Band Occupation Research Articles

Scrutinizing the electronic structure, magnetic, mechanical, thermodynamic, optical and thermoelectric properties of lead-free hafnium based Hf2VP (P = Si, Sn) full Heusler alloys

The study of energy harvesting and thermoelectric materials has drawn more attention in the last several years. The great thermal stability of these materials is advantageous for thermoelectric devices, in addition to their structural capacity for showcasing and incorporating diverse novel concepts to augment the thermoelectric figure of merit. In the current study, we have predicted the physical properties of Hf2VP (P = Si, Sn) Heuslers using density functional theory in conjunction with the Boltzmann transport scheme. The strength and ductility of these materials are ascertained by simulating elastic characteristics The exchange correlation potential is handled via the modified Becke–Johnson potential (mBJ) and the generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA-PBE). Near the Fermi level, the band profiles for Hf2VSi and Hf2VSn Heuslers were found to be n-type indirect band-gap respectively. The thermodynamic stability of these materials is approved by the formation and cohesive energy. The relationships between different transport parameters are predicted using the band occupation and density of states in the post DFT treatment. Slack’s equation has identified the most significant lattice component of heat conductivity with great precision. These materials are likely to find use in the design of memory devices and future thermoelectric and energy harvesting materials due to their half-metallic nature and efficient thermoelectric parameters, such as electrical conductivity, Seebeck coefficient, thermal conductivity, power factor, and ZT.

Exploring Co2TiRE (RE = Nd and Tb) as a promising shape memory alloy using DFT methods

Our study employs self-consistent ab-initio calculations using highly precise spin-polarized density functional theory with both GGA and GGA+U approaches, coupled with the Boltzmann transport scheme. This comprehensive approach investigated the structural stability, magneto-electronic behavior, thermophysical characteristics, and thermoelectric transport properties of Co2TiRE (RE = Nd, Tb) Heusler alloys. Through structural optimizations, we confirm that these materials exhibit ferromagnetic behavior. Analysis of band occupation and density of states using GGA and GGA+U methods reveals that both compounds are metallic in both spin configurations. We evaluated key transport properties such as the Seebeck coefficient, electrical and thermal conductivity and the figure of merit to understand their potential in thermoelectric applications. Conservative estimates of the Seebeck coefficient and the figure of merit suggest promising applications in thermoelectric energy harvesting technologies. Additionally, we provide a comprehensive analysis of the thermophysical behavior, including the Debye temperature, thermal expansion, and specific heat, to assess the alloys thermodynamic stability across varying temperature and pressure conditions.

Light-Induced Polaronic Crystals in Single-Layer Transition Metal Dichalcogenides.

Light-induced ordered states can emerge in materials after irradiation with ultrafast laser pulses. However, their prediction is challenging because the inverted band occupation confounds our chemical intuition. Hence, we use a recently developed constrained density functional perturbation theory approach to systematically screen single-layer transition metal dichalcogenides (TMDs) for light-induced ordered states. We demonstrate that all examined single-layer TMDs reveal similar light-induced charge orderings. The corresponding reconstructions are periodic arrangements of polarons (polaronic crystals), characterized by triangular metal clusters and having no equivalent at equilibrium conditions. The polarons are accompanied by localized midgap states in the electronic band structure, detectable by experimental methods. We assess the selenides as the most promising candidates for potential photoexcitation experiments because they transition at low critical fluences, have low transition barriers, and maintain an open band gap under photoexcitation. Our work paves the way for innovative material design approaches targeting light-induced phases.

Measurement Campaign of Radio Frequency Interference in a Portion of the C-Band (4-5.8 GHz) for the Sardinia Radio Telescope.

Radio frequency interference (RFI) analysis is crucial for ensuring the proper functioning of a radio telescope and the quality of astronomical observations, as human-generated interference can compromise scientific data collection. The aim of this study is to present the results of an RFI measurement campaign in the frequency range of 4-5.8 GHz, a portion of the well-known C-band, for the Sardinia Radio Telescope (SRT), conducted in October-November 2023. In fact, this Italian telescope, managed by the Astronomical Observatory of Cagliari (OAC), a branch of the Italian National Institute for Astrophysics (INAF), was recently equipped with a new C-band receiver that operates from 4.2 GHz to 5.6 GHz. The measurements were carried out at three strategically chosen locations around the telescope using the INAF mobile laboratory, providing comprehensive coverage of all possible antenna pointing directions. The results revealed several sources of RFI, including emissions from radar, terrestrial and satellite communications, and wireless transmissions. Characterizing these sources and assessing their frequency band occupation are essential for understanding the impact of RFI on scientific observations. This work provides a significant contribution to astronomers who will use the SRT for scientific observations, offering a suggestion for the development of mitigation strategies and safeguarding the radio astronomical environment for future observational campaigns.

