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"High-pressure behavior of A2B2O7 pyrochlore (A=Eu, Dy; B=Ti, Zr)" — Dylan R. Rittman: Katlyn M. Turner, Sulgiye Park, Antonio F. Fuentes, Jinyuan Yan, Rodney C.Ewing, and Wendy L. Mao; Journal of Applied Physics , 01/24/17.
In situ high-pressure X-ray diffraction and Raman spectroscopy were used to determine the influence of composition on the high-pressure behavior of A2B2O7 pyrochlore (A = Eu, Dy; B = Ti, Zr) up to ∼ 50 GPa. Based on X-ray diffraction results, all compositions transformed to the high-pressure cotunnite structure. The B-site cation species had a larger effect on the transition pressure than the A-site cation species, with the onset of the phase transformation occurring at ∼ 41 GPa for B = Ti and ∼ 16 GPa B = Zr. However, the A-site cation affected the kinetics of the phase transformation, with the transformation for compositions with the smaller ionic radii, i.e., A = Dy, proceeding faster than those with a larger ionic radii, i.e., A = Eu. These results were consistent with previous work in which the radius-ratio of the A- and B-site cations determined the energetics of disordering, and compositions with more similarly sized A- and B-site cations had a lower defect formation energy. Raman spectra revealed differences in the degree of short-range order of the different compositions. Due to the large phase fraction of cotunnite at high pressure for B = Zr compositions, Raman modes for cotunnite could be observed, with more modes recorded for A = Eu than A =Dy. These additional modes are attributed to increased short-to-medium range ordering in the initially pyrochlore structured Eu2Zr2O7 as compared with the initially defect-fluorite structured Dy2Zr2O7.
"Ultrafast light-induced symmetry changes in single BaTiO3 nanowires" — Yi-Hong Kuo: Sanghee Nah, Kai He, Te Hu, Aaron M. Lindenberg; Journal of Materials Chemistry C, 01/23/17.
The coupling of light to nanoscale ferroelectric materials enables novel means of controlling their coupled degrees of freedom and engineering new functionality. Here we present femtosecond time-resolution nonlinear-optical measurements of light-induced dynamics within single ferroelectric barium titanate nanowires. By analyzing the time-dependent and polarization-dependent second harmonic intensity generated by the nanowire, we identify its crystallographic orientation and then make use of this information in order to probe its dynamic structural response and change in symmetry. We show that photo-excitation leads to ultrafast, non-uniform modulations in the second order nonlinear susceptibility tensor, indicative of changes in the local symmetry of the nanostructure occurring on sub-picosecond time-scales.
"Cu2ZnSnSe4 Photovoltaic Absorber Layers Evaluated by Transmission X-Ray Microscopy Tomography: Composition Fluctuations on the Length Scale of Grains" — Dennis S. Pruzan: Anna E. Caruso, Dr. Yijin Liu, Dr. Yu Lin, Dr. Carolyn Beall, Dr. Ingrid Repins, Dr. Michael F. Toney and Michael A. Scarpulla; Solar RRL, 12/30/16.
The origins of open-circuit voltage deficits in Cu2ZnSnS(e)4-based solar cells have been an intense topic of research over the past few years as device efficiencies have never approached those of CuInGaSe2 based cells despite the materials sharing similar crystal and electronic structures. In this work, we use transmission X-ray microscopy tomography to investigate the length scales over which elemental fluctuations occur. We find and show evidence of micron-scale Cu to Zn anti-correlations over a previously inaccessible combination of resolution and sample size that is consistent with the length scale of grains in this material. This result yields further insight into the causes of the large open-circuit voltage deficits regularly seen in these devices as well as the challenges of achieving compositional homogeneity in this material.
"Hybrid metal–organic chalcogenide nanowires with electrically conductive inorganic core through diamondoid-directed assembly" — Hao Yan: J. Nathan Hohman, Fei Hua Li, Chunjing Jia, Diego Solis-Ibarra, Bin Wu, Jeremy E. P. Dahl, Robert M. K. Carlson, Boryslav A. Tkachenko, Andrey A. Fokin, Peter R. Schreiner, Arturas Vailionis, Taeho Roy Kim, Thomas P. Devereaux, Zhi-Xun Shen & Nicholas A. Melosh; Nature Materials, 12/26/16.
