Logarithmic Decay Rate Research Articles

Evidence for a time-dependent crossover from surfacelike to bulklike flux relaxation in YBa2Cu3O7- delta.

dc flux-relaxation experiments were performed on single crystals of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ at low magnetic fields. A sharp, time-dependent crossover in the logarithmic magnetization decay rate was observed in two different crystals. Consideration of the size dependence of the relaxation rate in one of the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ crystals suggests that the crossover in the relaxation rate is due to a transition from surface-controlled to bulk-controlled relaxation behavior. Our observations seem to indicate that, over a certain range of temperatures and magnetic fields, surface pinning may play a significant role in the flux-relaxation behavior of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$.

Magnetic relaxation in high Tc oxide YBa 2Cu 3O y superconductor

Flux creep in the high T c oxide YBa 2Cu 3O y superconductor in critical state has been investigated by observing the variation of magnetization with time. The experimental results show that in zero field cooled (ZFC) samples the magnetization decay is entirely logarithmic in time, and there is a magnetization jump during the logarithmic decay. The logarithmic decay rate d M/dln t changes with applied field, and reaches a maximum at a field of ≈ 3 H p. The appearance of a maximum is perhaps related to the existence of weak link superconductivity in our samples. At temperatures > 77.3 K, the rate d M/dlnt decreases monotonously with temperature. The thermal activation energy U o is also discussed.

Vortex-persistent current interaction, Meissner effect, and diamagnetic shielding in a ring of YBa2Cu3O7.

The vortex-persistent current interaction, Meissner effect, and diamagnetic shielding in a ring of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ have been studied by the measurement of the magnetic-field distributions across the sample and their time dependence at 77 K over an applied magnetic-field range 0--1000 G. The measurements were performed both for zero-field-cooling (ZFC) and field-cooling (FC) cases. Drilling a hole in a disk results in a time-dependent decay of the diamagnetic shielding and a time-dependent increase of the Meissner field being observed. The dependence of a trapped magnetic field on an applied magnetic field shows a maximum at an applied field of 150--200 G for the ZFC case and of 60--80 G for the FC case. For the ZFC case the magnetic field is trapped at applied magnetic fields as low as 10 G. The trapped flux (intergranular vortices) and macroscopic persistent current circulating around the ring contribute to the trapped field. The logarithmic decay rate of the trapped field is a linear function of an initial trapped field for the disk of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. The decay rate of the trapped field for the ring of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ was measured both at its center and its edge. The results revealed a jump in the decay rate at trapped fields in the range 5--10 G for the ZFC case. These results are interpreted as being due to the Josephson vortex-persistent current interaction in the ring. The Josephson vortex motion in the ring is the combination of flux-creep- and flux-flow-like processes.

Flux creep and penetration in Fe-doped YBa2Cu3O7.

Measurements of the magnetic-field distribution, flux penetration, and decay of trapped flux were performed for a disc-shaped sample of YBa{sub 2}Cu{sub 2.95}Fe{sub 0.05}O{sub 7{minus}{delta}} at 77 K. Time-dependent magnetic-flux penetration, i.e., a decay of diamagnetic shielding and the Meissner field, was observed for zero-field-cooled (ZFC) and field-cooled (FC) samples. The decays were found to be logarithmic in time. The logarithmic decay rate of diamagnetic shielding depends on an applied magnetic field and reaches its maximum at an applied field of about 30 G. The dependence of a trapped field versus an applied field shows a maximum at an applied field of 100 G for the FC case and 200 G for the ZFC case. The maximum amount of the trapped field is about two times less than that for a pure, undoped YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} sample of similar dimensions. The logarithmic decay rate of the trapped field versus the initial trapped field is described by a linear function with the change of its slope at the trapped field corresponding to an applied field of about 30 G for both the FC and ZFC cases. These results are related to the sample microstructure, i.e., grain size and porosity. Trapped fieldmore » decays can be explained in terms of the interaction between intergranular vortices and persistent current circulating around normal regions or voids, in the framework of the conventional flux-creep model recently proposed by Hagen, Griessen, and Salomons (Physica 157C, 199 (1989)).« less

Erratum: "Trapped flux, diamagnetic shielding, and Meissner effect in a disk of YBa2Cu3O7"

The trapped flux, diamagnetic shielding, and Meissner effect in polycrystalline ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ disks have been investigated by the measurement of the distribution of magnetic field across the sample at 77 K. The measurements were performed for samples cooled in zero and nonzero external magnetic fields. The distribution of the magnetic field was studied both in the presence of an applied magnetic field and when this field was switched off. The results show the relationship between shielding effect, Meissner effect, and flux pinning effect. Partial creep of flux into the disk was observed to occur for zero-field-cooled samples in applied magnetic fields as low as 1 G. The time decay of the trapped flux was observed to be logarithmic in time. The logarithmic decay rate increases linearly with the magnitude of the trapped magnetic field.

Radial oscillations of vapor bubbles in solutions

The dynamics and heat and mass transfer of vapor bubbles in binary systems are investigated. An anomalous effect of the component composition on the bubble dynamics in boiling nonideal solutions is established. It is shown that in some binary systems the value of the logarithmic decay rate for small free radial oscillations does not lie within the limiting values calculated for the pure components, which is associated with the cardinal importance of the effect of diffusion in the liquid phase on the intensity of the phase transitions.

