Heterodimensional Kondo superlattices with strong anisotropy.

Localized magnetic moments in non-magnetic materials, by interacting with the itinerary electrons, can profoundly change the metallic properties, developing various correlated phenomena such as the Kondo effect, heavy fermion, and unconventional superconductivity. In most Kondo systems, the localized moments are introduced through magnetic impurities. However, the intrinsic magnetic properties of materials can also be modulated by the dimensionality. Here, we report the observation of Kondo effect in a heterodimensional superlattice VS2-VS, in which arrays of the one-dimensional (1D) VS chains are encapsulated by two-dimensional VS2 layers. In such a heterodimensional Kondo superlattice, we observe the typical Kondo effect but with intriguing anisotropic field dependence. This unique anisotropy is determined to originate from the magnetic anisotropy which has the root in the unique 1D chains in the structure, as corroborated by the first-principles calculation. Our results open up a novel avenue of studying exotic correlated physics in heterodimensional materials.

Effective Manipulation of a Colossal Second-Order Transverse Response in an Electric-Field-Tunable Graphene Moiré System.

The second-order nonlinear transport illuminates a frequency-doubling response emerging in quantum materials with a broken inversion symmetry. The two principal driving mechanisms, the Berry curvature dipole and the skew scattering, reflect various information including ground-state symmetries, band dispersions, and topology of electronic wave functions. However, effective manipulation of them in a single system has been lacking, hindering the pursuit of strong responses. Here, we report on the effective manipulation of the two mechanisms in a single graphene moiré superlattice, AB-BA stacked twisted double bilayer graphene. Most saliently, by virtue of the high tunability of moiré band structures and scattering rates, a record-high second-order transverse conductivity ∼ 510 µm S V-1 is observed, which is orders of magnitude higher than any reported values in the literature. Our findings establish the potential of electrically tunable graphene moiré systems for nonlinear transport manipulations and applications.

Bibliometric and visual analysis of research on nutcracker syndrome from 1974 to 2021: A systematic review.

BACKGROUND: At present, researchers have obtained fruitful results in the study of nutcracker syndrome (NCS), but there is still a lack of systematic research on the overall status of this disease. This article aims to describe the past and current status of research into NCS, and predict future research trends and popular research topics. METHODS: Using bibliometric and visualization methods, 552 articles related to NCS collected from the Scopus database from 1974 to 2021 were analyzed from multiple perspectives. RESULTS: Overall, the amount of literature related to NCS is on the rise every year, and the number of citations is the turning point in 2006. The United States has the largest number of publications and has the most extensive cooperation with other countries. The main contents of the co-authored study focused on the symptoms, surgical procedures, and concomitant diseases of NCS. Keywords such as peak velocity, ultrasonography, orthostatic proteinuria, etc appeared earlier, whereas diagnosis, chronic pelvic pain, endovascular stents, etc appeared later. CONCLUSIONS: The literature utilization rate of NCS is relatively insufficient. The pathogenesis and pathological mechanisms need to be further studied, and the diagnostic criteria and surgical methods will continue to be favored by clinicians.

Mechanical Properties of TC11 Titanium Alloy and Graphene Nanoplatelets/TC11 Composites Prepared by Selective Laser Melting.

Titanium matrix composites (TMCs) with excellent mechanical properties, reinforced by graphene, is deemed the lightweight and high strength structural materials. In this study, TC11 titanium alloy powder and graphene nanosheets (GNPs) were used as raw materials, and the composite powder with good uniformity and fluidity was obtained through non-interventional homogeneous mixing by a planetary mixer. The microstructure and mechanical properties of the GNPs-TC11 composites and TC11 alloy were compared. The results showed that the microstructure of TC11 and the composites was acicular martensite α' phase under the process parameters of 280 W laser power, 1200 mm/s scanning speed, and 0.1 mm hatch spacing. The GNPs in addition, in the composites, reduced the acicular martensite particle size and expanded the proportion of low-angle grain boundaries. The tensile strength and percentage elongation after the fracture of the TC11 titanium alloy were 1265 MPa and 4.3%, respectively. Because of addition of the GNPs, the strength and percentage elongation after the fracture of the composite increased to 1384 MPa and 8.1%, respectively, at a GNPs mass content of 0.2%. The enhancement of mechanical properties can be attributed to grain refinement, dislocation strengthening, Orowan strengthening, and load transfer strengthening.

