Optical absorption of EuTe in high magnetic fields

For the first time, the optical absorption edge of the magnetic semiconductor europium telluride has been studied at low temperature (1.7 K) as a function of magnetic field in the high field range 0 ≤ H ≤ 9.5 Tesla, and related to the magnetic phase diagram of this material. The exponential absorption edge has been parameterized using a magnetic field dependent extension of Urbach's rule, and the observations are found to be consistent with a simple dependence of the gap parameter on polarization and magnetic field.     

Laser emission study of the pressure dependence of the energy gap in tellurium

The pressure dependence of the band gap of tellurium has been precisely measured to 8 kbar in a study of the laser emission of an optically-pumped sample at liquid nitrogen temperatures. A large negative pressure tuning coefficient is observed along with an anomalous nonlinearity in the tuning. These measurements are compared to previous studies using other optical and electrical techniques and are related to current theoretical models of the band structure of tellurium.     

Preconditioned HSS methods for the solution of non-Hermitian positive definite linear systems and applications to the discrete convection-diffusion equation

Summary. We study the role of preconditioning strategies recently developed for coercive problems in connection with a two-step iterative method, based on the Hermitian skew-Hermitian splitting (HSS) of the coefficient matrix, proposed by Bai, Golub and Ng for the solution of nonsymmetric linear systems whose real part is coercive. As a model problem we consider Finite Differences (FD) matrix sequences {A_n(a,p)}_n discretizing the elliptic (convection-diffusion) problem with being a plurirectangle of R^d with a(x) being a uniformly positive function and p(x) denoting the Reynolds function: here for plurirectangle we mean a connected union of rectangles in d dimensions with edges parallel to the axes. More precisely, in connection with preconditioned HSS/GMRES like methods, we consider the preconditioning sequence {P_n(a)}_n, P_n(a):= D_n^1/2(a)A_n(1,0) D_n^1/2(a) where D_n(a) is the suitably scaled main diagonal of A_n(a,0). If a(x) is positive and regular enough, then the preconditioned sequence shows a strong clustering at unity so that the sequence {P_n(a)}_n turns out to be a superlinear preconditioning sequence for {A_n(a,0)}_n where A_n(a,0) represents a good approximation of Re(A_n(a,p)) namely the real part of A_n(a,p). The computational interest is due to the fact that the preconditioned HSS method has a convergence behavior depending on the spectral properties of {P_n^-1(a)Re(A_n(a,p))}_n {P_n^-1(a)A_n(a,0)}_n: therefore the solution of a linear system with coefficient matrix A_n(a,p) is reduced to computations involving diagonals and to the use of fast Poisson solvers for {A_n(1,0)}_n.Some numerical experimentations confirm the optimality of the discussed proposal and its superiority with respect to existing techniques.Mathematics Subject Classification (1991): 65F10, 65N22, 15A18, 15A12, 47B65     

Temperature dependence of C-axis electrical resistivity and thermopower of graphite intercalation compounds

Results on the temperature variation from 2 ? T ? 300 K of the c-axis electrical resistivity and thermopower of a donor stage-5 potassium and an acceptor stage-2 FeCl₃ graphite intercalation compounds are reported. Comparing the data with those of the literature reveals for both compounds a different qualitative behavior for low and high stages.     

Stationary values of the ratio of quadratic forms subject to linear constraints

Sommario SiaA una matrice reale simmetrica di ordinen, B una matrice reale simmetrica di ordinen per cuix ^T B x>0 eC una matricen×p di rangor conrp. Si vogliono deteminare i vettorix per cui, JIMELè stazionaria eC ^T x= è il vettore nullo. È dato un algoritmo per generare un autosistema simmetrico i cui autovalori sono i valori stazionarì e per determinare i vettorix. Sono altresì presentate parecchie applicazioni dell'algoritmo.

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Dedicated to Professor L. Collatz on his sixtieth birthday

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This author was in part supported by the Atomic Energy Commission.     

Light scattering lineshape in opaque materials

The wavevector distribution excited by light scattering in opaque materials is calculated and compared to recently observed asymmetrical Brillouin lineshapes.