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The topological interpretation of phonons provides a new platform for new concepts in phonon physics. While reports on topological electronic excitations are plenty, the number of reports on phonons is extremely limited. In this paper, we present some realistic materials as examples and demonstrate the appearance of ideal nodal-net, nodal-chain, and nodal-cage phonons in these materials based on first-principle calculations

Nodal line spin-gapless semiconductors (NLSGSs) are a new type of topological spintronic material with possible high Curie temperature, 100% spin polarization, high carrier mobility, gapless nodal line states at the Fermi level in one spin channel, and spin-polarized drumhead-like surface states.

Thermal conductivity of two-dimensional (2D) materials has gained prominence due to the attractive applications in thermal management and thermoelectric devices. In this work, we present a new member of bilayer 2D boron allotropes, denoted as bilayer β12 borophene, and study the thermal transport properties by solving phonon Boltzmann transport equation based on density functional theory. Based on quantitative chemical bonding analysis, we identify large degrees of covalent bonding of the interlayer interaction. In comparison to its monolayer counterpart, the bilayer exhibits much higher in-plane thermal conductivity despite the lower phonon group velocity and buckling structure, inferring a new physical mechanism. The thermal conductivity (κ) of bilayer β12 borophene at 300 K is 140.5 (86.3) W·m−1·K−1 along armchair (zigzag) direction, and κarmchair is about 52.7% higher than that of monolayer β12 borophene. The abnormal enhancement is attributed to the suppressed phonon scattering possibility and elongation of phonon lifetime. More interesting, after forming bilayer β12 borophene through interlayer covalent bonding, the dominated phonon branch to thermal conductivity changes to transverse acoustic phonons from out-of-plane flexural acoustic (ZA) phonons in the monolayer borophene. Our study elucidates the rich thermal transport characteristics in bilayer covalently bonded 2D materials, and injects fresh insights into the phonon engineering of 2D borophene relevant for emergent thermal management applications.

Two-dimensional ferromagnetic (FM) half-metals are promising candidates for advanced spintronic devices with small size and high capacity. Motivated by a recent report on controlling the synthesis of FM Cr

Two-dimensional ferromagnetic (FM) half-metals are promising candidates for advanced spintronic devices with small-size and high-capacity. Motivated by recent report on controlling synthesis of FM Cr$_3$Te$_4$ nanosheet, herein, to explore the potential application in spintronics, we designed spintronic devices based on Cr$_3$X$_4$ (X=Se, Te) monolayers and investigated their spin transport properties. We found that Cr$_3$Te$_4$ monolayer based device shows spin filtering and dual spin diode effect when applying bias voltage, while Cr$_3$S$_4$ monolayer is an excellent platform to realize a spin valve. The different transport properties are primarily ascribed to the semiconducting spin channel, which is close to and away from the Fermi level in Cr$_3$Te$_4$ and Cr$_3$Se$_4$ monolayers, respectively. Interestingly, the current in monolayer Cr$_3$Se$_4$ based device also displays a negative differential resistance effect (NDRE) and a high magnetoresistance ratio (up to 2*10$^3$). Moreover, we found thermally induced spin filtering effect and NDRE in Cr$_3$Se$_4$ junction when applying temperature gradient instead of bias voltage. These theoretical findings highlight the potential of Cr$_3$X$_4$ (X=Se, Te) monolayers in spintronic applications and put forward realistic materials to realize nanosale spintronic device.

Topological phononic states with nodal lines not only have updated our knowledge of the phases of matter in a fundamental way, but also have become a major frontier research direction in condensed matter physics. From a mathematical perspective, nodal line phonons can be divided into open and closed types. The present attempt is a report on the coexistence of such open and closed nodal line phonons in two realistic solids, CoAsS and Na

