arXiv subjects

T. H. Johansen

Publications and source records attributed to T. H. Johansen.

At least 37 records · Page 2Linked to original sources

arXiv2013

Nonlocal electrodynamics of normal and superconducting films

Electrically conducting films in a time-varying transverse applied magnetic field are considered. Their behavior is strongly influenced by the self-field of the induced currents, making the electrodynamics nonlocal, and consequently difficult to analyze both numerically and analytically. We present a formalism which allows many phenomena related to superconducting and Ohmic films to be modelled and analyzed. The formalism is based on the Maxwell equations, and a material current-voltage characteristics, linear for normal metals, and nonlinear for superconductors, plus a careful account of the boundary conditions. For Ohmic films, we consider the response to a delta function source-field turned on instantly. As one of few problems in nonlocal electrodynamics, this has an analytical solution, which we obtain, in both Fourier and real space. Next, the dynamical behaviour of a square superconductor film during ramping up of the field, and subsequently returning to zero, is treated numerically. Then, this remanent state is used as initial condition for triggering thermomagnetic avalanches. The avalanches tend to invade the central part where the density of trapped flux is largest, forming dendritic patterns in excellent agreement with magneto-optical images. Detailed profiles of current and flux density are presented and discussed. Finally, the formalism is extended to multiply connected samples, and numerical results for a patterned superconducting film, a ring with a square lattice of antidots, are presented and discussed.

cond-mat.supr-con↗

arXiv2013

Modelling nonlocal electrodynamics in superconducting films: The case of a concave corner

We consider magnetic flux penetration in a superconducting film with a concave corner. Unlike convex corners, where the current flow pattern is easily constructed from Bean's critical state model, the current flow pattern at a concave corner is highly nontrivial. To address the problem, we do a numerical flux creep simulation, where particular attention is paid to efficient handling of the non-local electrodynamics, characteristic of superconducting films in the transverse geometry. We find that the current stream lines at the concave corner are close to circular, but the small deviation from exact circles ensure that the electric field is finite and continuous. Yet, the electric field is, as expected, very high at the concave corner. At low fields, the critical state penetration is deeper from the concave corner than from the straight edges, which is a consequence of the electrodynamic non-locality. A magneto-optical experiment on a YBCO displays an almost perfect match with the magnetic flux distribution from the simulation, hence verifying the necessity of including electrodynamic non-locality in the modelling of superconducting thin films.

cond-mat.supr-con↗

arXiv2013

Flux Penetration in Superconducting Strip with Edge-Indentation

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, $Δ$, can be significantly ($\sim$ 50%) larger than the indentation radius $r_0$, in contrast to a bulk supercondutor in the critical state where $Δ=r_0$. It is also shown that the flux creep tends to smoothen the flux front, i.e. reduce $Δ$. 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.

cond-mat.supr-con↗

arXiv2013

Dynamics and morphology of dendritic flux avalanches in superconducting films

We develop a fast numerical procedure for analysis of nonlinear and nonlocal electrodynamics of type-II superconducting films in transverse magnetic fields taking into account realistic boundary conditions. Using this procedure we explore stability of such films with respect to dendritic flux avalanches. The calculated flux patterns are very similar to experimental magneto-optical images of MgB2 and other superconductors, where the avalanche size and morphology change dramatically with temperature. We also find a threshold magnetic field, which agrees with both experiments and linear stability analysis. The simulations predict the temperature rise during an avalanche, where for a sub-microsecond time T ~ 1.5 Tc, and a precursor stage with large thermal fluctuations.

cond-mat.supr-con↗

arXiv2013

The thermomagnetic instability in superconducting films with adjacent metal layer

Dendritic flux avalanches is a frequently encountered consequence of the thermomagnetic instability in type-II superconducting films. The avalanches, potentially harmful for superconductor-based devices, can be suppressed by an adjacent normal metal layer, even when the two layers are not in thermal contact. The suppression of the avalanches in this case is due to so-called magnetic braking, caused by eddy currents generated in the metal layer by propagating magnetic flux. We develop a theory of magnetic braking by analyzing coupled electrodynamics and heat flow in a superconductor-normal metal bilayer. The equations are solved by linearization and by numerical simulation of the avalanche dynamics. We find that in an uncoated superconductor, even a uniform thermomagnetic instability can develop into a dendritic flux avalanche. The mechanism is that a small non-uniformity caused by the electromagnetic non-locality induces a flux-flow hot spot at a random position. The hot spot quickly develops into a finger, which at high speeds penetrates into the superconductor, forming a branching structure. Magnetic braking slows the avalanches, and if the normal metal conductivity is sufficiently high, it can suppress the formation of the dendritic structure. During avalanches, the braking by the normal metal layer prevents the temperature from exceeding the transition temperature of the superconductor. Analytical criteria for the instability threshold are developed using the linear stability analysis. The criteria are found to match quantitatively the instability onsets obtained in simulation.

