Field-induced uniaxial anisotropy in partially ordered Ni3Mn at low temperatures

A uniaxial anisotropy is induced in partially ordered MnNi alloys in the composition range near Ni₃Mn, by applying a magnetic field during cooling from room temperature to 77 K. The uniaxial anisotropy constant K_u depends on both the magnitude of the torque measuring field and that of the cooling field. The easy axis direction tends to be between the 〈100〉 and the direction of the cooling field. The value of K_u shows a maximum when the cooling field is applied along 〈100〉, amounting to 2 × 10⁴ crg/cm³. On the other hand, the crystalline anisotropy is not affected by field cooling. When the temperature is raised from 77 K to 300 K, neither K_u nor the rotational hysterisis loss is observed to have any critical temperature, for a torque measuring field of 20 kOe. As atomic ordering proceeds, these effects become smaller and appear to vanish in the perfectly ordered state.     

The voigt-type magneto-kerr effect in cubic semiconductors having ellipsoidal constant energy surfaces

A plane-wave analysis is given for the Voigt orientation of an applied static magnetic field ?B in cubic semiconductors having ellipsoidal constant energy surfaces. The effect is shown to be highly anisotropic with respect to the orientation of ?B (magneto-anisotropy) as well as to the initial polarization direction of the incident wave with respect to ?B (polarization anisotropy). The character and extent of the magneto-anisotropy depends not only on the location of the energy extrema, but on the anisotropy of the energy surfaces and on the energy dependence and anisotropy of the scattering processes.Calculations are given for the cases of propagation along 〈100〉 and 〈110〉 crystallographic directions in n-type germanium. High-field effects and infrared (IR) behavior are considered; however, the low-field quadratic range for microwaves in high-loss material is emphasized. A useful approximation for the high-loss case is given. Particular attention is given to the potential diagnostic applications of the effect.Room-temperature measurements of the Voigt-type Kerr effect for the TE₁₁ circular-waveguide mode at 35 GHz in n-type Ge are presented and compared with plane-wave calculations for propagation along 〈100〉 and 〈110〉 directions. Good quantitative agreement is obtained when the plane-wave calculations are reduced by the same mode-reduction factor as is applicable for the Faraday orientation.     

ESR study of color centers in yttria stabilized zirconia

The ESR of single crystals of yttria stabilized zirconia (YSZ), containing color centers due to reduction in a flowing H₂ atmosphere, has been measured at room temperature. The spin density is about 3 × 10¹⁷ cm^?3 and the strong ESR line displays axial symmetry with principal values g₆ = 1.989, g_⊥ = 1.860.The behaviour of the ESR spectra when the magnetic field is rotated about the 〈1 0 0〉 and 〈1 1 0〉 directions shows that the centers have axial symmetry, the symmetry axis being along the 〈111〉 and equivalent directions. The derivative peak to peak envelope width is g-dependent and varies from 18 Gauss at g = 1.989 to 65 Gauss at g = 1.860. The results are interpretable in terms of an electron trapped at an oxygen vacancy adjacent to an yttrium ion.     

Hysteresis phenomena in the course of polarization evolution in a sinusoidal electric field in lead magnesium niobate crystals

The processes of polarization evolution in single crystals of the PbMg_1/3Nb_2/3O₃ model ferroelectric relaxor in a sinusoidal electric field are investigated at temperatures near and above the temperature T _d 0 of destruction of the induced ferroelectric state upon heating in zero electric field. The polarization switching current loops are measured in the ac electric field applied along the 〈111〉 and 〈110〉 pseudocubic directions. The electroluminescence intensity loops are obtained under the combined action of ac and dc electric fields applied along the 〈100〉 direction. In a certain temperature range above T _d 0 and the freezing temperature T _f in lead magnesium niobate, there are electric current anomalies, that correspond to the dynamic formation and subsequent destruction of the ferroelectric macroregions throughout each half-cycle of the ac electric field. The measurements of electroluminescence hysteresis loops demonstrate that the observed depolarization delay (related to the ac electric field amplitude) increases with an increase in the dc electric field and decreases as the ac field amplitude increases. The nature of the observed phenomena is discussed.     

