Low Internal Field Research Articles

Vortex penetration and flux relaxation with arbitrary initial conditions in non-ideal and ideal superconductors

Vortex penetration and flux relaxation phenomena carry information about the pinning ability, and consequently current-carrying ability, of a type-II superconductor. However, the theoretical descriptions of these phenomena are currently limited to the cases with special initial conditions. A generalization to the recently developed infinite series models is presented here. It is shown that one can convert a vortex penetration process with a nonzero initial internal field into a process with a zero initial internal field by introducing some time parameters. Similarly, one can also convert a flux relaxation process starting with an arbitrary internal field into a process starting with a melting internal field by introducing a virtual time interval. Therefore, one can predict the melting internal field (or critical current density) from a flux relaxation process starting with a lower internal field. Finally, I show that the vortex penetration process in an ideal superconductor is strongly time dependent because of the surface barrier and internal field repulsive force. But the flux relaxation process does not occur in the ideal superconductor.

Measurement of the internal magnetic field in the correlated iridates Ca4IrO6, Ca5Ir3O12, Sr3Ir2O7and Sr2IrO4

Oxides containing iridium ions display a range of magnetic and conducting properties that depend on the delicate balance between interactions and are controlled, at least in part, by the details of the crystal architecture. We have used muon spin rotation ($\ensuremath{\mu}$SR) to study the local field in four iridium oxides, Ca${}_{4}$IrO${}_{6}$, Ca${}_{5}$Ir${}_{3}$O${}_{12}$, Sr${}_{3}$Ir${}_{2}$O${}_{7}$, and Sr${}_{2}$IrO${}_{4}$, which show contrasting behavior. Our $\ensuremath{\mu}$SR data on Ca${}_{4}$IrO${}_{6}$ and Ca${}_{5}$Ir${}_{3}$O${}_{12}$ are consistent with conventional antiferromagnetism where quasistatic magnetic order develops below ${T}_{\mathrm{N}}=13.85(6)$ and 7.84(7) K, respectively. A lower internal field is observed for Ca${}_{5}$Ir${}_{3}$O${}_{12}$, as compared to Ca${}_{4}$IrO${}_{6}$, reflecting the presence of both Ir${}^{4+}$ and Ir${}^{5+}$ ions and resulting in a more magnetically dilute structure. Muon precession is only observed over a restricted range of temperature in Sr${}_{3}$Ir${}_{2}$O${}_{7}$, while the Mott insulator Sr${}_{2}$IrO${}_{4}$ displays more complex behavior, with the $\ensuremath{\mu}$SR signal containing a single, well-resolved precession signal below ${T}_{\mathrm{N}}=230$ K, which splits into two precession signals at low temperature following a reorientation of the spins in the ordered state.

Crystal orientation effects on electronic and optical properties of wurtzite ZnO/MgZnO quantum well lasers

Crystal orientation effects on electronic and optical properties of ZnO/MgZnO QW structures are investigated by taking into account the non-Markovian gain model with many-body effects. These results are compared with those for GaN-based QW structures. In a range of small crystal angles, ZnO/MgZnO QW structures have a lower internal field than GaN/AlGaN and InGaN/GaN QW structures. However, ZnO/MgZnO QW structures show a larger internal field than GaN-based QW structures at crystal angles near $${\theta =50^{\circ}}$$ . The WZ ZnO/MgZnO QW structures are shown to have much larger optical gain than the GaN-based QW structures for small crystal angles. This is because WZ ZnO/MgZnO QW structures have larger matrix element and smaller effective masses than InGaN/GaN QW structures near the (0001) crystal orientation. On the other hand, in the case of the (10 $${\bar{1}}$$ 0) crystal orientation, the optical gain of ZnO/MgZnO QW structures becomes smaller than that of InGaN/GaN QW structures due to the increase of the effective mass. In addition, the ZnO/MgZnO QW structures have a maximum in the optical gain near $${\theta =50^{\circ}}$$ , which can be explained by the fact that the average hole effective mass increases although the matrix element at high carrier density is improved with increasing crystal angle.