Thermal Conductance of Graphene-Titanium Interface: A Molecular Simulation.

Titanium is a commonly used material in aviation, aerospace, and military applications, due to the outstanding mechanical properties of titanium and its alloys. However, its relatively low thermal conductivity restricts its extended usage. The use of graphene as a filler shows great potential for the enhancement of thermal conductivity in titanium-based metal-matrix composites (MMCs). We used classical molecular dynamics (MD) simulation methods to explore the thermal conductance at the titanium-graphene (Ti/Gr) interface for its thermal boundary conductance, which plays an important role in the thermal properties of Ti-based MMCs. The effects of system size, layer number, temperature, and strain were considered. The results show that the thermal boundary conductance (TBC) decreases with an increasing layer number and reaches a plateau at n = 5. TBC falls under tensile strain and, in turn, it grows with compressive strain. The variation of TBC is explained qualitatively by the interfacial atomic vibration coupling factor. Our findings also provide insights into ways to optimize future thermal management based on Ti-based MMCs materials.

Ultralight, Ultraflexible, Anisotropic, Highly Thermally Conductive Graphene Aerogel Films.

Graphene aerogels have attracted much attention as a promising material for various applications. The unusually high intrinsic thermal conductivity of individual graphene sheets makes an obvious contrast with the thermal insulating performance of assembled 3D graphene materials. We report the preparation of anisotropy 3D graphene aerogel films (GAFs) made from tightly packed graphene films using a thermal expansion method. GAFs with different thicknesses and an ultimate low density of 4.19 mg cm-3 were obtained. GAFs show high anisotropy on average cross-plane thermal conductivity (K⊥) and average in-plane thermal conductivity (K||). Additionally, uniaxially compressed GAFs performed a large elongation of 11.76% due to the Z-shape folding of graphene layers. Our results reveal the ultralight, ultraflexible, highly thermally conductive, anisotropy GAFs, as well as the fundamental evolution of macroscopic assembled graphene materials at elevated temperature.

Epitaxial growth of Bi(110) and Bi2Se3thin films on a ferromagnetic insulator substrate of Cr2Ge2Te6.

When a topological insulator (TI) is brought to the proximity of a ferromagnetic insulator (FMI), the breaking of the time-reversal symmetry may give rise to quantum anomalous Hall effect (QAHE). The physical properties of such TI-FMI systems are greatly affected by the interfacial structures of the components. Here, we report the growth and structural properties of Bi(110) and Bi2Se3thin films on a FMI of Cr2Ge2Te6(CGT) substrate by scanning tunneling microscopy. We observed various defects and impurities on the CGT surfaces, which serve as the preferential sites for initial nucleation and epitaxial growth of Bi(110) thin films. The exposure of the as-grown Bi(110) thin films to Se vapor leads to the formation of polycrystalline Bi2Se3thin films with randomly distributed holes. The structure and composition of the as-prepared Bi2Se3thin films were further confirmed by Raman spectroscopy and x-ray photoelectron spectroscopy. Our work shows that the quality of the CGT crystals is vital for the growth of high-quality TIs on CGT substrates for QAHE.

Magnetic and electric properties of single crystal MnI2.

Multiferroic materials endowed with both dielectric and magnetic orders, are ideal candidates for a wide range of applications. In this work, we reported two phase transitions of MnI2 at 3.45 K and 4 K by systemically measuring the magnetic-field and temperature-dependent magnetization of the MnI2 thin flakes. Furthermore, we observed similar temperature and field-dependent behaviours for the magnetic susceptibility of MnI2 and electronic capacitance of the Ag/MnI2/Ag devices below 3.5 K. Considering the related theory work, we discussed the relationship between the antiferromagnetic and ferroelectric orders in MnI2. Our work reveals the in-plane magnetic and electric properties of MnI2 materials, which might be helpful for the further investigation and application of MnI2 multiferroics in the future.

