Quadrupole Interaction Research Articles

Interplay between Quadrupolar and Magnetic Interactions in 5d1 Double Perovskite Ba2MgReO6 under Pressure

We measured the electrical resistivity, AC magnetic susceptibility, and specific heat of the cubic double perovskite Ba$_2$MgReO$_6$ under pressure. The application of pressure leads to a gradual increase in the transition temperature to the canted antiferromagnetic order, while the quadrupolar order is suppressed above 5 GPa, at which a collinear antiferromagnetic state appears. The obtained unit cell volume-temperature phase diagram for the compound as well as the related compounds suggests that Ba$_2$MgReO$_6$ exists near the phase boundary between the noncollinear and the collinear antiferromagnetic order with multipole decrees of freedom.

Cross-correlation spectra in interacting quantum dot systems

Two-color spin-noise spectroscopy of interacting electron spins in singly charged semiconductor quantum dots provides information on the interquantum dot interactions. We investigate the spin cross-correlation function in a quantum dot ensemble employing a modified semiclassical approach. Spin-correlation functions are calculated using a Hamilton quaternion approach that maintains local quantum mechanical properties of the spins. This method takes into account the effects of the nuclear-electric quadrupolar interactions, the randomness of the coupling constants, and the variation of the electron $g$ factor on the spin-noise power spectra. We demonstrate that the quantum dot ensemble can be mapped on an effective two-quantum dot problem and discuss how the characteristic length scale of the interdot interaction modifies the low-frequency cross-correlation spectrum. We argue that details on the interaction strength distribution can be extracted from the cross-correlation spectrum when applying a longitudinal or a transversal external magnetic field.

Probing multiferroic order parameters and domain population via nuclear spins

Quantifying domain population in multiferroics is required to understand domain nucleation/switching processes and achieve on-demand domain control. We report an approach based on nuclear magnetic resonance spectroscopy for the accurate measurement of volume fractions of multiferroic domains in bulk crystals. We demonstrate on a benchmarking system, ${\mathrm{GaV}}_{4}{\mathrm{Se}}_{8}$, that the electric quadrupole interaction of the $^{71}\mathrm{Ga}$ and the hyperfine field at the $^{51}\mathrm{V}$ nuclei are proper microscopic probes of the ferroelectric polarization and the ferromagnetic moment, respectively. We use the anisotropy of these local quantities to determine the multiferroic domain population, controlled here by both electric and magnetic fields. The sensitivity of this local-probe technique to site symmetries facilitates domain quantification in a wide range of anisotropic magnets, ferroelectrics, and multiferroics.

Quadrupolar Interaction with Zeolite and Pressure Swing Adsorption in Portable Medical Oxygen Concentrators for Breathing of Covid-19 and COPD Patients

Pressure swing adsorption (PSA) technology is used in medical oxygen concentrators for selective adsorption of nitrogen under pressure from air to produce oxygen-enriched air for breathing. The functional material in oxygen concentrators is aluminosilicate zeolite like Li-13X, 5A (LTA) with a relatively high Al-content. It selectively adsorbs nitrogen from the air into their molecular sieves due to the strong quadrupolar interaction between the adsorbed nitrogen and the electrical field of the charge balancing non-framework exchangeable cations of aluminosilicate zeolites. In an oxygen concentrator, two adsorbent vessels packed with the zeolite working alternately in a cyclic process are used for a near-continuous production of the oxygen-enriched air.

SUSY in the sky with gravitons

Picture yourself in the wave zone of a gravitational scattering event of two massive, spinning compact bodies (black holes, neutron stars or stars). We show that this system of genuine astrophysical interest enjoys a hidden mathcal{N} = 2 supersymmetry, at least to the order of spin-squared (quadrupole) interactions in arbitrary D spacetime dimensions. Using the mathcal{N} = 2 supersymmetric worldline action, augmented by finite-size corrections for the non-Kerr black hole case, we build a quadratic-in-spin extension to the worldline quantum field theory (WQFT) formalism introduced in our previous work, and calculate the two bodies’ deflection and spin kick to sub-leading order in the post-Minkowskian expansion in Newton’s constant G. For spins aligned to the normal vector of the scattering plane we also obtain the scattering angle. All D-dimensional observables are derived from an eikonal phase given as the free energy of the WQFT that is invariant under the mathcal{N} = 2 supersymmetry transformations.

