Band Structure Research Articles

High- Tc superconductivity in doped molecular superconductors of K4B8−xMxH32 (M = C, N) under high pressure

Stabilized and metallic light elements hydrides have provided a potential route to achieve the goal of room-temperature superconductors at moderate or ambient pressures. Here, we have performed systematic DFT theoretical calculations to examine the effects of different light elements C and N atoms doped in cubic K4B8H32 hydrides on the superconductivity at low pressures. As a result of various atoms substituting, we have found that metallic K4B _{8-x} M x H32 (M = C, N) hydrides are dynamically stable at 50 GPa, band structures and density of states (DOS) indicate that sizeable Tc correlates with a high B–H DOS at the Fermi level. With the increasing of B atoms in K4B _{8-x} M x H32 hydrides, the DOS values at Fermi level have been improved due to the delocalized electrons in B–H bonds, which result in strong electron–phonon coupling (EPC) interaction and increase the Tc from 19.04 to 77.07 K for KC2H8 and KB2H8 at 50 GPa. The NH4 unit in stable K4B7NH32 hydrides has weakened the EPC and led to low Tc value of 21.47 K. Our results suggest the light elements hydrides KB2H8 and K4B7CH32 could estimate high Tc values at 50 GPa, and the boron hydrides would be potential candidates to design or modulate hydrides superconductors with high Tc at moderate or ambient pressures.

Comparative study of the electronic and optical properties of Rhenium based Chalcogenides

Transition metal dichalcogenides are materials of growing interest due to their unique electronic properties and rich phase diagram, offering promising opportunities for various applications. In this study, we investigate the optoelectronic characteristics of two-dimensional rhenium (Re)-based chalcogenides. These materials are composed of rhenium atoms sandwiched between chalcogen layers. In this paper, we have selected three rhenium-based chalcogenide (ReS2, ReSe2 and ReTe2) compounds in their 1T structure and calculated their electronic properties. Our study aims to find the intricacies of theoretical band structures and optical properties, aiming to assess their viability as semiconducting materials for the optoelectronics industry employing density funtional theory. Furthermore, we explore the impact of varying chalcogen compositions on the optoelectronic behavior, uncovering the tunability of these materials for specific applications. The study provides insights into the role of Re-based chalcogenides as promising candidates for emerging technologies, including photodetectors, solar cells, and other optoelectronic devices. The observed band gaps highlight the potential of these materials within the infrared region of the electromagnetic spectrum. After observing optical properties derived from our calculations, we discuss the corresponding potentials of the chosen materials in optoelectronic applications.

Magnon bands and transverse transport in a proposed two-dimensional [formula omitted] ferrimagnet

The copper fluoride Cu2F5 is a proposed stable compound that can be seen as a layered lattice of S=1 and S=1/2 sites, corresponding to copper ions. Intending to cast light on the transport properties of ferrimagnetic magnons, we use the linear spin wave approach to study the magnon band structure of the 2D lattice in a ferrimagnetic off-plane order, as well as the transverse transport of magnons in the crystal bulk. A magnetic field or temperature gradient can induce transverse (Hall-like) transport within the linear response theory generated by the Berry curvature of the eigenstates. As in most cases involving magnons, the Berry curvature is related to Dzyaloshinskii-Moriya interactions between next-near-neighbors The band structure of the system is non-degenerate and the transport coefficients are non-null. The novel and interesting feature of the system is that, within certain conditions, the transport coefficients versus temperature curves are non-monotonic and present a sign change, opening the exciting possibility of controlling the transverse magnon flow direction and magnitude with the temperature change.

