bluuebunny/crossref_metadata_2025 huggingface dataset:
Uniaxial stress investigations of the three-level conduction band structure of GaAs
Authors: C Pickering  , A R Adams
Published: July 2002

Measurements are reported of low-field resistivity and Gunn effect threshold field as a function of (100) and (111) uniaxial stress for a number of epitaxial and bulk GaAs samples. The results provide the first direct evidence that the L1cminima are situated lower in energy than the X1cminima and are therefore involved in the operation of GaAs transferred-electron devices. Intervalley separations are obtained at 300K, but these are dependent on density-of-states values. Further information on band structure parameters such as intervalley coupling constants should now be obtainable by fitting the high-field data to Monte-Carlo calculations currently in progress. Deformation potentials for the satellite minima have been determined.
Electromechanical properties and electron tunnelling in KTaO3
Authors: U T Hochli  , L A Boatner
Published: July 2002

The electromechanical constants and polarisation moments of KTaO3have been determined for samples of varying size. The polarisation moments exhibit a large size dependence, and this effect is interpreted in terms of space charge, which is formed by ionisation of traps. Space-charge fields are shown to arise spontaneously and are also induced by external fields. Large external fields have been observed to alter the electro-mechanical response irreversibly. This observation is explained in terms of electron tunnelling between shallow traps and rectification of the metal-dielectric Schottky barrier.
The Haupt effect: coupled rotational and dipolar relaxation of methyl groups
Authors: P Beckmann  , S Clough  , J W Hennel  , J R Hill
Published: July 2002

A theory is described for the dynamic proton dipolar polarization observed by Haupt (1972) in 4-methylpyridine following a sudden temperature change. The theory differs from that of Haupt in assuming that transitions which change the rotational quantum number of the 4-methyl group by +or-3 occur very rapidly, maintaining thermal equilibrium within each of the three subsets of rotational levels corresponding to the three methyl group proton spin symmetry species A, Eaand Eb. The difference of A and E species populations approaches the new equilibrium value slowly and exponentially, following the temperature jump, and generates dipolar polarization in the process. Transitions between Eaand Ebspecies lead to destruction of the polarization, whose evolution from zero due to these competing processes has the simple form C(exp(-at)-exp(-bt)). This is checked by a modified version of Haupt's experiment in which the initial temperature jump is followed by a later burst of RF pulses which reduces the dipolar polarization to zero.
Investigation of resonant energy transfer for LaF3:Pr3+
Authors: E Strauss  , W J Miniscalco  , J Hegarty  , W M Yen
Published: July 2002

The problem of resonant energy transfer (RET) is addressed for diluted rare-earth and transition-metal ions in a solid. Such processes involve the redistribution of excitation among sites with degenerate states. A new spectroscopic double selection technique is presented which is capable of detecting RET for inhomogeneously broadened multilevel systems. The method has been applied to LaF3:Pr3
+ for concentrations from 5 to 80 at.%. The experiments indicate that no significant RET occurs in this system. Reasons for this result are discussed, and other interesting materials to which this technique may be applied are mentioned.
Strong collision approach of nuclear Zeeman relaxation time T1of a tetrahedrally coordinated four-spin-1/2system
Authors: K Morimoto
Published: July 2002

Proton spin-lattice relaxation time T1of an NH4ion in the laboratory frame has been calculated using a strong collision approach. The present model of T1is based on the calculation of the absorption spectra in which the effect of the non-secular part of the dipolar interaction is also included. The applicability of the calculation is restricted to the temperature where taur>>T2( gamma HLtaur>>1), where tauris the correlation time, T2spin-spin relaxation time for the rigid lattice and HLlocal field of proton spins. It is shown that the spin-lattice relaxation for taur>>T2is determined by the non-magnetic Orbach process between the ground and first-excited torsional states. To obtain a simple relation of T1by the Orbach process among multiple levels, a four-level model has been proposed. The absorption spectra of proton spins have also been calculated when the level crossing of NH4ion occurs.
A simple chemisorption theory and its application to transition adatoms on transition metals
Authors: M C Desjonqueres
Published: July 2002

Calculates the binding energy of an adatom on a transition metal substrate taking into account: the band term which is treated in the tight-binding approximation; and electronic correlation contribution obtained from a perturbation theory within the Hubbard model and a pairwise repulsive potential of the Born-Mayer type. The binding energy is minimised with respect to the position of the adsorbate keeping neutral all atoms involved in the bonding. The authors find that bond lengths increase with the coordination number of the adsorbate. The most stable site is generally the site with maximum coordination but exceptions exist and are discussed. Numerical results of binding energy and surface diffusion activation energies at 0K for transition adatoms on several faces of BCC transition metals are in good agreement with experiments.
Raman scattering from hydrogenated microcrystalline and amorphous silicon
Authors: Z Iqbal  , S Veprek
Published: July 2002

