Electronic Instability of Kagome Metal CsV_(3)Sb_(5) in the 2×2×2 Charge Density Wave State

Recently discovered kagome metals AV_(3)Sb_(5)(A=K,Rb,and Cs)provide an ideal platform to study the correlation among nontrivial band topology,unconventional charge density wave(CDW),and superconductivity.The evolution of electronic structures associated with the change of lattice modulations is crucial for understanding of the CDW mechanism,with the combination of angle-resolved photoemission spectroscopy(ARPES)measurements and density functional theory calculations,we investigate how band dispersions change with the increase of lattice distortions.In particular,we focus on the electronic states around M point,where the van Hove singularities are expected to play crucial roles in the CDW transition.Previous ARPES studies reported a spectral weight splitting of the van Hove singularity around M point,which is associated with the 3D lattice modulations.Our studies reveal that this“splitting”can be connected to the two van Hove singularities at k_(z)=0 and k_(z)=π/c in the normal states.When the electronic system enters into the CDW state,both van Hove singularities move down.Such novel properties are important for understanding of the CDW transition.

Experimental Observation of Electronic Structures of Kagome Metal YCr6Ge6

Using angle-resolved photoemission spectroscopy,we study electronic structures of a Kagome metal YCr6Ge6.Band dispersions along kz direction are significant,suggesting a remarkable interlayer coupling between neighboring Kagome planes.Comparing ARPES data with first-principles calculations,we find a moderate electron correlation in this material,since band calculations must be compressed in the energy scale to reach an excellent agreement between experimental data and theoretical calculations.Moreover,as indicated by band calculations,there is a flat band in the vicinity of the Fermi level at the Г–M–K plane in the momentum space,which could be responsible for the unusual transport behavior in YCr6Ge6.

Tailoring of Bandgap and Spin-Orbit Splitting in ZrSe_(2) with Low Substitution of Ti for Zr

Tuning the bandgap in layered transition metal dichalcogenides(TMDCs) is crucial for their versatile applications in many fields. The ternary formation is a viable method to tune the bandgap as well as other intrinsic properties of TMDCs, because the multi-elemental characteristics provide additional tunability at the atomic level and advantageously alter the physical properties of TMDCs. Herein, ternary Ti_(x)Zr_(1-x)Se_(2) single crystals were synthesized using the chemical-vapor-transport method. The changes in electronic structures of ZrSe_(2) induced by Ti substitution were revealed using angle-resolved photoemission spectroscopy. Our data show that at a low level of Ti substitution, the bandgap of Ti_(x)Zr_(1-x)Se_(2) decreases monotonically, and the electronic system undergoes a transition from a semiconducting to a metallic state without a significant variation of dispersions of valence bands. Meanwhile, the size of spin-orbit splitting dominated by Se 4p orbitals decreases with the increase of Ti doping. Our work shows a convenient way to alter the bandgap and spin-orbit coupling in TMDCs at the low level of substitution of transition metals.

Persistence of charge density wave against variation of band structures in V_(x)Ti_(1−x)Se_(2)(x=0–0.1)

Charge density wave(CDW)is a phenomenon that occurs in materials,accompanied by changes in their intrinsic electronic properties.The study of CDW and its modulation in materials holds tremendous significance in materials research,as it provides a unique approach to controlling the electronic properties of materials.TiSe_(2) is a typical layered material with a CDW phase at low temperatures.Through V substitution for Ti in TiSe_(2),we tuned the carrier concentration in V_(x)Ti_(1-x)Se_(2) to study how its electronic structures evolve.Angle-resolved photoemission spectroscopy(ARPES)shows that the band-folding effect is sustained with the doping level up to 10%,indicating the persistence of the CDW phase,even though the band structure is strikingly different from that of the parent compound TiSe_(2).Though CDW can induce the band fold effect with a driving force from the perspective of electronic systems,our studies suggest that this behavior could be maintained by lattice distortion of the CDW phase,even if band structures deviate from the electron-driven CDW scenario.Our work provides a constraint for understanding the CDW mechanism in TiSe_(2),and highlights the role of lattice distortion in the band-folding effect.

灭活疫苗或腺病毒载体疫苗初次免疫后不同疫苗加强针中和SARS-CoV-2 Omicron变异株的效果

SARS-CoV-2 Omicron变异株已在全球范围内广泛传播.包括中国在内的许多国家广泛接种了SARS-CoV-2灭活疫苗(IAV)、重组蛋白疫苗(PRV)和腺病毒载体疫苗(Ad V),以达到群体免疫效果.该研究系统评价了完成不同疫苗全程接种后以及接种不同疫苗加强针后的341份人血清对原始毒株和Omicron变异株的交叉中和活性.研究发现,所有类型疫苗诱导的针对Omicron变异株BA.1的中和抗体水平都明显低于原始毒株.初免疫IAV和Ad V后,分别接种Ad V和PRV的异源加强针可以最大程度地提高血清对Omicron中和活性.此外,该研究使用m RNA疫苗在小鼠模型中评估了初次免疫编码Spike蛋白的m RNA疫苗后,加强免疫编码原始毒株或者不同突变株RBD蛋白的m RNA疫苗对Omicron中和效果.结果发现,与原始毒株加强针相比,只有Omicron加强针疫苗显著地提升了Omicron的中和抗体水平.该研究对各种疫苗加强针的系统性评价可为接下来对抗Omicron变异株的免疫接种提供参考.

A non-rigid shift of band dispersions induced by Cu intercalation in 2H-TaSe2

The intercalation of metal is a promising method for the modulating electronic properties in transition metal dichalcogenides(TMDs).However,there still lacks enough knowledge about how the intercalated atoms directly impact the two-dimensional structural layers and modulate the band structures therein.Taking advantage of X-ray absorption fine structure and angle-resolved photoemission spectroscopy,we studied how Cu intercalation influences the host TaSe2 layers in Cu0.03TaSe2 crystals.The intercalated Cu atoms form bonds with Se of the host layers,and there is charge transfer from Cu to Se.By examining the changes of band dispersions,we show that the variation of electronic structures is beyond a simple rigid band model with merely charge doping effect.This work'reveals that the unusual change of band dispersions is associated with the formation of bonds between the intercalated metal elements and anion ions in the host layers,and provides a reference for the comprehensive understanding of the electronic structures in intercalated materials.

Tuning the electric transport behavior of AgCrSe_(2) by intrinsic defects

The two-dimensional(2 D)structure often leads to unusual phenomena for the impact of confined mean free path of carrier scattering.As a quasi-2 D layered material,Ag CrSe_(2)has a liquid-like phonon behavior for its unstable Ag atoms at service temperature,leading to the promising candidate for thermoelectricity and fast ionic conductor.However,the inferior electronic performance constrains its application prospects as a functional semiconductor,which provides broad opportunity to tune its electric behaviors by defect chemistry.In this work,we revealed abundant electric transport behaviors of Ag CrSe_(2)with different types of intrinsic defects.For example,the Ag CrSe_(2)changes from Anderson insulator to metal when Se defects become prevailing and the magnetoresistance alters its sign depending on the relative ratio of Ag and Cr defects.Our results reported here can give salutary boosting on regulating the electric properties of ternary transition metal selenide by defect chemistry.