Gap Splitting Research Articles

On the role of the exact Hartree–Fock exchange in determining the Jahn–Teller energy splitting and electronic band gap in the KBF3 (B=Sc, Ti, Fe, Co, Cr and Cu) perovskites. A quantum mechanical investigation

KBF3 perovskites present a partially filled t2g (Sc, Ti, Fe or Co), or eg (Cr or Cu) subshell. The role of the adopted functional on the Jahn–Teller (JT) splitting, band gap, and cell deformation is investigated. Results obtained with PBE, HSE06, B3LYP, PBE0, Hartree–Fock and a new functional PBE(X), in which X is the percentage of HF exchange (0 to 100%) are compared, revealing that the JT splitting and energy gap depend dramatically on X, while the correlation and local exchange terms composing the functionals have a slight influence on the investigated properties.

Strain-tuned optical conductivity of monolayer PbBiI

In this paper, we investigate the optical response of the PbBiI single-layer by developing a strain-induced Kane–Mele model from Peierls substitution and by employing the Kubo formula at low temperatures. We address three different regimes of uniform and non-uniform classes created by tuning the strength of the strain. From a detailed analysis of the electronic band structure, we find that the Rashba spin splitting gap is destroyed with strain, while the bulk gap slightly changes. We also find that interband optical transitions exhibit a blueshift spectrum with strain. Interestingly, all these findings are independent of the regime and class of strain. However, our simulations show that only the non-uniform class of strain leads to anisotropic optical conductivity. These results enhance optoelectronic applications of low-dimensional materials.

Magneto-electronic and thermopower properties of B, N and Si doped monolayer graphene

Pure graphene is limited by its gapless energy band structure in catalytic reactions that are important for energy conversion applications. However, impurities can create a band gap and improve the electrical and photocatalytic properties of graphene, leading to a wide range of energy-related applications. In this study, the Kubo-Greenwood formula was used to investigate the thermopower properties of monolayer graphene doped with B, N, and Si atoms under various concentrations of impurities and magnetic fields. The impurity concentration was controlled by varying the number of unit cells. The electronic structure of the doped graphene are strongly influenced by the type and concentration of impurities, with a created energy gap that was sensitive to the impurity concentration. Furthermore, applying a magnetic field resulted in subband splitting and band gap reduction for all selected structures, regardless of the impurity type. The thermal properties of the doped structures are dependent on the impurity type, concentration, and magnetic field strength. It was observed that the thermal properties decreased with increasing impurity concentration and were affected by the magnetic field. The thermal properties of the nitrogen-doped structure is higher than those of the boron-doped and silicon-carbide-doped structures. Overall, these findings are significant for developing more efficient energy conversion devices.

Ligand field luminescence in the honeycomb Mott insulator α-RuCl3

Novel electronic structure is central to Kitaev-like quantum spin liquid ground state in the van der Waals antiferromagnet α-RuCl3. In this octahedral low-spin system, the rich bandstructure includes Mott–Hubbard insulating bandgap of ∼1.1 eV and ligand field induced gap of ∼2 eV. Though the bandgap is extensively reported in previous optical and photoemission studies, the latter gap is less understood. In this regard, we probed the ligand field-induced electronic structure of bulk α-RuCl3 single crystals via unpolarized photoluminescence (PL) spectroscopy under linearly polarized excitation in the visible region. We observed the broad principal PL peak at E PL 1.87 eV, close to the energy required for the optical transition across the ligand field splitting gap (∼2 eV). This gap (E PL) monotonically increases with temperature, described by the thermal expansion of lattice parameters dominating the electron–phonon interaction in this Honeycomb Mott insulator. Moreover, we observed the multiplet structure of the PL spectra, which can be explained by parity-forbidden d–d transitions in the light of single ion ligand-field theory. We attribute the broad PL linewidths to strong vibronic coupling modeled by the Huang–Rhys parameter. Further we observed the signature of structural transitions, indicated by thermal hysteresis in normalized integrated PL intensity.