Exploring the structural, electronic, and optical properties of novel two-dimensional ABX-type materials

A first-principles study of the structural, electrical, and optical properties of graphene-like two-dimensional (2D) materials ABX (A=Na/K, B=C/Si/Ge, and X=N/P/As) such as NaCN, KCN, NaCP, KCP, etc was undertaken using state-of-the-art Density Functional Theory (DFT). The investigation encompasses essential parameters such as structural stability through ab initio molecular dynamics (AIMD), electronic structure, and dielectric constants. The AIMD measurements reveal that the structures stay stable for up to 10 picoseconds (ps). Band structure calculations at the PBE level of theory revealed that most materials are semi-conducting with a band gap of 1-3 eV, except NaCN and KCN, which exhibited insulating behaviour. Using hybrid functional (HSE), only eight materials were identified to have a band gap in the visible range. Optical properties have also been investigated to understand their interaction with light. Peaks in the imaginary component of the dielectric function were attributed to inter-band transitions. Several materials were discovered to be optimal for photo-catalysis, while six were found to exhibit conductivity of the order of ∼1012 (Ω−1cm−1s−1) at room temperature.

Prediction of a Rippled and Auxetic Two-Dimensional Sn9C15 Layers with Tunable Electronic Band Structure: A First-Principle Study

We predicted a two-dimensional Sn9C15 monolayer with regular ripple using first-principles methods by utilizing the VASP code based on density functional theory (DFT). The rippled Sn9C15 monolayer is an indirect bandgap semiconductor with anisotropic mechanical properties (Poisson’s ratio and Young’s modulus). It exhibits auxetic properties with a negative Poisson’s ratio (NPR) of up to −0.58 and exceptional flexibility in the x direction. Furthermore, its bandgap can be adjusted from 1.16 eV to 0.68 eV with a strain in the x direction of less than 12%. The rippled Sn9C15 monolayer is sensitive to polarized light and has a broad absorption spectrum from infrared to ultraviolet. In addition, the bilayer rippled Sn9C15 exhibits a regularly arranged nanotube-like structure and is a stable direct bandgap semiconductor material. These findings enrich the emerging landscape of 2D rippled monolayers and suggest potential applications in diverse fields such as optics, nanomechanics, electronic devices, and filtration.

Optimal half-metal band structure for large thermoelectric performance

Optimal half-metal band structure for large thermoelectric performance

Realization of Honeycomb Tellurene with Topological Edge States.

The two-dimensional (2D) honeycomb lattice has attracted intensive research interest due to the appearance of Dirac-type band structures as the consequence of two sublattices in the honeycomb structure. Introducing strong spin-orbit coupling (SOC) leads to a gap opening at the Dirac point, transforming the honeycomb lattice into a 2D topological insulator as a platform for the quantum spin Hall effect (QSHE). In this work, we realize a 2D honeycomb-structured film with tellurium, the heaviest nonradioactive element in Group VI, namely, tellurene, via molecular beam epitaxy. We revealed the gap opening of 160 meV at the Dirac point due to the strong SOC in the honeycomb-structured tellurene by angle-resolved photoemission spectroscopy. The topological edge states of tellurene are detected via scanning tunneling microscopy/spectroscopy. These results demonstrate that tellurene is a novel 2D honeycomb lattice with strong SOC, and they unambiguously prove that tellurene is a promising candidate for a room-temperature QSHE system.

Multimodal Approach Reveals the Symmetry-Breaking Pathway to the Broken Helix in EuIn2As2

