O 5 ), implying that they are entropy-stabilized and only stable above some minimum temperature. Hence, shear phases may represent a new and extensive class of "entropy-stabilized oxides". Their thermodynamic stability decreases with the increasing Nb content. Entropies of formation were calculated using the measured enthalpy of formation and assuming that their synthesis temperature is their lowest temperature of stability and using calculated configurational entropies arising from cation disorder. TiNb 24 O 62 has a high entropy consistent with extensive disorder, whereas TiNb 2 O 7 and TiNb 5 O 14.5 appear to be substantially more ordered. These entropy values are further constrained by considering the stability of the Wadsley−Roth phases with respect to each other. TiNb 2 O 7 and TiNb 5 O 14.5 can be relatively stable intercalating anode materials, while TiNb 24 O 62 is likely to decompose near room temperature during extended battery cycling. This work accentuates the underlying role of thermodynamic studies in engineering electrochemically active materials with enhanced stability for next-generation lithium-ion batteries and beyond.

Magnetic moments arranged at the corners of a honeycomb lattice are predicted to form a novel state of matter, the Kitaev quantum spin liquid, under the influence of frustration effects between bond-dependent Ising interactions. Some layered honeycomb iridates and related materials, such as Na 2 IrO 3 and α-RuCl 3 , are proximate to the Kitaev quantum spin liquid, but bosonic spin-wave excitations associated with undesirable antiferromagnetic long-range order mask the inherent properties of the Kitaev Hamiltonian. Here, we use 63 Cu nuclear quadrupole resonance to uncover the low-energy spin excitations in the nearly ideal honeycomb lattice of effective spin S ¼ 1=2 at the Ir 4þ sites in Cu 2 IrO 3 . We demonstrate that, unlike Na 2 IrO 3 , Ir spin fluctuations exhibit no evidence for critical slowing-down toward magnetic long-range order in zero external magnetic field. Moreover, the low-energy spin excitation spectrum is dominated by a mode that has a large excitation gap comparable to the Ising interactions, a signature expected for Majorana fermions of the Kitaev quantum spin liquid.

Van der Waals (VdW) materials have opened new directions in the study of low dimensional magnetism. A largely unexplored arena is the intrinsic tuning of VdW magnets toward new ground states. Chromium trihalides provided the first such example with a change of interlayer magnetic coupling emerging upon exfoliation. Here, we take a different approach to engineer previously unknown ground states, not by exfoliation, but by tuning the spin-orbit coupling (SOC) of the nonmagnetic ligand atoms (Cl, Br, I). We synthesize a three-halide series, CrCl3 − x − yBrxIy, and map their magnetic properties as a function of Cl, Br, and I content. The resulting triangular phase diagrams unveil a frustrated regime near CrCl3. First-principles calculations confirm that the frustration is driven by a competition between the chromium and halide SOCs. Furthermore, we reveal a field-induced change of interlayer coupling in the bulk of CrCl3 − x − yBrxIy crystals at the same field as in the exfoliation experiments.

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