Tuning d-Orbital Electronic Structure via Au-Intercalated Two-Dimensional Fe3GeTe2 to Increase Surface Plasmon Activity.

While extensive research has been dedicated to plasmon tuning within non-noble metals, prior investigations primarily concentrated on markedly augmenting the inherently low concentration of free carriers in materials with minimal consideration given to the influence of electron orbitals on surface plasmons. Here, we achieve successful intercalation of Au atoms into the layered structure of Fe3GeTe2 (FGT), thereby exerting control over the orbital electronic states or structure of FGT. This intervention not only amplifies the charge density and electron mobility but also mitigates the loss associated with interband transitions, resulting in increased two-dimensional FGT surface plasmon activity. As a consequence, Au-intercalated FGT detects crystal violet molecules as a surface-enhanced Raman scattering substrate, and the detection lines are 3 orders of magnitude higher than before Au intercalation. Our work provides insight for further studies on plasmon effects and the relation between surface plasmon resonance behavior and electronic structures.

Enhanced Superconductivity and Critical Current Density Due to the Interaction of InSe2 Bonded Layer in (InSe2)0.12NbSe2.

Superconductivity was discovered in (InSe2)xNbSe2. The materials are crystallized in a unique layered structure where bonded InSe2 layers are intercalated into the van der Waals gaps of 2H-phase NbSe2. The (InSe2)0.12NbSe2 superconductor exhibits a superconducting transition at 11.6 K and critical current density of 8.2 × 105 A/cm2. Both values are the highest among all transition metal dichalcogenide superconductors at ambient pressure. The present finding provides an ideal material platform for further investigation of superconducting-related phenomena in transition metal dichalcogenides.

Defect Emission and Its Dipole Orientation in Layered Ternary Znln2 S4 Semiconductor.

Defect engineering is promising to tailor the physical properties of 2D semiconductors for function-oriented electronics and optoelectronics. Compared with the extensively studied 2D binary materials, the origin of defects and their influence on physical properties of 2D ternary semiconductors are not clarified. Here, the effect of defects on the electronic structure and optical properties of few-layer hexagonal Znln2 S4 is thoroughly studied via versatile spectroscopic tools in combination with theoretical calculations. It is demonstrated that the Zn-In antistructural defects induce the formation of a series of donor and acceptor energy levels and sulfur vacancies induce donor energy levels, leading to rich recombination paths for defect emission and extrinsic absorption. Impressively, the emission of donor-acceptor pair in Znln2 S4 can be significantly tailored by electrostatic gating due to efficient tunability of Fermi level (Ef ). Furthermore, the layer-dependent dipole orientation of defect emission in Znln2 S4 is directly revealed by back focal plane imagining, where it presents obviously in-plane dipole orientation within a dozen-layer thickness of Znln2 S4 . These unique features of defects in Znln2 S4 including extrinsic absorption, rich recombination paths, gate tunability, and in-plane dipole orientation are definitely a benefit to the advanced orientation-functional optoelectronic applications.

A mixed-dimensional quasi-1D BiSeI nanowire-2D GaSe nanosheet p-n heterojunction for fast response optoelectronic devices.

Due to the unique combination configuration and the formation of a built-in electric field, mixed-dimensional heterojunctions present fruitful possibilities for improving the optoelectronic performances of low-dimensional optoelectronic devices. However, the response times of most photodetectors built from mixed-dimensional heterojunctions are within the millisecond range, limiting their applications in fast response optoelectronic devices. Herein, a mixed-dimensional BiSeI/GaSe van der Waals heterostructure is designed, which exhibits visible light detection ability and competitive photoresponsivity of 750 A W-1 and specific detectivity of 2.25 × 1012 Jones under 520 nm laser excitation. Excitingly, the device displays a very fast response time, e.g., the rise time and decay time under 520 nm laser excitation are 65 µs and 190 µs, respectively. Our findings provide a prospective approach to mixed-dimensional heterojunction photodetection devices with rapid switching capabilities.