We report a configuration strategy for improving the thermoelectric (TE) performance of two-dimensional (2D) transition metal dichalcogenide (TMDC) WS2 based on the experimentally prepared WS2/WSe2 lateral superlattice (LS) crystal. On the basis of density function theory combined with Boltzmann transport equation, we show that the TE figure of merit zT of monolayer WS2 is remarkably enhanced when forming into a WS2/WSe2 LS crystal. This is primarily ascribed to the almost halved lattice thermal conductivity due to the enhanced anharmonic processes. Electronic transport properties parallel (xx) and perpendicular (yy) to the superlattice period are highly symmetric for both p- and n-doped LS owing to the nearly isotropic lifetime of charger carriers. The spin-orbital effect causes a significant split of conduction band and leads to three-fold degenerate sub-bands and high density of states (DOS), which offers opportunity to obtain the high n-type Seebeck coefficient (S). Interestingly, the separated degenerate sub-bands and upper conduction band in monolayer WS2 form a remarkable stairlike DOS, yielding a higher S. The hole carriers with much higher mobility than electrons reveal the high p-type power factor and the potential to be good p-type TE materials with optimal zT exceeds 1 at 400K in WS2/WSe2 LS.

Graphical abstract Device model based on zigzag magnetic MoS2 nanotube and the calculated spin-dependent currents through the device., Bipolar magnetic semiconductors (BMSs) are a new member of spintornic materials. In BMSs, one can obtain 100% spin-polarized currents by means of the gate voltage. However, most of previous studies focused on their applications in spintronics instead of spin caloritronics. Herein, we show that BMS is an intrinsic model for spin Seebeck effect (SSE). Without any gate voltage and electric field, currents with opposite spin orientation are generated and flow in opposite directions with almost equal magnitude when simply applying a temperature bias. This is also due to the special electronic structure of BMS where the conduction and valence bands near the Fermi level belong to opposite spin orientation. Based on density function theory and non-equilibrium Green’s function methods, we confirm the thermal-induced SSE in BMS using a case of magnetic MoS2 nanotube. The magnitude of spin current in zigzag tube is almost four times higher than that in armchair tube. BMS is promising candidates for spin caloritronic applications.

Graphical abstract, Very recently, searching for new topological nodal line semimetals (TNLSs) and drum-head-like (DHL) surface states has become a hot topic in the field of physical chemistry of materials. Via first principles, in this study, a synthesized CsCl type binary alloy, TiOs, was predicted to be a TNLS with three topological nodal lines (TNLs) centered at the X point in the kx/y/z = π plane, and these TNLs, which are protected by mirror, time reversal (T) and spatial inversion (P) symmetries, are perpendicular to one another. The exotic drum-head-like (DHL) surface states can be clearly observed inside and outside the crossing points (CPs) in the bulk system. The CPs, TNLs, and DHL surface states of TiOs are very robust under the influences of uniform strain, electron doping, and hole doping. Spin-orbit coupling (SOC)-induced gaps can be found in this TiOs system when the SOC is taken into consideration. When the SOC is involved, surface Dirac cones can be found in this system, indicating that the topological properties are still maintained. Similar to TiOs, ZrOs and HfOs alloys are TNLSs under the Perdew-Burke-Ernzerhof method. The CPs and the TNLs in both alloys disappear, however, under the Heyd-Scuseria-Ernzerhof method. It is hoped that the DHL surface property in TiOs can be detected by surface sensitive probes in the near future.

To date, ideal topological nodal line semimetal (TNLS) candidates in high dynamically stable and high thermally stable two-dimensional (2D) materials are still extremely scarce. Herein, by performing first-principles calculations, on the one hand, we found that three-dimensional Nb3GeTe6 bulk possesses a single closed TNL in the kx = 0 plane and a fourfold TNL in the S–R direction without considering spin–orbit coupling (SOC). Under the SOC effect, a new topological signature, i.e., hourglass-like Dirac nodal line, occurs in Nb3GeTe6 bulk. On the other hand, we found that the 2D Nb3GeTe6 monolayer features a doubly degenerate TNL along surface X–S paths. Importantly, this monolayer enjoys the following advantages: (i) it has high thermal stability at room temperature and above; (ii) its TNL is nearly flat in energy and is very close to the Fermi level (EF), which provides a fantastic maximum value platform of the thermoelectric power factor around the EF; and (iii) no extraneous bands are close to the TNL, near the Fermi level. Moreover, we explore the entanglement between the topological states and thermolectric properties for the 2D Nb3GeTe6 monolayer. Our work not only reports the discovery of a novel TNL material, but also builds the link between the TNL and thermoelectric properties.

