Width Of Conduction Band Research Articles

Electron injection into superconducting trivalent fullerides close to the Mott transition boundary

To date, superconductivity in fullerides has been almost exclusively tuned by (chemical or physical) pressure control of the conduction bandwidth, W at half filling. This contrasts sharply with the extensive control of the superconducting transition temperature, [Formula: see text] in atom-based superconductors such as the cuprates and iron pnictides and chalcogenides via changes in valence (bandfilling). Here, we investigate the effect of doping away from the exactly half-filled [Formula: see text] level in quaternary face-centered-cubic (fcc) — structured fulleride solids with nominal composition ([Formula: see text]Csx)[Formula: see text][Formula: see text] ([Formula: see text]), in which divalent [Formula: see text] ions partially replace monovalent alkali [Formula: see text]/[Formula: see text] ions. The resulting charged-modified fullerides in which the [Formula: see text] bandwidth is also varied with changing x show a dome-shaped dependence of [Formula: see text] on interfullerene separation in analogy with their half-filled antecedents. However, following electron injection beyond half-filling, the superconductivity dome is found to shift towards shorter interfullerene separations, i.e. towards increased conduction bandwidths.

Promotion of nitrate reduction reaction activity by Co3O4@MoS2 Particle-Support system

Electrochemical reduction of nitrate to NH3 is a very promising alternative reaction to the Haber-Bosch process, and it is necessary to develop the efficient electrocatalysts. In this study, a particle-support mode Co3O4 catalyst was synthesized with ZIF-67 as the precursor, and then dispersed on MoS2 nanoflowers by hydrothermal method. The Co3O4 is anchored to MoS2 by forming Co-S coordination bond. Furthermore, the particle-supported Co3O4 exhibits better performance than Co3O4 alone, as is manifested by higher Faradaic efficiencies and NH3 yield rate at − 0.64 V (52.69% vs 32.03%; 4539.61 μg h−1 mg−1cat vs 2048.63 μg h−1 mg−1cat), lower energy barriers (0.96 eV vs 1.19 eV), and better electronic conductivity (Bandwidth: 0.581 eV vs 0.613 eV). In addition, this research provides an effective solution to solve the aggregation problem of metal oxide nanoparticles.

Realizing the Lowest Bandgap and Exciton Binding Energy in a Two-Dimensional Lead Halide System.

Finding stable analogues of three-dimensional (3D) lead halide perovskites has motivated the exploration of an ever-expanding repertoire of two-dimensional (2D) counterparts. However, the bandgap and exciton binding energy in these 2D systems are generally considerably higher than those in 3D analogues due to size and dielectric confinement. Such quantum confinements are most prominently manifested in the extreme 2D realization in (A)mPbI4 (m = 1 or 2) series of compounds with a single inorganic layer repeat unit. Here, we explore a new A-site cation, 4,4'-azopyridine (APD), whose size and hydrogen bonding properties endow the corresponding (APD)PbI4 2D compound with the lowest bandgap and exciton binding energy of all such compounds, 2.19 eV and 48 meV, respectively. (APD)PbI4 presents the first example of the ideal Pb-I-Pb bond angle of 180°, maximizing the valence and conduction bandwidths and minimizing the electron and hole effective masses. These effects coupled with a significant increase in the dielectric constant provide an explanation for the unique bandgap and exciton binding energies in this system. Our theoretical results further reveal that the requirement of optimizing the hydrogen bonding interactions between the organic and the inorganic units provides the driving force for achieving the structural uniqueness and the associated optoelectronic properties in this system. Our preliminary investigations in characterizing photovoltaic solar cells in the presence of APD show encouraging improvements in performances and stability.

Orbital Engineering in Sillén-Aurivillius Phase Bismuth Oxyiodide Photocatalysts through Interlayer Interactions.

