Pyrochlore electrons under pressure, heat and field: shedding light on the iridates

We study the finite temperature and magnetic field phase diagram of electrons on the pyrochlore lattice subject to a local repulsion as a model for the pyrochlore iridates. We provide the most general symmetry-allowed Hamiltonian, including next-nearest neighbour hopping, and relate it to a Slater-Koster based Hamiltonian for the iridates. It captures Lifshitz and/or thermal transitions between several phases such as metals, semimetals, topological insulators and Weyl semimetals, and gapped antiferromagnets with different orders. Our results on the charge conductivity, both DC and optical, Hall coefficient, magnetization and susceptibility show good agreement with recent experiments and provide new predictions. As such, our effective model sheds light on the pyrochlore iridates in a unified way.Comment: 6+3 pages, 7+2 figures. v3: expanded version, with 3 new figure

High electrochemical activity of the oxide phase in model ceria–Pt and ceria–Ni composite anodes

Fuel cells, and in particular solid-oxide fuel cells (SOFCs), enable high-efficiency conversion of chemical fuels into useful electrical energy and, as such, are expected to play a major role in a sustainable-energy future. A key step in the fuel-cell energy-conversion process is the electro-oxidation of the fuel at the anode. There has been increasing evidence in recent years that the presence of CeO_2-based oxides (ceria) in the anodes of SOFCs with oxygen-ion-conducting electrolytes significantly lowers the activation overpotential for hydrogen oxidation. Most of these studies, however, employ porous, composite electrode structures with ill-defined geometry and uncontrolled interfacial properties. Accordingly, the means by which electrocatalysis is enhanced has remained unclear. Here we demonstrate unambiguously, through the use of ceria–metal structures with well-defined geometries and interfaces, that the near-equilibrium H_2 oxidation reaction pathway is dominated by electrocatalysis at the oxide/gas interface with minimal contributions from the oxide/metal/gas triple-phase boundaries, even for structures with reaction-site densities approaching those of commercial SOFCs. This insight points towards ceria nanostructuring as a route to enhanced activity, rather than the traditional paradigm of metal-catalyst nanostructuring

Conformations and charge transport characteristics of biphenyldithiol self-assembled-monolayer molecular electronic devices: A multiscale computational study

We report a computational study of conformations and charge transport characteristics of biphenyldithiol (BPDT) monolayers in the (sqrt(3)×sqrt(3))R30° packing ratio sandwiched between Au(111) electrodes. From force-field molecular-dynamics and annealing simulations of BPDT self-assembled monolayers (SAMs) with up to 100 molecules on a Au(111) substrate, we identify an energetically favorable herringbone-type SAM packing configuration and a less-stable parallel packing configuration. Both SAMs are described by the (2sqrt(3)×sqrt(3))R30° unit cell including two molecules. With subsequent density-functional theory calculations of one unit cell of the (i) herringbone SAM with the molecular tilt angle theta[approximate]15°, (ii) herringbone SAM with theta[approximate]30°, and (iii) parallel SAM with theta[approximate]30°, we confirm that the herringbone packing configuration is more stable than the parallel one but find that the energy variation with respect to the molecule tilting within the herringbone packing is very small. Next, by capping these SAMs with the top Au(111) electrode, we prepare three molecular electronic device models and calculate their coherent charge transport properties within the matrix Green's function approach. Current–voltage (I–V) curves are then obtained via the Landauer–Büttiker formula. We find that at low-bias voltages (|V|~0.5 V), the I–V characteristics of the three models show noticeable differences due to different phenyl band structures. We thus conclude that the BPDT SAM I–V characteristics in the low-bias voltage region are mainly determined by the Si–Au interaction within the individual molecule-electrode contact, while both intramolecular conformation and intermolecular interaction can affect the BPDT SAM I–V characteristics in the high-bias voltage region

Possible performance improvement in [2]catenane molecular electronic switches

Mechanically interlocked bistable supramolecular complexes are promising candidates of molecular electronics. Applying a multiscale computational approach, here we study the coherent charge transport properties of catenane monolayers sandwiched between Cu(111) electrodes. We demonstrate the robust nature of electrical switching behavior with respect to the variations in the monolayer packing density and the type of electrodes, as well as the thermal fluctuations of the molecules. We propose that the asymmetry of molecule-electrode barriers can be utilized to improve the switching ratio

Ataxia-telangiectasia Mutated (ATM)-dependent Activation of ATR Occurs through Phosphorylation of TopBP1 by ATM