Magnetic relaxation effects in bulk CeCu 2Si 2 at H

A novel relaxation of the isothermal dc magnetization M has been observed in bulk polycrystalline and in powdered CeCu 2Si 2 similar to that in Sr La Cu O and Ba La Cu O oxides. If a field H i<H Cl is applied to a virgin specimen, the magnetization decays in time following accurately the law M ∝ ln(t/t 0 over observation times of 10 5 seconds. The same law is followed at H= 0 when the field H i is removed. In both cases, the logarithmic decay rates ∂M/∂lnt are proportional to H i 4 for H i < H Cl while for H i > H Cl the logarithmic rates are independent of H i.

Low-field magnetic relaxation effects in high-Tc superconductors Sr-La-Cu-O and Ba-La-Cu-O.

The relaxation of the isothermal dc magnetization $M$ in zero-field-cooled Ba-La-Cu-O and Sr-La-Cu-O has been investigated. After the external magnetic field has been changed or cycled, the resultant magnetization $M$ decays following the law $M\ensuremath{\propto}\mathrm{ln}(\frac{t}{{t}_{0}})$ with great accuracy over an observation time period of four decades. At $H=0$, the logarithmic decay rate $\frac{\ensuremath{\partial}M}{\ensuremath{\partial}\mathrm{ln}t}$ is proportional to the temperature $T$ and it is also proportional to ${{H}_{i}}^{3}$. Here ${H}_{i}$ is the external field which is reduced to zero before the decay rate of persistent currents is observed.

Influence of rotation on the damping of surface waves in a cylindrical layer of viscous fluid

A study is made of the oscillations of the free surface of a cylindrical layer of viscous incompressible fluid attracted by gravity to a solid, uniformly rotating cylinder. Logarithmic decay rates are found for the damping of the surface gravitational waves when large Reynolds numbers are assumed. It is shown that rotation introduces an asymmetry into the damping of the waves traveling in and against the direction of rotation of the fluid.

Measurement of microvascular blood flow in cancellous bone using laser Doppler flowmetry and 133Xe-clearance

Blood flow in cancellous bone with varying vascular density was investigated simultaneously with Laser Doppler Flowmetry (LDF) and 133Xe-clearance. The cancellous bone subapical to 2 contralateral incisors in the mandibles of 17 young pigs was used as an experimental model. Light from a 2 mW He-Ne-laser was guided through an optical fibre to a flowmeter probe. Stainless steel probe-holders firmly inserted in the pulpal canals of the two incisors served as the probe entrance to cancellous bone for blood flow recording. Due to the Doppler effect, the light scattered by circulating blood cells undergoes a frequency shift. The back-scattered light picked up by optical fibres in the probe, was guided to a photosensitive device, where it was demodulated. After signal processing, a signal referred to as the Blood Flow Value (BFV) was recorded on a pen recorder. Rhythmical variations (vasomotion) in BFV with frequencies from 2–11 cycles/min were observed in 6% of recordings made initially after probe-holder implantation, and in 34% of the recordings made 5 weeks later. On this occasion, a marked increase in BFV was recorded. Histological examination showed increased vascularity in the bone tissue. The reproducibility error of LDF was 7.4% and temporal changes in BFV, apart from vasomotion, were 8.3%, provided no injections or manipulations of the probe were made. Spatial variations in BFV were found to be related to the vascular density. 2 successive recordings by LDF from the same bone area were highly correlated ( r = 0.98). The corresponding figure for 2 logarithmic decay rates of locally injected 133Xe was 0.76. No correlation between BFV and 133Xe-clearance could be demonstrated.

Charge loss in metal-nitride-oxide-semiconductor (MNOS) devices at high temperatures

The charge loss from the nitride layer of MNOS transistors has been studied with emphasis on the temperature effects in the range from 25 to 175°C. The trend of major interest is the increase in the logarithmic decay rate of the dominant negative threshold voltage with increasing temperature. This result is believed due to a combination of three temperature related mechanisms: (1) thermal excitation (TE) discharge, (2) Poole-Frenkel emission-drift-capture events and (3) increasing nitride conductivity with increasing temperature.

Nonvolatile analog memory in MNOS capacitors

Long-term storage of analog signals of wide dynamic range has been successfully demonstrated for the first time in single MNOS capacitors. After a reset state is established by majority carrier tunneling, measured pulses of light are used to generate minority carriers which tunnel to nitride traps and in turn induce shifts in the flat-band voltage proportional to the minority carrier charge. Linear voltage windows of 12 volts are observed, and logarithmic decay rates are as low as 30 mV per decade of storage time per volt of initial flat-band shift. Analog signals can be stored linearly over a dynamic range of 40 dB for 30 hours.

The logarithmic decay of the Lewis-Rayleigh afterglow in nitrogen

Measurements on the logarithmic decay rate of the N2 bands in the afterglow of nitrogen with oxygen added as an impurity indicate that the catalytic efficiency gamma N for nitrogen atoms at the walls of a Pyrex glass vessel is very sensitive to traces of oxygen.