Showing papers in "Physica B-condensed Matter in 1976"

The theory of the transient hot-wire method for measuring thermal conductivity

Abstract: The paper contains a complete, modernized theory of the transient hot-wire method for measuring the thermal conductivity of fluids which can be employed in the form of an absolute instrument and which can be operated with a precision of 0.02% and an accuracy of 0.2%. It is a companion paper for ref. 1. The analysis demonstrates that the instrument can be designed to imitate very closely the behaviour of a finite portion of an infinite line source of constant heat flux, q, which transfers the heat radially into an infinite fluid. Expressions for the corrections are obtained by a general perturbation method which allows us to examine them one or several at a time. The principal corrections discussed in the form of nine subproblems are: finite inner cylinder, composite cylinders, Knudsen effects, radiation, outer cell circumference, compressibility and natural convection, finite cell dimensions, variable fluid properties and heating over a finite length. The last section summarizes the most important corrections for a reader who is interested in using them rather than in following the analysis itself. The main text supplies all data required by the designer of an instrument of this type.

1/f noise

Abstract: A survey is given of the recent literature on 1/ f noise. Proposals for mathematical models, for empirical relations and for physical models are discussed. The present situation is evaluated and some unsolved problems are indicated.

The physics of electron beam excited rare gases at high densities

Abstract: The unique properties of charged particle excited high density rare gases are reviewed in terms of the basic collisional and radiative processes upon which they depend. Both low and high excitation density conditions are considered and a kinetic model of the important processes is formulated using available and estimated rate coefficients.

Localized atomic states in metals

Abstract: An atom jumping between two equivalent positions in a metal is “dressed” by the conduction electrons, and the jumping frequency Δ/h is reduced by a factor which is essentially the overlap integral between two Slater determinants, one corresponding to the atomic position 1 and the other to the position 2. The reduction factor of the jumping frequency is expressed by Δ eff Δ = ( T D ) K or ( Δ D ) K , depending on T a Δ ot T « Δ. Here, K = V20ϱ2(1 − sin2 k2Fa2), T is the temperature , D ∼ the Fermi energy, V0ϱ the interaction constant between the atom and the conduction electrons, kF the Fermi wave vector and a the separation between the two positions.

Atomic radiative transition probabilities in a continuous medium

Abstract: A simple quantization procedure is applied to the electromagnetic radiation field in a nondissipative, weakly dispersive continuous medium. The resulting operator expressions for the electric and magnetic field strengths are used to study the influence of the dielectric and magnetic properties of the medium on the Einstein coefficients for spontaneous and stimulated radiative transitions of single atoms or molecules embedded in the medium. The expressions obtained are consistent with detailed-balance considerations.

Potentials supporting positive-energy eigenstates and their application to semiconductor heterostructures

Abstract: Herrick has proposed using semiconductor epitaxial heterostructures to create continuum embedded eigenstates (infinitely sharp resonances) of the Wigner-von Neumann type. This paper presents a modification of Herrick's construction rules which should simplify fabrication and improve operation of those heterostructures. The modified procedure has also been generalized to produce for the first time a family of local potentials which simultaneously support two continuum- embedded bound states. Finally, it is pointed out that cases can be generated for which infinitely sharp resonances exist which do not correspond to normalizable bound states.

Localized atomic states in metals: II. Resistivity and free energy

Abstract: Resistivity of a metal containing impurity atoms which can jump between two equivalent sites has been calculated to the fourth order of the Coulomb-type interaction between the atoms and the conduction electrons When wave-number dependence of the matrix elements of the interaction is taken into account, we obtain a logarithmic term in the fourth order, which is proportional to V 4 0 S 2 [log ( D/T )] 2 , where V 0 is the impurity potential of the atom, S ∼ the overlap integral between two localized atomic states, D ∼ the band-width, and T the temperature The change of the entropy of the same system has been found to be proportional to − V 4 0 S 2 log ( D/T ), which is interpreted as that the atom sets in a coupled motion with the conduction electrons

Construction of bound states in the continuum for epitaxial heterostructure superlattices

Abstract: We offer a theoretical method, based in part on an earlier mathematical model of Weidmann, for designing one-dimensional, rectangular, heterostructure superlattices which support square-integrable states at energies above the lattice barriers. The lattice potential vanishes asymptotically, giving a bound state in the continuum of the originally suggested by von Neumann and Wigner for smoothly oscillating potentials. Examples include wavefunctions and potentials having dimensions similar to those of real GaAs-Al x Ga 1− x As heterostructures, and analysis of the spontaneous ionization and transmission widths of the states for finite lattices.