Harnessing 4f Electron Itinerancy for Integrated Dual-Band Redox Systems Boosts Lithium-Oxygen Batteries Electrocatalysis.

In-depth comprehension and manipulation of band occupation at metal centers are crucial for facilitating effective adsorption and electron transfer in lithium-oxygen battery (LOB) reactions. Rare earth elements play a unique role in band hybridization due to their deep orbitals and strong localization of 4felectrons. Herein, we anchor single Ce atoms onto CoO, constructing a highly active and stable catalyst with d-fa dual-band redox center. It is discovered that the itinerant behavior of 4felectrons introduces an enhanced spin-orbit coupling effect, which facilitates ideal σ/π bonding and flexible adsorption between the Ce/Co active sites and *O. Simultaneously, the injection of localized Ce 4felectrons strengthens the orbital bonding capacity of Co-O, effectively inhibits the dissolution of Co sites and improves the structural stability of the cathode material. Bracingly, the Ce1/CoO-based LOB exhibits an ultra-low charge-discharge polarization (0.46 V) and stable cyclic performance (1088 hours). This work breaks through the traditional limitations in catalyst activity and stability, providing new strategies and theoretical insights for developing high-performance LOBs powered by rare-earth elements.

First principles study of electronic structure of x-form phthalocyanine crystals doped with one-dimensional iodine atomic chains

The x-form phthalocyanine (Pc) crystal is composed of a square-lattice arrangement of one-dimensional and double-period molecular chains with molecular planes normal to the stacking direction, and doped iodine (I) atomic chains between these molecular chains are known to induce the insulator-metal transition. Using the van der Waals density functional method, we investigate the electronic structure of a single x-form silicon Pc (x-SiPc) chain and the x-SiPc crystal undoped and doped with I atomic chains in a comparative manner. Although a SiPc molecule has a Si pz derived orbital just above the LUMO, the aligned Si atoms in x-crystals, each of which is at the center of the molecular plane, dimerize in the stacking direction, which prevents formation of a Si metallic band. In a single SiPc chain, two molecules in each primitive unit cell are stacked face-to-face with a staggering angle of 45°. However, when these molecular chains aggregate to create x-crystals, the staggering angle deviates from 45° to about 40° to form H–H bonding orbitals like H2 molecules between neighboring molecular planes in the lateral direction. Doping of the I atomic chains converts half-filling of the doubly degenerate bands to a lower band occupancy, which corresponds to the insulator-metal transition observed experimentally. The equally spaced I atomic chains create a metallic band due to pz-orbital overlapping with an effective-mass ratio of 0.15. Although the SiPc chains operate to create equally spaced I atomic chains, the effect of I atoms trying to trimerize is larger. This trimerization prevents pz orbitals of I atoms from making a metallic band.

Local gate control of Mott metal-insulator transition in a 2D metal-organic framework

Electron-electron interactions in materials lead to exotic many-body quantum phenomena, including Mott metal-insulator transitions (MITs), magnetism, quantum spin liquids, and superconductivity. These phases depend on electronic band occupation and can be controlled via the chemical potential. Flat bands in two-dimensional (2D) and layered materials with a kagome lattice enhance electronic correlations. Although theoretically predicted, correlated-electron Mott insulating phases in monolayer 2D metal-organic frameworks (MOFs) with a kagome structure have not yet been realised experimentally. Here, we synthesise a 2D kagome MOF on a 2D insulator. Scanning tunnelling microscopy (STM) and spectroscopy reveal a MOF electronic energy gap of ∼200 meV, consistent with dynamical mean-field theory predictions of a Mott insulator. Combining template-induced (via work function variations of the substrate) and STM probe-induced gating, we locally tune the electron population of the MOF kagome bands and induce Mott MITs. These findings enable technologies based on electrostatic control of many-body quantum phases in 2D MOFs.

