<i>Foundations of Applied Superconductivity</i>

Orlando, Terry P.; Delin, K. A.; Lobb, C. J.

doi:10.1063/1.2810145

Cited by 300 publications

(235 citation statements)

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“…An applied magnetic field in the geometry described above will produce the well known "V profile" of the magnetic flux density discussed, for example, in Orlando [44] and observed in many experiments, such as those of Behnia, et al [45]. This is shown in Fig.…”

Section: The Transport Current: Shifting the Boundary Conditionsmentioning

confidence: 88%

Simple model for plastic dynamics of a disordered flux-line lattice

2001

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We use a coarse-grained model of superconducting vortices driven through a random pinning potential to study the nonlinear current-voltage (IV) characteristics of flux flow in type II superconductors with pinning. In experiments, the IV relation measures flux flow down a flux density gradient. The work presented here treats this key feature explicitly. As the vortex repulsion weakens, the vortex pile maintains a globally steeper slope, corresponding to a larger critical current, for the same pinning potential. In addition, the magnitude of the peak in the differential resistance falls as the resistance peak shifts to higher currents. The model also exhibits so-called "IV fingerprints", and crossover to Ohmic (linear) behavior at high currents. Thus, many of the varieties of plastic behavior observed experimentally for soft flux line systems in the "peak regime" are reproduced in numerical simulations of the zero temperature model. This model describes a two-dimensional slice of the flux line system at the scale of the London length (λ). It does not include any degrees of freedom at scales much smaller than λ, which are required to specify the degree of disorder in a flux line lattice. Instead, the nonlinear transport behaviors are related to the self-organized, large scale morphologies of the vortex river flow down the slope of the vortex pile. These morphologies include isolated filamentary channels, which can merge at higher flow rates to make a braided river, and eventually give uniform flow at even higher flow rates. The filamentary structure is associated with an IV characteristic that has concave, or positive, curvature. The braided river is associated with the peak in the differential resistance, where the curvature of the IV relation changes to convex. The transition to Ohmic behavior comes about as the braided river floods when it cannot absorb a higher level of flow. We propose that these self-organized morphologies of flux flow down a flux gradient govern the various plastic flow behaviors, including nonlinear IV characteristics, observed in type II superconductors with random pinning.

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Section: The Transport Current: Shifting the Boundary Conditionsmentioning

confidence: 88%

Simple model for plastic dynamics of a disordered flux-line lattice

2001

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“…1, where a Josephson junction with the energy E J1 together with a superconducting loop forms an RF SQUID enclosing a bias flux Φ b . Magnetic field applied along the plane of the tunnel barrier, inside the junction [26], results in a flux of Φ 1 = 2e ϕ 1 . At Φ b = 0, the effective Josephson energy of the circuit can be…”

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confidence: 99%

Josephson Junction Microscope for Low-Frequency Fluctuators

Tian

Simmonds

2007

Phys. Rev. Lett.

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The high-Q harmonic oscillator mode of a Josephson junction can be used as a novel probe of spurious two-level systems (TLSs) inside the amorphous oxide tunnel barriers of the junction. In particular, we show that spectroscopic transmission measurements of the junction resonator mode can reveal how the coupling magnitude between the junction and the TLSs varies with an external magnetic field applied in the plane of the tunnel barrier. The proposed experiments offer the possibility of clearly resolving the underlying coupling mechanism for these spurious TLSs, an important decoherence source limiting the quality of superconducting quantum devices.Superconducting quantum circuits have been intensively tested in various regimes in the past few years, from superconducting qubits demonstrating long coherence times, to superconducting transmission line cavities coherently coupled to a Single Cooper Pair box [1,2,3,4,5,6]. Such circuits are extremely sensitive to very small quanta and defect states, and hence have the ability to detect individual microwave photons, charged quasiparticles, as well as spurious TLSs within or near Josephson junction tunnel barriers [7,8,9,10,11]. In recent experiments [10,11], TLSs were identified through spectroscopic measurements of a superconducting phase qubit appearing as 'gaps" or "splittings" in the energy spectrum.The TLS defects can be an unwanted source of decoherence for superconducting quantum bits. The lowfrequency noise, which has been shown to be a serious source of decoherence for superconducting qubits [10,11,12,13,14], is very probably induced by such amorphous fluctuators inside or near Josephson junctions [15,16]. Understanding the origin of these spurious TLSs, their coherent quantum behavior, and their connection to ubiquitous 1/f noise is hence a challenge that will be crucial to the future of superconducting quantum devices. The behavior of a distribution of these TLSs were studied theoretically in [17,18,19]. Recently, it was proposed that TLSs can be viewed as qubits themselves [20], given their relatively long coherence times. However, the microscopic origin and the coupling mechanism between the TLSs and the junction remains unresolved. Generally considered to be connected to the amorphous nature of the tunnel barrier [21], movement of unrestrained atoms or charges may lead to a number of possible coupling mechanisms. As originally proposed in Ref. [11], fluctuations of the TLS could lead to variations of the junction critical current. Another possibility, requires that the TLSs have fluctuating dipole moments which couple to the electric field found within the junction tunnel barrier [10].Here, we present a scheme that can resolve a variety of properties of the TLSs and distinguish between these two suggested coupling mechanisms through the use of an applied magnetic field. Consider a Josephson junction resonator operating as a high-Q, nonlinear cavity mode [22], coupling to a TLS through its canonical phase (or momentum) operator. This forms a cavity QED sys...