Near-Neighbor Electron Orbital Coupling Effect of Single-Atomic-Layer Au Cluster Intercalated Bilayer 2H-TaS2 for Surface Enhanced Raman Scattering Sensing.

It is difficult to perfectly analyze the enhancement mechanism of two-dimensional (2D) materials and their combination with precious metals as surface enhanced Raman scattering (SERS) substrates using chemical enhancement mechanisms. Here, we propose a new mentality based on the coupling effect of neighboring electron orbitals to elucidate the electromagnetic field enhancement mechanism of single-atom-layer Au clusters embedded in double-layer 2H-TaS2 for SRES sensing. The insertion of Au atoms into the 2H-TaS2 interlayer was verified by XRD, AFM, and HRTEM, and a SERS signal enhancement of 2 orders of magnitude was obtained compared to the pure 2H-TaS2. XPS and micro-UV/vis-NIR spectra indicate that the outer electrons of neighboring Au and 2H-TaS2 overlap and migrate from Au to 2H-TaS2. First-principles calculations suggest strong electronic coupling between Au and 2H-TaS2. This study offers insights into SERS enhancement in nonprecious metal compounds and guides the development of new SERS substrates.

Extending Plasmonic Enhancement Limit with Blocked Electron Tunneling by Monolayer Hexagonal Boron Nitride.

Fabricating ultrasmall nanogaps for significant electromagnetic enhancement is a long-standing goal of surface-enhanced Raman scattering (SERS) research. However, such electromagnetic enhancement is limited by quantum plasmonics as the gap size decreases below the quantum tunneling regime. Here, hexagonal boron nitride (h-BN) is sandwiched as a gap spacer in a nanoparticle-on-mirror (NPoM) structure, effectively blocking electron tunneling. Layer-dependent scattering spectra and theoretical modeling confirm that the electron tunneling effect is screened by monolayer h-BN in a nanocavity. The layer-dependent SERS enhancement factor of h-BN in the NPoM system monotonically increases as the number of layers decreases, which agrees with the prediction by the classical electromagnetic model but not the quantum-corrected model. The ultimate plasmonic enhancement limits are extended in the classical framework in a single-atom-layer gap. These results provide deep insights into the quantum mechanical effects in plasmonic systems, enabling the potential novel applications based on quantum plasmonic.

Poly(vinyl alcohol)-Assisted Exfoliation of van der Waals Materials.

We report a highly efficient and easily transferable poly(vinyl alcohol) (PVA)-assisted exfoliation method, which allows one to obtain van der Waals materials on large scales, e.g., centimeter-scale graphite flakes and hundred-micrometer-scale several layers of ZnIn2S4 and BN. The present exfoliation scheme is nondestructive, and the materials prepared by PVA-assisted exfoliation can be directly fabricated into devices. This exfoliation approach could be helpful in overcoming the preparation bottleneck for large-scale applications of two-dimensional (2D) materials.

A tellurium short-wave infrared photodetector with fast response and high specific detectivity.

Two-dimensional (2D) elementary tellurium (Te) has attracted intensive attention due to its potential applications in short-wave infrared photodetector devices. Here, we report hydrothermally synthesized 2D Te nanoflakes for short-wave infrared photodetectors with high performance. A Te-based photodetector exhibits a peak responsivity of 51.85 A W-1 at a 1550 nm wavelength, attributed to the efficient absorption of the phonons of 2D Te nanoflakes. Besides, the rising and decay time of the Te photodetector is calculated to be ∼19 µs and ∼21 µs, respectively, due to the rapid diffusion of charge carriers. In addition, Te-photodetectors exhibit a high specific detectivity (D*) of 1.88 × 1010 Jones and a superior external quantum efficiency (EQE) of 4148%. Our findings have demonstrated the development of high-performance short-wave infrared photodetectors with fast responses based on 2D Te nanoflakes.

Ultrasensitive, Ultrafast, and Gate-Tunable Two-Dimensional Photodetectors in Ternary Rhombohedral ZnIn2S4 for Optical Neural Networks.

