Quadrupolar Hyperfine Interactions Research Articles

Precise determination of the low-energy electronuclear Hamiltonian of LiY1−xHoxF4

We use complementary optical spectroscopy methods to directly measure the lowest crystal-field energies of the rare-earth quantum magnet LiY$_{1-x}$Ho$_{x}$F$_{4}$, including their hyperfine splittings, with more than 10 times higher resolution than previous work. We are able to observe energy level splittings due to the $^6\mathrm{Li}$ and $^7\mathrm{Li}$ isotopes, as well as non-equidistantly spaced hyperfine transitions originating from dipolar and quadrupolar hyperfine interactions. We provide refined crystal field parameters and extract the dipolar and quadrupolar hyperfine constants ${A_J=0.02703\pm0.00003}$ $\textrm{cm}^{-1}$ and ${B= 0.04 \pm0.01}$ $\textrm{cm}^{-1}$, respectively. Thereupon we determine all crystal-field energy levels and magnetic moments of the $^5I_8$ ground state manifold, including the (non-linear) hyperfine corrections. The latter match the measurement-based estimates. The scale of the non-linear hyperfine corrections sets an upper bound for the inhomogeneous line widths that would still allow for unique addressing of a selected hyperfine transition. e.g. for quantum information applications. Additionally, we establish the far-infrared, low-temperature refractive index of LiY$_{1-x}$Ho$_{x}$F$_{4}$.

Electronic Structure of the Primary Electron Donor P700+• in Photosystem I Studied by Multifrequency HYSCORE Spectroscopy at X- and Q-Band.

The primary electron donor P700 of the photosystem I (PSI) is a heterodimer consisting of two chlorophyll molecules. A series of electron-transfer events immediately following the initial light excitation leads to a stabilization of the positive charge by its cation radical form, P700+•. The electronic structure of P700+• and, in particular, its asymmetry with respect to the two chlorophyll monomers is of fundamental interest and is not fully understood up to this date. Here, we apply multifrequency X- (9 GHz) and Q-band (35 GHz) hyperfine sublevel correlation (HYSCORE) spectroscopy to investigate the electron spin density distribution in the cation radical P700+• of PSI from a thermophilic cyanobacterium Thermosynechococcus elongatus. Six 14N and two 1H distinct nuclei have been resolved in the HYSCORE spectra and parameters of the corresponding nuclear hyperfine and quadrupolar hyperfine interactions were obtained by combining the analysis of HYSCORE spectral features with direct numerical simulations. Based on a close similarity of the nuclear quadrupole tensor parameters, all of the resolved 14N nuclei were assigned to six out of total eight available pyrrole ring nitrogen atoms (i.e., four in each of the chlorophylls), providing direct evidence of spin density delocalization over the both monomers in the heterodimer. Using the obtained experimental values of the 14N electron-nuclear hyperfine interaction parameters, the upper limit of the electron spin density asymmetry parameter is estimated as RA/Bupper = 7.7 ± 0.5, while a tentative assignment of 14N observed in the HYSCORE spectra yields RB/A = 3.1 ± 0.5.

Multi-level relaxation model for describing the Mössbauer spectra of single-domain particles in the presence of quadrupolar hyperfine interaction

A multi-level stochastic model taking into account the magnetic anisotropy, precession and diffusion of the uniform magnetization of single-domain particles is developed in order to describe the Mössbauer absorption spectra of an ensemble of magnetic nanoparticles in the presence of quadrupolar hyperfine interaction with an arbitrary orientation of its principal axes. This model allows one to take into account physical mechanisms for forming the hyperfine structure in a real situation and can be easily realized even on a personal computer. In particular, now one can numerically describe qualitative features of temperature evolution of the Mössbauer spectral shape from a ‘symmetric’ magnetic sextet to a quadrupolar doublet of lines, which has been observed in a large number of experimental spectra of 57Fe nuclei in magnetic nanoparticles for almost half a century.