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“…Thus, C can be obtained from the typical charging energy for an oxide barrier tunnel junction E c S ∼ 1 μeV × μm 2 and L can be estimated as μ 0 l 2 /d, where l is the field penetration depth 16 . The actual value of l for a Nb film is strongly dependent on its thickness, degree of disorder and it is also influenced by the properties of the nonsuperconducting substrate on which it is deposited 17 .…”

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confidence: 99%

Entangled Andreev pairs and collective excitations in nanoscale superconductors

et al. 2007

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Nanoscale superconductors connected to normal metallic electrodes provide a potential source of entangled electron pairs 1-5 . Such states would arise from the splitting of Cooper pairs in the superconductor into two electrons with opposite spins, which then tunnel into different leads by means of a process known as crossed Andreev reflection (refs 6-8). In an actual system, the detection of these processes is hindered by the elastic transmission of individual electrons between the leads, which yields an opposite contribution to the non-local conductance. Here we demonstrate that low-energy collective excitations, which appear in superconducting structures of reduced dimensionality 9 , can have a significant influence on the transport properties of this type of hybrid nanostructure. When an electron tunnels into the superconductor it can excite such low-energy excitations that alter the balance between the different electronic processes, leading to a dominance of one over the other depending on the spatial symmetry of these excitations. These findings help to clarify some intriguing experimental results and provide future strategies for the detection of entangled electron pairs in solid-state devices for quantum computation.A generic set-up for the study of non-local transport through a superconductor is shown in Fig. 1a. It represents a superconducting region attached to three normal electrodes. Two of the leads (labelled 1 and 2 in Fig. 1a) are used to inject a current while the voltage drop is measured on the third one. The two basic microscopic processes contributing to the non-local conductance are illustrated in Fig. 1c,d. In the case of elastic cotunnelling (EC) processes, the injected electron tunnels elastically into the third lead, whereas in the case of crossed Andreev reflection (CAR) processes, it combines with an electron emerging from the third lead to form a Cooper pair in the superconductors. The probability of these processes decays exponentially on the scale of the superconducting coherence length, ξ, which can range between 10 and 100 nm for typical superconductors used in experiments 10,11 . On the other hand, the two processes yield opposite contributions to the non-local conductance (conventionally the CAR contribution is taken as positive) and, as demonstrated by previous theoretical studies 12-14 , tend to cancel each other in the case of Bardeen-Cooper-Schrieffer (BCS) superconductors weakly coupled to non-magnetic leads. Surprisingly, recent experiments by Russo et al. 11 have shown that even in this case the subgap non-local conductance can be appreciably large, exhibiting an intriguing behaviour in which either process can dominate depending on the energy of the injected electrons. This behaviour cannot be accounted for by the existing non-interacting theories.The importance of interactions in breaking the balance between EC and CAR processes can be understood by considering the case where the superconducting region is sufficiently small and can be characterized by a finite charging ...

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Foundations of Applied Superconductivity

Cited by 300 publications

References 0 publications

Simple model for plastic dynamics of a disordered flux-line lattice

Simple model for plastic dynamics of a disordered flux-line lattice

Josephson Junction Microscope for Low-Frequency Fluctuators

Entangled Andreev pairs and collective excitations in nanoscale superconductors

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