Electric-Field Control of Spin Polarization above Room Temperature in Single-Layer A-Type Antiferromagnetic Semiconductor

Two-dimensional (2D) antiferromagnets have drawn great interest for absence of stray fields in antiferromagnetic (AFM) spintronics. However, it remains challenging to manipulate their spin polarization above room temperature for practical applications. Herein, a general strategy is reported to realize the control of spin polarization above room temperature in 2D A-type AFM semiconductors by external electric field based on first-principles calculations, exemplified by transition metal monohalide MnCl and carbide MXenes Cr2CX2 (X = F, Cl, OH). It shows that 100% spin polarization can be induced around Fermi level with spin splitting gap related to the spatial distribution of spin density in real space. Meanwhile, the Neél temperature of 2D MnCl and Cr2CF2 remains above room temperature under external electric field up to 0.6 V/Å. This study exhibits the potential for application of 2D AFM semiconductors in electric-field-controlled spintronics.

Triggering single-molecule qubit spin dynamics via non-Abelian geometric phase effects.

We illustrate how macroscopic rotations can be utilised to trigger and control a spin dynamics within the ground doublet of both Kramers and non-Kramers-type molecular nanomagnets via the non-Abelian character of the time-evolution operator. For Kramers magnets, we show how this effect can be harnessed to realise single-qubit quantum gates and give the explicit example of a recently reported CoCl2(tu)4 single-molecule magnet (SMM). We demonstrate that gating operations could be performed on this magnet in as fast as 10 ps before the breakdown of adiabaticity, much faster than typical spin-lattice relaxation times. Based on this effect, we also suggest CoCl2(tu)4 as a quantum gyroscope for sensing yaw-axis rotations. For integer spin nanomagnets where non-axial crystal field interactions often lift ground state degeneracy, we show how spin dynamics from the non-Abelian geometric propagator can be recovered using non-adiabatic macroscopic rotations not-necessarily resonant with the tunnel splitting gap. Using the well-known TbPc2 single-ion magnet as a further example, we identify an experimentally plausible non-adiabatic rotation that induces a coherent superposition of tunnelling ground states, tantamount to preparing each member of a TbPc2 ensemble in the maximal angular momentum state |mJ = 6〉. The detection of an ensuing coherent oscillation of the macroscopic magnetisation polarised along the TbPc2 principal magnetic axis after the completed rotation could then proceed via time-resolved magnetisation measurements.

Structural Optimization of Single Crystal Turbine Blade Based on Recrystallization Critical Stress

In view of the problem that the surface structural recrystallization (SRR) is very easy to occur in the splitting gap area of the single crystal blade. A method to control the residual stress is proposed that based on the critical stress of single crystal recrystallization, and the structural details are optimized by combined with the numerical calculation and experimental verification. To reduce the residual stress level and change the residual stress distribution in the single crystal blade splitting gap. So as to control the level of residual stress, which is no more than the recrystallization critical stress. Finally, the experimental verification is completed by the optimized blade. The verification results show that the maximum residual stress is reduced by about 10% in the tail splitting region, and the recrystallization structure is completely eliminated in the original structure. It is proved that this method is a great effectiveness to control the structural integrity and surface structure connectivity of the single crystal blade.

Plasmon-phonon coupling between mid-infrared chiral metasurfaces and molecular vibrations.

Plasmon-phonon coupling between metamaterials and molecular vibrations provides a new path for studying mid-infrared light-matter interactions and molecular detection. So far, the coupling between the plasmonic resonances of metamaterials and the phonon vibrational modes of molecules has been realized under linearly polarized light. Here, mid-infrared chiral plasmonic metasurfaces with high circular dichroism (CD) in absorption over 0.65 in the frequency range of 50 to 60 THz are demonstrated to strongly interact with the phonon vibrational resonance of polymethyl methacrylate (PMMA) molecules at 52 THz, under both left-handed and right-handed circularly polarized (LCP and RCP) light. The mode splitting features in the absorption spectra of the coupled metasurface-PMMA systems under both circular polarizations are studied in PMMA layers with different thicknesses. The relation between the mode splitting gap and the PMMA thickness is also revealed. The demonstrated results can be applied in areas of chiral molecular sensing, thermal emission, and thermal energy harvesting.