Observation of antiphase boundaries inSr2FeMoO6

A high-purity ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$ sample was synthesized. According to Rietveld analysis of x-ray diffraction (XRD) data the occupancy of Fe at its regular B site was 95.6% with the rest occupying the Mo site, i.e., the antisite of Fe. Despite this, according to ${}^{57}\mathrm{Fe}$ M\"ossbauer spectra 22% of the Fe atoms resided in a trivalent state having an extremely low internal field of $\ensuremath{\sim}2.8\mathrm{T}$ even at 5 K, whereas a typical 46-T field was seen for the main component, which is known to arise from mixed-valence ${\mathrm{Fe}}^{\mathrm{I}\mathrm{I}/\mathrm{I}\mathrm{I}\mathrm{I}}.$ Since the low-field ${\mathrm{Fe}}^{\mathrm{III}}$ species were totally invisible to probing by XRD measurements, it is suggested that they are atoms located on antiphase boundaries (APB's). Magnetization measurements revealed a slightly lowered saturation magnetization ${(M}_{s})$ for the sample exhibiting the highest concentration of APB's, as compared to samples with low concentration of APB atoms. The two-Fe-atom-thick APB layer consists of Fe atoms antiferromagnetically coupled to each other. The spins of the APB layer atoms turned perpendicularly to an applied magnetic field, as observed by M\"ossbauer spectroscopy.

Effect of defect-induced internal field on the aging of relaxors

The effect of a defect-induced internal field on the dielectric response of relaxor ferroelectrics is investigated using a Monte Carlo simulation. It was observed that only at a small temperature range near the temperature of the dielectric maximum does the susceptibility decrease markedly due to the internal field. This temperature range increases with enhancing internal field. We found that the susceptibility is almost independent of the internal field width at low internal field width, and then decreases linearly with enhancing internal field width. This dependence of the susceptibility on the internal field width is very similar to the relation of the dielectric constant with logarithmic aging time, which probably suggests a linear dependence of the internal field on the logarithm of the aging time. The frequency dependence of the susceptibility aging is sensitive to the temperature. With increasing temperature, the curve of the susceptibility change against logarithmic frequency varies from concave to approximately linear, and then to convex, which is in agreement with the recent aging rate measurement.

Valence-state mixing and separation inSmBaFe2O5+w

A mixed-valence state, formally denoted as ${\mathrm{Fe}}^{2.5+},$ is observed in the 300 K M\"ossbauer spectra of the most reduced samples of ${\mathrm{SmBaFe}}_{2}{\mathrm{O}}_{5+w}.$ Upon cooling below the Verwey-type transition temperature ${(T}_{\mathrm{V}}\ensuremath{\approx}200\mathrm{K}),$ the component assigned to ${\mathrm{Fe}}^{2.5+}$ separates into a high-spin ${\mathrm{Fe}}^{3+}$ state and an ${\mathrm{Fe}}^{2+}$ state with an unusually low internal field. The separation of the mixed-valence state at ${T}_{\mathrm{V}}$ is also confirmed by magnetic susceptibility measurements and differential scanning calorimetry. A model is proposed which accounts for the variation of the amount of the mixed-valence state with the oxygen content parameter w.

Electric field screening in a multiple-quantum-well optically addressed spatial light modulator

The transmission and photocurrent of a multiple-quantum-well optically addressed spatial light modulator are measured to study the screening dynamics of the internal electric field under uniform illumination. The response is modeled with a numerical device simulator which solves the conventional drift-diffusion transport equations. Evidence is found for bipolar transport, and the photocurrent response at low internal fields is shown to decay due to the transit time of carriers across the quantum well region. For large internal fields the photocurrent response saturates and the corresponding screening rate is estimated using an analytical device model.

Scaling behavior of the homogeneous magnetization dynamics in the ferromagnetic state of EuS

Using a broadband vectorial microwave reflectometer, we measured the longitudinal dynamic susceptibility of a EuS sphere between 0.1 and 20 GHz in the temperature interval 4.2 K≤T≤Tc=16.5 K. For low internal fields, the shape corresponds to a heavily overdamped Lorentzian resonance, χ−1z(ω)= χ−1z(0)iω/Lz−(ω/Ωz)2 where Lz≤Ωz. The field and temperature dependence of the intrinsic kinetic coefficient and resonance is fully explained by that of the static susceptibility χz(0). Surprisingly, the resulting scaling function, Lz[χz(0)], agrees exactly with that observed earlier above Tc, where Lz[χz(0)] could quantitatively be explained by a crossover from a van Hove behavior for large χz(0), due to dipolar anisotropic fluctuations, to a critical speeding-up for χz(0)<1, where isotropic spin diffusion prevails. This finding suggests that the dipolar anisotropy dominates the magnetization dynamics also below Tc of EuS, which seems to be supported by the fact that the correlation frequency of the torques acting on the magnetization, ωc=Ω2z/Lz, agrees with the relaxation rate of the longitudinal magnetization fluctuations, δMq∥q.