cond-mat.supr-con↗

arXiv2013

Enhancement of pinning properties of superconducting thin films by graded pinning landscapes

A graded distribution of pinning centers (antidots) in superconducting MoGe thin films has been investigated by magnetization and magneto-optical imaging. The pinning landscape has maximum density at the border, decreasing progressively towards the center. At high temperatures and low fields, where this landscape mimics the vortex distribution predicted by the Bean model, an increase of the critical current is observed. At low temperatures and fields, the superconducting performance of the non-uniform sample is also improved due to suppression of thermomagnetic avalanches. These findings emphasize the relevance of non-uniform pinning landscapes, so far experimentally unexplored, on the enhancement of pinning efficiency.

cond-mat.supr-con↗

arXiv2012

Diversity of flux avalanche patterns in superconducting films

The variety of morphologies in flux patterns created by thermomagnetic dendritic avalanches in type-II superconducting films is investigated using numerical simulations. The avalanches are triggered by introducing a hot spot at the edge of a strip-shaped sample, which is initially prepared in a partially penetrated Bean critical state by slowly ramping the transversely applied magnetic field. The simulation scheme is based on a model accounting for the nonlinear and nonlocal electrodynamics of superconductors in the transverse geometry. By systematically varying the parameters representing the Joule heating, heat conduction in the film, and heat transfer to the substrate, a wide variety of avalanche patterns is formed, and quantitative characterization of areal extension, branch width etc. is made. The results show that branching is suppressed by the lateral heat diffusion, while large Joule heating gives many branches, and heat removal into the substrate limits the areal size. The morphology shows significant dependence also on the initial flux penetration depth.

cond-mat.supr-con↗

arXiv2012

Suppression of magnetic flux avalanches and recovery of the critical state in superconducting NbN films

Thermo-magnetic instability (TMI) in superconductors is known to destroy the critical state via magnetic flux avalanches, and hence it deteriorates the ability of the superconductors to shield external magnetic field. In this work, we quantify to what extent the shielding current is affected by TMI. We recover the critical state in one half of a thermo-magnetically unstable NbN film by coating it with a thin layer of Cu. Suppression of the instability in the metal coated part is confirmed by the results of a direct real time magneto-optical imaging. A pattern of discontinuity lines in the observed flux distribution indicates that only one quarter of the shielding current flows through the whole sample, including the unstable uncoated part, while three quarters flow in loops within the stable Cu-coated part.

cond-mat.supr-con↗

arXiv2012

Dendritic flux avalanches in rectangular superconducting films -- numerical simulations

Dendritic flux avalanches is a frequently encountered instability in the vortex matter of type II superconducting films at low temperatures. Previously, linear stability analysis has shown that such avalanches should be nucleated where the flux penetration is deepest. To check this prediction we do numerical simulations on a superconducting rectangle. We find that at low substrate temperature the first avalanches appear exactly in the middle of the long edges, in agreement with the predictions. At higher substrate temperature, where there are no clear predictions from the theory, we find that the location of the first avalanche is decided by fluctuations due to the randomly distributed disorder.

cond-mat.supr-con↗

arXiv2012

Mechanism for flux guidance by micrometric antidot arrays in superconducting films

A study of magnetic flux penetration in a superconducting film patterned with arrays of micron sized antidots (microholes) is reported. Magneto-optical imaging (MOI) of a YBCO film shaped as a long strip with perpendicular antidot arrays revealed both strong guidance of flux, and at the same time large perturbations of the overall flux penetration and flow of current. These results are compared with a numerical flux creep simulation of a thin superconductor with the same antidot pattern. To perform calculations on such a complex geometry, an efficient numerical scheme for handling the boundary conditions of the antidots and the nonlocal electrodynamics was developed. The simulations reproduce essentially all features of the MOI results. In addition, the numerical results give insight into all other key quantities, e.g., the electrical field, which becomes extremely large in the narrow channels connecting the antidots.

cond-mat.supr-con↗

arXiv2011

Anomalous flux avalanche morphology in a a-MoGe superconducting film with a square antidot lattice - experiment and simulation

We have employed magneto-optical imaging to visualize the occurrence of flux avalanches in a superconducting film of a-MoGe. The specimen was decorated with square antidots arranged in a square lattice. We observed avalanches with the anomalous habit of forming trees where the trunk is perpendicular to the main axis of the square lattice, whereas the branches form angles of 45 degrees. The overall features of the avalanches, and in particular the 45 degree direction of the branches, were confirmed by numerical simulations.

cond-mat.supr-con↗

arXiv2011

Effect of hydrogen plasma treatment and oxygen deficiency on conducting properties of In$_2$O$_3$:Sn thin films

