The layered honeycomb magnet α-RuCl3 has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled j eff = 1/2 Ru 3+ magnetic moments. Here we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on un-twinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a parent crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, in contrast with the currently-assumed trigonal 3-layer stacking periodicity. We report electronic band structure calculations for the monoclinic structure, which find support for the applicability of the j eff = 1/2 picture once spin orbit coupling and electron correlations are included. Of the three nearest neighbour Ru-Ru bonds that comprise the honeycomb lattice, the monoclinic structure makes the bond parallel to the b-axis non-equivalent to the other two, and we propose that the resulting differences in the magnitude of the anisotropic exchange along these bonds could provide a natural mechanism to explain the spin gap observed in powder inelastic neutron scattering [Banerjee et al.], in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as magnetic neutron powder diffraction show a single magnetic transition upon cooling below TN ≈13 K. The analysis of our neutron powder diffraction data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60 T show a single transition around 8 T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly-anisotropic exchange interactions.

Adenylyl cyclase (AC) converts adenosine triphosphate (ATP) to cyclic adenosine monophosphate, a ubiquitous second messenger that regulates many cellular functions. Recent structural studies have revealed much about the structure and function of mammalian AC but have not fully defined its active site or catalytic mechanism. Four crystal structures were determined of the catalytic domains of AC in complex with two different ATP analogs and various divalent metal ions. These structures provide a model for the enzyme-substrate complex and conclusively demonstrate that two metal ions bind in the active site. The similarity of the active site of AC to those of DNA polymerases suggests that the enzymes catalyze phosphoryl transfer by the same two-metal-ion mechanism and likely have evolved from a common ancestor.

Spin and orbital quantum numbers play a key role in the physics of Mott insulators, but in most systems they are connected only indirectly -via the Pauli exclusion principle and the Coulomb interaction. Iridium-based oxides (iridates) introduce strong spin-orbit coupling directly, such that the Mott physics has a strong orbital character.In the layered honeycomb iridates this is thought to generate highly spin-anisotropic magnetic interactions, coupling the spin orientation to a given spatial direction of exchange and leading to strongly frustrated magnetism. Here we report a new iridate structure that has the same local connectivity as the layered honeycomb and exhibits striking evidence for highly spin-anisotropic exchange. The basic structural units of this material suggest that a new family of three-dimensional structures could exist, the 'harmonic honeycomb' iridates, of which the present compound is the first example.

The recently-synthesized iridate β-Li2IrO3 has been proposed as a candidate to display novel magnetic behavior stabilized by frustration effects from bond-dependent, anisotropic interactions (Kitaev model) on a three-dimensional "hyperhoneycomb" lattice. Here we report a combined study using neutron powder diffraction and magnetic resonant x-ray diffraction to solve the complete magnetic structure. We find a complex, incommensurate magnetic order with non-coplanar and counter-rotating Ir moments, which surprisingly shares many of its features with the related structural polytype "stripyhoneycomb" γ-Li2IrO3, where dominant Kitaev interactions have been invoked to explain the stability of the observed magnetic structure. The similarities of behavior between those two structural polytypes, which have different global lattice topologies but the same local connectivity, is strongly suggestive that the same magnetic interactions and the same underlying mechanism governs the stability of the magnetic order in both materials, indicating that both β-and γ-Li2IrO3 are strong candidates to realize dominant Kitaev interactions in a solid state material.

In rhombohedral CaMn 7 O 12 , an improper ferroelectric polarization of magnitude 2870 µC m −2 is induced by an incommensurate helical magnetic structure that evolves below T N1 = 90 K. The electric polarization was found to be constrained to the high symmetry three-fold rotation axis of the crystal structure, perpendicular to the in-plane rotation of the magnetic moments. The multiferroicity is explained by the ferroaxial coupling mechanism, which in CaMn 7 O 12 gives rise to the largest magnetically induced, electric polarization measured to date.

The emission and absorption of Cs2AgBiBr6 are dominated by the strong carriers–phonon coupling.

ACs 1 catalyze the conversion of ATP into the second messenger cAMP, PP i being the second product of the cyclase reaction. Mammals express nine membranous ACs (ACs 1-9) (1, 2) and a sAC that is predominantly expressed in testis (3). Bacillus anthracis and Bacillus pertussis produce the AC toxins EF and ACT, respectively, that are activated by Ca 2ϩ /calmodulin and act through excessive cAMP accumulation in host cells (4,5). sGC is structurally related to ACs 1-9 in the catalytic site and is activated by [6][7][8]. sGC catalyzes the formation of the second messenger cGMP from GTP. ACs 1-9 contain a tandem repeat structure with two transmembrane domains and two cytosolic domains (1, 2). The cytosolic domains are referred to as C1 and C2, respectively. Together, C1 and C2 form the catalytic site of AC. C1 and C2 also contain the regulatory sites for the stimulatory G-protein, G␣ s , for the inhibitory G-protein, G␣ i , and for the diterpene, forskolin. Catalytic activity of all AC isoforms depends on the presence of divalent cations (Mg 2ϩ or Mn 2ϩ ). Membranous ACs possess two Me 2ϩ -binding sites (9 -11). When mixed together, purified C1 and C2 form a functional AC that is efficiently activated by forskolin and G␣ s -GTP␥S (12, 13).AC isoforms differ from each other in their regulation (1, 2). ACs 1-9 are all activated by G␣ s , whereas sAC is activated by HCO 3 Ϫ (14). Forskolin activates ACs 1-8 but not AC9 or sAC. G␣ i inhibits ACs 1, 5, and 6. G-protein ␤␥ subunits exhibit stimulatory or inhibitory effects on AC isoforms. Ca 2ϩ /calmodulin stimulates ACs 1, 3, and 8. In addition, Mg 2ϩ and Mn 2ϩ show differential stimulatory effects on AC isoforms (15). More-

We present measurements of conductance hysteresis on CH3NH3PbI3 perovskite thin films, performed using the double-wave method, in order to investigate the possibility of a ferroelectric response. A strong frequency dependence of the hysteresis is observed in the range of 0.1 Hz to 150 Hz, with a hysteretic charge density in excess of 1000 μC cm−2 at frequencies below 0.4 Hz—a behaviour uncharacteristic of a ferroelectric response. We show that the observed hysteretic conductance, as well as the presence of a double arc in the impedance spectroscopy, can be fully explained by the migration of mobile ions under bias on a timescale of seconds. Our measurements place an upper limit of ≈1 μC cm−2 on any intrinsic frequency-independent polarisation, ruling out ferroelectricity as the main cause of current-voltage hysteresis and providing further evidence of the importance of ionic migration in modifying the efficiency of CH3NH3PbI3 devices.