Anisotropic Hyperfine Coupling Tensors Research Articles

Site-Dependent Local Spin Susceptibility and Low-Energy Excitation in a Weyl Semimetal WTe2

Site-dependent local spin susceptibility is investigated with $^{125}$Te nuclear magnetic resonance in a Weyl semimetal WTe$_2$. The nuclear spin-lattice relaxation rate $1/T_1T$ shows a dependence of the square of temperature $T$ at high temperatures, followed by a constant behavior below 50 K. The temperature dependence features Weyl fermions appearing around the linearly crossing bands. The Knight shift $K$ scales to the square root of $1/T_1T$, corroborating a predominant spin contribution in low-lying excitation. The observed dependence of $K$ and $1/T_1T$ on the four Te sites shows the site-dependent electron correlation and density of states. The angular profile of the NMR spectrum gives the anisotropic hyperfine coupling tensor, consistent with $5p$ hole occupations on Te sites.

Modulated magnetic structure of Fe3PO7 as seen by Fe57 Mössbauer spectroscopy

The paper reports new results of the 57Fe M\"ossbauer measurements on Fe3PO4O3 powder sample recorded at various temperatures including the point of magnetic phase transition TN ~ 163K. The spectra measured above TN consist of quadrupole doublet with high quadrupole splitting of D300K ~ 1.10 mm/s, emphasizing that Fe3+ ions are located in crystal positions with a strong electric field gradient (EFG). In order to predict the sign and orientation of the main components of the EFG tensor we calculated monopole lattice contributions to the EFG. In the temperature range T < TN, the experimental spectra were fitted assuming that the electric hyperfine interactions are modulated when the Fe3+ spin (S) rotates with respect to the EFG axis and emergence of spatial anisotropy of the hyperfine field Hhf = S\~AI at 57Fe nuclei. These data were analyzed to estimate the components of the anisotropic hyperfine coupling tensor (\~A). The large anharmonicity parameter, m ~ 0.94, of the spiral spin structure results from easy-axis anisotropy in the plane of the iron spin rotation. The temperature evolution of the hyperfine field Hhf(T) was described by Bean-Rodbell model that takes into account that the exchange magnetic interactions are strong function of the lattice spacing. The obtained M\"ossbauer data are in qualitative agreement with previous neutron diffraction data for a modulated helical magnetic structure in strongly frustrated Fe3PO4O3.

The magnetic structure on the ground state of the equilateral triangular spin tube

The ground state of the frustrated equilateral triangular spin tube CsCrF4 is still hidden behind a veil though NMR spectrum broaden into 2 T at low temperature. In order to investigate the spin structure in an ordered state by 19F-NMR, we have determined the anisotropic hyperfine coupling tensors for each three fluorine sites in the paramagnetic state. The measurement field was raised up to 10 T to achieve highest resolution. The preliminary analysis using the obtained hyperfine tensors has shown that the archetypal 120°-type structure in ab-plane does not accord with the NMR spectra of ordered state.

Possible Mass Enhancement by Multipole Fluctuations Excited via the Singlet–Triplet Crystal Electric Field States in PrOs4Sb12: Sb-NMR Studies Using a Single Crystal

121,123 Sb nuclear magnetic resonance (NMR) measurements of a high-quality single crystalline PrOs 4 Sb 12 have been carried out in magnetic fields up to 98 kOe and in the temperature range of 0.4–30 K. From the analysis of the 121 Sb Knight shift ( K ) and magnetic susceptibility at H ≈90 kOe, the transferred hyperfine coupling constants ( A THF ) are obtained. The Sb isotropic hyperfine coupling constant and anisotropic hyperfine coupling tensor are determined as ≈27.6 Oe/µ B and ≈(149.9,42.3,-192.2) Oe/µ B , respectively. The anisotropic part originates from the spin–dipolar field due to the local spin density at Sb 5 p orbital transferred through the hybridization with 4 f electrons. The results are consistent with the Sb 12 5 p molecular orbital with a u symmetry predicted from the band structure calculation. A THF for H ∥[001] is dependent on the magnetic field, suggesting the coupling of the Sb nuclear dipole moment with not only dipole moments but also octupole moments of the Pr 4 f electrons with the same time reversal. The NMR relaxation rate depends both on magnetic field and temperature, and the temperature dependence of 1/ T 1 is explained by the phenomenological expression 1/( T 1 T A THF 2 )∝(1/ T ) exp [-Δ NMR ( H )/ T ] with excitation gap Δ NMR ( H ). Here, we found that Δ NMR ( H ) is proportional to the largest excitation gap between Γ 1 and Γ t - of the triplet Γ 4 (2) state under the magnetic field. These results indicate that the crystal electric field (CEF) excitation and the degeneracy of the triplet state are important for understanding Sb-NMR results. It is strongly indicated that the mass enhancement in PrOs 4 Sb 12 is caused by the scattering conduction electrons with multipole fluctuations via the singlet–triplet CEF excitation.

