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

Journal ArticleWe 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 h/4e2. 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 WLC's 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

Dichotomy in short superconducting nanowires: thermal phase slippage vs. Coulomb blockade

ManuscriptQuasi-one-dimensional superconductors or nanowires exhibit a transition into a nonsuperconducting regime, as their diameter shrinks. We present measurements on ultrashort nanowires (∼40-190 nm long) in the vicinity of this quantum transition. Properties of all wires in the superconducting phase, even those close to the transition, can be explained in terms of thermally activated phase slips. The behavior of nanowires in the nonsuperconducting phase agrees with the theories of the Coulomb blockade of coherent transport through mesoscopic normal metal conductors. Thus it is concluded that the quantum transition occurs between two phases: a "true superconducting phase" and an "insulating phase". No intermediate, "metallic" phase was found

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 consistent with the Chakravarty-Schmid-Bulgadaev phase boundary, namely with the critical resistance being equal to R_Q = h/4e^2. We find that transport properties of insulating nanowires exhibit a weak Coulomb blockade behavior.Comment: 5 pages, 4 figure

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

Journal ArticleWe 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 ¼ 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

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

Journal ArticleThe effects of a strong magnetic field on superconducting Nb and MoGe nanowires with diameter ~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, Tc, 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 τs.o. ~ τ (hc/Ze2)4, where τ is the elastic scattering time and Z is the atomic number

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

The effects of strong magnetic field on superconducting Nb and MoGe nanowires with diameter $\sim10$ 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 eleven 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 $\tau_{so}\simeq \tau(\hbar c/ Ze^{2})^{4}$, where $\tau$ is the elastic scattering time and $Z$ is the atomic number.Comment: accepted for publication in Physical Review Letter

Magnetic field enhancement of superconductivity in ultra-narrow wires

We study the effect of an applied magnetic field on sub-10nm wide MoGe and Nb superconducting wires. We find that magnetic fields can enhance the critical supercurrent at low temperatures, and does 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.Comment: 4 pages, 3 figures, 1 tabl