Normal metal - insulator - superconductor interferometer

Hybrid normal metal - insulator - superconductor microstructures suitable for studying an interference of electrons were fabricated. The structures consist of a superconducting loop connected to a normal metal electrode through a tunnel barrier . An optical interferometer with a beam splitter can be considered as a classical analogue for this system. All measurements were performed at temperatures well below 1 K. The interference can be observed as periodic oscillations of the tunnel current (voltage) through the junction at fixed bias voltage (current) as a function of a perpendicular magnetic field. The magnitude of the oscillations depends on the bias point. It reaches a maximum at energ…

Corrigendum: The quantum phase slip phenomenon in superconducting nanowires with a low-Ohmic environment

Microscopic model for multiple flux transitions in mesoscopic superconducting loops

A microscopic model is constructed which is able to describe multiple magnetic flux transitions as observed in recent ultra-low temperature tunnel experiments on an aluminum superconducting ring with normal metal - insulator - superconductor junctions [Phys. Rev. B \textbf{70}, 064514 (2004)]. The unusual multiple flux quantum transitions are explained by the formation of metastable states with large vorticity. Essential in our description is the modification of the pairing potential and the superconducting density of states by a sub-critical value of the persistent current which modulates the measured tunnel current. We also speculate on the importance of the injected non-equilibrium quasi…

Evidence of quantum phase slip effect in titanium nanowires

Electron transport properties of titanium nanowires were experimentally studied. Below the effective diameter $\lesssim$ 50 nm all samples demonstrated a pronounced broadening of the $R(T)$ dependencies, which cannot be accounted for thermal flcutuations. An extensive microscopic and elemental analysis indicates the absence of structural or/and geometrical imperfection capable to broaden the the $R(T)$ transition to such an extent. We associate the effect with quantum flucutuations of the order parameter.

Spatially resolved measurement of nonequilibrium quasiparticle relaxation in superconducting Al

Spatially resolved relaxation of nonequilibrium quasiparticles in a superconductor at ultralow temperatures was experimentally studied. It was found that the quasiparticle injection through a tunnel junction results in the modification of the shape of the I-V characteristic of a remote ``detector'' junction. The effect depends on the temperature, injection current, and proximity to the injector. The phenomena can be understood in terms of the creation of quasiparticle charge and energy disequilibrium characterized by two different length scales ${\ensuremath{\Lambda}}_{{Q}^{*}}~5$ and ${\ensuremath{\Lambda}}_{{T}^{*}}~40$ $\ensuremath{\mu}$m. The findings are in good agreement with existing…

Quantum size effect in single-crystalline bismuth nanorods

Quantum phase slip phenomenon in ultra-narrow superconducting nanorings

The smaller the system, typically - the higher is the impact of fluctuations. In narrow superconducting wires sufficiently close to the critical temperature Tc thermal fluctuations are responsible for the experimentally observable finite resistance. Quite recently it became possible to fabricate sub-10 nm superconducting structures, where the finite resistivity was reported within the whole range of experimentally obtainable temperatures. The observation has been associated with quantum fluctuations capable to quench zero resistivity in superconducting nanowires even at temperatures T-->0. Here we demonstrate that in tiny superconducting nanorings the same phenomenon is responsible for s…

Interference of nonequilibrium quasiparticles in a superconductor

Abstract We have observed an interference of nonequilibrium quasiparticles, injected from a copper electrode into an aluminium loop through a tunnel barrier. At temperatures below 1K the tunnel current at fixed voltage bias is periodically modulated by external magnetic field. The amplitude of the modulation reaches maximum at a bias slightly below the gap energy, and decreases with the further increase of the bias voltage. For a given voltage bias the amplitude of the current oscillations decreases with increase of the temperature and the loop circumference.

Unconventional behavior of superconducting nanostructures

Abstract Various aluminium nanostructures have been studied experimentally. Unusual nonmonotonous dependence of the critical current on external magnetic field has been observed. Pronounced nonlocal interaction has been found to decay exponentially on a scale of the superconducting coherence length ξ(T).