Electric field gradients in 111In-doped (Hf/Zr)3Al2 and (Hf/Zr)4Al3 mixed compounds: ab initio calculations, perturbed angular correlation measurements andsite preference

The quadrupolar hyperfine interactions of in-diffused probes in polycrystalline isostructuralZr4Al3 andHf4Al3 samples containing small admixtures of the phases(Zr/Hf)3Al2 were investigated. A strong preference of 111In solutes for thecontaminant (Zr/Hf)3Al2 minority phases was observed. Detailed calculations of the electric fieldgradient (EFG) at the Cd nucleus using the full-potential augmented planewave + local orbital formalism allowed us to assign the observed EFG fractions to the various lattice sites in the(Zr/Hf)3Al2 compounds and to understand the preferential site occupation of the minority phases by the 111In atoms. The effects of the size of the supercell and relaxation around the oversized In andCd probe atoms were investigated in detail.

Magnetic properties of solvothermally synthesized ZnFe2O4 nanoparticles

ZnFe2O4 nanoparticles with a mean size ranging from 3 nm to 10 nm, have been prepared using the solvothermal method. The TEM images reveal a narrow particle size distribution for all samples. Bulk magnetic measurements indicate superparamagnetic behaviour with a blocking temperature (TB) that increases from 10 K to 22 K upon increasing the particle size. At temperatures lower than TB the nanoparticles are in the "frozen" regime and the coercive field changes from 145 Oe to 220 Oe for nanoparticles with sizes from 3 nm to 10 nm. On heating, the Mössbauer spectroscopy data show a progressive crossover from a magnetic to a quadrupolar hyperfine interaction, with a blocking temperature coherent with that obtained from magnetization measurements. The spinel inversion parameter, which appears to be a function of the particle size, has been estimated from neutron diffraction experiments.

Hyperfine Interactions in La@C82 Studied by W-Band Electron Paramagnetic Resonance and Electron Nuclear Double Resonance

The analysis of dipolar and quadrupolar lanthanum hyperfine data measured with electron paramagnetic resonance (EPR) and electron nuclear double resonance reveals that at low temperatures no significant change occurs at the internal binding site of the endohedral complex. We interpret this result as indicative of freezing of the large-scale motion of the encased ion, which is observed at room temperature. Averaging of hyperfine interactions is fast on the time scale of the EPR experiment, preventing drastic changes of dipolar and quadrupolar hyperfine interaction (hfi), providing that the equilibrium position is unchanged. The detection of hfi in disordered samples was possible by invoking orientation selection in the 94-GHz EPR spectrum. Quadrupolar hfi could be directly measured for the first time in a metallo-endohedral fullerene complex.

Pulsed EPR and Ab Initio Calculation on [Ni(CN)4]3- in NaCl and KCl Host Lattices

Paramagnetic 3d9 [Ni(CN)4]3- complexes, with the unpaired electron in a dx2−y2 orbital, have been generated from diamagnetic Ni(II) 3d8 cyanide complexes in KCl or NaCl host lattices. The magnetic and quadrupolar hyperfine interactions with the four 14N, hidden in the CW-EPR (continuous wave electron paramagnetic ressonance) line width, are revealed by pulsed EPR and ENDOR (electron nuclear double resonance) angular variation studies. Ab initio embedded UMP2 cluster calculations, which take into account short- and long-range crystal interactions, confirm the unpaired electron orbital assignment and are in agreement with the measured hyperfine values. The trend of 14N Aiso values (7.7 MHz for NaCl and 6.8 MHz for KCl) is given by the Ni−CN distance, modified in each host lattice. Small asymmetry factors (about 0.04) for the 14N quadrupolar tensor are obtained both in experiment and in theory. The experimental lines and the calculations indicate spin density at the cations of both lattices. Experimental and...

Microwave spectroscopy of the NBr radical in the X 3Σ− state

The microwave spectrum of the NBr radical in the X 3Σ− ground electronic state has been observed by a source modulated spectrometer. The NBr radical was generated in a free space cell by a dc glow discharge in a mixture of N2, Br2, and He. The spectrum with three spin components of both two isotopomers, N79Br and N81Br, was observed. The spectrum showed complicated splitting by the hyperfine interactions due to both bromine and nitrogen nuclei. The molecular constants including the magnetic hyperfine and nuclear quadrupolar hyperfine interaction constants were determined by analyzing the observed spectrum. The spin density of the unpaired electrons was estimated from the observed hyperfine coupling constants to be 73.4% and 22.4% on the nitrogen and the bromine atoms, respectively.