Raman scattering measurements on hydrogenated microcrystalline silicon prepared in a hydrogen plasma at deposition temperatures between approximately=65 and 400 degrees C are presented and discussed. The shifts of the crystalline (c) and 'amorphous-like' (a) components of the spectra to lower frequencies with decreasing crystallite size have been correlated with the lattice expansion and the finite dimensions of the crystallites in these films. The roles of hydrogen and of the compressive stress in the samples have been investigated by annealing experiments and a deposition of the samples under negative bias of the substrate, respectively. These results point to a probable mechanism of the crystalline-amorphous transition in silicon. The data presented allow an assignment of the amorphous-like feature in the Raman spectra to surface-like modes at grain boundaries of the crystallites. Strong arguments are given that suggest that the 480 cm-1peak in the Raman spectra of X-ray amorphous silicon is of the same origin and is hence associated with some shearing modes of Si clusters rather than a broadened density of states. Results on the depolarisation ratio of Raman scattering in the microcrystalline and X-ray amorphous films are also presented and discussed.
Hydrodynamics of3HeA1and3He A2
Authors: T Dombre  , R Combescot
Published: July 2002

The authors show that in the A1and A2phases the spin pairing axis f cannot be considered as an independent hydrodynamic variable as has often been done previously. New equations are presented, where from the outset f is taken to be parallel to the magnetisation M. This is of special relevance for the transverse dynamics which, unfortunately, seems to be essentially governed by normal and dissipative processes. Some interesting physical points concerning the longitudinal dynamics are re-examined in some detail, including the spin-wave aspect of fourth and second sound, as well as the static magnetic fountain effect.
Multiphonon broadening of impact ionisation and Auger recombination involving traps in semiconductors
Authors: D J Robbins
Published: July 2002

The effect of lattice coupling upon impact ionisation and Auger recombination processes involving an arbitrary number of localised states is discussed within the framework of the static coupling scheme, and new expressions for the transition rates are obtained. These results are discussed in terms of the configurational coordinate diagram which highlights the close analogy that exists between (one-electron) radiative and (two-electron) impact ionisation and Auger recombination processes involving bound states. A calculation of Auger recombination rates involving a single trap is made, and some simple approximations are developed which are found to be in good agreement with more exacting numerical calculations.
Many-body effects in the (111)-1×1 surface of highly doped silicon
Authors: E Louis  , F Flores  , F Guinea  , J Tejedor
Published: July 2002

A two-dimensional model is proposed to explain, through the many-body properties of a strong correlated electron gas, the main electronic features of a (111)-1*1 surface of highly doped silicon.
Optical properties of small metallic particles in a continuous dielectric medium
Authors: A Liebsch  , B N J Persson
Published: July 2002

The dielectric function of an inhomogeneous system consisting of small particles in a dielectric host material is investigated using a lattice gas model to describe the topological disorder among the particles. By applying the coherent potential approximation, an effective single-particle polarisability is found, which contains the influence of the local electric fields arising from randomly distributed particles. In the Maxwell-Garnett theory it is assumed that these fields in a random distribution of dipoles are the same as those calculated for a cubic array of dipoles. The disorder is shown to give a substantial red shift of absorption peaks and a comparable blue shift of loss lines. Moreover, absorption as well as loss features are very strongly broadened. These kinds of shifts and enhanced linewidths have been observed experimentally for many systems. A detailed comparison with measured spectra is given for silver particles immersed in a gelatin matrix.
The number of spin symmetry species of tunnelling molecular groups
Authors: S Clough  , P J McDonald
Published: July 2002

It has been conventional for many years to discuss the quantum rotational motion of small symmetric molecular groups like CH3and NH4in a way which does not properly antisymmetrise state functions for single pair exchanges of nuclei, on the grounds that single pair exchanges occur too slowly to affect the energy levels. Though this is true, it also leads to a spurious factorisation of two-dimensional E nuclear spin symmetry species into two one-dimensional species Eaand Eb. This has led to widespread misconceptions and some erroneous interpretation of observations in terms of population differences of these fictitious species. It has also presented an obstacle to the development of a satisfactory description of the transition between quantum tunnelling motion at low temperatures and thermally activated hopping motions at high temperatures. Simple model calculations on methyl groups are presented to illustrate the true nature of the E species.
Spectroscopic investigation of the ferroelectric phase transition in tris-sarcosine calcium chloride: an order-disorder system with displacive features. I. Experimental results
Authors: T Chen  , G Schaack
Published: July 2002

The authors report infrared and Raman spectroscopic investigations of single crystals of tris-sarcosine calcium chloride (TSCC), of the isomorphous tris-sarcosine calcium bromide (TSCB) and of mixed crystals with different chlorine-bromine concentration ratios x (for TSCC1-xBx) in the whole wavenumber region below 4000 cm-1and in a temperature interval 2K<or=T<or=170 K. A general survey of the Raman and infrared reflection spectra is given and their temperature and concentration dependences are discussed. They concentrate on three different soft modes observed only in the ordered phase and on several 'hard-core' modes, which can be found in different spectral regions and which demonstrate a temperature-dependent frequency renormalization of the phonons on approaching Tc-. One of the soft modes is Raman-active with a negligible LO-TO splitting, the other two soft modes appear in the infrared spectrum with a small LO-TO splitting. Contrary to the usual behaviour of polar soft modes the LO frequencies also soften. The temperature and concentration dependences of these three modes are presented in detail. They are correlated with the presence of three different lattice sites for the sarcosine molecules in the low-temperature phase. The contribution of the soft modes to the anomaly of the dielectric constant at low frequencies is calculated. A microscopic order parameter is proposed that supports the assumption of a pseudo-proper phase transition in TSCC.
A theory for the electroreflectance spectra of quantum well structures
Authors: P C Klipstein  , N Apsley
Published: July 2002