Study of the doping effect on imperfect morphology at photovoltaic heterojunctions in bilayer organic solar cells

The efficient doping of a donor–acceptor copolymer is controlled by the charge–quadrupole interaction between the dopant and the respective units to improve the photovoltaic performance by alleviating the energy barrier for charge separation.

Hydrogen-Bonded Complexes of Neutral Nitroxide Radicals with 2-Propanol Studied by Multifrequency EPR/ENDOR

The hydrogen bond plays a key role in weak directional intermolecular interactions. It is operative in determining molecular conformation and aggregation, and controls the function of many chemical systems, ranging from inorganic, organic to biological molecules. Although an enormous amount of spectroscopic information has been collected about hydrogen-bond formation between molecules with closed-shell electronic configuration, the details of such interactions between open-shell radicals and closed-shell molecules are still rare. Here we report on an investigation of hydrogen-bonded complexes between pyrroline-type as well as piperidine-type neutral nitroxide radicals and an alcohol, i.e., 2-propanol. These nitroxide radicals are commonly used as EPR spin labels and probes. To obtain information on the geometry of the complexes and their electronic structure, multi-resonance EPR techniques at various microwave frequencies (X-, Q-, W-band, 244 GHz) have been employed in conjunction with DFT calculations. The planar five-membered ring system of the pyrroline-type nitroxide radical was found to form exclusively well-defined in-plane σ-type hydrogen-bonded complexes with one 2-propanol molecule in the first solvation shell in frozen solution. The measured hyperfine parameters of the hydrogen-bridge proton and the internal magnetic parameters describing the electron Zeeman and the electron-nuclear hyperfine and nuclear quadrupole interactions are in good agreement with values predicted by state-of-the-art DFT calculations. In contrast, multi-resonance EPR on the non-planar six-membered ring system of the piperidine-type nitroxide radical (TEMPOL) reveals a more complex situation, i.e., a mixture of a σ-type with, presumably, an out-of-plane π-type complex, both present in comparable fraction in frozen solution. For TEMPOL, the DFT calculations failed to predict magnetic interaction parameters that are in good agreement with experiment, apparently due to the considerable flexibility of the nitroxide and hydrogen-bonded complex. The detailed information about nitroxide/solvent complexes is of particular importance for Dynamic Nuclear Polarization (DNP) and site-directed spin-labeling EPR studies that employ nitroxides as polarizing agents or spin labels, respectively.

Magnetic transitions in two novel mixed-valence iron(II)–iron(III) metal formate frameworks: Two sublattice model

We study the two-sublattice model in the mean field theory by expanding the Gibbs free energy in terms of the magnetizations M1 (Mup) and M2 (Mdown) with the quadratic coupling M12M22 (quadrupolar interactions) for the order–disorder transition in the two mixed-valence iron (II)-iron (III) metal formate frameworks, C2H5NH3FeIIFeIIIHCOO6 and C2H52NH2FeIIFeIIIHCOO6. Expressions derived from the Gibbs free energy for the temperature dependence of the magnetizations M1 and M2, are fitted to the observed data (H=100Oe) from the literature for the two mixed-valence iron (II) – iron (III) system. The inverse susceptibilities (χ1-1 and χ2-1) are predicted as a function of temperature (H=100Oe) from the two-sublattice model. Additionally, the field dependence of the total magnetization (M=M1+M2) is analyzed using the literature data for the compounds studied.We find that the two-sublattice model with the quadrupolar interactions, is satisfactory to describe the order–disorder transition in the two mixed-valence iron (II) – iron (III) metal formate frameworks as studied here. It indicates that the two-sublattice model can also be applied to some other multiferroic mixed materials.