Theoretical prediction of g tensors and hyperfine coupling tensors of triplet molecules

This is the first time that all-valence-electron SCF MO calculations of g tensors and hyperfine coupling (HFC) tensors of some triplet molecules have been carried out. The molecular orbital spin density is partitioned into the σ and π-electron spin delocalization (SD) terms and the remaining terms of spin polarization (SP) which arise from the CI effect. The g tensors for CH2 and some other carbenes which have ground triplet states and the excited triplet formaldehyde are evaluated. The origin of HFC in CH2 is analyzed in detail by using the partitioning method for spin density matrices. The absolute values of the isotropic and anisotropic 13C HFC tensors of CH2 calculated by taking into account the SD terms only are in good agreement with those observed for CD2. The contribution of the SP terms to the isotropic HFC tensors is fairly large, while the contribution to the anisotropic HFC tensors is negligibly small.

Hyperfine interactions in the heavy fermion CeMIn5 systems

The CeMIn5 heavy fermion compounds have attracted enormous interest since their discovery six years ago. These materials exhibit a rich spectrum of unusual correlated electron behaviour, and may be an ideal model for the high-temperature superconductors. As many of these systems are either antiferromagnets, or lie close to an antiferromagnetic phase boundary, it is crucial to understand the behaviour of the dynamic and static magnetism. Since neutron scattering is difficult in these materials, often the primary source of information about the magnetic fluctuations is nuclear magnetic resonance. Therefore, it is crucial to have a detailed understanding of how the nuclear moments interact with conduction electrons and the local moments present in these systems. Here, we present a detailed analysis of the hyperfine coupling based on anisotropic hyperfine coupling tensors between nuclear moments and local moments. Because the couplings are symmetric with respect to bond axes rather than crystal lattice directions, the nuclear sites can experience non-vanishing hyperfine fields even in high symmetry sites.

Estimates of magnetic resonance parameters in semi-empirical quantum chemistry

The possibility to reliably estimate the magnetic resonance parameters (MRP) can significantly increase the information content of experimentally recorded spectra. Here we consider in detail semi-empirical estimates of the ESR spectra parameters: isotropic hyperfine coupling constants, anisotropic hyperfine coupling tensors and g-tensors. The results show that the semi-empirical procedures give estimates of MRP comparable in quality with those of complicated ab initio and DFT schemes. It was underlined that a special attention should be given to geometric parameters of free radicals. The automatic procedure to determine molecular geometries of free radicals on the ground of their spectral characteristics was discussed.

ENDOR study of the chlorinated thiocytosine radical in a crystal matrix

Interaction of protons with the unpaired electron in the σ2σ*1 three-electron S∴Cl bond has been investigated by electron nuclear double resonance (ENDOR) spectroscopy. The S∴Cl bond in the chlorinated thiocytosine radical, imbedded in the crystal lattice of cytosine hydrochloride, interacts significantly with at least seven protons. Five of these interactions have been analyzed. From the nature of the coupling tensors, it is concluded that three couplings are due to protons being constituents of the chlorine–thiocytosine radical, and the remaining two are due to protons belonging to neighboring cytosine molecules. The nature of the dipolar component of the coupling tensors is not consistent with the location of the entire electron spin density in the S∴Cl bond. From isotropic as well as anisotropic hyperfine coupling tensor components, it is concluded that the unpaired electron is somewhat delocalized over the radical, with a major portion being concentrated in the C(2)–S–Cl region but with a significant fraction occupying a π* molecular orbital extending over the cytosine ring atoms. Density functional theory molecular orbital calculations support such a kind of spin distribution. The data also suggest that the radical remains protonated at both the N(1) and N(3) positions.

