Lower Ground-state Energy Research Articles

Isomerization, Protonation, and Hydrolysis Properties of Naphthalimide-Containing Spiropyran in Aqueous Media.

Synthesis of spiropyrans exhibiting ring-opening/closing isomerization driven by external stimuli is a challenging subject for the development of molecular sensors. A napthalimide-containing spiropyran (1) promotes rapid isomerization between the spirocyclic (SP) form and ring-opened merocyanine (MC) form by the change in solvent polarity even under the dark condition at room temperature. In this work, the effect of water on the isomerization behavior of 1 was studied. The addition of water caused an MC-water H-bonding interaction and shifted the resonance MC structure to the zwitterionic form with a lower ground-state energy. The stabilized MC form promoted spontaneous SP → MC isomerization and increased the equilibrium MC concentration. The effect of pH on the behavior of 1 was also studied. In acidic-neutral media, protonation/deprotonation of the naphthalimide moiety led to rapid and reversible changes in the absorption spectra. In contrast, strongly basic media (pH > 12) promoted irreversible base-catalyzed hydrolysis of the alkene moiety.

Real-time observation of sub-100-fs charge and energy transfer processes in DNA dinucleotides.

Using as showcase the DNA dinucleotide 5'-dTpdG-3', in which the thymine (T) is located at the 5' end with respect to the guanine (G), we study the photoinduced electronic relaxation of coupled chromophores in solution with an unprecedented refinement. On the one hand, transient absorption spectra are recorded from 20 fs to 45 ps over the 330-650 nm range with a temporal resolution of 30 fs; on the other hand, quantum chemistry calculations determine the ground state geometry of the 4 possible conformers with stacked nucleobases, the associated Franck-Condon states, and map the relaxation pathways leading to excited state minima. Important spectral changes occurring before 100 fs are correlated with concomitant G+ → T- charge transfer and T → G energy transfer processes. The lifetime of the excited charge transfer state is only 5 ps and the absorption spectrum of a long-lived nπ*T state is detected. Our experimental results match the transient spectral properties computed for the anti-syn conformer of 5'-dTpdG-3', which is characterized by the lowest ground state energy and differs from that encountered in B-form duplexes.

First principle study of scandium-based novel ternary half Heusler ScXGe (X = Mn and Fe) alloys: insight into the spin-polarized structural, electronic, and magnetic properties.

The structural, electronic, and magnetic properties of novel half-Heusler alloys ScXGe (X = Mn, Fe) are investigated using the first principle full potential linearized augmented plane wave approach based on density functional theory (DFT). To attain the desired outcomes, we employed the exchange-correlation frameworks, specifically the local density approximation in combination with Perdew, Burke, and Ernzerhof's generalized gradient approximation plus the Hubbard U parameter method (GGA + U) to highlight the strong exchange-correlation interaction in these alloys. The structural parameter optimizations, whether ferromagnetic (FM) or nonmagnetic (NM), reveal that all ScXGe (where X = Mn, Fe) Heusler alloys attain their lowest ground state energy during FM optimization. The examination of the electronic properties of these alloys reveals their metallic character in both the spin-up and spin-down channels. The projected densities of states indicate that bonding is achieved through the hybridization of p-d and d-d states in all of the compounds. The investigation of the magnetic properties in ScXGe (where X = Mn, Fe) compounds indicates pronounced stability in their ferromagnetic state. Notably, the Curie temperatures for ScXGe (X = Mn, Fe) are determined to be 2177.02 K and 1656.09 K, respectively. The observation of metallic behavior and the strong ferromagnetic characteristics in ScXGe (X = Mn, Fe) half-Heusler alloys underscores their potential significance in the realm of spintronic devices. Consequently, our study serves as a robust foundation for subsequent experimental validation.

Mechanical and magneto-electronic properties of europium lanthanide-based cubic perovskites EuYO3 (Y=Cr, Mn, Fe): An ab initio study

Computational calculations using density functional theory (DFT) were performed for the first time using the full potential linearized augmented plane wave plus local orbital method (FP-LAPW + LO) to determine the structural, elastic, electronic and magnetic properties of europium-based cubic perovskites EuYO3 (Y=Cr, Mn, Fe). The exchange correlation potentials of GGA along with some analytical methods were adopted for the computation of structural and elastic properties. Moreover, the GGA + U formalism was also added for obtaining more precise electronic and magnetic properties, particularly to address the Eu-4f and Y-3d orientations in the spin-polarized double cell symmetry. The observed lattice parameters of these compounds are consistent with experiment. The observed bulk moduli predict that EuCrO3 is harder and less compressible than EuMnO3 and EuFeO3. The calculated tolerance factors of these compounds are within the cubic symmetry range. Our computed critical radius of EuCrO3 shows that EuCrO3 has a larger migration energy. Based on their elastic properties, these compounds are ductile in nature. We also computed the thermal properties of these compounds. The band structures and density of states show that these compounds are metallic in character. The lowest ground state energy and magnetic moments of these compounds expose their ferromagnetic nature. The metallic nature and strong ferromagnetism of these compounds make them promising applicants for application in spintronic.