The demand for high-performance semiconductors in electronics and optoelectronics has prompted the expansion of low-dimensional materials research to ternary compounds. However, photodetectors based on 2D ternary materials usually suffer from large dark currents and slow response, which means increased power consumption and reduced performance. Here we report a systematic study of the optoelectronic properties of well-characterized rhombohedral ZnIn2S4 (R-ZIS) nanosheets which exhibit an extremely low dark current (7 pA at 5 V bias). The superior performance represented by a series of parameters surpasses most 2D counterparts. The ultrahigh specific detectivity (1.8 × 1014 Jones), comparably short response time (τrise = 222 µs, τdecay = 158 µs), and compatibility with high-frequency operation (1000 Hz) are particularly prominent. Moreover, a gate-tunable characteristic is observed, which is attributed to photogating and improves the photoresponse by 2 orders of magnitude. Gating technique can effectively modulate the photocurrent-generation mechanism from photoconductive effect to dominant photogating. The combination of ultrahigh sensitivity, ultrafast response, and high gate tunability makes the R-ZIS phototransistor an ideal device for low-energy-consumption and high-frequency optoelectronic applications, which is further demonstrated by its excellent performance in optical neural networks and promising potential in optical deep learning and computing.

Enhanced magnetism and persistent insulating state in Mn doped Sr2IrO4.

The influences of Mn substitution at the Ir site of Sr2IrO4are investigated via a comprehensive study of the variation of structural parameters, the transport and magnetic properties of the Sr2Ir1-xMnxO4samples. The incorporation of Mn leads to an increase of the in-plane Ir-O-Ir bond angle, while it is not sufficient to drive the Mott-insulating state to a metallic state. Interestingly, we find a coexistence of Ir4+-O2--Ir4+super-exchange interaction and Mn3+-O2--Mn4+double exchange interaction inx⩾ 0.06 samples. The Mn3+-O2--Mn4+ferromagnetic domains are isolated by the Ir4+-O2--Ir4+antiferromagnetic areas, leading to a severely localized electronic and magnetic states. The electron hopping between the localized states dominates the conductivity of the Sr2Ir1-xMnxO4samples.

Estimating quantum steering and Bell nonlocality through quantum entanglement in two-photon systems.

Quantum entanglement, quantum steering and Bell nonlocality, as significant quantum resources in the field of quantum information science, can achieve variously valuable quantum information tasks. Among of them, quantum entanglement and Bell nonlocality are the weakest and strongest nonlocal correlations, respectively. One can capture the quantum steering and Bell nonlocality via violating steering inequality and Bell inequality, respectively. In general, the detections of quantum steering and Bell nonlocality are strictly harder than entanglement detection. Here, based on steering inequality test and quantum state tomography, we attain various nonlocal correlations and experimentally demonstrate that the estimations of quantum steering and Bell nonlocality can be realized according to the quantum entanglement of the prepared two-photon test states. The estimated efficiency of quantum steering is stronger than the one of Bell nonlocality in this scenario, i.e., more steerable two-photon test states can be verified through quantum entanglement. In addition, quantum steering and Bell nonlocality are bounded by the corresponding upper and lower bounds, and these bounds cannot be punctured by all prepared two-photon states in experiment. These results are conducive to understand the relations among these nonlocal correlations.

Global phase diagram of Coulomb-interacting anisotropic Weyl semimetal with disorder.

Taking into account the interplay between the disorder and Coulomb interaction, the phase diagram of three-dimensional anisotropic Weyl semimetal is studied by renormalization group (RG) theory. Weak disorder is irrelevant in anisotropic Weyl semimetal, while the disorder becomes relevant and drives a quantum phase transition (QPT) from semimetal to compressible diffusive metal (CDM) phases if the disorder strength is larger than a critical value. The long-range Coulomb interaction is irrelevant in clean anisotropic Weyl semimetal. However, interestingly, we find that the long-range Coulomb interaction exerts a dramatic influence on the critical disorder strength for phase transition to CDM. Specifically, the critical disorder strength can receive a prominent change even though an arbitrarily weak Coulomb interaction is included. This novel behavior is closely related to the anisotropic screening effect of Coulomb interaction, and essentially results from the specifical energy dispersion of the fermion excitations in anisotropic Weyl semimetal. The theoretical results are helpful for understanding the physical properties of the candidates of anisotropic Weyl semimetal, such as pressured BiTeI, and some other related materials.

