Spin glasses with short range interaction

The theory is presented of spin glasses for which the condition n _m ^1/3 l « dl is valid, l being the range of interaction forces between spins and n _m the volume concentration of spins. Various examples where such an interaction appears are considered. Thermodynamic and kinetic characteristics of such systems are calculated at high and low temperatures. Particular attention is paid to the transition region; for its analysis percolation theory methods are applied. The theory of one-dimensional spin glasses of several types is constructed. A ‘kinetic’ equation is derived for the cluster distribution function in the transition region and the behaviour of this function is studied.     

Neutron experiments to search for new spin-dependent interactions

The consideration is presented of possible neutron experiments to search for new short-range spin-dependent forces. The spin-dependent nucleon-nucleon interaction between neutron and nuclei may cause different effects: phase shift of a neutron wave in neutron interferometers of different kind, in particular of the Lloyd mirror configuration, neutron spin rotation in the pseudo-magnetic field, and transverse deflection of polarized neutron beam by a slab of substance. Estimates of sensitivity of these experiments are performed.     

Neutron diffraction study of the interaction of iron with amorphous fullerite

The amorphous fullerite C₆₀ has been prepared by mechanical activation (grinding in a ball mill), and its interaction with iron during sintering of powders with 0–95 at % Fe has been studied. After sintering in the range 800–1200°C under a pressure of 70 MPa, the samples have nonequilibrium structures different from the structures of both annealed and quenched steels. In this case, the carbon phase, i.e., amorphous fullerite, undergoes a polyamorphous transition to amorphous graphite. It has also been shown that the interaction of amorphous fullerite with iron is weaker compared to crystalline fullerite or crystalline graphite.     

Neutron diffraction on piezoelectric vibrating resonators

A proposal was made to use neutron diffraction to investigate the dynamics of lattice vibrations of piezoelectric resonators. The results obtained explain some experimental facts published in the papers [9, 10, 13]. The obtained spectral distribution of diffracted neutrons is analogous to the spectral distributions of acoustically modulated -rays in the Mössbauer effect.The author is indebted to Assoc. Prof. J. Tichý, RNDr. A. Vancura CSc. and Ing. V. A. Petrílka CSc. for their valuable discussions of the resluts of this paper.     

Neutron scattering study of the 1-D short range and 3-D long range magnetic order in TMNB

Quasielastic neutron scattering experiments on TMNB show for T > 2.8 K a diffraction pattern characteristic for a 1-D ferromagnet with an exchange energy along the chain J/k_B ? 10 K. For T < T_N =2.7 K the 3-D magnetic ordering was found to be of a simple antiferromagnetic type. A lattice distortion was observed as well below 200 K.     

Neutron diffraction studies of water

Neutron diffraction measurements have been made for heavy water at five temperatures in the range 11–75°C using the diffractometers D2 and D4 at ILL. A first-order difference method has been used to determine the change in the intermolecular contributions to the observed cross-section. Fourier transformation techniques are used to give the changes in the real space distribution function. The results confirm the preliminary observations obtained in an earlier experiment and give a more precise indication of the structural variation. A detailed analysis shows that the intermolecular OD separation which corresponds to an effective hydrogen bond distance increases as the temperature rises. At larger distances there is a systematic but more complex variation which may be attributed to a change in the relative orientations of neighbouring molecules.     

Neutron diffraction by lithium ferrite-aluminates

Neutron-diffraction results are presented for Li_0.5Fe_2.5-xA1_xO₄ for x from 0.2 to 1.0. Continuous solid solutions are formed, and the lattice constant varies from 8.33 to 8.17 Å. Ordering of type 13 occurs for the Li⁺ and Fe³⁺ ions in the octahedral sublattice. The Al content influences the oxygen parameter U and the distribution of the cations between the sublattices. The results are compared with those from magnetic measurements.Translated from Izvestiya VUZ. Fizika, Vol. 11, No. 11, pp. 88–92, November, 1968.I am indebted S. M. Zhilyakov for providing the specimens, to V. A. Somenkov and S. Sh. Shil'shtein for valuable discussions, and to V. M. Kuchin for assistance in the experiments.     

Universal spin-dependent variable range hopping in wide-band-gap oxide ferromagnetic semiconductors

This paper proposes a universal spin-dependent variable range hopping theoretical model to describe various experimental transport phenomena observed in wide-band-gap oxide ferromagnetic semiconductors with high transition metal concentration. The contributions of the `hard gap' energy, Coulomb interaction, correlation energy, and exchange interaction to the electrical transport are considered in the universal variable range hopping theoretical model. By fitting the temperature and magnetic field dependence of the experimental sheet resistance to the theoretical model, the spin polarization ratio of electrical carriers near the Fermi level and interactions between electrical carriers can be obtained.     