A theory is presented to model the lineshape observed in electroreflectance spectra of quantum well structures reported recently by the authors and elsewhere. The existence of such spectra is shown to be a direct consequence of the sensitivity to a perpendicular electric field of the quantum well sub-band energies and the mechanism is thus qualitatively different from the Franz-Keldysh effect, which may explain the electroreflectance spectra of bulk materials. The importance of optical interference in determining the observed lineshape in multilayer samples is emphasised, and good agreement is obtained between the observed spectrum and the spectrum predicted by the model of a single 100 AA quantum well of Ga0.8Al0.2As/GaAs/Ga0.8Al0.2As. From a careful analysis of absorption, photoluminescence excitation and electroreflectance spectra for this sample the authors calculate binding energies for the heavy and light hole excitons of 11.6+or-1.3 meV and 10.1+or-1.1 meV respectively.
The thermal expansion coefficient and Gruneisen parameter of InP crystal at low temperatures
Authors: K Haruna  , H Maeta  , K Ohashi  , T Koike
Published: July 2002

The thermal expansion of the lattice constant of InP crystal has been measured in the range 4.2-300 K by the Bond method and the results are shown graphically. The thermal expansion coefficient alpha calculated from the experimental results is negative between 15 and 80 K and positive below 15 K. The corresponding Gruneisen parameter gamma closely follows the behaviour of alpha and the values of alpha and gamma are tabulated and shown graphically. The results do not agree with the phenomenological theory in which only a negative alpha is predicted for InP at low temperatures.
Lattice dynamics of LiH, LiF and LiCl
Authors: M P Verma  , R K Singh
Published: July 2002

The electron charge density maps as determined by X-ray diffraction experiments lead to the conclusion that the charge transfer in lithium fluoride is about 0.88e rather than 1.0e usually accepted for all the alkali halides. A theoretical investigation of the lattice vibrations in LiH, LiF and LiCl has been carried out with a view to obtaining an independent check on this observation of partial charge transfer. The model used is a modified shell model which takes account of three body overlap forces arising in ionic solids due to nonorthogonality of free-ion wavefunctions and in which the valence Z of an ion enters as an independent parameter. It would appear that Z=0.88 is a more appropriate choice than Z=1.0.
Nuclear quadrupolar relaxation in liquid Ga-In alloys
Authors: G Cartledge  , R L Havill  , J M Titman
Published: July 2002

Measurements of the quadrupolar relaxation rate of the Ga spins in liquid Ga-In alloys are reported. The rate is an increasing function of composition and a decreasing function of temperature. The dependence on temperature leads to an interpretation of the data in terms of a contribution from each of two separate mechanisms. One is the coupling to the time dependent field gradients associated with the diffusive motion of the ions and the other is characteristic of the relaxation in pure liquid Ga metal.
Lattice dynamics of trigonal selenium. II. Model theories
Authors: H Wendel  , W Weber  , W D Teuchert
Published: July 2002

For pt.I see ibid., vol.8, p.3725 (1975). Phonon dispersion curves of trigonal selenium (Se) were studied using a valence force field model (VFFM) and a shell model (SM) with anisotropic core-shell coupling constants. The interchain coupling of the VFFM arises largely from forces representing interactions between adjacent bonds, suggesting covalent interchain bonding. Long range forces exist especially along the chains for the SM, which also gives correct values for the LO-TO splitting of the i.r. active A2and E2modes and the associated effective charges and also the low and high-frequency dielectric constants.
Raman spectrum of Zn4O(BO2)6
Authors: A F Murray  , D J Lockwood
Published: July 2002

The first-order Raman spectrum of cubic zinc metaborate has been measured at room temperature, and the number of q=0 phonon frequencies found exceeds the group theoretical prediction by one. The additional band appears as an interference feature at 122.5 cm-1and is thought to be attributable to some impurity introduced during crystal growth. The results are in reasonable agreement with an earlier qualitative investigation of the infrared transmission spectrum of this material, and the reported fluorescent properties are verified. A comparison between the Raman results for zinc metaborate and those for Cr3B7O13Cl shows some correlations. A simple force-constant calculation identified four bands in each compound that can be associated with vibrations of BO4tetrahedra contained within the boron-oxygen framework.
On the motion of small spheres in gases IV. Diffusion coefficient of porous membranes
Authors: M M R Williams
Published: July 2002

The formalism for scattering by spherical particles has been used for a study of the diffusion coefficient and separation efficiency of the porous membranes used in separating isotopes and gas mixtures. The membrane, or barrier, is replaced by the randomly dispersed sphere model, thereby enabling the ideas developed in earlier papers of this series to be employed. Calculations are made in the Knudsen régime, and from an asymptotic solution to the Boltzmann equation for gas flow in the barrier an expression is obtained for the diffusion coefficient in terms of an average over the solution of a certain integral equation whose kernel is determined by the barrier properties. The integral equation is solved by pertubation and variational methods, and the results indicate that, of the conventional gas-surface interaction parameters, it is the tangential momentum accommodation coefficient which is most effective in determining barrier efficiency. The energy accommodation enters only in a minor fashion. In general, it is found that the larger the tangential coefficient, the smaller is the diffusion coefficient, whilst the energy accommodation acts in the opposite sense although to a smaller extent. The limitations of the `dusty-gas' model are stressed by comparison with exact results.
Investigation of a cylindrical, axially blown, high-pressure arc
Authors: W Hermann  , U Kogelschatz  , K Ragaller  , E Schade
Published: July 2002