Spray: A Spectrum-efficient and Agile Concurrent Backscatter System

Recent works have achieved considerable success in improving the concurrency of backscatter network. However, they do not optimize the balance between throughput and spectrum occupancy, both of which serve as pivotal parameters in concurrent transmissions. Moreover, these works also introduce complex components on tag thereby increasing both power consumption and deployment costs. In this article, we propose Spray , a tag-lightweight system to achieve high throughput and narrow-band occupancy with low power. The key idea is to incorporate an agile channel allocating and scheduling mechanism into the backscatter network. This approach allows for efficient spectrum utilization and concurrency without the need for energy-intensive components. To optimize throughput in the presence of the challenge of harmonic interference, we introduce a novel algorithm that determines the channels with an optimal combination of central frequencies and bandwidths. Additionally, we propose a fair scheduling strategy to ensure equitable transmission opportunities for all tags. We prototype the Spray tag using commercial off-the-shelf components and implement the excitation and receiver with software-defined radio platform. Our evaluation shows that the system supports 30 parallel tags transmitting in the bandwidth of 600 kHz and the throughput can reach more than 280 kbps.

Reverse charge transfer and decomposition in Ca-Te compounds under high pressure.

Pressure alters the nature of chemical bonds and triggers novel reactions. Here, we employed first-principles calculations combined with the CALYPSO structural search technique to reveal the charge transfer reversal between Ca and Te under high pressure in the calcium-tellurium compound (CaxTe1-x, x = 1/4, 1/3, 1/2, 2/3). We predict several new phases with conventional and unconventional compounds and found an unfamiliar phenomenon: the Ca-Te compounds will reverse charge transfer between Ca and Te atoms and decompose into elemental solids under pressure. The Bader charge analyses indicate that the Ca2+ ion gains electrons and becomes an anion under high pressure. This leads to a weakened electrostatic interaction between Ca and Te and ultimately results in decomposition. The calculated band occupation number suggests that the occupation of Ca 3d orbitals under high pressure corresponds to this atypical phenomenon. Our results demonstrated the reverse charge transfer between Ca and Te and, in addition, clarified the mechanism of CaxTe1-x decomposition into solid Ca and Te elements under high pressure, providing important insights into the evolution of the properties of alkaline-earth chalcogenide compounds under high pressure.

DETERMINATION OF THE THRESHOLD SOLUTION FOR A CHANNEL WITH RAY-LEIG FADING WHEN PROBING THE COGNITIVE RADIO ENERGY DETECTOR SPECTRUM

Spectrum sensor system performance and detection of spectrum band occupancy in cognitive radio are among of the main research aspects. When considering a system in which the primary user changes or there are multiple types of primary users, an energy detector is used. For energy detector operation, the main parameters that determine probabilistic characteristics of detection (probability of user detection, probability of a «false alarm» error and probability of a «missing target» error) will depend on the correct determination of the decision threshold. This article reviews an analytical approach to the normalized decision threshold determination. The purpose is to analyze and determine the optimal decision threshold value for an energy detector in a channel with Rayleigh fading. A spectrum sensing system in cognitive radio is considered. Normalized values of the threshold solution for a channel with Rayleigh fading are determined. Graphic illustrations of the analysis results and calculations are presented. Results obtained for determining threshold solutions may be considered a proper approximation for calculating the characteristics of sensing systems in cognitive radio in channels with Rayleigh fading, providing the opportunity to determine frequency bands free of primary users more likely, thereby increasing the efficiency of use of the radio frequency spectrum.

Fermi-Dirac staircase occupation of Floquet bands and current rectification inside the optical gap of metals: An exact approach

We consider a model of a Bloch band subjected to an oscillating electric field and coupled to a featureless fermionic heat bath, which can be solved exactly. We demonstrate rigorously that in the limit of vanishing coupling to this bath (so that it acts as an ideal thermodynamic bath) the occupation of the Floquet band is not a simple Fermi-Dirac distribution function of the Floquet energy, but instead it becomes a ``staircase'' version of this distribution. We show that this distribution generically leads to a finite rectified electric current within the optical gap of a metal even in the limit of vanishing carrier relaxation rates, providing a rigorous demonstration that such rectification is generically possible and clarifying previous statements in the optoelectronics literature. We show that this current remains non-zero even up to the leading perturbative second order in the amplitude of electric fields, and that it approaches the standard perturbative expression of the Jerk current obtained from a simpler Boltzmann description within a relaxation time approximation when the frequencies are small compared to the bandwidth.

