Quadrupole Interaction Research Articles

Raman Evidence of Multiple Adsorption Sites and Structural Transformation in ZIF-4

The zeolitic imidazolate framework, ZIF-4, exhibits soft porosity and is known to show pore volume changes with temperatures, pressures, and guest adsorption. However, the mechanism and adsorption behavior of ZIF-4 are not completely understood. In this work, we report an open to narrow pore transition in ZIF-4 around T ∼ 253 K upon lowering the temperature under vacuum (10-6 Torr) conditions, facilitated by C-H···π interactions. In the gaseous environment of N2 and CO2 around the framework, characteristic Raman peaks of adsorbed gases were observed under ambient conditions of 293 K and 1 atm. A guest-induced transition at ∼153 K resulting in the opening of new adsorption sites was inferred from the Raman spectral changes in the C-H stretching modes and low-frequency modes (<200 cm-1). In contrast to a single vibrational mode generally reported for entrapped N2, we show three Raman modes of adsorbed N2 in ZIF-4. The adsorption is facilitated by dispersive and quadrupolar interactions. From our temperature-dependent Raman results and theoretical analysis based on the density functional tight-binding approach, we conclude that the C-Hs are the preferred adsorption sites on ZIF-4 in the following order: C4-H, C5-H > C2-H > center of the Im ring (interacting with C-H centers) > center of the cavity. We also show that with an increasing concentration of N2 adsorbed at low temperatures, the ZIF-4 structure undergoes shear distortion of the window formed by 4-imidazole rings and consequent volumetric expansion. Our results have immediate implications in the field of porous materials and could be vital in identifying subtle structural transformations that may favor or hinder guest adsorption.

Cubic water clusters as building blocks for self-assembly

The processes of self-organization in a system of cubic water clusters of D2d and S4 symmetry are studied. In the course of modeling, using the AMOEBA force field, a tendency to the formation of ice nanotubes and spatial nanostructures formed by tubular fragments is found. It was established that the quadrupole moments of the cubic isomers D2d and S4 are linear. However, the quadrupole–quadrupole interaction becomes decisive only at a great distance. It is very small in magnitude and therefore of little use. The structure of ice nanotubes obtained by combining D2d and S4 isomers has been studied. It is shown that these nanotubes significantly exceed all other configurations of square ice nanotubes in the stabilization energy. An atlas of the most probable structures with the number of cubes from 2 to 4 is presented. The methods of joining nanotubes and some peculiarities of their growth are discussed.

Phase structures and interaction of BEDT-TTF molecules

Structural phases in crystal BEDT-TTF under the influence of doping molecules were studied. The system demonstrates the transition from a parallel aligned molecular phase (β-phase) to a new equilibrium state, where the molecules are rotated at a certain angle (α- or θ-phase). The precursor of this transition is the electrostatic and quadrupole interaction between BEDT-TTF and doping molecules. The modified potential of the Girifalco type is introduced, it depends on both intermolecular distance and the angle of the molecular rotation. It is found that the equilibrium distance between molecules in a stack increases and the deflection angle arises with an increase of the charge of doping molecules, which leads to new equilibrium states.

Comparison of methods for 14N-1H recoupling in 14N-1H HMQC MAS NMR

1H-detected 14N heteronuclear multiple-quantum coherence (HMQC) magic-angle-spinning (MAS) NMR experiments performed at fast magic-angle spinning (≥50 kHz) are finding increasing application, e.g., to pharmaceuticals. Of importance to the efficacy of these techniques is the recoupling technique applied to reintroduce the 1H-14N dipolar coupling. In this paper, we compare, by experiment and 2-spin density matrix simulations, two classes of recoupling scheme: first, those based on n = 2 rotary resonance, namely R3 and spin-polarisation inversion SPI-R3, and the symmetry based SR412 method and, second, the TRAPDOR method. Both classes require optimisation depending on the magnitude of the quadrupolar interaction, and thus there is a compromise choice for samples with more than one nitrogen site, as is the case for the studied dipeptide β-AspAla that contains two nitrogen sites with a small and large quadrupolar coupling constant. Considering this, we observe better sensitivity for the TRAPDOR method, though noting the marked sensitivity of TRAPDOR to the 14N transmitter offset, with both SPI-R3 and SR412 giving similar recoupling performance.

