Band Occupation Research Articles

Nonequilibrium band occupation and optical response of gold after ultrafast XUV excitation

Free electron lasers offer unique properties to study matter in states far from equilibrium as they combine short pulses with a large range of photon energies. In particular, the possibility to excite core states drives new relaxation pathways that, in turn, also change the properties of the optically and chemically active electrons. Here, we present a theoretical model for the dynamics of the nonequilibrium occupation of the different energy bands in solid gold driven by exciting deep core states. The resulting optical response is in excellent agreement with recent measurements and, combined with our model, provides a quantitative benchmark for the description of electron–phonon coupling in strongly driven gold. Focusing on sub-picosecond time scales, we find essential differences between the dynamics induced by XUV and visible light.

Molecular-Level Insights into the Notorious CO Poisoning of Platinum Catalyst.

Carbon monoxide (CO) is notorious for its strong adsorption to poison platinum group metal catalysts in the chemical industry. Here, we conceptually distinguish and quantify the effects of the occupancy and energy of d electrons, emerging as the two vital factors in d-band theory, for CO poisoning of Pt nanocatalysts. The stepwise defunctionalization of carbon support is adopted to fine-tune the 5d electronic structure of supported Pt nanoparticles. Excluding other promotional mechanisms, the increase of Pt 5d band energy strengthens the competitive adsorption of hydrogen against CO for the preferential oxidation of CO, affording the scaling relationship between Pt 5d band energy and CO/H2 adsorption energy difference. The decrease of Pt 5d band occupancy lowers CO site coverage to promote its association with oxygen for the total oxidation of CO, giving the scaling relationship between Pt 5d occupancy and activation energy. The above insights outline a molecular-level understanding of CO poisoning.

Molecular‐Level Insights into the Notorious CO Poisoning of Platinum Catalyst

AbstractCarbon monoxide (CO) is notorious for its strong adsorption to poison platinum group metal catalysts in the chemical industry. Here, we conceptually distinguish and quantify the effects of the occupancy and energy of d electrons, emerging as the two vital factors in d‐band theory, for CO poisoning of Pt nanocatalysts. The stepwise defunctionalization of carbon support is adopted to fine‐tune the 5d electronic structure of supported Pt nanoparticles. Excluding other promotional mechanisms, the increase of Pt 5d band energy strengthens the competitive adsorption of hydrogen against CO for the preferential oxidation of CO, affording the scaling relationship between Pt 5d band energy and CO/H2 adsorption energy difference. The decrease of Pt 5d band occupancy lowers CO site coverage to promote its association with oxygen for the total oxidation of CO, giving the scaling relationship between Pt 5d occupancy and activation energy. The above insights outline a molecular‐level understanding of CO poisoning.

Comprehensive study of ferromagnetic MgNd2X4 (X = S, Se) spinels for spintronic and solar cells device applications

Full potential LAPW + lo method is used for exploring electronic, structural and thermoelectric properties for MgNd2X4 (X = S, Se) spinels that are found to show ferromagnetic-semiconductor behaviour in the spinel structure. The investigated negative value of formation energy and positive value of phonon spectra computed using PBEsol GGA indicates the energetic and dynamical stability of the studied cubic ferromagnetic-semiconductors. We have used TB-mBJ potential functional for electronic and magnetic properties, which lead to a reliable account of electronic structure, demonstrating band occupancy in the spinels along with a clear explanation of density of states. The stability of ferromagnetic state in the studied materials is because of the exchange splitting of Nd cations based on p-d hybridization which is in accordance with the results obtained for electronic band structure and density of states. The exchange splitting of bands can be justified by the spin magnetic moment between anions and cations, and sharing of charge. The computed values of dielectric constant and their associated optical parameters are used to explain the optical active behaviour of the spinels under investigation; indicating that the two spinels studied in the present work are suitable for solar cells device applications. The calculation of thermoelectric properties is very useful for determining a material's potential use in waste energy recovery systems and many other innovative applications.

Nonlinear Ballistic Response of Quantum Spin Hall Edge States.

