Low-lying Energy States Research Articles

Enhanced red luminescence from GdAl3(BO3)4: Pr3+ phosphors under NUV excitation.

The GdAl3(BO3)4:xPr3+ (0 ≤ x ≤ 5.0 mol%) phosphors were prepared through solid state reaction route and characterized for various lighting applications. Powder X-ray diffraction investigations revel rhombohedral structure matched to JCPDS card no. 83-1907. The morphological studies confirm the agglomeration of particles with different size and shape. The emission spectra show various emission transitions originating from Pr3+:(3P1,0, 1D2) emission states to their lower lying energy states upon 274 nm NUV excitation with a red shift for x > 0.5 mol%. The colour perception analysis results an intense red luminescence due to efficient energy transfer from Gd3+ to Pr3+ ions. The temperature-dependent luminescence investigations show good thermal stability even beyond 150°C with an activation energy of 0.24 eV. The observed experimental results show the potentiality of GdAl3(BO3)4:0.5 Pr3+ phosphor for red emitting devices and red component in phosphor converted white LEDs.

The impact of the laser field on the electronic properties of a hydrogen molecular ion in a Gaussian quantum dot

In this study, the influence of a circularly polarized non-resonant high-frequency laser field radiation on the electronic structure of a hydrogen molecular ion formed by two fixed hydrogen-like donor atoms bound to an electron confined in a two-dimensional Gaussian quantum dot is carried out. The low-lying energy states of the molecular complex were calculated by using the two-dimensional diagonalization method. The results show that the stability of the molecular complex is sensitive to the non-resonant laser field intensity and the internuclear distance. We conclude that the dissociation process of the molecular ion system can be tuned by changing the distance between donor atoms and the non-resonant laser field intensity. It was also concluded that if the molecular complex structure of the system is preserved, the binding energy is higher than in a fragmented state resulting from the dissociation process of the structure.

Structural evolution in 146–158 Nd isotopes using IBM-2 Hamiltonian

Neodymium isotopes (Z=60) lie in the traditional rotational to transitional-spherical region that occurs at the range of deformed nuclei. The identity of 146−158 Neodymium nuclei have been determined in framework of Interacting Boson Model-2, a simulation program with NPBOS has been built to obtain the theoretical energy levels for Neodymium isotopes. The parameters of the best fit to the measured data are determined.
 The energy positive parity bands of those isotopes were calculated using (IBM-2) and then it compared with the experimental values. The mentioned isotopes ratios have calculated and also E-GOS curves ) have drawn as a function of the spin I, to classify symmetry of the nuclei. The electromagnetic transition probabilities B(E2) of these nuclei was calculated as well, where a good agreement for low lying energy states were obtained between experimental results and theoretical calculations.
 The results have been shown that the interested nuclei 152-158Nd have rotational characters SU (3), and O (6) symmetry is substantiated for 146-148Nd, as well as the critical point X (5) has been determined for 150Nd isotope.

Experimental study of recombining nitrogen plasmas: II. Electronic population distributions and nonequilibrium radiation of atoms

We present optical emission spectroscopy measurements from the vacuum ultraviolet to the near-infrared in a recombining nitrogen plasma flow. An inductively coupled plasma torch is used to create an equilibrium plasma at atmospheric pressure. This plasma is then forced to recombine by flowing through a water-cooled tube. For certain conditions, the plasma is forced out of chemical equilibrium. The emission of excited nitrogen atoms, essential for reentry radiation, is measured to study the nonequilibrium distributions of the electronic states. The densities of the low-lying energy states are found to follow a Boltzmann distribution while the high-lying states are closer to a Saha–Boltzmann distribution—consistent with previous predictions of recombining nonequilibrium plasmas. The measured densities are then compared with the predictions of a two-temperature model, which is found to underpredict the measured densities. The total measured radiation is around 5 orders of magnitude stronger than the equilibrium radiation. The 2-T model estimate of this radiation is much closer yet still underestimates the measured radiation. A model using a single electronic temperature is therefore not sufficient to predict the measurements. The measurements presented in this paper will be useful to test more detailed state-to-state recombination models.