The exploration of topological states in materials is currently focused on metals and semimetals, and their linear band crossing features various topological states, including a nodal point, nodal line and nodal surface. Based on first principles calculations, we have predicted a ternary NdAsPd compound that exhibits multiple topological states of a type-II nodal ring, triply degenerate nodal point and nodal surface states in the absence of spin orbit coupling (SOC) effects. Its band formation mechanism has been analyzed and the corresponding nontrivial drumhead surface and Fermi arc have been confirmed. Under the consideration of SOC, these topological states evolve into a type-II nodal point and type-I nodal line. The remarkable topological diversity in the present material is very rare and can serve as a promising platform upon which to study the rich fermionic states in a single material.

Transcription factor EB (TFEB)-based gene therapy is a promising therapeutic strategy in treating neurodegenerative diseases by promoting autophagy/lysosome-mediated degradation and clearance of misfolded proteins that contribute to the pathogenesis of these diseases. However, recent findings have shown that TFEB has proinflammatory properties, raising the safety concerns about its clinical application. To investigate whether TFEB induces significant inflammatory responses in the brain, male C57BL/6 mice were injected with phosphate-buffered saline (PBS), adeno-associated virus serotype 8 (AAV8) vectors overexpressing mouse TFEB (pAAV8-CMV-mTFEB), or AAV8 vectors expressing green fluorescent proteins (GFPs) in the barrel cortex. The brain tissue samples were collected at 2 months after injection. Western blotting and immunofluorescence staining showed that mTFEB protein levels were significantly increased in the brain tissue samples of mice injected with mTFEB-overexpressing vectors compared with those injected with PBS or GFP-overexpressing vectors. pAAV8-CMV-mTFEB injection resulted in significant elevations in the mRNA and protein levels of lysosomal biogenesis indicators in the brain tissue samples. No significant changes were observed in the expressions of GFAP, Iba1, and proinflammation mediators in the pAAV8-CMV-mTFEB-injected brain compared with those in the control groups. Collectively, our results suggest that AAV8 successfully mediates mTFEB overexpression in the mouse brain without inducing apparent local inflammation, supporting the safety of TFEB-based gene therapy in treating neurodegenerative diseases.

Two-dimensional transition metals borides Ti

Topological phononic states with nodal lines not only have updated our knowledge of the phases of matter in a fundamental way, but also have become a major frontier research direction in condensed matter physics. From a mathematical perspective, nodal line phonons can be divided into open and closed types. The present attempt is a report on the coexistence of such open and closed nodal line phonons in two realistic solids, CoAsS and Na2CuP, based on first-principles calculations. Furthermore, it is shown that the closed and the open nodal line states in CoAsS and Na2CuP have touching points and can form a complex nodal structure phonon in a momentum space. Due to the topologically non-trivial behavior of the complex nodal structure in both phonons, evident phononic surface states occur in the projected surfaces of both materials. In this way, these states, arising from the projected crossing points, can benefit experimental detection in follow-up studies. It has been stated that the open and closed nodal line states are formed by the crossings of two phonon branches and, hence, these two types of nodal line phonons are coupled with each other. The results obtained here could be considered as a breakthrough in clearly demonstrating the coexistence of the open and closed nodal line states in phonons and, for this reason, may inspire researchers seeking materials with such topological states in other bosons, such as photons. © 2022 The Royal Society of Chemistry.

Searching for three-dimensional(3D) semiconducting carbon allotropes with proper bandgaps and excellent optoelectronic properties is always the chasing goal for the new emerging all-carbon optoelectronics. On the other side, 3D carbon materials have also been recognized as promising anode materials superior to commercialized graphite in Li-ion batteries (LIBs). Here, using first-principles calculations, we propose two novel 3D carbon allotropes through acetylenic linkages modification of two structurally intimately correlated 3D carbon structures - carbon kagome lattice(CKL)and interpenetrated graphene network(IGN). The modified CKL is a truly direct-gap semiconductor and possibly possesses the strongest optical transition coefficient amongst of all semiconducting carbon allotropes. The suitable bandgap and small effective masses also imply it can be a good electron transport material(ETM) for perovskite solar cells. As for the modified IGN, it is a topological nodal-line semimetal and shows greatly enhanced specific capacity as anode materials in LIBs comparing to that of IGN. Our work not only finds two new 3D carbon phases with fabulous physical and chemical properties for high-performance optoelectronics and Li-ion anode material, we also offer a fresh view to create various carbon structures with versatile properties., Comment: 7 figures

