D. V. Shantsev

Dendritic magnetic instability in superconducting MgB2 films

Magneto-optical imaging reveal that below 10 K the penetration of magnetic flux in MgB2 films is dominated by dendritic structures abruptly formed in response to an applied field. The dendrites show a temperature-dependent morphology ranging from quasi-1D at 4 K to large tree-like structures near 10 K. This behaviour is responsible for the anomalous noise found in magnetization curves, and strongly suppresses the apparent critical current. The instability is of thermo-magnetic origin, as supported by our simulations of vortex dynamics reproducing the variety of dendritic flux patterns.

Dendritic and uniform flux jumps in superconducting films

Recent theoretical analysis of spatially-nonuniform modes of the thermomagnetic instability in superconductors [Phys. Rev. B 70, 224502 (2004)] is generalized to the case of a thin film in a perpendicular applied field. We solve the thermal diffusion and Maxwell equations taking into account nonlocal electrodynamics in the film and its thermal coupling to the substrate. The instability is found to develop in a nonuniform, fingering pattern if the background electric field, E, is high and the heat transfer coefficient to the substrate, h0, is small. Otherwise, the instability develops in a uniform manner. We find the threshold magnetic field, H_fing(E,h0), the characteristic finger width, and the instability build-up time. Thin films are found to be much more unstable than bulk superconductors, and have a stronger tendency for formation of dendritic pattern.

Onset of dendritic flux avalanches in superconducting films

We report a detailed comparison of experimental data and theoretical predictions for the dendritic flux instability, believed to be a generic behavior of type-II superconducting films. It is shown that a thermomagnetic model published very recently [Phys. Rev. B 73, 014512 (2006)10.1103/PhysRevB.73.014512] gives an excellent quantitative description of key features like the stability onset (first dendrite appearance) magnetic field, and how the onset field depends on both temperature and sample size. The measurements were made using magneto-optical imaging on a series of different strip-shaped samples of MgB2. Excellent agreement is also obtained by reanalyzing data previously published for Nb.

Magnetostrictive behaviour of thin superconducting disks

Flux-pinning-induced stress and strain distributions in a thin disk superconductor in a perpendicular magnetic field are analysed. We calculate the body forces, solve the magneto-elastic problem and derive formulae for all stress and strain components, including the magnetostriction ΔR/R. The flux and current density profiles in the disk are assumed to follow the Bean model. During a cycle of the applied field the maximum tensile stress is found to occur approximately midway between the maximum field and the remanent state. An effective relationship between this overall maximum stress and the peak field is found.

Non-Gaussian low-frequency noise as a source of qubit decoherence.

With the growing efforts in isolating solid-state qubits from external decoherence sources, the material-inherent sources of noise start to play crucial role. One representative example is electron traps in the device material or substrate. Electrons can tunnel or hop between a charged and an empty trap, or between a trap and a gate electrode. A single trap typically produces telegraph noise and can hence be modeled as a bistable fluctuator. Since the distribution of hopping rates is exponentially broad, many traps produce flicker-noise with spectrum close to 1/f. Here we develop a theory of decoherence of a qubit in the environment consisting of two-state fluctuators, which experience transitions between their states induced by interaction with thermal bath. Due to interaction with the qubit the fluctuators produce 1/f-noise in the qubits eigenfrequency. We calculate the results of qubit manipulations - free induction and echo signals - in such environment. The main problem is that in many important cases the relevant random process is both non-Markovian and non-Gaussian. Consequently the results in general cannot be represented by pair correlation function of the qubit eigenfrequency fluctuations. Our calculations are based on analysis of the density matrix of the qubit using methods developed for stochastic differential equations. The proper generating functional is then averaged over different fluctuators using the so-called Holtsmark procedure. The analytical results are compared with simulations allowing checking accuracy of the averaging procedure and evaluating mesoscopic fluctuations. The results allow understanding some observed features of the echo decay in Josephson qubits.We study decoherence in a qubit with the distance between the two levels affected by random flips of bistable fluctuators. For the case of a single fluctuator we evaluate explicitly an exact expression for the phase-memory decay in the echo experiment with a resonant ac excitation. The echo signal as a function of time shows a sequence of plateaus. The position and the height of the plateaus can be used to extract the fluctuator switching rate gamma and its coupling strength v. At small times the logarithm of the echo signal is proportional to t3. The plateaus disappear when the decoherence is induced by many fluctuators. In this case the echo signal depends on the distribution of the fluctuators parameters. According to our analysis, the results significantly deviate from those obtained in the Gaussian model as soon as v greater than or approximately equal gamma.