Topological edge states (TES) exhibit dissipationless transport, yet their dispersion has never been probed. Here we show that the nonlinear electrical response of ballistic TES ascertains the presence of symmetry breaking terms, such as deviations from nonlinearity and tilted spin quantization axes. The nonlinear response stems from discontinuities in the band occupation on either side of a Zeeman gap, and its direction is set by the spin orientation with respect to the Zeeman field. We determine the edge dispersion for several classes of TES and discuss experimental measurement.

Experimental methods in chemical engineering:X‐ray absorption spectroscopy—XAS,XANES,EXAFS

AbstractAlthough X‐ray absorption spectroscopy (XAS) was conceived in the early 20th century, it took 60 years after the advent of synchrotrons for researchers to exploit its tremendous potential. Counterintuitively, researchers are now developing bench type polychromatic X‐ray sources that are less brilliant to measure catalyst stability and work with toxic substances. XAS measures the absorption spectra of electrons that X‐rays eject from the tightly bound core electrons to the continuum. The spectrum from 10 to 150 eV (kinetic energy of the photoelectrons) above the chemical potential—binding energy of core electrons—identifies oxidation state and band occupancy (X‐ray absorption near edge structure, XANES), while higher energies in the spectrum relate to local atomic structure like coordination number and distance, Debye‐Waller factor, and inner potential correction (extended X‐ray absorption fine structure, EXAFS). Combining XAS with complementary spectroscopic techniques like Raman, Fourier transform infrared (FTIR), X‐ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) elucidates the nature of the chemical bonds at the catalyst surface to better understand reaction mechanisms and intermediates. Because synchrotrons continue to be the light source of choice for most researchers, the number of articles Web of Science indexes per year has grown from 1000 in 1991 to 1700 in 2020. Material scientists and physical chemists publish an order of magnitude articles more than chemical engineers. Based on a bibliometric analysis, the research comprises five clusters centred around: electronic and optical properties, oxidation and hydrogenation catalysis, complementary analytical techniques like FTIR, nanoparticles and electrocatalysis, and iron, metals, and complexes.

Role of Ferroelectricity, Delocalization, and Occupancy of d States in the Electrical Control of Interface-Induced Magnetization

In this study, the influence of generalized-gradient-approximation (GGA) deficiencies, $d$-band occupancy, and itinerancy in the magnetoelectric (ME) effect is investigated from first principles by changing the interface composition of the ${\mathrm{Fe}}_{3}\mathrm{Si/}M{/\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_{3}$ heterostructure, where $M$ stands for an atomic layer of pure $3d$ or $4d$ metals (from Sc to $\mathrm{Cd}$). The lattice overestimation of the ferroelectric phase leads to the overestimation of the magnetoelectricity, which can be partially corrected by using the Perdew-Burke-Ernzerhof functional revised for solids. The inclusion of Hubbard-like corrections generally predicts larger changes of interface magnetization upon the reversal of polarization direction, although preserving the trends of plain GGA calculations. The itinerancy of $4d$ states does not favor the interface ME coupling due to the lower density of states near the Fermi level. Here, it is shown how the control of the $d$-state energy levels through their electronic occupancy has the potential to substantially enhance the interface ME effect induced by bonding effects. A substantial increase of magnetoelectricity in the ${\mathrm{Fe}}_{3}\mathrm{Si/}{\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_{3}$ heterostructure can be achieved by replacing interface $\mathrm{Fe}$ atoms with V or $\mathrm{Mn}$.

Atomic Line Defects and Topological Superconductivity in Unconventional Superconductors

Topological superconductors (TSCs) are correlated quantum states with simultaneous off-diagonal long-range order and nontrivial topological invariants. They produce gapless or zero energy boundary excitations, including Majorana zero modes and chiral Majorana edge states with topologically protected phase coherence essential for fault-tolerant quantum computing. Candidate TSCs are very rare in nature. Here, we propose a novel route toward emergent quasi-one-dimensional (1D) TSCs in naturally embedded quantum structures such as atomic line defects in unconventional spin-singlet $s$-wave and $d$-wave superconductors. We show that inversion symmetry breaking and charge transfer due to the missing atoms lead to the occupation of incipient impurity bands and mixed parity spin singlet and triplet Cooper pairing of neighboring electrons traversing the line defect. Nontrivial topological invariants arise and occupy a large part of the parameter space, including the time reversal symmetry breaking Zeeman coupling due to applied magnetic field or defect-induced magnetism, creating TSCs in different topological classes with robust Majorana zero modes at both ends of the line defect. Beyond providing a novel mechanism for the recent discovery of zero-energy bound states at both ends of an atomic line defect in monolayer Fe(Te,Se) superconductors, the findings pave the way for new material realizations of the simplest and most robust 1D TSCs using embedded quantum structures in unconventional superconductors with large pairing energy gaps and high transition temperatures.

