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"Current–phase relations of few-mode InAs nanowire Josephson junctions" — Eric M. Spanton: Mingtang Deng, Saulius Vaitiekėnas, Peter Krogstrup, Jesper Nygård, Charles M. Marcus & Kathryn A. Moler; Nature Physics, 08/14/17.
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Mingtang Deng, Saulius Vaitiekėnas, Peter Krogstrup, Jesper Nygård, Charles M. Marcus & Kathryn A. Moler
Abstract
Gate-tunable semiconductor nanowires with superconducting leads have great potential for quantum computation1, 2, 3 and as model systems for mesoscopic Josephson junctions4, 5. The supercurrent, I, versus the phase, ϕ, across the junction is called the current–phase relation (CPR). It can reveal not only the amplitude of the critical current, but also the number of modes and their transmission. We measured the CPR of many individual InAs nanowire Josephson junctions, one junction at a time. Both the amplitude and shape of the CPR varied between junctions, with small critical currents and skewed CPRs indicating few-mode junctions with high transmissions. In a gate-tunable junction, we found that the CPR varied with gate voltage: near the onset of supercurrent, we observed behaviour consistent with resonant tunnelling through a single, highly transmitting mode. The gate dependence is consistent with modelled subband structure that includes an effective tunnelling barrier due to an abrupt change in the Fermi level at the boundary of the gate-tuned region. These measurements of skewed, tunable, few-mode CPRs are promising both for applications that require anharmonic junctions6, 7 and for Majorana readout proposals8.
"Chiral Majorana fermion modes in a quantum anomalous Hall insulator–superconductor structure" — Qing Lin He: Qing Lin He, Lei Pan, Alexander L. Stern, Edward C. Burks, Xiaoyu Che, Gen Yin, Jing Wang, Biao Lian, Quan Zhou, Eun Sang Choi, Koichi Murata, Xufeng Kou, Zhijie Chen, Tianxiao Nie, Qiming Shao, Yabin Fan, Shou-Cheng Zhang, Kai Liu, Jing Xia, Kang L. Wang; Science , 07/21/17.
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Qing Lin He, Lei Pan, Alexander L. Stern, Edward C. Burks, Xiaoyu Che, Gen Yin, Jing Wang, Biao Lian, Quan Zhou, Eun Sang Choi, Koichi Murata, Xufeng Kou, Zhijie Chen, Tianxiao Nie, Qiming Shao, Yabin Fan, Shou-Cheng Zhang, Kai Liu, Jing Xia, Kang L. Wang
Abstract
Majorana fermion is a hypothetical particle that is its own antiparticle. We report transport measurements that suggest the existence of one-dimensional chiral Majorana fermion modes in the hybrid system of a quantum anomalous Hall insulator thin film coupled with a superconductor. As the external magnetic field is swept, half-integer quantized conductance plateaus are observed at the locations of magnetization reversals, giving a distinct signature of the Majorana fermion modes. This transport signature is reproducible over many magnetic field sweeps and appears at different temperatures. This finding may open up an avenue to control Majorana fermions for implementing robust topological quantum computing.
"Femtosecond electron-phonon lock-in by photoemission and x-ray free-electron laser" — S. Gerber: S.-L. Yang, D. Zhu, H. Soifer, J. A. Sobota, S. Rebec, J. J. Lee, T. Jia, B. Moritz, C. Jia, A. Gauthier, Y. Li, D. Leuenberger, Y. Zhang, L. Chaix, W. Li, H. Jang, J.-S. Lee, M. Yi, G. L. Dakovski, S. Song, J. M. Glownia, S. Nelson, K. W. Kim, Y.-D. Chuang, Z. Hussain, R. G. Moore, T. P. Devereaux, W.-S. Lee, P. S. Kirchmann, Z.-X. Shen; Science , 07/07/17.
