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Metal chalcogenide solid solution, especially ZnCdS, has been intensively investigated in photocatalytic H2 generation due to their cost-effective synthetic procedure and adjustable band structures. In this work, we report on the defect engineering of ZnCdS with surface disorder layer by simple room temperature Li-ethylenediamine (Li-EDA) treatment. Experimental results confirm the formation of unusual Zn and S dual vacancies, where rich S vacancies (VS) served as electron trapping sites, meanwhile Zn vacancies (VZn) served as hole trapping sites. The refined structure significantly facilitates the photo charge carrier transfer and improves photocatalytic properties of ZnCdS. The disordered ZnCdS shows a highest photocatalytic H2 production rate of 33.6 mmol·g−1·h−1 under visible light with superior photocatalytic stabilities, which is 7.3 times higher than pristine ZnCdS and 7 times of Pt (1 wt.%) loaded ZnCdS.

Self-assembly performed in ionic liquids (ILs) as a unique solvent promises distinct functions and applications in sensors, therapeutics, and optoelectronic devices due to the rich interactions between nanoparticle building blocks and ILs. However, the general consideration that common nanoparticles are readily destabilized by counterions in an IL has largely prevented researchers from investigating controlled nanoparticle assembly in IL-based systems. This study explores the assembling behaviour of double-stranded (ds) DNA-functionalized gold nanoparticles (dsDNA-AuNPs) in hydrated ionic liquids. The DNA base pair stacking assembly of dsDNA-AuNPs occurs at a low IL concentration (5%). However, a moderate ionic liquid concentration (5-40%) can de-hybridize dsDNA and leaves single-stranded (ss) DNA stabilizing the AuNPs. In concentrated ionic liquids (40%), interestingly, the higher ionic strength leads to the assembly of DNA-AuNPs. The triphasic assembly trend is also generally observed regardless of the type of IL. By down-regulation of DNA's melting temperature with the IL, the assembly of DNA-AuNPs affords robust response to a lower temperature range, promising applications in plasmonic devices and range-tunable temperature sensors.

The interactions between metal ions and biomolecules are crucial to various bioprocesses. Development of plasmon switching nanodevices that exploit these molecular interactions is of fundamental and technological interest. Here, we show plasmon switching based on rapid aggregation/dispersion of double-stranded DNA-modified gold nanorods (dsDNA–AuNRs) that exhibit colloidal behaviors depending on pairing/unpairing of the terminal bases. The dsDNA–AuNRs bearing a thymine–thymine (T–T) mismatch at the penultimate position undergo spontaneous non-cross-linking aggregation in the presence of Hg2+ due to T–Hg–T base pairing. Inversely, the subsequent addition of cysteine (Cys) gives rise to the removal of Hg2+ from the T–Hg–T base pair to reproduce the T–T mismatch, resulting in stable dispersion of the dsDNA–AuNRs. The chemical-responsive plasmon switch allows for the rapid and repeatable cycles at room temperature. The validity of the present method is further exemplified by developing another plasmon switch ...

Transition metal dichalcogenides, especially MoS2 , have been regarded as promising cocatalysts for the hydrogen evolution reaction (HER) because of a near-zero Gibbs free energy for H+ absorption. However, the HER activity of MoS2 is profoundly restricted by acidic media. Co-doped MoS2 (Co-MoS2 ) nanosheets are found to enable the highly efficient solar H2 evolution of CdS nanowires (NWs) in alkalescent electrolyte. The content of Co in MoS2 is optimized to 2.8 % and the Co-MoS2 content in Co-MoS2 /CdS NWs hybrids is 2.5 wt %; the optimized Co-MoS2 /CdS NWs shows a high H2 evolution rate of 14.22 mmol g-1 h-1 and 71 % apparent quantum efficiency in the Na2 SO3 electrolyte. Moreover, the H2 generation enabled by the Co-MoS2 cocatalyst can exhibit universality in alkalescent electrolytes, such as triethanolamine (TEA) and disodium ethylenediaminetetraacetic acid (EDTA), exhibiting greater photocatalytic H2 generation than Pt/CdS. The design of Co-MoS2 /CdS sheds light on the development of highly efficient low-cost photocatalysts for solar H2 generation from water reduction.

