Date: 01/09/2021

Advances and challenges in time-resolved macromolecular crystallography

GISELA BRÄNDÉN HTTPS://ORCID.ORG/0000-0001-5615-7187 AND RICHARD NEUTZE HTTPS://ORCID.ORG/0000-0003-0986-6153

Abstract

Conformational changes within biological macromolecules control a vast array of chemical reactions in living cells. Time-resolved crystallography can reveal time-dependent structural changes that occur within protein crystals, yielding chemical insights in unparalleled detail. Serial crystallography approaches developed at x-ray free-electron lasers are now routinely used for time-resolved diffraction studies of macromolecules. These techniques are increasingly being applied at synchrotron radiation sources and to a growing diversity of macromolecules. Here, we review recent progress in the field, including visualizing ultrafast structural changes that guide the initial trajectories of light-driven reactions as well as capturing biologically important conformational changes on slower time scales, for which bacteriorhodopsin and photosystem II are presented as illustrative case studies.

Back to the future: The original time crystal makes a comeback

By Adrian ChoNov. 27, 2019 , 11:35 AM

https://www.sciencemag.org/news/2019/11/back-future-original-time-crystal-makes-comeback

Ideal Weyl semimetal induced by magnetic exchange

J.-R. Soh, F. de Juan, M. G. Vergniory, N. B. M. Schröter, M. C. Rahn, D. Y. Yan, J. Jiang, M. Bristow, P. A. Reiss, J. N. Blandy, Y. F. Guo, Y. G. Shi, T. K. Kim, A. McCollam, S. H. Simon, Y. Chen, A. I. Coldea, and A. T. Boothroyd

Phys. Rev. B 100, 201102(R) (2019) – Published 13 November 2019 [https://journals.aps.org/prb/issues/100/20 ]

Weyl semimetals exhibit anomalous electronic transport due to the presence of topological band crossings, called Weyl nodes. In this work, experimental and theoretical results show that EuCd₂As₂ in a magnetic field is what Bernevig has termed the hydrogen atom of a Weyl semimetal, i.e. one with a single pair of Weyl nodes at the Fermi level and without overlapping electron bands. The discovery opens the door to a wide range of exotic physics, predicted for Weyl fermions in the solid state.

We report theoretical and experimental evidence that EuCd2As2 in magnetic fields greater than 1.6 T applied along the c axis is a Weyl semimetal with a single pair of Weyl nodes. Ab initio electronic structure calculations, verified at zero field by angle-resolved photoemission spectra, predict Weyl nodes with wave vectors k=(0,0,±0.03)×2π/c at the Fermi level when the Eu spins are fully aligned along the c axis. Shubnikov–de Haas oscillations measured in fields parallel to c reveal a cyclotron effective mass of mc*=0.08me and a Fermi surface of extremal area Aext=0.24nm−2, corresponding to 0.1% of the area of the Brillouin zone. The small values of mc* and Aext are consistent with quasiparticles near a Weyl node. The identification of EuCd2As2 as a model Weyl semimetal opens the door to fundamental tests of Weyl physics.

Leveraging electron-phonon interaction to enhance the thermoelectric power factor in graphene-like semimetals

Yi Xia, Junsoo Park, Vidvuds Ozoliņš, and Chris Wolverton

Phys. Rev. B 100, 201401(R) – Published 1 November 2019 [https://journals.aps.org/prb/abstract/10.1103/PhysRevB.100.201401 ]

Electron-phonon interactions (EPIs) are presumably detrimental for thermoelectric performance in semiconductors because they limit carrier mobility. Here we show that enhanced EPIs with strong energy dependence offer an intrinsic pathway to a significant increase in the Seebeck coefficient and the thermoelectric power factor, particularly in the context of two-dimensional (2D) graphene-like Dirac bands. The increase is realized by enabling electron energy filtering through preferential scattering of electron/hole carriers. We prove this concept by implementing first-principles computational methods with explicit treatment of EPIs for a 2D gapless MoS₂ allotrope, which has both massless Dirac bands and a heavy-fermion state that acts as the filter. We determine that the optimal location of the heavy state and hence the onset of the filtering process is at the Dirac point. Our study opens an avenue for attaining ultrahigh power factors via engineering the EPIs in graphene-like semimetals or identifying new compounds that intrinsically possess the featured electronic structure.

Received 25 April 2019

DOI:https://doi.org/10.1103/PhysRevB.100.201401