The realization of Kitaev spin liquid, where spins on a honeycomb lattice are coupled ferromagnetically by bond-dependent anisotropic interactions, has been a sought-after dream. 5d iridium oxides α-Li 2 IrO 3 and α-Na 2 IrO 3 with a honeycomb lattice of J eff = 1/2 moments recently emerged as a possible materialization. Strong signature of Kitaev physics, however, was not captured. Here we report the discovery of a complex iridium oxide β-Li 2 IrO 3 with J eff = 1/2 moments on "hyper-honeycomb" lattice, a three-dimensional analogue of honeycomb lattice. A positive Curie-Weiss temperature θ CW ~ 40 K indicated dominant ferromagnetic interactions among J eff = 1/2 moments in β-Li 2 IrO 3 . A magnetic ordering with a small entropy change was observed at T c = 38 K, which, with the application of magnetic field of only 3 T, changed to a fully polarized state of J eff = 1/2 moments. Those results imply that hyper-honeycomb β-Li 2 IrO 3 is located in the vicinity to a Kitaev spin liquid.

X-ray absorption spectroscopy studies of the magnetic-insulating ground state of Sr2IrO4 at ambient pressure show a clear deviation from a strong spin-orbit (SO) limit J(eff)=1/2 state, a result of local exchange interactions and a nonzero tetragonal crystal field mixing SO split J(eff)=1/2, 3/2 states. X-ray magnetic circular dichroism measurements in a diamond anvil cell show a magnetic transition at a pressure of ∼17  GPa, where the "weak" ferromagnetic moment is quenched despite transport measurements showing insulating behavior to at least 40 GPa. The magnetic transition has implications for the origin of the insulating gap and the nature of exchange interactions in this SO coupled system. The expectation value of the angular part of the SO interaction, , extrapolates to zero at ∼80-90  GPa where an increased bandwidth strongly mixes J(eff)=1/2, 3/2 states and SO interactions no longer dominate the electronic ground state of Sr2IrO4.

We demonstrate that the action of physical pressure, chemical compression, or aliovalent substitution in ACo2As2 (A = Eu and Ca) has a general consequence of causing these antiferromagnetic materials to become ferromagnets. In all cases, the mixed valence triggered at the electropositive A site results in the increase of the Co 3d density of states at the Fermi level. Remarkably, the dramatic alteration of magnetic behavior results from the very minor (<0.15 electron) change in the population of the 3d orbitals. The mixed valence state of Eu observed in the high-pressure (HP) form of EuCo2As2 exhibits a remarkable stability, achieving the average oxidation state of +2.25 at 12.6 GPa. In the case of CaCo2As2, substituting even 10% of Eu or La into the Ca site causes ferromagnetic ordering of Co moments. Similar to HP-EuCo2As2, the itinerant 3d ferromagnetism emerges from electronic doping into the Co layer because of chemical compression of Eu sites in Ca0.9Eu0.1Co1.91As2 or direct electron doping in Ca0.85La0.15Co1.89As2. The results reported herein demonstrate the general possibility of amplifying minor localized electronic effects to achieve major changes in material's properties via involvement of strongly correlated electrons.

While charge density wave (CDW) instabilities are ubiquitous to superconducting cuprates, the different ordering wavevectors in various cuprate families have hampered a unified description of the CDW formation mechanism. Here we investigate the temperature dependence of the low energy phonons in the canonical CDW ordered cuprate La1.875Ba0.125CuO4. We discover that the phonon softening wavevector associated with CDW correlations becomes temperature dependent in the hightemperature precursor phase and changes from a wavevector of 0.238 reciprocal space units (r.l.u.) below the ordering transition temperature up to 0.3 r.l.u. at 300 K. This high-temperature behavior shows that "214"-type cuprates can host CDW correlations at a similar wavevector to previously reported CDW correlations in non-"214"-type cuprates such as YBa2Cu3O 6+δ . This indicates that cuprate CDWs may arise from the same underlying instability despite their apparently different low temperature ordering wavevectors. arXiv:1712.04554v2 [cond-mat.supr-con]

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.

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UTe2 is a recently discovered unconventional superconductor that has attracted much interest because of its potentially spin-triplet topological superconductivity. Our ac calorimetry, electrical resistivity, and x-ray absorption study of UTe2 under applied pressure reveals key insights on the superconducting and magnetic states surrounding pressure-induced quantum criticality at Pc1 = 1.3 GPa. First, our specific heat data at low pressures, combined with a phenomenological model, show that pressure alters the balance between two closely competing superconducting orders. Second, near 1.5 GPa, we detect two bulk transitions that trigger changes in the resistivity, which are consistent with antiferromagnetic order, rather than ferromagnetism. Third, the emergence of magnetism is accompanied by an increase in valence toward a U4+ (5f2) state, which indicates that UTe2 exhibits intermediate valence at ambient pressure. Our results suggest that antiferromagnetic fluctuations may play a more substantial role on the superconducting state of UTe2 than previously thought.

We explore the response of Ir 5d orbitals to pressure in β-Li2IrO3, a hyperhoneycomb iridate in proximity to a Kitaev quantum spin liquid (QSL) ground state. X-ray absorption spectroscopy reveals a reconstruction of the electronic ground state below 2 GPa, the same pressure range where x-ray magnetic circular dichroism shows an apparent collapse of magnetic order. The electronic reconstruction, which manifests a reduction in the effective spin-orbit (SO) interaction in 5d orbitals, pushes β-Li2IrO3 further away from the pure J eff = 1/2 limit. Although lattice symmetry is preserved across the electronic transition, x-ray diffraction shows a highly anisotropic compression of the hyperhoneycomb lattice which affects the balance of bond-directional Ir-Ir exchange interactions driven by spin-orbit coupling at Ir sites. An enhancement of symmetric anisotropic exchange over Kitaev and Heisenberg exchange interactions seen in theoretical calculations that use precisely this anisotropic Ir-Ir bond compression provides one possible route to realization of a QSL state in this hyperhoneycomb iridate at high pressures.The novel electronic ground states of 5d-based compounds driven by spin-orbit interactions continue to provide an excellent playground for the realization of unconventional quantum phases of matter including topological insulators [1-4] and quantum spin-liquids (QSLs) [5][6][7]. One example of the latter is the non-trivial QSL ground state of the Kitaev model [8], a rare example of a solvable interacting quantum model with Majorana fermions as its elementary excitations. Material candidates for possible realization of the Kitaev model include honeycomb-based-lattice systems with strong spin-orbit coupling [6,9], such as the two and three-dimensional honeycomb iridates, α-Li(Na) [7,[20][21][22] as well as α-RuCl 3 [23,24]. However, it is experimentally established that these materials order magnetically at low temperatures [17,18,20,[25][26][27], spoiling numerous attempts to realize the Kitaev QSL. Hence, tuning structure and related intricate interactions present in these materials through chemical or physical pressure provides a potential route to introduce magnetic frustration and realize novel phases of matter.In this Letter we have investigated the electronic and structural response of β-Li 2 IrO 3 to high pressure. Xray absorption near edge structure (XANES) measurements at Ir L-edges reveal a dramatic suppression of the isotropic Ir (L 3 /L 2 ) branching ratio at P ∼ 1.5 GPa, signaling a reduction in the effective strength of spinorbit interactions in the 5d band. This is the same pressure at which net magnetization in applied field collapses [17]. The reconstructed electronic state preserves the L z / S z orbital-to-spin moment ratio of Ir magnetic moments and the insulating ground state indicating that spin-orbit interactions and Mott physics con-