Multicomponent inorganic compounds containing post-transition-metal cations such as Sn, Pb, and Bi are a promising class of photocatalysts, but their structure-property relationships remain difficult to decipher. Here, we report three novel bismuth-based layered oxyiodides, the Sillén-Aurivillius phase Bi4NbO8I, Bi5BaTi3O14I, and Bi6NbWO14I. We show that the interlayer Bi-Bi interaction is a key to controlling the electronic structure. The replacement of the halide layer from Cl to I negatively shifts not only the valence band but also the conduction band, thus providing lower electron affinity without sacrificing photoabsorption. The suppressed interlayer chemical interaction between the 6p orbitals of the Bi lone-pair cations reduces the conduction bandwidth. These oxyiodides have narrower band gaps and show much higher water oxidation activities under visible light than their chloride counterparts. The design strategy has not only provided three novel Bi-based photocatalysts for water splitting but also offers a pathway to control the optoelectronic properties of a wider class of lone-pair (ns2np0) semiconductors.

Coherent heavy charge carriers in an organic conductor near the bandwidth-controlled Mott transition

The physics of the Mott metal-insulator transition (MIT) has attracted huge interest in the last decades. However, despite broad efforts, some key theoretical predictions are still lacking experimental confirmation. In particular, it is not clear whether the large coherent Fermi surface survives in immediate proximity to the bandwidth-controlled first-order MIT. A quantitative experimental verification of the predicted behavior of the quasiparticle effective mass, renormalized by many-body interactions, is also missing. Here we address these issues by employing organic $\kappa$-type salts as exemplary quasi-two-dimensional bandwidth-controlled Mott insulators and gaining direct access to their charge carrier properties via magnetic quantum oscillations. We trace the evolution of the effective cyclotron mass as the conduction bandwidth is tuned very close to the MIT by means of precisely controlled external pressure. We find that the sensitivity of the mass renormalization to tiny changes of the bandwidth is significantly stronger than theoretically predicted and is even further enhanced upon entering the transition region where the metallic and insulating phases coexist. On the other hand, even at its very edge of stability the metallic ground state preserves a large coherent Fermi surface with no significant enhancement of scattering.

Understanding the structural and optical characteristics in Sr2xLa2-2xLi2xTe2-2xO6 perovskites

Perovskites of the type Sr2xLa2-2xLi2xTe2-2xO6; x = 0.25, 0.4, 0.5, 0.6, 0.75 were analyzed for the first time for their structural and optical characteristics. All the compounds were found to be monoclinic with the P21/n structure thereby indicating a double perovskite structure formation. The x = 0.5 composition with Li+ and Te6+ was found to represent an ordered double perovskite having the least B-site disorder while x = 0.4 and x = 0.6 were found to have the highest disorder based on the refined occupancies. The tolerance factor calculations indicated that the symmetry is lowered on account of the octahedral tilting and Li-O-Te bending in making room for any divergence in symmetry that may arise due to a change to monoclinic from cubic structure. A greater tilting of the octahedral were observed in all compositions except x = 0.5 in terms of the tilting angle resulting in a drop in the bond angle of Li-O-Te. FTIR measurements indicate the existence of symmetric and antisymmetric stretching vibrations of the Li(Te)O6 octahedra. Energy band gap measurements indicated a depletion in the conduction band width and an increase in the energy gap. The ability for conducting is found to be the best in x = 0.25 and x = 0.6 as these compositions have the least value of energy band gap. This behavior can be accounted for by the possible presence of polarons in these compositions. Also, an increase in the energy band gap is observed when going from x = 0.25 to x = 0.5 due to an increase in cell volume for these compositions. SEM and EDX measurements carried out for x = 0.25, 0.5 and 0.75 indicate rough and agglomerated particles with a near uniform distribution in terms of their shape and the presence of elements as per the stoichiometric formula.