Understanding and manipulating emergent phases, which are themes at the forefront of quantum-materials research, rely on identifying their underlying symmetries. This general principle has been particularly prominent in materials with coupled electronic and magnetic degrees of freedom, in which magnetic order influences the electronic band structure and can lead to exotic topological effects. However, identifying symmetry of a magnetically ordered phase can pose a challenge, particularly in the presence of small domains. Here we introduce a multimodal approach for determining magnetic structures, which combines symmetry-sensitive optical probes, scattering, and group-theoretical analysis. We apply it to EuIn2As2, a material that has received attention as a candidate axion insulator. While first-principles calculations predict this state on the assumption of a simple collinear antiferromagnetic structure, subsequent neutron-scattering measurements reveal a much more intricate magnetic ground state characterized by two coexisting magnetic wave vectors reached by successive thermal phase transitions. The proposed high- and low-temperature phases are a spin helix and a state with interpenetrating helical and Néel antiferromagnetic order termed a “broken helix,” respectively. Employing a multimodal approach, we identify the magnetic structure associated with these two phases of EuIn2As2. We find that the higher-temperature phase is characterized by a variation of the magnetic moment amplitude from layer to layer, with the moment vanishing entirely in every third Eu layer. The lower-temperature structure is similar to the broken helix, with one important difference: Because of local strain, the relative orientation of the magnetic structure and the lattice is not fixed. Consequently, the symmetry required to protect the axion phase is not generically protected in EuIn2As2, but we show that it can be restored if the magnetic structure is tuned with uniaxial strain. Finally, we present a spin Hamiltonian that identifies the spin interactions that account for the complex magnetic order in EuIn2As2. Our work highlights the importance of a multimodal approach in determining the symmetry of complex order parameters. Published by the American Physical Society 2024

CaLi2PN3 - A Quaternary Chain-Type Nitridophosphate by Medium-Pressure Synthesis.

Nitridophosphates are in the focus of current research interest due to their structural versatility and properties, such as ion conductivity, ultra-incompressibility and luminescent properties when doped with suitable activator ions. Multinary representatives often require thorough investigation due to the competition with the thermodynamically more stable binary and ternary compounds. Another point of concern is the synthetic control of structural details, which is usually limited by conventional bottom-up syntheses. In this study, we report on the synthesis and characterization of the quaternary nitridophosphate CaLi2PN3. Various synthesis protocols were used for the preparation of CaLi2PN3, including the novel nitridophosphate double salt approach. The crystal structure was solved and refined from single-crystal X-ray diffraction data and confirmed by Rietveld refinement, solid-state NMR spectroscopy, EDX measurements and low-cost crystallographic calculations. The experimental results were corroborated by DFT calculations, which revealed the electronic band structure. Formation energy calculations allowed conclusions to be drawn about the stability in comparison to the initial ternary nitridophosphates. The synthesis of CaLi2PN3 exemplifies the enormous potential of medium-pressure syntheses in the field of nitridophosphate research. Furthermore, the presented new synthesis route allows a certain degree of structural control, which is a promising addition to previous synthesis strategies in nitridophosphate chemistry.

Tunable charge transport properties in non-stoichiometric SrIrO3 thin films

Delving into the intricate interplay between spin-orbit coupling and Coulomb correlations in strongly correlated oxides, particularly perovskite compounds, has unveiled a rich landscape of exotic phenomena ranging from unconventional superconductivity to the emergence of topological phases. In this study, we have employed pulsed laser deposition technique to grow SrIrO3 (SIO) thin films on SrTiO3 substrates, systematically varying the oxygen content during the post-deposition annealing. X-ray photoelectron spectroscopy (XPS) provided insights into the stoichiometry and spin-orbit splitting energy of Iridium within the SIO film, while high-resolution x-ray studies meticulously examined the structural integrity of the thin films. Remarkably, our findings indicate a decrease in the metallicity of SIO thin films with reduced annealing O2 partial pressure. Furthermore, we carried out magneto-transport studies on the SIO thin films, the results revealed intriguing insights into spin transport as a function of oxygen content. The tunability of the electronic band structure of SIO films with varying oxygen vacancy is correlated with the density functional theory calculations. Our findings elucidate the intricate mechanisms dictating spin transport properties in SIO thin films, offering invaluable guidance for the design and optimization of spintronic devices based on complex oxide materials. Notably, the ability to tune bandwidth by varying post-annealing oxygen partial pressure in iridate-based spintronic materials holds significant promise for advancing technological applications in the spintronics domain.

Even-integer quantum Hall effect in an oxide caused by a hidden Rashba effect.

