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Register a new userFlux Penetration in Superconducting Strip with Edge-Indentation
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J{\\o}rn Inge Vestg{\\aa}rden
Publication date
2013
fields
Physics
and research's language is
English
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The flux penetration near a semicircular indentation at the edge of a thin superconducting strip placed in a transverse magnetic field is investigated. The flux front distortion due to the indentation is calculated numerically by solving the Maxwell equations with a highly nonlinear $E(j)$ law. We find that the excess penetration, $Delta$, can be significantly ($sim$ 50%) larger than the indentation radius $r_0$, in contrast to a bulk supercondutor in the critical state where $Delta=r_0$. It is also shown that the flux creep tends to smoothen the flux front, i.e. reduce $Delta$. The results are in very good agreement with magneto-optical studies of flux penetration into an YBa$_2$Cu$_3$O$_x$ film having an edge defect.
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Flux penetrations into three-dimensional Nb superconducting strip arrays, where two layers of strip arrays are stacked by shifting a half period, are studied using a magneto-optical imaging method. Flux avalanches are observed when the overlap between the top and bottom layers is large even if the width of each strip is well below the threshold value. In addition, anomalous linear avalanches perpendicular to the strip are observed in the shifted strip array when the overlap is very large and the thickness of the superconductor is greater than the penetration depth. We discuss possible origins for the flux avalanches, including linear ones, by considering flux penetration calculated by the Campbell method assuming the Bean model.
The magneto-optical imaging technique is used to visualize the penetration of the magnetic induction in YBa$_{2}$Cu$_{3}$O$_{7-delta}$ thin films during surface resistance measurements. The in-situ surface resistance measurements were performed at 7 GHz using the dielectric resonator method. When only the microwave magnetic field $H_{rf}$ is applied to the superconductor, no $H_{rf}$-induced vortex penetration is observed, even at high rf power. In contrast, in the presence of a constant magnetic field superimposed on $H_{rf}$ we observe a progression of the flux front as $H_{rf}$ is increased. A local thermometry method based on the measurement of the resonant frequency of the dielectric resonator placed on the YBa$_{2}$Cu$_{3}$O$_{7-delta}$ thin film shows that the $H_{rf}$--induced flux penetration is due to the increase of the film temperature.
We have studied the impact of low-frequency magnetic flux noise upon superconducting transmon qubits with various levels of tunability. We find that qubits with weaker tunability exhibit dephasing that is less sensitive to flux noise. This insight was used to fabricate qubits where dephasing due to flux noise was suppressed below other dephasing sources, leading to flux-independent dephasing times T2* ~ 15 us over a tunable range of ~340 MHz. Such tunable qubits have the potential to create high-fidelity, fault-tolerant qubit gates and fundamentally improve scalability for a quantum processor.
By means of small magnetic field ac susceptibility measurement at 10 kHz we found that the real and imaginary parts of ceramic $YBa_2Cu_3O_ x$ susceptibility in presence of the external low frequency field close to 0.1 Hz exhibit frequency dependence. The wide maximum of hysteresis losses and {color{black} the exponential dependence} of the {color{black} effectiveness of flux penetration} with increasing of external field frequency were obtained. We observed a nonlinear dynamic magnetic response in presence dc field and we suggest that this behavior is due to the dynamic and static Josephson vortex-vortex interaction.
Surface distributions of two level system (TLS) defects and magnetic vortices are limiting dissipation sources in superconducting quantum circuits. Arrays of flux-trapping holes are commonly used to eliminate loss due to magnetic vortices, but may increase dielectric TLS loss. We find that dielectric TLS loss increases by approximately 25% for resonators with a hole array beginning 2 $mu text{m}$ from the resonator edge, while the dielectric loss added by holes further away was below measurement sensitivity. Other forms of loss were not affected by the holes. Additionally, we estimate the loss due to residual magnetic effects to be $9times 10^{-10} /mutext{T} $ for resonators patterned with flux-traps and operated in magnetic fields up to $5$ $mutext{T}$. This is orders of magnitude below the total loss of the best superconducting coplanar waveguide resonators.
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