Effect of Pd alloying on structural, electronic and magnetic properties of L10 Fe–Ni

We present a systematic study of the effect of Pd-alloying on phase stability, electronic structure, and elastic properties in L10 Fe–Ni using density-functional theory. Being from the same group of the periodic table, Pd is the best candidate for chemical alloying. The Fe–Ni/Fe–Pd/Ni–Pd bond-length increases with increasing Pd-concentration, which weakens the hybridization between low lying energy states below Fermi-level. The reduced hybridization decreases the relative thermodynamic stability of L10 Fe(Ni1−x Pd x ) until x = 0.75. Beyond this concentration, the relative stability gets enhanced, which is attributed to a unique change in the lattice distortion (c/a). The elastic properties show a non-monotonous behavior as a function of x, which is again due to a specific change-over in the uniaxial strain. We found that Pd alloying increases the local Fe moment and structural anisotropy of L10 FeNi, which are important for applications such as microwave absorption, refrigeration systems, recording devices, imaging and sensors. We believe that the present study for the chemical alloying effect can provide critical insights toward the understanding of electronic-structure and elastic behavior of other technologically important materials.

Nuclear structure and phase transition of odd-odd Cu isotopes: A neutron-proton interacting boson-fermion-fermion model calculation

Low-lying energy states of the odd-odd C60−66u isotopic chain are studied within the framework of interacting boson-fermion-fermion model (IBFFM-1) and the neutron-proton interacting boson-fermion-fermion model (IBFFM-2). The obtained results compare well with the available experimental data and IBFFM-1, which augment the reliability of the wave function and Hamiltonian applied within the IBFFM-2 model. The ground-state phase transitions in Cu odd-A and odd-odd nuclei are investigated. Some signs of shape-phase transitions are seen in odd-odd nuclei. The obtained results indicate that the spectra of these isotopes similar to Cu odd-A nuclei provide good examples of vibrational symmetry without any significant deformed gamma-unstable structure. So, well known properties in the odd mass nuclei persist also in the odd-odd mass system. The calculations reproduce this behavior.

Coarse-grained modeling of thermochemical nonequilibrium using the multigroup maximum entropy quadratic formulation.

This work addresses the construction of a reduced-order model based on a multigroup maximum entropy formulation for application to high-enthalpy nonequilibrium flows. The method seeks a piecewise quadratic representation of the internal energy-state populations by lumping internal energy levels into groups and by applying the maximum entropy principle in conjunction with the method of moments. The use of higher-order polynomials allows for an accurate representation of the logarithm of the distribution of the low-lying energy states, while preserving an accurate description of the linear portions of the logarithm of the distribution function that characterize the intermediate- and high-energy states. A comparison of the quadratic and the linear reconstructions clearly demonstrates how the higher-order reconstruction provides a more accurate representation of the internal population distribution function at a modest increase in the computational cost. Numerical simulations carried out under conditions relevant to hypersonic flight reveal that the proposed model is able to capture the dynamics of the nonequilibrium distribution function using as few as three groups, thereby reducing the computational costs for simulations of nonequilibrium flows.

