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We analyze electric transport and noise properties at 4.2 K of self-shunted superconductor-normal metal-superconductor (SNS) sandwich-type Josephson junctions, comprising Nb as the superconductor and Hf-Ti as the normal conducting material, with lateral dimensions down to approximately 80 nm. The junctions are fabricated with an optimized multilayer Nb technology based on nanopatterning by electron-beam lithography and chemical-mechanical polishing. The dependence of transport properties on the junction geometry (lateral size and barrier thickness d(Hf-Ti)) is studied, yielding a characteristic voltage V-c up to approximately 100 mu V for the smallest d(Hf-Ti) = 17 nm. The observed small hysteresis in the current-voltage curves of devices with high V-c and large size can be attributed to self-heating of the junctions and fitted with an extended version of the resistively shunted junction model. Measurements of voltage noise of single junctions are consistent with the model including self-heating effects. The potential of our technology for further miniaturization of nanoscale superconducting quantum interference devices and for the improvement of their performance is discussed.

We report on the fabrication and characterization of nanopatterned dc SQUIDs with grain boundary Josephson junctions based on heteroepitaxially grown YBa$_2$Cu$_3$O$_7$ (YBCO)/ SiTrO$_3$ (STO) superlattices on STO bicrystal substrates. Nanopatterning is performed by Ga focused-ion-beam milling. The electric transport properties and thermal white flux noise of superlattice nanoSQUIDs are comparable to single layer YBCO devices on STO bicrystals. However, we find that the superlattice nanoSQUIDs have more than an order of magnitude smaller low-frequency excess flux noise, with root-mean-square spectral density $S_\Phi^{1/2}\sim 5-6\,\mu\Phi_0/\sqrt{\rm Hz}$ at 1 Hz ($\Phi_0$ is the magnetic flux quantum). We attribute this improvement to an improved microstructure at the grain boundaries forming the Josephson junctions in our YBCO nanoSQUDs., Comment: 9 pages, 7 figures

By scanning with a $30\, \mathrm{keV}$ focused He ion beam (He-FIB) across YBa$_2$Cu$_3$O$_7$ (YBCO) thin film microbridges, we create Josephson barriers with critical current density $j_\mathrm{c}$ adjustable by irradiation dose $D$. The dependence $j_\mathrm{c} (D)$ yields an exponential decay. At $4.2\, \mathrm{K}$, a transition from flux-flow to Josephson behavior occurs when $j_\mathrm{c}$ decreases below $\approx 2\, \mathrm{MA/cm^2}$. The Josephson junctions exhibit current-voltage characteristics (IVCs) that are well described by the resistively and capacitively shunted junction model, without excess current for characteristic voltages $V_\mathrm{c} \lesssim 1\, \mathrm{mV}$. Devices on MgO and LSAT substrates show non-hysteretic IVCs, while devices on SrTiO$_3$ show a small hysteresis. For all junctions an approximate scaling $V_\mathrm{c} \propto j_\mathrm{c}^{1/2}$ is found. He-FIB irradiation with high dose produces barriers with $j_\mathrm{c}=0$ and high resistances of $10\, \mathrm{k\Omega} \ldots 1\, \mathrm{G\Omega}$. This provides the possibility to write highly resistive walls or areas into YBCO using a He-FIB. Transmission electron microscopy reveals an amorphous phase within the walls, whereas for lower doses the YBCO stays crystalline. We have also ``drawn'' superconducting quantum interference devices (SQUIDs) by using a He-FIB for definition of the SQUID hole and the junctions. The SQUIDs show high performance, with flux noise $< 500\, \mathrm{n \Phi_0/Hz^{1/2}}$ in the thermal white noise limit for a device with $19\, \mathrm{pH}$ inductance.

We report on doping and undoping experiments of terahertz (THz) emitting intrinsic Josephson junction stacks, where the change in charge carrier concentration is achieved by heavy current injection. The experiments were performed on stand-alone structures fabricated from a Bi2Sr2CaCu2O8+delta single crystal near optimal doping. The stacks contained about 930 intrinsic Josephson junctions. On purpose, the doping and undoping experiments were performed over only a modest range of charge carrier concentrations, changing the critical temperature of the stack by less than 1 K. We show that both undoping and doping is feasible also for the large intrinsic Josephson junction stacks used for THz generation. Even moderate changes in doping introduce large changes in the THz emission properties of the stacks. The highest emission power was achieved after doping a pristine sample.

We demonstrate experimentally the operation of a deterministic Josephson ratchet with tunable asymmetry. The ratchet is based on a $\varphi$ Josephson junction with a ferromagnetic barrier operating in the underdamped regime. The system is probed also under the action of an additional dc current, which acts as a counter force trying to stop the ratchet. Under these conditions the ratchet works against the counter force, thus producing a non-zero output power. Finally, we estimate the efficiency of the $\varphi$ Josephson junction ratchet.