Robust edge photocurrent response on layered type II Weyl semimetal WTe2.

Photosensing and energy harvesting based on exotic properties of quantum materials and new operation principles have great potential to break the fundamental performance limit of conventional photodetectors and solar cells. Weyl semimetals have demonstrated novel optoelectronic properties that promise potential applications in photodetection and energy harvesting arising from their gapless linear dispersion and Berry field enhanced nonlinear optical effect at the vicinity of Weyl nodes. In this work, we demonstrate robust photocurrent generation at the edge of Td-WTe2, a type-II Weyl semimetal, due to crystalline-symmetry breaking along certain crystal fracture directions and possibly enhanced by robust fermi-arc type surface states. This edge response is highly generic and arises universally in a wide class of quantum materials with similar crystal symmetries. The robust and generic edge current response provides a charge separation mechanism for photosensing and energy harvesting over broad wavelength range.

Observation of Topological Edge States at the Step Edges on the Surface of Type-II Weyl Semimetal TaIrTe4.

Topological materials harbor topologically protected boundary states. Recently, TaIrTe4, a ternary transition-metal dichalcogenide, was identified as a type-II Weyl semimetal with the minimal nonzero number of Weyl points allowed for a time-reversal invariant Weyl semimetal. Monolayer TaIrTe4 was proposed to host topological edge states, which, however, lacks of experimental evidence. Here, we report on the topological edge states localized at the monolayer step edges of the type-II Weyl semimetal TaIrTe4 using scanning tunneling microscopy. One-dimensional electronic states that show substantial robustness against the edge irregularity are observed at the step edges. Theoretical calculations substantiate the topologically nontrivial nature of the edge states and their robustness against the edge termination and layer stacking. The observation of topological edge states at the step edges of TaIrTe4 surfaces suggests that monolayer TaIrTe4 is a two-dimensional topological insulator, providing TaIrTe4 as a promising material for topological physics and devices.

Pressure-induced phase transitions and superconductivity in a quasi-1-dimensional topological crystalline insulator α-Bi4Br4.

Great progress has been achieved in the research field of topological states of matter during the past decade. Recently, a quasi-1-dimensional bismuth bromide, Bi4Br4, has been predicted to be a rotational symmetry-protected topological crystalline insulator; it would also exhibit more exotic topological properties under pressure. Here, we report a thorough study of phase transitions and superconductivity in a quasihydrostatically pressurized α-Bi4Br4 crystal by performing detailed measurements of electrical resistance, alternating current magnetic susceptibility, and in situ high-pressure single-crystal X-ray diffraction together with first principles calculations. We find a pressure-induced insulator-metal transition between ∼3.0 and 3.8 GPa where valence and conduction bands cross the Fermi level to form a set of small pockets of holes and electrons. With further increase of pressure, 2 superconductive transitions emerge. One shows a sharp resistance drop to 0 near 6.8 K at 3.8 GPa; the transition temperature gradually lowers with increasing pressure and completely vanishes above 12.0 GPa. Another transition sets in around 9.0 K at 5.5 GPa and persists up to the highest pressure of 45.0 GPa studied in this work. Intriguingly, we find that the first superconducting phase might coexist with a nontrivial rotational symmetry-protected topology in the pressure range of ∼3.8 to 4.3 GPa; the second one is associated with a structural phase transition from monoclinic C2/m to triclinic P-1 symmetry.

Intrinsic alterations in the hydrogen desorption of Mg2NiH4 by solid dissolution of titanium.