Real-space numerical renormalization group computation of transport properties in side-coupled geometry

A real-space formulation of the numerical renormalization group (NRG) procedure is introduced. The real-space construction, dubbed eNRG, is more straightforward than the NRG discretization, and allows a faithful description of the coupling between quantum dots and conduction states even if the design of the couplings is intricate. General features of the two procedures are discussed comparatively. A more specific comparison is then developed, based on computations of the zero-bias transport properties for an Anderson-model description of a quantum wire side-coupled to a single quantum dot. An eNRG computation is shown to reproduce accurately the temperature-dependent electrical conductance for the uncorrelated model in the continuum limit, while significant deviations mark the corresponding NRG computation for thermal energies comparable to the conduction bandwidth. A combination of analytical and numerical results for the transport properties of the correlated model then provides a more exacting check on the accuracy of the eNRG procedure. A recent NRG analysis mapping the transport properties of a single-electron transistor onto universal functions of the temperature scaled by the Kondo temperature is extended to the side-coupled device, on the basis of eNRG reasoning. Numerical results for the electrical conductance, thermopower, and thermal conductance in side-coupled geometry are then shown to agree very well with the mappings. The numerical results are also checked against the thermal dependence of the thermopower measured by K\"ohler et al. [Phys. Rev. B 77, 104412 (2008)] in ${\mathrm{Lu}}_{0.9}^{\phantom{\ifmmode\dagger\else\textdagger\fi{}}}{\mathrm{Yb}}_{0.1}^{\phantom{\ifmmode\dagger\else\textdagger\fi{}}}{\mathrm{Rh}}_{2}^{\phantom{\ifmmode\dagger\else\textdagger\fi{}}}{\mathrm{Si}}_{2}^{\phantom{\ifmmode\dagger\else\textdagger\fi{}}}$, and the remarkably accurate, recent conductance measurements by Xu et al. [Chin. Phys. Lett. 38, 087101 (2021)].

120 MeV Ag[formula omitted] induced modifications in the structural, electrical and optical properties of La-doped SrSnO[formula omitted] thin films

Effect of 120 MeV Ag-ion irradiation at a fluence of 2 × 1012 ions/cm2 was studied on La-doped SrSnO3 thin films. The structural study revealed grain growth with irradiation caused by local crystallization in the films due to the annealing effect below the threshold value of the electronic energy loss. The spectral profile and a shift in the absorption edge of X-ray absorption near-edge spectra suggested the oxygen vacancies and conduction band broadening in the films. The electrical transport properties revealed that the irradiation decreased the resistivity of the films while the mobility increased from 228 cm2(Vs)−1 for the pristine film to 244 cm2(Vs)−1. The enhancement in the conduction bandwidth, grain growth, and carrier hopping through Sn2+ is explained based on the transport results. The irradiated films are highly transparent to the visible spectrum region with transmittance above 95%.

Chiral Bloch states in single-layer graphene with Rashba spin-orbit coupling: Equilibrium spin current

We study the Bloch spectrum and spin physics of 2D massless Dirac electrons in single layer graphene subject to a one dimensional periodic Kronig-Penney potential and Rashba spin-orbit coupling. The Klein paradox exposes novel features in the band dispersion and in graphene spintronics. In particular it is shown that: (1) The Bloch energy dispersion $\veps(p)$ has unusual structure: There are {\it two Dirac points} at Bloch momenta $\pm p \ne 0$ and a narrow band emerges between the wide valence and conduction bands. (2) The charge current and the spin density vector vanish. (3) Yet, all the non-diagonal elements of the spin current density tensor are finite and their magnitude increases linearly with the spin-orbit strength. In particular, there is a spin density current whose polarization is perpendicular to the graphene plane. (4) The spin density currents are space-dependent, hence their continuity equation includes a finite spin torque density.

Correlated electronic structure and optical response of rare-earth based semiconductors

The coexistence of Mott localized $f$ states with wide conduction and valence bands in $f$-electron semiconductors results, quite generically, in a complex optical response with the nature of the absorption edge difficult to resolve both experimentally and theoretically. Here, we combine a dynamical mean-field theory approach to localized $4f$ shells with an improved description of band gaps by a semilocal exchange-correlation potential to calculate the optical properties of the light rare-earth fluorosulfides $Ln\mathrm{SF}$ ($Ln=\mathrm{Pr}$, Nd, Sm, Gd) from first principles. In agreement with experiment, we find the absorption edge in SmSF to stem from $\mathrm{S}\text{\ensuremath{-}}3p$ to $\mathrm{Sm}\text{\ensuremath{-}}4f$ transitions, while the Gd compound behaves as an ordinary $p\text{\ensuremath{-}}d$ gap semiconductor. In the unexplored PrSF and NdSF systems we predict a rather unique occurrence of strongly hybridized $4f\text{\ensuremath{-}}5d$ states at the bottom of the conduction band. The nature of the absorption edge results in a characteristic anisotropy of the optical conductivity in each system, which may be used as a fingerprint of the relative energetic positions of different states.