Assessing the quality of transmission of lightpaths in multiband C+L networks through Gaussian noise models

In an optical network scenario, wavelength division-multiplexing (WDM) channels are constantly being added and dropped, leading to dynamic traffic variations in the lightpaths. In this work, the impact of the network traffic load and spectral occupancy on the quality of transmission, namely on the normalized nonlinear interference (NLI) power, power transfer due to stimulated Raman scattering (SRS) and optical signal-to-noise ratio (OSNR) of the lightpaths in a C+L multiband optical network is assessed using the recently proposed closed-form interchannel SRS Gaussian noise model (ISRS GN-model). We show that, due to the dynamic traffic behavior, the normalized NLI power can oscillate up to 2 dB in the highest frequency channels due to NLI variations when the tested channels have unequal spacing along the spectrum. For the optimum channel launch power and by increasing the network traffic load, the power transfer between the outer channels can increase up to 5.1 dB due to the SRS effect. With 201 WDM channels, high traffic load and for the optimum channel power, we obtained a maximum OSNR variation along the channel frequencies of only about 0.7 dB. A comparison between the OSNR predictions of the closed-form ISRS GN-model and a closed-form Gaussian noise (GN) model that does not take into account the SRS effect is also performed. In all results obtained, the maximum difference between the OSNR predictions of GN (without SRS) and ISRS GN models is below 0.7 dB at optimum OSNR and maximum C+L band occupancy. For channel launch powers higher than the optimum, the OSNR differences increase up to 3 dB.

Filtering and Imaging of Frequency-Degenerate Spin Waves Using Nanopositioning of a Single-Spin Sensor.

Nitrogen-vacancy (NV) magnetometry is a new technique for imaging spin waves in magnetic materials. It detects spin waves by their microwave magnetic stray fields, which decay evanescently on the scale of the spin-wavelength. Here, we use nanoscale control of a single-NV sensor as a wavelength filter to characterize frequency-degenerate spin waves excited by a microstrip in a thin-film magnetic insulator. With the NV probe in contact with the magnet, we observe an incoherent mixture of thermal and microwave-driven spin waves. By retracting the tip, we progressively suppress the small-wavelength modes until a single coherent mode emerges from the mixture. In-contact scans at low drive power surprisingly show occupation of the entire isofrequency contour of the two-dimensional spin-wave dispersion despite our one-dimensional microstrip geometry. Our distance-tunable filter sheds light on the spin-wave band occupation under microwave excitation and opens opportunities for imaging magnon condensates and other coherent spin-wave modes.

Rashba spin splitting in two-dimensional electron gas in polar-polar perovskite oxide heterostructure LaVO3/KTaO3: A DFT investigation

Perovskite oxide surfaces and their interfaces host two-dimensional electron gas (2DEG) and many other emergent properties. Polar-polar perovskite oxide heterostructure LaVO3/KTaO3 with the n-type interface is studied using comprehensive first principle calculations. Our calculations involve crystal orientation-dependent investigation of 2DEG for two types of terminations of KTO surface slab namely asymmetric and symmetric terminations in [001] and [111] orientations. Electronic structure calculations show n-type conducting interface and the Rashba spin splitting in the bands with αR in the range 0.37−0.9eVÅ and ER in the range of 0.6−4.5eV owing to the high spin–orbit coupling of Ta and symmetry breaking electric field present in the heterostructure. The occupation of heavy bands dyz/dxz and the additional occupation of dz2 bands in [111] orientation give rise to orbital-dependent Rashba splitting. The charge density difference plots corroborate the transfer of charge from the surface to the interfacial region. Constant energy contour plots of spin texture confirm the 2D Rashba spin splitting of 2DEG in [001] orientation, while a complex spin structure is observed in [111] orientation. Thus, our findings extend the scope of perovskite-based heterostructures for future spintronic applications maneuvering the Rashba effect with a fascinating spin structure.

Spectrum Sensing Utilizing Power Threshold and Artificial Intelligence in Cognitive Radio

The purpose of this paper was to take a preventative approach to predict the transmission status of the primary user (PU) on the white spectrum throughout the duration of the simulation. We intend to reduce the computational simulation budget to eliminate user interference by precise spectrum sensing handoff. We develop PU behaviors to use a time waiting estimate methodology that gives band occupancy time intervals. The proposed strategy may also lessen risks associated with flexible clients and adapt to changing channels. Real-time applications will use ANN range sharing, which reduces transmission delay, while high-throughput applications will use PTHD range sharing. Since noise as well as obscuring effects after each transmission plan, their proximity in the channel can influence range detection. This finding is in line with published investigations. Most range detection systems compare frequency test results with the operating channel's control scope thickness. The paper met design objectives by developing a noise-independent sensing methodology with proactive time estimation. The trial is accomplished with reduced cost and latency.