Novel 1:1 stoichiometric rare-earth HoX (X $$=$$ Pd, Ag and Cd) intermetallic compounds: DFT-based study

A systematic investigation on structural, electronic, elastic, mechanical and optical properties of novel 1:1 stoichiometric rare-earth $$\mathrm{HoX}\ (\mathrm{X}=\text {Pd, Ag and Cd})$$ intermetallic compounds has been made via ab-initio density functional theory-based linearized augmented plane wave method as coded in wien2k. An outline of local density approximation and generalized gradient approximation have been employed. Structural optimization established the stable CsCl-type cubic structure of $$\mathrm{HoX}\ (\mathrm{X}=\mathrm{Pd, Ag and Cd})$$ compounds. The determined crystal structure stability, compressibility and fracture strength of compounds are enhanced with X in the order $$\mathrm{HoPd}>\mathrm{HoAg}>\mathrm{HoCd}$$ . The interlacing electron dispersion curves at Fermi-level in band structure and density of states substantiate the metallic nature of compounds. The splitting gap between the lower and upper valence bands monotonously becomes narrower with the substitution of heavier core-element $$(\hbox {Pd}\rightarrow \hbox {Ag}\rightarrow \hbox {Cd})$$ . The mechanical constants i.e., bulk modulus (B), Young’s modulus (E), shear modulus ( $${G}_{\mathrm{H}}$$ ), Pugh’s ratio $$(B/G_{\mathrm{H}})$$ , Poisson’s ratio (v) and anisotropic factor (A) have been calculated to corroborate the mechanical properties of compounds. The cationic nature of Ho-atoms and anionic nature of X-atoms has been evaluated through the effective charge $$(\textit{Q}^{*}$$ ) computations. The observed peaks in the low energy region of optical conductivity spectra attribute the intra-band, while the high energy structures are associated with inter-band transitions in $$\mathrm{HoX}\ (\mathrm{X}=\text {Pd, Ag and Cd})$$ compounds. The bond order calculations demonstrate the highest strength of HoCd compound among the herein studied compounds.

THz filter based on the Si microdisk array

Compared to plasmonic metasurface, all-dielectric metasurface can suppress radiation loss at terahertz (THz) frequencies due to the low intrinsic loss of dielectric. Here, we propose a THz filter based on all-dielectric metasurface composed of Si microdisk (SiMD) array standing on TPX substrate. Numerical simulation results demonstrate that both electric dipole and magnetic dipole resonances are excited in the SiMD, resulting in a high reflection coefficient of 100%. The working frequency of the designed filter can be passively tuned over a wide range from 1.0 THz to 1.5 THz by manipulating the radius of the SiMD. In addition, the proposed filter is robust to the incident polarizations (x-/y-linear polarizations) and the incident angles (ranging from 0° to 25°). Besides, a new degree of freedom is introduced by cutting a split in the SiMD. The filter can work well even with a split in the SiMD since the presence of the split does not induce additional losses. We further demonstrate the resonant modes can also be modified by tuning the width of the splitting gap, resulting in a tunable THz filter with high efficiency.

Open spin levels effect in the localized states on nanosilicon doped with oxygen under the magnetic moment of metal atoms

Magnetism in a non-magnetic material by manipulating its structure at the nanoscale is created, where many structural impurities and their defects have unpaired spins to create a magnetically ordered state. The magnetic properties of the non-magnetic metal atoms (Au or Al) adsorbed on impuritied nanosilicon surface were investigated, where the opening spin levels (OSL) effect in the localized states was obviously observed on nanosilicon doped with oxygen under the magnetic moment of these metal atoms. Here, the splitting gap of individual two-level spin ±1/2 states isolated in the localized states increases to order of 100 meV in the photovaltaic system consisting of the quantum dots or the quantum layers of silicon prepared by using pulsed laser in oxygen environment. It is interesting to make a comparison with the OSL effect in the localized states under the magnetic moment of magnetic metal Fe or Co adsorbed on nanosilicon surface in the detailed simulating calculations.

Electrical control of magnetic proximity effect in a graphene/multiferroic heterostructure

The proximity effect, which offers a proper route to extend the properties of 2D materials, is of great current interest. In hybrid systems formed by graphene and multiferroic materials, effective manipulation of the proximity effect is expected through magneto-electric coupling. In this work, we report the electrical control of the magnetic proximity effect in graphene/BiFeO3 heterostructures. The obvious ferroelectric gating effect on graphene is achieved using BiFeO3 as a top gate. The interfacial magnetic exchange field has a notable dependence on the top gate voltage, giving rise to an electrical modulation on Zeeman splitting and energy gap inside N = 0 Landau level of graphene. Our findings suggest graphene/BiFeO3 heterostructures provide a broad avenue for realization of future multiferroic electronics and spintronics.