Record-High Superconductivity in Niobium-Titanium Alloy.

The extraordinary superconductivity has been observed in a pressurized commercial niobium-titanium alloy. Its zero-resistance superconductivity persists from ambient pressure to the pressure as high as 261.7 GPa, a record-high pressure up to which a known superconducting state can continuously survive. Remarkably, at such an ultra-high pressure, although the ambient pressure volume is shrunk by 45% without structural phase transition, the superconducting transition temperature (TC ) increases to ≈19.1 K from ≈9.6 K, and the critical magnetic field (HC2 ) at 1.8 K has been enhanced to 19 T from 15.4 T. These results set new records for both the TC and the HC2 among all the known alloy superconductors composed of only transition metal elements. The remarkable high-pressure superconducting properties observed in the niobium-titanium alloy not only expand the knowledge on this important commercial superconductor but also are helpful for a better understanding on the superconducting mechanism.

Opposite pressure effects in the orbitally-induced Peierls phase transition systems CuIr2S4 and MgTi2O4.

The iso-spinel structural systems CuIr2S4 and MgTi2O4 exhibit phase transitions of a similar nature at ∼230 K and ∼260 K respectively, which are explained as an orbitally-induced Peierls phase transition. However, in this work, we uncover that the applied pressure has opposite pressure effects on the phase transitions in CuIr2S4 and MgTi2O4. As the pressure increases, the phase transition temperature (TMI) for CuIr2S4 increases while that for MgTi2O4 decreases. In addition, the phase transition intensity becomes weaker for CuIr2S4 but gets stronger for MgTi2O4 under pressure. Our results indicate that the applied pressure suppresses the metallic phase in CuIr2S4, while enhances that in MgTi2O4. Combining the experimental observations with first-principles electronic structure calculations, we suggest that the opposite pressure effects in CuIr2S4 and MgTi2O4 originate from the different orbital ordering configurations (dxy, dyz/dxz) caused by different lattice distortions in these two systems. Our findings indicate directly that the interplay between the orbital and lattice degrees of freedom plays an important role in the orbitally-induced Peierls phase transition.

Drive the Dirac electrons into Cooper pairs in SrxBi2Se3.

Topological superconductors are a very interesting and frontier topic in condensed matter physics. Despite the tremendous efforts in exploring topological superconductivity, its presence is however still under heavy debate. The Dirac electrons have been proven to exist on the surface of a topological insulator. It remains unclear whether and how the Dirac electrons fall into Cooper pairing in an intrinsic superconductor with the topological surface states. Here we show the systematic study of scanning tunnelling microscope/spectroscopy on the possible topological superconductor SrxBi2Se3. We first demonstrate that only the intercalated Sr atoms can induce superconductivity. Then we show the full superconducting gaps without any in-gap density of states as expected theoretically for a bulk topological superconductor. Finally, we find that the surface Dirac electrons will simultaneously condense into the superconducting state within the superconducting gap. This vividly demonstrates how the surface Dirac electrons are driven into Cooper pairs.

Anisotropic magnetic coupling with a two-dimensional characteristic in noncentrosymmetric Cr11Ge19.

In this work, we successfully synthesize the single crystal Cr11Ge19. The magnetism of the noncentrosymmetric Cr11Ge19 with itinerant ferromagnetic ground state is thoroughly investigated on the single crystal. Based on the variation measurements including the angular rotation, temperature, and magnetic field dependence of magnetization, we find that this material exhibits strong magnetic anisotropy along the c-axis. To clearly reveal the magnetic interactions, the critical behavior is studied using the modified Arrott plot, the Kouvel-Fisher method, and the critical isotherm technique. Combining these different methods, three main critical exponents (ß, γ, and δ) are obtained. The critical exponent ß is close to the theoretical prediction of a three-dimensional XY model with spin-dimensionality n = 2, indicating two-dimensional magnetic coupling. Meanwhile, the critical exponent γ suggests that the magnetic interaction is of long-range type with magnetic exchange distance decaying as J(r) ≈ r-4.61. We propose that the ferromagnetic ground state of Cr11Ge19 is formed by the polarized magnetic moments along the c-axis, while the long-range magnetic coupling is established within the ab plane.

