Giant intrinsic photovoltaic effect in one-dimensional van der Waals grain boundaries.

The photovoltaic effect lies at the heart of eco-friendly energy harvesting. However, the conversion efficiency of traditional photovoltaic effect utilizing the built-in electric effect in p-n junctions is restricted by the Shockley-Queisser limit. Alternatively, intrinsic/bulk photovoltaic effect (IPVE/BPVE), a second-order nonlinear optoelectronic effect arising from the broken inversion symmetry of crystalline structure, can overcome this theoretical limit. Here, we uncover giant and robust IPVE in one-dimensional (1D) van der Waals (vdW) grain boundaries (GBs) in a layered semiconductor, ReS2. The IPVE-induced photocurrent densities in vdW GBs are among the highest reported values compared with all kinds of material platforms. Furthermore, the IPVE-induced photocurrent is gate-tunable with a polarization-independent component along the GBs, which is preferred for energy harvesting. The observed IPVE in vdW GBs demonstrates a promising mechanism for emerging optoelectronics applications.

High-temperature magnetically topological candidate material Mn3Bi2Te6.

The Mn-Bi-Te family displaying magnetism and non-trivial topological properties has received extensive attention. Here, we predict that the antiferromagnetic structure of Mn3Bi2Te6with three MnTe layers is energetically stable and the magnetic energy difference of Mn-Mn is enhanced four times compared with that in the single MnTe layer of MnBi2Te4. The predicted Néel transition point is raised to 102.5 K, surpassing the temperature of liquid nitrogen. The topological properties show that with the variation of the MnTe layer from a single layer to three layers, the system transforms from a non-trivial topological phase to a trivial topological phase. Interestingly, the ferromagnetic state of Mn3Bi2Te6is a topological semimetal and it exhibits a topological transition from trivial to non-trivial induced by the magnetic transition. Our results enrich the Mn-Bi-Te family system, offer a new platform for studying topological phase transitions, and pave a new way to improve the working temperature of magnetically topological devices.

Strong bulk photovoltaic effect in engineered edge-embedded van der Waals structures.

Bulk photovoltaic effect (BPVE), a second-order nonlinear optical effect governed by the quantum geometric properties of materials, offers a promising approach to overcome the Shockley-Quiesser limit of traditional photovoltaic effect and further improve the efficiency of energy harvesting. Here, we propose an effective platform, the nano edges embedded in assembled van der Waals (vdW) homo- or hetero-structures with strong symmetry breaking, low dimensionality and abundant species, for BPVE investigations. The BPVE-induced photocurrents strongly depend on the orientation of edge-embedded structures and polarization of incident light. Reversed photocurrent polarity can be observed at left and right edge-embedded structures. Our work not only visualizes the unique optoelectronic effect in vdW nano edges, but also provides an effective strategy for achieving BPVE in engineered vdW structures.

Role of Doping Effect and Chemical Pressure Effect Introduced by Alkali Metal Substitution on 1144 Iron-Based Superconductors.

CaAFe4As4 with A = K, Rb, and Cs are close to the doped 122 system, and the parent material can reach a superconducting transition temperature of 31-36 K without doping. To study the role of alkali metals, we investigated the induced hole doping and chemical pressure effects as a result of the introduction of alkali metals using density-functional-based methods. These two effects can affect the superconducting transition temperature by changing the number of electrons and the structure of the FeAs conductive layer, respectively. Our study shows that the dxz and dyz orbitals, which are degenerate in CaFe2As2, become nondegenerate in CaAFe4As4 due to two nonequivalent arsenic atoms (As1 and As2). The unusual oblate ellipsoid hole pocket at Γ point in CaAFe4As4 results from a divalent cation Ca2+ replaced by a monovalent cation A+. It shows one of the main differences in fermiology compared to a particular form of CaFe2As2 with reduced 1144 symmetry, due to the enhancement of As2-Fe hybridization. The unusual band appears in CaFe2As2 (1144) and gradually disappears in the change of K to Cs. Further analysis shows that this band is contributed by As1 and has strong dispersion perpendicular to the FeAs layer, suggesting that it is related to the peculiar van Hove singularity below the Fermi level. In addition, various aspects of CaFe2As2 (1144) and CaAFe4As4 in the ground state are discussed in terms of the influence of hole doping and chemical pressure.