ATM (ataxia-telangiectasia mutated) is necessary for activation of Chk1 by ATR (ATM and Rad3-related) in response to double-stranded DNA breaks (DSBs) but not to DNA replication stress. TopBP1 has been identified as a direct activator of ATR. We show that ATM regulates Xenopus TopBP1 by phosphorylating Ser-1131 and thereby strongly enhancing association of TopBP1 with ATR. Xenopus egg extracts containing a mutant of TopBP1 that cannot be phosphorylated on Ser-1131 are defective in the ATR-dependent phosphorylation of Chk1 in response to DSBs but not to DNA replication stress. Thus, TopBP1 is critical for the ATM-dependent activation of ATR following production of DSBs in the genome

Xenopus Drf1, a Regulator of Cdc7, Displays Checkpoint-dependent Accumulation on Chromatin during an S-phase Arrest

We have cloned a Xenopus Dbf4-related factor named Drf1 and characterized this protein by using Xenopus egg extracts. Drf1 forms an active complex with the kinase Cdc7. However, most of the Cdc7 in egg extracts is not associated with Drf1, which raises the possibility that some or all of the remaining Cdc7 is bound to another Dbf4-related protein. Immunodepletion of Drf1 does not prevent DNA replication in egg extracts. Consistent with this observation, Cdc45 can still associate with chromatin in Drf1-depleted extracts, albeit at significantly reduced levels. Nonetheless, Drf1 displays highly regulated binding to replicating chromatin. Treatment of egg extracts with aphidicolin results in a substantial accumulation of Drf1 on chromatin. This accumulation is blocked by addition of caffeine and by immunodepletion of either ATR or Claspin. These observations suggest that the increased binding of Drf1 to aphidicolin-treated chromatin is an active process that is mediated by a caffeine-sensitive checkpoint pathway containing ATR and Claspin. Abrogation of this pathway also leads to a large increase in the binding of Cdc45 to chromatin. This increase is substantially reduced in the absence of Drf1, which suggests that regulation of Drf1 might be involved in the suppression of Cdc45 loading during replication arrest. We also provide evidence that elimination of this checkpoint causes resumed initiation of DNA replication in both Xenopus tissue culture cells and egg extracts. Taken together, these observations argue that Drf1 is regulated by an intra-S-phase checkpoint mechanism that down-regulates the loading of Cdc45 onto chromatin containing DNA replication blocks

Element-centric clustering comparison unifies overlaps and hierarchy

Clustering is one of the most universal approaches for understanding complex data. A pivotal aspect of clustering analysis is quantitatively comparing clusterings; clustering comparison is the basis for many tasks such as clustering evaluation, consensus clustering, and tracking the temporal evolution of clusters. In particular, the extrinsic evaluation of clustering methods requires comparing the uncovered clusterings to planted clusterings or known metadata. Yet, as we demonstrate, existing clustering comparison measures have critical biases which undermine their usefulness, and no measure accommodates both overlapping and hierarchical clusterings. Here we unify the comparison of disjoint, overlapping, and hierarchically structured clusterings by proposing a new element-centric framework: elements are compared based on the relationships induced by the cluster structure, as opposed to the traditional cluster-centric philosophy. We demonstrate that, in contrast to standard clustering similarity measures, our framework does not suffer from critical biases and naturally provides unique insights into how the clusterings differ. We illustrate the strengths of our framework by revealing new insights into the organization of clusters in two applications: the improved classification of schizophrenia based on the overlapping and hierarchical community structure of fMRI brain networks, and the disentanglement of various social homophily factors in Facebook social networks. The universality of clustering suggests far-reaching impact of our framework throughout all areas of science

Universal transport near a quantum critical Mott transition in two dimensions

We discuss the universal transport signatures near a zero-temperature continuous Mott transition between a Fermi liquid (FL) and a quantum spin liquid in two spatial dimensions. The correlation-driven transition occurs at fixed filling and involves fractionalization of the electron: upon entering the spin liquid, a Fermi surface of neutral spinons coupled to an internal gauge field emerges. We present a controlled calculation of the value of the zero temperature universal resistivity jump predicted to occur at the transition. More generally, the behavior of the universal scaling function that collapses the temperature and pressure dependent resistivity is derived, and is shown to bear a strong imprint of the emergent gauge fluctuations. We further predict a universal jump of the thermal conductivity across the Mott transition, which derives from the breaking of conformal invariance by the damped gauge field, and leads to a violation of the Wiedemann-Franz law in the quantum critical region. A connection to organic salts is made, where such a transition might occur. Finally, we present some transport results for the pure rotor O(N) CFT.Comment: 27 pages, 16 figure