Contrasting Transport Performance of Electron-and Hole-Doped Epitaxial Graphene for Quantum Resistance Metrology

Epitaxial graphene grown on silicon carbide(Si C/graphene)is a promising solution for achieving a highprecision quantum Hall resistance standard.Previous research mainly focused on the quantum resistance metrology of n-type Si C/graphene,while a comprehensive understanding of the quantum resistance metrology behavior of graphene with different doping types is lacking.Here,we fabricated both n-and p-type Si C/graphene devices via polymer-assisted molecular adsorption and conducted systematic magneto-transport measurements in a wide parameter space of carrier density and temperature.It is demonstrated that n-type devices show greater potential for development of quantum resistance metrology compared with p-type devices,as evidenced by their higher carrier mobility,lower critical magnetic field for entering quantized Hall plateaus,and higher robustness of the quantum Hall effect against thermal degeneration.These discrepancies can be reasonably attributed to the weaker scattering from molecular dopants for n-type devices,which is further supported by the analyses on the quantum interference effect in multiple devices.These results enrich our understanding of the charged impurity on electronic transport performance of graphene and,more importantly,provide a useful reference for future development of graphene-based quantum resistance metrology.

Synergistic effect of heterogeneous single atoms and clusters for improved catalytic performance

Electrocatalytic water splitting provides an efficient method for the production of hydrogen.In electrocatalytic water splitting,the oxygen evolution reaction(OER)involves a kinetically sluggish four-electron transfer process,which limits the efficiency of electrocatalytic water splitting.Therefore,it is urgent to develop highly active OER catalysts to accelerate reaction kinetics.Coupling single atoms and clusters in one system is an innovative approach for developing efficient catalysts that can synergistically optimize the adsorption and configuration of intermediates and improve catalytic activity.However,research in this area is still scarce.Herein,we constructed a heterogeneous single-atom cluster system by anchoring Ir single atoms and Co clusters on the surface of Ni(OH)_(2)nanosheets.Ir single atoms and Co clusters synergistically improved the catalytic activity toward the OER.Specifically,Co_(n)Ir_(1)/Ni(OH)_(2)required an overpotential of 255 mV at a current density of 10 mA·cm^(−2),which was 60 mV and 67 mV lower than those of Co_(n)/Ni(OH)_(2)and Ir1/Ni(OH)_(2),respectively.The turnover frequency of Co_(n)Ir_(1)/Ni(OH)_(2)was 0.49 s^(−1),which was 4.9 times greater than that of Co_(n)/Ni(OH)_(2)at an overpotential of 300 mV.

Bio-compatibility and cytotoxicity studies of water-soluble CuInS_2-ZnS-AFP fluorescence probe in liver cancer cells

BACKGROUND： The oncogenesis of hepatocellular carcinoma（HCC） is not clear. The current methods of the pertinent studies are not precise and sensitive. The present study was to use liver cancer cell line to explore the bio-compatibility and cytotoxicity of ternary quantum dots（QDs） probe and to evaluate the possible application of QDs in HCC.METHODS： CuInS_2-ZnS-AFP fluorescence probe was designed and synthesized to label the liver cancer cell HepG 2. The cytotoxicity of CuInS_2-ZnS-AFP probe was evaluated by MTT experiments and flow cytometry. RESULTS： The labeling experiments indicated that CuInS_2-ZnS QDs conjugated with AFP antibody could enter HepG 2 cells effectively and emit intensive yellow fluorescence by ultraviolet excitation without changing cellular morphology. Toxicity tests suggested that the cytotoxicity of CuInS_2-ZnS-AFP probe was significantly lower than that of CdT e-ZnS-AFP probe（t test, F=0.8, T=-69.326, P〈0.001）. For CuInS_2-ZnS-AFP probe, timeeffect relationship was presented in intermediate concentration（〉20%） groups（P〈0.05） and dose-effect relationship was presented in almost all of the groups（P〈0.05）. CONCLUSION： CuInS_2-ZnS-AFP QDs probe had better biocompatibility and lower cytotoxicity compared with CdT e-ZnS-AFP probe, and could be used for imaging the living cells in vitro.

