Higher-order topological superconductivity from repulsive interactions in kagome and honeycomb systems

We discuss a pairing mechanism in interacting two-dimensional multipartite lattices that intrinsically leads to a second order topological superconducting state with a spatially modulated gap. When the chemical potential is close to Dirac points, oppositely moving electrons on the Fermi surface undergo an interference phenomenon in which the Berry phase converts a repulsive electron-electron interaction into an effective attraction. The topology of the superconducting phase manifests as gapped edge modes in the quasiparticle spectrum and Majorana Kramers pairs at the corners. We present symmetry arguments which constrain the possible form of the electron-electron interactions in these systems and classify the possible superconducting phases which result. Exact diagonalization of the Bogoliubov-de Gennes Hamiltonian confirms the existence of gapped edge states and Majorana corner states, which strongly depend on the spatial structure of the gap. Possible applications to vanadium-based superconducting kagome metals AV$_3$Sb$_3$ (A=K,Rb,Cs) are discussed.

Improving the Privacy-Preserving of Covid-19 Bluetooth-Based Contact Tracing Applications Against Tracking Attacks

Bluetooth is an essential wireless standard for short-distance and low-power wireless networks. Health departments’ contact-tracing applications depended on Bluetooth technology to prevent infectious diseases from spreading, especially COVID-19. The security threats of the Bluetooth-based contact-tracing applications increased because an adversary can use them as surveillance tools that violate the user’s privacy and revealpersonal information. The Bluetooth standard mainly depends on the device address in its authenticated pairing mechanism (Secure Simple Pairing), which can collect with off-the-shelf hardware and software and leads to a tracking attack. To avoid the risk of tracking based on this security vulnerability in the Bluetooth protocol, we suggest a novel authentication protocol based on a noninteractive zero-knowledge scheme to substitute the authentication protocol used in the Bluetooth standard. The new protocol can replace the authentication protocol in the Bluetooth stack without any modification in the device pairing flow. Finally, we prove the security of our proposed scheme against the man-in-themiddle attack and tracking attack. A performance comparison with the authentication algorithm in the BLE standard shows that our method mitigates the tracking attack with low communication messages. Our results help enhance the contact-tracing application’s security in which Bluetooth access is available.

Targeted Poverty Alleviation in China: A Typology of Official–Household Relations

‘Targeted Poverty Alleviation’ (TPA) is the Chinese government’s latest anti-poverty policy, aiming to lift the remaining 70 million Chinese citizens above the poverty line by 2020. The TPA scheme is novel in that every impoverished household is paired one-on-one with a local government official, who then bears responsibility for the eradication of their poverty. Despite being at the core of TPA, this pairing mechanism has received little academic attention. Based on an empirical case study of ten households across two villages in rural Shaanxi Province, China, this article aims to investigate this pairing mechanism at the micro level and its outcomes for poverty alleviation, in order to better understand how the notion of ‘precision’ is being realized through TPA. Two distinct traits that influence the TPA pairing system emerged: first, the ranking of the assigned local official is important in that higher-ranked officials have greater social and financial resources at their disposal, bringing about enhanced poverty alleviation outcomes for their households compared with lower-ranked officials. Secondly, the willingness and ability of impoverished households to actively participate in their poverty alleviation programme is beneficial within the TPA scheme, achieving better outcomes in the long-term compared with households who are passive receivers. TPA has the potential to work effectively and to achieve China’s poverty reduction goals; however, our analysis shows that some pairing mechanisms are more effective in achieving poverty alleviation goals than others.

Evolution of spin excitations from bulk to monolayer FeSe

In ultrathin films of FeSe grown on SrTiO3 (FeSe/STO), the superconducting transition temperature Tc is increased by almost an order of magnitude, raising questions on the pairing mechanism. As in other superconductors, antiferromagnetic spin fluctuations have been proposed to mediate SC making it essential to study the evolution of the spin dynamics of FeSe from the bulk to the ultrathin limit. Here, we investigate the spin excitations in bulk and monolayer FeSe/STO using resonant inelastic x-ray scattering (RIXS) and quantum Monte Carlo (QMC) calculations. Despite the absence of long-range magnetic order, bulk FeSe displays dispersive magnetic excitations reminiscent of other Fe-pnictides. Conversely, the spin excitations in FeSe/STO are gapped, dispersionless, and significantly hardened relative to its bulk counterpart. By comparing our RIXS results with simulations of a bilayer Hubbard model, we connect the evolution of the spin excitations to the Fermiology of the two systems revealing a remarkable reconfiguration of spin excitations in FeSe/STO, essential to understand the role of spin fluctuations in the pairing mechanism.