SnO2-based anodes with high capacity are appealing for Li-ion batteries (LIBs). However, the large volume change and inferior cycling stability limit their practical application. To mitigate these problems, a novel nanocomposite of silicon-incorporated SnO2 with graphene sheets (STOG) has been successfully fabricated as anode material for LIBs. Through a simple hydrolysis process, ultrafine Si-incorporated SnO2 (STO) nanoparticles are uniformly loaded on the graphene sheets. Further it is found that Si incorporation brings about the Si O Sn bonding in the SnO2 matrix and strengthens the Sn O C bonding between STO and graphene. These merits can enhance the structural stability and electron/ion transport of STOG nanocomposite, facilitating the reversible conversion of Sn–SnO2. As a result, this STOG material delivers a high discharge capacity of 1117.8 mAh g−1 and retains 92.5% of the second capacity after 100 cycles at 0.1 A g−1. Furthermore, an excellent rate capacity of 683.9 mAh g−1 can be obtained at a high current of 1 A g−1. This work provides an effective way to design high-performance SnO2-based anode material for LIBs.

In the past decade, crystal facet engineering of TiO2 though various controllable strategies has attracted a lot of attention due to its strong facet dependent photoactivities. In this work, we report on nontopological transformation of titanium oxydifluoride (TiOF2) to hierarchical TiO2 structures by hydrothermal tailoring. The fluorine-containing species in TiOF2 crystal perform the roles as either an etching agent or a capping agent, which can conveniently modulate morphologies and facets of TiO2 crystals via dissolution or oriented collapse under hydrothermal condition. The Wulff construction and different surface energies of shape and facet lead to the controlled formation of hierarchical TiO2, respectively with (1 0 1) faceted nanowalls, (0 0 1) and (1 1 1) faceted nanosheets, or their multi-facets. All TiO2 with single exposed facet exhibit higher photocatalytic H2 evolution performance than the commercial P25 TiO2. Moreover, the (1 0 1) faceted nanowalls demonstrate superior photocatalytic H2 evolution to (1 1 1) and (0 0 1) faceted TiO2 nanosheets in presence of hole scavenger (methanol) due to favoured hole transfer at (1 0 1) facet/methanol interface. This work presents novel methodology for shape and facet control of TiO2 towards solar to fuel conversion.

Tailored defects on a semiconductor surface can provide active catalytic sites and effectively tune the electronic structure for suitable optical properties. Herein, we report that surface modification of WO3 with a disordered layer enables the photochemical hydrogen production from water. A simple room temperature solution process with lithium-ethylenediamine (Li-EDA) alters the surface of WO3 with localized defects that form a thin disordered layer. Both structural and optical characterization reveal that such a disordered layer induces an upshift in the Fermi level and the elevation of the conduction band of WO3 above the hydrogen reduction potential. Using an alkaline sacrificial agent, Li-EDA treated WO3 shows a co-catalyst-free photochemical hydrogen evolution rate of 94.2 μmol g−1 h−1 under simulated sunlight. To the best of our knowledge, this is the first example of using WO3 as a direct photocatalyst for hydrogen generation from water via simple surface defect engineering.

Nanophase graphene frameworks (NGFs) assembled by interconnected domains have massive interfaces, where the interfacial interaction and the compact architectures drastically elevate the durability of graphene towards physical and chemical destruction. The excellent electrical conductivity of the NGFs can be perfectly maintained even after 1500 friction cycles or 3 h flame treatment.