Band Structures of Ultrathin Bi(110) Films on Black Phosphorus Substrates Using Angle-Resolved Photoemission Spectroscopy

The band structures of two-monolayer Bi（110） films on black phosphorus substrates are studied using angleresolved photoemission spectroscopy. Within the band gap of bulk black phosphorus, the electronic states near the Fermi level are dominated by the Bi（110） film. The band dispersions revealed by our data suggest that the orientation of the Bi（110） film is aligned with the black phosphorus substrate. The electronic structures of the Bi（110） film strongly deviate from the band calculations of the free-standing Bi（110） film, suggesting that the substrate can significantly affect the electronic states in the Bi（110） film. Our data show that there are no non-trivial electronic states in Bi（110） films grown on black phosphorus substrates.

Effect of Boundary Scattering on Magneto-Transport Performance in BN-Encapsulated Graphene

For conductors in the ballistic regime, electron-boundary scattering at the sample edge plays a dominant role in determining the transport performance, giving rise to many intriguing phenomena like low-field negative magnetoresistance effect. We systematically investigate the magneto-transport behaviors of BN-encapsulated graphene devices with narrow channel width W, wherein the bulk mean free path Lmfp can be very large and highly tunable. By comparing the magnetoresistance features and the amplitude of Lmfp in a large parameter space of temperature and carrier density, we reveal that the boundary-scattering-dominated negative magnetoresistance effect can still survive even when the ballistic ratio(Lmfp/W) is as low as 0.15. This striking value is much smaller than the expected value for achieving(quasi-) ballistic transport regime(Lmfp/W ≥ 1), and can be attributed to the ultra-low specularity of the sample edge of our graphene devices. These findings enrich our understanding of the effects of boundary scattering on channel transport, which is of vital importance for future designs of two-dimensional electronic devices with limited lateral sizes.

Magneto-transport properties of thin flakes of Weyl semiconductor tellurium

As an elemental semiconductor,tellurium has recently attracted intense interest due to its non-trivial band topology,and the resulted intriguing topological transport phenomena.In this study we report systematic electronic transport studies on tellurium flakes grown via a simple vapor deposition process.The sample is self-hole-doped,and exhibits typical weak localization behavior at low temperatures.Substantial negative longitudinal magnetoresistance under parallel magnetic field is observed over a wide temperature region,which is considered to share the same origin with that in tellurium bulk crystals,i.e.,the Weyl points near the top of valence band.However,with lowering temperature the longitudinal magnetoconductivity experiences a transition from parabolic to linear field dependency,differing distinctly from the bulk counterparts.Further analysis reveals that such a modulation of Weyl behaviors in this low-dimensional tellurium structure can be attributed to the enhanced inter-valley scattering at low temperatures.Our results further extend Weyl physics into a low-dimensional semiconductor system,which may find its potential application in designing topological semiconductor devices.

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.

Spin Resolved Zero-Line Modes in Minimally Twisted Bilayer Graphene from Exchange Field and Gate Voltage

The reliance on spin-orbit coupling or strong magnetic fields has always posed significant challenges for the mass production and even laboratory realization of most topological materials. Valley-based topological zero-line modes have attracted widespread attention due to their substantial advantage of being initially realizable with just an external electric field. However, the uncontrollable nature of electrode alignment and precise fabrication has greatly hindered the advancement in this field. By utilizing minimally twisted bilayer graphene and introducing exchange fields from magnetic substrates, we successfully realize a spin-resolved, electrode-free topological zeroline mode. Further integration of electrodes that do not require alignment considerations significantly enhances the tunability of the system's band structure. Our approach offers a promising new support for the dazzling potential of topological zero-line mode in the realm of low-energy-consumption electronics.

