arXiv subjects

D. Koelle

Publications and source records attributed to D. Koelle.

At least 55 records · Page 3Linked to original sources

arXiv2012

Electrical Breakdown in a V2O3 device at the Insulator to Metal Transition

We have measured the electrical properties of a V2O3 thin film micro bridge at the insulator metal transition (IMT). Discontinuous jumps to lower voltages in the current voltage characteristic (IV) followed by an approximately constant voltage progression for high currents indicate an electrical breakdown of the device. In addition, the IV curve shows hysteresis and a training effect, i.e. the subsequent IV loops are different from the first IV loop after thermal cycling. Low temperature scanning electron microscopy (LTSEM) reveals that the electrical breakdown over the whole device is caused by the formation of electro-thermal domains (ETDs), i.e. the current and temperature redistribution in the device. On the contrary, at the nanoscale, the electrical breakdown causes the IMT of individual domains. In a numerical model we considered these domains as a network of resistors and we were able to reproduce the electro-thermal breakdown as well as the hysteresis and the training effect in the IVs.

cond-mat.mtrl-sci↗

arXiv2012

The emergence of atomic semifluxons in optical Josephson junctions

We propose to create pairs of semifluxons starting from a flat-phase state in long, optical 0-pi-0 Josephson junctions formed with internal electronic states of atomic Bose-Einstein condensates. In this optical system, we can dynamically tune the length of the pi-junction, the detuning of the optical transition, or the strength of the laser-coupling, to induce transitions from the flat-phase state to such a semifluxon-pair state. Similarly as in superconducting 0-pi-0 junctions, there are two, energetically degenerate semifluxon-pair states. A linear mean-field model with two internal electronic states explains this degeneracy and shows the distinct static field configuration in a phase-diagram of the junction parameters. This optical system offers the possibility to dynamically create a coherent superposition of the distinct semifluxon-pair states and observe macroscopic quantum oscillation.

cond-mat.quant-gas↗

arXiv2012

Magnetic field dependence of the critical current in YBa_2Cu_3O_{7-δ}/Au/Nb ramp-zigzag Josephson junctions

We study the critical current I_c dependence on applied magnetic field H for multifacet YBa_2Cu_3O_{7-δ}-Au-Nb ramp-type zigzag Josephson junctions. For many experiments one would like to apply a homogeneous field in the junction plane. However, even tiny misalignments can cause drastic deviations from homogeneity. We show this explicitly by measuring and analyzing I_c vs. H for an 8 facet junction, forming an array of 4\times(0-π)-segments. The ramp angle is θ_r=8^\circ. The facet width is 10\,\mum. H is applied under different angles θrelative to the substrate plane and different angles ϕrelative to the in-plane orientation of the zigzags. We find that a homogeneous flux distribution is only achieved for an angle θ_h\approx 1^\circ - 2^\circ and that even a small misalignment \sim 0.1^\circ relative to θ_h can cause a substantial inhomogeneity of the flux density inside the junction, drastically altering its I_c vs. H interference pattern. We also show, that there is a dead angle θ^*_d relative to θ_h of similar magnitude, where the average flux density completely vanishes.

cond-mat.supr-con↗

arXiv2012

Sub-μm Josephson Junctions for Superconducting Quantum Devices

For high-performance superconducting quantum devices based on Josephson junctions (JJs) decreasing lateral sizes is of great importance. Fabrication of sub-μm JJs is challenging due to non-flat surfaces with step heights of up to several 100 nm generated during the fabrication process. We have refined a fabrication process with significantly decreased film thicknesses, resulting in almost flat surfaces at intermediate steps during the JJ definition. In combination with a mix-&-match process, combining electron-beam lithography (EBL) and conventional photolithography, we can fabricate JJs with lateral dimensions down to 0.023 μm^2. We propose this refined process as an alternative to the commonly used chemical-mechanical polishing (CMP) procedure. We present transport measurements of JJs at 4.2 K that yield critical-current densities in the range from 50 to 10^4 A/cm^2. Our JJ process yields excellent quality parameters, Rsg/Rn up to ~50 and Vgap up to 2.81 mV, and also allows the fabrication of high-quality sub-μm wide long JJs (LJJs) for the study of Josephson vortex behavior. The developed technique can also be used for similar multilayer processes and is very promising for fabricating sub-μm JJs for quantum devices such as SQUIDs, qubits and SIS mixers.