Improving the Accuracy of Variational Quantum Eigensolvers with Fewer Qubits Using Orbital Optimization

Near-term quantum computers will be limited in the number of qubits on which they can process information as well as the depth of the circuits that they can coherently carry out. To date, experimental demonstrations of algorithms such as the Variational Quantum Eigensolver (VQE) have been limited to small molecules using minimal basis sets for this reason. In this work we propose incorporating an orbital optimization scheme into quantum eigensolvers wherein a parametrized partial unitary transformation is applied to the basis functions set in order to reduce the number of qubits required for a given problem. The optimal transformation is found by minimizing the ground state energy with respect to this partial unitary matrix. Through numerical simulations of small molecules up to 16 spin orbitals, we demonstrate that this method has the ability to greatly extend the capabilities of near-term quantum computers with regard to the electronic structure problem. We find that VQE paired with orbital optimization consistently achieves lower ground state energies than traditional VQE when using the same number of qubits and even frequently achieves lower ground state energies than VQE methods using more qubits.

Ab-initio study of half-metallic ferromagnetism and transport characteristics of MgV2S/Se4 spinels for spintronics and thermoelectric applications

Understanding ferromagnetism and thermoelectric behavior are crucial in spintronics and thermoelectric device applications. Using density functional theory-based WIEN2k code, we have examined the physical properties of vanadium-based MgV2S/Se4 spinels. The calculated negative formation energies and positive phonon frequency indicate the stability of the studied system. The lowest energy ground state has been predicted to be a ferromagnetic phase. The calculated electronic band structure and density of states show that these materials are half-metallic ferromagnetic. The existence of the ferromagnetic phase is described using the pd hybridization, double exchange interaction model by computing the exchange energy and constants. In addition, the quantum coupling of electrons is caused by the shift of the magnetic moment from the V site to non-magnetic sites (S/Se, Mg). Finally, electronic transport parameters like the Seebeck coefficient, electric and thermal conductivity, and power factor are also determined.

Synthesis and characterization of copper complexes of phenylpiperazine ring-based ligands and evaluation of their antimicrobial, BSA binding, computational, and molecular docking studies

This paper describes the synthesise of copper complexes of phenylpiperazine ring-based ligands and characterization via physical and spectroscopic methods. Structure of complexes has been proposed on the basis of UV-vis, IR and Mass fragmentation pattern and additionally supported by electrochemical and thermogravimetric analysis. Low molar conductance value of metal complexes in water and DMSO suggest non-ionic nature of complexes which is in agreement of proposed structure. TGA curve, where loss of coordinated solvent molecules starts early followed by loss of ligand itself also support the proposed structures. Percentage ash content analysis also indicate that only one copper metal is binding to the ligands reported here. Antibacterial activity of metal complexes has been performed by agar well diffusion method and indicated these metal complexes possess higher activity than the corresponding ligands. Binding interaction of these complexes were studied with BSA protein by UV-vis spectroscopic methods and binding constant was calculated which indicated that the complexes are having moderate affinity. Some complexes were also geometrically optimized to lowest energy ground state by using DFT calculations. Docking study of these optimized structures with BSA protein revealed these metal complexes exhibit hydrophobic interaction owing to the aromatic bulk in the ligand.

A First Look at Structural Properties of Long HP Model Sequences

The longest sequence in the literature of the HP model of protein folding is studied on a simple cubic lattice using replica-exchange Wang-Landau sampling. We find a lower ground state energy than found in previous studies, and, for the first time, study the structural and thermal behavior of this sequence during the folding process.

