Nuclear spin-lattice relaxation of impurities in ferromagnetic iron

Due to the development of Green's function method the calculation of the nuclear spin-lattice relaxation time of impurities in ferromagnets has become feasible in the last years. We present the result of calculations for allsp andd impurities in ferromagnetic iron. The calculations are based on the density functional formalism. They well, reproduce the experimental trend of the relaxation timeT ₁ for bothsp andd impurities. By decomposing the relaxation rate into various contributions, we explain the observed systematic behavior ofT ₁ T in terms of the local electronic structure.     

Al nuclear magnetic resonance study of synthetic and natural corundum (α-Al2O3) Some experimental aspects of quantitative Al nuclear magnetic resonance spectroscopy

The ²⁷Al nuclear magnetic resonance (NMR) response of a series of natural and synthetic corundum (α-Al₂O₃) samples is studied quantitatively by short-pulse excitation and frequency-stepped adiabatic half-passage (FSAHP). Using on- and off-resonance nutation NMR, it was established that the quadrupole coupling parameters of visible Al is identical in all samples. Remarkably, the relaxation behavior for the aluminum is very different in the various samples and has a marked effect on the quantitative response. In natural corundum samples the ²⁷Al spin-lattice relaxation is very efficient as these samples contain paramagnetic impurities. As a result, however, the full signal could not be recovered, which is attributed to relaxation broadening of spins in the vicinity of these impurities. In synthetic samples, containing no impurities, the full signal could be recovered, although the relaxation behaviour appeared to depend strongly on the preparation method. We observed differences in the spin-lattice relaxation by a factor 20; the longest T₁ was observed in a crushed single crystal. This implies that α-Al₂O₃ can only be used as a standard in quantitative analyses if it has been characterized thoroughly. Furthermore, the effective relaxation behaviour for different types of excitation is studied. Finally, a method to measure the spin-lattice relaxation of half-integer quadrupole nuclei is introduced, using a frequency-stepped adiabatic passage (FSAP) to invert the spin system.     

Proton hyperfine interactions in the gas-phase E.P.R. spectra of SH (2Π3/2) in two lowest rotational levels

Gas-phase E.P.R. spectra of the SH radical (²Π_3/2) in the J = 5/2 and J = 3/2 rotational levels are described. Analysis of the hyperfine splittings gives the values of axial and non-axial components of the proton hyperfine interaction and the value of the constant d. The frequencies of the two strong lines in the zero-field spectrum are estimated by the use of the observed value of the hyperfine interaction constant d.     

N.M.R. relaxation time studies in liquid methyl iodide

The spin-lattice relaxation times of the various nuclei in methyl iodide, methyl iodide-d ₃, and carbon-13 methyl iodide (¹³C, ¹H, ²D) were measured between 210 and 350 K. The separation of the proton-proton intermolecular relaxation was accomplished by a dilution study in methyl iodide-d ₃; the resulting intermolecular contribution agreed well with the existing theories for this mechanism. It was found that the spin-rotation interaction contributed significantly to the intramolecular relaxation of both the protons and the carbon-13. For both nuclei the separation of the spin-rotation interaction from the intramolecular dipole-dipole interaction was accomplished without making any assumptions about the temperature dependence of the spin-rotation relaxation time. The resulting spin-rotation relaxation times for both carbon-13 and protons offer evidence that the large spin-rotation effects are due to the methyl group reorientation.     

P.M.R. relaxation time studies in some binary liquid systems

Proton magnetic resonance relaxation time measurements have been carried out in mixtures of water with formic acid, acetic acid, propionic acid and n-butyric acid. On account of molecular association, the results reveal strong deviation of these mixtures from the ideal behaviour. The molecular association effect is discussed in some detail. The molecular association ratios determined from the present studies are in excellent agreement with those obtained by the ultrasonic absorption technique. It is further shown that N.M.R. relaxation time studies reveal more information about molecular interactions in liquids than the ultrasonic absorption studies.     

Dynamic frequency shift in the E.S.R. spectra of transition metal ions

The factors which determine the electron spin resonance lineshapes and shifts of transition metal complexes in solution are discussed. It is shown that for ions with S > ½ the dynamic frequency shift may have pronounced effects on the E.S.R. lineshapes, and experimental evidence for this effect in complexes of Cr³⁺, Fe³⁺ and Gd³⁺ is presented. A quantitative interpretation of these spectra in terms of a relaxation mechanism due to fluctuation of the quadratic zero field splitting interaction is given.     