Achieving rapid hydrogen desorption at low temperatures is one of the main challenges for Mg-based alloys in hydrogen storage. Here, we report that the dehydrogenation process of Mg2NiH4 can be intrinsically improved with the solid dissolution of titanium into its lattices. The hydrogen desorption is suggested to be promoted by shifting the rate-limiting step from proven nucleation and/or diffusion to two-dimensional phase boundary migration based on kinetic modeling studies, for which few additions have been obtained, probably due to the understanding being confounded by other impurity phases. This alteration intrinsically results in enhanced desorption properties of Mg2NiH4 when Ti is dissolved, i.e., not only does it exhibit a decreased peak desorption temperature with a reduction in the activation energy, but it also positively changes the enthalpy value in comparison with that of the pure compound. These obtained results allow for a deep understanding of how the intrinsic desorption features are enhanced by solid dissolution, and also provide insight into improving hydrogen storage in Mg-based alloys.

Anisotropic Broadband Photoresponse of Layered Type-II Weyl Semimetal MoTe2.

Photodetectors based on Weyl semimetal promise extreme performance in terms of highly sensitive, broadband and self-powered operation owing to its extraordinary material properties. Layered Type-II Weyl semimetal that break Lorentz invariance can be further integrated with other two-dimensional materials to form van der Waals heterostructures and realize multiple functionalities inheriting the advantages of other two-dimensional materials. Herein, we report the realization of a broadband self-powered photodetector based on Type-II Weyl semimetal Td -MoTe2 . The prototype metal-MoTe2 -metal photodetector exhibits a responsivity of 0.40 mA W-1 and specific directivity of 1.07 × 108 Jones with 43 µs response time at 532 nm. Broadband responses from 532 nm to 10.6 µm are experimentally tested with a potential detection range extendable to far-infrared and terahertz. Furthermore, we identify the response of the detector is polarization angle sensitive due to the anisotropic response of MoTe2 . The anisotropy is found to be wavelength dependent, and the degree of anisotropy increases as the excitation wavelength gets closer to the Weyl nodes. In addition, with power and temperature dependent photoresponse measurements, the photocurrent generation mechanisms are investigated. Our results suggest this emerging class of materials can be harnessed for broadband angle sensitive, self-powered photodetection with decent responsivities.

Ultrafast Broadband Photodetectors Based on Three-Dimensional Dirac Semimetal Cd3As2.

Photodetection with extreme performances in terms of ultrafast response time, broad detection wavelength range, and high sensitivity has a wide range of optoelectronic and photonic applications, such as optical communications, interconnects, imaging, and remote sensing. Graphene, a typical two-dimensional Dirac semimetal, has shown excellent potential toward a high-performance photodetector with high operation speed, broadband response, and efficient carrier multiplications benefiting from its linear dispersion band structure with a high carrier mobility and zero bandgap. As the three-dimensional analogues of graphene, Dirac semimetal Cd3As2 processes all advantages of graphene as a photosensitive material but potentially has stronger interaction with light as a bulk material and thus enhanced responsivity. In this work, we report the realization of an ultrafast broadband photodetector based on Cd3As2. The prototype metal-Cd3As2-metal photodetector exhibits a responsivity of 5.9 mA/W with a response time of about 6.9 ps without any special device optimization. Broadband responses from 532 nm to 10.6 µm are achieved with a potential detection range extendable to far-infrared and terahertz. Systematical studies indicate that the photothermoelectric effect plays an important role in photocurrent generation. Our results suggest this emerging class of exotic quantum materials can be harnessed for photodetection with a high sensitivity and high speed (∼145 GHz) over a broad wavelength range.

Photovoltaic Effect and Evidence of Carrier Multiplication in Graphene Vertical Homojunctions with Asymmetrical Metal Contacts.

