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Zhao J.,University of California at Berkeley | Zhao J.,Miller Institute for Basic Research in Science | Cao H.,Oak Ridge National Laboratory | Bourret-Courchesne E.,Lawrence Berkeley National Laboratory | And 3 more authors.
Physical Review Letters

The recently discovered K-Fe-Se high-temperature superconductor has caused heated debate regarding the nature of its parent compound. Transport, angle-resolved photoemission spectroscopy, and STM measurements have suggested that its parent compound could be insulating, semiconducting, or even metallic. Because the magnetic ground states associated with these different phases have not yet been identified and the relationship between magnetism and superconductivity is not fully understood, the real parent compound of this system remains elusive. Here, we report neutron-diffraction experiments that reveal a semiconducting antiferromagnetic (AFM) phase with rhombus iron vacancy order. The magnetic order of the semiconducting phase is the same as the stripe AFM order of the iron pnictide parent compounds. Moreover, while the √5×√5 block AFM phase coexists with superconductivity, the stripe AFM order is suppressed by it. This leads us to conjecture that the new semiconducting magnetic ordered phase is the true parent phase of this superconductor. © 2012 American Physical Society. Source

Shekhawat A.,Lawrence Berkeley National Laboratory | Shekhawat A.,University of California at Berkeley | Shekhawat A.,Miller Institute for Basic Research in Science | Ritchie R.O.,Lawrence Berkeley National Laboratory | Ritchie R.O.,University of California at Berkeley
Nature Communications

Pristine monocrystalline graphene is claimed to be the strongest material known with remarkable mechanical and electrical properties. However, graphene made with scalable fabrication techniques is polycrystalline and contains inherent nanoscale line and point defects - grain boundaries and grain-boundary triple junctions - that lead to significant statistical fluctuations in toughness and strength. These fluctuations become particularly pronounced for nanocrystalline graphene where the density of defects is high. Here we use large-scale simulation and continuum modelling to show that the statistical variation in toughness and strength can be understood with 'weakest-link' statistics. We develop the first statistical theory of toughness in polycrystalline graphene, and elucidate the nanoscale origins of the grain-size dependence of its strength and toughness. Our results should lead to more reliable graphene device design, and provide a framework to interpret experimental results in a broad class of two-dimensional materials. © 2016, NPG. All rights reserved. Source

Zhao J.,Fudan University | Zhao J.,University of California at Berkeley | Zhao J.,Miller Institute for Basic Research in Science | Rotundu C.R.,Lawrence Berkeley National Laboratory | And 8 more authors.
Physical Review Letters

Magnetic correlations in isovalently doped Ba(Fe1-xRu x)2As2 (x=0.25, Tc=14.5 K; x=0.35, Tc=20 K) are studied by elastic and inelastic neutron scattering techniques. A relatively large superconducting spin gap accompanied by a weak resonance mode is observed in the superconducting state in both samples. In the normal state, the magnetic excitation intensity is dramatically reduced with increasing Ru doping toward the optimally doped regime. Our results favor that the weakening of the electron-electron correlations by Ru doping is responsible for the dampening of the resonance mode, as well as the suppression of the normal state antiferromagnetic correlations near the optimally doped regime in this system. © 2013 American Physical Society. Source

Liu C.C.,University of California at Berkeley | Liu C.C.,Miller Institute for Basic Research in Science | Qi L.,University of California at Berkeley | Yanofsky C.,Stanford University | And 2 more authors.
Nature Biotechnology

Small-molecule regulation of gene expression is intrinsic to cellular function and indispensable to the construction of new biological sensing, control and expression systems. However, there are currently only a handful of strategies for engineering such regulatory components and fewer still that can give rise to an arbitrarily large set of inducible systems whose members respond to different small molecules, display uniformity and modularity in their mechanisms of regulation, and combine to actuate universal logics. Here we present an approach for small-molecule regulation of transcription based on the combination of cis-regulatory leader-peptide elements with genetically encoded unnatural amino acids (amino acids that have been artificially added to the genetic code). In our system, any genetically encoded unnatural amino acid (UAA) can be used as a small-molecule attenuator or activator of gene transcription, and the logics intrinsic to the network defined by expanded genetic codes can be actuated. © 2011 Nature America, Inc. All rights reserved. Source

Haber L.H.,Lawrence Berkeley National Laboratory | Haber L.H.,Columbia University | Doughty B.,Lawrence Berkeley National Laboratory | Leone S.R.,Lawrence Berkeley National Laboratory | Leone S.R.,Miller Institute for Basic Research in Science
Molecular Physics

Time-resolved anisotropy parameters and cross-section ratios of the positive and negative sidebands from two-colour two-photon above threshold ionization of helium atoms are measured using photoelectron velocity map imaging with the selected 19th high-order harmonic at 29.1 eV in an 810 nm perturbative dressing field. The intensities of both the sidebands and the single-photon ionization depletion follow a Gaussian correlation function where the photoelectron angular distributions and cross-section ratios of the sidebands do not change as a function of the temporal delay between the extreme ultraviolet and infrared pulses. The experimental results are compared with theoretical predictions using the soft-photon approximation, showing poor agreement, and analytical expressions are derived using second-order perturbation theory to determine the relative magnitudes of the resulting S and D partial waves of the above threshold ionization features. © 2010 Taylor & Francis. Source

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