Control and readout of a superconducting qubit using a photonic link.

Delivering on the revolutionary promise of a universal quantum computer will require processors with millions of quantum bits (qubits)1-3. In superconducting quantum processors4, each qubit is individually addressed with microwave signal lines that connect room-temperature electronics to the cryogenic environment of the quantum circuit. The complexity and heat load associated with the multiple coaxial lines per qubit limits the maximum possible size of a processor to a few thousand qubits5. Here we introduce a photonic link using an optical fibre to guide modulated laser light from room temperature to a cryogenic photodetector6, capable of delivering shot-noise-limited microwave signals directly at millikelvin temperatures. By demonstrating high-fidelity control and readout of a superconducting qubit, we show that this photonic link can meet the stringent requirements of superconducting quantum information processing7. Leveraging the low thermal conductivity and large intrinsic bandwidth of optical fibre enables the efficient and massively multiplexed delivery of coherent microwave control pulses, providing a path towards a million-qubit universal quantum computer.

Optoelectronic properties of diketopyrrolopyrrole homopolymers compared to donor-acceptor copolymers.

Diketopyrrolopyrrole (DPP) is a component of a large number of materials used for optoelectronic applications. As it is exclusively used in combination with aromatic donors, the properties of its homopolymers are unknown. Because donor-acceptor character has been shown for other systems to reduce bandwidths, DPP homopolymers should have even larger conduction bands and better n-type conductivity than the thiophene-flanked systems, which have exceptional n-type conductivity and ambipolar character. Therefore, a theoretical study was carried out to elucidate the properties of the unknown DPP homopolymer. Calculations were done with density functional theory and with the complete active space self-consistent field method plus n-electron valence state perturbation theory for the dynamic correlation. Poly-DPP is predicted to have radical character and an extremely wide low-lying conduction band. If it were possible to produce this material, it should have unprecedented n-type conductivity and might be a synthetic metal. A comparison with various unknown donor-acceptor systems containing vinyl groups and thienyl rings with a higher concentration of DPP than the known copolymers reveals how donor-acceptor substitution reduces bandwidths and decreases electron affinities.

Microscopic Study of Two Band Superconductivity in Magnesium Diboride Superconductor (MgB<sub>2</sub>)

We formulate a Model Hamiltonian of two band superconductivity for Magnesium Diboride superconductors (MgB₂). It is a conventional BCS type metallic superconductor which has the highest critical temperature T_c=39K. It is assumed that the superconductivity in MgB₂ arises due to metallic nature of the 2D sheets. From band structure calculations, it is observed that two types of bands i.e. σ and π bands are located at Fermi surface. Here, we consider phonon mediated superconductivity in which σ band is dominant over π band i.e. σ band is more coupled to a superconductor with much higher coupling. We consider a model Hamiltonian with mean field approach and solve this by calculating equations of motion of Green functions for a single particle. We determine the quasi-particle energy from the poles of the Green functions. We derive the single particle correlation functions and determine the two SC order parameters for both σ and π band. Here, the two SC order parameters for the bands are solved self- consistently and numerically. The conduction bandwidth (W) is considered as W=8t₀, where t₀ is the hopping integral. To make all the physical quantities dimensionless, we divide 2t₀ in each of the physical quantities. We then calculate the gap ratio 2∆(0)/K<SUB>B</SUB>T_c for both the bands. It is seen form our theoretical model that the two bands of MgB₂ superconductors have two different SC gaps with the same critical temperature. We also observe the variation of dispersion curves of quasi-particles for different temperature parameters for both σ and π band.