Bereziskii-Kosterlitz-Thouless transition in the Weyl system PtBi2

Symmetry breaking in topological matter became, in the last decade, a key concept in condensed matter physics to unveil novel electronic states. In this work, we reveal that broken inversion symmetry and strong spin-orbit coupling in trigonal PtBi2 lead to a Weyl semimetal band structure, with unusually robust two-dimensional superconductivity in thin fims. Transport measurements show that high-quality PtBi2 crystals are three-dimensional superconductors (Tc≈600 mK) with an isotropic critical field (Bc≈50 mT). Remarkably, we evidence in a rather thick flake (60 nm), exfoliated from a macroscopic crystal, the two-dimensional nature of the superconducting state, with a critical temperature Tc≈370 mK and highly-anisotropic critical fields. Our results reveal a Berezinskii-Kosterlitz-Thouless transition with TBKT≈310 mK and with a broadening of Tc due to inhomogenities in the sample. Due to the very long superconducting coherence length ξ in PtBi2, the vortex-antivortex pairing mechanism can be studied in unusually-thick samples (at least five times thicker than for any other two-dimensional superconductor), making PtBi2 an ideal platform to study low dimensional superconductivity in a topological semimetal.

Insights into genome recoding from the mechanism of a classic +1-frameshifting tRNA

While genome recoding using quadruplet codons to incorporate non-proteinogenic amino acids is attractive for biotechnology and bioengineering purposes, the mechanism through which such codons are translated is poorly understood. Here we investigate translation of quadruplet codons by a +1-frameshifting tRNA, SufB2, that contains an extra nucleotide in its anticodon loop. Natural post-transcriptional modification of SufB2 in cells prevents it from frameshifting using a quadruplet-pairing mechanism such that it preferentially employs a triplet-slippage mechanism. We show that SufB2 uses triplet anticodon-codon pairing in the 0-frame to initially decode the quadruplet codon, but subsequently shifts to the +1-frame during tRNA-mRNA translocation. SufB2 frameshifting involves perturbation of an essential ribosome conformational change that facilitates tRNA-mRNA movements at a late stage of the translocation reaction. Our results provide a molecular mechanism for SufB2-induced +1 frameshifting and suggest that engineering of a specific ribosome conformational change can improve the efficiency of genome recoding.

Insights into Genome Recoding from the Mechanism of a Classic +1-Frameshifting tRNA

ABSTRACTWhile genome recoding using quadruplet codons to incorporate non-proteinogenic amino acids is attractive for biotechnology and bioengineering purposes, the mechanism through which such codons are translated is poorly understood. Here we investigate translation of quadruplet codons by a +1-frameshifting tRNA, SufB2, that contains an extra nucleotide in its anticodon loop. Natural post-transcriptional modification of SufB2 in cells prevents it from frameshifting using a quadruplet-pairing mechanism such that it preferentially employs a triplet-slippage mechanism. We show that SufB2 uses triplet anticodon-codon pairing in the 0-frame to initially decode the quadruplet codon, but subsequently shifts to the +1-frame during tRNA-mRNA translocation. SufB2 frameshifting involves perturbation of an essential ribosome conformational change that facilitates tRNA-mRNA movements at a late stage of the translocation reaction. Our results provide a molecular mechanism for SufB2-induced +1 frameshifting and suggest that engineering of a specific ribosome conformational change can improve the efficiency of genome recoding.

Single particle tunneling spectrum of superconducting Nd1-xSrxNiO2 thin films

The pairing mechanism in cuprates remains as one of the most challenging issues in condensed matter physics. Recently, superconductivity was discovered in thin films of the infinite-layer nickelate Nd1-xSrxNiO2 (x = 0.12–0.25) which is believed to have the similar 3d9 orbital electrons as that in cuprates. Here we report single-particle tunneling measurements on the superconducting nickelate thin films. We find predominantly two types of tunneling spectra, one shows a V-shape feature which can be fitted well by a d-wave gap function with gap maximum of about 3.9 meV, another one exhibits a full gap of about 2.35 meV. Some spectra demonstrate mixed contributions of these two components. Combining with theoretical calculations, we attribute the d-wave gap to the pairing potential of the $${\mathrm{Ni - }}3d_{x^2 - y^2}$$ Ni- 3 d x 2 − y 2 orbital. Several possible reasons are given for explaining the full gap feature. Our results indicate both similarities and distinctions between the newly found Ni-based superconductors and cuprates.