Two-dimensional antiferromagnetic nodal-line semimetal and quantum anomalous Hall state in the van der Waals heterostructure germanene/Mn2S2.

Materials with interactions between the topology and magnetism are triggering increasing interest. We constructed a two-dimensional (2D) van der Waals heterostructure germanene/Mn2S2, where the germanene is a quantum spin Hall insulator and Mn2S2provides antiferromagnetic (AFM) interactions. In this structure, a 2D AFM nodal-line semimetal (NLSM) phase is expected without the spin-orbit coupling (SOC), which is of a high density of states around the Fermi level. The band touching rings originate from the intersection between different spin components ofporbitals of germanene. This result provides a possible 2D realization of NLSMs, which are usually realized in three-dimensional systems. When the SOC is present, a quantum anomalous Hall (QAH) state emerges with the annihilation of the band-touching rings. The nontrivial topology is determined by calculating the Chern number and Wannier charge centers. This provides an alternative platform to realize QAH states. These results could also provide the possibility of further understanding the topological states in NLSM and electronic applications.

Multivalley Superconductivity in Monolayer Transition Metal Dichalcogenides.

In transition metal dichalcogenides (TMDs), Ising superconductivity with an antisymmetric spin texture on the Fermi surface has attracted wide interest due to the exotic pairing and topological properties. However, it is not clear whether the Q valley with a giant spin splitting is involved in the superconductivity of heavily doped semiconducting 2H-TMDs. Here by taking advantage of a high-quality monolayer WS2 on hexagonal boron nitride flakes, we report an ionic-gating induced superconducting dome with a record high critical temperature of ∼6 K, accompanied by an emergent nonlinear Hall effect. The nonlinearity indicates the development of an additional high-mobility channel, which (corroborated by first principle calculations) can be ascribed to the population of Q valleys. Thus, multivalley population at K and Q is suggested to be a prerequisite for developing superconductivity. The involvement of Q valleys also provides insights to the spin textured Fermi surface of Ising superconductivity in the large family of transition metal dichalcogenides.

The generation of switchable polarized currents in nodal ring semimetals using high-frequency periodic driving.

A nodal ring semimetal (NRSM) can be driven to a spin-polarized NRSM or a spin-polarized Weyl semimetal (WSM) by a high-frequency electromagnetic field. We investigate the conditions in realizing these phases and propose a switchable spin-polarized currents generator based on periodically driven NRSMs. Both bulk and surface polarized currents are investigated. The polarization of bulk current is sensitive to the amplitude of the driving field and robust against the direction and polarization of the driving, the opaqueness of the lead-device interface and the misalignment between the nodal ring and the interface, which provides sufficient flexibility in manipulating the devices. Similar switchable polarized surface currents are also expected, which is contributed by the Fermi arc surface state associated with the WSM phases. The generation of polarized currents and the polarization switching effect offer opportunities to design periodic driving controlled topological spintronics devices based on NRSMs.

Electrically controlled spin polarized current in Dirac semimetals.

We propose a highly tunable [Formula: see text] spin-polarized current generated in a spintronic device based on a Dirac semimetal (DSM) under a magnetic field, which can be achieved merely by controlling electrical parameters, i.e. the gate voltage, the chemical potential in the lead and the coupling strength between the leads and the DSM. These parameters are all related to the special properties of a semimetal. The spin polarized current generated by gate voltage is guaranteed by its semimetallic feature, because of which the density of state vanishes near Dirac nodes. The barrier controlled current results from the different distance of Weyl nodes generated by the Zeeman field. And the coupling strength controlled spin polarized current originates from the surface Fermi arcs. This DSM-based spintronic device is expected to be realized in [Formula: see text] experimentally.