Ab initio study of ultrafast charge dynamics in graphene

Monolayer graphene provides an ideal material to explore one of the fundamental light-field driven interference effects: Landau-Zener-St\"uckelberg interference. However, direct observation of the resulting interference patterns in momentum space has not proven possible, with Landau-Zener-St\"uckelberg interference observed only indirectly through optically induced residual currents. Here we show that the transient electron momentum density (EMD), an object that can easily be obtained in experiment, provides an excellent description of momentum resolved charge excitation. We employ state-of-the-art time-dependent density function theory calculations, demonstrating by direct comparison of EMD with conduction band occupancy, obtained from projecting the time propagated wavefunction onto the ground state, that the two quantities are in excellent agreement. For even the most intense laser pulses we find that the electron dynamics to be almost completely dominated by the $\pi$-band, with transitions to other bands strongly suppressed. Simple model based tight-binding approaches can thus be expected to provide an excellent description for the laser induced electron dynamics in graphene.

Strain effects on band structure and Dirac nodal-line morphology of ZrSiSe

The Dirac nodal-line semimetals are new promising materials for technological applications due to their exotic properties, which originate from band structure dispersion and nodal-line behavior. We report strain effects on the band structure of ZrSiSe Dirac nodal-line semimetal through the density functional theory calculations. We found that the kz=0 Dirac nodal-line of ZrSiSe is robust to all strains under reasonable magnitude although there are significant changes in the band oscillation amplitude, bandgap, and band occupancy due to orbital interactions and the Fermi energy shift upon strains. We also found that the effective strains to tune the nodal-line and band structure are equi-biaxial tensile, uniaxial (100) tensile, and xz-plane shear strains.

Layered autonomous TSCH scheduler for minimal band occupancy with bounded latency

Robustness and bounded latency are among the key requirements in wireless industrial networks. A typical traffic patterns is convergecast, where sensors sends data towards a sink, resulting in a funneling effect with increased traffic intensity. This letter proposes the Layered autonomous Time Slotted Channel Hopping (TSCH) scheduler for convergecast traffic, which addresses the funneling effect while reducing the band occupancy. It divides the slotframe into layers where all nodes have one timeslot reserved for forwarding its own traffic. Layers are organized to allow spatially reuse such that traffic belonging to a node may be forwarded at multiple hops simultaneously. This is achieved autonomously by exploiting a node's knowledge of the routing topology. We evaluate the Layered scheduler through theoretical analysis and simulations in Cooja. Results show that Layered attains bounded latency even when nodes are utilizing all their available resources. This is achieved with significantly reduced band occupancy compared to Escalator, an autonomous scheduler for convergecast traffic. The performance is traded‐off by increased maximum latency.

Robustness in ferromagnetic phase stability, half‐metallic behavior and transport properties of cobalt‐based full‐Heuslers compounds: A first principles approach