Electronic origin of x-ray absorption peak shifts

Encoded in the transient x-ray absorption (XAS) and magnetic circular (MCD) response functions resides a wealth of information of the microscopic processes of ultrafast demagnetization. Employing state-of-the-art first-principles dynamical simulations we show that the experimentally observed energy shift of the ${L}_{3}$ XAS peak in Ni, and the absence of a corresponding shift in the dichroic MCD response, can be explained in terms of laser-induced changes in band occupation. Strikingly, we predict that for the same ultrashort pump pulse applied to Co the opposite effect will occur: a substantial shift upward in energy of the MCD peaks will be accompanied by very small change in the position of XAS peaks, a fact we relate to the reduced $d$-band filling of Co that allows a greater energetic range above the Fermi energy into which charge can be excited. We also carefully elucidate the dependence of this effect on pump pulse parameters. These findings (i) establish an electronic origin for early-time peak shifts in transient XAS and MCD spectroscopy and (ii) illustrate the rich information that may be extracted from transient response functions of the underlying dynamical system.

Inspecting the Thermoelectric Response and Mechanical Stability of Novel Cobalt‐Based Heusler Alloys: A DFT Insight

The potential of Heusler materials to provide interesting aspects predominantly of vital physical properties along with typical variations has extended their prospects for thermoelectric applicability. Herein, highly precise and high‐throughput density functional theory (DFT) calculations along with Boltzmann transport simulation scheme are intensely applied to deliver the numerous physical properties of two Cobalt‐based Heusler materials. The geometric structural optimization of both these alloys suitably approves the cubic crystalline geometry at elevated temperatures. The electronic band profile validates the half‐metallic behavior. The fetched density of states and band occupation reflect the ferromagnetic half‐metallic character of the materials. The calculated elastic parameters reveal that the presented set of Heusler materials is mechanically stable holding ductile nature. These cobalt materials exhibit anisotropic performance for both longitudinal and transverse velocities, so these demonstrate directional‐dependent velocities. The utmost significant lattice part of thermal conductivity is accurately figured out by the help of Slack's equation. The current exploration demonstrates spin half metallicity as well as good value of transport parameters, which opens up a route typical for numerous research areas like thermoelectrics and spintronic applicability.

Orbital magnetization of Floquet topological systems

A general expression for the orbital magnetization of a Floquet system is derived. The expression holds for a clean system and is valid for any driving protocol and arbitrary occupation of the bands. The orbital magnetization is shown to be large not only for Chern insulators, but also for anomalous phases where the Chern number does not fully account for the topology. In addition, the orbital magnetization is shown to take significant values both for a thermal equilibrium occupation of the Floquet bands and for occupations determined by a quantum quench from an initial state with zero orbital magnetization. For the latter case, the orbital magnetization is shown to be highly sensitive to van Hove singularities of the Floquet bands.

Cooperative Cache in Cognitive Radio Networks: A Heterogeneous Multi-Agent Learning Approach

Deploying distributed cache in cognitive radio networks (CRNs), which spreads popular contents to the edge of network during the off-peak time through spectrum sharing, can reduce the deliver delay to users nearby without causing severe interference to the primary network. However, due to the un-predicable contents requirement as well as the band occupation of primary users, it is non-trivial to optimize the cache storage and contents fetching strategy of users dynamically. The letter proposes a heterogeneous multi-agent deep deterministic policy gradient (MADDPG) approach, which takes users and cache servers as two different types of agents to learn the cooperation and competition for mutual benefits. The numeral simulation demonstrates that comparing with the other single or homogeneous deep reinforcement learning (DRL) approaches, the proposed heterogeneous MADDPG can further reduce the delivery delay of users and enhance the cache efficiency of SBSs.

Interplay of band occupation, localization, and polaron renormalization for electron transport in molecular crystals: Naphthalene as a case study

Understanding electronic properties and charge transport in organic semiconductors is important for improving organic electronic materials and devices. Here we investigate the impact of electronic band occupation, charge-carrier concentration, and symmetry of phonon modes on the electron mobility in naphthalene crystals for various temperatures. Our theoretical approach is based on the description of the electron-phonon coupling (EPC), where the coupling to low-frequency modes is treated by an effective vibrational disorder potential with local and nonlocal contributions and the coupling to high-frequency modes is included by a polaron treatment. Surprisingly, the coupling to high-frequency modes leads to an increase in the mobility in presence of the low-frequency modes, which is explained by localization and band occupation effects that further depend on the carrier density. A symmetry analysis sheds additional light on the energy dependence of the EPC, which is important to describe transport properties as a function of charge density and temperature. We also find that coupling to low-frequency phonons together with band occupation effects can lead to a vanishing slope of the mobility versus temperature that is known from experiments.