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The obtained periodic magnetic-field dependences I c+(Φ/Φ0) and I c−(Φ/Φ0) of the critical current measured in opposite directions on asymmetric superconducting aluminum rings has made it possible to explain previously observed quantum oscillations of dc voltage as a result of alternating current rectification. It was found that a higher rectification efficiency of both single rings and ring systems is caused by hysteresis of the current-voltage characteristics. The asymmetry of current-voltage characteristics providing the rectification effect is due to the relative shifts of the magnetic dependences I c−(Φ/Φ0) = I c+(Φ/Φ0 + Δϕ) of the critical current measured in opposite directions. This shift means that the position of I c+(Φ/Φ0) and I c−(Φ/Φ0) minima does not correspond to n + 0.5 magnetic flux Φ quanta, which is in direct contradiction to measured Little-Parks resistance oscillations. Despite this contradiction, the amplitude I c, an(Φ/Φ0) = I c+(Φ/Φ0) − I c−(Φ/Φ0) of critical current anisotropy oscillations and its variations with temperature correspond to the expected amplitude of persistent current oscillations and its variations with temperature. [ABSTRACT FROM AUTHOR]

Little-Parks oscillations are observed in a system of 110 series-connected aluminum rings 2 μm in diameter with the use of measuring currents from 10 nA to 1 μA. The measurements show that the amplitude and character of oscillations are independent of the relation between the measuring current and the amplitude of the persistent current. By using asymmetric rings, it is demonstrated that the persistent current has a direction. This means that, in the Little-Parks experiment, the total current in one of the half-rings may be directed against the electric field. [ABSTRACT FROM AUTHOR]

Magnetic field dependences of critical current, resistance, and rectified voltage of asymmetric (half circles of different widths) and symmetrical (half circles of equal widths) aluminum rings close to the super-conducting transition were measured. All these dependences are periodic magnetic field functions with periods corresponding to the flux quantum in the ring. The periodic dependences of critical current measured in opposite directions were found to be close to each other for symmetrical rings and shifted with respect to each other by half the flux quantum in asymmetric rings with ratios between half circle widths of from 1.25 to 2. This shift of the dependences by a quarter of the flux quantum as the ring becomes asymmetric makes critical current anisotropic, which explains the effect of alternating current rectification observed for asymmetric rings. Shifts of the extrema of the periodic dependences of critical current by a quarter of the flux quantum directly contradict the results obtained by measuring asymmetric ring resistance oscillations, whose extrema are, as for symmetrical rings, observed at magnetic fluxes equal to an integer and a half of flux quanta. [ABSTRACT FROM AUTHOR]

The paper presents the results obtained in a study of electron transport in split-gate structures prepared from heterostructures with self-organizing InAs quantum dots situated close to a two-dimensional electron gas. Coulomb oscillations of current through InAs quantum dots depending on the voltage on the gate were observed. Coulomb current oscillations persisted up to about 20 K. The Coulomb energy Δ E C = 12.5 meV corresponding to theoretical estimates for the p-states of quantum dots in our structures was determined. [ABSTRACT FROM AUTHOR]

Quantum electrodynamics (resulting from the merger of quantum mechanics and relativity theory) has provided a clear understanding of phenomena ranging from particle physics to cosmology and from astrophysics to quantum chemistry. The ideas underlying quantum electrodynamics also influence the theory of condensed matter, but quantum relativistic effects are usually minute in the known experimental systems that can be described accurately by the non-relativistic Schrödinger equation. Here we report an experimental study of a condensed-matter system (graphene, a single atomic layer of carbon) in which electron transport is essentially governed by Dirac's (relativistic) equation. The charge carriers in graphene mimic relativistic particles with zero rest mass and have an effective ‘speed of light’ c* ≈ 106 m s-1. Our study reveals a variety of unusual phenomena that are characteristic of two-dimensional Dirac fermions. In particular we have observed the following: first, graphene's conductivity never falls below a minimum value corresponding to the quantum unit of conductance, even when concentrations of charge carriers tend to zero; second, the integer quantum Hall effect in graphene is anomalous in that it occurs at half-integer filling factors; and third, the cyclotron mass mc of massless carriers in graphene is described by E = mcc*2. This two-dimensional system is not only interesting in itself but also allows access to the subtle and rich physics of quantum electrodynamics in a bench-top experiment. [ABSTRACT FROM AUTHOR]

The electron transport is studied in split-gate structures fabricated on the basis of a modulation-doped heterostructure that contains a single quantum well and self-assembled InAs quantum dots near the 2D electron gas regions. The current passing through the channel with a denumerable set of InAs quantum dots is found to exhibit Coulomb oscillations as a function of the gate voltage. The oscillations are associated with the excited p states of InAs quantum dots, which are characterized by opposite spins and caused by lifting of the spin degeneracy of the p state due to the Coulomb interaction. The Coulomb oscillations of the current are observed up to a temperature of ∼20 K. The Coulomb energy is found to be ΔEc = 12.5 meV, which agrees well with the theoretical estimates for the p states of quantum dots in the structures under study. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