In the presence of a high magnetic field, quantum Hall systems usually host both even- and odd-integer quantized states because of lifted band degeneracies. Selective control of these quantized states is challenging but essential to understand the exotic ground states and manipulate the spin textures. Here we demonstrate the quantum Hall effect in Bi2O2Se thin films. In magnetic fields as high as 50 T, we observe only even-integer quantum Hall states, but there is no sign of odd-integer states. However, when reducing the thickness of the epitaxial Bi2O2Se film to one unit cell, we observe both odd- and even-integer states in this Janus (asymmetric) film grown on SrTiO3. By means of a Rashba bilayer model based on the ab initio band structures of Bi2O2Se thin films, we can ascribe the only even-integer states in thicker films to the hidden Rasbha effect, where the local inversion-symmetry breaking in two sectors of the [Bi2O2]2+ layer yields opposite Rashba spin polarizations, which compensate with each other. In the one-unit-cell Bi2O2Se film grown on SrTiO3, the asymmetry introduced by the top surface and bottom interface induces a net polar field. The resulting global Rashba effect lifts the band degeneracies present in the symmetric case of thicker films.

Lattice Distortion Induced Ta‐doped BaTiO3 for Efficient Photocatalytic Water Splitting for Hydrogen Production

AbstractBaTiO3, as a perovskite type material with excellent chemical stability and suitable conduction, has been widely studied in the field of photocatalysis. However, it can only absorb ultraviolet light because of its wide band gap. Herein, the lifetime of photogenerated electrons of BaTiO3 doped with Ta5+ can be enhanced. To study the band structure and photocatalytic performance of Ta‐doped BaTiO3, sol‐gel assisted solid‐phase method was employed to prepare BaTiO3 doped with varying Ta5+ doping amounts, and the structure characteristic and formation mechanism of the Ta‐doped BaTiO3 were analyzed. The results showed that the heat treatment temperature reached 1052.7 °C, sufficient thermodynamic conditions were obtained for Ta5+ to dope into the BaTiO3 lattice, and lattice distortion occurred in BaTiO3. Meanwhile, the particle size after doping decreased with the increase of Ta5+ doping amount. The 1.25 mol %Ta5+‐doped BaTiO3 had the lowest band gap (3.077 eV), and the photocatalytic water splitting had the best hydrogen evolution activity, which was 2.4 times that of BaTiO3. Furthermore, the conductance potential of 1.25 mol %Ta5+‐doped BaTiO3 was more negative than that of BaTiO3, which improved the thermodynamic advantage of the photocatalytic water splitting, and it had higher and more stable photoresponse and photogenerated carrier migration rate.

Construction of a BiOCl/Bi2O2CO3 S-Scheme Heterojunction Photocatalyst via Sharing [Bi2O2]2+ Slabs with Enhanced Photocatalytic H2O2 Production Performance.

The construction of a close contact interface is key to enhancing the photocatalytic activity in heterojunctions. In the work, the BiOCl/Bi2O2CO3 of sharing [Bi2O2]2+ slabs S-scheme heterojunction was prepared by a HCl in situ etching method. The optimal composite photocatalyst could accomplish sizable productivity of H2O2 to 2562.95 μmol g-1 h-1 under simulated solar irradiation, higher than that of primitive Bi2O2CO3 and BiOCl. Moreover, the synthesized catalysts showed good stability. The band structures of BiOCl and Bi2O2CO3 were determined, confirming the formation of BiOCl/Bi2O2CO3 S-scheme heterojunction The BiOCl/Bi2O2CO3, which obviously improved the separation efficiency of photoinduced carriers and effectively enhanced the redox ability of the photocatalyst. In addition, density functional theory (DFT) calculations were utilized to analyze the electron transfer properties and the constitution of the built-in electric field at the interface of BiOCl and Bi2O2CO3. The photocatalytic reaction process was further researched by electron paramagnetic resonance (EPR), indicating the active species in the photocatalytic production of hydrogen peroxide. Eventually, a feasible S-scheme electron transfer mechanism on the BiOCl/Bi2O2CO3 heterojunction during the photocatalytic H2O2 production process was proposed and discussed. This work provides a reliable strategy for the fine design of the S-scheme heterojunction.