Instantaneous population densities of the excited levels of hydrogen atoms and hydrogen-like ions in plasmas

Abstract: The statistical rate equations for the number densities of excited atomic (ionic) states have been solved for atomic hydrogen and hydrogen-like ions assuming a non L.T.E. distribution between states of different principal quantum number but statistical equilibrium amongst the sublevel populations belonging to a given principal quantum number. The calculations have been performed for the optically thin as well as for three different degrees of optical reabsorption. A detailed analysis of existing cross-section data is included. The numerical results are listed in tables containing the two-dimensionless global excitation coefficients r ( 0 ) z−1 , j and r ( 1 ) z−1 , j from which the absolute instantaneous populations are obtained. A new set of collisional-radiative recombination and ionization coefficients is presented in graphical form. Comparison with existing literature values for the optically thin case shows discrepancies which exceed a factor of two at low and medium electron densities at nearly all electron temperatures.

Plasma Focus Devices

Abstract: Plasma focus devices are presently investigated in about 20 laboratories. All these devices generate hot (some 100 eV-some keV), dense (10 17 −10 20 particles cm −3 ) and short-lived (5−200 ns) plasmas. Due to improved diagnostics and theoretical models considerable progress in the understanding of these devices has been made since their invention in the early 1960 ies. The principle of operation, the important parameters for the mode of operation, the characteristics plasma properties and the possible applications (as realized or planned) are described.

Observation of lattice- and spin-dimensionality crossovers in the susceptibility of quasi 2-dimensional Heisenberg ferromagnets

Abstract: We have studied the magnetic susceptibility (χ) for T → T + c in the quasi 2-dimensional (2-d) Heisenberg ferromagnets ( C n H 2 n + 1 NH 3 ) 2 CuCl 4 ( n = 1, 2, 3, 10). Through the series of compounds the ratio | R | = | J ′/ J | of inter- to intralayer interactions varies from 10 −3 − 10 −6 . In comparing χ curves for different R , clear evidence is obtained for crossover from 2-d to 3-d behaviour as T → T + c . In agreement with theoretical predictions, the quantity χ| J ′| determines the ‘crossover temperature’, the sign of J ′ determining whether χ deviates upward or downward from 2-d behavior. In addition, spin-dimensionality crossovers are observed, due to the presence of small XY anisotropy (0.3%) and Ising anisotropy (0.02%) in the interactions. For compounds with | R | −4 the lattice-dimensionality crossover is preceded by spin-dimensionality crossovers, from 2-d Heisenberg to 2-d XY behavior ( at ϵ ≏ 1 − T c /T ≅ 0.5) and subsequently to 2-d Ising behavior (at ϵ ≅ 0.2). The critical behavior of χ (for ϵ ϵ = 0.02−0.05) entail a change in the critical exponent γ from the 2-d Ising to the 3-d Ising values (1.75 and 1.25, resp.), the crossover revealing itself as a “kink” in the log χ versus log ϵ plot. In comparing χ curves for materials with different amounts of spin-space anisotropy, extrapolation to the case of zero anisotropy is made, and an estimate of the χ behavior for the 2-d Heisenberg ferromagnet is obtained down to kT / J = 0.5. This temperature is a factor of three lower than that at which the high-temperature-series prediction for this model becomes unreliable.

Review of glow discharge instabilities

Abstract: In a diffusion dominated discharge striations occur because excitations are reduced if the electrons can gain ionization energies in short distances and then ionize further on, but this process requires an energy transport distance Λ. This process no longer works in a recombination dominated discharge because any increase in electron density results in a decreased field and hence reduced ionization. However, if dissociative attachment is present the decreased field may result in decreased attachment and hence further increase the density, which is destabilizing. It seems that a negative differential conductivity dJ dE , which must be stabilized externally by a series resistance, remains unstabilized internally between adjacent sections of the plasma. The negative thermal coefficient ( ∂J ∂T g ) p,E is always destabilizing and produces a transverse contraction unless it is neutralized by thermal conduction or electron diffusion. Recent work by Haas, Nigan and Wiegand and Douglas-Hamilton and Mani are reviewed.