cond-mat.supr-con↗

arXiv2012

Josephson junction with magnetic-field tunable current-phase relation

We consider a 0-$π$ Josephson junction consisting of asymmetric 0 and $π$ regions of different lengths $L_0$ and $L_π$ having different critical current densities $j_{c,0}$ and $j_{c,π}$. If both segments are rather short, the whole junction can be described by an \emph{effective} current-phase relation for the spatially averaged phase $ψ$, which includes the usual term $\propto\sin(ψ)$, a \emph{negative} second harmonic term $\propto\sin(2ψ)$ as well as the unusual term $\propto H \cosψ$ tunable by magnetic field $H$. Thus one obtains an electronically tunable current-phase relation. At H=0 this corresponds to the $φ$ Josephson junction.

cond-mat.supr-con↗

arXiv2012

Experimental evidence of a ϕ Josephson junction

We demonstrate experimentally the existence of Josephson junctions having a doubly degenerate ground state with an average Josephson phase ψ=\pmϕ. The value of ϕ can be chosen by design in the interval 0<ϕ<π. The junctions used in our experiments are fabricated as 0-π Josephson junctions of moderate normalized length with asymmetric 0 and π regions. We show that (a) these ϕ Josephson junctions have two critical currents, corresponding to the escape of the phase ψ from -ϕ and +ϕ states; (b) the phase ψ can be set to a particular state by tuning an external magnetic field or (c) by using a proper bias current sweep sequence. The experimental observations are in agreement with previous theoretical predictions.

cond-mat.supr-con↗

arXiv2012

Direct current superconducting quantum interferometers with asymmetric shunt resistors

We have investigated asymmetrically shunted Nb/Al-AlO$_x$/Nb direct current (dc) superconducting quantum interference devices (SQUIDs). While keeping the total resistance $R$ identical to a comparable symmetric SQUID with $R^{-1} = R_1^{-1} + R_2^{-1}$, we shunted only one of the two Josephson junctions with $R = R_{1,2}/2$. Simulations predict that the optimum energy resolution $ε$ and thus also the noise performance of such an asymmetric SQUID can be 3--4 times better than that of its symmetric counterpart. Experiments at a temperature of 4.2\,K yielded $ε\approx 32\,\hbar$ for an asymmetric SQUID with an inductance of $22\,\rm{pH}$. For a comparable symmetric device $ε= 110\,\hbar$ was achieved, confirming our simulation results.

cond-mat.supr-con↗

arXiv2012

Excess flux stability fingerprints in the I_c(B)-T_c(B) phase boundary of superconducting thin films with quasiperiodic microtopology

We experimentally investigate the magnetic field $B$ dependence of the critical current I_c and the transition temperature T_c, i.e. the I_c(B)-T_c(B) phase boundary, of superconducting niobium thin films patterned with periodic and quasiperiodic antidot arrays on the submicron scale. For this purpose we monitor current-voltage characteristics at different values of B and T. We investigate samples with antidots positioned at the vertices of two different tilings with quasiperiodic symmetry, namely the Shield Tiling and the Tuebingen Triangle Tiling. For reference we investigate a sample with a triangular antidot lattice. We find modulations of the phase boundary for both quasiperiodic tilings, which were predicted by numerical simulations but not observed in experiments yet. The particularity of these commensurability effects is that they correspond to excess flux densities, which are slightly higher than the matching flux. The observed matching effects can be explained by quasiperiodic caging of interstitial vortices and/or the formation of symmetry induced giant vortices.

cond-mat.supr-con↗

arXiv2012

Unusual linewidth dependence of coherent THz emission measured from intrinsic Josephson junction stacks in the hot-spot regime

We report on measurements of the linewidth Δf of THz radiation emitted from intrinsic Josephson junction stacks, using a Nb/AlN/NbN integrated receiver for detection. Previous resolution limited measurements indicated that Δf may be below 1 GHz - much smaller than expected from a purely cavity-induced synchronization. While at low bias we found Δf to be not smaller than ? 500 MHz, at high bias, where a hotspot coexists with regions which are still superconducting, Δf turned out to be as narrow as 23 MHz. We attribute this to the hotspot acting as a synchronizing element. Δf decreases with increasing bath temperature, a behavior reminiscent of motional narrowing in NMR or ESR, but hard to explain in standard electrodynamic models of Josephson junctions.