Influence of Residual Quadrupolar Interaction on Quantitative Sodium Brain Magnetic Resonance Imaging of Patients With Multiple Sclerosis.

The purpose of this work was to evaluate the influence of residual quadrupolar interaction on the determination of human brain apparent tissue sodium concentrations (aTSCs) using quantitative sodium magnetic resonance imaging ( 23 Na MRI) in healthy controls (HCs) and patients with multiple sclerosis (MS). Especially, it was investigated if the more detailed examination of residual quadrupolar interaction effects enables further analysis of the observed 23 Na MRI signal increase in MS patients. 23 Na MRI with a 7 T MR system was performed on 21 HC and 50 MS patients covering all MS subtypes (25 patients with relapsing-remitting MS, 14 patients with secondary progressive MS, and 11 patients with primary progressive MS) using 2 different 23 Na pulse sequences for quantification: a commonly used standard sequence (aTSC Std ) as well as a sequence with shorter excitation pulse length and lower flip angle for minimizing signal loss resulting from residual quadrupolar interactions (aTSC SP ). Apparent tissue sodium concentration was determined using the same postprocessing pipeline including correction of the receive profile of the radiofrequency coil, partial volume correction, and relaxation correction. Spin dynamic simulations of spin-3/2 nuclei were performed to aid in the understanding of the measurement results and to get deeper insight in the underlying mechanisms. In normal-appearing white matter (NAWM) of HC and all MS subtypes, the aTSC SP values were approximately 20% higher than the aTSC Std values ( P < 0.001). In addition, the ratio aTSC SP /aTSC Std was significantly higher in NAWM than in normal-appearing gray matter (NAGM) for all subject cohorts ( P < 0.002). In NAWM, aTSC Std values were significantly higher in primary progressive MS compared with HC ( P = 0.01) as well as relapsing-remitting MS ( P = 0.03). However, in contrast, no significant differences between the subject cohorts were found for aTSC SP . Spin simulations assuming the occurrence of residual quadrupolar interaction in NAWM were in good accordance with the measurement results, in particular, the ratio aTSC SP /aTSC Std in NAWM and NAGM. Our results showed that residual quadrupolar interactions in white matter regions of the human brain have an influence on aTSC quantification and therefore must be considered, especially in pathologies with expected microstructural changes such as loss of myelin in MS. Furthermore, the more detailed examination of residual quadrupolar interactions may lead to a better understanding of the pathologies themselves.

Eu2+/Ce3+-doped NaAlSiO4 selective occupation-induced emission structure unit evolution and its luminescence characteristics

NaAlSiO4: xEu2+,yCe3+ blue-red phosphors were prepared by high-temperature melting-icy quenching-heat treatment. The effects of emission structure unit evolution, microstrain, microcrystalline size and selective occupation on the luminescence characteristics of the prepared phosphor were systematically studied by powder X-ray diffraction pattern (XRD) and photoluminescence spectroscopy (PL). In the structure of NaAlSiO4: 2%Eu2+, 2%Ce3+ hexagonal crystalline sodium nepheline, Eu2+ ions occupy the Na+ (II) position to form a EuNa+(II)2+ luminescent structure unit, and Ce3+ ions occupy the Na+ (I) position to form a CeNa+(I)3+ luminescent structure unit. The NaAlSiO4: 2%Eu2+/NaAlSiO4: 2%Ce3+ emits 555 nm yellow-green light/395 nm blue-violet light under the excitation of 354 nm/298 nm, respectively, which is derived from the 5d-4f transition of Eu2+ and Ce3+. There is energy transfer of Ce3+→Eu2+ in the sodium nepheline matrix, of which the transport pathway is m EuNa+(II)2+ ←O←n CeNa+(I)3+ →O→m EuNa+(II)2+, and the transport mechanism is electric quadrode-electric quadrupole interaction, and the energy transfer efficiency reaches 98.6%. The sodium nepheline NaAlSiO4: 2%Eu2+, 2%Ce3+ phosphor with energy level barrier 0.386 eV, possesses good thermal stability. This single-phase aluminosilicate multicolor phosphor has potential application value for WLED illumination and PDP display.