Warm-up spectroscopy of quadrupole-split nuclear spins in n -GaAs epitaxial layers

The efficiency of the adiabatic demagnetization of the nuclear spin system (NSS) of a solid is limited, if quadrupole effects are present. Nevertheless, despite a considerable quadrupole interaction, recent experiments validated the thermodynamic description of the NSS in GaAs. This suggests that nuclear spin temperature can be used as a probe of nuclear magnetic resonances. We implement this idea by analyzing the modification of the NSS temperature in response to an oscillating magnetic field at various frequencies, an approach termed as the warm-up spectroscopy. It is tested in a $n$-GaAs sample where both mechanical strain and built-in electric field may contribute to the quadrupole splitting, yielding the parameters of electric field gradient tensors for $^{75}\mathrm{As}$ and both Ga isotopes, $^{69}\mathrm{Ga}$ and $^{71}\mathrm{Ga}$.

Probing Fermi liquid exceptional points through AC conductivity

Exceptional points, which are topological non-Hermitian degeneracies, show up in the collective mode spectrum of Fermi liquids with high angular momentum interactions. In this paper, we look for signatures of these nontrivial singularities by computing the AC conductivity of Fermi liquids with dipolar and quadrupolar interactions in a narrow slab. We show that the finite size of the slab imprints clear signatures of the structures of the collective mode spectrum in the conductivity as well as in the dephasing between the electric field and the current density in a wide range of coupling constants. In particular, we show the fingerprints of exceptional points, observed in the weak attractive dipolar and/or quadrupolar regime. The main result does not depend on the specific model of quasiparticle interactions. We also discuss some actual compounds where these phenomena could be experimentally observed.

D‐Band EPR and ENDOR Spectroscopy of 15N‐Labeled Photosystem I

For billions of years, nature has optimized the photosynthetic machinery that converts light energy into chemical energy. Key primary reactions of photosynthesis occur in large membrane protein–cofactor complexes. The light-induced sequential electron transfer reactions occur through a chain of donor/acceptor cofactors embedded in the protein matrix resulting in a long-lived transmembrane charge-separated state. EPR is the method of choice to study electron transfer and the interaction of protein environment with redox-active cofactors. However, the spectra of organic cofactor radicals typically are not fully resolved and severely overlap at conventional X-band EPR. Even at Q-band EPR, this overlap is present and often a serious problem. As a result, there is a large variation of the reported EPR data and limited understanding of electronic structures of several redox-active cofactors. These serious problems can often be overcome by the excellent spectral resolution provided by high-frequency EPR (HF EPR). Here, we study the electronic structure of the primary electron donor P700 and the secondary electron acceptor A1 of Photosystem I (PSI) using 130 GHz (D-band) EPR and Electron–Nuclear-Double-Resonance (ENDOR) spectroscopy. PSI was isotopically labeled with 15N (I = ½) to avoid quadrupolar interactions in the most abundant nitrogen isotope 14N (I = 1) and simplify the ENDOR spectra. ENDOR spectroscopy is central for determining the hyperfine coupling of nitrogen atoms of the two chlorophyll molecules comprising oxidized P700 and the involvement of protein nitrogen atoms with reduced A1. While HF ENDOR of A1− allows identification of two nitrogen atoms, HF ENDOR of P700+ still does not permit unique assignment of the recorded hyperfine couplings.

M\xf6ssbauer measurements of GaFeO3 single crystal multiferroic

Mössbauer measurements on single crystal absorbers at room and at low temperatures were performed. The results are fully consistent with previously published reports by other groups. Spectra of single crystals were simultaneously analyzed including magnetic dipole and electric quadrupole interactions. The analysis shows that there is a small component of magnetic moments perpendicular to the magnetic easy axis. Mössbauer data seem not agree with commonly accepted ferrimagnetic structure of GaFeO3.