Multipartite nonlocality in the Lipkin-Meshkov-Glick model

Lipkin-Meshkov-Glick (LMG) models describe a set of $N$ qubits which are located at the corners of an $N$-dimensional simplex and embedded in an external magnetic field. Because of its high coordinate number, quantum correlations in the models are quite nontrivial. For instance, in the large-$N$ limit, two-site entanglement concurrence vanishes for any magnetic field. Multisite global entanglement and generalized global entanglement also tend to vanish in some quantum regions. We characterize the quantum correlations in the LMG model with multipartite nonlocality. In LMG models with anisotropy in the $x\ensuremath{-}y$ plane, for any fixed magnetic field, the ground-state nonlocality (denoted by $\mathcal{S}$) is found to scale as ${log}_{10}\mathcal{S}\ensuremath{\sim}aN+b$, with $a$ and $b$ the fitting parameters. For LMG models which are isotropic in the $x\ensuremath{-}y$ plane, nevertheless, the nonlocality for each Dicke state scales as ${log}_{10}\mathcal{S}\ensuremath{\sim}a\frac{1}{N}+b$, with $bg0$. Signals for the quantum phase transitions of the models will also be discussed. These results indicate that multipartite nonlocality captures some key ingredient of the ground-state quantum correlations in the models. In addition to the ground states, we also study multipartite nonlocality in the LMG models at finite temperatures. In the anisotropic models, the thermal stability of the nonlocality at low temperatures is determined by the energy gap. For the isotropic models, nevertheless, as the temperature increases, the nonlocality presents a sharp valley which is followed by a round peak. The behavior is far beyond the above-mentioned ``thermal stability vs energy gap'' picture. The underlying mechanics is the ``contribution inversion'' of low-lying energy states in the thermal-state nonlocality $\mathcal{S}({\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\rho}}}_{T})$; that is, in a low-temperature region, the first-excited state ``defeats'' the ground state and plays a dominant role in thermal-state nonlocality.

Localized states induced by variations in the curvature of the path of an electron in a closed nanoscopic loop

We analyzed the effect of the curvature of the path on the energy spectrum of an electron confined in a closed nanoscopic loop in the presence of magnetic and electric external fields has been studied. The system was modeled using the stationary Schrödinger equation in the framework of the approximation of effective mass and enveloping function, which was solved using the finite element method. The closed loop has been modeled theoretically as a flat waveguide, whose width is small compared to the length of the path. These geometries allowed putting in evidence the variation in the confinement potential of the electron due to changes in the curvature. The variation of the electronic spectrum and the electronic densities for some low-lying energy states were analyzed as a function of the intensity of a magnetic field applied in the direction of growth and an electric field applied in the structural plane. The results demonstrate, with clarity, the high sensitivity of the electronic spectrum of a closed nanoscopic loop in the presence of changes in the curvature of the path, which translates into high sensitivity in electronic, magnetic and optical properties.

Annealing by simulating the coherent Ising machine.

The coherent Ising machine (CIM) enables efficient sampling of low-lying energy states of the Ising Hamiltonian with all-to-all connectivity by encoding the spins in the amplitudes of pulsed modes in an optical parametric oscillator (OPO). The interaction between the pulses is realized by means of measurement-based optoelectronic feedforward, which enhances the gain for lower-energy spin configurations. We present an efficient method of simulating the CIM on a classical computer that outperforms the CIM itself, as well as the noisy mean-field annealer in terms of both the quality of the samples and the computational speed. It is furthermore advantageous with respect to the CIM in that it can handle Ising Hamiltonians with arbitrary real-valued node coupling strengths. These results illuminate the nature of the faster performance exhibited by the CIM and may give rise to a new class of quantum-inspired algorithms of classical annealing that can successfully compete with existing methods.

A Dy4 Cubane: A New Member in the Single-Molecule Toroics Family.

Molecular materials that possess a toroidal moment associated to a non-magnetic ground state are known as single-molecule toroics (SMTs) and are usually planar molecules. Herein, we report a Dy4 cubane, namely [Dy4 (Bppd)4 (μ3 -OH)4 (Pa)4 (H2 O)4 ]⋅0.333 H2 O (where BppdH=1,3-Bis(pyridin-4-yl)propane-1,3-dione and PaH=2-Picolinic acid) for which magnetometry measurements and state-of-art ab initio calculations highlight SMT behavior in a tridimensional structure (3D-SMT). The in-depth theoretical analysis on the resulting low-lying energy states, along with their variation in function of the magnetic exchange pathways, allows further light to be shed on the description of single-molecule toroics and identify the coupling scheme that better reproduces the observed data.