A Theoretical Study on Electronic Behavior and Mechanical Properties of Ferromagnetic Manganese Selenide: AgMn2Se4

In this study, the electronic behavior and mechanical properties of the ferromagnetic chalcospinel manganese-based selenide (AgMn2Se4) which crystallized in face centered cubic structure with space group Fd3 ̅m and space number 227, were investigated. All ab initio calculations were carried out by Generalized Gradient Approximation (GGA) under spin polarization. For this composition, three different type magnetic orders were considered to detect the most stable magnetic character. For the given composition, the results of the computations indicate its ferromagnetic nature since this compound has a lower ground state energy in this magnetic order than in other magnetic phases. After determining the most energetically stable magnetic phase, the electronic behavior in this magnetic arrangement was examined. The observed electronic band structure under spin polarization of this compound shows that this selenide system is almost half-metallic material due to having small band gap (Eg = 0.09 eV) in the minority spin state. In addition, the mechanical stability was determined with the help of elastic constants which were also employed to determine the mechanical characteristics of this compound.

Structural and magnetic properties of two-dimensional layered BiFeO3 from first principles

Using first-principles calculations, we predict a unique $\mathrm{BiFe}{\mathrm{O}}_{3}$ (BFO) phase, which is coined $I$-BFO, with layered characteristics based on the ilmenite structure. The interactions between interlayer bismuth atoms make it easy to exfoliate monolayer BFO with a configuration in which Bi ions are above and below two-dimensional (2D) iron-oxygen octahedra sharing common edges. The calculated phonon spectra reveal the dynamic stability of both bulk and monolayer forms of $I$-BFO. Combining total energy computation and crystal structure search, we corroborate that the $I$-BFO monolayer maintains the lowest-energy ground state, as compared to other 2D BFO exfoliations. We also discover that monolayer $I$-BFO is a robust antiferromagnetic material with a N\'eel temperature (${T}_{N}$) of 4 K in the fully relaxed case. Such ${T}_{N}$ can be prominently improved by strain, for example, ${T}_{N}=25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at 6% tension. Furthermore, our electronic structure calculations with PBE and hybrid HSE functionals show the $I$-BFO monolayer to be an indirect-gap semiconductor. Finally, we indicate that other ternary $AB{\mathrm{O}}_{3}$ compounds also have the potential to produce layered materials from the ilmenite structure. This work thus provides guidance to explore unconventional layered materials, and further broadens the application of BFO and other promising ilmenite oxides in two dimensions.

Replica exchange Wang–Landau sampling of long HP model sequences

Long sequences of the HP model of protein folding are studied using replica exchange Wang–Landau sampling. We find lower ground state energies than found from earlier simulations using PERM and replica exchange Monte Carlo and, for the first time, extract specific heat curves.

An intervening metallic phase at the CDW–SDW transition region in the one-dimensional Holstein-Hubbard model at half filling: a semi-exact solution

The one-dimensional Holstein–Hubbard model is analyzed at half filling at the CDW-SDW transition region by using a series of unitary transformations. The phonon degrees of freedom are eliminated by averaging the transformed Hamiltonian with respect to a generalized many-phonon state and the resulting effective electronic Hamiltonian is finally solved exactly by employing the Bethe-ansatz technique. The present variational state leads to a lower ground state energy as compared to those obtained from the previous variational calculations and yields a wider intermediate metallic phase at the SDW-CDW transition region confirming the prediction of Takada and Chatterjee.

Non-Gaussian variational approach to Fermi polarons in one- and two-dimensional lattices

We study the Fermi polaron problem of one mobile spin-up impurity immersed atop the bath consisting of spin-down fermions in one- and two-dimensional square lattices. We solve this problem by applying a variational approach with non-Gaussian states after separating the impurity and the background by the Lee-Low-Pines transformation. The ground state for a fixed total momentum can be obtained via imaginary time evolution for the variational parameters. For the one-dimensional case, the variational results are compared with numerical solutions of the matrix product state method with excellent agreement. In two-dimensional lattices, we focus on the dilute limit, and find a polaron--molecule evolution in consistence with previous results obtained by variational and quantum Monte Carlo methods for models in continuum space. Comparing to previous works, our method provides the lowest ground state energy in the entire parameter region considered, and has an apparent advantage as it does not need to assume {\it in priori} any specific form of the variational wave function.

Optimal Orbital Selection for Full Configuration Interaction (OptOrbFCI): Pursuing the Basis Set Limit under a Budget.