Origin of Metallic Nature of Na3N.

Among inorganic clathrates, the inner cavity space rarely affects the electronic structure of the framework. We report that the anti-ReO3-type compound Na3N has a metallic nature irrespective of the stoichiometric chemical composition of simple representative elements and that this unusual nature originates from the collapse of the bandgap owing to the presence of a crystallographic cavity. We synthesized Na3N by the plasma-assisted nitridation of alkali metals, and diffuse reflectance measurements indicated a metallic nature. The introduction of nitrogen into the Na metal induced the formation of both the Na+ ion and the crystallographic cavity. The former increased the density of the lattice of Na+ ions to form a wide Na 3s conduction band. The latter interacted with the Na 3s band to enhance the bandwidth, resulting in the collapse of the bandgap. Na3N is a unique nitride, which possesses an electronically active cavity space.

High refractive index and extreme biaxial optical anisotropy of rhenium diselenide for applications in all-dielectric nanophotonics

Abstract We establish a simple quantitative criterium for the search of new dielectric materials with high values of refractive index in the visible range. It is demonstrated, that for light frequencies below the bandgap, the latter is determined by the dimensionless parameter η calculated as the ratio of the sum of the widths of conduction and valence bands and the bandgap. Small values of this parameter, which can be achieved in materials with almost flat bands, lead to dramatic increase of the refractive index. We illustrate this rule with a particular example of rhenium dichalcogenides, for which we perform ab initio calculations of the band structure and optical susceptibility and predict the values of the refractive index n &gt; 5 $n{ &gt;}5$ in a wide frequency range around 1 eV together with comparatively low losses. Our findings open new perspectives in search for the new high-index/low-loss materials for all-dielectric nanophotonics.

STUDY ELECTROMAGNETIC WAVE INTERACTION OF ACTIVE-MATRIX THIN FILM TRANSISTORS

Active-matrix thin film transistors (TFTs) on glass substrates with a metal backplane, that are applied for flat panel displays, can be considered as a metamaterial absorber. In this study, TFT structures using doped silicon at source, drain, and channel terminals are investigated. These terminals are unchanged in size of 75 µm square and thickness of 5.3 µm. The electric conductivity is varied at the channel. The simulation results show that the structures with 500 S\m electric conductivity channels absorb incident electromagnetic waves with appropriately 100% at 758 GHz and a wide bandwidth of 20 GHz. As the electrical conductivity increases, the absorption and bandwidth are smaller at the main resonance peak. As the electrical conductivity decreases, the absorption falls at the resonance frequency, but the bandwidth is broadened. In addition, the electric field in the channel may influence the electron in the semiconductor and the electrical current between the source and drain terminals. By observing the electric field at the resonance frequency, we found that it is focused on the sides of channel terminals.

Electromagnetic shielding property of laminated carbon fiber tape reinforced thermoplastics

ABSTRACT Carbon fiber reinforced thermoplastic (CFRTP) has a good electromagnetic shielding property due to its higher conductivity and broad absorption frequency bandwidth while presenting high specific strength and stiffness, their easy process-ability, cost-effectiveness, and recyclability. In the present study, laminated CFRTP made of carbon fiber-polyamide 6 unidirectional tape (UD tape) were fabricated with different laminate configurations and arrangements (unidirectional, bidirectional, and quasi–isotropic) and then were compared with randomly chopped-tape CFRTP. Thereafter, estimated electromagnetic interference shielding effectiveness (EMI-SE) using Simon formalism and flexural properties of CFRTP were evaluated. The result showed a remarkable total EMI-SE of 31–44 dB in the Ultra High Frequency (UHF) range and strongly correlated with the laminate configuration of CFRTP. Whereas, randomly chopped-tape CFRTP has a lower total EMI-SE of 23–27 dB in the same frequency range. In addition, the flexural test result showed the flexural strength and modulus are strongly influenced by the tape layer configuration in the laminated CFRTP. Moreover, microscopy analysis was also conducted to verify the interlaminar structure and fiber-to-fiber contact in the composite. In conclusion, laminated CFRTP made of UD tape can be considered as electromagnetic interference shielding material for both functional and structural applications.