Calculation of some of the nuclear properties of even-even 172-176Hf isotopes using IBM-1

A description of the even-even Hf isotopes for A = 172 ‒ 176 is carried out in the framework of the interacting boson model (IBM-1). The energy levels, B(E2) transition probabilities, electric quadrupole moment QL, and potential energy surface of those nuclei have been calculated. The calculated results are compared with the most recent experimental data. A good agreement for low lying energy states is obtained between experimental results and theoretical calculations. The contour plots of the potential energy surfaces show that the interested nuclei have rotational characters.

Retardation effects in spectroscopic measurements of the Casimir-Polder interaction

Spectroscopy is a unique experimental tool for measuring the fundamental Casimir-Polder interaction between excited state atoms, or other polarisable quantum objects, and a macroscopic surface. Spectroscopic measurements probe atoms at nanometric distances away from the surface where QED retardation is usually negligeable and the atom-surface interaction is proportional to the inverse cube of the separation distance, otherwise known as the van der Waals regime. Here we focus on selective reflection, one of the main spectroscopic probes of Casimir-Polder interactions. We calculate for the first time selective reflection spectra using the full, distance dependent, Casimir-Polder energy shift and linewidth. We demonstrate that retardation can have significant effects, in particular for experiments with low lying energy states. We also show that the effective probing depth of selective reflection spectroscopy depends on the transition linewidth. Our analysis allows us to calculate selective reflection spectra with composite surfaces, such as metasurfaces, dielectric stacks, or even bi-dimensional materials.

Interatomic potentials of van der Waals dimers Hg2 and Cd2: Probing discrepancies between theory and experiment

Results of new all-electron ab initio calculations and revisit of experimental studies of the interatomic potentials of lower-lying ungerade excited and ground electronic energy states of the Hg2 and Cd2 van der Waals complexes are used as probes of discrepancies between theory and experiment. From simulations of the previously and presently measured LIF excitation and dispersed emission spectra new analytical representations of the excited- and the ground-state interatomic potentials are proposed. An inverted perturbation approach was also used to improve the studied interatomic potentials. The comparison of the new ab-initio calculated potentials with the results of the analyses illustrates an improve theory-to-experiment agreement for such a demanding system like Hg2 or Cd2.

Geometry-Driven Folding of a Floating Annular Sheet.

Predicting the large-amplitude deformations of thin elastic sheets is difficult due to the complications of self contact, geometric nonlinearities, and a multitude of low-lying energy states. We study a simple two-dimensional setting where an annular polymer sheet floating on an air-water interface is subjected to different tensions on the inner and outer rims. The sheet folds and wrinkles into many distinct morphologies that break axisymmetry. These states can be understood within a recent geometric approach for determining the gross shape of extremely bendable yet inextensible sheets by extremizing an appropriate area functional. Our analysis explains the remarkable feature that the observed buckling transitions between wrinkled and folded shapes are insensitive to the bending rigidity of the sheet.

The electron-furfural scattering dynamics for 63 energetically open electronic states.

We report on integral-, momentum transfer- and differential cross sections for elastic and electronically inelastic electron collisions with furfural (C5H4O2). The calculations were performed with two different theoretical methodologies, the Schwinger multichannel method with pseudopotentials (SMCPP) and the independent atom method with screening corrected additivity rule (IAM-SCAR) that now incorporates a further interference (I) term. The SMCPP with N energetically open electronic states (N(open)) at either the static-exchange (N(open) ch-SE) or the static-exchange-plus-polarisation (N(open) ch-SEP) approximation was employed to calculate the scattering amplitudes at impact energies lying between 5 eV and 50 eV, using a channel coupling scheme that ranges from the 1ch-SEP up to the 63ch-SE level of approximation depending on the energy considered. For elastic scattering, we found very good overall agreement at higher energies among our SMCPP cross sections, our IAM-SCAR+I cross sections and the experimental data for furan (a molecule that differs from furfural only by the substitution of a hydrogen atom in furan with an aldehyde functional group). This is a good indication that our elastic cross sections are converged with respect to the multichannel coupling effect for most of the investigated intermediate energies. However, although the present application represents the most sophisticated calculation performed with the SMCPP method thus far, the inelastic cross sections, even for the low lying energy states, are still not completely converged for intermediate and higher energies. We discuss possible reasons leading to this discrepancy and point out what further steps need to be undertaken in order to improve the agreement between the calculated and measured cross sections.