Ab-initio study of electronic properties of Si(C) honeycomb structures

In this work, the electronic structures of pure and concentrated graphene and Silicene have been studied by performing first-principles pseudo potential plane-wave calculations. The concentrated structures have been obtained by the substitution of Si(C) atoms in the graphene (silicene), respectively. Firstly, the calculations are performed for pure graphene and continued for its concentrations. The concentrated graphene is obtained by substitution of Si atoms (with: 12.5, 25, 37.5 and 50 mol percentage) at different positions in the unit cell of graphene. Similar to graphene, the same calculations are performed for pure silicene as well as for silicene after substitution of C atoms. We have modeled the lattice constant, the band structure and its directivity, while the position and mole fractions of the substituted atoms are changed in the unit cell of the studied compound. Our results showed that: the total energy, the density of States (DOS), the charge density (CD), the opening of the band gap and its directivity are strongly dependent both on the position and mole fraction of the substituted Si(C) atoms. As an interesting result, we found an indirect open band gap, as large as 2.53 eV for silicon doped graphene. Also, it was found that both the elemental concentration and unit cell geometry could offer remarkable advantages for band splitting and band gap opening in these graphene like structures, which have known as ideal structures with many promising potential applications in the electronic, optoelectronic and spintronic.

Different approach to the modeling of nonfree particle diffusion.

A new approach to the modeling of nonfree particle diffusion is presented. The approach uses a general setup based on geometric graphs (networks of curves), which means that particle diffusion in anything from arrays of barriers and pore networks to general geometric domains can be considered and that the (free random walk) central limit theorem can be generalized to cover also the nonfree case. The latter gives rise to a continuum-limit description of the diffusive motion where the effect of partially absorbing barriers is accounted for in a natural and non-Markovian way that, in contrast to the traditional approach, quantifies the absorptivity of a barrier in terms of a dimensionless parameter in the range 0 to 1. The generalized theorem gives two general analytic expressions for the continuum-limit propagator: an infinite sum of Gaussians and an infinite sum of plane waves. These expressions entail the known method-of-images and Laplace eigenfunction expansions as special cases and show how the presence of partially absorbing barriers can lead to phenomena such as line splitting and band gap formation in the plane wave wave-number spectrum.

Growth of CaAl2Se4: Co Single Crystal Thin Film for Solar Cell Development and Its Solar Cell Application

After the CuInSe₂films grown by hot-wall epitaxy method were annealed in Cu-, Se- and In-atmospheres, the point defects were investigated by using the photoluminescence (PL) experiment. In the as-grown CuInSe₂films, the free excitons corresponding to the light hole and the heavy hole have been observed, and their splitting gap was 8.2 meV. The gap is associated with the strain caused by the lattice mismatch between the substrate and the film in the heterojunction growth. By means of thermal annealing in various atmospheres, the (D˚, X) emission was observed to originate from the Vse or CUint. From this emission, the ED value of the donor impurity level is extracted to 0.1950 eV. Also, the emission between the free electrons and the acceptor holes (FA) became a dominant peak after Se atmosphere treatment, and the EA value of the acceptor level is turned out to be 0.2371 eV. Thus, the origin of the FA emission is related to Vcu or Seint. These PL results led us to confirm that CuInSe₂:Se is converted into the optical n-type. In addition, the origin of the donor-acceptor pair emissions is caused by the recombination between donors such as Vse or CUint and acceptors such as Vcu or Seint. Based on these PL results, we have schemed a new energy-level diagram of the recombination process in CuInSe₂.

Magnetic and structural correlations in [Fe(nsal2trien)] salts: the role of cation–anion interactions in the spin crossover phenomenon

Cation–anion and cation–solvent–anion interactions determine the SCO behaviour of six [FeIII(nsal2trien)] salts.

Tunable gap opening and spin polarization of two dimensional graphene/hafnene van der Waals heterostructures

Spin polarization of graphene is the current subject of intense investigation efforts. Herewith, the interfacial electronic structure of graphene/hafnene two dimensional van der Waals heterostructures with different stacking configurations are investigated by first-principles calculations. It is found that the distinct electronic distribution and spin-polarized characteristic of the vdW heterostructures come from the intensity of orbital overlap induced by diverse stacking configurations. The Pauli-exclusion principle takes effect between the hafnene and graphene, and subsequently drive interfacial charges to transfer, which results in n-doped graphene without breaking the p-electron Dirac points. Nevertheless, the orbital hybridization motivates a π-band gap opening with metallic gap states and spin polarization in graphene. Particularly, the electronic structure, spin splitting and band gap depend on the interfacial interactions, which can be tailored by strain and electric field. Field-effect transistor based on graphene/hafnene heterostructures has been proposed. These results strongly revive this novel system as a candidate for future graphene-based electronics.