Exact Results for Conditional Means of a Passive Scalar in Certain Statistically Homogeneous Flows

For a passive scalar T(r, t) randomly advected by a statistically homogeneous flow, the probability density function (pdf) of its fluctuation can in general be expressed in terms of two conditional means: 〈?² T|T〉 and 〈|?T|²|T〉. We find that in some special cases, either one of the two conditional means can be obtained explicitly from the equation of motion. In the cases when there is no external source and that the normalized fluctuation reaches a steady state or when a steady state results from a negative damping, 〈?² T|T〉=?(〈|?T|²〉/〈T ²〉)T. The linearity of the conditional mean in these cases follows directly from the fact that the advection equation of a passive scalar is linear. On the other hand, when the scalar is supported by a homogeneous white-in-time external source, 〈|?T|²|T〉=〈|?T|²〉.     

Polarized photoluminescence from bound magnetic polarons in (Cd,Mn) Se

Luminescence from Cd_1-xMn_xSe, with x～10^-1, shows strong circular polarization in the presence of a weak magnetic field. The polarization saturates at fields much smaller than does the magnetization. The carrier-Mn²⁺ exchange interaction is responsible for the field and temperature dependence of the polarization. At high temperatures, the polarization is similar to that inferred from band splittings; at lower temperatures, it is modified by bound magnetic polaron effects and a spin diffusion bottleneck.     

Large magnetic anisotropy in single crystal us

It is shown for ferromagnetic US that an extremely large anisotropy restrains magnetic moments to 〈111〉 easy axes, resulting in a near cos θ angular dependence of the magnetization away from the 〈111〉 axes. This is further confirmed by torque measurements, which in addition show large hysteresis effects upon rotation through the hard axes. It is illustrated through torque and magnetization measurements that a near stable domain configuration can be established by field rotation through decreasing angular amplitude around a hard 〈001〉 direction. The anisotropy constant K₁(T) is estimated by computing the small angle through which the magnetization deviates from the 〈111〉 axes when a field is applied along the [001] direction.     

Spinpolarisation von Feldemissionselektronen aus einkristallinem Wolfram

Field-emitted electrons from tungsten tips extracted in magnetic fields of 3–25 kG at 80 K show spin polarization. The observed polarization is always found to be parallel to the external magnetic field for the polycrystalline and oriented tips (〈100〉, 〈110〉 and 〈111〉) investigated. The maximum values of polarization obtained with these monocrystalline emitters range betweenP _max=11–13 percent.     

Anomalous fluctuations of 2D-electron recombination radiation intensity in the quantum Hall regime

Anomalous intensity fluctuations are observed in the spectrum of radiative ecombination of quasi-two-dimensional (2D) electrons with photoexcited holes in a single quantum well. The fluctuations are observed exclusively under the conditions of the quantum Hall effect (QHE). It is shown that, if the QHE conditions are not fulfilled, the radiation intensity fluctuates strictly following the Poisson distribution 〈δN ²〉/〈N〉= 1), whereas in the QHE regime the fluctuation amplitude increases by several orders of magnitude (〈δ N ²〉/〈N〉～10²). It is demonstrated that the maxima of the emission noise amplitude coincide with the maxima of inverse magnetoresistance of 2D electrons in the QHE regime and correspond to establishing an anomalously high uniformity of the system.     

Averaged local field intensities in composite films

The photophysical properties of molecules adsorbed in composite films (e.g. surface island films) depend on the local electromagnetic field within the film. The ratio between the average field intensity 〈|E|²〉 in the film and the intensity |E_I|² associated with the incident field is a measure of the electromagnetic contribution to the surface influence on molecular photophysical phenomena. This ratio depends on the film composition and morphology, on the dielectric properties of the pure components making the film and on the frequency, direction and polarization of the incident radiation. Calculations of this ratio as a function of these parameters for several models of composite films are presented. Image interactions and retardation effects as well as radiative damping and finite size contributions to the dielectric response of the films are taken into account. In addition, an estimate of the field inhomogeneity within the film is obtained by calculating also the ratio 〈|E|²〉_shell/|E_I|² associated with the field in thin shells surrounding the dielectric particles which constitute the film.     