Evaluation of MNDO approximation in quantum-chemical calculations of anisotropic hyperfine coupling tensors in free radicals

The MNDO approximation was tested for applicability in quantum-chemical calculations of anisotropic hyperfine coupling tensors using the same set of 17 free radicals as that used previously in evaluating a new procedure for quantum-chemical estimates of constants of isotropic hyperfine coupling with protons in the framework of the same approach.

Analysis of13C−NMRspectra inC60superconductors: Hyperfine coupling constants, electronic correlation effect, and magnetic penetration depth

A C-NMR anisotropic hyperfine coupling tensor was determined as 2p(21.68, 21.68, 3.37) 310 rad/sec for C60 32 in A3C60 superconductors, where A is an alkali metal, by analyzing C-NMR spectra below 85 K. Combined with an isotropic coupling constant of (2p30.69)310 rad/sec, the 2s and 2p characters of the electronic wave functions at the Fermi level were deduced. The results were compatible with local-density-approximation band calculations. From a simulation of C-NMR spectra at superconducting state, the traceless chemical ~orbital! shift tensor and isotropic chemical shift were determined as ~67, 34, 2101! ppm and ;150 ppm, respectively. An estimated magnetic penetration depth is larger than 570 nm in K3C60. Furthermore, the modified Korringa relation, T1TK ;bS ~with Knight shift K, spin-lattice relaxation time T1 , and Korringa constant S!, clearly showed the existence of weak but substantial antiferromagnetic spin fluctuation in A3C60; b50.40– 0.58 with an error of 620%. The Stoner enhancement factor was also determined as 1–1.5 from a comparison between spin susceptibility obtained from NMR and band-calculation results. @S0163-1829~98!00542-6#

Magnetic anisotropies in the nuclear magnetic resonance of the Heisenberg-ferromagnet EuO

We investigated the field- and temperature dependence of the nuclear magnetic resonance of153Eu in an EuO single crystal sphere at low temperatures in fields up to 7T. The spin-wave analysis yields an ecellent, parameter free description of the magnetization with the exchange constantsJ1=0.55±1 K,J2=0.15±1 K and an anisotropy fieldBα=50±1 mT from neutron scattering and ferromagnetic resonance respectively. The influence of a biquadratic exchange within the spin-wave analysis is discussed. The hyperfine field varies with the orientation of the magnetization by 200 kHz. The temperature and field dependence of this anisotropy indicates that it is due to an anisotropic hyperfine coupling tensor. The effect was used to observe the rotation of the domain magnetization in small internal fields and determineBα with greater accuracy than from the spin-wave analysis.

ESR study of irradiated single crystals of the cocrystalline complex of cytidine: Salicylic acid

Irradiation at 77 K of single crystals of the 1:1 complex of cytidine and salicylic acid produces a phenoxyl radical formed by oxidation of the salicylic acid. Anisotropic hyperfine coupling tensors have been determined for this radical which are associated with the para and ortho hydrogens. No cytidine oxidation products (alkoxy or hydroxyalkyl radicals) were observed at 77 K. Following the decay of the phenoxyl radical at room temperature, four radicals were detected. These include the cytosine 5–yl and 6–yl radicals, formed by H addition to the cytosine ring, and an anisotropic doublet. By UV irradiation at room temperature, it is possible to convert a significant fraction of 6-yl radicals into 5-yl radicals. Hyperfine coupling and g tensors determined for the anisotropic doublet indicate that this radical is formed in the C1'-C2' region of the sugar moiety. These results indicate a shift in radiation damage away from the salicylic acid upon warming, and show that the radiation chemistry of the cocrystalline complex is different from that of the isolated bases.

57Fe Mossbauer studies of the (Fe1-x)Sn system for 0⩽x⩽0.2:estimation of the anisotropy of the hyperfine coupling tensor

57Fe Mossbauer investigations have been reanalyzed to estimate the components of the anisotropic hyperfine coupling tensor Aapproximately ; from the complex spectra of FeSn at temperatures below TN, and those of Mn-substituted compounds at low temperatures in which spins are in the basal plane, Ax and Az have been evaluated. Due to spin reorientation from the basal compounds resemble a single nuclear Zeeman pattern and this has been analysed to estimate the value of Ay. The angle of deviation of the hyperfine field from the magnetisation direction has also been estimated for the component patterns arising from the crystallography equivalent 3f Fe sites. Explicit expressions for various parameters, which have been used to define the anisotropic hyperfine fields, have been given in terms of the components of the hyperfine coupling tensor and the dipolar fields arising from external magnetic moments. An estimate of the sign and magnitude of all these parameters has also been made for the present system.

