Precise in situ tuning of the critical current of a superconducting nanowire using high bias voltage pulses

Eliminating Quantum Phase Slips in Superconducting Nanowires

In systems with reduced dimensions, quantum fluctuations have a strong influence on the electronic conduction, even at very low temperatures. In superconductors, this is especially interesting, since the coherent state of the superconducting electrons strongly interacts with these fluctuations and therefore is a sensitive tool to study them. In this paper, we report on comprehensive measurements of superconducting nanowires in the quantum phase slip regime. Using an intrinsic electromigration process, we have developed a method to lower the nanowire's resistance in situ and therefore eliminate quantum phase slips in small consecutive steps. We observe critical (Coulomb) blockade voltages and superconducting critical currents, in good agreement with theoretical models. Between these two regimes, we find a continuous transition displaying a nonlinear metallic-like behavior. The reported intrinsic electromigration technique is not limited to low temperatures, as we find a similar change in resistance that spans over 3 orders of magnitude also at room temperature. Aside from superconducting quantum circuits, such a technique to reduce the resistance may also have applications in modern electronic circuits.

Analysis of the geometric phase for a nanowire-bridged superconducting Fabry-Perot resonator

The geometric phases of a nanowire-bridged superconducting Fabry-Perot resonator subjected to a microwave transmission have been investigated through its modelling into a RLC-circuit. Because the Hamiltonian of the system is a somewhat complicated form, special mathematical techniques, such as the invariant operator method and the unitary transformation approach, have been adopted in order to treat the system; These methods are very useful for managing complicated time-dependent Hamiltonian systems. We have rigorously evaluated the analytical geometric phases in both the Fock and coherent states. Typically, the geometric phases oscillate and the amplitude of such oscillations tend to grow over time. The influence of parameters of the system on the geometric phases has been analyzed in detail through the relevant illustrations. From our research, the concept of geometric phases and associated quantum mechanical characters of the system has been clarified. Our investigation for the geometric phases is useful for understanding topological features of the system, that take place through the evolution of the wave functions.

Critical temperature in feedback-controlled electromigration of gold nanostructures

This paper presents several experiments demonstrating the need for a more nuanced picture of electromigration (EM) than that of a fixed critical junction temperature at which EM onset occurs. Our data suggests that even for a fixed cross-sectional geometry the critical junction temperature for EM, T _c , varies with environmental temperature, thermal resistance of adjacent regions, and even the direction of the current flow in asymmetric structures. We have performed feedback-controlled EM on nanowires at environmental temperatures between 75 and 260 K and fit the EM onset points with a constant junction power model. We find that average fit critical power is monotonically increasing with decreasing temperature, but is decidedly nonlinear at lower temperatures. We extract and compare the corresponding T _c values using several different thermal models which utilize measured values of nanowire thermal conductivity for our devices: these models all agree on a moderately increasing T _c with decreasing environmental temperature. This is tentatively explained by enhanced current-driven annealing on the voltage ramp prior to EM onset which decreases structural scattering, thereby increasing the critical temperature at which wind-force-driven hopping events will achieve a critical atomic flux. We also obtain fit critical power for a series of bowtie structures of identical constriction but varying adjacent thermal resistance (R _th ), and estimate that T _c in the constriction varies with R _th for higher resistance structures. Critical power measurements on a second series of asymmetric bowties further suggests that T _c also depends on the alignment of the electron flow with the temperature gradient at the constriction.

In situ tailoring of superconducting junctions via electro-annealing

We demonstrate the in situ engineering of superconducting nanocircuitry by targeted modulation of material properties through high applied current densities. We show that the sequential repetition of such customized electro-annealing in a niobium (Nb) nanoconstriction can broadly tune the superconducting critical temperature T_c and the normal-state resistance R_n in the targeted area. Once a sizable R_n is reached, clear magneto-resistance oscillations are detected along with a Fraunhofer-like field dependence of the critical current, indicating the formation of a weak link but with further adjustable characteristics. Advanced Ginzburg-Landau simulations fully corroborate this picture, employing the detailed parametrization from the electrical characterization and high resolution electron microscope images of the region within the constriction where the material has undergone amorphization by electro-annealing.

Healing Effect of Controlled Anti-Electromigration on Conventional and High-Tc Superconducting Nanowires

The electromigration process has the potential capability to move atoms one by one when properly controlled. It is therefore an appealing tool to tune the cross section of monoatomic compounds with ultimate resolution or, in the case of polyatomic compounds, to change the stoichiometry with the same atomic precision. As demonstrated here, a combination of electromigration and anti-electromigration can be used to reversibly displace atoms with a high degree of control. This enables a fine adjustment of the superconducting properties of Al weak links, whereas in Nb the diffusion of atoms leads to a more irreversible process. In a superconductor with a complex unit cell (La_2-x Ce_x CuO₄ ), the electromigration process acts selectively on the oxygen atoms with no apparent modification of the structure. This allows to adjust the doping of this compound and switch from a superconducting to an insulating state in a nearly reversible fashion. In addition, the conditions needed to replace feedback controlled electromigration by a simpler technique of electropulsing are discussed. These findings have a direct practical application as a method to explore the dependence of the characteristic parameters on the exact oxygen content and pave the way for a reversible control of local properties of nanowires.

Statistics of localized phase slips in tunable width planar point contacts

The main dissipation mechanism in superconducting nanowires arises from phase slips. Thus far, most of the studies focus on long nanowires where coexisting events appear randomly along the nanowire. In the present work we investigate highly confined phase slips at the contact point of two superconducting leads. Profiting from the high current crowding at this spot, we are able to shrink in-situ the nanoconstriction. This procedure allows us to investigate, in the very same sample, thermally activated phase slips and the probability density function of the switching current I_sw needed to trigger an avalanche of events. Furthermore, for an applied current larger than I_sw, we unveil the existence of two distinct thermal regimes. One corresponding to efficient heat removal where the constriction and bath temperatures remain close to each other, and another one in which the constriction temperature can be substantially larger than the bath temperature leading to the formation of a hot spot. Considering that the switching current distribution depends on the exact thermal properties of the sample, the identification of different thermal regimes is of utmost importance for properly interpreting the dissipation mechanisms in narrow point contacts.