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S.-L. Yang, D. Zhu, H. Soifer, J. A. Sobota, S. Rebec, J. J. Lee, T. Jia, B. Moritz, C. Jia, A. Gauthier, Y. Li, D. Leuenberger, Y. Zhang, L. Chaix, W. Li, H. Jang, J.-S. Lee, M. Yi, G. L. Dakovski, S. Song, J. M. Glownia, S. Nelson, K. W. Kim, Y.-D. Chuang, Z. Hussain, R. G. Moore, T. P. Devereaux, W.-S. Lee, P. S. Kirchmann, Z.-X. Shen
Abstract
The interactions that lead to the emergence of superconductivity in iron-based materials remain a subject of debate. It has been suggested that electron-electron correlations enhance electron-phonon coupling in iron selenide (FeSe) and related pnictides, but direct experimental verification has been lacking. Here we show that the electron-phonon coupling strength in FeSe can be quantified by combining two time-domain experiments into a “coherent lock-in” measurement in the terahertz regime. X-ray diffraction tracks the light-induced femtosecond coherent lattice motion at a single phonon frequency, and photoemission monitors the subsequent coherent changes in the electronic band structure. Comparison with theory reveals a strong enhancement of the coupling strength in FeSe owing to correlation effects. Given that the electron-phonon coupling affects superconductivity exponentially, this enhancement highlights the importance of the cooperative interplay between electron-electron and electron-phonon interactions.
"Quantum spin Hall state in monolayer 1T'-WTe2" — Shujie Tang: Chaofan Zhang, Dillon Wong, Zahra Pedramrazi, Hsin-Zon Tsai, Chunjing Jia, Brian Moritz, Martin Claassen, Hyejin Ryu, Salman Kahn, Juan Jiang, Hao Yan, Makoto Hashimoto, Donghui Lu, Robert G. Moore, Chan-Cuk Hwang, Choongyu Hwang, Zahid Hussain, Yulin Chen, Miguel M. Ugeda, Zhi Liu, Xiaoming Xie, Thomas P. Devereaux, Michael F. Crommie, Sung-Kwan Mo & Zhi-Xun Shen; Nature Physics, 06/26/17.
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Chaofan Zhang, Dillon Wong, Zahra Pedramrazi, Hsin-Zon Tsai, Chunjing Jia, Brian Moritz, Martin Claassen, Hyejin Ryu, Salman Kahn, Juan Jiang, Hao Yan, Makoto Hashimoto, Donghui Lu, Robert G. Moore, Chan-Cuk Hwang, Choongyu Hwang, Zahid Hussain, Yulin Chen, Miguel M. Ugeda, Zhi Liu, Xiaoming Xie, Thomas P. Devereaux, Michael F. Crommie, Sung-Kwan Mo & Zhi-Xun Shen
Abstract
A quantum spin Hall (QSH) insulator is a novel two-dimensional quantum state of matter that features quantized Hall conductance in the absence of a magnetic field, resulting from topologically protected dissipationless edge states that bridge the energy gap opened by band inversion and strong spin–orbit coupling1, 2. By investigating the electronic structure of epitaxially grown monolayer 1T’-WTe2 using angle-resolved photoemission (ARPES) and first-principles calculations, we observe clear signatures of topological band inversion and bandgap opening, which are the hallmarks of a QSH state. Scanning tunnelling microscopy measurements further confirm the correct crystal structure and the existence of a bulk bandgap, and provide evidence for a modified electronic structure near the edge that is consistent with the expectations for a QSH insulator. Our results establish monolayer 1T’-WTe2 as a new class of QSH insulator with large bandgap in a robust two-dimensional materials family of transition metal dichalcogenides (TMDCs).
"Dispersive charge density wave excitations in Bi2Sr2CaCu2O8+δ" — L. Chaix: G. Ghiringhelli, Y. Y. Peng, M. Hashimoto, B. Moritz, K. Kummer, N. B. Brookes, Y. He, S. Chen, S. Ishida, Y. Yoshida, H. Eisaki, M. Salluzzo, L. Braicovich, Z.-X. Shen, T. P. Devereaux & W.-S. Lee; Nature Physics, 06/12/17.