Incommensurate magnetic structure and crystal field in ErNi2B2C Studied by 166Er Mössbauer spectroscopy and specific heat measurements

166Er Mossbauer absorption spectra have been recorded in the superconductor ErNi2B2C (Tc = 10 K), in zero field and in the temperature range 1.4 K–40 K. The spectra in the magnetically ordered phase (TN ≃ 5.5 K) are characteristic of an incommensurate magnetic structure with a maximum Er3+ moment of 8.2 μB. We show that the magnetic transition is first order, with a small temperature range where magnetically ordered and paramagnetic domains coexist. The analysis of the magnetic and quadrupolar hyperfine interactions below and above TN suggests that the Er3+ ground doublet is close to |J = 15/2; Jz = ± 1/2 > and shows that there is a low lying (≃ 10 K) excited doublet. The measured Er3+ electronic relaxation rate 1/T1 shows an anomaly at 10 K (Tc suggesting that the conduction electrons that are exchange coupled to the 4f spin take part in the formation of the superconducting state. We also present specific heat data for ErxY1 − xNi2B2C and Er0.2Lu0.8Ni2B2C which also evidence a low lying doublet and from which we propose a modified (improved) Er3+ crystal field level scheme (0, 9, 62, 71, 73, 181, 212 and 216 K) where the energies of the higher levels are consistent with published neutron scattering data.

Nuclear-spin-lattice relaxation in rhenium metal.

Nuclear-spin-lattice relaxation measurements are presented for $^{187}\mathrm{Re}$ in Re metal as a function of temperature. The relaxation transition probabilities were extracted from the nuclear magnetization recovery curves both in high magnetic field (${H}_{0}$=8 T) nuclear-magnetic-resonance experiments and in nuclear-quadrupole-resonance (${\mathrm{H}}_{0}$=0) experiments. It is found that the dominant relaxation mechanisms is due to magnetic rather then quadrupolar hyperfine interaction with ${W}_{M}$=1.32 T. The data are analyzed in terms of the electronic structure of Re metal. The analysis confirms that Re is a ``weakly enhanced'' transition metal with a nuclear relaxation rate dominated by the s-contact hyperfine interaction.

Calculated perturbed angular correlations for111In doped cubic single crystals

The perturbed angular correlation functions pertaining to the quadrupolar hyperfine interaction of111In activity in cubic single crystals were calculated for various low-index crystallographic orientations of the principle axes of the electric field gradient (efg). The asymmetry of the efg was varied. The capacity and limitations of the method are discussed, especially for the case of non-axial efg.

Structural aspecis of quadrupolar hyperfine interactions in liquid metallic alloys

The quadrupolar part of the nuclear spin re laxat ion rate RQ in l i qu id al loys shows large changes with a l loy partner, concentrat ion, or temperature. Using the Shell-Warren theory, these are traced back to changes of the instantaneous f luc tuat ion of the e l e c t r i c f i e l d gradient due to the spat i a l s t ructure in the a l loy . Reference in made to examples from the s-p,metals. The changes in cor re la t ion time may act in such a way as to enlarge even fur ther the changes in RQ induced by the structure.

Relativistic quadrupolar contribution in systems with nonvanishing orbital magnetism

The relativistic contribution to the quadrupolar hyperfine interaction in systems with nonvanishing orbital moment is treated in terms of a nonrelativistic description of the wave function in these systems. The Sandars-Beck theory, originally treating $S$-state ions only, is extended to treat non-$S$ systems. The results are utilized to derive the relativistic quadrupolar contribution in two realistic situations; (a) ionic systems with an orbital singlet ground state, and (b) metallic systems amenable to orthogonal-plane-waves-interpolation-scheme band-structure calculation.