Investigation of Angle‐Dependent Shubnikov‐de Haas Oscillations in Topological Insulator Bismuth

The current article investigates the band structure in the presence and absence of spin‐orbit coupling (SOC), examines the Z2 invariants, and investigates the detailed angle‐dependent magneto‐transport of up to 10 T (Tesla) and down to 2 K for the bismuth crystal. The out‐of‐plane field‐dependent magnetoresistance (MR) is positive and is huge to the order of ≈104% at 2 K and 10 T. On the contrary, the longitudinal (in‐plane) field‐dependent MR is relatively small and is negative. The thermal activation energy is also estimated by using the Boltzmann formula from resistivity versus temperature measurement under applied transverse magnetic fields. The topological nature of Bi is confirmed by Z2 invariant calculation using density functional theory (DFT). PBESol bands show trivial but hybrid functional (HSE) bands show non‐trivial topology being present in Bismuth. This article comprehensively studies the dependence of MR oscillations upon the angle between the applied field and the current. The observed oscillations fade away as the angle is increased. This article is an extension of our previous work on bismuth (J. Sup. Novel Mag. 2023, 36, 389), in which a comprehensive analysis of its structural and micro‐structural properties is conducted along with its transport behavior in an applied transverse magnetic field.

Reactive oxygen boosts photocatalytic oxytetracycline degradation and H2O2 production by K, O and cyano co-modified carbon nitride

Engineering photocatalysts to enhance the production of reactive oxygen species holds great importance in realizing effective water purification and hydrogen peroxide (H2O2) generation. Herein, the K, O, and cyano co-modified carbon nitride (KOCN) was constructed using thermal polymerization. Under visible light irradiation, KOCN showed excellent oxytetracycline hydrochloride (OTC) degradation efficiency and H2O2 generation rate. During the photocatalysis, rich reactive oxygens involving ·O2-, 1O2, H2O2, and ·OH were detected. Additionally, the production of H2O2 was attributed to a two-step single-electron oxygen reduction reaction (ORR). DFT and characterization analyses confirmed that the doping of K and O as well as the formation of cyano groups expanded the visible light response range, adjusted the band structure, and improved the separation of photogenerated carriers. This study presents a novel approach for developing efficient, stable, and reusable photocatalysts to address contemporary environmental challenges.

Impact of ternary nanocomposite In2O3/SnO2/F-CDs composition on low ppm NO2 sensing performance

Nitrogen dioxide (NO2) is a significant air pollutant with a notable environmental impact, necessitating precise monitoring. However, most existing NO2 sensing materials exhibit low sensitivity and require high-temperature operation. In this study, we employed a hydrothermal and co-precipitation method to synthesize a novel ternary nanocomposite, In2O3/SnO2 nanoparticles (NPs)/fluorine-doped carbon quantum dots (F-CDs). By varying the mass ratio of In2O3 to SnO2 NPs, we investigated the gas-sensing performance of the composite towards NO2. The results demonstrate that when the ratio of SnO2 and In2O3 reaches to 1 : 2, the 2-In2O3/SnO2/F-CDs gas sensor exhibited the highest response, excellent selectivity, and remarkable stability under ultraviolet (UV) light irradiation at room temperature (∼25 °C). Furthermore, the response of the 2-In2O3/SnO2/F-CDs gas sensor displayed a strong linear relationship with the concentration of NO2 gas. At a NO2 gas concentration of 3 ppm, the sensitivity was 238, which is significantly higher than that of In2O3/SnO2 NPs and In2O3 sensors. The response time and recovery time were measured at 62 s and 33 s, respectively. Finally, based on experimental characterization results and band structure analysis, we confirmed the promising effects of ternary nanocomposite in enhancing gas sensing.

Ultrafast spin transfer and its impact on the electronic structure.

