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Proton-assisted growth of ultra-flat graphene films 期刊论文
NATURE, 2020, 577 (7789) : 204-+
作者:  Yuan, Guowen;  Lin, Dongjing;  Wang, Yong;  Huang, Xianlei;  Chen, Wang;  Xie, Xuedong;  Zong, Junyu;  Yuan, Qian-Qian;  Zheng, Hang;  Wang, Di;  Xu, Jie;  Li, Shao-Chun;  Zhang, Yi;  Sun, Jian;  Xi, Xiaoxiang;  Gao, Libo
收藏  |  浏览/下载:29/0  |  提交时间:2020/07/03

Graphene films grown by chemical vapour deposition have unusual physical and chemical properties that offer promise for applications such as flexible electronics and high-frequency transistors(1-10). However, wrinkles invariably form during growth because of the strong coupling to the substrate, and these limit the large-scale homogeneity of the film(1-4,11,12). Here we develop a proton-assisted method of chemical vapour deposition to grow ultra-flat graphene films that are wrinkle-free. Our method of proton penetration(13-17) and recombination to form hydrogen can also reduce the wrinkles formed during traditional chemical vapour deposition of graphene. Some of the wrinkles disappear entirely, owing to the decoupling of van der Waals interactions and possibly an increase in distance from the growth surface. The electronic band structure of the as-grown graphene films shows a V-shaped Dirac cone and a linear dispersion relation within the atomic plane or across an atomic step, confirming the decoupling from the substrate. The ultra-flat nature of the graphene films ensures that their surfaces are easy to clean after a wet transfer process. A robust quantum Hall effect appears even at room temperature in a device with a linewidth of 100 micrometres. Graphene films grown by proton-assisted chemical vapour deposition should largely retain their intrinsic performance, and our method should be easily generalizable to other nanomaterials for strain and doping engineering.


  
Synchrotron infrared spectroscopic evidence of the probable transition to metal hydrogen 期刊论文
NATURE, 2020, 577 (7792) : 631-+
作者:  Zhuang, Zhe;  Yu, Jin-Quan
收藏  |  浏览/下载:33/0  |  提交时间:2020/07/03

Hydrogen has been an essential element in the development of atomic, molecular and condensed matter physics(1). It is predicted that hydrogen should have a metal state(2)  however, understanding the properties of dense hydrogen has been more complex than originally thought, because under extreme conditions the electrons and protons are strongly coupled to each other and ultimately must both be treated as quantum particles(3,4). Therefore, how and when molecular solid hydrogen may transform into a metal is an open question. Although the quest for metal hydrogen has pushed major developments in modern experimental high-pressure physics, the various claims of its observation remain unconfirmed(5-7). Here a discontinuous change of the direct bandgap of hydrogen, from 0.6 electronvolts to below 0.1 electronvolts, is observed near 425 gigapascals. This result is most probably associated with the formation of the metallic state because the nucleus zero-point energy is larger than this lowest bandgap value. Pressures above 400 gigapascals are achieved with the recently developed toroidal diamond anvil cell(8), and the structural changes and electronic properties of dense solid hydrogen at 80 kelvin are probed using synchrotron infrared absorption spectroscopy. The continuous downward shifts of the vibron wavenumber and the direct bandgap with increased pressure point to the stability of phase-III hydrogen up to 425 gigapascals. The present data suggest that metallization of hydrogen proceeds within the molecular solid, in good agreement with previous calculations that capture many-body electronic correlations(9).


  
Investigation of the fine structure of antihydrogen 期刊论文
NATURE, 2020, 578 (7795) : 375-+
作者:  Zhang, Bing;  Ma, Sai;  Rachmin, Inbal;  He, Megan;  Baral, Pankaj;  Choi, Sekyu;  Goncalves, William A.;  Shwartz, Yulia;  Fast, Eva M.;  Su, Yiqun;  Zon, Leonard I.;  Regev, Aviv;  Buenrostro, Jason D.;  Cunha, Thiago M.;  Chiu, Isaac M.;  Fisher, David E.;  Hsu, Ya-Chieh
收藏  |  浏览/下载:56/0  |  提交时间:2020/07/03