Tunneling interstitial impurity in iron-chalcogenide-based superconductors

A pronounced local in-gap zero-energy bound state (ZBS) has been observed by recent scanning tunneling microscopy (STM) experiments on the interstitial Fe impurity (IFI) and its nearest-neighboring (nn) sites in $\mathrm{FeTe_{0.5}Se_{0.5}}$ superconducting (SC) compound. By introducing a new impurity mechanism, the so-called tunneling impurity, and based on the Bogoliubove-de Gennes (BDG) equations we investigated the low-lying energy states of the IFI and the underlying Fe-plane. We found the peak of ZBS does not shift or split in a magnetic field as long as the tunneling parameter between IFI and the Fe-plane is sufficiently small and the Fe-plane is deep in the SC state. Our results are in good agreement with the experiments. We also predicted that modulation of spin density wave (SDW), or charge density wave (CDW) will suppress the intensity of the ZBS.

Singlet–triplet competition in the low-lying energy states of C4O4−n S n (n = 1–3) molecules

Cyclobutanetetrone, C4O4, has a triplet ground state, although the ground state of C4S4 is singlet. This computational study focuses on the mono-, di-, and trithiosquarate, C4O4−n S n (n = 1–3), molecules as transition stages between the two ending points (C4O4 and C4S4), and investigates the trends for the changes in the energies, geometry, partial atomic charges, partial spin densities, as well as orbital energies of the four low-lying electronic states. As the number of the sulfur atoms is increasing, the singlet spin state becomes energetically more and more preferred. For C4O3S molecule, where only one oxygen atom is substituted by sulfur, the CCSD(T) calculations predict a triplet ground state, but the error of the calculations is most likely higher than the calculated 0.5 kcal/mol singlet–triplet energy gap.

Effect of mesoscale ordering on the density of States of polymeric semiconductors.

A multiscale simulation scheme, which incorporates both long-range conformational disorder and local molecular ordering, is proposed for predicting large-scale morphologies and charge transport properties of polymeric semiconductors. Using poly(3-hexylthiophene) as an example, it is illustrated how the energy landscape and its spatial correlations evolve with increasing degree of structural order in mesophases with amorphous, uniaxial, and biaxial nematic ordering. It is shown that the formation of low-lying energy states in more ordered systems is mostly due to larger (on average) conjugation lengths and not due to electrostatic interactions. The proposed scheme is general and can be applied to a wide range of polymeric organic materials.

Efficient one-pot synthesis of monodisperse alkyl-terminated colloidal germanium nanocrystals

An efficient one-pot method for fabricating alkyl-capped germanium nanocrystals (Ge NCs) is reported. Ge NCs with a size of 3.9 ± 0.5 nm, are formed by co-reduction of germanium tetrachloride in the presence of n-butyltrichlorogermane, producing NCs with butyl-terminated surfaces. The advantage of this method is that it allows rapid synthesis and functionalisation of NCs with minimal post-synthetic purification requirements. TEM imaging showed that the Ge NCs are monodisperse and highly crystalline, while EDX and SAED confirmed the chemical identity and crystal phase of the NCs. FTIR and XPS confirmed that the Ge NCs were well passivated, with some oxidation of the nanocrystal surface. Optical spectroscopy of the NCs showed a strong absorbance in the UV region and an excitation wavelength dependent photoluminescence in the UV/violet. Time resolved photoluminescence measurements showed the presence of two nanosecond lifetime components, consistent with recombination of photogenerated excitons at low lying energy states present at the nanocrystal surface. Photoluminescence quantum yields were determined to be 37 %, one of the highest values reported for organically terminated Ge NCs.