Characteristics of π−-meson multiplicity distributions in central collisions of12C and16O with nuclei at a momentum of 4.5 GeV/c per incident nucleon

Multiplicities of π^?-mesons from central collisions of¹²C and¹⁶O(4.5 GeV/c/n momentum) with several target nuclei were studied in a streamer chamber experiment. The parameter η=(〈n _? ² 〉?〈n_?〉²)/〈n_?〉 was determined for several samples of events characterized by different values of Θ _ch — the “vetoangle” for emission of the projectile charged fragments. The value of η is shown to decrease when the projectile mass number increases from 12 to 16 and to decrease significantly when Θ _ch increases from 0° to 4°, whereas it remains nearly constant when Θ _ch increases further to 14°. The results are compared with predictions of some theoretical models.     

〈S0〉Cs and 〈S0〉Rb relaxation in the presence of buffer gas and the spin exchange between Cs and Rb

〈S⁰〉_Rb and 〈S⁰〉_Cs relaxation is studied in the presence of buffer gas and spin exchange between Cs and Rb. Spin exchange collisions are found to equalize the relaxation rates of Rb and Cs polarization. This result is completely confirmed experimentally for the relaxation of the first diffusion mode.     

Molecular-field theory of moment reduction in Kondo systems with crystal-field effects

Several dense Kondo compounds have a low-temperature ordered phase in which the magnetic moments are reduced with respect to the values expected for the crystal-field (CEF) ground state. In the present work the phenomenon of moment reduction is studied within a molecular-field theory combined with a variational solution of the one-impurity Anderson model with CEF effects. The calculated zero-temperature magnetization and susceptibility agree well with available exact results; the present method is easily applied to systems of any symmetry. We first study the f ¹ configuration in cubic symmetry, for small values of the ratio T _K/Δ between Kondo temperature and CEF splitting. With a Γ _∞ ground state and a field along a 〈100〉 direction, an inflection point occurs in the magnetization curve, which gives rise to a first order transition in the zero-temperature phase diagram. This feature is not found for a field along 〈110〉 or 〈111〉, for which the transition is second order. For a Γ ₇ ground state and small values of T _K/Δ, the magnetic-nonmagnetic transition is second order for all field directions. On increasing T _K/Δ an inflection point in the magnetization curve appears first for a field along 〈111〉. The theory is applied to a study of cubic CeAg, CeAl₂, CePb₃, CeIn₃, CeTe, and hexagonal CePd₂Ga₃. The bare value of the moment is found to be strongly increased by exchange coupling to excited CEF states, and the amount of Kondo reduction is found to be substantial, confirming the importance of the Kondo effect in these compounds.     

Spin polarized field emission from Fe and Co-coated W tips

Spin polarized field emission from thin film Fe- and Co-coated W(001) and W(111) tips has been investigated. A transverse spin polarization component is detected in each case. For W(111) tips, the azimuthal orientation of polarization showed only a weak preference for the set of 〈1?10〉 and 〈112?〉 tip directions due to the competing influence of magnetocrystalline anisotropy and tip morphology on tip magnetization. On the contrary, the polarization direction for W(001) tips exhibited a strong preference for alignment with the transverse low-index crystallographic directions, i.e. the 〈110〉 for Co and 〈100〉 for Fe. Superparamagnetic fluctuations of the tip magnetization are evident in the polarization direction of emitted electrons when film coatings are very thin. At marginally larger thickness, long-term stability of the polarization magnitude and direction is observed at 300 K. A method for changing the stable spin polarization direction is also presented that exploits spontaneous flipping of the tip magnetization at elevated temperature.     

High-field autosolitons in an electron-hole plasma in <Emphasis Type="Italic">p</Emphasis>-Ge

The formation of high-field thermodiffusion autosolitons was studied experimentally in a photogenerated electron-hole plasma heated up by an electric field in p-Ge samples oriented along the 〈111〉 axis at T=77 K. Measurements of the current-voltage characteristics, electric field distributions along the samples, and IR emission in the wavelength range λ=1.65–10 μm showed that the arising of an autosoliton was accompanied by the appearance of N-shaped current-voltage characteristic regions. Autosolitons were formed at electron-hole plasma concentrations n≥1×10¹⁴ cm^?3 and field strengths E≥500 V/cm. They manifested themselves as static, moving, and pulsating strata with field strengths E _as =1000–20000 V/cm and carrier temperatures T _e ≥1000 K. We also observed a turbulent electron-hole plasma state when autosolitons chaotically appeared and disappeared in the samples. The multivalley band structure of germanium influenced the dynamics of autosoliton formation; intervalley transfer of electrons in the strong field of autosolitons caused a three-step autosoliton field growth.     