Angular dependence of low-field remanent magnetization in YBa 2Cu 3O 7−δ

The remanent magnetization in single crystals of YBa 2Cu 3O 7−δ has been measured at 77 K as a function of the applied magnetic field orientation relative to the sample. The remanent magnetization was generated by an externally applied field of 10 G. After correction for the demagnetization factor due to the shape of the crystal samples, it is found that the remanent magnetization, plotted as a function of the internal field, has two distinct slopes. Furthermore, for the same internal field, the remanent magnetization in the a-b-plane is shown to be larger than that along the c-axis. The experimental data suggest that at low internal fields the remanent magnetization prefers to orient in a direction parallel to the a-b-plane. We discuss these findings in terms of relevant vortex-flux penetration and flux pinning mechanisms.

A comparison of spherical wave boundary value matching versus integral equation scattering solution for a perfectly conducting body

A spherical-wave expansion (SPEX) technique for calculating the scattering from a smooth perfectly conducting body is presented. Sample case results are compared with the well-known Lawrence Livermore National Laboratory Numerical Electromagnetics code (NEC), which is based on the integral equation formulation. The internal fields are computed for both results using a third surface current integration program, which is totally independent of both SPEX and NEC. The internal fields, which would be zero for a perfect solution, are much more sensitive to the currents than the scattered fields. The SPEX solution, which uses fewer unknowns and less computer time than NEC, also produces a lower internal field. The SPEX technique also allows a direct check on satisfaction of the boundary condition at any set of points on the surface, independent of the points used to obtain the solution. This provides a valuable built-in test feature for quickly validating results, which is one of the most attractive features of the technique.

Thermoelectric power of magnetic and nonmagnetic amorphous metals

The thermoelectric power of a number of magnetic and nonmagnetic amorphous metallic alloys has been measured as a function of temperature between 15 and 580 K. The temperature dependence of the thermopower for the magnetic alloys is qualitatively different from that for the nonmagnetic alloys. The data for the nonmagnetic alloys are consistent with the extended Ziman theory for transport in liquid and amorphous metals. It is argued that the data for the magnetic alloys are consistent with a model, based on the Kondo scattering of conduction electrons by spins situated in low internal fields, which was recently proposed to explain the resistivity minima observed in these materials.

THE EFFECT OF HYDROGEN ON THE MAGNETIC STRUCTURE OF INVAR ALLOYS

The effect of absorbed hydrogen on the magnetic structure of f.c.c. invar alloys, Fee5Ni35, Fe6GNi31Mn3 and Fe69Ni2eC3, was studied by means of Fe Mossbauer effect. As the electrolytic hydrogenation preceeded, the characteristic absorption spectrum with low internal field components, which is believed to be an origin of the invar effect of these alloys, was drastically changed and hydride phases appeared in large fractions. The spectral analysis showed that the hydrides with the same f.c. c. structure as the matrix phase lost the weak field components and only strong ferromagnetic components remained. This effect of hydrogen on the magnetic structure of invar alloys was interpreted in terms of 3d hole filling with electrons from solute hydrogen in the Stoner's band model. A reverse effect found by a previous study on a non-invar Fe Ni alloy that the internal field of the hydride became about 15% smaller than that of the matrix phase was explained by the same model.

Magnetic Study of Fe3Si and Fe5Si3 by Mössbauer Effect

Mossbauer measurements were made on ferromagnetic Fe 3 Si and Fe 5 Si 3 to investigate the magnetic structure. In the ordered crystal of Fe 3 Si, two internal fields were found to be 320 kOe and 205 kOe at liquid nitrogen temperature, corresponding to the two crystallographically different sites, Fe(I) and Fe(II). Since Fe(II) has a small magnetic moment, a low internal field and a large isomer shift, its electronic structure may be fairly modified by a formation of covalent bond with Si atom. On the other hand, the situation of Fe(I) seems to be little changed from that of Fe atom in pure iron. Fe 5 Si 3 also has two different sites and two internal fields of 230 kOe and 130 kOe at liquid nitrogen temperature. Both kinds of Fe seem to be considerably different from Fe in pure iron.