Two-dimensional ferromagnetic monolayers have attracted growing interest due to their promising applications in spintronic devices. To explore the potential application of monolayer VS2 and VCl2 in spintronic devices, previously reported ferromagnetic semiconductor and half-metal, respectively, we investigate the spin transport properties of VS2 homo-junction, VCl2 homo-junction, and lateral VS2–VCl2 heterostructure using first-principles combined with non-equilibrium Green's function. We show that monolayer VS2 exhibit superior spin Seebeck effect along an armchair direction, monolayer VCl2 is an excellent platform to realize a spin valve, and the magnetoresistance ratio is up to 1.3 × 104. Moreover, the VS2–VCl2 heterostructure exhibits an excellent spin diode effect. We explain these effects from the calculated spin-dependent band structure and transmission spectrum. The superior spin transport properties make monolayer VS2 and VCl2 promising candidates for spintronic applications.

A heavy element is a special character for high thermoelectric performance since it generally guarantees a low lattice thermal conductivity. Here, we unexpectedly found a promising thermoelectric performance in a two-dimensional semiconducting monolayer consisting of a light boron element. Using first-principles combined with the Boltzmann transport theory, we have shown that in contrast to graphene or black phosphorus, the boron monolayer has a low lattice thermal conductivity arising from its complex crystal of hexagonal vacancies. The conduction band with an intrinsic camelback shape leads to the high DOS and a high n-type Seebeck coefficient, while the highly degenerate valence band along with the small hole effective mass contributes to the high p-type power factor. As a result, we obtained the p-type thermoelectric figure of merit up to 0.96 at 300 K, indicating that the boron monolayer is a promising p-type thermoelectric material.

In this study, an experimentally synthesized DHM LaNiO3 is discovered with many Dirac cones and complete spin polarization near the Fermi level via first principles; it is also shown that the crystal structures of these materials are strongly correlated with their physical properties., In the past three years, Dirac half-metals (DHMs) have attracted considerable attention and become a high-profile topic in spintronics becuase of their excellent physical properties such as 100% spin polarization and massless Dirac fermions. Two-dimensional DHMs proposed recently have not yet been experimentally synthesized and thus remain theoretical. As a result, their characteristics cannot be experimentally confirmed. In addition, many theoretically predicted Dirac materials have only a single cone, resulting in a nonlinear electromagnetic response with insufficient intensity and inadequate transport carrier efficiency near the Fermi level. Therefore, after several attempts, we have focused on a novel class of DHMs with multiple Dirac crossings to address the above limitations. In particular, we direct our attention to three-dimensional bulk materials. In this study, the discovery via first principles of an experimentally synthesized DHM LaNiO3 with many Dirac cones and complete spin polarization near the Fermi level is reported. It is also shown that the crystal structures of these materials are strongly correlated with their physical properties. The results indicate that many rhombohedral materials with the general formula LnNiO3 (Ln = La, Ce, Nd, Pm, Gd, Tb, Dy, Ho, Er, Lu) in the space group R 3 c are potential DHMs with multiple Dirac cones.

A remarkable recent advancement has been the successful synthesis of two-dimensional boron monolayers on metal substrates. However, although up to 16 possible bulk allotropes of boron have been reported, none of them possess van der Waals (vdW) layered structures. In this work, starting from the experimentally synthesized monolayer boron sheet (β12 borophene), we explored the possibility for forming vdW layered bulk boron. We found that two β12 borophene sheets cannot form a stable vdW bilayer structure, as covalent-like B–B bonds are formed between them because of the peculiar bonding. Interestingly, when the covalently bonded bilayer borophene sheets are stacked on top of each other, three-dimensional (3D) layered structures are constructed via vdW interlayer interactions, rather than covalent. The 3D vdW layered structures were found to be dynamically stable. The interlayer binding energy is about 20 meV/Å2, which is close to the weakly bound graphene layers in graphite (∼16 meV/Å2). Furthermore, the density functional theory predicted electronic band structure testifies that these vdW bulk boron crystals can behave as good conductors. The insights obtained from this work suggest an opportunity to discover new vdW layered structures of bulk boron, which is expected to be crucial to numerous applications ranging from microelectronic devices to energy storage devices.

