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- ItemNesting-driven multipolar order in CeB6 from photoemission tomography(London : Nature Publishing Group, 2016) Koitzsch, A.; Heming, N.; Knupfer, M.; Büchner, B.; Portnichenko, P.Y.; Dukhnenko, A.V.; Shitsevalova, N.Y.; Filipov, V.B.; Lev, L.L.Some heavy fermion materials show so-called hidden-order phases which are invisible to many characterization techniques and whose microscopic origin remained controversial for decades. Among such hidden-order compounds, CeB6 is of model character due to its simple electronic configuration and crystal structure. Apart from more conventional antiferromagnetism, it shows an elusive phase at low temperatures, which is commonly associated with multipolar order. Here we show that this phase roots in a Fermi surface instability. This conclusion is based on a full 3D tomographic sampling of the electronic structure by angle-resolved photoemission and comparison with inelastic neutron scattering data. The hidden order is mediated by itinerant electrons. Our measurements will serve as a paradigm for the investigation of hidden-order phases in f-electron systems, but also generally for situations where the itinerant electrons drive orbital or spin order.
- ItemMisorientation-angle-dependent electrical transport across molybdenum disulfide grain boundaries(London : Nature Publishing Group, 2016) Ly, Thuc Hue; Perello, David J.; Zhao, Jiong; Deng, Qingming; Kim, Hyun; Han, Gang Hee; Chae, Sang Hoon; Jeong, Hye Yun; Lee, Young HeeGrain boundaries in monolayer transition metal dichalcogenides have unique atomic defect structures and band dispersion relations that depend on the inter-domain misorientation angle. Here, we explore misorientation angle-dependent electrical transport at grain boundaries in monolayer MoS2 by correlating the atomic defect structures of measured devices analysed with transmission electron microscopy and first-principles calculations. Transmission electron microscopy indicates that grain boundaries are primarily composed of 5–7 dislocation cores with periodicity and additional complex defects formed at high angles, obeying the classical low-angle theory for angles <22°. The inter-domain mobility is minimized for angles <9° and increases nonlinearly by two orders of magnitude before saturating at ∼16 cm2 V−1 s−1 around misorientation angle≈20°. This trend is explained via grain-boundary electrostatic barriers estimated from density functional calculations and experimental tunnelling barrier heights, which are ≈0.5 eV at low angles and ≈0.15 eV at high angles (≥20°).
- ItemStrongly frustrated triangular spin lattice emerging from triplet dimer formation in honeycomb Li2IrO3(London : Nature Publishing Group, 2016) Nishimoto, Satoshi; Katukuri, Vamshi M.; Yushankhai, Viktor; Stoll, Hermann; Rößler, Ulrich K.; Hozoi, Liviu; Rousochatzakis, Ioannis; van den Brink, JeroenIridium oxides with a honeycomb lattice have been identified as platforms for the much anticipated Kitaev topological spin liquid: the spin-orbit entangled states of Ir4+ in principle generate precisely the required type of anisotropic exchange. However, other magnetic couplings can drive the system away from the spin-liquid phase. With this in mind, here we disentangle the different magnetic interactions in Li2IrO3, a honeycomb iridate with two crystallographically inequivalent sets of adjacent Ir sites. Our ab initio many-body calculations show that, while both Heisenberg and Kitaev nearest-neighbour couplings are present, on one set of Ir–Ir bonds the former dominates, resulting in the formation of spin-triplet dimers. The triplet dimers frame a strongly frustrated triangular lattice and by exact cluster diagonalization we show that they remain protected in a wide region of the phase diagram.
- ItemWavelength-tunable sources of entangled photons interfaced with atomic vapours(London : Nature Publishing Group, 2016) Trotta, Rinaldo; Martín-Sánchez, Javier; Wildmann, Johannes S.; Piredda, Giovanni; Reindl, Marcus; Schimpf, Christian; Zallo, Eugenio; Stroj, Sandra; Edlinger, Johannes; Rastelli, ArmandoThe prospect of using the quantum nature of light for secure communication keeps spurring the search and investigation of suitable sources of entangled photons. A single semiconductor quantum dot is one of the most attractive, as it can generate indistinguishable entangled photons deterministically and is compatible with current photonic-integration technologies. However, the lack of control over the energy of the entangled photons is hampering the exploitation of dissimilar quantum dots in protocols requiring the teleportation of quantum entanglement over remote locations. Here we introduce quantum dot-based sources of polarization-entangled photons whose energy can be tuned via three-directional strain engineering without degrading the degree of entanglement of the photon pairs. As a test-bench for quantum communication, we interface quantum dots with clouds of atomic vapours, and we demonstrate slow-entangled photons from a single quantum emitter. These results pave the way towards the implementation of hybrid quantum networks where entanglement is distributed among distant parties using optoelectronic devices.