An experimental arrangement for the production of a quasi-stationary, high-current arc is described. It is stabilized by an axial gas flow in a high-pressure environment (current: 1900 A, pressure: 23 atm, gas: nitrogen). The conditions are described under which part of the arc assumes a cylindrical form. For such an arc the radial temperature distribution was measured. Because of the cylindrical shape a relatively simple evaluation yields quantitative data about the local radiative energy balance and the other energy transport mechanisms. This leads, for the first time, to a quantitative understanding of the different physical processes in this arc, and makes it possible to formulate a simplified arc model which is useful in practical applications.
X-ray wave fields in perfect and nearly perfect crystals-theoretical background and recent applications
Authors: G Hildebrandt
Published: July 2002

The fundamental theories of X-ray diffraction are outlined and the following experimental techniques are dealt with: X-ray diffraction topography; X-ray interferometry; and synchrotron radiation as an improved X-ray source.
A new method for decoupling perpendicular modes in electron spin induction cavities
Authors: A Witte
Published: July 2002

A decoupling device for induction cavities is described, consisting of tuning screws which affect the magnetic field pattern in a rectangular waveguide and therewith the coupling angle to a resonant cavity.
An electronic correlator for frequencies of 1-100 kHz
Authors: P F Gascoyne  , P E Stott  , K B Scholefield
Published: July 2002

An electronic correlator for the frequency range 1-100 kHz is described. Time delays between 1 mu s and 1 ms are produced by a lumped circuit delay line which is automatically switched. The output is plotted on a XY recorder.
A class of unitarily invariant norms on 𝐵(𝐻)
Authors: Chan Jor-Ting , Li Chi-Kwong , Tu Charlies
Published: July 2002

Let

H
H

be a complex Hilbert space and let

B
(
H
)

B(H)

be the algebra of all bounded linear operators on

H
H

. For

c
=
(

c

1

,
…
,

c

k

)

c=(c_{1},\dots ,c_{k})

, where

c

1

≥
⋯
≥

c

k

>
0

c_{1}\ge \cdots \ge c_{k}>0

and

p
≥
1

p\ge 1

, define the

(
c
,
p
)

(c,p)

-norm of

A
∈
B
(
H
)

A\in B(H)

by
\[
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="double-vertical-bar upper A double-vertical-bar Subscript c comma p Baseline equals left-parenthesis sigma-summation Underscript i equals 1 Overscript k Endscripts c Subscript i Baseline s Subscript i Baseline left-parenthesis upper A right-parenthesis Superscript p Baseline right-parenthesis Superscript StartFraction 1 Over p EndFraction Baseline comma">

‖
A

‖

c
,
p

=

(

∑

i
=
1

k

c

i

s

i

(
A

)

p

)

1
p

,

\|A\|_{c,p}=\left (\sum _{i=1}^{k} c_{i} s_{i}(A)^{p}\right )^{\frac {1}{p}} ,

\]
where

s

i

(
A
)

s_{i}(A)

denotes the

i
i

th

s
s

-numbers of

A
A

. In this paper we study some basic properties of this norm and give a characterization of the extreme points of its closed unit ball. Using these results, we obtain a description of the corresponding isometric isomorphisms on

B
(
H
)

B(H)

.
Commutators for the maximal and sharp functions
Authors: Bastero Jesús , Milman Mario , Ruiz Francisco
Published: July 2002

The class of functions for which the commutator with the Hardy-Littlewood maximal function or the maximal sharp function are bounded on

L
q

L^q

are characterized and proved to be the same.
Sufficient conditions for one domain to contain another in a space of constant curvature
Authors: Zhou Jiazu
Published: July 2002

As an application of the analogue of C-S. Chen’s kinematic formula in the 3-dimensional space of constant curvature

ϵ
\epsilon

, that is, Euclidean space

R

3

{\mathbb {R}}^{3}

,

3
3

-sphere

S

3

S^{3}

, hyperbolic space

H

3

{\mathbb {H}}^{3}

(

ϵ
=
0
,

+
1
,

−
1

\epsilon =0,\,+1,\,-1

, respectively), we obtain sufficient conditions for one domain to contain another domain in either an Euclidean space

R

3

\mathbb {R}^{3}

, or a

3
3

-sphere

S

3

S^{3}

or a hyperbolic space

H

3

\mathbb {H}^{3}

.
Associative rings satisfying the Engel condition
Authors: Riley D. , Wilson Mark
Published: July 2002

Let

C
C

be a commutative ring, and let

R
R

be an associative

C
C

-algebra generated by elements

{

x
1

,
…
,

x
d

}

\{x_1,\ldots ,x_d\}

. We show that if

R
R

satisfies the Engel condition of degree

n
n

, then

R
R

is upper Lie nilpotent of class bounded by a function that depends only on

d
d

and

n
n

. We deduce that the Engel condition in an arbitrary associative ring is inherited by its group of units, and implies a semigroup identity.
Existence of convex hypersurfaces with prescribed Gauss-Kronecker curvature
Authors: Wang Xu-Jia
Published: July 2002

Let

f
(
x
)

f(x)

be a given positive function in

R

n
+
1

R^{n+1}

. In this paper we consider the existence of convex, closed hypersurfaces

X
X

so that its Gauss-Kronecker curvature at

x
∈
X

x\in X

is equal to

f
(
x
)

f(x)