Movements of individual domain walls in a ferromagnetic garnet were studied with angstrom resolution. The measurements reveal that domain walls can be locked between adjacent crystallographic planes and propagate by distinct steps matching the lattice periodicity. [ABSTRACT FROM AUTHOR]

Luminescence properties of thin-film cylindrical ZnO microcavities 1.8 μm in diameter obtained by electron-beam lithography and reactive ion etching were studied. Narrow luminescence peaks were detected in the excitonic and green spectral regions of zinc oxide and were attributed to spatial quantization of photon wave functions. It was found that stimulated ultraviolet luminescence arises with coupled photon modes of microcavities as optical pump power increases. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

A dc voltage induced by an external ac current was observed in a system of asymmetric aluminum loops at temperatures corresponding to 0.95-0.98 of the superconducting transition temperature. The voltage magnitude and sign change periodically in a magnetic field with a period corresponding to the magnetic flux quantum through the loop. The amplitude of these oscillations depends nonmonotonically on the amplitude of ac current and is almost independent of its frequency in the range from 100 Hz to 1 MHz. The observed phenomenon is interpreted as the result of displacing the loop into a dynamic resistive state by the external current, where the loop is "switched" back and forth between the closed superconducting state with a nonzero steady current and the nonclosed state with a nonzero resistance along the loop circle. It is shown that voltages are summed up in a system of loops connected in series. For systems with one, three, and twenty loops, the voltage reaches 10, 40, and 300μV, respectively. [ABSTRACT FROM AUTHOR]

The proximity effect in SF structures is examined. It is shown that, due to the oscillations of the induced superconducting order parameter in a ferromagnet, the critical temperature of an SF bilayer becomes minimal when the thickness of the ferromagnetic layer is close to a quarter of the period of spatial oscillations. It is found that the spontaneous vortex state arising in the superconductor due to the proximity of the magnetic domain structure of a ferromagnet brings about noticeable magnetoresistive effects. [ABSTRACT FROM AUTHOR]

The luminescence of interwell excitons in double quantum wells GaAs/AlGaAs (n–i–n heterostructures) with large-scale fluctuations of random potential in the heteroboundary planes was studied. The properties of excitons whose photoexcited electron and hole are spatially separated in the neighboring quantum wells were studied as functions of density and temperature within the domains on the scale less than one micron. For this purpose, the surfaces of the samples were coated with a metallic mask containing specially prepared holes (windows) of a micron size an less for the photoexcitation and observation of luminescence. For weak pumping (less than 50 μW), the interwell excitons are strongly localized because of small-scale fluctuations of a random potential, and the corresponding photoluminescence line is inhomogeneously broadened (up to 2.5 meV). As the resonant excitation power increases, the line due to the delocalized excitons arises in a thresholdlike manner, after which its intensity linearly increases with increasing pump power, narrows (the smallest width is 350 μeV), and undergoes a shift (of about 0.5 μeV) to lower energies, in accordance with the filling of the lowest state in the domain. With a rise in temperature, this line disappears from the spectrum (T[sub c]≤3.4 K). The observed phenomenon is attributed to Bose–Einstein condensation in a quasi-two-dimensional system of interwell excitons. In the temperature range studied (1.5–3.4 K), the critical exciton density and temperature increase almost linearly with temperature. © 2002 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

The deposition of bismuth films under a residual gas pressure 10[sup -6] Pa results in the "metal-type" variation of conductivity at low temperatures. Conductivity of the cross-shaped microstructures based on these films exhibits the properties analogues to those of the ballistic electron transport in 2D systems. [ABSTRACT FROM AUTHOR]

In simply-connected, micron-size, aluminum thin-film structures in a perpendicular magnetic field, resistance oscillations with a period close to the quantum of magnetic flux through the area of the structure are observed. The results obtained attest to the fact that when Abrikosov vortices penetrate into a small-area superconducting film (mesoscopic superconducting film), the film behaves similarly to a Little–Parks thin-film interferometer. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Silicon membranes with through pores have been manufactured based on silicon plates with a gradient-porous structure, in which the pore cross section and structure varies with depth. In order to increase the specific surface of membranes, a layer of carbon nanofibers and nanotubes has been formed on the surface of macropores in silicon. The obtained structure can serve as a basis for the development of electrodes for solid polymer fuel cells with a specific area of conducting surface (with respect to carbon) of no less than 100 m2/g. [ABSTRACT FROM AUTHOR]