NMR Evidence for Universal Pseudogap Behavior in Quasi-Two-Dimensional FeSe-Based Superconductors

Recently,by intercalating organic ions into bulk FeSe superconductors,two kinds of layered FeSe-based superconductors[(TBA)xFeSe and(CTA)xFeSe]with superconducting transition temperatures(Tc)above 40 K have been discovered.Due to the large interlayer distance(~15A),these new layered superconductors have a large resistivity anisotropy analogous to bismuth-based cuprate superconductors.Moreover,remarkable pseudogap behavior well above Tcis revealed by nuclear magnetic resonance(NMR)measurements on77Se nuclei,suggesting a preformed pairing scenario similar to that of cuprates.Here,we report another new kind of organic-ion-intercalated FeSe superconductor,(PY)xFeSe,with a reduced interlayer distance(~10A)compared to(TBA)xFeSe and(CTA)xFeSe.By performing77Se NMR and transport measurements,we observe a similar pseudogap behavior well above Tcof~40 K and a large resistivity anisotropy of~10~4 in(PY)xFeSe.All these facts strongly support a universal pseudogap behavior in these layered FeSe-based superconductors with quasi-two-dimensional electronic structures.

Two-dimensional Sb cluster superlattice on Si substrate fabricated by a two-step method

Nanoclusters consisting of a few atoms have attracted a lot of research interests due to their exotic size-dependent properties. Here, well-ordered two-dimensional Sb cluster superlattice was fabricated on Si substrate by a two-step method and characterized by scanning tunneling microscopy. High resolution scanning tunneling microscope measurements revealed the fine structures of the Sb clusters, which consist of several Sb atoms ranging from 2 to 7. Furthermore, the electronic structure of the nanocluster displays the quantized energy-level which is due to the single-electron tunneling effects. We believe that the fabrication of Sb cluster superlattice broadens the species of the cluster superlattice and provides a promising candidate to further explore the novel physical and chemical properties of the semimetal nanocluster.

In-Plane Magnetization-Induced Corner States in Bismuthene

We theoretically demonstrate that the electronic second-order topological insulator with robust corner states,having a buckled honeycomb lattice, can be realized in bismuthene by inducing in-plane magnetization. Based on the sp^(3) Slater–Koster tight-binding model with parameters extracted from first-principles results, we show that spin-helical edge states along zigzag boundaries are gapped out by the in-plane magnetization whereas four robust in-gap electronic corner states at the intersection between two zigzag boundaries arise. By regulating the orientation of in-plane magnetization, we show different position distribution of four corner states with different energies. Nevertheless, it respects some spatial symmetries and thus can protect the higher-order topological phase. Combined with the Kane–Mele model, we discuss the influence of the magnetization orientation on the position distribution of corner states.

Temperature Dependent In-Plane Anisotropic Magnetoresistance in HfTe_(5) Thin Layers

We report the observation of in-plane anisotropic magnetoresistance and planar Hall effect in non-magnetic HfTe_(5) thin layers.The observed anisotropic magnetoresistance as well as its sign is strongly dependent on the critical resistivity anomaly temperature T_(p).Below T_(p),the anisotropic magnetoresistance is negative with large negative magnetoresistance.When the in-plane magnetic field is perpendicular to the current,the negative longitudinal magnetoresistance reaches its maximum.The negative longitudinal magnetoresistance effect in HfTe_(5) thin layers is dramatically different from that induced by the chiral anomaly as observed in Weyl and Dirac semimetals.One potential underlying origin may be attributed to the reduced spin scattering,which arises from the in-plane magnetic field driven coupling between the top and bottom surface states.Our findings provide valuable insights for the anisotropic magnetoresistance effect in topological electronic systems and the device potential of HfTe5 in spintronics and quantum sensing.