Test strips represent a class of point-of-care testing (POCT) tools for analysis of a variety of biomarkers towards diagnostics. Conventional test strips offer benefits of simple operation, visualization, and short detection time, along with the drawbacks of relatively low sensitivity and unavailability of quantitative analysis. Recently, the combination of surface-enhanced Raman scattering (SERS) and test strips have evolved to provide a powerful platform capable of ultrasensitive and multiplex detection of extensive analytes of interest. In this review, we focus on the working principles, design strategies and POCT applications of SERS-based test strips. Initially, both lateral and vertical flow test strips are briefly introduced, followed by presentation of various strategies for reforming SERS-based test strips with better detection performance. Applications of SERS-based test strips in diagnosis of disease biomarkers, nucleic acids and toxins are reviewed, with an emphasis on SERS tag design, sensitivity and analytical applicability. Finally, conclusions are made and perspectives on futuristic research directions are given.

Coding metasurfaces have drawn great attention for its digital wave manipulation in deep subwavelength-scale in the last decade, more sophisticated and flexible coding strategies suitable for terahertz wavefront manipulations are becoming more urgently demanded. Due to its rigidity in phase gradient division, both phase gradient metasurfaces and conventional phase coding technique lack the flexibility to expand applications in a large field of view and accurate targeting. This study presents a generalized coding method by precisely reconfiguring the array factor based on the phased array theory and metasurface concept, which can be applied for anomalous scattering and ultrafine radiation patterning. According to our quantitative analysis on the relationship between the deflected angles and the supercell spacing, a fractional coding method for arbitrary phase gradient distribution has been attained by logically discretizing the spacing scale of supercells. By switching on different coding sequences or incident frequencies, a single beam to multiple beam scanning in an expanded angular range with minimal step can be achieved on the fractional phase-coding metasurfaces. As a proof of concept, the 2-bit coding metasurfaces arranged by four fractional coding sequences have been fabricated and measured, demonstrating a consecutive single-beam steering pattern ranging from 22° to 74° in 0.34-0.5 THz. Crosswise verified by the good accordance among numerical prediction, simulation and experiment, the proposed coding strategy paves a path to delicate beam regulation for high-resolution imaging and detection.

The iron-doped SnO2/reduced graphene oxide (Fe-SnO2/rGO) composite has been facilely synthesized through a simple wet chemical method. It is shown that the Fe-SnO2 nanocrystals (∼8 nm) are homogeneously anchored onto rGO sheets with the content of 69.5 wt%. As the anode material for lithium-ion batteries, the as-prepared Fe-SnO2/rGO composite exhibits a reversible capacity of 1353 mAh g−1 after 100 cycles at 0.1 A g−1. Even at a high specific current of 1 A g−1, it still maintains a high reversible capacity of 691 mAh g−1, and a capacity of 1248 mAh g−1 can be delivered when the specific current returns back to 0.1 A g−1 after 50 cycles at different specific currents. The excellent electrochemical performance of Fe-SnO2/rGO composite can be attributed to the Fe doping, which can enhance the interfacial interaction between Fe-SnO2 nanocrystals and rGO sheets, improve the electrical conductivity of the composite and more importantly, facilitate the conversion reaction between Li2O and SnO2 during cycling.

Considerable works have been devoted on developing high-efficiency nonplatinum electrocatalysts for hydrogen evolution reaction (HER). Herein, 3D heteromorphic NiCo2S4/Ni3S2 nanosheets network has been constructed on Ni foam (denoted as NiCo2S4/Ni3S2/NF) serving as a self-standing electrocatalyst through directly thermal sulfurization of a single-source NiCo-layered double hydroxide precursor. The resultant NiCo2S4/Ni3S2/NF electrode exhibits outstanding electrocatalytic HER performance with an extremely low onset overpotential of 15 mV and long-term durability in alkaline solution. Such enhanced HER performance can be credited to (1) the massive exposed active sites provided by mixed transition metal chalcogenides (NiCo2S4 and Ni3S2), (2) the strong interfacial interaction at NiCo2S4/Ni3S2 heterojunction interfaces with the strengthened H binding, and (3) the porous highly conductive Ni foam substrate with accelerated electron transfer. This work opens up a new direction to fabricate effective and non-no...

