Electronic states of silicon in Ni-silicides by nuclear magnetic resonance

Spin-lattice relaxation time (T₁) of ²⁹Si nuclei in several Ni-silicides (Ni_1?xSi_x:0.25?x?0.67) were studied at low temperature (1.4?T?4.2 K) by spin-echo technique. The relation of T₁T = const. and also very short T₁ were observed indicating that the silicides studied were metallic with enough densities of state of 3s-electrons at the Fermi energy (E_F). Another feature of the results was that T₁ decreased with the increment of silicon concentration. This effect was discussed in connection with the soft X-ray spectroscopy (SXS) data on Ni-silicides and Ni—Al compounds.     

Stability of nuclear rotational states

The stability of rotational states described by using a procedure of the Hartree-Fock-Bogoliubov method is discussed. The stability condition is obtained in a general form and some of its properties are examined with the aid of simplifying approximations. It is pointed out in conclusion that the Coriolis force might lead to the instability of rotational states for sufficiently large angular momenta. The instability of rotational states is caused by the instability of the pairing density, and also by that of the nuclear density distribution. In the former case it is shown that below the critical angular momentum, a first-order phase transition takes place and in some cases the rotational band breaks off. In the latter case, it might be considered that the assumption of an axially symmetric deformation is broken or large change of the equilibrium deformation is brought suddenly. Then discontinuous change of the rotational level might be seen at the unstable point.     

Tunneling of coupled methyl groups

A method is developed which allows the calculation of tunneling frequencies for coupled methyl groups in strong or weak orientational potentials to any desired accuracy. The method is applied to two coupled methyl groups and the results of the calculation are compared with a recent experiment. The comparison allows conclusions on the strength and symmetry of the potential. A further important point is the prediction of coupling effects in rotational tunneling also in the limit of strong potentials.     

The rotational energy levels of four methyl groups

The energy level spectrum for a system of four interacting methyl groups belonging to an X(CH₃)₄ type molecule is calculated by numerical methods. The rotational potential at the site of each methyl group is assumed to be of a three-fold symmetry. The torsion-torsion interaction is defined as a term in the multipole expansion of the electrostatic interaction of two rigid charge distributions. It is shown, that the tunneling frequency characterizing the ground state manifold in the absence of methyl-methyl interaction, splits into a set of closely spaced frequencies.     

Preparing high purity initial states for nuclear magnetic resonance quantum computing

Here we demonstrate how parahydrogen can be used to prepare a two-spin system in an almost pure state which is suitable for implementing nuclear magnetic resonance quantum computation. A 12 ns laser pulse is used to initiate a chemical reaction involving pure parahydrogen (the nuclear spin singlet of H2). The product, formed on the micros time scale, contains a hydrogen-derived two-spin system with an effective spin-state purity of 0.916. To achieve a comparable result by direct cooling would require an unmanageable (in the liquid state) temperature of 6.4 mK or an impractical magnetic field of 0.45 MT at room temperature. The resulting spin state has an entanglement of formation of 0.822 and cannot be described by local hidden variable models.     

Surface phenomena investigated by nuclear magnetic resonance

This paper reviews recent achievements in the application of Nuclear Magnetic Resonance to surface phenomena, especially to molecules adsorbed on surfaces of porous crystals. Basic principles of Nuclear Magnetic Resonance are treated only as far as it is necessary to understand potentialities of this method in a study of absolute number (section 2.1), electronic environment (section 2.2), arrangement (section 2.3), and of thermal motion (section 2.4) of nuclei which are part of the absorbate or adsorbent. The structure of an important group of porous crystals, called zeolites of faujasite type, is briefly discussed (section 3.1) and recent results concerning the state of water (section 3.2) and of simple cyclic hydrocarbons (section 3.3) adsorbed in these crystals are reviewed.     

Near-zero-field nuclear magnetic resonance

We investigate nuclear magnetic resonance (NMR) in near zero field, where the Zeeman interaction can be treated as a perturbation to the electron mediated scalar interaction (J?coupling). This is in stark contrast to the high-field case, where heteronuclear J?couplings are normally treated as a small perturbation. We show that the presence of very small magnetic fields results in splitting of the zero-field NMR lines, imparting considerable additional information to the pure zero-field spectra. Experimental results are in good agreement with first-order perturbation theory and with full numerical simulation when perturbation theory breaks down. We present simple rules for understanding the splitting patterns in near-zero-field NMR, which can be applied to molecules with nontrivial spectra.     

g-Factors of magnetic–rotational states in 85Zr

The properties of the double iron and tungsten carbide prepared by mechanical alloying technique (MA) from elemental powders are reported. The samples were milled for 1, 3, 5, 10, 15, 20, 25 and 30 h. The alloy progress for each milling time was evaluated by X-ray diffraction (XRD) and ⁵⁷Fe Mössbauer spectrometry. Once the alloy was consolidated two sorts of paramagnetic sites and a magnetic distribution were detected according to the Mössbauer fitting. The majority doublet could correspond to Fe₆W₆C ternary carbide as X-ray diffraction suggests, and the other could be Fe₃W₃C. The hyper fine parameters are reported. Vickers microhardness measurements of 30 h milled sample was conducted at room temperature with a load of 0.245 N for 20 s.     

Two-dimensional nuclear magnetic resonance petrophysics

Two-dimensional nuclear magnetic resonance (2D NMR) opens a wide area for exploration in petrophysics and has significant impact to petroleum logging technology. When there are multiple fluids with different diffusion coefficients saturated in a porous medium, this information can be extracted and clearly delineated from CPMG measurements of such a system either using regular pulsing sequences or modified two window sequences. The 2D NMR plot with independent variables of T2 relaxation time and diffusion coefficient allows clear separation of oil and water signals in the rocks. This 2D concept can be extended to general studies of fluid-saturated porous media involving other combinations of two or more independent variables, such as chemical shift and T1/T2 relaxation time (reflecting pore size), proton population and diffusion contrast, etc.     

Measurement of heat transfer coefficients by nuclear magnetic resonance

We demonstrate an experimental method for the measurement of heat transfer coefficient for a fluid system by magnetic resonance imaging. In this method, the temporal variation of thermally induced nuclear shielding is monitored and the average heat transfer coefficient is measured as a function of fluid velocity. We examine the cases of natural convection and forced convection at fluid velocity up to 0.8 m s(-1). These cases correspond to low dimensionless Biot (Bi) number where the heat transfer is limited by thermal convection. We demonstrate the NMR method for two simple geometries, a cylinder and a sphere, to experimentally determine the heat transfer coefficient (h) in two NMR imaging and spectroscopy systems through measuring three NMR parameters, the chemical shift, magnetization and spin self diffusion coefficient.     

Interfacial density of states in magnetic tunnel junctions

Large zero-bias resistance anomalies as well as a collapse of magnetoresistance were observed in Co/Al2O3/Co magnetic tunnel junctions with thin Cr interfacial layers. The tunnel magnetoresistance decays exponentially with nominal Cr interlayer thickness with a length scale of approximately 1 A more than twice as fast as for Cu interlayers. The strong suppression of magnetoresistance, as well as the zero-bias anomalies, can be understood by considering a strong spin-dependent modification of the density of states at Co/Cr interfaces. The role of the interfacial density of states is shown by the use of specially engineered structures. Similar effects are predicted and observed in junctions with Ru interfacial layers.