Tunneling spectroscopy of giant vorticity states in superconducting micro- and nanorings at ultra-low temperatures

Abstract When a superconducting contour is exposed to a magnetic field screening currents are induced. At temperatures well below the critical temperature Tc periodicity of the persistent currents can significantly exceed the superconducting flux quantum h / 2 e due to formation of metastable energy states with high quantum winding numbers (vorticity). We have studied the effect in normal metal-insulator-superconductor (NIS) and SIS’IS tunnel structures with the superconducting (S or S′) electrode in a shape of a loop. The tunnel current oscillates due to the modulation of the superconducting density of states by the persistent currents reaching the sub-critical values. In the limit of loop…

Resistive state of quasi-one-dimensional superconductors: Fluctuations vs. sample inhomogeneity

The shape of experimentally observed R(T) transition of thin superconducting wires is analyzed. Broadening of the transition in quasi-1-dimensional superconducting channels is typically associated with phase slip mechanism provided by thermal or quantum fluctuations. It is shown that consideration of inevitable geometrical inhomogeneity and finite dimensions of real samples studied in experiments is of primary importance for interpretation of results. The analysis is based on experimental fact that for many superconducting materials the critical temperature is a function of characteristic dimension of a low-dimensional system: film thickness or wire cross section

Nonlocality in superconducting microstructures

We discuss experimental evidence of nonlocality in electron transport of small structures. It is shown that for superconductors reasonable agreement with experiment can be achieved by assuming exponential decay of the nonlocal interaction ∝ exp(—Lξ), where L is the distance between the interacting points and ξ is the correlation length. ξ is associated with the Ginzburg - Landau coherence length ξGL.

Superconducting transition of single-crystal tin microstructures

Single-crystal superconducting microstructures have been fabricated. The resistances of tin whiskers were measured in a multiprobe configuration. Contacts were made of copper, gold, or niobium films using e-beam lithography followed by a lift-off process. Structures with normal metal probes showed unusual behavior: below the critical temperature of bulk tin, the resistance decreases in distinct steps and does not reach zero even when cooled down to 1 K. The origin of these phenomena is not clear but is likely related to a proximity effect.

Coulomb Blockade and Bloch Oscillations in Superconducting Ti Nanowires

Quantum fluctuations in quasi-one-dimensional superconducting channels leading to spontaneous changes of the phase of the order parameter by $2\pi$, alternatively called quantum phase slips (QPS), manifest themselves as the finite resistance well below the critical temperature of thin superconducting nanowires and the suppression of persistent currents in tiny superconducting nanorings. Here we report the experimental evidence that in a current-biased superconducting nanowire the same QPS process is responsible for the insulating state -- the Coulomb blockade. When exposed to RF radiation, the internal Bloch oscillations can be synchronized with the external RF drive leading to formation of…

Solid-State Analog of an Optical Interferometer

To some extend one may treat a metal ring with two probes as a solid-state analog of an optical interferometer. One node can be considered as a beam splitter (bi-prism, for example), and the electric current at the other node as an equivalent to a light intensity of an interference pattern formed at a screen. In optics, to obtain a stationary pattern one should use a monochromatic source of radiation, as afterwards in a conventional passive media (i.e. air) the phase of the radiation is preserved. On the contrary, in solids the phase of a conducting electron wavefunction is randomly altered due to inelastic collisions (mainly phonons at high temperatures). Hence, to satisfy the condition of…

Tunneling spectroscopy of persistent currents in superconducting microrings

It is shown that in a structure consisting of a superconducting ring-shaped electrode overlapped by a normal metal contact through a thin oxide barrier, measurements of the tunnel current in magnetic field can probe persistent currents in the ring. The effect manifests itself as periodic oscillations of the tunnel current through the junction at a fixed bias voltage as function of perpendicular magnetic field. The magnitude of oscillations depends on bias point. It reaches maximum at energy eV which is close to the superconducting gap and decreases with increase of temperature. The period of oscillations dF in units of magnetic flux is equal neither to h/e nor to h/2e, but significantly exc…

Towards quantum phase slip based standard of electric current

An accurate standard of electric current is a long-standing challenge of modern metrology. It has been predicted that a superconducting nanowire in the regime of quantum fluctuations can be considered as the dynamic equivalent of a chain of conventional Josephson junctions. In full analogy with the quantum standard of electric voltage based on the Josephson effect, the quantum phase slip phenomenon in ultrathin superconducting nanowires could be used for building the quantum standard of electric current. This work presents advances toward this ultimate goal.

Ion beam shaping and downsizing of nanostructures

We report a new approach for progressive and well-controlled downsizing of nanostructures below the 10 nm scale. Low energetic ion beam (Ar+) is used for gentle surface erosion, progressively shrinking the dimensions with ~ 1 nm accuracy. The method enables shaping of nanostructure geometry and polishing the surface. The process is clean room / high vacuum compatible being suitable for various applications. Apart from technological advantages, the method enables study of various size phenomena on the same sample between sessions of ion beam treatment.