Inspired by the increasing use of plasmonic gold and silver nanoplates as probes for diverse analytes, the research community often questions which metal nanoplates should be chosen for a given application. A comparative study was performed on the performance and physicochemical properties of three types of metal nanoplates for use in plasmonic detection of Hg(II) ion. Specifically, gold, silver and Ag@Au nanoplates were studied. The established amalgamation method integrated into a detection scheme using nanoplates affords a unique yet straightforward signaling and extraction route for selective recognition of Hg(II) ion. Upon transformation of Hg(II) ion to metallic mercury, nanoplate amalgamation takes place instantly. This reshapes both the morphology and the optical characteristics of nanoplates. It is found that gold and Ag@Au nanoplates enable highly selective quantitation of Hg(II) ion by using a DNA oligomer consisting of poly-deoxycytidine (poly(C)) as a masking agent against Ag(I) ion. The silver nanoplates, in turn, display the best sensitivity owing to the chemical instability. The induced surface plasmonic shifts (of up to 250 nm and color changes from red to green) allows for determination of Hg(II) over a wide range and with a limit of detection of ~10 nM. It is recommended that the gold and Ag@Au nanoplates are used in relatively complex systems, while silver nanoplates are suited for simple matrices. Graphic abstract The amalgamation process integrated with metal (e.g., Au, Ag and Ag@Au) nanoplates affords plasmonic detection of Hg(II) ion with the aid of a poly(c) DNA sequence as the masking agent for Ag(I) ion.

In this paper, a broadband terahertz phase-shifter based on metamaterial is designed by integrating High Electron Mobility Transistor (HEMT) into a well-designed multi-layer structure unit. It can be independent electrically tuned for three-dimensional (3D) beam control. The variation of carrier density in the HEMT results in a transition of the resonant mode in the multilayer resonant structure. The change of mutual-coupling magnetic resonance provide a phase shift of 180°±15° with a 20% bandwidth in 0.35-0.42 THz. This work facilitated the implementation of 3D terahertz beam control.

A novel 2-bit coding metasurface for terahertz beam steering is proposed here. By fractionally dividing the phase gradient distribution, a higher-precision beam steering has been implemented. Compared with uniform coding manner, the novel non-uniform coding strategy employed on the proposed metasurface can shorten prominently the step length of beam steering, highly being valued for high resolution imaging and detecting.

A terahertz digital phase modulator based on an ultrafast electrically controlled HEMT-embedded metasurface is theoretically and experimentally demonstrated here.Different from traditional delay-line phase shifter, the proposed phase modulator combined the active HEMT 2DEGs with the resonant structure, demonstrating higher reflective efficiency and 2πfull coverage phase shift under low electron densities around 1015/m2. The preliminary measurement results show a π phase difference occurs at 0.358 THz with a -8.5 dB attenuation on average, which is in good accordance with the simulation.

The interplay between electronic correlations and topological protection may offer a rich avenue for discovering emergent quantum phenomena in condensed matter. However, electronic correlations have so far been little investigated in Weyl semimetals (WSMs) by experiments. Here, we report a combined optical spectroscopy and theoretical calculation study on the strength and effect of electronic correlations in a magnet Co3Sn2S2. The electronic kinetic energy estimated from our optical data is about half of that obtained from single-particle ab initio calculations in the ferromagnetic ground state, which indicates intermediate-strength electronic correlations in this system. Furthermore, comparing the energy and side-slope ratios between the interband-transition peaks at high energies in the experimental and single-particle-calculation-derived optical conductivity spectra with the bandwidth-renormalization factors obtained by many-body calculations enables us to estimate the Coulomb-interaction strength (U ∼ 4 eV) in Co3Sn2S2. Besides, a sharp experimental optical conductivity peak at low energy, which is absent in the single-particle-calculation-derived spectrum but is consistent with the optical conductivity peaks obtained by many-body calculations with U ∼ 4 eV, indicates that an electronic band connecting the two Weyl cones is flattened by electronic correlations and emerges near the Fermi energy in Co3Sn2S2. Our work paves the way for exploring flat-band-generated quantum phenomena in WSMs., How electron correlation interplays with topological states remains rarely explored. Here, the authors report flat band arising due to electron correlations in magnetic Weyl semimetal Co3Sn2S2 from a combined optical-spectroscopy and simulation study.