Controlling inorganic structure and dimensionality through structure-directing agents is a versatile approach for new materials synthesis that has been used extensively for metal–organic frameworks and coordination polymers. However, the lack of ‘solid’ inorganic cores requires charge transport through single-atom chains and/or organic groups, limiting their electronic properties. Here, we report that strongly interacting diamondoid structure-directing agents guide the growth of hybrid metal–organic chalcogenide nanowires with solid inorganic cores having three-atom cross-sections, representing the smallest possible nanowires. The strong van der Waals attraction between diamondoids overcomes steric repulsion leading to a cis configuration at the active growth front, enabling face-on addition of precursors for nanowire elongation. These nanowires have band-like electronic properties, low effective carrier masses and three orders-of-magnitude conductivity modulation by hole doping. This discovery highlights a previously unexplored regime of structure-directing agents compared with traditional surfactant, block copolymer or metal–organic framework linkers.
"Distinct Electronic Structure for the Extreme Magnetoresistance in YSb" — Junfeng He: Chaofan Zhang, Nirmal J. Ghimire, Tian Liang, Chunjing Jia, Juan Jiang, Shujie Tang, Sudi Chen, Yu He, S.-K. Mo, C. C. Hwang, M. Hashimoto, D. H. Lu, B. Moritz, T. P. Devereaux, Y. L. Chen, J. F. Mitchell, and Z.-X. Shen; Physical Review Letters, 12/23/16.
An extreme magnetoresistance (XMR) has recently been observed in several nonmagnetic semimetals. Increasing experimental and theoretical evidence indicates that the XMR can be driven by either topological protection or electron-hole compensation. Here, by investigating the electronic structure of a XMR material, YSb, we present spectroscopic evidence for a special case which lacks topological protection and perfect electron-hole compensation. Further investigations reveal that a cooperative action of a substantial difference between electron and hole mobility and a moderate carrier compensation might contribute to the XMR in YSb.
"Ideal charge-density-wave order in the high-field state of superconducting YBCO" — H. Jang: W.-S. Lee, H. Nojiri, S. Matsuzawa, H. Yasumura, L. Nie, A. V. Maharaj, S. Gerber, Y.-J. Liu, A. Mehta, D. A. Bonn, R. Liang, W. N. Hardy, C. A. Burns, Z. Islam, S. Song, J. Hastings, T. P. Devereaux, Z.-X. Shen, S. A. Kivelson, C.-C. Kao, D. Zhu, and J.-S. Lee; Proceedings of the National Academy of Sciences, 12/20/16.
The existence of charge-density-wave (CDW) correlations in cuprate superconductors has now been established. However, the nature of the CDW ground state has remained uncertain because disorder and the presence of superconductivity typically limit the CDW correlation lengths to only a dozen unit cells or less. Here we explore the field-induced 3D CDW correlations in extremely pure detwinned crystals of YBa2Cu3O2 (YBCO) ortho-II and ortho-VIII at magnetic fields in excess of the resistive upper critical field (Hc2Hc2) where superconductivity is heavily suppressed. We observe that the 3D CDW is unidirectional and possesses a long in-plane correlation length as well as significant correlations between neighboring CuO2planes. It is significant that we observe only a single sharply defined transition at a critical field proportional to Hc2Hc2, given that the field range used in this investigation overlaps with other high-field experiments including quantum oscillation measurements. The correlation volume is at least two to three orders of magnitude larger than that of the zero-field CDW. This is by far the largest CDW correlation volume observed in any cuprate crystal and so is presumably representative of the high-field ground state of an “ideal” disorder-free cuprate.
"2D materials advances: from large scale synthesis and controlled heterostructures to improved characterization techniques, defects and applications" — Zhong Lin: Amber McCreary, Natalie Briggs, Shruti Subramanian, Kehao Zhang, Yifan Sun, Xufan Li, Nicholas J Borys, Hongtao Yuan, Susan K Fullerton-Shirey, Alexey Chernikov, Hui Zhao, Stephen McDonnell, Aaron M Lindenberg, Kai Xiao, Brian J LeRoy, Marija Drndić, James C M Hwang, Jiwoong Park, Manish Chhowalla, Raymond E Schaak, Ali Javey, Mark C Hersam, Joshua Robinson and Mauricio Terrones; IOP Science, 12/08/16.