Strontium ruthenate–anatase titanium dioxide heterojunctions from first-principles: Electronic structure, spin, and interface dipoles

The epitaxial integration of functional oxides with wide band gap semiconductors offers the possibility of new material systems for electronics and energy conversion applications. We use first principles to consider an epitaxial interface between the correlated metal oxide SrRuO3 and the wide band gap semiconductor TiO2, and assess energy level alignment, interfacial chemistry, and interfacial dipole formation. Due to the ferromagnetic, half-metallic character of SrRuO3, according to which only one spin is present at the Fermi level, we demonstrate the existence of a spin dependent band alignment across the interface. For two different terminations of SrRuO3, the interface is found to be rectifying with a Schottky barrier of ≈1.3–1.6 eV, in good agreement with experiment. In the minority spin, SrRuO3 exhibits a Schottky barrier alignment with TiO2 and our calculated Schottky barrier height is in excellent agreement with previous experimental measurements. For majority spin carriers, we find that SrRuO3 recovers its exchange splitting gap and bulk-like properties within a few monolayers of the interface. These results demonstrate a possible approach to achieve spin-dependent transport across a heteroepitaxial interface between a functional oxide material and a conventional wide band gap semiconductor.

First-principles study of crystal and electronic structure of rare-earth cobaltites

Using density functional theory plus self-consistent Hubbard U (DFT + Usc) calculations, we have investigated the structural and electronic properties of the rare-earth cobaltites RCoO3 (R = Pr – Lu). Our calculations show the evolution of crystal and electronic structure of the insulating low-spin RCoO3 with increasing rare-earth atomic number (decreasing ionic radius), including the invariance of the Co-O bond distance (dCo–O), the decrease of the Co-O-Co bond angle (Θ), and the increase of the crystal field splitting (ΔCF) and band gap energy (Eg). Agreement with experiment for the latter improves considerably with the use of DFT + Usc and all trends are in good agreement with the experimental data. These trends enable a direct test of prior rationalizations of the trend in spin-gap associated with the spin crossover in this series, which is found to expose significant issues with simple band based arguments. We also examine the effect of placing the rare-earth f-electrons in the core region of the pseudopotential. The effect on lattice parameters and band structure is found to be small, but distinct for the special case of PrCoO3 where some f-states populate the middle of the gap, consistent with the recent reports of unique behavior in Pr-containing cobaltites. Overall, this study establishes a foundation for future predictive studies of thermally induced spin excitations in rare-earth cobaltites and similar systems.

Systematic analysis of geometrical based unequal droplet splitting in digital microfluidics

This paper presents the thorough analysis of a new operator developed for unequal droplet splitting by geometrical modification of a conventional electrode in digital microfluidic (DMF) platforms. This operator functions in an area as small as the size of a conventional electrode divided into several smaller sub-electrodes addressable independently. Using this operator, droplets can be precisely split unequally with a high volume ratio, as well as being dispensed with a wide range of sizes from a reservoir. This operator functions without complications in the fabrication process and the need for additional external modules such as feedback control systems. To characterize the range of the applicability of this operator, the effects of the applied voltage, the gap height between the two plates, and the sub-electrode geometry on the splitting performance are studied. For high applied voltages, splitting is performed with an error close to 5% (similar to the splitting precision obtained in conventional DMF devices); whereas, for voltages close to the threshold value, the error is less than 1%. In this way the droplet size becomes fairly independent of the geometry and the gap height. The threshold splitting voltage versus gap height has also been studied and shows a linear behavior, facilitating the selection of proper voltages for high precision splitting. For the three sub-electrode patterns studied here (i.e. square, horizontal stripe, and vertical stripe), the results show that the square sub-electrodes provide higher reliability and a wider range of sizes for the split droplet as compared to the other patterns. The final part of the study illustrates that there is a linear relation between the area of the split droplets and the number of actuated sub-electrodes. This linear behavior allows for the selection of an appropriate number of the sub-electrodes to be actuated based on the desired volume of the droplet.