cond-mat.supr-con↗

arXiv2012

Localization of superconductivity in superconductor-electromagnet hybrids

We investigate the nucleation of superconductivity in a superconducting Al strip under the influence of the magnetic field generated by a current-carrying Nb wire, perpendicularly oriented and located underneath the strip. The inhomogeneous magnetic field, induced by the Nb wire, produces a spatial modulation of the critical temperature T_c, leading to a controllable localization of the superconducting order parameter (OP) wave function. We demonstrate that close to the phase boundary T_c(B_ext) the localized OP solution can be displaced reversibly by either applying an external perpendicular magnetic field B_ext or by changing the amplitude of the inhomogeneous field.

cond-mat.supr-con↗

arXiv2012

Phase-sensitive evidence for dx2-y2-pairing symmetry in the parent-structure high-Tc cuprate superconductor Sr1-xLaxCuO2

Even after 25 years of research the pairing mechanism and - at least for electron doped compounds - also the order parameter symmetry of the high transition temperature (high-Tc) cuprate superconductors is still under debate. One of the reasons is the complex crystal structure of most of these materials. An exception are the infinite layer (IL) compounds consisting essentially of CuO2 planes. Unfortunately, these materials are difficult to grow and, thus, there are only few experimental investigations. Recently, we succeeded in depositing high quality films of the electron doped IL compound Sr1-xLaxCuO2 (SLCO), with x approximately 0.15, and on the fabrication of well-defined grain boundary Josephson junctions (GBJs) based on such SLCO films. Here we report on a phase sensitive study of the superconducting order parameter based on GBJ SQUIDs from a SLCO film grown on a tetracrystal substrate. Our results show that also the parent structure of the high-Tc cuprates has dx2-y2-wave symmetry, which thus seems to be inherent to cuprate superconductivity.

cond-mat.supr-con↗

arXiv2011

Edge superconductivity in Nb thin film microbridges revealed by integral and spatially resolved electric transport

The resistance $R$ vs perpendicular external magnetic field $H$ was measured for superconducting Nb thin--film microbridges with and without microholes [antidots (ADs)]. Well below the transition temperature, integral $R(H)$ measurements of the resistive transition to the normal state on the plain bridge show two distinct regions, which can be identified as bulk and edge superconductivity, respectively. The latter case appears when bulk superconductivity becomes suppressed at the upper critical field $H_{c2}$ and below the critical field of edge superconductivity $H_{c3}\approx 1.7\, H_{c2}$. The presence of additional edges in the AD bridge leads to a different shape of the $R(H)$ curves. We used low-temperature scanning laser microscopy (LTSLM) to visualize the current distribution in the plain and AD bridge upon sweeping $H$. While the plain bridge shows a dominant LTSLM signal at its edges for $H > H_{c2}$ the AD bridge also gives a signal from the inner parts of the bridge due to the additional edge states around the ADs. LTSLM reveals an asymmetry in the current distribution between left and right edges, which confirms theoretical predictions. Furthermore, the experimental results are in good agreement with our numerical simulations (based on the time-dependent Ginzburg--Landau model) yielding the spatial distribution of the order parameter and current density for different bias currents and $H$ values.

cond-mat.supr-con↗

arXiv2011

Reducing vortex losses in superconducting microwave resonators with microsphere patterned antidot arrays

We experimentally investigate the vortex induced energy losses in niobium coplanar waveguide resonators with and without quasihexagonal arrays of nanoholes (antidots), where large-area antidot patterns have been fabricated using self-assembling microsphere lithography. We perform transmission spectroscopy experiments around 6.25 and 12.5 GHz in magnetic field cooling and zero field cooling procedures with perpendicular magnetic fields up to B=27 mT at a temperature T=4.2 K. We find that the introduction of antidot arrays into resonators reduces vortex induced losses by more than one order of magnitude.