The Imbert-Fedorov (IF) shift is the transverse shift of a beam at a surface or an interface. It is a manifestation of the three-component Berry curvature in three dimensions and has been studied in optical systems and Weyl semimetals. Here we investigate the IF shift in two types of topological systems: topological semimetals with pseudospin $N/2$ for an arbitrary integer $N$, and nodal-line semimetals (NLSMs). For the former, we find that the IF shift depends on the components of the pseudospin, with the sign depending on the chirality. We term this phenomenon the pseudospin Hall effect of topological fermions. The shift can also be interpreted as a consequence of the conservation of the total angular momentum. For the latter, if the NLSM has both time-reversal and inversion symmetries, the IF shift is zero; otherwise it could be finite. We take the NLSM with a vortex ring, which breaks both symmetries, as an example, and show that the IF shift can be used to detect topological Lifshitz transitions. Finally, we propose experimental designs to detect the IF shift.

Hourglass-like band structures protected by nonsymmorphic space group symmetries can appear along high-symmetry lines or in high-symmetry surfaces in the Brillouin zone. In this work, from symmetry analysis, we demonstrate that $n$-hourglass-like band structures, a generalization of hourglass-like band structures, which host a number of Weyl points, are enforced along screw-invariant lines in non-magnetic materials with a single $N$-fold screw axis when spin-orbit coupling is finite, where $n$, a non-unity factor of $N$, denotes the degree of the screw-invariant line. The "standard" $n$-hourglass has $n-1$ crossings, which is minimal, and its variants can have more crossings. Purely by symmetry considerations, we find there are minimally two particle and two hole Fermi pockets enclosing Weyl points with opposite monopole charges at proper fillings, which can result in distinct physical effects including the possible formation of topological density waves and the quantum nonlinear Hall effect. We construct a minimal model which respects all the symmetries, and from which we see how the $n$-hourglasses appear when spin-orbit coupling is turned on. The same results are derived from compatibility relations. As exemplary, BiPd, ZnTe under high pressure, and the high-temperature phase of Tl$_3$PbBr$_5$, are shown from first-principles calculations to exhibit $n$-hourglass-like band structures, with $N=2,3,4$, respectively, which confirms our symmetry analysis and minimal model., Comment: 6 pages, 4 figures + supplemental materials

BiVO4 theoretically has a thermodynamic activity trend toward highly selective water oxidative H2 O2 formation, but it is more inclined to generate O2 in practical. The influence of intrinsic oxygen vacancy (Ovac ), especially, on surface reactivity, has never been considered as a possible activity loss mechanism in the synthetic BiVO4 . In this work, it is theoretically and experimentally demonstrated that the intrinsic surface Ovac is responsible for lower H2 O2 evolution activity via promoting water dissociation to form intermediate. Through an annealing process under a V2 O5 rich atmosphere, the surface Ovac can be eliminated that awakens the photoelectrochemical (PEC) water oxidative H2 O2 activity in a NaHCO3 electrolyte, which achieves an average of 58.4%, and increases by up to 4.28 times of the one annealed in air. This work offers a general understanding of catalytic activity loss and may be extended to other photo or electrocatalysts for catalytic selectivity regulation.

Surface engineering of TiO2 is faced with the challenge of high solar-to-hydrogen conversion efficiency. Recently, surface-disordered TiO2, referred to as black TiO2, which can absorb both visible and near-infrared solar light, has triggered an explosion of interest in many important applications. Unfortunately, the mechanism underlying the improved photocatalytic effect from an amorphous surface layer remains unclear and seems to contradict conventional wisdom. Here, we demonstrate selectively “disorder engineered” Degussa P-25 TiO2 nanoparticles using simple room-temperature solution processing, which maintain the unique three-phase interfaces composed of ordered white-anatase and disordered black-rutile with open structures for easy electrolyte access. The strong reducing agent in a superbase, which consists of lithium in ethylenediamine (Li-EDA), can disorder only the white-rutile phase of P-25, leaving behind blue coloured TiO2 nanoparticles. The order/disorder/water junction created by the blue P-25 can not only efficiently internally separate electrons/holes through type-II bandgap alignment but can also induce a strong hydrogen (H2) evolution surface reaction in the sacrificial agent containing electrolyte. As a result, the blue P-25 exhibited outstanding H2 production rates of 13.89 mmol h−1 g−1 using 0.5 wt% Pt (co-catalyst) and 3.46 mmol h−1 g−1 without using any co-catalyst.