The rise of two-dimensional(2D) materials research took place following the isolation of graphene in 2004. These new 2D materials include transition metal dichalcogenides, mono-elemental 2D sheets, and several carbide- and nitride-based materials. The number of publications related to these emerging materials has been drastically increasing over the last five years. Thus, through this comprehensive review, we aim to discuss the most recent groundbreaking discoveries as well as emerging opportunities and remaining challenges. This review starts out by delving into the improved methods of producing these new 2D materials via controlled exfoliation, metal organic chemical vapor deposition, and wet chemical means.We look into recent studies of doping as well as the optical properties of 2D materials and their heterostructures. Recent advances towards applications of these materials in 2D electronics are also reviewed, and include the tunnel MOSFET and ways to reduce the contact resistance for fabricating highquality devices. Finally, several unique and innovative applications recently explored are discussed as well as perspectives of this exciting and fast moving field.
"Direct and continuous strain control of catalysts with tunable battery electrode materials" — Haotian Wang: Shicheng Xu, Charlie Tsai, Yuzhang Li, Chong Liu, Jie Zhao, Yayuan Liu, Hongyuan Yuan, Frank Abild-Pedersen, Fritz B. Prinz, Jens K. Nørskov, Yi Cui; Science , 11/25/16.
We report a method for using battery electrode materials to directly and continuously control the lattice strain of platinum (Pt) catalyst and thus tune its catalytic activity for the oxygen reduction reaction (ORR). Whereas the common approach of using metal overlayers introduces ligand effects in addition to strain, by electrochemically switching between the charging and discharging status of battery electrodes the change in volume can be precisely controlled to induce either compressive or tensile strain on supported catalysts. Lattice compression and tension induced by the lithium cobalt oxide substrate of ~5% were directly observed in individual Pt nanoparticles with aberration-corrected transmission electron microscopy. We observed 90% enhancement or 40% suppression in Pt ORR activity under compression or tension, respectively, which is consistent with theoretical predictions.
"Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO3" — F. Chen: Y. Zhu, S. Liu,Y. Qi, H. Y. Hwang, N. C. Brandt, J. Lu, F. Quirin, H. Enquist, P. Zalden, T. Hu, J. Goodfellow, M.-J. Sher, M. C. Hoffmann, D. Zhu, H. Lemke, J. Glownia, M. Chollet, A. R. Damodaran, J. Park, Z. Cai, I. W. Jung, M. J. Highland, D. A. Walko, J. W. Freeland, P. G. Evans, A. Vailionis, J. Larsson, K. A. Nelson, A. M. Rappe, K. Sokolowski-Tinten, L. W. Martin, H. Wen, and A. M. Lindenberg; Physical Review B, 11/22/16.
The dynamical processes associated with electric field manipulation of the polarization in a ferroelectric remain largely unknown but fundamentally determine the speed and functionality of ferroelectric materials and devices. Here we apply subpicosecond duration, single-cycle terahertz pulses as an ultrafast electric field bias to prototypical BaTiO3 ferroelectric thin films with the atomic-scale response probed by femtosecond x-ray-scattering techniques. We show that electric fields applied perpendicular to the ferroelectric polarization drive large-amplitude displacements of the titanium atoms along the ferroelectric polarization axis, comparable to that of the built-in displacements associated with the intrinsic polarization and incoherent across unit cells. This effect is associated with a dynamic rotation of the ferroelectric polarization switching on and then off on picosecond time scales. These transient polarization modulations are followed by long-lived vibrational heating effects driven by resonant excitation of the ferroelectric soft mode, as reflected in changes in the c-axis tetragonality. The ultrafast structural characterization described here enables a direct comparison with first-principles-based molecular-dynamics simulations, with good agreement obtained.
"All-optical materials design of chiral edge modes in transition-metal dichalcogenides" — Martin Claassen: Chunjing Jia, Brian Moritz & Thomas P. Devereaux; Nature Communications, 10/10/16.
Monolayer transition-metal dichalcogenides are novel materials which at low energies constitute a condensed-matter realization of massive relativistic fermions in two dimensions. Here, we show that this picture breaks for optical pumping—instead, the added complexity of a realistic materials description leads to a new mechanism to optically induce topologically protected chiral edge modes, facilitating optically switchable conduction channels that are insensitive to disorder. In contrast to graphene and previously discussed toy models, the underlying mechanism relies on the intrinsic three-band nature of transition-metal dichalcogenide monolayers near the band edges. Photo-induced band inversions scale linearly in applied pump field and exhibit transitions from one to two chiral edge modes on sweeping from red to blue detuning. We develop an ab initio strategy to understand non-equilibrium Floquet–Bloch bands and topological transitions, and illustrate for WS2 that control of chiral edge modes can be dictated solely from symmetry principles and is not qualitatively sensitive to microscopic materials details.