Heat capacity of Cs2CoCl4 below 1 K, compared with the S = 12 linear chain XY model

Abstract: Heat-capacity data on Cs2CoCl4 below 1 K are reported. A magnetic exchange contribution is found, showing a broad, finite maximum at T ≃ 0.9 K, and a small λ-type anomaly at Tc = 0.222 ± 0.005 K. The data agree quite well with the prediction for the heat capacity of the S = 1 2 , linear chain, XY model. The applicability of the S = 1 2 , XY model is shown to follow from crystal-field arguments. Furthermore, it is argued that the existence of magnetic chains can be understood by considering the various superexchange paths occurring in the structure.

On the origin of visible continuum radiation in rare gas glow discharges

Abstract: To prove the electron-atom bremsstrahlung origin of visible continuum radiation in rare gas medium-pressure glow discharges some new experiments and theoretical considerations were carried out. Arguments for the bremsstrahlung theory are supported by experimental results for (i) the behaviour of the radiation in ArHg mixtures; (ii) the spectral distribution in the visible and near UV range (Ne, Ar, Xe); (iii) the change of intensity during the transition from the diffuse to the constricted mode of the positive column (Ne) and (iv) the behaviour of the radiation in the Faraday dark space (He). Together with some other facts a foundation of the bremsstrahlung theory is given.

Relaxation times for establishing quasi-stationary state populations in non-thermal plasmas

Abstract: The temporal relaxation of excited state populations towards a quasi-stationary state has been calculated by solving the corresponding time-dependent rate equations. The relaxation times which follow from these calculations give values which differ for the higher excited levels up to one order of magnitude from exissting literature values. The calculation of relaxation times is re-examined on the basis of time-dependent coupling between all excited states. A new formula for the relaxation times is obtained in which intervene the initial conditions and the collisional-radiative interaction frequencies between all levels. It gives numerical results in agreement with the time-dependent solutions of the coupled system of rate equations.

High frequency parametric wave phenomena and plasma heating: A review

Abstract: A survey of parametric instabilities in plasma, and associated particle heating, is presented. A brief summary of linear theory is given. The physical mechanism of decay instability, the purely growing mode (oscillating two-stream instability) and soliton and density cavity formation is presented. Effects of density gradients are discussed. Possible nonlinear saturation mechanisms are pointed out. Experimental evidence for the existence of parametric instabilities in both unmagnetized and magnetized plasmas is reviewed in some detail. Experimental observation of plasma heating associated with the presence of parametric instabilities is demonstrated by a number of examples. Possible application of these phenomena to heating of pellets by lasers and heating of magnetically confined fusion plasmas by high power microwave sources is discussed.

Evidence for giant moments in the transition towards the ferromagnetic state in the Pd1−xNix alloys

Abstract: We have shown by magnetization measurements in high fields and susceptibility measurements at very low temperatures that the magnetic nickel atoms in Pd Ni alloys form giant moments. These giant moments are centered on groups of three nickel which polarize the host around them. This effect is quite similar to the effect of an atom of iron in the Pd NiFe alloys. Our results confirm previous ones and indicate that in Pd Ni alloys the transition from nonmagnetism has an inhomogenous character.

Stochastic NH3-rotation in [Ni(NH3)6](ClO4)2 crystals studied by the neutron incoherent scattering method

Abstract: Quasielastic neutron scattering were spectra measured for polycrystalline [Ni(NH3)6](ClO4)2. The spectra have been interpreted using models describing the stochastic NH3-rotational motions. The two models: 120°-jumps and the uniaxial rotational diffusion were used in the fitting procedure. The results of the fitting show that a preference should be given to the 120†-jump model. A comparison with an IR profile study was also made. Both the existence of 120°-jumps and the discontinuity in the correlation times observed at the phase transition can qualitatively be explained by the Bates and Stevens picture.

Revised analysis of the Cu IV spectrum

Abstract: An extensive revision of the analysis of the Cu IV spectrum is presented together with results of parametrical energy-level calculations; configurations considered are 3d8, 3d74s and 3d74p. The importance of calculated line strengths is demonstrated. Comparison with earlier published material learns that only 48 out of the 163 old level values hold; 107 new values are given, but levels of 3d75s could not be found, so that the old value of the ionization potential must be considered as dubious. Unresolved Zeeman patterns have been obtained at 6.7 tesla; observed splittings between σ+ and σ- components are compared with calculations.