AbstractHeusler alloys fill up the demanding gaps and becomes a point of prestige due to their interesting multi‐dimensional and remarkable applications in future technologies like spintronics and thermoelectrics. Self‐consistent ab‐initio calculations with highly precise spin‐polarized density functional theory associated with Boltzmann transport scheme have been perform to investigate the structural stability, mechanical, magneto‐electronic, thermo‐physical and thermoelectric transport properties of Co2VX (X = Sn, Sb) Heuslers. The equilibrium lattice parameters at the cost of structural optimizations are observed to agree with the available experimental data. The occurrence of perfect band occupation and interpretation of density of states through the modified version of Becke Johnson scheme delivers more precise and accurate results rather than GGA and GGA + U, possessing an indirect band gap of 1.12 eV and 1.34 eV in spin down channel of Co2VX (X = Sn, Sb) Heuslers respectively. Robustness of various elastic constants and its other associated mechanical constituents against external forces is checked to define the stability of these alloys. The transport parameters like Seebeck coefficient, electrical and thermal conductivity, power factor and zT have been put together to express their thermoelectric response. Conservative estimates of Seebeck coefficient and zT demonstrate its extending application stand in thermoelectric energy harvesting technologies. Furthermore complete and precise description of thermo‐physical behavior of the vital quantities like Debye temperature, thermal expansion, Grüneisen parameter and specific heat were examined to check its thermo‐dynamical stability against temperature and pressure variation.

Design of Tracking, Telemetry, Command (TT&C) and Data Transmission Integrated Signal in TDD Mode

For satellite or aircraft networks, tracking, telemetry, and control (TT&C) and data transmission between different nodes are necessary. Traditional measurement mostly adopts the frequency division duplex (FDD) mode and uses a continuous measurement system to achieve high-precision measurement. However, as the number of network nodes increases, the mode suffers from complex frequency domain allocation, and high-cost measurement and data transmission equipment is required. This paper proposes the integrated signal in time division duplex (TDD) mode to improve frequency utilization to address these circumstances. The proposed signal can transmit the TT&C and data at the same frequency. In addition, the high-precision time-frequency synchronization and relative measurement technology in the TDD mode for distributed spacecraft or aircraft networks are studied. The simulation results show that the signal can work normally when the Doppler extrapolation error is less than a quarter of the integration frequency. The distance extrapolation error should be less than a quarter of the length of a chip. The integrated signal reduces the frequency band occupation and realizes the integration of TT&C and data transmission. In addition, the measurement performance is reduced by only 2~3 dB compared with that of the traditional pure TT&C signal.

Ultrafast dynamics of hot charge carriers in an oxide semiconductor probed by femtosecond spectroscopic ellipsometry

Many linked processes occur concurrently in strongly excited semiconductors, such as interband and intraband absorption, scattering of electrons and holes by the heated lattice, Pauli blocking, bandgap renormalization and the formation of Mahan excitons. In this work, we disentangle their dynamics and contributions to the optical response of a ZnO thin film. Using broadband pump-probe ellipsometry, we can directly and unambiguously obtain the real and imaginary part of the transient dielectric function which we compare with first-principles simulations. We find interband and excitonic absorption partially blocked and screened by the photo-excited electron occupation of the conduction band and hole occupation of the valence band (absorption bleaching). Exciton absorption turns spectrally narrower upon pumping and sustains the Mott transition, indicating Mahan excitons. Simultaneously, intra-valence-band transitions occur at sub-picosecond time scales after holes scatter to the edge of the Brillouin zone. Our results pave new ways for the understanding of non-equilibrium charge-carrier dynamics in materials by reliably distinguishing between changes in absorption coefficient and refractive index, thereby separating competing processes. This information will help to overcome the limitations of materials for high-power optical devices that owe their properties from dynamics in the ultrafast regime.

Topological pumping assisted by Bloch oscillations

This paper provides a scheme for quantized pumping without a uniform band occupation in a time-modulated and tilted lattice. The authors propose using Bloch oscillations to sample the momentum space uniformly.

Linear response theory and optical conductivity of Floquet topological insulators

Motivated by the quest for experimentally accessible dynamical probes of Floquet topological insulators, we formulate the linear response theory of a periodically driven system. We illustrate the applications of this formalism by giving general expressions for optical conductivity of Floquet systems, including its homodyne and heterodyne components and beyond. We obtain the Floquet optical conductivity of specific driven models, including two-dimensional Dirac material such as the surface of a topological insulator, graphene, and the Haldane model irradiated with circularly or linearly polarized laser, as well as semiconductor quantum well driven by an ac potential. We obtain approximate analytical expressions and perform numerically exact calculations of the Floquet optical conductivity in different scenarios of the occupation of the Floquet bands, in particular, the diagonal Floquet distribution and the distribution obtained after a quench. We comment on experimental signatures and detection of Floquet topological phases using optical probes.