The quantum phase slip phenomenon in superconducting nanowires with a low-Ohmic environment

In a number of recent experiments it has been demonstrated that in ultra-narrow superconducting channels quantum fluctuations of the order parameter, alternatively called quantum phase slips, are responsible for the finite resistance well below the critical temperature. The acceptable agreement between those experiments and the models describing quantum fluctuations in quasi-one-dimensional superconductors has been established. However the very concept of the phase slip is justified when these fluctuations are the relatively rare events, meaning that the effective resistance of the system should be much smaller than the normal state equivalent. In this paper we study the limit of the strong…

Influence of magnetic field on cooling by normal-insulator–superconductor junctions

Cooling by normal-insulator–superconductor junctions in external magnetic field has been studied experimentally. For all orientations of magnetic field the cooling performance correlates with the magnetic field dependent superconducting energy gap Δ(H). In perpendicular orientation of magnetic field with respect to the sample plane, additional degradation of the cooling power originates from scattering of nonequilibrium quasiparticles in the superconductor on magnetic vortices. The effect is hysteretic and its magnitude depends on the shape of the superconducting probes.

Superconductivity in one dimension

Superconducting properties of metallic nanowires can be entirely different from those of bulk superconductors because of the dominating role played by thermal and quantum fluctuations of the order parameter. For superconducting wires with diameters below $ \sim 50$ nm quantum phase slippage is an important process which can yield a non-vanishing wire resistance down to very low temperatures. Further decrease of the wire diameter, for typical material parameters down to $\sim 10$ nm, results in proliferation of quantum phase slips causing a sharp crossover from superconducting to normal behavior even at T=0. A number of interesting phenomena associated both with quantum phase slips and with …

Coherent quantum phase slip

A hundred years after discovery of superconductivity, one fundamental prediction of the theory, the coherent quantum phase slip (CQPS), has not been observed. CQPS is a phenomenon exactly dual to the Josephson effect: whilst the latter is a coherent transfer of charges between superconducting contacts, the former is a coherent transfer of vortices or fluxes across a superconducting wire. In contrast to previously reported observations of incoherent phase slip, the CQPS has been only a subject of theoretical study. Its experimental demonstration is made difficult by quasiparticle dissipation due to gapless excitations in nanowires or in vortex cores. This difficulty might be overcome by usin…

Negative magnetoresistance of ultra-narrow superconducting nanowires in the resistive state

We present a phenomenological model qualitatively explaining negative magnetoresistance in quasi-one-dimensional superconducting channels in the resistive state. The model is based on the assumption that fluctuations of the order parameter (phase slips) are responsible for the finite effective resistance of a narrow superconducting wire sufficiently close to the critical temperature. Each fluctuation is accompanied by an instant formation of a quasi-normal region of the order of the non-equilibrium quasiparticle relaxation length 'pinned' to the core of the phase slip. The effective time-averaged voltage measured in experiment is a sum of two terms. First one is the conventional contributio…

Experimental Study of the Fluctuation-Governed Resistive State in Quasi-One-Dimensional Superconductors

Physical properties of quasi-one-dimensional superconducting channels (nanowires) can differ significantly from those of bulk superconductors. The reason behind is the impact of thermal and quantum fluctuations. In the particular case of resistive measurements the fluctuations may significantly broaden the superconducting transition R(T). Here we review the activities in the field with main emphasis on experiment, while brief theoretical background is also presented.

Quantum fluctuations in ultranarrow superconducting aluminum nanowires

Progressive reduction of the effective diameter of a nanowire is applied to trace evolution of the shape of the superconducting transition $R(T)$ in quasi-one-dimensional aluminum structures. In nanowires with effective diameter $\ensuremath{\leqslant}15\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ the $R(T)$ dependences are much wider than predicted by the model of thermally activated phase slips. The effect can be explained by quantum fluctuations of the order parameter. Negative magnetoresistance is observed in the thinnest samples. Experimental results are in reasonable agreement with existing theoretical models. The effect should have a universal validity, indicating a breakdown of the zero-…

Superconductivity suppression in Fe-implanted thin Al films

At present, ion implantation into metallic systems is given increasing attention, aiming at achieving properties and functionalities of technologically valuable materials not easily available via conventional techniques. In our experiments thin Al films were implanted with Fe ions in order to find out how the superconductive properties of the metal can be modified at will. The purpose was twofold, viz., first, to study the basic physics of superconductivity in low-dimensional metallic structures doped with impurities. The second purpose was to apply ion implantation for the suppression of undesired superconductivity in aluminum widely used for fabrication of micro- and nanodevices operated …

Experimental limits of the observation of thermally activated phase-slip mechanism in superconducting nanowires

The shape of experimentally observed $R(T)$ transition of thin superconducting wires is analyzed. From theoretical point of view, broadening of the transition in quasi-one-dimensional superconducting channels is typically associated with phase-slip mechanism. It is shown that such interpretation can be misleading if to consider geometrical inhomogeneity and finite dimensions of real samples studied in experiments. The analysis is based on experimental fact that for many superconducting materials the critical temperature depends on the characteristic dimension of a sample: film thickness or wire cross section.