Single-crystal x-ray diffraction studies with synchrotron radiation on the honeycomb iridate α-Li2IrO3 reveal a pressure-induced structural phase transition with symmetry lowering from monoclinic to triclinic at a critical pressure of Pc = 3.8 GPa. According to the evolution of the lattice parameters with pressure, the transition mainly affects the ab plane and thereby the Ir hexagon network, leading to the formation of Ir-Ir dimers. These observations are independently predicted and corroborated by our ab initio density functional theory calculations where we find that the appearance of Ir-Ir dimers at finite pressure is a consequence of a subtle interplay between magnetism, correlation, spin-orbit coupling, and covalent bonding. Our results further suggest that at Pc the system undergoes a magnetic collapse. Finally we provide a general picture of competing interactions for the honeycomb lattices A2M O3 with A= Li, Na and M = Ir, Ru.PACS numbers: 61.05.cp,61.50. Ks,71.15.Mb In recent years, layered honeycomb 4d and 5d metal oxides, such as Na 2 IrO 3 , α-Li 2 IrO 3 , and α-RuCl 3 , have been intensively scrutinized as Kitaev physics candidates [1-6] due to the presence of sizable nearest-neighbor bond-dependent spin-orbital 1/2 Ising interactions. However, instead of the expected Z 2 spin liquid groundstate, as shown by Kitaev [1], these materials order magnetically either in a zig-zag structure [4, 7-9] (Na 2 IrO 3 , α-RuCl 3 ) or an incommensurate spiral structure [10] (α-Li 2 IrO 3 ). This magnetic long-range order has been suggested to originate from further non-Kitaev interactions and a present debate is whether the magnetic excitations in these materials nevertheless retain some of the non-trivial features of the Kitaev model, such as fractionalization [9,11,12]. It might be expected that one route to enhance Kitaev interactions would be by applying pressure or by doping. However, the physics of this structural family is much richer and there are many more instabilities that interfere with the Kitaev interactions, in particular under pressure. Indeed, Li 2 RuO 3 is nonmagnetic and strongly dimerized at ambient pressure [13][14][15], while SrRu 2 O 6 is an ultra-high-temperature antiferromagnet [16,17], despite having the same planar geometry, and shows no bond disproportionation.Many factors control the competition between Kitaev physics, magnetism, and dimerization [18] in A 2 M O 3 honeycomb networks, such as the number of transition metal M d-electrons, the strength of spin-orbit coupling, * valenti@th.physik.uni-frankfurt.de † christine.kuntscher@physik.uni-augsburg.de the strength of correlation effects and Hund's rule coupling, or the ionic radii of the buffer element A. In this context it is particularly instructive to compare α-Li 2 IrO 3 with Li 2 RuO 3 , which contains the same buffer element (Li). α-Li 2 IrO 3 is less correlated than Li 2 RuO 3 (5d versus 4d electrons, resp.) so that one would expect in the former a reduced tendency to magnetism in favor of dimerization. On the other hand, ...

Time-resolved two-photon photoemission was used to study the electronic structure and dynamics at the surface of SnSb 2 Te 4 , a p-type topological insulator. The Dirac point is found 0.32 ± 0.03 eV above the Fermi level. Electrons from the conduction band minimum are scattered on a time scale of 43 ± 4 fs to the Dirac cone. From there they decay to the partly depleted valence band with a time constant of 78 ± 5 fs. The significant interaction of the Dirac states with bulk bands is attributed to their bulk penetration depth of ∼3 nm as found from density functional theory calculations. While topological insulators (TIs) are bulk insulators, they exhibit a spin-polarized metallic topological surface state (TSS) with linear dispersion (Dirac cone) [1,2]. The helical spin structure of the Dirac cone was predicted to constrain intraband scattering in the absence of spin-flipping events, resulting in long carrier lifetimes [3]. Evidence for the suppression of elastic spin-flipping scattering events was given by Fourier-transformed scanning tunneling spectroscopy [4]. Time-resolved photoemission measures the transient population of initially empty electronic states following an optical pump pulse, and therefore allows us to access electron dynamics directly in the time domain. Previous studies have focused on carrier cooling in bismuth chalcogenides which are intrinsically n type [5][6][7] and can be p doped by Mg [8]. These studies showed that the electron dynamics of TSSs is dominated by the bulk conduction band, but did not provide scattering rates between TSS and the conduction or valence band. Such information could be related to results obtained from transport measurements and would be important for device applications.Complex ternary Sb 2 Te 3 -based alloys possess a layer structure with a van der Waals gap between Te layers and were recently proposed to exhibit a topological surface state [9][10][11]. However, intrinsic p doping of antimony-containing materials does not permit to access the Dirac point by conventional angle-resolved photoelectron spectroscopy [12] even after doping by alkali-metal atoms [13]. Angle-resolved two-photon photoemission (2PPE) uses a pump-probe process to access the unoccupied electronic states as indicated by the arrows in Fig. 1(a). Here, we show that SnSb 2 Te 4 is a p-doped TI and obtain energy and dispersion of the TSS as well as bulk conduction and valence bands by 2PPE. The results agree very well with results from density functional theory (DFT) calculations. Time-resolved 2PPE is used to measure the transient population dynamics. It is dominated by refilling from the conduction band and scattering to the valence band, which is partly depleted at the present doping level. The strong interaction between bulk and surface is attributed to the large penetration depth of the TSS. Two-photon photoemission experiments used the fundamental (1.63 eV, IR pump) and the third harmonic (4.89 eV, UV probe) of a Ti:sapphire oscillator with a repetition rate of 90 MHz. The width of the cro...