Fredholm Modules on P.C.F. Self-Similar Fractals and Their Conformal Geometry

The aim of the present work is to show how, using the differential calculus associated to Dirichlet forms, it is possible to construct non-trivial Fredholm modules on post critically finite fractals by regular harmonic structures (D, r). The modules are (d _S , ∞)–summable, the summability exponent d _S coinciding with the spectral dimension of the generalized Laplacian operator associated with (D, r). The characteristic tools of the noncommutative infinitesimal calculus allow to define a d _S -energy functional which is shown to be a self-similar conformal invariant. Thiswork has been supported by the project “Teoria ellittica e forme di Dirichlet su spazi frattali” G.N.A.M.P.A. 2008 and by the G.R.E.F.I.-G.E.N.C.O. French-Italian Research Group.     

Electron paramagnetic relaxation in aqueous solutions of manganese (II) ions

The processes of the electron paramagnetic relaxation, molecular motions and structural changes in aqueous solutions of manganese nitrate have been investigated by direct measurement of spin-lattice (T ₁) and spin-spin (T ₂) relaxation times for a wide range of concentrations, temperatures and viscosities. T ₁ and T ₂ were measured by a non-resonance absorption method.

It was discovered that some structural regions exist at the different concentrations of Mn(II) ions in solution. So, the structure of highly concentrated solutions may be considered as one of the corresponding crystallohydrate. The structural microinhomogeneities were observed also in the intermediate concentration range at definite temperatures. It is shown that the relaxation mechanism proposed by Bloembergen and Morgan is not effective in the concentration range studied by us.

The analysis of relaxation times and E.P.R. spectra has shown the formation of ‘liquid microphases’ at the freezing point of the solution. Such microphases can exist at temperatures a few tenths of a degree below the solvent freezing point, and its composition considerably differs from the initial solution.

The correlation times for intramolecular and intermolecular electron relaxation mechanisms are evaluated and their nature is discussed.     

On the electronic structure of N,N′-ethylenebis(acetylacetonatiminato)Co(II), Co(II)acacen

Single crystal E.P.R. and cobalt ENDOR measurements on N,N′-ethylenebis(acetylacetonatiminato)Co(II), Coacacen, diluted in Niacacen·1/2 H₂O are reported. Forbidden Δm _Co = 1,2 transitions in the E.P.R. spectra have been observed. The g-tensor (E.P.R.) and the cobalt hyperfine and quadrupole tensors (ENDOR) have been determined. The g- and cobalt hyperfine tensors are discussed. They support the |² A ₂,yz? groundstate proposed for four coordinated low-spin Schiff base complexes of cobalt(II). The measured quadrupole coupling is also compatible with a |² A ₂,yz? groundstate, if anisotropic contraction of the cobalt 3d orbitals is taken into consideration.     

Spectrophotometric and E.P.R. study of TCNE charge-transfer complexes

Liquid state charge-transfer complexes formed from the strong acceptor tetracyanoethylene (TCNE) and four donors of like donor properties are studied by spectrophotometric and E.P.R. techniques. If the enthalpy of complex formation ΔH is sufficiently large, the triplet state of the complex ³C will be appreciably populated thermally during complex formation. The E.P.R. signal of the TCNE anion promptly arises because of a dissociation of ³C, reaches a maximum, and then follows second-order decay kinetics. The higher the dielectric constant of the donor, the slower the decay rate. The initial intensity of the E.P.R. signal is proportional to the original TCNE concentration in the donor-solvent. Reversible photoinduced TCNE anions are formed by irradiating the complexes in the charge-transfer band. The relationship between the prompt and photoinduced E.P.R. signals is shown. Singlet-triplet separation energies are estimated.     

1H NMR relaxation investigation of herbicides interacting with photosystem II preparations fromSpinacia oleracea L.