- ItemThe effect of acid-base clustering and ions on the growth of atmospheric nano-particles(London : Nature Publishing Group, 2016) Lehtipalo, Katrianne; Rondo, Linda; Kontkanen, Jenni; Schobesberger, Siegfried; Jokinen, Tuija; Sarnela, Nina; Kürten, Andreas; Ehrhart, Sebastian; Franchin, Alessandro; Nieminen, Tuomo; Kulmala, Markku; Riccobono, Francesco; Sipila, Mikko; Yli-Juuti, Taina; Duplissy, Jonathan; Adamov, Alexey; Ahlm, Lars; Almeida, Joa˜o; Amorim, Antonio; Bianchi, Federico; Breitenlechner, Martin; Dommen, Josef; Downard, Andrew J.; Dunne, Eimear M.; Flagan, Richard C.; Guida, Roberto; Hakala, Jani; Hansel, Armin; Jud, Werner; Kangasluoma, Juha; Kerminen, Veli-Matti; Keskinen, Helmi; Kim, Jaeseok; Kirkby, Jasper; Kupc, Agnieszka; Kupiainen-Määttä, Oona; Laaksonen, Ari; Lawler, Michael J.; Leiminger, Markus; Mathot, Serge; Olenius, Tinja; Ortega, Ismael K.; Onnela, Antti; Petäjä, Tuukka; Praplan, Arnaud; Rissanen, Matti P.; Ruuskanen, Taina; Santos, Filipe D.; Schallhart, Simon; Schnitzhofer, Ralf; Simon, Mario; Smith, James N.; Tröstl, Jasmin; Tsagkogeorgas, Georgios; Tomé, António; Vaattovaara, Petri; Vehkamäki, Hanna; Vrtala, Aron E.; Wagner, Paul E.; Williamson, Christina; Wimmer, Daniela; Winkler, Paul M.; Virtanen, Annele; Donahue, Neil M.; Carslaw, Kenneth S.; Baltensperger, Urs; Riipinen, Ilona; Curtius, Joachim; Worsnop, Douglas R.; Kulmala, MarkkuThe growth of freshly formed aerosol particles can be the bottleneck in their survival to cloud condensation nuclei. It is therefore crucial to understand how particles grow in the atmosphere. Insufficient experimental data has impeded a profound understanding of nano-particle growth under atmospheric conditions. Here we study nano-particle growth in the CLOUD (Cosmics Leaving OUtdoors Droplets) chamber, starting from the formation of molecular clusters. We present measured growth rates at sub-3 nm sizes with different atmospherically relevant concentrations of sulphuric acid, water, ammonia and dimethylamine. We find that atmospheric ions and small acid-base clusters, which are not generally accounted for in the measurement of sulphuric acid vapour, can participate in the growth process, leading to enhanced growth rates. The availability of compounds capable of stabilizing sulphuric acid clusters governs the magnitude of these effects and thus the exact growth mechanism. We bring these observations into a coherent framework and discuss their significance in the atmosphere.
- ItemMagnon spectrum of the helimagnetic insulator Cu2OSeO3(London : Nature Publishing Group, 2016) Portnichenko, P.Y.; Romhányi, J.; Onykiienko, Y.A.; Henschel, A.; Schmidt, M.; Cameron, A.S.; Surmach, M.A.; Lim, J.A.; Park, J.T.; Schneidewind, A.; Abernathy, D.L.; Rosner, H.; van den Brink, Jeroen; Inosov, D.S.Complex low-temperature-ordered states in chiral magnets are typically governed by a competition between multiple magnetic interactions. The chiral-lattice multiferroic Cu2OSeO3 became the first insulating helimagnetic material in which a long-range order of topologically stable spin vortices known as skyrmions was established. Here we employ state-of-the-art inelastic neutron scattering to comprehend the full three-dimensional spin-excitation spectrum of Cu2OSeO3 over a broad range of energies. Distinct types of high- and low-energy dispersive magnon modes separated by an extensive energy gap are observed in excellent agreement with the previously suggested microscopic theory based on a model of entangled Cu4 tetrahedra. The comparison of our neutron spectroscopy data with model spin-dynamical calculations based on these theoretical proposals enables an accurate quantitative verification of the fundamental magnetic interactions in Cu2OSeO3 that are essential for understanding its abundant low-temperature magnetically ordered phases.