Synthesis and up-conversion luminescence of Ho3+ and Yb3+ co-doped La7P3O18 phosphors

Ho3+ and Yb3+ co-doped La7P3O18 up-conversion phosphors were prepared by a high-temperature solid-state process. XRD results reveal that the samples are mixtures of monoclinic structure La7P3O18 crystals with P21/n space group and minor LaPO4 crystals. UV—Vis DRS results indicate that La7P3O18 crystal is an indirect semiconductor with an optical band gap of 4.10 eV. After 980 nm laser excitation, the Ho3+ and Yb3+ co-doped La7P3O18 phosphors show the characteristic blue (486 nm), green (550 nm), and red (661 nm) luminescence peaks of Ho3+ ions. The peak at 661 nm dominates in the up-conversion luminescence spectra of samples. Meanwhile, with the increase of Ho3+ and Yb3+ doping content, the up-conversion luminescence intensity increases first and then decreases. By increasing the doping contents of Ho3+ and Yb3+ up to 1 at.% and 10 at.%, respectively, concentration quenching may appear by an electric quadrupole– electric quadrupole interaction process. The pumping power dependence of luminescence indicates that the green and red emissions of samples are excited by a two-photon absorption process. The color coordinates of Ho3+ and Yb3+ co-doped La7P3O18 crystals locate in the orange—red region.

First-Principles Calculation of the Temperature-Dependent Transition Energies in Spin Defects.

Spin qubits associated with color centers are promising platforms for various quantum technologies. However, to be deployed in robust quantum devices, the variations of their intrinsic properties with the external conditions, in particular temperature and strain, should be known with high precision. Unfortunately, a predictive theory on the temperature dependence of the resonance frequency of electron and nuclear spin defects in solids remains lacking. In this work, we develop a first-principles method for the temperature dependence of the zero-field splitting, hyperfine interaction, and nuclear quadrupole interaction of color centers. As a testbed, we compare our ab initio calculations with experiments for the nitrogen-vacancy (NV-) center in diamond, finding good agreements. We identify the major origin of the temperature dependence as a second-order effect of dynamic phonon vibrations, instead of the thermal-expansion strain. The method can be applied to different color centers and provides a theoretical tool for designing high-precision quantum sensors.

β -decay half-lives of the r -process waiting-point isotones of N=81 and 82 nuclei

In this paper we perform systematic calculations of $\ensuremath{\beta}$-decay half-lives for the $r$-process waiting-point isotones of $N=81$ and 82, in the framework of nucleon-pair approximation. A phenomenological form of the Hamiltonian, which includes single-particle energies, pairing plus quadrupole interactions, and Gamow-Teller interactions is assumed, and the theoretical $\ensuremath{\beta}$-decay energies are taken based on the Bayesian machine learning. A good agreement with experimental values of half-lives is achieved by using the nucleon-pair truncated model space in which the $S$-pair condensation dominates. Our calculated half-lives are found to be very sensitive to $\ensuremath{\beta}$-decay energies on the one hand, and are quite robust with respect to extension of the model space and variation of interaction parameters on the other hand.