Progresses on carbon dioxide electroreduction into methane

The conversion of CO_(2) into value‐added chemicals and fuels via electrochemical methods paves a promising avenue to mitigate both energy and environmental crisis.Among all the carbonaceous products derived from CO_(2) electroreduction,CH_(4) is one of the most important carriers for chemical bond energy storage due to the highest value of mass heat.Herein,starting from the proposed reaction mechanisms reported previously,we summarized the recent progresses on CO_(2) electroreduction into CH_(4) from the perspective of catalyst design strategies including construction of subnanometer catalytic sites,modulation of interfaces,in‐situ structural evolution,and engineering of tandem catalysts.On the basis of both the previously theoretical predictions and experimental results,we aimed to gain insights into the reaction mechanism for the formation of CH_(4),which,in turn,would provide guidelines for the design of highly efficient catalysts.

Electron-Exciton Coupling in 1T-TiSe_(2)Bilayer

Excitons in solid state are bosons generated by electron-hole pairs as the Coulomb screening is sufficiently reduced.The exciton condensation can result in exotic physics such as super-fluidity and insulating state.In charge density wave(CDW)state,1T-TiSe_(2) is one of the candidates that may host the exciton condensation.However,to envision its excitonic effect is still challenging,particularly at the two-dimensional limit,which is applicable to future devices.Here,we realize the epitaxial 1T-TiSe_(2) bilayer,the two-dimensional limit for its 2×2×2 CDW order,to explore the exciton-associated effect.By means of high-resolution scanning tunneling spectroscopy and quasiparticle interference,we discover an unexpected state residing below the conduction band and right within the CDW gap region.As corroborated by our theoretical analysis,this mysterious phenomenon is in good agreement with the electron-exciton coupling.Our study provides a material platform to explore exciton-based electronics and opto-electronics.

Butterfly-Like Anisotropic Magnetoresistance and Angle-Dependent Berry Phase in a Type-ⅡWeyl Semimetal WP2

The Weyl semimetal has emerged as a new topologically nontrivial phase of matter,hosting low-energy excitations of massless Weyl fermions.Here,we present a comprehensive study of a type-ⅡWeyl semimetal WP2.Transport studies show a butterfly-like magnetoresistance at low temperature,reflecting the anisotropy of the electron Fermi surfaces.This four-lobed feature gradually evolves into a two-lobed variant with an increase in temperature,mainly due to the reduced relative contribution of electron Fermi surfaces compared to hole Fermi surfaces for magnetoresistance.Moreover,an angle-dependent Berry phase is also discovered,based on quantum oscillations,which is ascribed to the effective manipulation of extremal Fermi orbits by the magnetic field to feel nearby topological singularities in the momentum space.The revealed topological character and anisotropic Fermi surfaces of the WP2 substantially enrich the physical properties of Weyl semimetals,and show great promises in terms of potential topological electronic and Fermitronic device applications.

Interplay between the atomic structures and superconductivity of two-monolayer Pb films

Superconductors with reduced dimensionality have been widely explored for their exotic superconducting behaviors.Especially,at the two-dimensional limit,two-monolayer Pb films with two types of structures provide an ideal platform to unveil the underlying superconducting mechanism[Science 324,1314(2009)].Here,by combining scanning tunneling microscopy(STM)with the first-principle calculations,we successfully identify that these two types have different atomic lattice structures with varying stacking phases,which further enables us to calculate the phonon spectrum and electron phonon coupling strength of each type.The theoretical calculations are in good agreement with tunneling spectroscopy measurements of the superconducting transition temperatures(T_(c)),which established a correlation between atomic structures and superconductivity.Moreover,it was observed that the higher T_(c)of these two types also possess higher out-of-plane upper critical magnetic fields(Hc2).These findings will provide important new insights into two-dimensional superconductivity at the atomic level.

Structural and physical properties of the new layered transition metal material Na4Cu3TaAs4

We report the synthesis,structural and physical properties of a new layered transition metal arsenide Na4Cu3TaAs4.This material adopts the space groupⅠ√2 m,with lattice parameters of a=5.9101(3)?and c=13.8867(12)?.This structure contains two layers of Na sandwiched by antiPb O-type(Cu/Ta)As layers,similar to the"111"-type ironbased superconductor Na Fe As.The transition metal sites are occupied by 75%Cu and 25%Ta,with Ta forming a well-defined superstructure.Cu and Ta were determined to be+1 and+5 oxidation state respectively.The band structure of the Na4Cu3TaAs4 measured by angle resolved photoemission spectroscopy(ARPES)is in good agreement with the density functional theory(DFT)calculation.Both ARPES and resistivity measurement indicate that this material exhibits metallic behavior with p-type carriers.Magnetic susceptibility measurement shows that the material exhibits nearly T-independent diamagnetism.This new material extends the material system with anti-Pb O-type layers and offers a good playground to investigate this material system further.

