We have investigated the effect of the long-range Coulomb interaction on the one-particle excitation spectrum of n-type germanium, using tunneling spectroscopy on mechanically controllable break junctions. At low temperatures, the tunnel conductance shows a minimum at zero bias voltage due to the Coulomb gap. Above 1 K, the gap is filled by thermal excitations. This behavior is reflected in the variable-range hopping resistivity measured on the same samples: up to a few degrees Kelvin the Efros-Shklovskii lnR infinity T(-1/2) law is obeyed, whereas at higher temperatures deviations from this law occur. The type of crossover differs from that considered previously in the literature.

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A B S T R A C TThis study investigates the generation of the low-frequency borehole Stoneley wave (tube wave) by a plane P-wave propagating through the surrounding elastic formation, which is intersected by a fluid-filled fracture. A model is constructed taking into account the dynamic fluid coupling between the borehole interior and the fluid-filled fracture of infinite extent with parallel walls. The basic mechanism of such coupling is due to the contraction of the fracture walls by the incident P-wave, leading to seismic radiation into the fracture. The dynamic fluid flux from the fracture into the borehole interior, and vice versa, is the source of the low-frequency Stoneley wave. An expression for the monopole pressure source, exciting the tube wave, is obtained.The tube-wave equation in the long-wave approximation is derived in the presence of a fluid-filled fracture of infinite extent. Amplitudes and waveforms of Stoneley waves are analysed in the seismic wavelength range for P-wave pulses of various shapes. It is shown that the amplitude and waveform of the Stoneley wave depends significantly on the two dimensionless parameters of the problem: (1) the ratio of the borehole radius to the dominant wavelength of the incident pulse; (2) the ratio of the fracture width to the borehole radius. It is found that the amplitude of the generated Stoneley wave can be of the order of the P-wave amplitude in the borehole fluid. Stoneley waveforms are found to be completely different from those of the incident pulse.

We present magnetic-, resistivity-, and specific-heat measurements of the title compounds and found that in Eu0.7Sr0.3MnO3 an insulator-metal transition can be induced by an external field below 60 K while Eu0.7Ca0.3MnO3 remains insulating. EuMnO3 exhibits two magnetic transitions at 35 and 47 K which are associated with the appearance of a weak ferromagnetic component and antiferromagnetic ordering. With respect to the parent compound, Ca and Sr substitution enhance the ferromagnetic component significantly, however, for low fields and after zero-field cooling the antiferromagnetic ground state is preserved to a large extent.

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R. R.ENTZSCH e t al. : Hopping in Keutron-Transmutatim-Doped Gallium Arsenide 691 phys. stat. sol. (b) 137, 691 (1986) First investigations are reported on the temperature dependence of the dc-conductivity a t T = = 0.05 to 300 K on neutron-transmutation-doped (NTD) n-GaAs in the vicinity of the metalinsulator transition (MIT). At a medium compensation of K = 0.60 to 0.77 the MIT takes place a t the critical electron concentration of nc = 2.3 X lOla ~m -~. On the dielectric side of the MIT in the variable-range hopping regime (VRH) a t T 5 10 K the conductivity obeys the equation

Regular single and triple step arrays with different periodicities have been fabricated in ultrahigh vacuum on clean Si͑557͒ surfaces at various thermal treatment procedures. The atomic structure of the triple step staircases has been studied with high resolution scanning tunneling microscopy ͑STM͒. The results of atomically resolved STM experiments demonstrate a number of possible triple step configurations on Si͑hhm͒ surfaces. The triple step models consistent with atomically resolved STM data obtained on regular Si͑223͒ and Si͑556͒ triple step staircases are presented. Possible driving forces for self-assembling regular step arrays on large scale areas are discussed.

SUMMARY Tube‐wave characteristics generated in a fluid‐filled borehole may be used to determine enclosing‐media parameters and, in particular, to localize petroliferous strata, if any, and estimate their permeabilities. As a rule, tube waves are generated by an external seismic source of explosive type located some distance away from a borehole embedded in a layered medium. The modelling of tube‐wave excitation and propagation is a complicated problem, whose solution is necessary for solving the inverse problem. In the present study, an efficient numerical algorithm is presented to compute the tube waves' propagation, caused by external explosive perturbations in a fluid‐filled borehole located in a layered enclosing medium. Fluid flow between the borehole interior and the enclosing saturated porous rocks can easily be taken into account within the framework of the algorithm. The solution is obtained by Sommerfeldintegral representation of external field potentials on the borehole axis and subsequent analytical integration of the external field along the borehole intervals of homogeneity; the resultant tridiagonal amplitude matrix is solved numerically. The method was tested in detail. Numerical results for simple medium configurations surrounding a fluid‐filled borehole have been obtained. The modelling of tube waves, caused by the incident P, reflected PP, PS and the Rayleigh wave in a fluid‐filled borehole of finite length located in an elastic half‐space, adequately describes the main features of tube‐wave propagation. Tube‐wave modelling that includes the presence of a layer with hydraulic conductivity shows a high efficiency of tube‐wave generation at the layer boundaries and significant amplitude attenuation inside it. It follows from the results of testing that the algorithm developed can be used for adequate modelling of VSP data.