cond-mat.supr-con↗

arXiv2011

Spectroscopy of a fractional Josephson vortex molecule

In long Josephson junctions with multiple discontinuities of the Josephson phase, fractional vortex molecules are spontaneously formed. At each discontinuity point a fractional Josephson vortex carrying a magnetic flux $|Φ|<Φ_0$, $Φ_0\approx 2.07\times 10^{-15}$ Wb being the magnetic flux quantum, is pinned. Each vortex has an oscillatory eigenmode with a frequency that depends on $Φ/Φ_0$ and lies inside the plasma gap. We experimentally investigate the dependence of the eigenfrequencies of a two-vortex molecule on the distance between the vortices, on their topological charge $\wp=2πΦ/Φ_0$ and on the bias current $γ$ applied to the Josephson junction. We find that with decreasing distance between vortices, a splitting of the eigenfrequencies occurs, that corresponds to the emergence of collective oscillatory modes of both vortices. We use a resonant microwave spectroscopy technique and find good agreement between experimental results and theoretical predictions.

cond-mat.supr-con↗

arXiv2011

Josephson junction with magnetic-field tunable ground state

We consider an asymmetric 0-pi Josephson junction consisting of 0 and pi regions of different lengths L_0 and L_pi. As predicted earlier this system can be described by an effective sine-Gordon equation for the spatially averaged phase psi so that the effective current-phase relation of this system includes a \emph{negative} second harmonic ~sin(2 psi). If its amplitude is large enough, the ground state of the junction is doubly degenerate psi=\pmvarphi, where varphi depends on the amplitudes of the first and second harmonics. We study the behavior of such a junction in an applied magnetic field H and demonstrate that H induces an additional term ~H cos(psi) in the effective current-phase relation. This results in a non-trivial ground state \emph{tunable} by magnetic field. The dependence of the critical current on H allows for revealing the ground state experimentally.

cond-mat.supr-con↗

arXiv2011

Deterministic Josephson Vortex Ratchet with a load

We investigate experimentally a deterministic underdamped Josephson vortex ratchet -- a fluxon-particle moving along a Josephson junction in an asymmetric periodic potential. By applying a sinusoidal driving current one can compel the vortex to move in a certain direction, producing average dc voltage across the junction. Being in such a rectification regime we also load the ratchet, i.e., apply an additional dc bias current I_dc (counterforce) which tilts the potential so that the fluxon climbs uphill due to the ratchet effect. The value of the bias current at which the fluxon stops climbing up defines the strength of the ratchet effect and is determined experimentally. This allows us to estimate the loading capability of the ratchet, the output power and efficiency. For the quasi-static regime we present a simple model which delivers simple analytic expressions for the above mentioned figures of merit.

cond-mat.supr-con↗

arXiv2011

Crossover between different regimes of inhomogeneous superconductivity in planar superconductor-ferromagnet hybrids

We studied experimentally the effect of a stripe-like domain structure in a ferromagnetic BaFe_{12}O_{19} substrate on the magnetoresistance of a superconducting Pb microbridge. The system was designed in such a way that the bridge is oriented perpendicular to the domain walls. It is demonstrated that depending on the ratio between the amplitude of the nonuniform magnetic field B_0, induced by the ferromagnet, and the upper critical field H_{c2} of the superconducting material, the regions of the reverse-domain superconductivity in the H-T plane can be isolated or can overlap (H is the external magnetic field, T is temperature). The latter case corresponds to the condition B_0/H_{c2}<1 and results in the formation of superconductivity above the magnetic domains of both polarities. We discovered the regime of edge-assisted reverse-domain superconductivity, corresponding to localized superconductivity near the edges of the bridge above the compensated magnetic domains. Direct verification of the formation of inhomogeneous superconducting states and external-field-controlled switching between normal state and inhomogeneous superconductivity were obtained by low-temperature scanning laser microscopy.

cond-mat.supr-con↗

arXiv2011

Superconductor-insulator-ferromagnet-superconductor Josephson junction: From the dirty to the clean limit

The proximity effect and the Josephson current in a superconductor-insulator-ferromagnet-superconductor (SIFS) junction are investigated within the framework of the quasiclassical Eilenberger equations. This investigation allows us to compare the dirty and the clean limits, to investigate an arbitrary impurity scattering, and to determine the applicability limits of the Usadel equations for such structures. The role of different types of the FS interface is analyzed. It is shown that the decay length and the spatial oscillation period of the Eilenberger function may exhibit a nonmonotonic dependence on the properties of the ferromagnetic layer such as exchange field or electron mean free path. The results of our calculations are applied to the interpretation of experimentally observed dependencies of the critical current density on the ferromagnet thickness in Josephson junctions containing a Ni layer with an arbitrary scattering.

cond-mat.supr-con↗