Engineering local strain for single-atom nuclear acoustic resonance in silicon

Mechanical strain plays a key role in the physics and operation of nanoscale semiconductor systems, including quantum dots and single-dopant devices. Here, we describe the design of a nanoelectronic device, where a single nuclear spin is coherently controlled via nuclear acoustic resonance (NAR) through the local application of dynamical strain. The strain drives spin transitions by modulating the nuclear quadrupole interaction. We adopt an AlN piezoelectric actuator compatible with standard silicon metal–oxide–semiconductor processing and optimize the device layout to maximize the NAR drive. We predict NAR Rabi frequencies of order 200 Hz for a single 123Sb nucleus in a wide region of the device. Spin transitions driven directly by electric fields are suppressed in the center of the device, allowing the observation of pure NAR. Using electric field gradient-elastic tensors calculated by the density-functional theory, we extend our predictions to other high-spin group-V donors in silicon and to the isoelectronic 73Ge atom.

Pure Nuclear Bragg Reflection due to Combined Magnetic and Quadrupole Interaction in Fe3O4

A pure nuclear Bragg reflection due to the combined magnetic and quadrupole interaction in the B-site Fe of Fe3O4 has been investigated. The 10 10 0 reflection (Bragg angle θB = 46.48°), which is e...

Extension of Spectral Shift Controls from Equivalent Substitution to an Energy Migration Model Based on Eu2+/Tb3+-Activated Ba4-xSrxGd3-xLuxNa3(PO4)6F2 Phosphors.

Single-phase phosphors with tunable emission colors are crucial to develop high-performance white light-emitting diodes since they are valuable to improve the energy efficiency, color rendering index, and correlated color temperature. Most of the studies have been conducted to control the spectral shifts via a polyhedral distortion or chemical unit cosubstitution strategy. The combination of host optimization and dopant activator design in a single-phase phosphor system is very rare. Herein, a partial substitution strategy of [Ba2+-Gd3+] by [Sr2+-Lu3+] has been employed in Ba4-xSrxGd3-x-yLuxNa3(PO4)6F2/5% Eu2+ (x = 0-0.40) phosphors. Also, the energy migration from Eu2+ to Tb3+ ions has been investigated in as-prepared samples. Consequently, the emitted signal is observed to shift from 470 to 575 nm derived from equivalent substitutions, which is attributed to specific performance by the emission profile of Eu2+, and such results are closely related to splitting of the crystal field and energy transfer among various luminescent centers. Moreover, the tunable yellowish-green emitting material has been assembled by incorporating ion pairs (Eu2+ → Tb3+) into the Ba3.85Sr0.15Gd2.85Lu0.15Na3(PO4)6F2, and their relative ratios are varied. The corresponding Eu2+ → Tb3+ energy migration process is assigned to be the dipole-quadrupole interaction by the Inokuti-Hirayama model. This work provides rational guidance for the design and discovery of new products with tunable emission colors, originating from the cosubstitution strategy and energy conversion model.

Hyperfine and Quadrupole Interactions in EPR of 153Eu2+ and 151Eu2+ in Yttrium–Aluminum Garnet

Hyperfine and Quadrupole Interactions in EPR of 153Eu2+ and 151Eu2+ in Yttrium–Aluminum Garnet

Nuclear magnetic relaxation studies of BH4 in metal Borohydrides

AbstractA different approach of nuclear magnetic resonance (NMR) relaxation of the rotational motion of BH4 tetrahedron in light metal borohydrides was introduced. Previous NMR relaxation studies were mainly performed using an approximation which is based on a single exponential transition approach for the spin–lattice relaxation of the spin under quadrupolar interaction. In this new approach, a relaxation rate equation was derived by strictly taking into account both electric quadrupolar and magnetic dipolar contributions to the transitions between Zeeman energy states of quadrupolar 11B nucleus that was coupled to many proton spins. Two exponential relaxation times derived from our theoretical treatment were applied to the analysis of 11B spin–lattice relaxation of LiBH4 and Ca(BH4)2. The activation energies and the correlation times of the reorientational motions of BH4 were evaluated from the two experimental spin–lattice relaxation times. The results were compared to previous activation energies and correlation times.

Separation of quadrupolar and paramagnetic shift interactions in high-resolution nuclear magnetic resonance of spinning powders.

