D. W. Deis

Magnetic Studies of the Alloy System SnTe–MnTe

Magnetic ordering and superconductivity have been investigated in the system Sn0.97‐xMnxTe for 0.00001≤x≤0.16 over the temperature range 0.02≤T≤300°K. Reciprocal Hall coefficient, 1/Re, for all these samples was ∼1021 cm−3. Samples with x>0.005 showed ferromagnetic ordering with the Curie temperatures varying linearly with x. For 0.0001<x<0.005, neither magnetic ordering nor superconductivity were observed above 0.02°K. In samples with x<0.0001 superconductivity has been observed with the transition temperatures being depressed below that for Sn0.97Te (Tc≃0.20°K). Measurements have also been made on samples with constant manganese concentration and varying carrier density. Heat capacity measurements between 2°–30°K showed anomalies attributable to the magnetic coupling of the Mn ions. These results will be discussed in terms of current long‐range indirect‐exchange interaction theories.

Chapter 5 Superconductivity in Semiconductors and Semi-Metals

Publisher Summary The main object of this chapter is to review the present state of knowledge on low carrier concentration superconductors. The subject is a fairly new one, since until seven years ago the possibility of the occurrence of superconductivity in materials with appreciably less than one carrier per atom was essentially ignored. This chapter also describes that heavily doped, multivalley semiconductors such as germanium, silicon, and their alloys should become superconducting below 1 K. This possibility has not yet been verified for these materials. Carrier density as an experimental variable in superconductors was ignored for many years because most superconductors are metals, in which the carrier density is difficult to change substantially. This difficulty does not exist for the superconducting semiconductors already mentioned. All of these compounds have the common feature that the carrier density can be varied in a controlled fashion by at least one order of magnitude. This wide variation produces quite dramatic changes in the superconducting critical temperature.

Study of Magnetic Interactions in (SnTe–MnTe) and (SnTe–EuTe) Systems

As reported earlier the alloy system Sn0.97−xMnxTe orders ferromagnetically with Curie temperatures varying linearly with x. The present work has extended this and includes: (i) More superconductivity results which indicate that for low carrier densities (∼1021 cm−3), the paramagnetic impurity influences Tc very drastically, e.g., 100‐ppm Mn destroys the superconductivity completely. However, if the carrier density is increased to 4×1021 cm−3, the transition temperature decreases gradually with increasing Mn content and superconductivity persists even for x=0.001. In both cases, however, the transition is broadened probably due the presence of the internal fields. (ii) Electrical resistivity measurements made on these samples (0.00001<x<0.10) between 4° and 100°K, show a characteristic metallic behavior but with no maxima or minima appearing in the residual part. (iii) A study of s‐f interactions by means of Eu doping for comparison with s‐d interaction results (Mn doped). Eu‐doped samples are single phas...

Lower critical field measurements in NbN bulk and thin films

Low‐field magnetization measurements have been made at 4.2°K on thin‐film and bulk NbN samples using a vibrating‐sample Foner magnetometer. These data can be used to calculate the upper critical field, Hc2, without having to resort to resistivity data, which, particularly in the case of NbN, yield very anomalous results. In the present work the experimental value of Hc1 is obtained from the magnetization curves and Hc2 is then calculated using the GLAG equations. These calculations and the significance of the Pauli spin paramagnetism and spin‐orbit scattering in these materials are discussed.

Scaling laws for flux pinning in Nb and NbTi multifilamentary conductors

Magnetization measurements M(H,T) are made on several commercially drawn Nb–Cu and NbTi–Cu multifilamentary conductors as a function of applied magnetic field and temperature. The samples are distinguished by their filament sizes or the amount of cold work. From these measurements the pinning force Fp(H,T) and the critical current characteristics Jc(H) are deduced for all samples. Using this information Kramers scaling laws for flux pinning in multifilamentary conductors were tested. It is found that the model with some reservations seems to account for most of the observed behavior in the Nb–Cu and NbTi–Cu systems.

Superconductivity as a function of carrier density and magnetic spin concentration in the SnTe-MnTe system☆

Abstract Superconductivity results on self doped as-cast tin telluride show a strong particle size dependence of the superconducting critical temperature ( T c , °K). The T c of the powdered as-cast material (size ⩽ 100μ) approaches the T c of the pressed and sintered samples. A similar but larger effect was observed in Sn 0·87 Ag 0·10 Te, where the T c of the as-cast material (1·8°K) decreased to about 1°K on grinding (size ⩽ 100μ). Pressing and sintering the same material did not increase the T c Complete T c data on the as-cast Sn 0·97− x Ag x Te samples with 0·01 T c of Sn 0·97− x−y Ag x Mn y Te samples was also studied by changing the parameters and the results are discussed on the basis of prevalent theories.

Anisotropy of the critical current density in the NbTi filaments of round composite superconductors

Magnetization measurements have been performed on several multifilament NbTi wires in transverse and longitudinal magnetic fields. The ratio of circumferential and axial critical current densities for the individual filaments is deduced from the measurements and appears to be an increasing function of the magnitude of the magnetic field and the amount of cold work in the filaments. The results suggest that filaments may be less stable in longitudinal magnetic fields than calculations utilizing standard critical current data would indicate. Transverse swept field magnetization data confirm the theory of coupling in the composite.

Alternating‐Current Loss Measurements in Thin‐Film Type II Superconductors

An electrical technique is described for measuring the audio frequency power dissipation in superconducting thin films as a function of the ac current density, frequency, and applied dc magnetic field. This system has been used to study the ac losses in thin films of Nb and NbN, and also the ac and dc critical current densities in these materials. The results can be explained by assuming that both hysteresis and eddy current losses contribute.

Multifilament Nb3Sn conductors

The superconducting and mechanical properties of multifilament Nb3Sn wires prepared by the surface diffusion technique are described. Materials tested were 0.021‐ and 0.051‐cm‐diam wires with 392 filaments 6.6 or 16 μ in diameter. Electron microprobe and scanning electron microscope studies of Nb3Sn layer thickness and matrix contamination have been carried out also.

Critical currents of superconducting conductors measured by a new technique

A magnetic torque technique for measuring critical current densities is described. Results obtained on Nb–Cu and (Nb–Ti)–Cu composite conductors by this technique are presented and compared with direct transport current measurements.