Spectral features of vibrational Te isotopes

The variation of collective vibrational spectra with varying $N$ displayed by the $Z=52$ Te isotopes, for less than $N=82$ closed shell, are of special interest. The spectral features of the ground band and the two and three phonon excited multiplets in $^{118--128}\mathrm{Te}$, are studied here empirically. The state energy $E({2}_{1}{}^{+})$ increases, and the energy ratio ${R}_{4/2}$ decreases with increasing neutron number $N$. Also the ${0}_{2}{}^{+}$ state rises up sharply, and the ${0}_{3}{}^{+}$ state follows it, unlike their relation in the E(5) symmetry. It is shown that $^{120}\mathrm{Te}$ isotope displays the weakly deformed status, and the heavier isotopes move towards the spherical harmonic vibrator status. The interacting boson model (IBM-1) is used to reproduce these spectral features of $^{118--124}\mathrm{Te}$, with use of slight variation of boson energy \ensuremath{\epsilon} and of the quadrupole interaction strength. In the microscopic approach, the dynamic pairing plus quadrupole model of Kumar and Baranger also reproduces these basic spectral features.

Correlation of Neutrinoless Double-β Decay Nuclear Matrix Element with E2 Strength

We explore the correlation of the neutrinoless double-β decay nuclear matrix element (NME) with electric quadrupole (E2) strength in the framework of the Hamiltonian-based generator-coordinate method, which is a configuration-mixing calculation of symmetry-restored intrinsic basis states. The restoration of symmetries that are simultaneously broken in the mean-field states allows us to compute the structural and decay properties associated with wave functions characterized by good quantum numbers. Our calculations show a clear anti-correlation between the neutrinoless double-β decay NME and the transition rate of the collective quadrupole excitation from the ground state in response to artificial changes of the quadrupole–quadrupole interaction. The anti-correlation is more remarkable in the decay from a weakly deformed parent nucleus to a more deformed grand-daughter nucleus. This interrelation may provide a way to reduce the uncertainty of the nuclear matrix element.

Symmetry of the Hyperfine and Quadrupole Interactions of Boron Vacancies in a Hexagonal Boron Nitride

The concept of optically addressable spin states of deep level defects in wide band gap materials is successfully applied for the development of quantum technologies. Recently discovered negatively charged boron vacancy defects (VB) in hexagonal boron nitride (hBN) potentially allow a transfer of this concept onto atomic thin layers due to the van der Waals nature of the defect host. Here, we experimentally explore all terms of the VB spin Hamiltonian reflecting interactions with the three nearest nitrogen atoms by means of conventional electron spin resonance and high frequency (94 GHz) electron-nuclear double resonance. We establish symmetry, anisotropy, and principal values of the corresponding hyperfine interaction (HFI) and nuclear quadrupole interaction (NQI). The HFI can be expressed in the axially symmetric form as Aperp = 45.5 MHz and Apar = 87 MHz, while the NQI is characterized by quadrupole coupling constant Cq = 1.96 MHz with slight rhombisity parameter n = (Pxx - Pyy)/Pzz = -0.070. Utilizing a conventional approach based on a linear combination of atomic orbitals and HFI values measured here, we reveal that almost all spin density (84 %) of the VB electron spin is localized on the three nearest nitrogen atoms. Our findings serve as valuable spectroscopic data and direct experimental demonstration of the VB spin localization in a single two dimensional BN layer.

Electrostatic multipole contributions to the binding energy of electrons

The interaction of an electron with a local static charge distribution (e.g., an atom or molecule) is dominated by the radial 1/r Coulomb potential at large distances. The second-order effect is generated by the non-central electric dipole contribution cosθ/r2. The third-order effect is attributed to the electric quadrupole potential (3cos2θ-1)/2r3. In this study, we used the tridiagonal representation approach to produce a reasonably accurate account for the combined effects of every contribution to the binding energy of the electron with an effective quadrupole interaction. As a result, we obtained the bound states of a valence electron in an atom with both electric dipole and quadrupole moments.