. This problem has variational structure and the existence of stable solutions has been discussed by Tso (J. Diff. Geom. 34 (1991), 389–410). Using the Mountain Pass Lemma and the Gauss curvature flow we prove the existence of unstable solutions to the problem.
Bifurcation problems for the 𝑝-Laplacian in 𝑅ⁿ
Authors: Drábek Pavel , Huang Yin
Published: July 2002

In this paper we consider the bifurcation problem<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="minus d i v left-parenthesis StartAbsoluteValue nabla u EndAbsoluteValue Superscript p minus 2 Baseline nabla u right-parenthesis equals lamda g left-parenthesis x right-parenthesis StartAbsoluteValue u EndAbsoluteValue Superscript p minus 2 Baseline u plus f left-parenthesis lamda comma x comma u right-parenthesis comma">−div⁡(|∇u|p−2∇u)=λg(x)|u|p−2u+f(λ,x,u),\begin{equation*} -\operatorname {div} (|\nabla u|^{p-2}\nabla u)=\lambda g(x)|u|^{p-2}u+f(\lambda , x, u), \end{equation*}inRNR^Nwithp>1p>1. We show that a continuum of positive solutions bifurcates out from the principal eigenvalueλ1\lambda _{1}of the problem<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="minus d i v left-parenthesis StartAbsoluteValue nabla u EndAbsoluteValue Superscript p minus 2 Baseline nabla u right-parenthesis equals lamda g left-parenthesis x right-parenthesis StartAbsoluteValue u EndAbsoluteValue Superscript p minus 2 Baseline u period">−div⁡(|∇u|p−2∇u)=λg(x)|u|p−2u.\begin{equation*}-\operatorname {div} (|\nabla u|^{p-2}\nabla u)=\lambda g(x)|u|^{p-2}u. \end{equation*}Here both functionsffandggmay change sign.
Analysis and convergence of the MAC scheme. II. Navier-Stokes equations
Authors: Nicolaides R. , Wu X.
Published: July 2002

The MAC discretization scheme for the incompressible Navier-Stokes equations is interpreted as a covolume approximation to the equations. Using some results from earlier papers dealing with covolume error estimates for div-curl equation systems, and under certain conditions on the data and the solutions of the Navier-Stokes equations, we obtain first-order error estimates for both the vorticity and the pressure.
On the Sauer-Xu formula for the error in multivariate polynomial interpolation
Authors: de Boor Carl
Published: July 2002

Use of a new notion of multivariate divided difference leads to a short proof of a formula by Sauer and Xu for the error in interpolation, by polynomials of total degree

≤
n

\le n

in

d
d

variables, at a ‘correct’ point set.
A space-time finite element method for the nonlinear Schrödinger equation: the discontinuous Galerkin method
Authors: Karakashian Ohannes , Makridakis Charalambos
Published: July 2002

The convergence of the discontinuous Galerkin method for the nonlinear (cubic) Schrödinger equation is analyzed in this paper. We show the existence of the resulting approximations and prove optimal order error estimates in

L

∞

(

L

2

)
.

L^{\infty }(L^{2} ) .

These estimates are valid under weak restrictions on the space-time mesh.
The “hot spots” conjecture for domains with two axes of symmetry
Authors: Jerison David , Nadirashvili Nikolai
Published: July 2002

Consider a Neumann eigenfunction with lowest nonzero eigenvalue of a convex planar domain with two axes of symmetry. We show that the maximum and minimum of the eigenfunction are achieved at points on the boundary only. We deduce J. Rauch’s “hot spots” conjecture: if the initial temperature distribution is not orthogonal to the first nonzero eigenspace, then the point at which the temperature achieves its maximum tends to the boundary. This was already proved by Bañuelos and Burdzy in the case in which the eigenspace is one dimensional. We introduce here a new technique based on deformations of the domain that applies to the case of multiple eigenvalues.
Polynomial extensions of van der Waerden’s and Szemerédi’s theorems
Authors: Bergelson V. , Leibman A.
Published: July 2002

An extension of the classical van der Waerden and Szemerédi theorems is proved for commuting operators whose exponents are polynomials. As a consequence, for example, one obtains the following result: Let

S
⊆

Z

l

S\subseteq \mathbb {Z}^l

be a set of positive upper Banach density, let

p
1

(
n
)
,
…
,

p
k

(
n
)

p_1(n),\dotsc ,p_k(n)

be polynomials with rational coefficients taking integer values on the integers and satisfying

p
i

(
0
)
=
0

p_i(0)=0

,

i
=
1
,
…
,
k
;

i=1,\dotsc ,k;

then for any

v
1

,
…
,

v
k

∈

Z

l

v_1,\dotsc ,v_k\in \mathbb {Z}^l

there exist an integer

n
n

and a vector

u
∈

Z

l

u\in \mathbb {Z}^l

such that

u
+

p
i

(
n
)

v
i

∈
S

u+p_i(n)v_i\in S

for each

i
≤
k

i\le k

.
Pythagoras numbers of fields
Authors: Hoffmann Detlev
Published: July 2002

A field

F
F

of characteristic

≠
2

\neq 2

is said to have finite Pythagoras number if there exists an integer

m
≥
1

m\geq 1

such that each nonzero sum of squares in

F
F

can be written as a sum of

≤
m

\leq m

squares, in which case the Pythagoras number

p
(
F
)

p(F)

of

F
F

is defined to be the least such integer. As a consequence of Pfister’s results on the level of fields,

p
(
F
)

p(F)

of a nonformally real field

F
F

is always of the form

2
n

2^n

or

2
n

+
1

2^n+1

, and all integers of such type can be realized as Pythagoras numbers of nonformally real fields. Prestel showed that values of the form