Ultraefficient Terahertz Emission Mediated by Shift-Current Photovoltaic Effect in Layered Gallium Telluride.

Generating terahertz waves using thin-layered materials holds great potential for the realization of integrated terahertz devices. However, previous studies have been limited by restricted radiation intensity and finite efficiency. Exploiting materials with higher efficiency for terahertz emission has attracted increasing interest worldwide. Herein, with visible-light excitation, a thin-layered GaTe film is demonstrated to be a promising emitter of terahertz radiation induced by the shift-current photovoltaic effect. Through theoretical calculations, a transient charge-transfer process resulting from the asymmetric structure of GaTe is shown to be the origin of an ultrafast shift current. Furthermore, it was found that the amplitude of the resulting terahertz signals can be manipulated by both the fluence of the pump laser and the orientation of the sample. Such high emission efficiency from the shift current indicates that the layered material (GaTe) is an excellent candidate for photovoltaics and terahertz emitters.

[HPLC fingerprint of famous traditional formula Sanpian Decoction and quality value transmitting of Chuanxiong Rhizoma].

Famous traditional formula Sanpian Decoction(SPD)comes from Dialectical Records of Chen Shiduo of the Qing Dynasty,and ranks among 100 classic prescriptions of Classic Famous Traditional Formula catalogue(the First Batch). SPD was prepared according to Management Standards for Traditional Chinese Medicine Decoction Room in Medical Institutions. According to the polarity of different components in SPD,two HPLC fingerprints were established, in which six herbs, namely Chuanxiong Rhizoma, Paeoniae Randix Alba, Sinapis Semen, Glycyrrhizae Radix et Rhizoma, Pruni Semen, Angelicae Dahuricae Radix,are all reflected in the fingerprints; The dry extract rate, transfer rate and similarities of fingerprints were used as indicators to study the relationship between the quality value transmitting of medicinal herbs-decoction pieces-whole decoction of Chuanxiong Rhizoma. Experiment result shows that,the transfer rate of ferulic acid from medicinal herbs to decoction pieces is between 72.00% and 108.36%; the transfer rate of ferulic acid from decoction pieces to SPD is between 31.76% and 64.09%; the dry extract rate of the whole decoction is between 14.69% and 20.16%;The similarity range of fingerprint 1 of 15 batches of SPD is between 0.971 and 0.998, and the similarity range of fingerprint 2 is between 0.980 and 0.996. The established fingerprint has rich information,and the established quality evaluation method is suitable for the quality control of medicinal herbs-decoction pieces-whole decoction of Chuanxiong Rhizoma, which can provide a certain reference for developing the quality control evaluation method for formulated granules, famous formulae and other terminal products derived from traditional Chinese medicine decoction.

[Study on benchmark preparation technology of boiled and powdered classical prescriptions by taking Houpo Wenzhong Decoction as an example].

In recent years,the development and application of classical famous prescriptions have attracted much attention. However,the differences between ancient and modern conditions lead to difficulties in carrying out practical work. In this paper,with Houpu Wenzhong Decoction as an example,the key technologies of boiling granularity,water addition,boiling time and sample pretreatment methods were investigated on the basis of sufficient literature research. The experimental results showed that there was no significant difference in the concentration of index components between those with different granularity( 2 mm and 3-5 mm) and different decocting time( 30 min and 60 min),but the extraction rate of index components was relatively high when the granularity of powder was 2 mm and decocting time was 30 min. With the increase of water content,the concentration of index components and the extraction rate were increased in varying degrees. A certain proportion of methanol aqueous solution was used as the resolvent before content determination of the reference sample of Houpu Wenzhong Decoction,which could take into account both the spectral information of water-soluble components and fat-soluble components in the prescription,and help to display the overall information of the prescription' s chemical components more comprehensively. At the same time,the boiling and dispersing classical prescriptions in the Catalogue of Ancient Classical Prescriptions( the first batch) were collected and summarized in this study; the key influencing factors of decocting process were analyzed from different angles,and preliminary research suggestions were put forward,so as to provide a certain direction and reference for the establishment of quality standard of Houpu Wenzhong Decoction,as well as for the development,research and clinical application of boiling and dispersing classical prescriptions.

