Observation of a giant mass enhancement in the ultrafast electron dynamics of a topological semimetal

Topological phases of matter offer exciting possibilities to realize lossless charge and spin information transport on ultrafast time scales. However, this requires detailed knowledge of their nonequilibrium properties. Here, we employ time-, spin- and angle-resolved photoemission to investigate the ultrafast response of the Sb(111) spin-polarized surface state to femtosecond-laser excitation. The surface state exhibits a giant mass enhancement which is observed as a kink structure in its energy-momentum dispersion above the Fermi level. The kink structure, originating from the direct coupling of the surface state to the bulk continuum, is characterized by an abrupt change in the group velocity by ~70%, in agreement with our GW-based band structure calculations. Our observation of this connectivity in the transiently occupied band structure enables the unambiguous experimental verification of the topological nature of the surface state. The influence of bulk-surface coupling is further confirmed by our measurements of the electron dynamics, which show that bulk and surface states behave as a single thermalizing electronic population with distinct contributions from low-k electron-electron and high-k electron-phonon scatterings. These findings are important for future applications of topological semimetals and their excitations in ultrafast spintronics.

Decoding on the ribosome depends on the structure of the mRNA phosphodiester backbone

During translation, the ribosome plays an active role in ensuring that mRNA is decoded accurately and rapidly. Recently, biochemical studies have also implicated certain accessory factors in maintaining decoding accuracy. However, it is currently unclear whether the mRNA itself plays an active role in the process beyond its ability to base pair with the tRNA. Structural studies revealed that the mRNA kinks at the interface of the P and A sites. A magnesium ion appears to stabilize this structure through electrostatic interactions with the phosphodiester backbone of the mRNA. Here we examined the role of the kink structure on decoding using a well-defined in vitro translation system. Disruption of the kink structure through site-specific phosphorothioate modification resulted in an acute hyperaccurate phenotype. We measured rates of peptidyl transfer for near-cognate tRNAs that were severely diminished and in some instances were almost 100-fold slower than unmodified mRNAs. In contrast to peptidyl transfer, the modifications had little effect on GTP hydrolysis by elongation factor thermal unstable (EF-Tu), suggesting that only the proofreading phase of tRNA selection depends critically on the kink structure. Although the modifications appear to have no effect on typical cognate interactions, peptidyl transfer for a tRNA that uses atypical base pairing is compromised. These observations suggest that the kink structure is important for decoding in the absence of Watson–Crick or G–U wobble base pairing at the third position. Our findings provide evidence for a previously unappreciated role for the mRNA backbone in ensuring uniform decoding of the genetic code.

A Deformation Mechanism for Ridge-Shaped Kink Structure in Layered Solids

A deformation mechanism for ridge-shaped kink structure (RSKS), a type of localized deformation, is studied with a discussion focusing on the kink deformation of a monocrystal with a single-slip system under a plane-strain condition. From a geometrical study of displacement continuity it is found that, to satisfy displacement continuity, the kink boundary formed by the deformation from an initially homogeneous structure must have symmetry. We propose a simple model of the RSKS deformation mode to accomplish plastic deformation from a compressive force parallel to the slip direction. First, important geometrical knowledge related to the RSKS formation mechanism is formulated analytically. Then, a simulation of a spring–mass model is performed to clarify the RSKS formation mechanism. The intensity of the angle-dependent force field is found to affect the deformation mode.