2
n

2^n

,

2
n

+
1

2^n+1

, and

∞
\infty

can always be realized as Pythagoras numbers of formally real fields. We will show that in fact to every integer

n
≥
1

n\geq 1

there exists a formally real field

F
F

with

p
(
F
)
=
n

p(F)=n

. As a refinement, we will show that if

n
,
m
≥
2

n,m\geq 2

and

k
≥
1

k\geq 1

are integers such that

2
m
≥

2

k

≥
n

2m\geq 2^{k}\geq n

, then there exists a uniquely ordered field

F
F

with

p
(
F
)
=
n

p(F)=n

and

u
(
F
)
=

u
~

(
F
)
=
2
m

u(F)=\tilde {u}(F)=2m

(resp.

u
(
F
)
=

u
~

(
F
)
=
∞

u(F)=\tilde {u}(F)=\infty

), where

u
u

(resp.

u
~

\tilde {u}

) denotes the supremum of the dimensions of anisotropic forms over

F
F

which are torsion in the Witt ring of

F
F

(resp. which are indefinite with respect to each ordering on

F
F

).
Computing roadmaps of semi-algebraic sets on a variety
Authors: Basu Saugata , Pollack Richard , Roy Marie-Françoise
Published: July 2002

We consider a semi-algebraic set

S
S

defined by

s
s

polynomials in

k
k

variables which is contained in an algebraic variety

Z
(
Q
)

Z(Q)

. The variety is assumed to have real dimension

k
′

,

k’,

the polynomial

Q
Q

and the polynomials defining

S
S

have degree at most

d
d

. We present an algorithm which constructs a roadmap on

S
S

. The complexity of this algorithm is

s

k
′

+
1

d

O
(

k
2

)

s^{k’+1}d^{O(k^2)}

. We also present an algorithm which, given a point of

S
S

defined by polynomials of degree at most

τ
\tau

, constructs a path joining this point to the roadmap. The complexity of this algorithm is

k
′

s

τ

O
(
1
)

d

O
(

k
2

)

.

k’ s \tau ^{O(1)} d^{O(k^2)}.

These algorithms easily yield an algorithm which, given two points of

S
S

defined by polynomials of degree at most

τ
\tau

, decides whether or not these two points of

S
S

lie in the same semi-algebraically connected component of

S
S

and if they do computes a semi-algebraic path in

S
S

connecting the two points.
Antibiotic resistance in Gram-positive bacteria: epidemiological aspects
Authors: Witte Wolfgang
Published: July 2002

The emergence and spread of antibiotic resistance in Gram-positive bacterial pathogens has become an increasing problem. There has been a dramatic increase in the prevalence of methicillin-resistant Staphylococcus aureus (MRSA), coagulase-negative staphylococci and enterococci. This is mainly due to the clonal dissemination of certain epidemic multiply- resistant strains, for example, those of MRSA and S. pneumoniae, as well as to the spread of resistance genes as exemplified by those causing glycopeptide resistance in enterococci.
The regular packing of fibres in three dimensions
Authors: Parkhouse J. G. , Kelly A.
Published: July 2002

The possible tightest packing of bars of variously shaped cross–sections is investigated in detail. The bars are taken to lie along well–defined directions, e.g. cube edges, face diagonals, etc. The results are used to explore the feasibility of designing a fibrous composite which is both elastically isotropic and contains an appreciable volume fraction of reinforcement. Such is shown not to be possible for a regular array of bars all of the same cross–section. A volume fraction of about 0.25 can be obtained by arranging bars in a quasi–periodic fashion, akin to a three–dimensional Penrose tiling. Large volume fractions, greater than one half, together with isotropy may be obtained with two sets of bars each with two different cross–sections, e.g. 60% parallel to cube body diagonals and 40% parallel to cube edges.
Beijerinck's work on tobacco mosaic virus: historical context and legacy
Authors: Bos L.
Published: July 2002

Beijerinck's entirely new concept, launched in 1898, of a filterablecontagium vivum fluidumwhich multiplied in close association with the host's metabolism and was distributed in phloem vessels together with plant nutrients, did not match the then prevailing bacteriological germ theory. At the time, tools and concepts to handle such a new kind of agent (the viruses) were non–existent. Beijerinck's novel idea, therefore, did not revolutionize biological science or immediately alter human understanding of contagious diseases.

That is how bacteriological dogma persisted, as voiced by Loeffler and Frosch when showing the filterability of an animal virus (1898), and especially by Ivanovsky who had already in 1892 detected filterability of the agent of tobacco mosaic but kept looking for a microbe and finally (1903) claimed its multiplication in an artificial medium. The dogma was also strongly advocated by Roux in 1903, when writing the first review on viruses, which he named ‘so–called “invisible” microbes’, unwittingly including the agent of bovine pleuropneumonia, only much later proved to be caused by a mycoplasma. In 1904, Baur was the first to advocate strongly the chemical view of viruses. But uncertainty about the true nature of viruses, with their similarities to enzymes and genes, continued until the 1930s when at long last tobacco mosaic virus particles were isolated as an enzyme–like protein (1935), soon to be better characterized as a nucleoprotein (1937). Physicochemical virus studies were a key element in triggering molecular biology which was to provide further means to reveal the true nature of viruses ‘at the threshold of life’.