Evolution of the topological properties of two-dimensional group IVA materials and device design.

Two-dimensional group IVA materials (graphene, silicene, germanene, stanene, and plumbene) are promising candidates for realization of the quantum spin Hall effect and for future device applications. We employ density functional theory, tight-binding models, and a Green's function method to systematically investigate their topological properties. From graphene to plumbene, the strength of spin-orbit coupling and the bulk gap increases with increasing atomic mass, and plumbene, as a normal insulator, is totally different from the other four materials, whose ground states are topological insulators. Through detailed analyses of orbital character weights and the evolution of low-energy states around the Γ point, we explain why plumbene is so different. Our quantum transport calculations also indicate that there exist electronic transport channels along edges within the bulk gap of topological insulators. By investigating the effects of external fields on the electronic structures of silicene, germanene, and stanene, we reveal a rich phase diagram and propose two filters with nearly 100% spin polarization. In addition, we present a theoretical design for a spin twister, based on curved two-dimensional topological insulators.

Theoretical study of HgCr2Se3.5Te0.5: a doping-site-dependent semimetal.

Weyl semimetals have recently attracted enormous attention due to their unusual features. So far, this novel state has been predicted theoretically and confirmed experimentally in several materials, such as HgTe, LaPtBi, Y2Ir2O7, TaAs, TaP, NbAs, NbP and HgCr2Se4. Doping plays an important role in the research of condensed-matter materials. However, its influence on the Weyl semimetal has been little investigated. Here, we present detailed first-principles and theoretical studies on HgCr2Se4 with doping of Te atoms at the Se sites. A special case where only one pair of crossing points locates at the Fermi level is realized in HgCr2Se3.5Te0.5 where one of the Se atoms in the primitive unit cell is replaced by a Te atom. A further study of k·p theory shows that the two points constitute a pair of Weyl nodes with opposite chiralities in the momentum space, and only one edge state and one single Fermi arc are obtained at each boundary of a film. Moreover, through investigations and analyses of different doping cases of HgCr2Se3.5Te0.5, we find that when the type of doping induces inversion symmetry or positional disorder, the Weyl nodes transform into Dirac points resulting in a change from a Weyl semimetal to a Dirac semimetal.

Band filling and correlation controlling electronic properties and magnetism in K(x)Fe(2-y)Se2: a slave boson study.

We investigate the electronic and magnetic properties of K(x)Fe(2-y)Se2 materials at different band fillings utilizing the multi-orbital Kotliar-Ruckenstein slave boson mean-field approach. We find that the ground state of KFe2Se2 is a paramagnetic (PM) bad metal with intermediate correlation, in contrast with the previous antiferromagnetic (AFM) results obtained by the local density approximation. Our PM metallic ground state suggests that KFe2Se2 is the parent phase of superconducting K(x)Fe(2-y)Se2, supporting a recent scanning tunneling spectroscopy experiment. For pure Fe2+-based systems, the ground state is a striped AFM (SAFM) metal with a spin density wave gap partially opened near the Fermi level. In comparison, for Fe3+-based compounds, besides SAFM, a Néel AFM metal without orbital ordering is observed, and an orbital selective Mott phase (OSMP) accompanied by an intermediate-spin to high-spin transition is also found, giving a possible scenario of an OSMP in K(x)Fe(2-y)Se2. These results demonstrate that the band filling and correlation control the Fermi surface topology, electronic state and magnetism in K(x)Fe(2-y)Se2.