Low-energy scatterings and pseudopotential of polarized quadrupoles

We investigate the low-energy scattering properties of two identical particles interacting via the polarized quadrupolar interaction. It is shown that a series of s- and p-wave resonances appear for identical bosons and fermions, respectively, as the strength of the quadrupolar interaction increases. Interestingly, scattering resonances also appear on the generalized scattering length corresponding to the coupling between the s and d waves. This observation inspires us to propose a new pseudopotential for the quadrupolar interaction. We also explore the bound-state properties of two particles in both free space and harmonic traps.

Interaction of two Rydberg atoms in the vicinity of an optical nanofibre

We consider two rubidium atoms, prepared in the same S or P Rydberg states, near an optical nanofibre, and we determine their van der Waals interaction potential as a function of their separation along the nanofibre axis, their distance to the nanofibre axis, and their relative azimuthal angle. We compare results obtained through direct diagonalisation of the Hamiltonian (including quadrupolar interaction terms) with second-order perturbation calculations, and we identify which couplings mainly contribute to the potential in the presence of the nanofibre and in free-space. We relate the appearance of new allowed couplings to the broken rotation symmetry around the interatomic axis due to the presence of the fibre. These couplings induce novel features and cause a reshaping of the interaction anisotropy and formation of an interaction potential well for P states near the nanofibre. Our work constitutes an important step in the assessment of Rydberg atom-nanofibre quantum interfaces and devices.

Urea-doped hierarchical porous carbons derived from sucrose precursor for highly efficient CO2 adsorption and separation

Over the past few years, heteroatom-doped microporous carbons have engrossed significant interest as emergent CO2 adsorbents. However, the clear role of small-sized micropores and heteroatom content on CO2 adsorption is rarely been reported. In this study, a single-step synthetic approach is proposed to prepare a series of sucrose-derived porous carbons by choosing different ratios of KOH and urea (as nitrogen dopant) content (up to ∼ 11 at.%). The role of both KOH and urea variation on textural features (such as specific surface area: SSA, and pore volume: PV) and resultant CO2 adsorption capacity has been studied. Based on KOH and urea content variation, the prepared carbons achieved a remarkable surface area (342-2231 m2g−1), and a large micropore volume (0.24-1.32 cm3g−1). The optimized sample “SUK-113” with sucrose: urea: KOH ratio of 1:1:3 demonstrated a CO2 uptake of 8.19 mmol g−1 at 273K/1 bar. Such remarkable capacity value is accredited to the presence of the highest micropore volume for pores < 1nm for the investigated material. Furthermore, the calculated isosteric heat of adsorption (28.7-34.5 kJ mol−1) and CO2/N2 selectivity (28-137) revealed the significance of nitrogen moieties which offers quadrupole interactions for CO2 molecules and therefore, improved selective separation of CO2 from N2 gases. Conclusively, the narrow micropores and optimum nitrogen content played a decisive role in achieving excellent CO2 adsorption performance in these carbons.

Novel Sm3+/Eu3+ co-doped Sr7Sb2O12 red-emitting phosphor for white LED

Novel red-emitting phosphors Sr7Sb2O12: Sm3+ and Sr7Sb2O12: Sm3+, Eu3+ were successfully synthesized through a high temperature solid-state reaction method. The materials were characterized by X-ray diffraction patterns, scanning electron microscopy, Raman spectra, diffuse reflection spectra, photoluminescence excitation and emission spectra, luminescence decay curves and temperature-dependent emission spectra. The energy transfer between Sm3+ and Eu3+ ions in Sr7Sb2O12 was realized via electric quadrupole–quadrupole interaction with about 42.4% transfer efficiency. The phosphor exhibited good thermal stability with activation energy of 0.180 eV. Color coordinates and color purity of Sr7Sb2O12: 0.08Sm3+, 0.09Eu3+ was (0.620, 0.368) and 87.4% respectively. Combined the red phosphor with the near-UV or blue chips, the fabricated light emitting diode (LED) lamp emitted white light with Ra = 83.7 or 82.4 color rendering index (CRI) respectively. These results demonstrated that Sm3+/Eu3+ co-doped Sr7Sb2O12 red phosphor has a great potential application in white LED.