Beijerinck's 1898 vision was not appreciated or verified during his lifetime. But Beijerinck already had a clear notion of the mechanism behind the phenomena he observed. Developments in virology and molecular biology since 1935 indicate how close Beijerinck (and even Mayer, Beijerinck's predecessor in research on tobacco mosaic) had been to the mark. The history of research on tobacco mosaic and the commitments of Mayer, Beijerinck and others demonstrate that progress in science is not only a matter of mere technology but of philosophy as well. Raemaekers' Mayer cartoon, inspired by Beijerinck, artistically represents the crucial question about the reliability of our images of reality, and about the scope of our technological interference with nature.
The role of thermal activation in motion and force generation by molecular motors
Authors: Astumian R. Dean
Published: July 2002

The currently accepted mechanism for ATP–driven motion of kinesin is called the hand–over–hand model, where some chemical transition during the ATP hydrolysis cycle stretches a spring, and motion and force production result from the subsequent relaxation. It is essential in this mechanism for the moving head of kinesin to dissociate, while the other head remains firmly attached to the microtubule. Here we propose an alternative Brownian motor model where the action of ATP modulates the interaction potential between kinesin and the microtubule rather than a spring internal to the kinesin molecule alone. In this model neither head need dissociate (which predicts that under some circumstances a single–headed kinesin can display processive motion) and the transitions by which the motor moves are best described as thermally activated steps. This model is consistent with a wide range of experimental data on the force–velocity curves, the one ATP to one–step stoichiometry observed at small load, and the stochastic properties of the stepping.
How F
o
–ATPase generates rotary torque
Authors: Oster George , Wang Hongyun , Grabe Michael
Published: July 2002

The F–ATPases synthesize ATP using a transmembrane ionmotive force (IMF) established by the electron transport chain. This transduction involves first converting the IMF to a rotary torque in the transmembrane F
o
portion. This torque is communicated from F
o
to the F
1
portion where the energy is used to release the newly synthesized ATP from the catalytic sites according to Boyer's binding change mechanism. Here we explain the principle by which an IMF generates this rotary torque in the F
o
ion engine.
Heparin binding by the HIV‐1 tat protein transduction domain
Authors: Hakansson Susanna , Jacobs Amy , Caffrey Michael
Published: July 2002

The protein transduction domain from the HIV‐1 tat protein (termed PTD‐tat) has been fused to the C‐terminus of a model cargo protein, the IgG binding domain of streptococcal protein G. We demonstrate that PG‐Ctat (PTD‐tat fused to the C‐terminus of protein G) binds to a heparin affinity column. PG‐Ctat binds with relatively high affinity, as shown by its elution at 1.6 M NaCl. The heparin binding properties of PTD‐tat are consistent with the idea that heparan sulfate, an analog of heparin found at the cell surface, plays a role in the translocation of PTD‐tat fusions. We suggest that the heparin‐binding properties of PTD‐tat can be exploited for purification of PTD‐tat fusions in the absence of affinity tags.
An innovative digital imaging set‐up allowing a low‐dose approach to phase contrast applications in the medical field
Authors: Olivo A. , Arfelli F. , Cantatore G. , Longo R. , Menk R. H. , Pani S. , Prest M. , Poropat P. , Rigon L. , Tromba G. , Vallazza E. , Castelli E.
Published: July 2002

Recently, new imaging modalities based on the detection of weak phase perturbations effects, among which are phase contrast and diffraction imaging, have been developed by several researchers. Due to their high sensitivity to weakly absorbing details, these techniques seem to be very promising for applications in the medical field. On the other hand, digital radiology is undergoing a wide diffusion, and its benefits are presently very well understood. Up to now, however, the strong pixel size constraints associated with phase contrast pattern detection limited the possibility of exploiting the advantages of phase contrast in digital radiology applications. In this paper, an innovative setup capable of removing the pixel size constraints, and thus opening the way to low dose digital phase contrast imaging, is described. Furthermore, we introduce an imaging technique based on the detection of radiation scattered at small angles: the information extracted from the sample is increased at no dose expense. We believe that several radiological fields, mammography being the first important example, may benefit from the herein described innovative imaging techniques.
Neural network based automated algorithm to identify joint locations on hand/wrist radiographs for arthritis assessment
Authors: Duryea J. , Zaim S. , Wolfe F.
Published: July 2002