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G. Ghiringhelli, Y. Y. Peng, M. Hashimoto, B. Moritz, K. Kummer, N. B. Brookes, Y. He, S. Chen, S. Ishida, Y. Yoshida, H. Eisaki, M. Salluzzo, L. Braicovich, Z.-X. Shen, T. P. Devereaux & W.-S. Lee
Abstract
Experimental evidence on high-Tc cuprates reveals ubiquitous charge density wave (CDW) modulations1, 2, 3, 4, 5, 6, 7, 8, 9, 10, which coexist with superconductivity. Although the CDW had been predicted by theory11, 12, 13, important questions remain about the extent to which the CDW influences lattice and charge degrees of freedom and its characteristics as functions of doping and temperature. These questions are intimately connected to the origin of the CDW and its relation to the mysterious cuprate pseudogap10, 14. Here, we use ultrahigh-resolution resonant inelastic X-ray scattering to reveal new CDW character in underdoped Bi2.2Sr1.8Ca0.8Dy0.2Cu2O8+δ. At low temperature, we observe dispersive excitations from an incommensurate CDW that induces anomalously enhanced phonon intensity, unseen using other techniques. Near the pseudogap temperature T∗, the CDW persists, but the associated excitations significantly weaken with an indication of CDW wavevector shift. The dispersive CDW excitations, phonon anomaly, and analysis of the CDW wavevector provide a comprehensive momentum-space picture of complex CDW behaviour and point to a closer relationship with the pseudogap state.
"Structure and chemistry of epitaxial ceria thin films on yttria-stabilized zirconia substrates, studied by high resolution electron microscopy" — Robert Sinclair: Sang Chul Lee, Yezhou Shi, William C. Chueh; Ultramicroscopy, 03/18/17.
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Sang Chul Lee, Yezhou Shi, William C. Chueh
Abstract
We have applied aberration-corrected transmission electron microscopy (TEM) imaging and electron energy loss spectroscopy (EELS) to study the structure and chemistry of epitaxial ceria thin films, grown by pulsed laser deposition onto (001) yttria-stabilized zirconia (YSZ) substrates. There are few observable defects apart from the expected mismatch interfacial dislocations and so the films would be expected to have good potential for applications. Under high electron beam dose rate (above about 6000 e-/Å2s) domains of an ordered structure appear and these are interpreted as being created by oxygen vacancy ordering. The ordered structure does not appear at lower lose rates (ca. 2600 e-/Å2s) and can be removed by imaging under 1 mbar oxygen gas in an environmental TEM. EELS confirms that there is both oxygen deficiency and the associated increase in Ce3+ versus Ce4+ cations in the ordered domains. In situ high resolution TEM recordings show the formation of the ordered domains as well as atomic migration along the ceria thin film (001) surface.
"Pressure-Induced Metallization of the Halide Perovskite (CH3NH3)PbI3" — Adam Jaffe: Yu Lin, Wendy L. Mao, Hemamala I. Karunadasa; Journal of the American Chemical Society, 03/14/17.
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Yu Lin, Wendy L. Mao, Hemamala I. Karunadasa
Abstract
We report the metallization of the hybrid perovskite semiconductor (MA)PbI3 (MA = CH3NH3 + ) with no apparent structural transition. We tracked its bandgap evolution during compression in diamond-anvil cells using absorption spectroscopy and observed strong absorption over both visible and IR wavelengths at pressures above ca. 56 GPa, suggesting the imminent closure of its optical bandgap. The metallic character of (MA)PbI3 above 60 GPa was confirmed using both IR reflectivity and variable-temperature dc conductivity measurements. The impressive semiconductor properties of halide perovskites have recently been exploited in a multitude of optoelectronic applications. Meanwhile, the study of metallic properties in oxide perovskites has revealed diverse electronic phenomena. Importantly, the mild synthetic routes to halide perovskites and the templating effects of the organic cations allow for fine structural control of the inorganic lattice. Pressure-induced closure of the 1.6 eV bandgap in (MA)PbI3 demonstrates the promise of the continued study of halide perovskites under a range of thermodynamic conditions, toward realizing wholly new electronic properties.