Motivated by the experimental synthesis of two-dimensional MSe2 (M = Zr, Hf) thin films, we set out to investigate the electronic, thermal, and thermoelectric transport properties of 1T-phase MSe2 (M = Zr, Hf) bilayers on the basis of first-principles calculations and Boltzmann transport theory. Both bilayer ZrSe2 and HfSe2 are indirect band gap semiconductors possessing degenerate conduction bands and stair-like-shaped DOS, which provide a high n-doped power factor. In combination with the low lattice thermal conductivity that originated from the low phonon frequency of acoustic modes and the coupling of acoustic modes with optical modes, the maximum figure of merits ZT at room temperature for n-type doping are predicted as 1.84 and 3.83 for ZrSe2 and HfSe2 bilayers, respectively. Our results suggest that bilayer conformation of ZrSe2 and HfSe2 are promising thermoelectric materials with superior performance to their bulk counterparts.

Manipulation of the electronic properties of 2D materials with molecular adsorption has been widely reported in the past decade. However, the impact of adsorption of molecules on the magnetism of intrinsic 2D magnets is not yet sufficiently understood. Inspired by the recent discovery of intrinsic 2D ferromagnetism in atomically thin Cr2Ge2Te6 and CrI3 crystals, in this investigation, the adsorption of gas molecules including CO, CO2, H2O, N2, NH3, NO, NO2, O2 and SO2 on the basal plane of a Cr2Ge2Te6 (CGT) monolayer was systematically investigated by density functional theory. The structures, energy, charge transfer, work function, and electronic and magnetic properties of the gas molecule adsorbed Cr2Ge2Te6 monolayers were explored. It has been found that the adsorption of gas molecules significantly influences the electronic and magnetic properties of a CGT sheet. It was observed that the adsorption of molecules could effectively enhance the ferromagnetism and Curie temperature (TC) of the CGT sheet. Notably, a remarkable enhancement of ferromagnetism and TC by 38% and 32% was achieved by the adsorption of NO and NO2, respectively. Moreover, NO2 adsorbed CGT exhibits a semiconductor-to-metal transition. The enhanced ferromagnetism and tunable electronic structure could be ascribed to the considerable charge transfer as well as to the alignment of the frontier molecular orbital. The outcome of this study reveals not only exciting insights into the environmental effect on 2D magnets but also provides a new candidate as a NO2 gas sensor.

Antiferromagnetic (AF) spin devices could be one of the representative components for applications of spintronics thanks to the numerous advantages such as resistance to magnetic field perturbation, stray field-free operation, and ultrahigh device operation speed. However, detecting and manipulating the spin of AF materials is still a major challenge due to the absence of a net magnetic moment and spin degeneracy in the band structure. Bipolar antiferromagnetic semiconductors are promising solutions to these problems. Herein, using density functional theory calculations, we present asymmetrical functionalized double MXenes (Cr2TiC2FCl) that behave as a novel bipolar antiferromagnetic semiconductor (BAFS) with vanishing magnetism, in which the valence band and conduction band around the Fermi level exhibit opposite spin directions. Remarkably, gate voltage can manipulate the spin orientation of the AF Cr2TiC2FCl and lead to a transition from BAFS to half-metal antiferromagnets (HMAF). Moreover, the mixed functionalized double MXenes with various F/Cl concentrations show the BAFS feature due to the different chemical environment for the Cr atom. Our results presented herein open a new strategy towards AF spintronics and the realization of the AF spin field effect transistor.

In general, maximum power point tracking (MPPT) method is employed to ensure the maximum power extraction from photovoltaic (PV) arrays. The perturbation and observation (P&O) algorithm is widely used in PV system due to its simplicity and ease of implementation. However, this algorithm will introduce perturbation which is the main reason of interharmonics injection in PV system. In this paper, an interharmonics suppression control scheme with modified MPPT algorithm was proposed. In the proposed control scheme, the dc-link voltage reference generated by MPPT was used to deduce whether the PV system was operated near the MPP. When the dc-link reference voltages meet the characteristics of three points oscillation three consecutive times, it was meaning that the PV system was operated at the MPP. Then the constant voltage tracking algorithm was trigged. Otherwise, it was operated under conventional P&O MPPT algorithm. The proposed control scheme can suppress interharmonics emission and improve power quality. Meanwhile, it can maintain the MPPT efficiency. Simulations have been carried out to verify the effectiveness of the proposed control scheme.

