Yu. V. Bomze

Phonon Bottleneck in Graphene-Based Josephson Junctions at Millikelvin Temperatures

We examine the nature of the transitions between the normal and superconducting branches in superconductor-graphene-superconductor Josephson junctions. We attribute the hysteresis between the switching (superconducting to normal) and retrapping (normal to superconducting) transitions to electron overheating. In particular, we demonstrate that the retrapping current corresponds to the critical current at an elevated temperature, where the heating is caused by the retrapping current itself. The superconducting gap in the leads suppresses the hot electron outflow, allowing us to further study electron thermalization by phonons at low temperatures (T≲1 K). The relationship between the applied power and the electron temperature was found to be P∝T3, which we argue is consistent with cooling due to electron-phonon interactions.

Resonant tunneling in a dissipative environment

We measure tunneling through a single quantum level in a carbon nanotube quantum dot connected to resistive metal leads. For the electrons tunneling to/from the nanotube, the leads serve as a dissipative environment, which suppresses the tunneling rate. In the regime of sequential tunneling, the height of the single-electron conductance peaks increases as the temperature is lowered, although it scales more weekly than the conventional 1/T. In the resonant tunneling regime (temperature smaller than the level width), the peak width approaches saturation, while the peak height starts to decrease. Overall, the peak height shows a non-monotonic temperature dependence. We associate this unusual behavior with the transition from the sequential to the resonant tunneling through a single quantum level in a dissipative environment.

Structural investigations of superconducting multilayers consisting of semiconducting materials

There are rather exotic semiconductor superlattices (SL) consisting of monochalcogenides of Pb, Sn, and rare-earth metals which exhibit superconductivity at temperature as high as 6 K. Here we report the results of precision x-ray diffractometry and TEM investigations as well as Auger spectroscopy data obtained on some of the epitaxially grown superconducting semiconductor SLs. It is established that the essential features of the SL structure determining the appearance of superconductivity are the perfect single-crystallinity of the samples and the presence of continuous dense grids of misfit dislocations on the interfaces between two semiconductors. The segregation of free Pb which was observed in some cases does not correlate, according to experimental data, with the appearance of superconductivity.

Mo/Si superlattices: phase transitions between the commensurate states in the vortex ensemble

The avalanche-type jumps of the resistivity in the mixed state have been observed on the artificial superconducting multilayers Mo/Si with weak Josephson interlayer coupling when an applied magnetic field was oriented along the layer planes. These jumps are periodic with an inverse magnetic field. The simple model is proposed which explains the jump patterns on different samples as the manifestation of the phase transitions between various commensurate states of the vortex lattice. The values of the jump fields and the distances between these fields are quantitatively consistent with the model considered. It is assumed that the set of the jumps corresponds to the sequence of the upper critical fields for different commensurate lattices in the strongly layered superconductor. These phase transitions belong to the class of the first order transitions in distinction to the corresponding transition in homogeneous type-II superconductors. The resistive curves are hysteretic and reveal considerable supercooling.

Mo/Si and W/Si multilayers: commensurated vortex lattice structures and superconductivity reentrance

Abstract We report on the unusual behavior of Mo/Si and W/Si multilayers (MLs) in parallel and slightly inclined to the ML planes magnetic fields. Field dependencies of resistivity are nonmonotonic and display minima or zero resistance regions (ZRR) in magnetic fields different from zero. We show that such a behavior is connected with strong intrinsic pinning and vortex lattice (VL) commensurability with underlying layered structure. The locations of ZRR correspond to stable VL configurations.

Commensurability effect and lock-in transition in Mo/Si superconducting superlattices

We report the first observation of the lock-in transition in artificial superconducting superlattices, which takes place in tilted magnetic fields. The measurements were carried out on the Mo/Si layered system. The temperature dependence of the critical angle for the trapping of the vortices in the orientation parallel to the layer planes is determined by the previously known resistive method and by a new method based on the effect of commensurability between the intervortex distance and the superlattice wavelength. The temperature dependences of the critical angle obtained by the two methods practically coincide. The experimental results are consistent with the theoretical predictions of Feinberg and Villard.

Observation of Unitary Conductance for Resonant Tunneling with Dissipation

We investigate tunneling through a resonant level formed in a carbon nanotube quantum dot contacted by resistive metal wires. These contacts create a dissipative environment for the electrons tunneling across the nanotube, thus suppressing the tunneling rate. We study the shape of the resonant peak in the nanotube conductance, with the expectation that the peak width and height, both dependent on the tunneling rate, will be suppressed. Instead, we find that the behavior crucially depends on the ratio of the tunneling rates from the resonant level to the two contacts. We discuss the implication of our findings for a boundary quantum phase transition in this system.

Semiconducting chalcogenide multilayers: structure and superconductivity

Abstract Superconducting properties study, X-ray diffractometry and TEM investigations of the structure have been carried out on rather exotic superconducting multilayers consisting only of semiconducting materials (chalcogenides of Pb, Sn and rare-earth elements). Superconductivity is discovered in a new combination of semiconductors PbSe/EuS. It is shown that essential features of the multilayers structure determining the appearance of superconductivity are the perfect single crystallinity of entire sample and regular grids of misfit dislocations on the interfaces between two semiconductors. The segregation of free Pb observed in some cases does not correlate with appearance of superconductivity. The possibility of dislocation-induced superconductivity is discussed.

Superconductivity reentrance in strong magnetic fields in Mo/Si multilayers

Abstract Novel reentrance phenomenon is discovered on Mo/Si multilayers which occurs in parallel to the layers and slightly inclined magnetic fields. This effect may be explained in terms of the intrinsic pinning and vortex lattice (VL) commensurability with underlying layered structure. The locations of the zero resistance regions correspond to the stable VL configurations or to the transitions between two commensurate lattices. It is suggested that resistive method may be used as a new tool of the VL structure study in layered superconductors.