Changes in nuclear mean square charge radii of stable Krypton isotopes

The isotope shifts of stable even Kr isotopes (A=78 throughA=86) in the optical transitions at 432 nm and 557 nm were measured by means of polarization laser spectroscopy. The observed shifts are consistent with earlier results for other transitions. From the isotope shifts the changes in the nuclear mean square charge radiiδ〈r ²〉 were inferred using preliminary muonic isotope shift data. Starting from⁷⁸Kr, a monotonic decrease of 〈r ²〉 with increasing mass number is found throughout theg _9/2 neutron shell. The effect onδ〈r ²〉 of nuclear deformations as well as possible contributions due to changes in the skin thickness of the nuclear charge distribution are discussed.     

Signal for intermittency in e+e− reactions obtained from negative binomial fits

With a simple relation between the parameter k of the negative binomial distribution and the factorial moments 〈Fⁱ〉 we reanalyse published data from e⁺e⁻ reactions at √s=29 GeV on multiplicity distributions in rapidity bins to show that there is a significant rise of the 〈Fⁱ〉 with decreasing bin size δy along the thrust axis. The rise for δy<1 can be regarded as a signal for intermittency. For further discussion we compare the data with the Lund shower Monte Carlo. We suggest a direct measurement of factorial moments in e⁺e⁻ jets.     

Top quark polarization in polarized e+e− annihilation near threshold

Top quark polarization in e⁺e^{t -} annihilation into tt? is calculated for linearly polarized beams. The Green function formalism is applied to this reaction near threshold. The Lippmann—Schwinger equations for the S-wave and P-wave Green functions are solved numerically for the QCD chromostatic potential given by the two-loop formula for large momentum transfer and Richardson’s ansatz for intermediate and small momenta. S- P— wave interference contributes to all components of the top quark polarization vector. Rescattering of the decay products is considered. The mean values 〈nl〉 of the charged lepton four-momentum projections on appropriately chosen directions n in semileptonic top decays are proposed as experimentally observable quantities sensitive to top quark polarization. The results for 〈nl〉 are obtained including S- P— wave interference and rescattering of the decay products. It is demonstrated that for the longitudinally polarized electron beam a highly polarized sample of top quarks can be produced.     

Classical Motion in Force Fields with Short Range Correlations

We study the long time motion of fast particles moving through time-dependent random force fields with correlations that decay rapidly in space, but not necessarily in time. The time dependence of the averaged kinetic energy 〈p ²(t)〉/2 and mean-squared displacement 〈q ²(t)〉 is shown to exhibit a large degree of universality; it depends only on whether the force is, or is not, a gradient vector field. When it is, 〈p ²(t)〉～t ^2/5 independently of the details of the potential and of the space dimension. The stochastically accelerated particle motion is then superballistic in one dimension, with 〈q ²(t)〉～t ^12/5, and ballistic in higher dimensions, with 〈q ²(t)〉～t ². These predictions are supported by numerical results in one and two dimensions. For force fields not obtained from a potential field, the power laws are different: 〈p ²(t)〉～t ^2/3 and 〈q ²(t)〉～t ^8/3 in all dimensions d≥1.     

Phononenfreie Absorptions- und Emissionsübergänge in strahlungsverfärbten NaF-Kristallen

Inγ-irradiated NaF crystals at 5456, 5754, 5837, 6070 and 6146 å zero-phonon lines are observed in absorption and emission. These resonance lines are accompanied by multiphonon absorption and emission bands with their spectral positions nearly mirror symmetric with respect to the zero-phonon line position. By excitation spectra higher excited states are detected. Uniaxial stress splitting of the lines 5754 and 6070 å is studied in absorption and emission. The color centers associated with the lines are found to exhibit rhombic symmetry with rotation axes along 〈110〉 and 〈100〉 directions and monoclinic symmetry with a rotation axis along 〈110〉 direction. The transitions in both centers correspond to 〈110〉 dipoles. In the monoclinic center the transition between the ground and the second excited state seems to correspond to a 〈112〉 dipole.