De Hass-van Alphen and magnetoresistance reveal predominantly single-band transport behavior in PdTe2.

Research on two-dimensional transition metal dichalcogenides (TMDs) has grown rapidly over the past several years, from fundamental studies to the development of next generation technologies. Recently, it has been reported that the MX2-type PdTe2 exhibits superconductivity with topological surface state, making this compound a promising candidate for investigating possible topological superconductivity. However, due to the multi-band feature of most of TMDs, the investigating of magnetoresistance and quantum oscillations of these TMDs proves to be quite complicated. Here we report a combined de Hass-van Alphen effect and magnetoresistance studies on the PdTe2 single crystal. Our high-field de Hass-van Alphen data measured at different temperature and different tilting angle suggest that though these is a well-defined multi-band feature, a predominant oscillation frequency has the largest oscillation magnitude in the fast Fourier transformation spectra, which is at least one order of magnitude larger than other oscillation frequencies. Thus it is likely that the transport behavior in PdTe2 system can be simplified into a single-band model. Meanwhile, the magnetoresistance results of the PdTe2 sample can be well-fitted according to the single-band models. The present results could be important in further investigation of the transport behaviors of two-dimensional TMDs.

Critical phenomenon of the near room temperature skyrmion material FeGe.

The cubic B20 compound FeGe, which exhibits a near room temperature skyrmion phase, is of great importance not only for fundamental physics such as nonlinear magnetic ordering and solitons but also for future application of skyrmion states in spintronics. In this work, the critical behavior of the cubic FeGe is investigated by means of bulk dc-magnetization. We obtain the critical exponents (ß = 0.336 ± 0.004, γ = 1.352 ± 0.003 and ß = 5.276 ± 0.001), where the self-consistency and reliability are verified by the Widom scaling law and scaling equations. The magnetic exchange distance is found to decay as r(-4.9), which is close to the theoretical prediction of 3D-Heisenberg model (r(-5)). The critical behavior of FeGe indicates a short-range magnetic interaction. Meanwhile, the critical exponents also imply an anisotropic magnetic coupling in this system.

Superconductivity with Topological Surface State in SrxBi2Se3.

By intercalation of alkaline earth metal Sr in Bi2Se3, superconductivity with large shielding volume fraction (∼91.5% at 0.5 K) has been achieved in Sr0.065Bi2Se3. Analysis of the Shubnikov-de Hass oscillations confirms the half-shift expected from a Dirac spectrum, giving transport evidence of the existence of surface states. Importantly, SrxBi2Se3 superconductor is stable under air, making SrxBi2Se3 an ideal material base for investigating topological superconductivity.

Superconductor-insulator transition in quasi-one-dimensional single-crystal Nb2PdS5 nanowires.

Superconductor-insulator transition (SIT) in one-dimensional (1D) nanowires attracts great attention in the past decade and remains an open question since contrasting results were reported in nanowires with different morphologies (i.e., granular, polycrystalline, or amorphous) or environments. Nb2PdS5 is a recently discovered low-dimensional superconductor with typical quasi-1D chain structure. By decreasing the wire diameter in the range of 100-300 nm, we observed a clear SIT with a 1D transport character driven by both the cross-sectional area and external magnetic field. We also found that the upper critical magnetic field (Hc2) decreases with the reduction of nanowire cross-sectional area. The temperature dependence of the resistance below Tc can be described by the thermally activated phase slip (TAPS) theory without any signature of quantum phase slips (QPS). These findings demonstrated that the enhanced Coulomb interactions with the shrinkage of the wire diameter competes with the interchain Josephson-like coupling may play a crucial role on the SIT in quasi-1D system.