Full configuration interaction (FCI) solvers are limited to small basis sets due to their expensive computational costs. An optimal orbital selection for FCI (OptOrbFCI) is proposed to boost the power of existing FCI solvers to pursue the basis set limit under a computational budget. The optimization problem coincides with that of the complete active space SCF method (CASSCF), while OptOrbFCI is algorithmically quite different. OptOrbFCI effectively finds an optimal rotation matrix via solving a constrained optimization problem directly to compress the orbitals of large basis sets to one with a manageable size, conducts FCI calculations only on rotated orbital sets, and produces a variational ground-state energy and its wave function. Coupled with coordinate descent full configuration interaction (CDFCI), we demonstrate the efficiency and accuracy of the method on the carbon dimer and nitrogen dimer under basis sets up to cc-pV5Z. We also benchmark the binding curve of the nitrogen dimer under the cc-pVQZ basis set with 28 selected orbitals, which provide consistently lower ground-state energies than the FCI results under the cc-pVDZ basis set. The dissociation energy in this case is found to be of higher accuracy.

Exploration of novel High-Pressure Structures of Hf2O3

In the high-speed development of information technology, hafnium oxides are the most prospective alternative material for the resistive random-access memory, field-effect transistors and complementary metal oxide semiconductor transistors. Here, we carry out a systematic study on the phase transition, stability and electronic properties of Hf2O3 by using the first-principles calculations. The enthalpy pressure relations show that the P-4m2 phase has the lowest ground state energy under ambient condition, which is correspond well with the reported literatures. With the increasing pressure, at 186.1 GPa, the P-4m2 phase transforms to C2∕c phase. The calculated phonons and elastic constants of P-4m2 and C2∕c phases suggest that they are both dynamically and elastically stable. The energy band structure and the density of state calculations show that the P-4m2 phase acts as semimetal, while the C2∕c phase acts as metallic. The hardness of the P-4m2 phase and C2∕c phase have been computed based on the semiempirical method. The results indicate that P-4m2 and C2∕c phases are both hard materials with hardness values of 10.16 GPa and 19.81 GPa, respectively. We expect that our results will promote future experimental studies on Hf2O3.

Beyond the one-dimensional configuration coordinate model of photoluminescence

The one-dimensional configuration coordinate model (1D-CCM) is widely used for the analysis of photoluminescence in molecules and doped solids, and relies on a linear combination of the equilibrium nuclear configurations of ground and excited states. It delivers an estimation of the energy barrier at which ground and excited state curves cross, semi-classically linked to non-radiative transition rate and thermal quenching. To assess its predictive power for the latter properties, we propose a new \textit{optimized configuration path (OCP) method in which} the ground-state and excited-state forces are mixed instead of their configurations. We also define another one-parameter model thanks a double energy parabola hypothesis (DEPH). We compare the OCP method and the DEPH reference with the 1D-CCM for three paradigmatic 4f-5d phosphors Y$_3$Al$_5$O$_{12}$:Ce, Lu$_2$SiO$_5$:Ce, and YAlO$_3$:Ce. We find that the OCP and DEPH methods yield similar results with geometries that have significantly lower ground-state energies than the 1D-CCM for the same 4f-5d energy difference. However, the OCP method suffers from the appearance of multiple local minima, rendering the clear determination of the optimal geometry very difficult in practice. Still the OCP method allows one to quantify the deviations from the 1D-CCM, therefore increasing confidence in the lower bound obtained from the DEPH for the 4f-5d crossing barrier, and its comparison with the energy of the auto-ionization thermal quenching mechanism. We expect the OCP approach to be applicable to other luminescent materials or molecules.

Interplay between lattice topology, frustration, and spin quantum number in quantum antiferromagnets on Archimedean lattices

The interplay between lattice topology, frustration, and spin quantum number, $s$, is explored for the Heisenberg antiferromagnet (HAFM) on the 11 two-dimensional Archimedean lattices (square, honeycomb, CaVO, SHD, SrCuBO, triangle, bounce, trellis, maple-leaf, star, and kagome). We show that the coupled cluster method (CCM) provides consistently accurate results when compared to the results of other approximate methods. The CCM also provides valuable information relating to the selection of ground states and we find that this depends on spin quantum number for the kagome and star lattices. Specifically, the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ model state provides lower ground-state energies than those of the $q=0$ model state for the kagome and star lattices for most values of $s$. The $q=0$ model state provides lower ground-state energies only for $s=1/2$ for the kagome lattice and $s=1/2$ and $s=1$ for the star lattice. The kagome and star lattices demonstrate the least amount of magnetic ordering and the unfrustrated lattices (square, honeycomb, SHD, and CaVO) demonstrate the most magnetic ordering for all values of $s$. The SrCuBO and triangular lattices also demonstrate high levels of magnetic ordering, while the remaining lattices (bounce, maple-leaf, and trellis) tend to lie between these extremes, again for all values of $s$. These results also clearly reflect the strong increase in magnetic order with increasing spin quantum number $s$ for all lattices. The ground-state energy, ${E}_{g}/(NJ{s}^{2})$, scales with ${s}^{\ensuremath{-}1}$ to first order, as expected from spin-wave theory, although the order parameter, $M/s$, scales with ${s}^{\ensuremath{-}1}$ for most of the lattices only. Self-consistent spin-wave theory calculations indicated previously that $M/s$ scales with ${s}^{\ensuremath{-}2/3}$ for the kagome lattice HAFM, whereas previous CCM results (replicated here also) suggested that $M/s$ scales with ${s}^{\ensuremath{-}1/2}$. It is probable, therefore, that different scaling for $M/s$ than with ${s}^{\ensuremath{-}1}$ does indeed occur for the kagome lattice. By using similar arguments, we find here also that $M/s$ scales with ${s}^{\ensuremath{-}1/3}$ for the star lattice and with ${s}^{\ensuremath{-}2/3}$ for the SrCuBO lattice.