At the historic Shelter Island Conference on the Foundations of Quantum Mechanics in 1947, Willis Lamb reported an unexpected feature in the fine structure of atomic hydrogen: a separation of the 2S(1/2) and 2P(1/2) states(1). The observation of this separation, now known as the Lamb shift, marked an important event in the evolution of modern physics, inspiring others to develop the theory of quantum electrodynamics(2-5). Quantum electrodynamics also describes antimatter, but it has only recently become possible to synthesize and trap atomic antimatter to probe its structure. Mirroring the historical development of quantum atomic physics in the twentieth century, modern measurements on anti-atoms represent a unique approach for testing quantum electrodynamics and the foundational symmetries of the standard model. Here we report measurements of the fine structure in the n = 2 states of antihydrogen, the antimatter counterpart of the hydrogen atom. Using optical excitation of the 1S-2P Lyman-alpha transitions in antihydrogen(6), we determine their frequencies in a magnetic field of 1 tesla to a precision of 16 parts per billion. Assuming the standard Zeeman and hyperfine interactions, we infer the zero-field fine-structure splitting (2P(1/2)-2P(3/2)) in antihydrogen. The resulting value is consistent with the predictions of quantum electrodynamics to a precision of 2 per cent. Using our previously measured value of the 1S-2S transition frequency(6,7), we find that the classic Lamb shift in antihydrogen (2S(1/2)-2P(1/2) splitting at zero field) is consistent with theory at a level of 11 per cent. Our observations represent an important step towards precision measurements of the fine structure and the Lamb shift in the antihydrogen spectrum as tests of the charge-parity-time symmetry(8) and towards the determination of other fundamental quantities, such as the antiproton charge radius(9,10), in this antimatter system.


Precision measurements of the 1S-2P transition in antihydrogen that take into account the standard Zeeman and hyperfine effects confirm the predictions of quantum electrodynamics.


  
Quantum crystal structure in the 250-kelvin superconducting lanthanum hydride 期刊论文
NATURE, 2020, 578 (7793) : 66-+
作者:  Gate, David;  Saligrama, Naresha;  Leventhal, Olivia;  Yang, Andrew C.;  Unger, Michael S.;  Middeldorp, Jinte;  Chen, Kelly;  Lehallier, Benoit;  Channappa, Divya;  De Los Santos, Mark B.;  McBride, Alisha;  Pluvinage, John;  Elahi, Fanny;  Tam, Grace Kyin-Ye;  Kim, Yongha;  Greicius, Michael;  Wagner, Anthony D.;  Aigner, Ludwig;  Galasko, Douglas R.;  Davis, Mark M.;  Wyss-Coray, Tony
收藏  |  浏览/下载:33/0  |  提交时间:2020/07/03

The discovery of superconductivity at 200 kelvin in the hydrogen sulfide system at high pressures(1) demonstrated the potential of hydrogen-rich materials as high-temperature superconductors. Recent theoretical predictions of rare-earth hydrides with hydrogen cages(2,3) and the subsequent synthesis of LaH10 with a superconducting critical temperature (T-c) of 250 kelvin(4,5) have placed these materials on the verge of achieving the long-standing goal of room-temperature superconductivity. Electrical and X-ray diffraction measurements have revealed a weakly pressure-dependent T-c for LaH10 between 137 and 218 gigapascals in a structure that has a face-centred cubic arrangement of lanthanum atoms(5). Here we show that quantum atomic fluctuations stabilize a highly symmetrical Fm (3) over barm crystal structure over this pressure range. The structure is consistent with experimental findings and has a very large electron-phonon coupling constant of 3.5. Although ab initio classical calculations predict that this Fm (3) over barm structure undergoes distortion at pressures below 230 gigapascals(2,3,) yielding a complex energy landscape, the inclusion of quantum effects suggests that it is the true ground-state structure. The agreement between the calculated and experimental Tc values further indicates that this phase is responsible for the superconductivity observed at 250 kelvin. The relevance of quantum fluctuations calls into question many of the crystal structure predictions that have been made for hydrides within a classical approach and that currently guide the experimental quest for room-temperature superconductivity(6-8). Furthermore, we find that quantum effects are crucial for the stabilization of solids with high electron-phonon coupling constants that could otherwise be destabilized by the large electron-phonon interaction(9), thus reducing the pressures required for their synthesis.


  
Updated SABER Night Atomic Oxygen and Implications for SABER Ozone and Atomic Hydrogen 期刊论文
GEOPHYSICAL RESEARCH LETTERS, 2018, 45 (11) : 5735-5741
作者:  Mlynczak, Martin G.;  Hunt, Linda A.;  Russell, James M., III;  Marshall, B. Thomas
收藏  |  浏览/下载:19/0  |  提交时间:2019/04/09
atomic oxygen  atomic hydrogen  ozone  mesosphere  energy budget