混合溶剂调控插层FeSe基超导体的层间距

近年来,通过湿化学方法探索合成新型插层FeSe基超导体一直是凝聚态物理的研究热点之一.本文中,我们利用溶剂热离子交换技术成功合成出两种FeSe衍生物:c轴晶胞参数为9.93Å的Li_(0.34)Se_(0.05)(EG)_(0.14)-FeSe和c轴晶胞参数为13.85Å的Li_(0.21)Se_(0.05)(EG)_(0.26)FeSe.通过调整反应溶剂可以调控以上两种FeSe衍生物的层间距.磁化率和电阻率测试表明它们的超导转变温度(Tc)均为30 K左右,似乎与FeSe层层间距的大小无关.与原始FeSe单晶相比,X射线光电子能谱结果显示FeSe层Fe的价态降低,这表明Tc的提高可能是由于电子掺杂效应引起的.我们的研究为探索合成插层铁基超导体提供了有价值的参考,并为在超导领域探索新型插层材料提供了新的可能性.

Imaging momentum-space Cooper pair formation and its competition with the charge density wave gap in a kagome superconductor

The superconducting ground state of kagome metals AV_(3)Sb_(5)(where A stands for K,Rb,or Cs)emerges from an exotic charge density wave(CDW)state that potentially breaks both rotational and time reversal symmetries.However,the specifics of the Cooper pairing mechanism,and the nature of the interplay between these two states remain elusive,largely due to the lack of momentum-space(k-space)superconducting energy gap structure.By implementing Bogoliubov quasiparticle interference(B QPI)imaging,we obtain k-space information on the multiband superconducting gap structureΔ_(SC)^(i)(k)in pristine CsV_(3)Sb_(5).We show that the estimated energy gap on the vanadium d_(xy/x^(2)-y^(2))orbital is anisotropic but nodeless,with a minimal value located near the M point.Interestingly,a comparison ofΔ_(SC)^(i)(k)with the CDW gapΔ_(CDW)^(i)(k)obtained by angle-re solved photoemission spectro scopy(ARPES)reveals direct k-space competition between the se two order parameters,i.e.,the opening of a large(small)CDW gap at a given momentum corresponds to a small(large)superconducting gap.When the long-range CDW order is suppressed by replacing vanadium with titanium,we find a nearly isotropic energy gap on both the V and Sb bands.This information will be critical for identifying the microscopic pairing mechanism and its interplay with intertwined electro nic orders in this kagome superconductor family.

Multi-conditioned controlled growth of CoBi nanostructures on SrTiO_(3)

Cobalt pnictides have been theoretically proposed to be attractive candi-dates for high-temperature superconductors.Additionally,monolayered CoX(X=As,Sb,Bi)on SrTiO_(3) systems present a potential new platform for realizing topological superconductors in the two-dimensional limit,due to their nontrivial band topology.To this end,we have successfully fabricated high-quality CoBi nanoislands on SrTiO_(3)(001)substrates by molecular beam epitaxy followed by an investigation of their atomic struc-ture and electronic properties via in situ scanning tunneling microscopyl spectroscopy.Beyond the previously predicted lattice with a=b=3.5 A,2×1 dimer row was observed in this study.Furthermore,our results reveal that the topography of CoBi islands is strongly influenced by various growth conditions,such as substrate temperature,the flux ratio between Co and Bi,and the annealing process.This study paves the way for further explorations of the superconductivity and topological properties of cobalt pnictidesystems.