Tuning vortex fluctuations and the resistive transition in superconducting films with a thin overlayer

It is shown that the temperature of the resistive transition $T_r$ of a superconducting film can be increased by a thin superconducting or normal overlayer. For instance, deposition of a highly conductive thin overlayer onto a dirty superconducting film can give rise to an "anti-proximity effect" which manifests itself in an initial increase of $T_r(d_2)$ with the overlayer thickness $d_2$ followed by a decrease of $T_r(d_2)$ at larger $d_2$. Such a nonmonotonic thickness dependence of $T_r(d_2)$ results from the interplay of the increase of a net superfluid density mitigating phase fluctuations and the suppression of the critical temperature $T_c$ due to the conventional proximity effect. This behavior of $T_r(d_2)$ is obtained by solving the Usadel equations to calculate the temperature of the Berezinskii-Kosterletz-Thouless transition, and the temperature of the resistive transition due to thermally-activated hopping of single vortices in dirty bilayers. The theory incorporates relevant materials parameters such as thicknesses and conductivities of the layers, interface contact resistance between them and the subgap quasiparticle states which affect both phase fluctuations and the proximity effect suppression of $T_c$. The transition temperature $T_r$ can be optimized by tuning the overlayer parameters, which can significantly weaken vortex fluctuations and nearly restore the mean-field critical temperature. The calculated behavior of $T_r(d_2)$ may explain the nonmonotonic dependence of $T_r(d_2)$ observed on (Ag, Au, Mg, Zn)-coated Bi films, Ag-coated Ga and Pb films or NbN and NbTiN films on AlN buffer layers. These results suggest that bilayers can be used as model systems for systematic investigations of optimization of fluctuations in superconductors

Fragmentation of Fast Josephson Vortices and Breakdown of Ordered States by Moving Topological Defects

Topological defects such as vortices, dislocations or domain walls define many important effects in superconductivity, superfluidity, magnetism, liquid crystals, and plasticity of solids. Here we address the breakdown of the topologically-protected stability of such defects driven by strong external forces. We focus on Josephson vortices that appear at planar weak links of suppressed superconductivity which have attracted much attention for electronic applications, new sources of THz radiation, and low-dissipative computing. Our numerical simulations show that a rapidly moving vortex driven by a constant current becomes unstable with respect to generation of vortex-antivortex pairs caused by Cherenkov radiation. As a result, vortices and antivortices become spatially separated and accumulate continuously on the opposite sides of an expanding dissipative domain. This effect is most pronounced in thin film edge Josephson junctions at low temperatures where a single vortex can switch the whole junction into a resistive state at currents well below the Josephson critical current. Our work gives a new insight into instability of a moving topological defect which destroys global long-range order in a way that is remarkably similar to the crack propagation in solids.Comment: Sci. Rep. 5, 1782

Surface impedance and optimum surface resistance of a superconductor with imperfect surface

We calculate a low-frequency surface impedance of a dirty, s-wave superconductor with an imperfect surface incorporating either a thin layer with a reduced pairing constant or a thin, proximity-coupled normal layer. Such structures model realistic surfaces of superconducting materials which can contain oxide layers, absorbed impurities or nonstoichiometric composition. We solved the Usadel equations self-consistently and obtained spatial distributions of the order parameter and the quasiparticle density of states which then were used to calculate a low-frequency surface resistance $R_s(T)$ and the magnetic penetration depth $\lambda(T)$ as functions of temperature in the limit of local London electrodynamics. It is shown that the imperfect surface in a single-band s-wave superconductor results in a non-exponential temperature dependence of $Z(T)$ at $T\ll T_c$ which can mimic the behavior of multiband or d-wave superconductors. The imperfect surface and the broadening of the gap peaks in the quasiparticle density of states $N(\epsilon)$ in the bulk give rise to a weakly temperature-dependent residual surface resistance. We show that the surface resistance can be optimized and even reduced below its value for an ideal surface by engineering $N(\epsilon)$ at the surface using pairbreaking mechanisms, particularly, by incorporating a small density of magnetic impurities or by tuning the thickness and conductivity of the normal layer and its contact resistance. The results of this work address the limit of $R_s$ in superconductors at $T\ll T_c$, and the ways of engineering the optimal density of states by surface nano-structuring and impurities to reduce losses in superconducting micro-resonators, thin film strip lines, and radio frequency cavities for particle accelerators

Dynamic transition of vortices into phase slips and generation of vortex-antivortex pairs in thin film Josephson junctions under dc and ac currents