We demonstrate experimentally the operation of a deterministic Josephson ratchet with tunable asymmetry. The ratchet is based on a $��$ Josephson junction with a ferromagnetic barrier operating in the underdamped regime. The system is probed also under the action of an additional dc current, which acts as a counter force trying to stop the ratchet. Under these conditions the ratchet works against the counter force, thus producing a non-zero output power. Finally, we estimate the efficiency of the $��$ Josephson junction ratchet.

We study a 0-$\pi$ dc superconducting quantum interference device (SQUID) with asymmetric inductances and critical currents of the two Josephson junctions (JJs). By considering such a dc SQUID as a black box with two terminals, we calculate its effective current-phase relation $I_s(\psi)$ and the Josephson energy $U(\psi)$, where $\psi$ is the Josephson phase across the terminals. We show that there is a domain of parameters where the black box has the properties of a $\varphi$ JJ with degenerate ground state phases $\psi=\pm\varphi$. The $\varphi$ domain is rather large, so one can easily construct a $\varphi$ JJ experimentally. We derive the current phase relation and show that it can be tuned \emph{in situ} by applying an external magnetic flux resulting in a continuous transition between the systems with static solutions $\psi=\pm\varphi$, $\psi=\varphi_0$ ($\varphi_0 \neq 0,\pi$) and even $\psi=\varphi_0\pm\varphi$. The dependence of $\varphi_0$ on applied magnetic flux is not $2\pi$ (one flux quantum) periodic., Comment: to be published in Phys. Rev. B (status on June 1st, 2015)

We study a 0-$��$ dc superconducting quantum interference device (SQUID) with asymmetric inductances and critical currents of the two Josephson junctions (JJs). By considering such a dc SQUID as a black box with two terminals, we calculate its effective current-phase relation $I_s(��)$ and the Josephson energy $U(��)$, where $��$ is the Josephson phase across the terminals. We show that there is a domain of parameters where the black box has the properties of a $��$ JJ with degenerate ground state phases $��=\pm��$. The $��$ domain is rather large, so one can easily construct a $��$ JJ experimentally. We derive the current phase relation and show that it can be tuned \emph{in situ} by applying an external magnetic flux resulting in a continuous transition between the systems with static solutions $��=\pm��$, $��=��_0$ ($��_0 \neq 0,��$) and even $��=��_0\pm��$. The dependence of $��_0$ on applied magnetic flux is not $2��$ (one flux quantum) periodic., to be published in Phys. Rev. B (status on June 1st, 2015)

We used one-dimensional coupled sine-Gordon equations combined with heat diffusion equations to numerically investigate the thermal and electromagnetic properties of a $300\,\mu\mathrm{m}$ long intrinsic Josephson junction stack consisting of $N = 700$ junctions. The junctions in the stack are combined to $M$ segments where we assume that inside a segment all junctions behave identically. Most simulations are for $M = 20$. For not too high bath temperatures there is the appearence of a hot spot at high bias currents. In terms of electromagnetic properties, robust standing wave patterns appear in the current density and electric field distributions. These patterns come together with vortex/antivortex lines across the stack that correspond to $\pi$ kink states, discussed before in the literature for a homogeneous temperature distribution in the stack. We also discuss scaling of the thermal and electromagnetic properties with $M$, on the basis of simulations with $M$ between 10 and 350., Comment: 17 pages, 17 figures

Using the Usadel equation approach, we provide a compact formalism to calculate the critical current density of 21 different types of ferromagnetic (F) Josephson junctions containing insulating (I) and normal metal (N) layers in the weak link regions. In particular, we obtain that even a thin additional N layer may shift the 0-$\pi$ transitions to larger or smaller values of the thickness $d_F$ of the ferromagnet, depending on its conducting properties. For certain values of $d_F$, a 0-$\pi$ transition can even be achieved by changing only the N layer thickness. We use our model to fit experimental data of SIFS and SINFS tunnel junctions, where S is a superconducting electrode.

We consider a $��$ Josephson junction, which has a bistable zero-voltage state with the stationary phases $��=\pm��$. In the non-zero voltage state the phase "moves" viscously along a tilted periodic double-well potential. When the tilting is reduced quasistatically, the phase is retrapped in one of the potential wells. We study the viscous phase dynamics to determine in which well ($-��$ or $+��$) the phase is retrapped for a given damping, when the junction returns from the finite-voltage state back to zero-voltage state. In the limit of low damping the $��$ Josephson junction exhibits a butterfly effect --- extreme sensitivity of the destination well on damping. This leads to an impossibility to predict the destination well.