Here we demonstrate a novel method to synthesize secondary-structured LiFePO4 (SS-LFP) with ultrahigh tap density (1.3 g/cm3) by using LiClO4 and NH4OH precursors for tuning the acidity. Comparing to the other LFP cathode, SS-LFP based lithium ion batteries show a high reversible capacity of 166 mA/h/g at 0.1 C and long-term stability (1000 cycles).

Silicon is regarded as the most promising candidate for the next-generation anode materials, because of its high specific capacity and wide availability. However, silicon anode materials suffer from large volume change during the charging/discharging processes, which leads to a too fast capacity degradation. To alleviate the volume change and capacity degradation, preforming pore structure in silicon anode materials is the most frequently adopted strategy. The pores are usually formed either by using templates or through selective etching, which involves complicated preparation process and consumes hazard chemicals. Here, we propose a nanostructured silicon anode derived from low-cost raw materials without the use of templates. The composite is composed of dense active cores and fluffy cocoon-like buffer layer. The buffer layer is a network of carbon nanobelts, which accommodates the volume change and maintains the electrical contact during the charging/discharging processes. As a result, the porous composite with the buffer layer displays a high cyclability, with a capacity retention of 104% after 100 cycles, while the dense sample without the buffer layer has a capacity retention of only 36%. Moreover, the process is green, effective and scalable, which could promote the application of silicon based anode materials.

This research reports the effects of factors such as metal ratio, thermal treatment temperature, time and atmosphere in a previously developed path that recycled spent lithium-ion batteries for OER catalysts. With regards to the uncontrollable metal-ratio in recycled material, for the first time, the metal ratio of the Li-Ni-Co-Mn oxides’ influence on their electrochemical performance is studied. In thermal treatment, the atmosphere is newly evaluated as a determining factor, while factors like temperature and time are also further compared. The mechanism of the lattice change resulted from the reductive decomposition reaction of Li x CoO2(x ≠ 1) ( α -NaFeO2-type), generating Co3O4(spinel) is introduced. Finally, the recycled material is innovatively applied to CO2 electrolysis as anode OER catalyst with high current efficiency (97%) towards CO production under a total current density of 32 mA cm−2 at 2.25 V, with an energy efficiency of 63%, which shows a promising future in the industry use.

Terahertz metasurface has always aroused extensive attention for many years. However, terahertz reconfigurable metasurface is still a significant challenge. The broadband terahertz reconfigurable metasurface based on 1-bit asymmetric coding metamaterial is presented in this paper. The asymmetric metamaterial breaks the charge balance and provides multiple resonances, which is conducive to expanding the operating bandwidth. Conventional coding metamaterials change phase by varying the shape or size of the patch. The proposed metamaterial only disconnects or connects the metal lines in the metamaterial to obtain 1-bit phase information, which is beneficial for designing reconfigurable coding metasurface. Both simulated and experimental results show that the asymmetric metamaterial efficiently reflects the normal incident waves at 0.34–0.49 THz with a relative bandwidth of 36%, and the reflection coefficients are measured to exceeds 80%. Furthermore, the simulation and experimental results of the metasurface show that the normal incident wave is reflected as a double beam of the expected angle by configuring metamaterials with different coding sequences. This work provides important potential applications for the development of valuable dynamic terahertz beam scanning devices.