Electrical conductivity, Hall effect and magnetoresistance of In$_2$O$_3$:Sn thin films deposited on a glass substrates at different temperatures and oxygen pressures, as well as the films treated in a hydrogen plasma, have been investigated in the temperature range 1.5-300 K. The observed temperature dependences of resistivity were typical for metallic transport of electrons except temperature dependence of resistivity of the In$_2$O$_3$:Sn film deposited in the oxygen deficient atmosphere. The electron concentration and mobility for the film deposited at 230$^\circ$C was larger than that for the film deposited nominally at room temperature. Short (5 minutes) treatment of the films in hydrogen plasma leads to the enhancement of electrical conductivity while longer (30 minute) treatment has the opposite effect. The electrical measurements were accompanied by AFM and SEM studies of structural properties, as well as by XPS analysis. Basic on structural and electrical measurements we conclude the reduction process initiated by the hydrogen plasma provides essential modification of the ITO films surface. At the same time, electrical properties of the remaining (located beneath the surface layer) parts of the ITO films remain mostly unchangeable. XPS analysis shows that grown in situ oxygen deficient ITO films have enhanced DOS between the Fermi level and the valence band edge. The extra localized states behave as acceptors leading to a compensation of $n$-type ITO. That can explain lower $n$-type conductivity in this material crossing over to a Mott-type hopping.

cond-mat.mtrl-sci↗

arXiv2010

Very strong intrinsic supercurrent carrying ability and vortex avalanches in (Ba,K)Fe2As2 superconducting single crystals

We report that single crystals of (Ba,K)Fe2As2 with Tc = 32 K have a pinning potential, U0, as high as 10^4 K, with U0 showing very little field depend-ence. In addition, the (Ba,K)Fe2As2 single crystals become isotropic at low temperatures and high magnetic fields, resulting in a very rigid vortex lattice, even in fields very close to Hc2. The rigid vortices in the two dimensional (Ba,K)Fe2As2 distinguish this compound from 2D high Tc cuprate superconductors with 2D vortices, and make it being capable of cearrying very high critical current.Flux jumping due to high Jc was also observed in large samples at low temperatures.

cond-mat.supr-con↗

arXiv2009

Two-stage dissipation in a superconducting microbridge: Experiment and modeling

Using fluorescent microthermal imaging we have investigated the origin of "two-step" behavior in I-V curves for a current-carrying YBa_2Cu_3O_x superconducting bridge. High resolution temperature maps reveal that as the applied current increases the first step in the voltage corresponds to local dissipation (hot spot), whereas the second step is associated with onset of global dissipation throughout the entire bridge. A quantitative explanation of the experimental results is provided by a simple model for an inhomogeneous superconductor, assuming that the hot spot nucleates at a location with slightly depressed superconducting properties.

cond-mat.supr-con↗

arXiv2007

Flux saturation number of superconducting rings

The distributions of electrical current and magnetic field in a thin-film superconductor ring is calculated by solving the London equation. The maximum amount of flux trapped by the hole, the fluxoid saturation number, is obtained by limiting the current density by the depairing current. The results are compare it with similar results derived for the bulk case of a long hollow cylinder [Nordborg & Vinokur, Phys. Rev. B 62, 12408 (2000)]. In the limit of small holes our result reduces to the Pearl solution for an isolated vortex in a thin film. For large hole radius, the ratio between saturation numbers in bulk and film superconductors is proportional to the square root of the hole size.

cond-mat.supr-con↗

arXiv2007

Flux Distribution in Superconducting Films with Holes

Flux penetration into type-II superconducing films is simulated for transverse applied magnetic field and flux creep dynamics. The films contain macroscopic, non-conducting holes and we suggest a new method to introduce the holes in the simulation formalism. The method implies reconstruction of the magnetic field change inside the hole. We find that in the region between the hole and the edge the current density is compressed so that the flux density is slightly reduced, but the traffic of flux is significantly increased. The results are in good agreement with magneto-optical studies of flux distributions in YBCO films.

cond-mat.supr-con↗

arXiv2007

Interaction between superconducting vortices and Bloch wall in ferrite garnet film

Interaction between a Bloch wall in a ferrite-garnet film and a vortex in a superconductor is analyzed in the London approximation. Equilibrium distribution of vortices formed around the Bloch wall is calculated. The results agree quantitatively with magneto-optical experiment where an in-plane magnetized ferrite-garnet film placed on top of NbSe2 superconductor allows observation of individual vortices. In particular, our model can reproduce a counter-intuitive attraction observed between vortices and a Bloch wall having the opposite polarity. It is explained by magnetic charges appearing due to discontinuity of the in-plane magnetization across the wall.

cond-mat.supr-con↗

arXiv2007

Reentrant stability of superconducting films

We propose a mechanism responsible for the abrupt vanishing of the dendritic flux instability found in many superconducting films when an increasing magnetic field is applied. The onset of flux avalanches and the subsequent reentrance of stability in NbN films was investigated using magneto-optical imaging, and the threshold fields were measured as functions of critical current density, $j_c$. The results are explained with excellent quantitative agreement by a thermomagnetic model published recently, Phys. Rev. B73, 014512 (2006), showing that the reentrant stability is a direct consequence of a monotonously decreasing $j_c$ versus field.

cond-mat.supr-con↗