The interaction of three common herbicides, paraquat, acifluorfen and alachlor, with spinach chloroplast photosystem II (PS II) was investigated by measuring¹H nuclear magnetic resonance spin-lattice relaxation rates, transient nuclear Overhauser effect (NOE) and NOE spectroscopy (NOESY) spectra. Binding to PS II was detected by (i) the enhancement of single-selective relaxation rates and (ii) the decrease in the optimal mixing time providing maximal cross-peak intensity in NOESY spectra. Titration of relaxation enhancements was used to calculate the dissociation constants (K _d) from the bound state for paraquat (K _d = 292 ± 71 μM⁻¹) and acifluorfen (K _d = 311 ± 58 μM⁻¹). A similarK _d was apparent for alachlor. Double-selective relaxation rates allowed the isolation of dipolar relaxation terms between selected proton pairs wherefrom dynamic features of the bound state were evaluated. In all cases the motional correlation time of bound herbicide (τ_c = 0.1−0.4 ns at 300 K) was found two orders of magnitude slower than in the free-solution state. In the case of alachlor the E and Z isomers were observed to bind differently to PS II and a change in conformation could be hypothesized.     

Orbital angular momentum of 3d impurities in metals

It is shown that 3d impurities often have a significant orbital moment due to the high symmetry of the crystalline electric field in typical metallic hosts. The EPR behavior which has been observed for such impurities is clarified and further predictions are made. The configurations of 3d impurities in noble metals are established: for example, Fe has 6 3d electrons andS=2. A spin-orbit splitting of about 0.02 eV is expected for Fe impurities, which may be observable by inelastic neutron scattering. The influence of orbital angular momentum on the Kondo effect is briefly discussed.     

On the critical dynamics of disordered spin systems

The dynamic critical behaviour of spin systems with quenched impurities, and of amorphous spin systems as characterized by the additional presence of random anisotropy directions, is studied by renormalization group methods to second order in=4–d. For the Halperin-Hohenberg-Ma model with purely relaxational dynamics it is concluded that in three dimensions (d=3) the critical slowing down should be enhanced by impurities for systems with Ising type statics, whereas there is no change forXY- and Heisenberg systems. For amorphous systems, however, the critical dynamics should change also in theXY- and Heisenberg cases. Furthermore, it is concluded that additional conserved, but noncritical modes become always statically decoupled from the order parameter for systems with impurities, but not for amorphous systems. Thus, for the impure system, the energy density mode and the asymmetric models of Halperin, Hohenberg and Siggia are ruled out. But the effects of dynamic coupling remain: Especially, the relationz=d/2 for the dynamic exponent of planar and isotropic antiferromagnets is modified for impure or amorphous systems.     

N.M.R. study of solid perfluorocyclobutane

spin-lattice relaxation times and linewidth measurements for fluorine nuclei in solid perfluorocyclobutane are presented. The results are discussed in terms of D _2d molecular species performing fast internal motion. The relaxation measurements corroborate the existance of four solid-solid phase transitions and give some insight into their nature. The activation energies for molecular reorientation and self-diffusion processes are found to be 28·0 and 32·2 kJ mol^-1, respectively.     

Properties of vanadium in InP

Experiments are performed on InP/V single crystals extracted from a polycristalline ingot grown in a two zones gradient freeze furnace; in photoluminescence (PL) we observe a band with two zero phonon lines at 5692.5 cm^-1 and 5705.5 cm^-1. Photoluminescence excitation (P.L.E.) spectra have also been observed. This type of luminescence spectra has already been observed in V doped GaAs and GaP and attributed to an internal transition of V²⁺ (3 d³). D.L.T.S. and O.D.L.T.S. experiments do not reveal any deep level in the forbidden band gap. These two results seem to indicate a puzzling behaviour of V in InP as in GaAs which is discussed in the framework of a model proposed for GaAs/V : the V²⁺ ground state has a large relaxation energy and so, the excited states giving rise to the P.L. and P.L.E. spectra are not resonant with the conduction band.     

E.P.R. study of Mn2+ in lithium potassium tartrate (LPT) and its isomorphism with lithium ammonium tartrate (LAT)

An E.P.R. study of Mn²⁺ in single crystals of LiKC₄H₄O₆. H₂O (LPT) was carried out from ～300 K to 77 K and four magnetically differently oriented but symmetry related triclinic complexes of Mn²⁺ were observed. Spin hamiltonian parameters were obtained at ～ 300 K. The E.P.R. results suggest that LPT is not only isomorphous but also isostructural to LiNH₄C₄H₄O₆ . H₂O (LAT). In LPT no structural phase transition could be detected in the temperature range 300 K to 77 K. The observed small magnitude of the low symmetry effect is attributed to the small contribution of fourth order crystal field terms relative to that of the second order crystal field terms, to the zero-field splittings of the ⁶ S ground state of Mn²⁺. A temperature dependent anomalous behaviour is observed in the low field region of the E.P.R. spectrum for B//[010] and its neighbourhood.     