Isotope shift studies in gadolinium spectra

Abstract: Isotope shift studies have been carried out in the gadolinium spectrum using a recording Fabry-Perot spectrometer and gadolinium samples enriched in 156 Gd and 160 Gd isotopes. Isotope shifts Δσ(156–160) have been recorded in 134 lines in the region 3930–4140 A. Some of these lines involve the recently identified even configuration 4f 8 5d6s of Gd I and the newly classified transition 4f 8 6s–4f 8 6p of Gd II. From the isotope shift measurements of lines involving the 4f 8 6s–4f 8 6p transition in Gd II, the isotope shift, ΔT (156–160) = 87 mK, has been obtained for the 4f 8 6s configuration. Electronic configurations have been suggested for a number of energy levels and configuration mixing has been pointed out in certain cases.

Term analysis of the system 3d6−3d54p of the fifth spectrum of nickel (Ni V)

Abstract: The spectrum of Ni V has been measured in the region from approximately 300 A to 450 A where the transitions between the 3d 6 −3d 5 4p configurations have been located. Out of the 34 theoretically possible levels of the 3d 6 configuration 33 levels have been determined, only the higher 1 S 0 level is missing. Out of the 214 levels of the 3d 5 4p configuration 177 levels are known. All these level values and the classified lines are tabulated at the end of this paper. Parametric (configuration) calculations of the levels of the 3d 6 and 3d 5 4p configurations have been made.

Experimental and theoretical study of the antiferromagnetic double-layer compounds Rb3Mn2F7 and K3Mn2F7

Abstract: We report on susceptibility measurements on K 3 Mn 2 F 7 and Rb 3 Mn 2 F 7 . The structure of these compounds consists of an array of antiferromagnetic double-layers, separated from each other by two non-magnetic layers. The high-temperature susceptibility series for n -layer Heisenberg films ( n = 1–7, and general S ), reported by Ritchie and Fisher, have been analyzed for the case of antiferromagnetic interaction. Furthermore, the low-temperature behaviour is calculated from spin-wave theory, including renormalization effects. The experimental data are in good agreement with the predictions for an S = 5 2 double-layer Heisenberg antiferromagnet (with free-surface boundary conditions), yielding the exchange constants J k B ) = -4.04(16) K and -3.56(20) K for K 3 Mn 2 F 7 and Rb 3 Mn 2 F 7 , respectively, which are close to the values expected from the interatomic distances. Experimental values for the anisotropy fields have been deducted from measurements of the spin-flop fields. Within the experimental errors the obtained anisotropy fields can be accounted for by the dipolar anisotropy, as calculated for the antiferromagnetic double-layer. The data for Rb 3 Mn 2 F 7 are compared with results for K 2 MnF 4 (mono-layer; n = 1) and RbMnF 3 (s.c.; n = ∞). Data for Rb 3 Mn 2 Cl 7 are also briefly discussed.

Experimental study of the simple cubic, S = 12, XY magnet. specific heat measurement on Co(C5H5NO)6(ClO4)2 and Co(C5H5NO)6(BF4)2 below 1 K

Abstract: The specific heats of Co(C5H5NO)6(ClO4)2 and Co(C5H5NO)6(BF4)2 have been studied below 1 K. Transitions to long-range antiferromagnetic order are observed at Tc = 0.428 ± 0.0003 K, respectively. The thermal data on both compounds agree very well with theoretical predictions for the s.c., S = 1 2 , XY model, yielding the respective values J k = -0.212 ± 0.0001 K and -0.180 ± 0.0001 K for the exchange constants. The predictions include Pade-approximants to the high-temperatures series, and spin-wave theory at the lowest temperatures. The observed critical behaviour agrees for 1 − T c T > 0.03 with the predicted logarithmic divergence in the specific heat.

Microwave-enhanced superconductivity in aluminium films

Abstract: Measurements are reported of the critical current of long, narrow superconducting aluminium thin film strips subjected to high-frequency radiation (10 MHz–10 GHz). Above a critical frequency considerable enhancement of the critical current is observed with increasing hf amplitude. In addition the critical temperature shows an increase of up to 16 mK. The results are consistent with predictions from the gap enhancement theory of Eliashberg et al. They suggest that the Dayem-Wyatt effect observed in short microbridges is due to the (microscopic) interaction between the superconductor and the hf field.

