Andrey Rogachev

Superconducting properties of polycrystalline Nb nanowires templated by carbon nanotubes

Continuous Nb wires, 7–15 nm in diameter, have been fabricated by sputter-coating single fluorinated carbon nanotubes. Transmission electron microscopy revealed that the wires are polycrystalline, having grain sizes of about 5 nm. The critical current of wires thicker than ∼12 nm is very high (107 A/cm2) and comparable to the expected depairing current. The resistance versus temperature curves measured down to 0.3 K are well described by the Langer–Ambegaokar–McCumber–Halperin theory of thermally activated phase slips. Quantum phase slips are suppressed.

Individual topological tunnelling events of a quantum field probed through their macroscopic consequences

Measurements of the distribution of stochastic switching currents in homogeneous, ultra-narrow superconducting nanowires provide strong evidence that the low-temperature current-switching in such systems occurs through quantum phase slips—topological quantum fluctuations of the superconducting order parameter via which tunnelling occurs between current-carrying states.

Influence of high magnetic fields on the superconducting transition of one-dimensional Nb and MoGe nanowires.

The effects of a strong magnetic field on superconducting Nb and MoGe nanowires with diameter approximately 10 nm have been studied. We have found that the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory of thermally activated phase slips is applicable in a wide range of magnetic fields and describes well the temperature dependence of the wire resistance, over 11 orders of magnitude. The field dependence of the critical temperature, T(c), extracted from the LAMH fits is in good quantitative agreement with the theory of pair-breaking perturbations that takes into account both spin and orbital contributions. The extracted spin-orbit scattering time agrees with an estimate tau(s.o.) approximately tau(variant Plancks over 2pic/Ze(2))(4), where tau is the elastic scattering time and Z is the atomic number.

Magnetic-field enhancement of superconductivity in ultranarrow wires.

We study the effect of an applied magnetic field on sub-10-nm wide MoGe and Nb superconducting wires. We find that magnetic fields can enhance the critical supercurrent at low temperatures, and do so more strongly for narrower wires. We conjecture that magnetic moments are present, but their pair-breaking effect, active at lower magnetic fields, is suppressed by higher fields. The corresponding microscopic theory, which we have developed, quantitatively explains all experimental observations, and suggests that magnetic moments have formed on the wire surfaces.

Determination of the superconductor-insulator phase diagram for one-dimensional wires.

We establish the superconductor-insulator phase diagram for quasi-one-dimensional wires by measuring a large set of MoGe nanowires. This diagram is roughly consistent with the Chakravarty-Schmid-Bulgadaev phase boundary, namely, with the critical resistance being equal to RQ=h/4e2. Deviations from this boundary for a small fraction of the samples prompt us to suggest an alternative phase diagram, which matches the data exactly. Transport properties of wires in the superconducting phase are dominated by phase slips, whereas insulating nanowires exhibit a weak Coulomb blockade behavior.

Effect of morphology on the superconductor-insulator transition in one-dimensional nanowires

We study the effect of morphology on the low-temperature behavior of superconducting nanowires which vary in length from 86 nm to 188 nm. A well-defined superconductor-insulator transition is observed only in the family of homogeneous wires, in which case the transition occurs when the normal resistance is close to

Admittance spectroscopy study of polymer diodes in small magnetic fields

{h/4e}^{2}.

Magnetic-field dependent differential capacitance of polymer diodes

Inhomogeneous wires, on the other hand, exhibit a mixed behavior, such that signatures of the superconducting and insulating regimes can be observed in the same sample. The resistance versus temperature curves of inhomogeneous wires show multiple steps, each corresponding to a weak link constriction (WLC) present in the wire. Similarly, each WLC generates a differential resistance peak when the bias current reaches the critical current of the WLC. Due to the presence of WLCs an inhomogeneous wire splits into a sequence of weakly interacting segments where each segment can act as a superconductor or as an insulator. Thus the entire wire then shows a mixed behavior.

Enhancing superconductivity: Magnetic impurities and their quenching by magnetic fields

We performed a systematic study of bipolar and unipolar organic diodes based on the π-conjugated polymer, 2-methoxy-5-(2′-ethylhexyloxy) (MEH-PPV), using electronic and magneto-transport measurements with magnetic field in the range 0–180 mT and admittance spectroscopy in the frequency range 1 Hz < f < 10 MHz. The admittance spectra of bipolar devices reveal two relaxation processes with distinct time scales that are influenced by the magnetic field. The slower process, which dominates the device capacitance at f < 10 Hz, is attributed to the trap-assisted monomolecular recombination. The faster process is attributed to the electron-hole bimolecular recombination kinetics. We found that the time scale of this process, τ2, decreases exponentially with the bias voltage. Application of magnetic field, B = 30 mT decreases τ2 by ∼30%. We also found that the magneto-conductance, ΔG(ω,B)/G(ω,0), has a characteristic cutoff frequency that shifts to higher frequencies with increasing bias voltage. In particular, t...

Cross-Correlated Current Noise Spectroscopy and Detection of Shot Noise in Organic Light-Emitting Diodes

Using admittance spectroscopy, we found that bipolar organic diodes based on pi-conjugated polymer, 2-methoxy-5-(2′-ethylhexyloxy), MEH-PPV, have strong divergent contribution to the device differential capacitance. It is positive at low bias voltages, turns negative at intermediate biases, and becomes positive again at stronger biases. In addition, we found that at certain biases, a small magnetic field can change the capacitance from divergent negative to divergent positive. Possible physical processes responsible for this anomalous behavior of the capacitance and its relation to the phenomenon of organic magnetoresistance are discussed.