S. V. Dubonos

Ballistic hall micromagnetometry

We report a magnetization measurement technique which allows quantitative studies of thermodynamic properties of individual submicron superconducting and ferromagnetic particles.

Paramagnetic Meissner effect in small superconductors

A superconductor placed in a magnetic field and cooled down through the transition temperature expels magnetic flux. This phenomenon, known as the Meissner effect, is arguably the most essential property of superconductors and implies zero resistivity. Surprisingly, several recent experiments have shown that some superconducting samples may attract magnetic field—the so-called paramagnetic Meissner effect. The scarce, if not controversial, experimental evidence for this effect makes it difficult to identify the origin of this enigmatic phenomenon, although a large number of possible explanations have been advanced. Here we report observations of the paramagnetic Meissner effect with a resolution better than one quantum of magnetic flux. The paramagnetic Meissner effect is found to be an oscillating function of the magnetic field (due to flux quantization) and replaces the normal Meissner effect only above a certain field when several flux quanta are frozen inside a superconductor. The paramagnetic state is found to be metastable and the Meissner state can be restored by external noise. We conclude that the paramagnetic Meissner effect is related to the surface superconductivity and, therefore, represents a general property of superconductors: on decreasing temperature, the flux captured at the third (surface) critical field inside the superconducting sheath compresses into a smaller volume, allowing extra flux to penetrate at the surface.

Submicron sensors of local electric field with single-electron resolution at room temperature

We describe probes of a local electric field, which are capable of detecting an electric charge as small as the charge of one electron e, operational under ambient conditions and having a spatial resolution down to 100nm. The submicron-sized probes were made from a high-density high-mobility two-dimensional electron gas, which is sensitive to the presence of electric charges near its surface. We demonstrate the possibility of using such microprobes for life-science applications by measuring an electric response of individual yeast cells to abrupt changes in their environment.

Direct observation of vortex shells and magic numbers in mesoscopic superconducting disks.

We have studied vortex configurations in mesoscopic superconducting disks using the Bitter decoration technique. For a broad range of vorticities L the circular geometry is found to lead to the formation of concentric shells of vortices. From images obtained on disks of different sizes in a range of magnetic fields we traced the evolution of vortex states and identified stable and metastable configurations of interacting vortices. Furthermore, the analysis of shell filling with increasing L allowed us to identify magic numbers corresponding to the appearance of consecutive new shells.

Observation of the external-ac-current-induced dc voltage proportional to the steady current in superconducting loops

A dc voltage induced by an external ac current is observed in system of asymmetric mesoscopic superconducting loops. The value and sign of this dc voltage, like the one of the persistent current, depend in a periodical way on a magnetic field with period corresponded to the flux quantum within the loop. The amplitude of the oscillations does not depend on the frequency of the external ac current (in the investigated region 100 Hz - 1 MHz) and depends on its amplitude. The latter dependence is not monotonous. The observed phenomenon of rectification is interpreted as a consequence of a dynamic resistive state induced by superposition of the external current and the persistent current. It is shown that the dc voltage can be added in system of loops connected in series: the dc voltage oscillations with amplitude up to 0.00001 V were observed in single loop, up to 0.00004 V in a system of 3 loops and up to 0.0003 V in a system of 20 loops.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 300 μ V, respectively.

Bose Condensation of Interwell Excitons in Double Quantum Wells

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 (Tc ≤ 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.

Submicron probes for Hall magnetometry over the extended temperature range from helium to room temperature

We report on mesoscopic Hall sensors made from various materials and their suitability for accurate magnetization studies of submicron samples over a wide temperature range and, especially, at room temperature. Among the studied devices, the best stability and sensitivity have been found for Hall probes made from a high-concentration two-dimensional electron gas (HC-2DEG). Even at 300 K, such submicron probes can reliably resolve local changes in dc magnetic field of ≈1 G, which corresponds to a flux sensitivity of less than 0.1 φ0 (φ0=h/e is the flux quantum). The resolution increases 100 times at temperatures below 80 K. It is also much higher for the detection of ac magnetic fields because resistance fluctuations limiting the low-frequency stability of the studied devices can be eliminated. Our second choice for room-temperature Hall micromagnetometry is gold Hall probes, which can show a sensitivity of the order of 10 G. The capabilities of HC-2DEG and gold micromagnetometers are demonstrated by measu...

Proximity effect and spontaneous vortex phase in planar SF structures

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.

Vortex configurations in mesoscopic superconducting triangles: Finite-size and shape effects

Triangular-shaped mesoscopic superconductors are consistent with the symmetry of the Abrikosov vortex lattice resulting in a high stability of vortex patterns for commensurate vorticities. However, for non-commensurate vorticities, vortex configurations in triangles are not compatible with the sample shape. Here we present the first direct observation of vortex configurations in μm-sized niobium triangles using the Bitter decoration technique, and we analyze the vortex states in triangles by analytically solving the London equations and performing molecular-dynamics simulations. We found that filling rules with increasing vorticity can be formulated for triangles in a similar way as for mesoscopic disks where vortices form shells.

Little-Parks Effect in a System of Asymmetric Superconducting Rings

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.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 the 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 clockwise or contra-clockwise direction. This means that the total current in one of the semi-rings may be directed against the electric field at measurement of the Little-Parks oscillations. The measurements at zero and low measuring current have revealed that the persistent current, like the conventional circulating current, causes a potential difference on the semi-rings with different cross sections in spite of the absence of the Faraday’s voltage.