Defect levels and hyperfine constants of hydrogen in beryllium oxide from hybrid-functional calculations and muonium spectroscopy

The atomistic and electronic structures of isolated hydrogen states in BeO were studied by ab initio calculations and muonium spectroscopy (SR). Whereas standard density-functional theory with a semi-local GGA functional led to a detailed probing of all possible minimum-energy configurations of hydrogen further calculations with the hybrid HSE06 functional provided improved properties avoiding band-gap and self-interaction errors. Similarly to earlier findings for the other wide-gap alkaline-earth oxide, MgO, hydrogen in BeO is also predicted to be an amphoteric defect with the pinning level, E(), positioned in the mid-gap region. Both donor and acceptor levels were found too deep in the gap to allow for hydrogen to act as a source of free carriers. Whereas, hydrogen in its positively-charged state, , adopts exclusively hydroxide-bond OH configurations, and instead show a preference to occupy cage-like interstitial sites in the lattice. in particular displays a multitude of minimum-energy configurations: its lowest-energy ground state resembles closely a trapped-atom state with a nearly spherical spin-density profile. In contrast to the strongly ionic MgO, in BeO was further found to stabilise in additional higher-energy elongated-bond OH configurations whose existence should be traced to a partial covalent character of the Be–O bonding. Calculations of the proton-electron hyperfine coupling for all states showed that the ground-state interstitial configuration is dominated by an isotropic hyperfine interaction with a magnitude very close to the vacuum value, a finding corroborated by the SR-spectroscopy data.

Numerical analysis and simulation for a generalized planar Ginzburg–Landau equation in a circular geometry

In this paper, a numerical scheme for a generalized planar Ginzburg-Landau energy in a circular geometry is studied. A spectral-Galerkin method is utilized, and a stability analysis and an error estimate for the scheme are presented. It is shown that the scheme is unconditionally stable. We present numerical simulation results that have been obtained by using the scheme with various sets of boundary data, including those the form u(θ) = exp(idθ), where the integer d denotes the topological degree of the solution. These numerical results are in good agreement with the experimental and analytical results. Results include the computation of bifurcations from pure bend or splay patterns to spiral patterns for d = 1, and computations of metastable or unstable higher-energy solutions as well as the lowest energy ground state solutions for values of d ranging from two to five.

Photo-induced electronic properties in single quantum well system: effect of excitonic lifetime

In the present study, we have established a correlation between the photo-induced electronic phenomena and excited state lifetime of the photo generated carriers in double barrier Al0.3Ga0.7As\\GaAs quantum well (QW) structures. The excited state lifetime was measured experimentally by picosecond time resolved photoluminescence spectroscopy for two samples with different well widths (5.3 nm and 16.5 nm). The faster nonradiative decay time of the narrower well can be attributed to the facile escape of electrons from well to barrier due to lower associated energy compared to that of the thicker well which resembles the simulated results of the energy level distribution. The proposed mechanism of carrier escape is further proven from the higher value of unconventional excitonic capacitance value in the thicker well, measured by impedance spectroscopy. The dependence of photo-induced capacitance on well thickness is explained by the lifetime of the excited carriers in this study. Dependence of the photo-generated capacitance (C) on externally applied bias voltage (V) was also studied to quantitatively establish a proportional relation between the carrier holding capacity of the well and the excitonic lifetime. The higher accumulation of charge and lower ground state energy of the thicker well is evident from the higher tunnelling current found for the same in the photocurrent (I) versus voltage (V) measurement. Thus the escape of electrons from the well to barrier is the key factor affecting the photo generated charge accumulation and its holding capacity which in turn influences the device performances.