Nonlinear Hall effect of magnetized two-dimensional spin- 32 heavy holes

We identify a sizable nonlinear Hall effect of spin-3/2 heavy holes in zincblende nanostructures, driven by a quadrupole interaction with the electric field formerly believed to be negligible. The interaction is enabled by ${T}_{d}$-symmetry, reflects inversion breaking, and in two dimensions results in an electric-field correction to the in-plane $g$ factor. The effect can be observed in state-of-the-art heterostructures, either via magnetic doping or by using a vector magnet, where even for small perpendicular magnetic fields it is comparable in magnitude to topological materials.

Quantifying quadrupole effects in the NMR spectra of spin-1/2 nuclei in rotating solids.

Quantifying the NMR spectra of spin I = 1/2 nuclei coupled to quadrupolar spins (nuclei with spin quantum number, S > 1/2) has remained an arduous task in solid-state magic angle spinning (MAS) NMR experiments. In particular, extraction of chemical shift anisotropy (CSA) tensors from line-shapes of spin I = 1/2 nuclei coupled to quadrupolar spin (S = 1) in MAS experiments has remained challenging owing to the simultaneous presence of both heteronuclear dipolar interactions and quadrupolar interactions. Unlike experiments that involve only spin-1/2 nuclei, both faster spinning frequencies and stronger decoupling field strengths on the quadrupolar spins are essential to average/minimize the contributions from heteronuclear dipolar interactions. To this end, a quantitative theory based on the concept of "effective fields" is proposed to deduce optimal conditions in experiments that involve simultaneous recoupling and decoupling of heteronuclear dipolar interactions. Through analytic expressions, the spectral frequencies and intensities observed in experiments are quantified and rigorously verified. Since extraction of molecular constraints in NMR experiments involves iterative fitting of experimental data, we believe that the analytic expressions derived would speed up and be beneficial in quantifying such experiments.

Probing molecular interactions of semiquinone radicals at quinone reduction sites of cytochrome bc1 by X-band HYSCORE EPR spectroscopy and quantum mechanical calculations.

Quinone redox reactions involve a semiquinone (SQ) intermediate state. The catalytic sites in enzymes stabilize the SQ state via various molecular interactions, such as hydrogen bonding to oxygens of the two carbonyls of the benzoquinone ring. To understand how these interactions contribute to SQ stabilization, we examined SQ in the quinone reduction site (Qi) of cytochrome bc1 using electron paramagnetic resonance (ESEEM, HYSCORE) at the X-band and quantum mechanical (QM) calculations. We compared native enzyme (WT) with a H217R mutant (replacement of histidine that interacts with one carbonyl of the occupant of Qi to arginine) in which the SQ stability has previously been shown to markedly increase. The 14N region of the HYSCORE 2D spectrum for SQi in WT had a shape typical of histidine residue, while in H217R, the spectrum shape changed significantly and appeared similar to the pattern described for SQ liganded natively by arginine in cytochrome bo3. Parametrization of hyperfine and quadrupolar interactions of SQi with surrounding magnetic nuclei (1H, 14N) allowed us to assign specific nitrogens of H217 or R217 as ligands of SQi in WT and H217R, respectively. This was further substantiated by qualitative agreement between the experimental (EPR-derived) and theoretical (QM-derived) parameters. The proton (1H) region of the HYSCORE spectrum in both WT and H217R was very similar and indicative of interactions with two protons, which in view of the QM calculations, were identified as directly involved in the formation of a H-bond with the two carbonyl oxygens of SQ (interaction of H217 or R217 with O4 and D252 with O1). In view of these assignments, we explain how different SQ ligands effectively influence SQ stability. We also propose that the characteristic X-band HYSCORE pattern and parameters of H217R are highly specific to the interaction of SQ with the nitrogen of arginine. These features can thus be considered as potential markers of the interaction of arginine with SQ in other proteins.