A. Levchenko

Distribution of Supercurrent Switching in Graphene under the Proximity Effect

We study the stochastic nature of switching current in hysteretic current-voltage characteristics of superconductor-graphene-superconductor junctions. We find that the dispersion of the switching current distribution scales with temperature as σ(I) proportional to T(α(G)) with α(G) as low as 1/3. This observation is in sharp contrast to the known Josephson junction behavior where σ(I) proportional to T(α(J)) with α(J)=2/3. We propose an explanation using a generalized version of Kurkijärvis theory for the flux stability in rf-SQUID and attribute this anomalous effect to the temperature dependence of the critical current which persists down to low temperatures.

Enhancement of the London penetration depth in pnictides at the onset of spin-density-wave order under superconducting dome.

Recent measurements of the doping dependence of the London penetration depth λ(x) at low T in clean samples of isovalent BaFe2(As(1-x)P(x))2 at T≪T(c) [Hashimoto et al., Science 336, 1554 (2012)] revealed a peak in λ(x) near optimal doping x=0.3. The observation of the peak at T≪T(c), points to the existence of a quantum critical point beneath the superconducting dome. We associate such a quantum critical point with the onset of a spin-density-wave order and show that the renormalization of λ(x) by critical magnetic fluctuations gives rise to the observed feature. We argue that the case of pnictides is conceptually different from a one-component Galilean invariant Fermi liquid, for which correlation effects do not cause the renormalization of the London penetration depth at T=0.

Quantitative analysis of quantum phase slips in superconducting Mo76Ge24 nanowires revealed by switching-current statistics

We measure quantum and thermal phase-slip rates using the standard deviation of the switching current in superconducting nanowires at high bias current. Our rigorous quantitative analysis provides firm evidence for the presence of quantum phase slips (QPS) in homogeneous nanowires. We observe that as temperature is lowered, thermal fluctuations freeze at a characteristic crossover temperature Tq, below which the dispersion of the switching current saturates to a constant value, indicating the presence of QPS. The scaling of the crossover temperature Tq with the critical temperature Tc is linear, which is consistent with the theory of macroscopic quantum tunneling. We can convert the wires from the initial amorphous phase to a single crystal phase, in situ, by applying calibrated voltage pulses. This technique allows us to probe directly the effects of the wire resistance, critical temperature and morphology on thermal and quantum phase slips.

Universal Features of Counting Statistics of Thermal and Quantum Phase Slips in Nanosize Superconducting Circuits

We perform measurements of phase-slip-induced switching current events on different types of superconducting weak links and systematically study statistical properties of the switching current distributions. We employ two types of devices in which a weak link is formed either by a superconducting nanowire or by a graphene flake subject to proximity effect. We demonstrate that independently of the nature of the weak link, higher moments of the distribution take universal values. In particular, the third moment (skewness) of the distribution is close to -1 both in thermal and quantum regimes. The fourth moment (kurtosis) also takes a universal value close to 5. The discovered universality of skewness and kurtosis is confirmed by an analytical model. Our numerical analysis shows that introduction of extraneous noise into the system leads to significant deviations from the universal values. We suggest using the discovered universality of higher moments as a robust tool for checking against undesirable effects on noise in various types of measurements.

Relaxation and edge reconstruction in integer quantum Hall systems

The interplay between the confinement potential and the electron–electron interactions causes reconstructions of quantum Hall edges. We study the consequences of this edge reconstruction for the relaxation of hot electrons injected into integer quantum Hall edge states. In translationally invariant edges, the relaxation of hot electrons is governed by three-body collisions, which are sensitive to the electron dispersion and thus to reconstruction effects. We show that the relaxation rates are significantly altered in different reconstruction scenarios.

Phase-sensitive bichromatic photoresistance in a two-dimensional electron gas

We have studied microwave photoresistance in a two-dimensional electron system subject to two radiation fields (frequencies

Magnetic penetration depth in disordered iron-based superconductors

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Voltage-driven quantum oscillations of conductance in graphene

and

Effect of spin-density wave fluctuations on the specific heat jump in iron pnictides at the superconducting transition

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Quantum-fluctuation effects in the transport properties of ultrathin films of disordered superconductors above the paramagnetic limit.

) using quantum kinetic equation. We have found that when