We present theoretical and numerical investigations of vortices driven by strong dc and ac currents in long Josephson junctions described by a nonlinear integro-differential equation which takes into account nonlocal electrodynamics of films, vortex bremsstrahlung and Cherenkov radiation amplified by the attraction of vortices to the edges of the junction. The work focuses on the dynamics of vortices in Josephson junctions in thin films where the effects of Josephson nonlocality dominate but London screening is negligible. We obtained an exact solution for a vortex driven by an arbitrary time-dependent current in an overdamped junction where the vortex turns into a phase slip if the length of the junction is shorter than a critical length which depends on current. Our analytical and numerical results show that the dynamic behavior of vortices depends crucially on the ohmic damping parameter. In overdamped junctions vortices expand as they move faster and turn into phase slips as current increases. In underdamped junctions vortices entering from the edges produce Cherenkov radiation generating cascades of expanding vortex-antivortex pairs, which ultimately drive the entire junction into a resistive phase slip state. Simulations revealed a variety of complex dynamic states of vortices under dc and ac currents which can manifest themselves in hysteretic current-voltage characteristics with jumps and regions with negative differential resistance resulting from transitions from oscillating to ballistic propagation of vortices, their interaction with pinning centers and standing nonlinear waves in the junction.Comment: 19 page

Tuning Microwave Losses in Superconducting Resonators

Performance of superconducting resonators, particularly cavities for particle accelerators and micro cavities and thin film resonators for quantum computations and photon detectors has been improved substantially by recent materials treatments and technological advances. As a result, the niobium cavities have reached the quality factors Q ~ 1011 at 1-2 GHz and 1.5 K and the breakdown radio-frequency (rf) fields H close to the dc superheating eld of the Meissner state. These advances raise the question whether the state-of-the-art cavities are close to the fundamental limits, what these limits actually are, and to what extent the Q and H limits can be pushed by the materials nano structuring and impurity management. These issues are also relevant to many applications using high-Q thin film resonators, including single-photon detectors and quantum circuits. This topical review outlines basic physical mechanisms of the rf nonlinear surface impedance controlled by quasiparticles, dielectric losses and trapped vortices, as well as the dynamic field limit of the Meissner state. Sections cover ways of engineering an optimum quasiparticle density of states and superfluid density to reduce rf losses and kinetic inductance by pairbreaking mechanisms related to magnetic impurities, rf currents, and proximity-coupled metallic layers at the surface. A section focuses on mechanisms of residual surface resistance which dominates rf losses at ultra low temperatures. Microwave losses of trapped vortices and their reduction by optimizing the concentration of impurities and pinning potential are also discussed

The Human race

To tell a story and to tell it well is a paramount function in human society. Whether the desired outcome is to engage, educate, or entertain the viewer the goal is the same. Through story we endeavor to learn something more about ourselves and hopefully show others something new within themselves. As the old adage goes, Give a man a fish you feed him for a day. Teach that man how to fish you feed him for a lifetime. One could simply give that hungry man a pole, a line, a hook, and a worm and give him directions on how to fish and he may or may not get the general idea. But if you tell him a story about how a starving man was able to feed himself and his entire family because another man taught him to fish, he will be engaged. He will want to be like this man and a good story would teach him all he needs to know. Stories are explorations into the state of the human condition. Through humorous stories we can more easily engage viewers with difficult subjects. They will let down their various filters and allow themselves to be confronted with a troubling topic, usually without their realization. One purpose of this story is to teach people something about the way we reproduce in a highly abstracted and entertaining way. An admittedly more selfish goal of mine was to learn how to better tell a story visually using the established film grammar through the use of computer generated imagery (CGI)

Tuning Vortex Fluctuations and the Resistive Transition in Superconducting Films With a Thin Overlayer

It is shown that the temperature of the resistive transition Tr of a superconducting film can be increased by a thin superconducting or normal overlayer. For instance, deposition of a highly conductive thin overlayer onto a dirty superconducting film can give rise to an “antiproximity effect,” which manifests itself in an initial increase of Tr (d2) with the overlayer thickness d2 followed by a decrease of Tr (d2)at larger d2. Such a nonmonotonic thickness dependence of Tr (d2) results from the interplay of the increase of a net superfluid density mitigating phase fluctuations and the suppression of the critical temperature Tc due to the conventional proximity effect. This behavior of Tr (d2) is obtained by solving the Usadel equations to calculate the temperature of the Berezinskii-Kosterlitz-Thouless transition, and the temperature of the resistive transition due to thermally activated hopping of single vortices in dirty bilayers. The theory incorporates relevant material parameters such as thicknesses and conductivities of the layers, the interface contact resistance between them, and the subgap quasiparticle states, which affect both phase fluctuations and the proximity effect suppression of Tc. The transition temperature Tr can be optimized by tuning the overlayer parameters, which can significantly weaken vortex fluctuations and nearly restore the mean-field critical temperature. The calculated behavior of Tr (d2) may explain the nonmonotonic dependence of Tr (d2) observed on (Ag,Au,Mg,Zn)-coated Bi films, Ag-coated Ga and Pb films, or NbN and NbTiN films on AlN buffer layers. These results suggest that bilayers can be used as model systems for systematic investigations of optimization of fluctuations in superconductors