17O-ligand hyperfine structure in single crystal ESR spectra of copper(II)hydroxyquinolinate: phthalimide and copper(II)picolinate:zincpicolinate tetrahydrate

K-band ESR spectra of 17O labelled Copper(II)oxyquinolinate (CuOX 2) substituted in phthalimide single crystals and of Copper(II)picolinate (Cupic 2) substituted in single crystals of Zinc picolinate tetrahydrate are reported. The spectra were analyzed as a superposition of spectra of 3 isotopic 16O, 17O species, yielding anisotropic hyperfine coupling tensors for the 17O ligands of the two complexes. The magnetic data will then be used for derivation of bonding parameters in a one hole MO scheme and of spin densities at oxygen ligands by two different approaches π o values of 0.13 and 0.085 (per oxygen atom) for CuOx 2 and Cupic 2 were found. f 0 p/f 0 s ratios amount to 11.0 and 8.7 for the two systems, respectively, thereby ranging considerably higher then found hitherto.

Ab initio unrestricted Hartree–Fock calculations. Part 9.–Hyperfine coupling constants in the ethyl radical

Calculated values for the anisotropic hyperfine coupling tensors for the ethyl radical by the ab initio UHF method with spin annihilation are in good agreement with experiment. The results are critically compared with those from semi-empirical calculations.

SCF MO INDO calculation of anisotropie hyperfine coupling tensors for ?-type radicals

The anisotropic hyperfine coupling tensors for some σ-type radicals (HCO, FCO, NO2, CO 2 − , CN, phenyl, vinyl) were calculated. The calculation was performed with an all-valence-electron approximate open-shell MO method using the INDO approximation and with the dipolar integrals evaluated over Slater-type AO's. The diagonalyzed tensors were in reasonable agreement with the available data of experiment.

Calculation of the Anisotropic Hyperfine Coupling in Cu(II) Bis(dithiocarbamate) and Cu(II) Bis(diselenocarbamate). A Formula for the Anisotropic Hyperfine Coupling Tensor

For a molecule with one unpaired electron a general formula is derived for the elements of the anisotropic hyperfine coupling tensor, taking into account all possible electron excitations. For a check of the formula the hyperfine coupling tensors are calculated for the copper atom in bis(N, N-diethyl dithiocarbamato) copper(II) and for the copper and selenium atoms in bis(N, N-diethyldiselenocarbamato) copper(II), using the iterative extended Hückel method. Both the calculated principal values of these tensors and the direction cosines of the principal axes are in good agreement with the experimentally observed ones.

Electron Resonance Studies of Perchlorodiphenylmethyl and Perchlorotriphenylmethyl Free Radicals

The stable free radicals perchlorodiphenylmethyl, (C6Cl5)2ĊCl, and perchlorotriphenylmethyl, (C6Cl5)3C, have been examined by electron resonance. In solution, the isotropic α-chlorine hyperfine splitting in PDM was found to be 6.08 Mc/sec. Splittings from 13C in natural abundance at the methyl carbon (about 100 Mc/sec) and at additional phenyl–carbon sites were measured for both radicals. By incorporating PDM in the liquid crystal p-azoxyanisole, the principal values of the anisotropic hyperfine coupling tensor of the α chlorine were derived and found to be A11 = − 9.2, A22 = − 18.9, and A33 = 47.2 Mc/sec. The variation with temperature of the degree of ordering of the radicals in the nematic mesophase was determined.

Electron spin resonance studies on oriented polyoxymethylene

AbstractElectron spin resonance spectra of stretched polyoxymethylene irradiated with γ‐rays from a Co60 source were studied. With suitable treatment three kinds of spectra were selectively observed: doublet with irradiation and measurement at room temperature; singlet with irradiation and measurement at about −180°C.; triplet with the irradiation at about −180°C. and the measurement at −102°C. The doublet and the triplet spectra showed anisotropy, while the singlet did not. These observations confirm the assignment of the spectra so far reported with unoriented polyoxymethylene, i.e., CH2O, OĊHO, OĊH2 for the singlet, doublet, and triplet, respectively. The doublet spectrum is treated as related to the anisotropic hyperfine coupling tensor, the anisotropic g value and the chain conformation in crystalline polyoxymethylene. A rather small value, p≈0.45, was obtained for the spin density at the central carbon atom.