Topological nonmagnetic impurity states in topological Kondo insulators

We examine the presence of nonmagnetic impurities in a hybridization gap model of a Kondo insulator which has band inversion. The model has been used to predict that ${\mathrm{SmB}}_{6}$ is a topological insulator. We show that there are two types of nonmagnetic impurity states in a Kondo insulator. The type of states can be categorized as deep impurity states, similar to the impurity states in an ordinary metal, and impurity states which have energies within the hybridization gap. Unlike the deep impurity states which only form if the impurity potential exceeds a critical value of the order of the conduction bandwidth, the in-gap impurity states form for exceptionally small values of the impurity potential comparable to the hybridization gap. This result may explain why Kondo insulators are found to be exceptionally sensitive to impurities. We show that these in-gap states are caused by band inversion and have properties similar to those expected for impurities in a topological insulator.

MAC Protocol for Underwater Sensor Networks Using EM Wave With TDMA Based Control Channel

Underwater communication (UWC) growing over the past few decades is the major tool for exploring the vast underwater space. Research and development of UWC networks are dominantly acoustic (Ac) wave based due to its long coverage capability. However, the demand of low latency and high throughput applications, and the emergence of short-range UWCs in modern times has drawn attention towards the development of electromagnetic (EM) wave based underwater sensor networks (UWSNs). In light of this, this paper proposes novel medium access control (MAC) protocols by integrating EM-based communications in UWSNs. Proposed protocols use one TDMA-based control channel and one or multiple data channel(s) for control and data packet transmissions respectively. The control channel uses EM wave, while the data channels use either EM or Ac wave for communications. Proposed MAC protocols are developed considering both protocol interference model and physical interference model by taking both propagation delay and underwater channel impairments into account. Performance of the protocols is evaluated through extensive simulations in terms of throughput, packet collision rate, waiting time, energy requirement and network coverage. Impact of system parameters including offered load, network size, data packet size, control packet power, water conductivity and channel bandwidth are analyzed thoroughly. A comprehensive feasibility study by comparing the proposed MAC protocols with the traditional protocols as well as with the full Ac-wave based counterpart is also presented for further validations.

Charge‐Transport Properties of F6TNAP‐Based Charge‐Transfer Cocrystals

AbstractThe crystal structures of the charge‐transfer (CT) cocrystals formed by the π‐electron acceptor 1,3,4,5,7,8‐hexafluoro‐11,11,12,12‐tetracyanonaphtho‐2,6‐quinodimethane (F6TNAP) with the planar π‐electron‐donor molecules triphenylene (TP), benzo[b]benzo[4,5]thieno[2,3‐d]thiophene (BTBT), benzo[1,2‐b:4,5‐b′]dithiophene (BDT), pyrene (PY), anthracene (ANT), and carbazole (CBZ) have been determined using single‐crystal X‐ray diffraction (SCXRD), along with those of two polymorphs of F6TNAP. All six cocrystals exhibit 1:1 donor/acceptor stoichiometry and adopt mixed‐stacking motifs. Cocrystals based on BTBT and CBZ π‐electron donor molecules exhibit brickwork packing, while the other four CT cocrystals show herringbone‐type crystal packing. Infrared spectroscopy, molecular geometries determined by SCXRD, and electronic structure calculations indicate that the extent of ground‐state CT in each cocrystal is small. Density functional theory calculations predict large conduction bandwidths and, consequently, low effective masses for electrons for all six CT cocrystals, while the TP‐, BDT‐, and PY‐based cocrystals are also predicted to have large valence bandwidths and low effective masses for holes. Charge‐carrier mobility values are obtained from space‐charge limited current (SCLC) measurements and field‐effect transistor measurements, with values exceeding 1 cm2 V−1 s1 being estimated from SCLC measurements for BTBT:F6TNAP and CBZ:F6TNAP cocrystals.