"Field-theoretic simulations of random copolymers with structural rigidity" — Shifan Mao: Quinn MacPherson, Jian Qin, Andrew J. Spakowitz; The Royal Society of Chemistry, 03/13/17.
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Quinn MacPherson, Jian Qin, Andrew J. Spakowitz
Abstract
Copolymers play an important role in a range of soft-materials applications and biological phenomena. Prevalent works on block copolymer phase behavior use flexible chain models and incorporate interactions using a mean-field approximation. However, when phase separation takes place on length scales comparable to a few monomers, the structural rigidity of the monomers becomes important. In addition, concentration fluctuations become significant at short length scales, rendering the mean-field approximation invalid. In this work, we use simulation to address the role of finite monomer rigidity and concentration fluctuations in microphase segregation of random copolymers. Using a field-theoretic Monte-Carlo simulation of semiflexible polymers with random chemical sequences, we generate phase diagrams for random copolymers. We find that the melt morphology of random copolymers strongly depends on chain flexibility and chemical sequence correlation. Chemically anti-correlated copolymers undergo first-order phase transitions to local lamellar structures. With increasing degree of chemical correlation, this first-order phase transition is softened, and melts form microphases with irregular shaped domains. Our simulations in the homogeneous phase exhibit agreement with the density–density correlation from mean-field theory. However, conditions near a phase transition result in deviations between simulation and mean-field theory for the density–density correlation and the critical wavemode. Chain rigidity and sequence randomness lead to frustration in the segregated phase, introducing heterogeneity in the resulting morphologies.
"Nonequilibrium lattice-driven dynamics of stripes in nickelates using time-resolved x-ray scattering" — W.S. Lee: Y. F. Kung, B. Moritz, G. Coslovich, R. A. Kaindl, Y. D. Chuang, R. G. Moore, D. H. Lu, P. S. Kirchmann, J. S. Robinson, M. P. Minitti, G. Dakovski, W. F. Schlotter, J. J. Turner, S. Gerber, T. Sasagawa, Z. Hussain, Z. X. Shen, and T. P. Devereaux; Physical Review B , 03/13/17.
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Y. F. Kung, B. Moritz, G. Coslovich, R. A. Kaindl, Y. D. Chuang, R. G. Moore, D. H. Lu, P. S. Kirchmann, J. S. Robinson, M. P. Minitti, G. Dakovski, W. F. Schlotter, J. J. Turner, S. Gerber, T. Sasagawa, Z. Hussain, Z. X. Shen, and T. P. Devereaux
Abstract
We investigate the lattice coupling to the spin and charge orders in the striped nickelate, La1.75Sr0.25NiO4, using time-resolved resonant x-ray scattering. Lattice-driven dynamics of both spin and charge orders are observed when the pump photon energy is tuned to that of an Eu bond- stretching phonon. We present a likely scenario for the behavior of the spin and charge order parameters and its implications using a Ginzburg-Landau theory.
"Reviving the lithium metal anode for high-energy batteries" — Dingchang Lin: Yayuan Liu & Yi Cui; Nat Nano, 03/07/17.
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Yayuan Liu & Yi Cui
Abstract
Lithium-ion batteries have had a profound impact on our daily life, but inherent limitations make it difficult for Li-ion chemistries to meet the growing demands for portable electronics, electric vehicles and grid-scale energy storage. Therefore, chemistries beyond Li-ion are currently being investigated and need to be made viable for commercial applications. The use of metallic Li is one of the most favoured choices for next-generation Li batteries, especially Li–S and Li–air systems. After falling into oblivion for several decades because of safety concerns, metallic Li is now ready for a revival, thanks to the development of investigative tools and nanotechnology-based solutions. In this Review, we first summarize the current understanding on Li anodes, then highlight the recent key progress in materials design and advanced characterization techniques, and finally discuss the opportunities and possible directions for future development of Li anodes in applications.