- ItemSpin-orbit-driven magnetic structure and excitation in the 5d pyrochlore Cd2Os2O7(London : Nature Publishing Group, 2016) Calder, S.; Vale, J.G.; Bogdanov, N.A.; Liu, X.; Donnerer, C.; Upton, M.H.; Casa, D.; Said, A.H.; Lumsden, M.D.; Zhao, Z.; Yan, J.-Q.; Mandrus, D.; Nishimoto, S.; van den Brink, J.; Hill, J.P.; McMorrow, D.F.; Christianson, A.D.Much consideration has been given to the role of spin-orbit coupling (SOC) in 5d oxides, particularly on the formation of novel electronic states and manifested metal-insulator transitions (MITs). SOC plays a dominant role in 5d5 iridates (Ir4þ), undergoing MITs both concurrent (pyrochlores) and separated (perovskites) from the onset of magnetic order. However, the role of SOC for other 5d configurations is less clear. For example, 5d3 (Os5þ) systems are expected to have an orbital singlet with reduced effective SOC. The pyrochlore Cd2Os2O7 nonetheless exhibits a MIT entwined with magnetic order phenomenologically similar to pyrochlore iridates. Here, we resolve the magnetic structure in Cd2Os2O7 with neutron diffraction and then via resonant inelastic X-ray scattering determine the salient electronic and magnetic energy scales controlling the MIT. In particular, SOC plays a subtle role in creating the electronic ground state but drives the magnetic order and emergence of a multiple spin-flip magnetic excitation.
- ItemUnconventional charge order in a co-doped high-Tc superconductor(London : Nature Publishing Group, 2016) Pelc, D.; Vucˇkovic, M.; Grafe, H.-J.; Baek, S.-H.; Požek, M.Charge-stripe order has recently been established as an important aspect of cuprate high-Tc superconductors. However, owing to the complex interplay between competing phases and the influence of disorder, it is unclear how it emerges from the parent high-temperature state. Here we report on the discovery of an unconventional ordered phase between charge-stripe order and (pseudogapped) metal in the cuprate La1.8xEu0.2SrxCuO4. We use three complementary experiments—nuclear quadrupole resonance, nonlinear conductivity and specific heat—to demonstrate that the order appears through a sharp phase transition and exists in a dome-shaped region of the phase diagram. Our results imply that the new phase is a state, which preserves translational symmetry: a charge nematic. We thus resolve the process of charge-stripe development in cuprates, show that this nematic phase is distinct from high-temperature pseudogap and establish a link with other strongly correlated electronic materials with prominent nematic order.
- ItemSee–saw relationship of the Holocene East Asian–Australian summer monsoon(London : Nature Publishing Group, 2016) Eroglu, Deniz; McRobie, Fiona H.; Ozken, Ibrahim; Stemler, Thomas; Wyrwoll, Karl-Heinz; Breitenbach, Sebastian F.M.; Marwan, Norbert; Kurths, JürgenThe East Asian–Indonesian–Australian summer monsoon (EAIASM) links the Earth’s hemispheres and provides a heat source that drives global circulation. At seasonal and inter-seasonal timescales, the summer monsoon of one hemisphere is linked via outflows from the winter monsoon of the opposing hemisphere. Long-term phase relationships between the East Asian summer monsoon (EASM) and the Indonesian–Australian summer monsoon (IASM) are poorly understood, raising questions of long-term adjustments to future greenhouse-triggered climate change and whether these changes could ‘lock in’ possible IASM and EASM phase relationships in a region dependent on monsoonal rainfall. Here we show that a newly developed nonlinear time series analysis technique allows confident identification of strong versus weak monsoon phases at millennial to sub-centennial timescales. We find a see–saw relationship over the last 9,000 years—with strong and weak monsoons opposingly phased and triggered by solar variations. Our results provide insights into centennial- to millennial-scale relationships within the wider EAIASM regime.
- ItemTwinned growth behaviour of two-dimensional materials(London : Nature Publishing Group, 2016) Zhang, Tao; Jiang, Bei; Xu, Zhen; Mendes, Rafael G.; Xiao, Yao; Chen, Linfeng; Fang, Liwen; Gemming, Thomas; Chen, Shengli; Rümmeli, Mark H.; Fu, LeiTwinned growth behaviour in the rapidly emerging area of two-dimensional nanomaterials still remains unexplored although it could be exploited to fabricate heterostructure and superlattice materials. Here we demonstrate how one can utilize the twinned growth relationship between two two-dimensional materials to construct vertically stacked heterostructures. As a demonstration, we achieve 100% overlap of the two transition metal dichalcogenide layers constituting a ReS2/WS2 vertical heterostructure. Moreover, the crystal size of the stacked structure is an order of magnitude larger than previous reports. Such twinned transition metal dichalcogenides vertical heterostructures exhibit great potential for use in optical, electronic and catalytic applications. The simplicity of the twinned growth can be utilized to expand the fabrication of other heterostructures or two-dimensional material superlattice and this strategy can be considered as an enabling technology for research in the emerging field of two-dimensional van der Waals heterostructures.