High Transverse Field Research Articles

Slow relaxation and electric field quenching of persistent conductivity in GaAs metal-semiconductor field-effect transistors with different buffer layer structures

Slow relaxation in photoconductivity and high transverse field effects have been investigated in GaAs metal-semiconductor field-effect transistors with undoped GaAs buffers and AlGaAs/GaAs superlattice buffers. Persistent photoconductivity (PPC) has been observed in both types of devices at 77 K. Complete electric field quenching of the PPC is possible in devices with a GaAs buffer. A high electric field resulted in more profound effects on the channel conductance of the superlattice buffered devices. The barrier established by the superlattice precludes complete elimination of the PPC and thermal quenching is necessary.

Diffusion and localization of positive muons in niobium single crystals at low temperatures in the presence of impurities measured in zero and high transverse fields

We studied the trapping and diffusion behaviour of positive muons ( μ +) in niobium single crystals by comparing the measured depolarization rate with model calculations. Zero-field muon spin resonance measurements reveal the temperature dependence of the diffusion coefficient and also clearly indicate that the different trap configurations above and below about 22 K are correlated. The activation energy needed to escape from the low temperature traps was found to be 200 ± 20 K (17 ± 1.5 meV). Subsequently, the angular dependence of the depolarization rate was measured at 14.0 and 36.8 K in a transverse field of 7.47 kG by rotating a niobium single crystal around its 〈110〉 axis, which was kept perpendicular to both the field and the μ + polarization. This allows the site symmetry of the trapped muon to be determined as well as the lattice distortions around it. The trapping site at 36.8 K could be identified as a tetrahedral site next to a tantalum impurity, and possibly also close to an interstitial (nitrogen or oxygen) impurity. A local lattice relaxation ΔR R of 6.7(6)% for nearest neighbours and of −(6 ± 2)% for next-nearest neighbours has been deduced. The low temperature curve shows a much more pronounced angular dependence than the high temperature curve, indicating a completely different μ + environment. A satisfactory explanation for these data has not yet been found.

Detailed investigation of the “DIP” structure in the μ+-relaxation rate in Nb

The depolarization rate of positive muons implanted in a number of nominally pure, cylindrical Nb single crystals (maximal 250 ppm Ta, 100 ppm N + O) was investigated at two temperatures, viz. 14.0 and 36.8 K, in a high transverse field of 7.5 kG with the stroboscopicμSR technique in order to study the nature of the “dip” at 22 K. To determine the sites at which the muon is trapped on both sides of this dip, the full angular dependence of the depolarization rate was measured by rotating a large single crystal around its 〈110〉 cylinder axis in a transverse magnetic field. The resulting curves for both temperatures are quite different, reflecting clearly the different environment in which the muon is trapped above and below 22 K. The trapping site at 36.8 K was identified to be of tetrahedral symmetry, located near a Ta substitutional impurity and possibly associated with an interstitial impurity. Lattice distortions due to these impurities and radial relaxation around the muon,δR/R, were determined. The latter is +6.7(6)% for nearest neighbors and −6(2)% for next nearest neighbors. The 14.0 K angular dependence could not be fitted by considering distorted tetrahedral and octahedral sites and pointlike muons.

AC Loss in Superconducting Wires in a Rotating Magnetic Field. I. Low-Frequency Loss in a Wire in a High Transverse Field

The magnetic behavior of a nonideal type II superconducting wire in a rotating magnetic field is discussed with the aid of the critical-state model. The discussion in this paper is confined to the case where an external magnetic field He rotates very slowly in a plane perpendicular to the wire axis with a magnitude much larger than Hp, the value of He at which a flux front reaches the center of the wire. The present paper provides analytical expressions for the flux distribution, the magnetization, the AC loss, and the critical angle at which the steady state is reached.

High field critical current densities and resistive transition of NbCN superconducting films

We studied the influence of the carbon density on the critical current densities of niobium carbonitride films in high magnetic fields. The samples were about 2 200 Å thick and were obtained by reactive sputtering either from a Nb-C target in a residual Ar + N 2 atmosphere or from a bulk Nb target in a residual Ar + N 2 + CH 4 atmosphere. They all had sufficiently high critical temperature, around 15 K, which implies that the ratio of carbon atoms over the total supplementary nitrogen and carbon atoms lay in the range 0.06 to 0.35. The samples were immersed in the liquid helium at 4.2 K. The registered V(I) curves either showed an hysteretic feature or not, depending on the composition and/or the applied magnetic field. The best critical current densities, about several 10 10 Am − 2 in a zero field were still more than 3 × 10 8 in a 14 T field, irrespective of direction, transverse or perpendicular. In the former case, some samples were tested up to 18 T, a value for which the best critical density was 10 7 Am − 2 . Heterogeneous samples which have periodic variations in carbon concentration, have still greater critical densities in high transverse fields. The J c (B) curves show a plateau or a slight hump when the field is such that the flux line lattice parameter fits the period of the carbon ratio variations. The resistivities of the different films are also high enough, more than 10 − 6 Ωm. So these alloys might be of interest in a superconducting switchgear, using the resistive transition to the normal state in order to limit a short circuit current.

Losses in superconducting composites under high rate pulsed transverse field

In case of a plasma current disruption in a tokomak the conductor of the toroidal field coil may be subjected to a field varying exponentially with a time constant as low as a few milliseconds. Expressions of losses produced by the transverse component of the changing field can be written in the form <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">W = \Delta B^{2}\theta / 2 \mu_{0}(\theta+\tau)</tex> Where θ is the time constant of the coupling currents in the superconducting multifilament composite and τ the time constant of the magnetic field. The value of the time constant θ evaluated in other theories is discussed For small τ, the outer layers of filaments saturate leading to some decrease of the coupling losses together with the appearance of hysteretic losses in these layers. Both losses are calculated for this case. Measured losses are within 20% of the theoretical predictions, in both two and three component composites. There is a noticeable decrease of losses in rectangular composites when the field is parallel to the broad face of the conductor and the presence of a transport current does not lead to a significant increase of overall losses.

A new “shelf structure” for lithium drifted silicon detectors

A lithium drifted semiconductor detector develops an n-type surface layer where the intrinsic region is exposed to normal ambient atmosphere 1,2). The presence of this layer results in an undesirable increase in leakage current and noise, above a certain detector voltage. This difficulty must be overcome in order to achieve a short collection time while still maintaining a low noise level. Lothrop and Goulding 3) propose to solve the difficulty by a double drift geometry in which this n-type channel is pinched off by providing a high transverse field region. The geometry proposed in this paper also ensures a high transverse field region, but it is achieved by different means. A source limited diffusion on certain parts of the wafer is accomplished by controlling the local deposition rate of lithium on a silicon wafer during the lithiu, diffusion. A shelf-like structure automatically results after drifting. This shelf has a field gradient which ensures the pinch-off of the n-type channel beginning at low voltages.