New universal short-time scaling behaviour of critical relaxation processes

We study the critical relaxation properties of Model A (purely dissipative relaxation) starting from a macroscopically prepared initial state characterised by non-equilibrium values for order parameter and correlations. Using a renormalisation group approach we observe that even (macroscopically)early stages of the relaxation process display universal behaviour governed by a new, independent initial slip exponent. For large times, the system crosses over to the well-known long-time relaxation behaviour.The new exponent is calculated toO(²) in =4–d, whered is the spatial dimension of the system. The initial slip scaling form of general correlation and response functions as well as the order parameter is derived, exploiting a short-time operator expansion. The leading scaling behaviour is determined by initial states with sharp values of the order parameter. Non-vanishing correlations generate corrections to scaling.     

Improvements for MeasuringH–H Coupling Constants in DNA via New Stripe-COSY and Superstripe-COSY Pulse Sequences Combined with a Novel Strategy of Selective Deuteration

Three bond proton-proton vicinal coupling constants are of potential value for analyzing sugar conformations in DNA. However, self-cancellation in antiphase cross peaks and modulation of peak splittings by transverse cross relaxation can alter the apparent coupling constants such that they do not accurately reflect the sugar conformations. Transverse cross relaxation is most effective between strongly coupled geminal proton pairs. Here we report the use of stereospecific deuteration at the H2″ position in the A5 and A6 residues in the 12 base pair DNA sequence [d(CGCGAATTCGCG)2] as a means of investigating the effect of transverse cross relaxation on P.E.COSY type cross peaks. Deuteration of the H2″ proton is expected to reduce the transverse cross relaxation rate by the square of ratio of the proton to deuteron gyromagnetic ratios, i.e., by a factor of 42. Additionally, a striking eight- to ninefold increase in the signal intensity was observed for cross peaks involving the remaining H2′ proton resulting from diminished dipolar relaxation. Further improvements in signal-to-noise ratio were realized by collecting P.E.COSY spectra in strips, using an experiment referred to as stripe-COSY, employing selective excitation pulses which reduced the number of requiredt₁increments by a factor of four. A final improvement was achieved by employing selective time-shared homonuclear decoupling during the acquisition period, in an experiment referred to as superstripe-COSY, to collapse splittings due to passive couplings. Collectively, these approaches provide P.E.COSY-type spectra with two to three orders of magnitude increased sensitivity per unit time and that are relatively free from artifacts.     

Combined effect of nonmagnetic and magnetic scatterers on the critical temperatures of superconductors with different anisotropies of the gap

The combined effect of nonmagnetic and magnetic defects and impurities on the critical temperatures of superconductors with different anisotropies of the gap is studied theoretically within the weak coupling limit of the BCS model. An expression is derived which relates the critical temperature to the relaxation rates of charge carriers on non-magnetic and magnetic scatterers and to the coefficient of anisotropy of the superconducting order parameter on the Fermi surface. The particular cases of d-wave, (s+d)-wave, and anisotropic s-wave superconductors are briefly discussed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 10, 627–632 (25 November 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Study of exchange and dipolar interaction in MCuX 3 systems by E.P.R.

Compounds of the ABX ₃ type (where A = K, NH₄, Cs, Rb, B = Cu, Ni, Cr, X = Cl, Br, F, I) make an interesting study by the presence of BX ₆, B ₂ X ₄, B ₃ X ₉, linear chains which also show interesting magnetic properties and crystal structures. Single crystals of KCuCl₃, NH₄CuCl₃, RbCuCl₃ and CsCuCl₃, in the polycrystalline state have been studied by E.P.R. measurements. These compounds show simple E.P.R. spectra but have interesting dependences of line widths on temperature and g values. The line width dependence has been shown to arise from unresolved hyperfine and dipolar interactions, confirmed by second moment calculations. The quasi axial nature of the g value has been explained as being due to antiferro-distortive ordering of the Cu₂ X 6/2- octahedra. The ammonium ion acts as a better exchange pathway than potassium in the two respective crystals. The E.P.R. intensities have a Curie type dependence and has no contribution from the dimeric coppers.