Separation and correlation of the shift anisotropy and the first-order quadrupolar interaction of spin I = 1 nuclei under magic-angle spinning (MAS) are achieved by the phase-adjusted spinning sideband (PASS) nuclear magnetic resonance (NMR) experiment. Compared to methods for static samples, this approach has the benefit of higher sensitivity and resolution. Moreover, the PASS experiment has the advantage over previous MAS sequences in the ability to completely separate the shift anisotropy and first-order quadrupolar interactions. However, the main drawback of the pulse sequence is the lower excitation bandwidth. The sequence is comprehensively evaluated using theoretical calculations and numerical simulations and applied experimentally to the 2H NMR of a range of paramagnetic systems: deuterated nickel(II) acetate tetrahydrate, deuterated copper(II) chloride dihydrate, and two forms of deuterated oxyhydride ion conductor BaTiO3-xHy. Our results show that despite the issue with broadband excitation, the extracted shift and quadrupolar interaction tensors and the Euler angles relating the two tensors match well with the NMR parameters obtained with static NMR methods. Therefore, the new application of the PASS experiment is an excellent addition to the arsenal of NMR experiments for 2H and potentially 14N in paramagnetic solids.

Observation of Sub-Microsecond Protein Methyl-Side Chain Dynamics by Nanoparticle-Assisted NMR Spin Relaxation.

Amino-acid side-chain properties in proteins are key determinants of protein function. NMR spin relaxation of side chains is an important source of information about local protein dynamics and flexibility. However, traditional solution NMR relaxation methods are most sensitive to sub-nanosecond dynamics lacking information on slower ns-μs time-scale motions. Nanoparticle-assisted NMR spin relaxation (NASR) of methyl-side chains is introduced here as a window into these ns-μs dynamics. NASR utilizes the transient and nonspecific interactions between folded proteins and slowly tumbling spherical nanoparticles (NPs), whereby the increase of the relaxation rates reflects motions on time scales from ps all the way to the overall tumbling correlation time of the NPs ranging from hundreds of ns to μs. The observed motional amplitude of each methyl group can then be expressed by a model-free NASR S2 order parameter. The method is demonstrated for 2H-relaxation of CH2D methyl moieties and cross-correlated relaxation of CH3 groups for proteins Im7 and ubiquitin in the presence of anionic silica-nanoparticles. Both types of relaxation experiments, dominated by either quadrupolar or dipolar interactions, yield highly consistent results. Im7 shows additional dynamics on the intermediate time scales taking place in a functionally important loop, whereas ubiquitin visits the majority of its conformational substates on the sub-ns time scale. These experimental observations are in good agreement with 4-10 μs all-atom molecular dynamics trajectories. NASR probes side-chain dynamics on a much wider range of motional time scales than previously possible, thereby providing new insights into the interplay between protein structure, dynamics, and molecular interactions that govern protein function.

Radiation-Induced Stable Radicals in Calcium Phosphates: Results of Multifrequency EPR, EDNMR, ESEEM, and ENDOR Studies

This article presents the results of a study of radiation-induced defects in various synthetic calcium phosphate (CP) powder materials (hydroxyapatite—HA and octacalcium phosphate—OCP) by electron paramagnetic resonance (EPR) spectroscopy at the X, Q, and W-bands (9, 34, 95 GHz for the microwave frequencies, respectively). Currently, CP materials are widely used in orthopedics and dentistry owing to their high biocompatibility and physico-chemical similarity with human hard tissue. It is shown that in addition to the classical EPR techniques, other experimental approaches such as ELDOR-detected NMR (EDNMR), electron spin echo envelope modulation (ESEEM), and electron-nuclear double resonance (ENDOR) can be used to analyze the electron–nuclear interactions of CP powders. We demonstrated that the value and angular dependence of the quadrupole interaction for 14N nuclei of a nitrate radical can be determined by the EDNMR method at room temperature. The ESEEM technique has allowed for a rapid analysis of the nuclear environment and estimation of the structural positions of radiation-induced centers in various crystal matrices. ENDOR spectra can provide information about the distribution of the nitrate radicals in the OCP structure.