Optically induced intersite spin transfer (OISTR) promises manipulation of spin systems within the ultimate time limit of laser excitation. Following its prediction, signatures of ultrafast spin transfer between oppositely aligned spin sublattices have been observed in magnetic alloys and multilayers. However, it is known neither from theory nor from experiment whether the band structure immediately follows the ultrafast change in spin polarization or whether the exchange split bands remain rigid. We show that ultrafast spin transfer occurs even in ferromagnetic gadolinium metal. Charge transfer between localized surface and extended valence-band states leads to a decrease of the surface spin polarization. This synchronously alters the exchange splitting of the bulk valence bands during laser excitation. Moreover, the onset of demagnetization can be tuned by over 200 fs by changing the temperature-dependent spin mixing. Our results show a promising route to ultrafast control of the magnetization, widening the impact and applicability of OISTR.

Effect of Co- and Fe-doping on magnetic and optical properties of SnS2 monolayer

The magnetic and optical properties of pure, Co/Fe single- and co-doped of SnS2 monolayers are studied by using the first-principles calculation. Results show that all the doping systems are stable. The band structures of Sn15Co1S32 and the configuration I of the Sn14Fe1Co1S32 monolayers show metal behavior. While the band structure of Sn15Fe1S32 monolayer show half-metal behavior. The estimated TC value for the configuration I of the Sn14Fe1Co1S32 monolayer is bout 696.27 K. The absorption spectrums of doping systems have a red shift. The results indicate that Co/Fe doped SnS2 monolayer systems are promising candidate materials for new spintronics and optoelectronic device.

Atomically Precise Regulation of the N-Heterocyclic Microenvironment in Triazine Covalent Organic Frameworks for Coenzyme Photocatalytic Regeneration.

Artificial photosynthesis represents a sustainable strategy for accessing high-value chemicals; however, the conversion efficiency is significantly limited by its difficulty in the cycle of coenzymes such as NADH. In this study, we report a series of isostructural triazine covalent organic frameworks (COFs) and explore their N-substituted microenvironment-dependent photocatalytic activity for NADH regeneration. We discovered that the rational alteration of N-heterocyclic species, which are linked to the triazine center through an imine linkage, can significantly regulate both the electron band structure and planarity of a COF layer. This results in different separation efficiencies of the photoinduced electron-hole pairs and electron transfer behavior within and between individual layers. The optimal COF catalyst herein achieves an NADH regeneration capacity of 89% within 20 min, outperforming most of the reported nanomaterial photocatalysts. Based on this, an artificial photosynthesis system is constructed for the green synthesis of a high-value compound, L-glutamate, and its conversion efficiency significantly surpasses the enzymatic approach without the NADH photocatalytic cycle. This work offers new insights into the coenzyme regeneration by means of regulating the distal heterocyclic microenvironment of a COF skeleton, holding great potential for the green photosynthesis of important chemicals.

Energy Band-Modulated SnO Anodes with Improved Rate Capacity and Initial Coulombic Efficiency for Sodium-Ion Batteries.

An efficient interface and composition modification strategy is proposed to significantly increase the rate capacity and initial Coulombic efficiency (ICE) of SnO anodes via energy band structure modulation for Na-ion batteries (SIBs). The as-fabricated SnO@Bi@C material with Bi nanocrystals homogeneously dispersed on and around the SnO surface is prepared by an electrospinning method. The excellently dispersed Bi nanocrystals realize an effective interface contact with SnO and Na2O, which effectively lowers the electron conduction barriers and promotes sodium-ion release and transport upon the desodiation process, increasing the rate capacity and ICE of the SnO anode. The prepared SnO@Bi@C exhibits a much higher rate capacity up to 10 A g-1, except an improved ICE of 80.67%, compared to 57.38% of SnO@C. Importantly, the intercalation of Na+ between layer-structured Bi galleries provides a good basis for excellent cycle stability due to the highways constructed inside the prepared composites. This finding provides an effective tactic to establish ion transport high-speed channels, which essentially increase the rate performance and the ICE of the SnO-based anodes.