Magneto-optical imaging

Preface. Group photo. List of contributors. Overview. Paving the way for the success of magneto-optics H.-U. Habermeier. Comparison of magneto-optical imaging with other local magnetic probes S.J. Bending, et al. MOI of superconductors. Magneto-optical investigation of superconducting materials A.A. Polyanskii, et al. Quantitative magneto-optics: Flux, current and electrical field imaging Ch. Jooss, et al. Magneto-optical imaging of Josephson vortices in layered superconductors V.K. Vlasko-Vlasov, et al. Magneto-optic investigation of magnetic flux penetration on a nanosecond timescale B. Biehler, et al. Magneto-optical imaging of superconducting vortices T.H. Johansen, et al. Magneto-optical imaging of pattern formation in the vortex landscape R.J. Wijngaarden, et al. MO-Imaging of granular and structured high temperature superconductors M.R. Koblischka, A. Koblischka-Veneva. First order transition of the vortex lattice in disordered Bi-2212 crystals K. van der Beek, et al. Magneto-optical measurements of the lifetime spectrum of transient vortex states in BSCCO B. Kalisky, et al. Magneto-optical imaging of crossing-lattices state in Bi2Sr2CaCu2O8+y T. Tamegai, et al. Strong 3D-correlation in the vortex system of Bi2212:Pb L.S. Uspenskaya, et al. Magneto-optical investigation of the vortex order-disorder phase transition in BSCCO B. Kalisky, et al. Magneto-optical studies of chemical inhomogeneities in Bi2212 single crystals N. Chikumoto, et al. Optimization of Bi2223 tape fabrication procedure with the help of magneto-optical imaging M. Roussel, et al. Magneto-optical imaging of cracking in high temperature superconducting films and tapes under tensile strain I.B. Rutel,et al. Magneto-optical imaging of magnetic screening in superconducting wires A.V. Pan, et al. Magneto-optical imaging of small angle grain boundaries on different bi-crystalline substrates K. Guth, et al. Low-angle grain boundaries of YBCO in external magnetic fields J. Albrecht. Electrodynamics of superconducting YBCO films with confined correlated nanodefects E. Mezzetti, et al. Magneto-optical studies of YBCO thick films in the critical state Z.X. Ye, et al. Modifying the current distribution of grain boundaries in YBCO films E. Brinkmeier, et al. Magneto-optical imaging of vortex penetration in patterned YBCO thin film near Tc S. Flament, et al. Proton irradiation induced effects on YBCO films analysed by magneto-optics L. Gozzelino, et al. Flux dynamics in current-carrying superconductors A.V. Bobyl, et al. Effect of substrate orientation and hydrogen impurities on flux penetration in Nb thin films M.S. Welling, et al. Flux jumps in magnesium diboride D.V. Shantsev, et al. Dendritic flux instabilities in Nb3Sn and NbN thin films I. Rudnev, et al. Thermo-magnetic instability as limiting mechanism for electrical current density in MgB2 thin films F. Laviano, et al. Surface penetration of high-Tc superconductors for MO imaging A. Koblischka-Veneva, M.R. Koblischka. Superconductor magneto-optics and theory E.H. Brandt. Inversion of the Biot-Savart law: An approach based on discrete sine and cosine transforms M. Dirickx, et al. MOI of magnetic materials. Domain structure and magnetic anisotrophy in Ga1-xMnxAs U. Welp, et al. Forensic imaging of magnetic tapes using magnetic garnet indicator films C. Krafft, et al. Manipulating magnetic particles

Dendritic flux patterns in MgB2 films

Magneto-optical studies of a c-oriented MgB2 film with a critical current density of 107?A?cm-2 demonstrate a breakdown of the critical state at temperatures below 10?K. Instead of conventional uniform and gradual flux penetration in an applied magnetic field, we observe an abrupt invasion of complex dendritic structures. When the applied field subsequently decreases, similar dendritic structures of the return flux penetrate the film. The static and dynamic properties of the dendrites are discussed.

Avalanche-driven fractal flux distributions in NbN superconducting films

Flux distributions in thin superconducting NbN films placed in a perpendicular magnetic field have been studied using magneto-optical imaging. Below 5.5K the flux penetrates in the form of abrupt avalanches resulting in dendritic structures. Magnetization curves in this regime exhibit extremely noisy behavior. Stability is restored both above a threshold temperature T* and applied field H*, where H* is smaller for increasing field than during descent. The dendrite size and morphology are strongly T dependent, and fractal analysis of the first dendrites entering into a virgin film shows that dendrites formed at higher T have larger fractal dimension.

Finger patterns produced by thermomagnetic instability in superconductors

A linear analysis of thermal diffusion and Maxwell equations is applied to study the thermomagnetic instability in a type-II superconducting slab. It is shown that the instability can lead to formation of spatially nonuniform distributions of magnetic field and temperature. The distributions acquire a finger structure with fingers perpendicular to the screening current direction. We derive the criterion for the instability, and estimate its build-up time and characteristic finger width. The fingering instability emerges when the background electric field is larger than a threshold field,

THIN SUPERCONDUCTING DISK WITH B-DEPENDENT JC : FLUX AND CURRENT DISTRIBUTIONS

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