Mid-IR Cathodoluminescence of Fe:ZnSe

Optics and Spectroscopy - The dependences of the integral intensity and kinetics of mid-IR cathodoluminescence (CL) of Fe:ZnSe crystals on an iron concentration varying from 0.01 to 14 wt % are...     

Oscillatory persistent currents in self-assembled quantum rings

We report the direct measurement of the persistent current carried by a single electron by means of magnetization experiments on self-assembled InAs/GaAs quantum rings. We measured the first Aharonov-Bohm oscillation at a field of 14 T, in perfect agreement with our model based on the structural properties determined by cross-sectional scanning tunneling microscopy measurements. The observed oscillation magnitude of the magnetic moment per electron is remarkably large for the topology of our nanostructures, which are singly connected and exhibit a pronounced shape asymmetry.     

Vortex core deformation and stepper-motor ratchet behavior in a superconducting aluminum film containing an array of holes

We investigated experimentally the frequency dependence of a superconducting vortex ratchet effect by means of electrical transport measurements and modeled it theoretically using the time-dependent Ginzburg-Landau formalism. We demonstrate that the high frequency vortex behavior can be described as a discrete motion of a particle in a periodic potential, i.e., the so-called stepper-motor behavior. Strikingly, in the more conventional low frequency response a transition takes place from an Abrikosov vortex rectifier to a phase slip line rectifier. This transition is characterized by a strong increase in the rectified voltage and the appearance of a pronounced hysteretic behavior.     

The Influence of Annealing in Zinc Vapor on the Visible and Mid-IR Luminescence of ZnSe:Fe2+

Optics and Spectroscopy - The influence of annealing in zinc vapor on the impurity-defect composition and the IR cathodoluminescence (CL) of ZnSe:Te plates diffusion-doped with iron is studied....     

Oscillatory persistent currents in quantum rings: Semiconductors versus superconductors

Persistent currents are a hallmark of superconductivity in metals. To observe those dissipationless currents in a non-superconducting ring, the circumference of the ring must be short enough so that the phase coherence of the electronic wave functions is preserved around the loop. Recent progress in the fabrication of self-assembled semiconductor quantum rings (SAQRs), which can be filled with only a few (1–2) electrons, has offered the unique possibility to study the magnetic-field-induced oscillations in the persistent current carried by a single electron. In this paper, we discuss similarities and distinctions between the behavior of persistent currents in semiconductor and superconductor samples and give an overview of the recent results for oscillatory persistent currents in SAQRs. Although the real SAQR shape differs strongly from an idealized circular-symmetric open ring structure, the Aharonov–Bohm oscillations of the magnetization survive, as observed in low temperature magnetization measurements on In_xGa_1−xAs/GaAs SAQRs.     

Impurity spin magnetization of thin Fe doped Au films

In order to probe the influence of the surface-induced anisotropy on the impurity spin magnetization, we measure the anomalous Hall effect in thin AuFe films at magnetic fields up to 15 T. The observed suppression of the anomalous Hall resistivity at low fields as well as the appearance of a minimum in the differential Hall resistivity at higher fields can be explained by our theoretical model, which takes into account the influence of a polycrystalline film structure on the surface-induced anisotropy. Our results imply that the apparent discrepancy between different experimental results for the size effects in dilute magnetic alloys can be linked to a different microstructure of the samples.     

Negative-μ regime in the ac magnetic response of superconductor nanoshells

The time-dependent Ginzburg–Landau formalism is applied to analyze the vortex states and vortex dynamics in superconducting spherical nanoshells, subjected to mutually perpendicular strong dc and weak ac magnetic fields. We demonstrate that nonuniformity of the shell thickness can dramatically affect the ac magnetic response of a 3D array of superconducting nanoshells. Remarkably, this response is strongly influenced not only by the relevant geometric and material parameters and the ac-field frequency but also by the magnitude of the applied dc field: by changing this field the real part of the effective ac magnetic permeability can be tuned from positive values significantly larger than one down to negative values.     

Electroluminescence spectra of an STM-tip-induced quantum dot

We analyze the electroluminescence spectrum of an STM-tip-induced quantum dot in a GaAs surface layer. A flexible model has been developed, that combines analytical and numerical methods and describes the key features of many-particle states in the STM-tip-induced quantum dot. The dot is characterized by its depth and lateral width, which are experimentally controlled by the bias and the tunneling current. We find, in agreement with experiment, that increasing voltage on the STM-tip results in a red shift of the electroluminescence peaks, while the peak positions as a function of the electron tunneling current through the STM-tip reveal a blue shift.     

Modeling of the dolphin’s clicking sound source: The influence of the critical parameters

A physical and a mathematical models of the dolphin’s source of echolocation clicks have been recently proposed. The physical model includes a bottle of pressurized air connected to the atmosphere with an underwater rubber tube. A compressing rubber ring is placed on the underwater portion of the tube. The ring blocks the air jet passing through the tube from the bottle. This ring can be brought into self-oscillation by the air jet. In the simplest case, the ring displacement follows a repeated triangular waveform. Because the acoustic pressure gradient is proportional to the second time derivative of the displacement, clicks arise at the bends of the displacement waveform. The mathematical model describes the dipole oscillations of a sphere “frozen” in the ring and calculates the waveform and the sound pressure of the generated clicks. The critical parameters of the mathematical model are the radius of the sphere and the peak value and duration of the triangular displacement curve. This model allows one to solve both the forward (deriving the properties of acoustic clicks from the known source parameters) and the inverse (calculating the source parameters from the acoustic data) problems. Data from click records of Odontocetes were used to derive both the displacement waveforms and the size of the “frozen sphere” or a structure functionally similar to it. The mathematical model predicts a maximum source level of up to 235 dB re 1 μPa at 1-m range when using a 5-cm radius of the “frozen” sphere and a 4-mm maximal displacement. The predicted sound pressure level is similar to that of the clicks produced by Odontocetest.