We study the effect of isoelectronic doping and external pressure in tuning the ground state of the honeycomb iridate Na2IrO3 by combining optical spectroscopy with synchrotron x-ray diffraction measurements on single crystals. The obtained optical conductivity of Na2IrO3 is discussed in terms of a Mott insulating picture versus the formation of quasimolecular orbitals and in terms of Kitaevinteractions. With increasing Li content x, (Na1−xLix)2IrO3 moves deeper into the Mott insulating regime and there are indications that up to a doping level of 24% the compound comes closer to the Kitaev-limit. The optical conductivity spectrum of single crystalline α-Li2IrO3 does not follow the trends observed for the series up to x = 0.24. There are strong indications that α-Li2IrO3 is less close to the Kitaev-limit compared to Na2IrO3 and closer to the quasimolecular orbital picture. Except for the pressure-induced hardening of the phonon modes, the optical properties of Na2IrO3 seem to be robust against external pressure. Possible explanations of the unexpected evolution of the optical conductivity with isolectronic doping and the drastic change between x = 0.24 and x = 1 are given by comparing the pressure-induced changes of lattice parameters and the optical conductivity with the corresponding changes induced by doping.

Background A recent association study identified a common variant (rs9790517) at 4q24 to be associated with breast cancer risk. Independent association signals and potential functional variants in this locus have not been explored. Methods We conducted a fine-mapping analysis in 55,540 breast cancer cases and 51,168 controls from the Breast Cancer Association Consortium. Results Conditional analyses identified two independent association signals among women of European ancestry, represented by rs9790517 (conditional p = 2.51 × 10−4; OR = 1.04; 95% CI 1.02–1.07) and rs77928427 (p = 1.86 × 10−4; OR = 1.04; 95% CI 1.02–1.07). Functional annotation using data from the Encyclopedia of DNA Elements (ENCODE) project revealed two putative functional variants, rs62331150 and rs73838678 in linkage disequilibrium (LD) with rs9790517 (r2 ≥ 0.90) residing in the active promoter or enhancer, respectively, of the nearest gene, TET2. Both variants are located in DNase I hypersensitivity and transcription factor binding sites. Using data from both The Cancer Genome Atlas (TCGA) and Molecular Taxonomy of Breast Cancer International Consortium (METABRIC), we showed that rs62331150 was associated with level of expression of TET2 in breast normal and tumor tissue. Conclusion Our study identified two independent association signals at 4q24 in relation to breast cancer risk and suggested that observed association in this locus may be mediated through the regulation of TET2. Impact Fine-mapping study with large sample size warranted for identification of independent loci for breast cancer risk.

We studied the effect of external pressure on the electrodynamic properties of α-Li2IrO3 single crystals in the frequency range of the phonon modes and the Ir d-d transitions. The abrupt hardening of several phonon modes under pressure supports the onset of the dimerized phase at the critical pressure P c=3.8 GPa. With increasing pressure an overall decrease in spectral weight of the Ir d-d transitions is found up to P c. Above P c, the local (on-site) d-d excitations gain spectral weight with increasing pressure, which hints at a pressure-induced increase in the octahedral distortions. The non-local (intersite) Ir d-d transitions show a monotonic blue-shift and decrease in spectral weight. The changes observed for the non-local excitations are most prominent well above P c, namely for pressures ≥12 GPa, and only small changes occur for pressures close to P c. The profile of the optical conductivity at high pressures (∼20 GPa) appears to be indicative for the dimerized state in iridates.