Background A growing body of evidence shows that indoor concentrations of airborne particles are often higher than is typically encountered outdoors. Since exposure to indoor PM2.5 is thought to be associated with cardiovascular disease, the health impacts of indoor air pollution need to be explored. Based on animal models, ambient particulate matter has been proved to promote coagulation which is very likely involved in the pathogenic development of cardiovascular disease. However, animal models are insufficient to predict what will happen with any certainty in humans. For this reason, the precise pathogenic mechanisms behind the development of cardiovascular disease in humans have not yet been determined. Results We generated a 3D functional human microvascular network in a microfluidic device. This model enables human vascular endothelial cells to form tissue-like microvessels that behave very similarly to human blood vessels. The perfusable microvasculature allows the delivery of particles introduced into these generated human-like microvessels to follow the fluid flow. This exposure path effectively simulates the dynamic movement of airborne nanoscale particles (ANPs) within human vessels. In this study, we first identified the existence of ANPs in indoor air pollution. We then showed that ANPs could activate endothelial cells via ROS induced inflammation, and further resulted in abnormal expression of the coagulation factors (TF, TM and t-PA) involved in coagulation cascades. In addition, we found that a protein could cover ANPs, and this biointeraction could interfere with heparan sulfate (HS). Human organotypic 3D microvessel models provide a bridge for how research outcomes can translate to humans. Conclusions The 3D human microvessel model was used to determine the physiological responses of human vessels to ANP stimulation. Based on the obtained data, we concluded that ANPs not only disrupts normal coagulation functions, but also act directly on anticoagulant factors in human vessels. These experimental observations provide a potential biological explanation for the epidemiologically established link between ANPs and coagulation abnormality. This organ-on-chip model may provide a bridge from in vitro results to human responses.

We report infrared studies of $A{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($A=\mathrm{Ba}$, Sr), two representative parent compounds of iron-arsenide superconductors, at magnetic fields ($B$) up to 17.5 T. Optical transitions between Landau levels (LLs) were observed in the antiferromagnetic states of these two parent compounds. Our observation of a $\sqrt{B}$ dependence of the LL transition energies, the zero-energy intercepts at $B=0\text{ }\text{ }\mathrm{T}$ under the linear extrapolations of the transition energies and the energy ratio ($\ensuremath{\sim}2.4$) between the observed LL transitions, combined with the linear band dispersions in two-dimensional (2D) momentum space obtained by theoretical calculations, demonstrates the existence of massless Dirac fermions in the antiferromagnet ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$. More importantly, the observed dominance of the zeroth-LL-related absorption features and the calculated bands with extremely weak dispersions along the momentum direction ${k}_{z}$ indicate that massless Dirac fermions in ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ are 2D. Furthermore, we find that the total substitution of the barium atoms in ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ by strontium atoms not only maintains 2D massless Dirac fermions in this system, but also enhances their Fermi velocity, which supports that the Dirac points in iron-arsenide parent compounds are topologically protected.

It was recently shown that nonsymmorphic space group symmetries can protect novel surface states with hourglass-like dispersions. In this paper, we show that such dispersions can also appear in the bulk of three-dimensional (3D) systems which respect nonsymmorphic symmetries. Specifically, we construct 3D lattice models featuring hourglass-like dispersions in the bulk, which are protected by nonsymmorphic and time-reversal symmetries. We call such systems hourglass semimetals, as they have point or line nodes associated with hourglass-like dispersions. Hourglass nodal lines appear in glide-invariant planes, while hourglass Weyl points can occur on screw-invariant axes. The Weyl points and surface Fermi arcs in hourglass Weyl semimetals are stable against weak perturbations breaking those nonsymmorphic symmetries. Our results may shed light on searching for exotic Weyl semimetals in nonsymmorphic materials., 6 pages, 5 figures + Supplemental Material, published versioin