Arthritis is a significant and costly healthcare problem that requires objective and quantifiable methods to evaluate its progression. Here we describe software that can automatically determine the locations of seven joints in the proximal hand and wrist that demonstrate arthritic changes. These are the five carpometacarpal (CMC1, CMC2, CMC3, CMC4, CMC5), radiocarpal (RC), and the scaphocapitate (SC) joints. The algorithm was based on an artificial neural network (ANN) that was trained using independent sets of digitized hand radiographs and manually identified joint locations. The algorithm used landmarks determined automatically by software developed in our previous work as starting points. Other than requiring user input of the location of nonanatomical structures and the orientation of the hand on the film, the procedure was fully automated. The software was tested on two datasets: 50 digitized hand radiographs from patients participating in a large clinical study, and 60 from subjects participating in arthritis research studies and who had mild to moderate rheumatoid arthritis (RA). It was evaluated by a comparison to joint locations determined by a trained radiologist using manual tracing. The success rate for determining the CMC, RC, and SC joints was 87%–99%, for normal hands and 81%–99% for RA hands. This is a first step in performing an automated computer‐aided assessment of wrist joints for arthritis progression. The software provides landmarks that will be used by subsequent image processing routines to analyze each joint individually for structural changes such as erosions and joint space narrowing.
Analytical considerations of beam hardening in medical accelerator photon spectra
Authors: Kleinschmidt Christoph
Published: July 2002

Beam hardening is a well‐known phenomenon for therapeutic accelerator beams passing through matter in narrow beam geometry. This study assesses quantitatively the magnitude of beam hardening of therapeutic beams in water. A formal concept of beam hardening is proposed which is based on the decrease of the mean attenuation coefficient with depth. On the basis of this concept calculations of beam hardening effects are easily performed by means of a commercial spreadsheet program. Published accelerator spectra and the tabulated values of attenuation coefficients serve as input for these calculations. It is shown that the mean attenuation coefficient starts at depth zero with an almost linear decrease and then slowly levels off to a limit value. A similar behavior is found for the beam hardening coefficient. A physically reasonable, semianalytical model is given which fits the data better than previously published functions. The energy dependence of the initial attenuation coefficient is evaluated and shown. It fits well to published experimental data. The initial beam hardening coefficient, however, shows no energy dependence. Its mean value is also in close agreement to the measured data.
Computer sound card assisted measurements of the acoustic Doppler effect for accelerated and unaccelerated sound sources
Authors: Bensky T. J. , Frey S. E.
Published: July 2002

An approach to experimentally measuring the speed of a moving object by direct application of the Doppler effect for sound is discussed. The method presented here uses a Windows computer and sound card to record Doppler shifted sound from a moving source. This sound card approach allows for direct acquisition of Doppler shifted sound intensity as a function of time, affording much analytical and pedagogical freedom in undergraduate lab instruction. In addition, the acquisition of such data allows for the experimental study of not only constant velocity sound sources, but of accelerated sound sources as well.
Quantum jumps and classical harmonics
Authors: Fedak William A. , Prentis Jeffrey J.
Published: July 2002

We present an introduction to quantum mechanics based on the formal correspondence between the atomic properties of quantum jumps and the classical harmonics of the electron’s periodic motion. By adding a simple quantum condition to the classical Fourier analysis, we readily find the energies of the stationary states, calculate the transition probabilities between the states, and construct the line spectrum of the emitted light. We provide examples to illustrate the asymptotic, and sometimes exact, agreement between the classical-quantum results (Fourier harmonics) and the exact quantum results (Heisenberg harmonics).
The relationship between speech perception and electrode discrimination in cochlear implantees
Authors: Henry Belinda A. , McKay Colette M. , McDermott Hugh J. , Clark Graeme M.
Published: July 2002

Speech Intelligibility Index (SII) procedures were used to measure the amount of speech information perceived in five frequency bands (170–570, 570–1170, 1170–1768, 1768–2680, and 2680–5744 Hz) by 15 users of the Cochlear Ltd. CI-22M implant and Spectra-22/SPEAK processor. The speech information perceived was compared to that perceived by normal-hearing listeners. The ability of these subjects to discriminate between stimulation on adjacent electrodes corresponding to each frequency band was also investigated, using a 4IFC procedure with random current level variations of between 0% and 60% of the dynamic range. Relative to normal-hearing listeners, speech information was, on average, significantly more reduced in the four frequency regions between 170 and 2680 Hz than in the region 2680–5744 Hz. There was a significant correlation between electrode discrimination ability (when the random level variation encompassed 20% or more of the dynamic range) and the amount of speech information perceived in the four frequency regions between 170 and 2680 Hz. There was no such correlation in the region 2680–5744 Hz, regardless of the extent of random level variation. These results indicate that speech information in the low to medium frequencies is more difficult for implantees to perceive, that this difficulty is correlated with the difficulty in discriminating electrode place in the presence of random loudness variations, and that fine spectral discrimination may be relatively more important in the vowel-formant regions than in higher frequency regions.
The prediction of speech intelligibility in underground stations of rectangular cross section
Authors: Yang Lening , Shield Bridget M.
Published: July 2002

Long enclosures are spaces with nondiffuse sound fields, for which the classical theory of acoustics is not appropriate. Thus, the modeling of the sound field in a long enclosure is very different from the prediction of the behavior of sound in a diffuse space. Ray-tracing computer models have been developed for the prediction of the sound field in long enclosures, with particular reference to spaces such as underground stations which are generally long spaces of rectangular or curved cross section. This paper describes the development of a model for use in underground stations of rectangular cross section. The model predicts the sound-pressure level, early decay time, clarity index, and definition at receiver points along the enclosure. The model also calculates the value of the speech transmission index at individual points. Measurements of all parameters have been made in a station of rectangular cross section, and compared with the predicted values. The predictions of all parameters show good agreement with measurements at all frequencies, particularly in the far field of the sound source, and the trends in the behavior of the parameters along the enclosure have been correctly predicted.