Neutron diffraction experiments with macroscopic objects

To discover the reasons for the failure in the performance of a neutron interferometer with a Fresnel biprism, proposed by Maier-Leibnitz and Springer, the experiments were repeated with a simple diffraction apparatus.We found that the distortion of the diffraction pattern only arises in incorrect adjustment of the probes or in bad probe edges. There is therefore no reason to introduce a special coherent inelastic scattering mechanism into theory as suggested by Landkammer. It is shown that the biprism can be advantageously substituted by a quartz thread surrounded by a D₂O—H₂O mixture.     

Neutron diffraction study of liquid iodine

A theory of molecular diffusion in liquids that takes into account the shape of intermolecular potential wells with a cosine potential produces fine spectral detail in the far infrared. The effect of a static external electric field is to shift this panoply of peaks to higher frequency, increase the number of peaks, and shift their individual relative frequencies and intensities.     

Bound states in a two-dimensional short range potential induced by the spin-orbit interaction

We have discovered an unexpected and surprising fact: a 2D axially symmetric short-range potential contains an infinite number of the levels of negative energy if one takes into account the spin-orbit (SO) interaction. For a shallow well (meU0R2/h2<1, where me is the effective mass and U0 and R are the depth and the radius of the well, respectively) and weak SO coupling (/alpha/meR/h<1, where alpha is the SO coupling constant) exactly one twofold degenerate bound state exists for each value of the half-integer moment j=m+1/2, and the corresponding binding energy Em extremely rapidly decreases with increasing m.     

Neutron diffraction studies of liquid carbon suboxide

The structure factor S _m(Q) for liquid carbon suboxide has been determined for a Q-value range of 0·4 to 60 Å^-1 by neutron diffraction measurements using a steady-state (reactor) and a pulsed (linac) neutron source. The bond lengths of the molecule have been determined from the data and give good agreement with the results of electron diffraction measurements on the vapour phase after application of a molecular recoil correction term. The quasi-linear nature of the molecule is confirmed but the shape of the form factor indicates that large amplitude bending motion probably occurs in the liquid phase.

Oscillations in the intermolecular pair correlation function are observed to have a regular periodicity extending to 12 Å but details of orientational effects cannot be established from a single diffraction measurement.     

Neutron diffraction studies of transition metal nitrides

The magnetic structures of Fe₄N and Mn₄N have been redetermined using neutron diffraction. The magnetic form factors, obtained from polarised neutron data have been shown to be different for the face-centred and corner atoms. A qualitative explanation of the structures of Fe₄N and Mn₄N has been provided from the shapes of the magnetic form factors. Since deceased.     

Neutron forward diffraction by single crystal prisms

We have derived analytic expressions for the deflection as well as transmitted fraction of monochromatic neutrons forward diffracted by a single crystal prism. In the vicinity of a Bragg reflection, the neutron deflection deviates sharply from that for an amorphous prism, exhibiting three orders of magnitude greater sensitivity to the incidence angle. We have measured the variation of neutron deflection and transmission across a Bragg reflection, for several single crystal prisms. The results agree well with theory.      

Neutron diffraction studies of simple molecular liquids

Summary A brief review is presented of the structural characteristics of diatomic and tetrahedral molecular liquids. Paper presented at the workshop ?Highlights on Simple Liquids?, held in Turin at ISI on 1–3 May, 1989.     

Neutron diffraction by laser-generated volume phase gratings

Cold neutrons are diffracted by thick phase gratings which have been produced by light-optical holography in poly-(methyl methacrylate) (PMMA). The neutron diffraction efficiency is investigated in the Laue case as a function of the angle of incidence by rocking the sample. The dynamical theory of neutron diffraction allows to relate the measured diffraction efficiency to the modulation amplitude of the neutron-optical refractive index. The absence of higher order diffraction peaks proves that this modulation is sinusoidal. Further experimental parameters are shown to influence the diffraction in accordance with the dynamical theory: the wavelength of neutrons, the sample thickness, and the grating period. Laser diffraction experiments allow us to give upper limits for the uniformity of the grating constant throughout the sample volume. The gratings show properties which are promising for the fabrication of high-precision control samples for cold and very cold neutron spectrometers, for the construction of neutron-optical instruments, and the possibility to measure photoinduced changes of neutron-optical potentials.