The Imbert-Fedorov (IF) shift in optics describes the transverse shift of light beams at the reflection interface. Recently, the IF shift of Weyl fermions at the interface between Weyl semimetals (WSMs) with single monopole charge has been studied. Here, we study the IF shift at the interface between two WSMs, each of which is carrying an arbitrary integer monopole charge. We find a general relation between the monopole charges of the two WSMs and the IF shift. In particular, the IF shift is proportional to the monopole charge if both WSMs have the same one. Our results can be used to infer the topology of the materials by experimentally measuring their IF shift. Furthermore, we consider the possibility that the Weyl fermions are scattered to other Weyl cones during the reflection, which results in qualitatively different behavior of the IF shift. While we use a quantum mechanical approach to solve the problem, semiclassical equations of motion and the conservation of total angular momentum can help us intuitively interpret our results in special cases., Comment: 7 pages, 3 figures, published version

We investigate the superconducting instability of a two-dimensional repulsive fermion gas with Rashba spin-orbit coupling $\al_R$. Using renormalization group approach, we find the superconducting transition temperature as a function of the dimensionless ratio $\Theta={1}{2}m\al_R^2/E_F$ where $E_F=0$ when the smaller Fermi surface shrinks to a (Dirac) point. The general trend is that superconductivity is enhanced as $\Theta$ increases, but in an intermediate regime $\Theta\sim0.1$, a dome-like behavior appears. At a very small value of $\Theta$, the angular momentum channel $j_z$ in which superconductivity occurs is quite high. With increasing $\Theta$, $j_z$ decreases with a step of 2 down to $j_z=6$, after which we find the sequence $j_z=6, 4, 6, 2$, the last value of which continues to $\Theta\rightarrow\infty$. In an extended range of $\Theta$, the superconducting gap predominantly resides on the large Fermi surface, while Josephson coupling induces a much smaller gap on the small Fermi surface. Below the superconducting transition temperature, we apply mean field theory to derive the self-consistent equations and find the condensation energies. The state with the lowest condensation energy is an unconventional superconducting state which breaks time reversal symmetry, and in which singlet and triplet pairings are mixed. In general, these states are topologically nontrivial, and the Chern number of the state with total angular momentum $j_z$ is $C=2j_z$., Comment: 19 pages, 7 figures

A generation of reactive oxygen species (ROS) from TiO2 under solar light has been long sought since the ROS can disinfect organic pollutants. We found that newly developed crystalline/amorphous reduced TiO2 (rTiO2) that has low energy bandgap can effectively generate ROS under solar light and successfully remove a bloom of algae. The preparation of rTiO2 is a one-pot and mass productive solution-process reduction using lithium-ethylene diamine (Li-EDA) at room temperature. Interestingly only the rutile phase of TiO2 crystal was reduced, while the anatase phase even in case of both anatase/rutile phased TiO2 was not reduced. Only reduced TiO2 materials can generate ROS under solar light, which was confirmed by electron spin resonance. Among the three different types of Li-EDA treated TiO2 (anatase, rutile and both phased TiO2), the both phased rTiO2 showed the best performance to produce ROS. The generated ROS effectively removed the common green algae Chlamydomonas. This is the first report on algae degradation under solar light, proving the feasibility of commercially available products for disinfection.

Weyl points in three-dimensional photonic crystals behave as monopoles of Berry flux in momentum space. Here, based on general symmetry analysis, we show that a minimal number of four symmetry-related (consequently equifrequency) Weyl points can be realized in time-reversal invariant photonic crystals. We further propose an experimentally-feasible way to modify double-gyroid photonic crystals to realize four equifrequency Weyl points, which is explicitly confirmed by our first-principle photonic band-structure calculations. Remarkably, photonic crystals with equifrequency Weyl points are qualitatively advantageous in applications including angular selectivity, frequency selectivity, invisibility cloaking, and three dimensional imaging., 5 pages + Supplemental Material, 4 figures

We study a model of a two-dimensional repulsive Fermi gas with Rashba spin-orbit coupling $\alpha_R$, and investigate the superconducting instability using renormalization group approach. We find that in general superconductivity is enhanced as the dimensionless ratio 1/2m\alpha_R^2/E_F$ increases, resulting in unconventional superconducting states which break time reversal symmetry., Comment: 5 pages, 4 figures