A molecular dynamics study of the intensity and band shape of depolarized light scattered from rare-gas crystals

Abstract: The induced dipole-induced dipole autocorrelation functions for systems of atoms interacting with the Lennard-Jones, the purely repulsive Lennard-Jones, and the hard sphere potentials in the solid phase are calculated by molecular dynamics. The calculated band shape for the Lennard-Jones system agrees with the experimental measurements of rare gas solids at their triple points as well as at lower temperatures where more phonon structure is evident. The band shape calculated for the hard sphere system shows no discrete phonon structure even at very high density and disagrees at the triple point of argon. A system of particles interacting with the purely repulsive Lennard-Jones potential shows an intermediate level of phonon contributions. The hard sphere and the Lennard-Jones potentials, however, each lead to a calculated intensity which is an order of magnitude too large, but give about the correct ratio of the liquid to the solid intensity at the triple point. This shows that the polarizability distortion is of similar magnitude in both the phases of argon.

Field-induced magnetic ordering in the S = 1 singlet ground-state system Ni(C5H5NO)6(ClO4)2 studied by specific heat

Abstract: Specific-heat data on Ni(C 5 H 5 NO) 6 (ClO 4 ) 2 are presented for fields 0 ⩽ H ⩽ 60 kOe and temperatures 0.1 ⩽ T ⩽ 1.3 K. In this material the S = 1 ground triplet is split by a trigonal crystal field term into a singlet and a higher lying doublet, such that ordering cannot occur in zero field. The field-induced magnetic phase transitions are studied both experimentally and theoretically, using molecular field theory, high-temperature series expansions and spin-wave theory. We obtain the values J / k = -(0.20 ± 0.01) K and D / k = 6.6 ± 0.2 K for the antiferromagnetic (subcritical) exchange constant and for the crystal-field splitting parameter, respectively.

Fifth spectrum of copper: Cu V

Abstract: The analysis of the Cu V spectrum has been considerably extended on the basis of new observations on the 6.650 m normal and the 6.600 m grazing incidence spectrographs at the Zeeman-Laboratorium in Amsterdam and on the 10.7 m normal and grazing incidence spectrographs at the National Bureau of Standards Laboratory in Washington D.C. The sources used were a sliding spark and a triggered spark. All levels of the 3d 7 configuration have been located; 53 out of 63 levels of the 3d 6 4s configuration have been determined and 175 out of 180 levels of the 3d 6 4p configuration have been established. Parametric calculations of the energy-level values agree very well with the experimental values. We classified 1165 lines in this spectrum. Tables of the classified lines, the energy levels and the values of the parameters of the configurations mentioned above are given.

Determination of trapping parameters from the conventional model for thermally stimulated luminescence and conductivity

Abstract: The conventional model for thermally stimulated luminescence and conductivity is solved by an expansion in a small parameter. A stable numerical method is given to obtain luminescence and conductivity curves from this model. Some relevant trapping parameters can be determined by a process of comparing theoretical calculations with experimental data with the help of nonlinear regression analysis.

The melting temperature of platinum measured from continually melting and freezing ribbons

Abstract: The melting temperature of platinum can be measured by using pure platinum ribbons, heated by a modulated direct electric current. The modulation of the emitted radiation shows a sharply increasing distortion when the current is raised to a value whereby in each period the maximum of the temperature reaches the melting point. During a fraction of the period a small center dot of the ribbon melts, causing a rise in emissivity of the center. It has been found that the radiance temperature at which the melting starts can be measured with a sensitivity of 0.02 K. Furthermore, using an opto-electronic stabilization, platinum ribbons in free air can be kept within 0.2 K at a constant mean temperature close to the melting point for one hour or more. From measurements of the radiance temperature and the spectral emissivity the melting temperature of platinum on the IPTS-68 was found to be 2041.0 ± 0.5 K.

On the cyclotron resonance width due to the electron-charged impurity interaction

Abstract: The cyclotron resonance width Γ for a very prue semi-conductor is calculated, starting with Kubo's formula and employing the proper connected diagram; the unscreened Coulomb, interaction, non-degenerate electrons, low temperature (below 4.2 K0 and high resonance frequency ω0 (≈ 50 Gc/s) are assumed. In the very low temperature region, we found that Γ = 1.403 h - 3 2 ( e 2 κ ) M - 1 4 ω 0 - 1 4 n s 1 2 where κ and M are respectively dieletric constant and (electron) effective mass. The square-root concentration n dependence is noteworthy. This formula is essentially identical with the formula obtained by Kawamura et al. on semi-classical arguments; only the numerical factor 0.915 is now replaced by 1.403. As temperature T is raised, Γ gradually decreases in value and asymptotically behaves like (k B T)[ - 1 2 ln (k B T) . These theoretical results are in good agreement with the experimental data obtained by Kawamura et al. for very pure Ge, n ≈ ns ≈ 1012 cm-3.