High Pressure-Temperature study on thermodynamics, half-metallicity, transport, elastic and structural properties of Co-based Heusler alloys: A first-principles study

Employing first-principles based on density functional theory, we have investigated thermodynamics, half-metallicity, transport, elastic and structural properties of Co2ScSb and Co2TiSb full-Heuslers at high temperature and pressure. The existence of perfect band structure and density of states through the modified version of Becke Johnson (mBJ) system delivers more precise and accurate results rather than GGA and GGA ​+ ​U for both these Heuslers. Structural optimizations support the stability in AlCu2Mn-prototype with space symmetry of Fm3−m (#225) space group in both these alloys. The density of states and band occupation describes the semiconducting nature. Co2ScSb and Co2TiSb possess an indirect band-gap of 1.08 ​eV and 1.32 ​eV respectively. The estimation of elastic constants and its other associated mechanical constituents exhibits ductile behavior along with high melting temperature. The thermoelectric coefficients are used to check the applicability of the material for waste heat recovery systems and technological purposes. Thermodynamic potentials which include heat capacity, thermal expansion and Grüneisen parameter have been keenly predicted by employing quasi harmonic Debye model to exhibit its stability at high temperature and pressure varying conditions. Hence, the overall theme of this current study creates an application stand in spintronics, power generation as well as green energy sources for future technologies.

Exploration of electronic structure, mechanical stability, magnetism, and thermophysical properties of L21structured Co2XSb (X = Sc and Ti) ferromagnets

The origin of half-metallicity, structural, thermoelectric, elasto-mechanical, and thermodynamic properties of Co2ScSb and Co2TiSb full-Heuslers has been examined by full potential methods. Structural optimizations support the stability of both alloys in AlCu2Mn-prototype with space symmetry of (#225) space group. The occurrence of perfect band occupation and interpretation of density of states through the modified version of the Becke Johnson (mBJ) scheme for both these Heusler systems delivers more precise and accurate results rather than GGA and GGA + U. The density of states and band occupation describes the semiconducting nature with an indirect band-gap 1.08 eV for Co2ScSb and 1.32 eV Co2TiSb. Robustness of various elastic constants against external forces is checked to descripted stability of these alloys. The evaluation of elastic constants and its other associated mechanical constituents reveals ductile behavior along with high melting temperature. The thermoelectric coefficients are used to check the applicability of the material for waste heat recovery systems and technological purposes. Thermal parameters support the material's low anharmonicity and hence predict the stability of these alloys at the wide range of temperatures and pressure. The thermoelectric parameters pose its applications and stand to develop devices based on spintronics and thermoelectric purposes.

Electronic structure, magnetism and elastic properties of Inverse Perovskite Carbide: A first principles study

Antiperovskite materials display many fascinating properties like that of perovskites. We summed up the electronic, magnetic and mechanical properties of Ni3LiC compound in this work, via density functional theory calculations. The structural optimization via the Birch-Murnaghan approach supply the lattice parameters and ground state energy within the Fm-3m space group. Spin resolved band occupation and density of states define its metallic nature with a total ferromagnetic moment of 0.76 μB. The effect of uniform strain is analysed, which shows the increase in magnetic moment with the expansion of the lattice and vice versa. The speculated elastic constants and their different combinations conclude the mechanical strength and stability of this material. The large bulk modulus and B/G ratio claims its ductile and incompressible nature and the Zener's parameter reflects the exhibition of anisotropic properties.

Physics of arbitrarily doped Kondo lattices: From a commensurate insulator to a heavy Luttinger liquid and a protected helical metal

Kondo lattices are systems where itinerant and localized electrons coexist. If the density of the localized electrons is high and the space is one dimensional, the physics is determined by the Ruderman-Kittel-Kasuya-Yosida interaction, which dominates over the Kondo effect and governs a rich phase diagram. Depending on the band occupation, the ground state realizes commensurate insulators (at fillings $\frac{1}{2}$, $\frac{1}{4}$, or $\frac{3}{4}$), Tomonaga-Luttinger liquids, or 4${k}_{F}$ charge density wave phases. The latter are exotic helical metals with protected transport.