Due to the Randomness and fluctuation of the photovoltaic (PV) system, Maximum Power Point Tracking (MPPT) algorithm is employed to ensure the maximum power extraction from PV arrays, when it is connected to the grid. The previous studies have indicated that the MPPT algorithm is one of the main causes of interharmonics emission in PV systems. The MPPT algorithm parameters such as perturbation step size inevitably have a strong impact on the interharmonics in the current injected to the grid. In general, a smaller MPPT perturbation step size will reduce the interharmonics emission level, but it will result in poor tracking performance of the MPPT algorithm. Thus, there is a trade-off between the MPPT efficiency and the interharmonics emission in PV system. In this paper, an interharmonics suppression scheme with variable step size MPPT algorithm was proposed. In the suppression scheme, two voltage values near by the MPPT voltage and two tracking step size are preset. When the dc-link voltage of PV is between the two preset voltage values, it means that the PV power near its MPP and the smaller perturbation step size implemented in the MPPT algorithm. Otherwise, the bigger perturbation step size is employed. It can greatly reduce interharmonics emission level and maintain the MPPT efficiency. Simulations have been carried out to verify the effectiveness of the suppression scheme.

To date, a handful of topological semimetals (TMs) with multiple types of topological nodal line (TNL) states have been theoretically predicted in novel materials. However, their TNLs are often affected by many factors, such as spin-orbit coupling (SOC) effect, strain, and the extraneous bands near the band crossing points, and therefore, the TNL states cannot be easily verified by experiments. Here, by using first-principles calculations, we report that the Pnma LiBH is a potential TM with twofold and quadruple degenerate topological nodal lines. These TNLs situate very close to the Fermi level, and do not coexist with other extraneous bands. More importantly, the TNLs of this material are very robust to the effect of SOC, uniform strain, and biaxial strain. The nontrivial band structure in LiBH produces drum-head-like surface states in the (001) surface projection. Our result reveals that LiBH material is an excellent candidate to study the multiple kinds of TNLs. © 2020 IOP Publishing Ltd.

Electron-crystal and phonon-glass are regarded as two essential factors for ideal thermoelectric materials, which require both an enhanced electronic transport and a depressed phononic transport. These two characteristics usually cannot coexist in complete bulk semiconductors because of the strong interaction between electrons and phonons. Introducing nanostructures into bulk thermoelectric materials has long been applying to allow for diverse phonon scattering mechanisms to reduce lattice thermal conductivity while keeping the electrical conductivity intact. However, some two-dimensional materials do the opposite. Here, we propose an example of monolayer SnSe introduced from newly researched bulk thermoelectric material SnSe. Through first-principles electronic structure calculations and Boltzmann transport theory, we show that the lattice thermal conductivity of monolayer SnSe is instead higher than that of bulk SnSe, leading to a depressed thermoelectric figure of merit compared with bulk one. Besides, we also find the anisotropic level in monolayer SnSe is reduced, and better thermoelectric performance can be realized in electron doping. The present work may provide a fundamental understanding of thermoelectric transport in SnSe-based materials. Keywords: First-principles, Thermoelectric, SnSe, Boltzmann theory

Thallium compounds have appeared as a new class of advanced thermoelectric (TE) materials owing to their extremely low lattice thermal conductivity. Using a first-principles approach and Boltzmann transport theories, we investigate the electronic and phonon transport properties of a ternary thallium telluride, TlInTe2. Unlike the Bi2Te3 and PbTe used in current TE devices, a multi-valley band structure with intrinsic degeneracy is obtained for TlInTe2, which contributes to its high Seebeck coefficient and relatively high power factor. Also, because of its weak bonding stiffness and strong phonon anharmonicity, TlInTe2 has an extremely low lattice thermal conductivity of about 0.37 W m−1 K−1 at room temperature, which is almost one third of the values for Bi2Te3 and PbTe. Consequently, competitive ZT values of 1.78 and 1.84 are obtained at 300 K for p- and n-doped TlInTe2, respectively, indicating that thallium compounds are promising TE materials for practical applications.