Graphene exhibits exciting potentials for high-speed wideband photodetection and high quantum efficiency solar energy harvest because of its broad spectral absorption, fast photoelectric response, and potential carrier multiplication. Although photocurrent can be generated near a metal-graphene interface in lateral devices, the photoactive area is usually limited to a tiny one-dimensional line-like interface region. Here, we report photoelectric devices based on vertical graphene two-dimensional homojunction, which is fabricated via vertically stacking four graphene monolayers with asymmetric metal contacts. The devices show excellent photovoltaic output with excitation wavelength ranging from visible light to mid-infrared. The wavelength dependence of the internal quantum efficiency gives direct evidence of the carrier multiplication effect in graphene. The simple fabrication process, easy scale-up, large photoresponsive active area, and broadband response of the vertical graphene device are very promising for practical applications in optoelectronics and photovoltaics.

Dynamical Evolution of Anisotropic Response in Black Phosphorus under Ultrafast Photoexcitation.

Black phosphorus has recently emerged as a promising material for high-performance electronic and optoelectronic device for its high mobility, tunable mid-infrared bandgap, and anisotropic electronic properties. Dynamical evolution of photoexcited carriers and the induced transient change of electronic properties are critical for materials' high-field performance but remain to be explored for black phosphorus. In this work, we perform angle-resolved transient reflection spectroscopy to study the dynamical evolution of anisotropic properties of black phosphorus under photoexcitation. We find that the anisotropy of reflectivity is enhanced in the pump-induced quasi-equilibrium state, suggesting an extraordinary enhancement of the anisotropy in dynamical conductivity in hot carrier dominated regime. These results raise attractive possibilities of creating high-field, angle-sensitive electronic, optoelectronic, and remote sensing devices exploiting the dynamical electronic anisotropy with black phosphorus.

Coherent longitudinal acoustic phonon approaching THz frequency in multilayer Molybdenum Disulphide.

Coherent longitudinal acoustic phonon is generated and detected in multilayer Molybdenum Disulphide (MoS2) with number of layers ranging from 10 to over 1300 by femtosecond laser pulse. For thin MoS2, the excited phonon frequency exhibits a standing wave nature and shows linear dependence on the sample thickness. The frequency varies from 40 GHz to 0.2 THz (10 layers), which promises possible application in THz frequency mechanical resonators. This linear thickness dependence gradually disappears in thicker samples above about 150 layers, and the oscillation period shows linear dependence on the probe wavelength. From both the oscillation period of the coherent phonon and the delay time of acoustic echo, we can deduce a consistent sound velocity of 7.11*10(3) m/s in MoS2. The generation mechanisms of the coherent acoustic phonon are also discussed through pump power dependent measurement.

Topological surface state enhanced photothermoelectric effect in Bi2Se3 nanoribbons.

The photothermoelectric effect in topological insulator Bi2Se3 nanoribbons is studied. The topological surface states are excited to be spin-polarized by circularly polarized light. Because the direction of the electron spin is locked to its momentum for the spin-helical surface states, the photothermoelectric effect is significantly enhanced as the oriented motions of the polarized spins are accelerated by the temperature gradient. The results are explained based on the microscopic mechanisms of a photon induced spin transition from the surface Dirac cone to the bulk conduction band. The as-reported enhanced photothermoelectric effect is expected to have potential applications in a spin-polarized power source.

Valley carrier dynamics in monolayer molybdenum disulfide from helicity-resolved ultrafast pump-probe spectroscopy.

We investigate the valley-related carrier dynamics in monolayer molybdenum disulfide using helicity-resolved nondegenerate ultrafast pump-probe spectroscopy at the vicinity of the high-symmetry K point under the temperature down to 78 K. Monolayer molybdenum disulfide shows remarkable transient reflection signals, in stark contrast to bilayer and bulk molybdenum disulfide due to the enhancement of many-body effect at reduced dimensionality. The helicity-resolved ultrafast time-resolved result shows that the valley polarization is preserved for only several picoseconds before the scattering process makes it undistinguishable. We suggest that the dynamical degradation of valley polarization is attributable primarily to the exciton trapping by defect states in the exfoliated molybdenum disulfide samples. Our experiment and a tight-binding model analysis also show that the perfect valley circular dichroism selectivity is fairly robust against disorder at the K point but quickly decays from the high-symmetry point in the momentum space in the presence of disorder.