We calculate the current phase relation of a planar Josephson junction with a ferromagnetic weak link located on top of a thin normal metal film. Following experimental observations we assume transparent superconductor-ferromagnet interfaces. This provides the best interlayer coupling and a low suppression of the superconducting correlations penetrating from the superconducting electrodes into the ferromagnetic layer. We show that this Josephson junction is a promising candidate for an experimental �� junction realization., References updated

We report on the observation of dendritic flux avalanches in a large Niobium single crystal. In contrast to avalanches observed in thin films, they appear only in a very narrow temperature interval of about a tenth of a Kelvin near the critical temperature of Nb. At a fixed temperature, we find two sets of dendritic structures, which differ by the magnetic field required for their formation and by the maximum distance the dendrites penetrate into the sample. The effect is caused by dendritic flux penetration into thin superconducting surface layers formed in the single crystal close to the critical temperature.

The $��$ Josephson junction has a doubly degenerate ground state with the Josephson phases $\pm��$. We demonstrate the use of such a $��$ Josephson junction as a memory cell (classical bit), where writing is done by applying a magnetic field and reading by applying a bias current. In the "store" state, the junction does not require any bias or magnetic field, but just needs to stay cooled for permanent storage of the logical bit. Straightforward integration with Rapid Single Flux Quantum logic is possible., to be published in APL

We consider an infinitely long 0-$��$ Josephson junction consisting of 0 and $��$ regions having different critical current densities $j_{c,0}$ and $j_{c,��}$. The ground state of such a junction corresponds to a spontaneosly formed asymmetric semifluxon with tails decaying on different length scales. We calculate the depinning current of such a fractional vortex and show that it is different for positive and negative bias polarity. We also show that upon application of a bias current, the fractional flux (topological charge) associated with the vortex changes. We calculate the range of fractional flux associated with the vortex when the bias changes from negative to positive critical (depinning) values., submitted to Phys. Rev. B

We study the critical current I_c dependence on applied magnetic field H for multifacet YBa_2Cu_3O_{7-\delta}-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-\pi)-segments. The ramp angle is \theta_r=8^\circ. The facet width is 10\,\mum. H is applied under different angles \theta relative to the substrate plane and different angles \phi relative to the in-plane orientation of the zigzags. We find that a homogeneous flux distribution is only achieved for an angle \theta_h\approx 1^\circ - 2^\circ and that even a small misalignment \sim 0.1^\circ relative to \theta_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 \theta^*_d relative to \theta_h of similar magnitude, where the average flux density completely vanishes., Comment: in press (PRB), 11 pages, 9 figures

We have studied experimentally and numerically temperature profiles and the formation of hot spots in intrinsic Josephson junction stacks in Bi2Sr2CaCu2O8 (BSCCO). The superconducting stacks are biased in a state where all junctions are resistive. The formation of hot spots in this system is shown to arise mainly from the strongly negative temperature coefficient of the c-axis resistivity of BSCCO at low temperatures. This leads to situations where the maximum temperature in the hot spot can be below or above the superconducting transition temperature Tc. The numerical simulations are in good agreement with the experimental observations.

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.

We report on measurements of the linewidth {\Delta}f of THz radiation emitted from intrinsic Josephson junction stacks, using a Nb/AlN/NbN integrated receiver for detection. Previous resolution limited measurements indicated that {\Delta}f may be below 1 GHz - much smaller than expected from a purely cavity-induced synchronization. While at low bias we found {\Delta}f to be not smaller than ? 500 MHz, at high bias, where a hotspot coexists with regions which are still superconducting, {\Delta}f turned out to be as narrow as 23 MHz. We attribute this to the hotspot acting as a synchronizing element. {\Delta}f 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., Comment: 4 figures, 5 pages

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., Submitted to PRL, 5 pages, 3 figures, supplementary information included (4 pages, 4 figures)

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

We demonstrate experimentally the existence of Josephson junctions having a doubly degenerate ground state with an average Josephson phase ��=\pm��. The value of �� can be chosen by design in the interval 0

We study the critical current I_c dependence on applied magnetic field H for multifacet YBa_2Cu_3O_{7-��}-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-��)-segments. The ramp angle is ��_r=8^\circ. The facet width is 10\,\mum. H is applied under different angles ��relative to the substrate plane and different angles ��relative to the in-plane orientation of the zigzags. We find that a homogeneous flux distribution is only achieved for an angle ��_h\approx 1^\circ - 2^\circ and that even a small misalignment \sim 0.1^\circ relative to ��_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 ��^*_d relative to ��_h of similar magnitude, where the average flux density completely vanishes., in press (PRB), 11 pages, 9 figures

We report on imaging of the nonuniform superconducting states in a Pb thin film bridge on top of a ferromagnetic BaFe_{12}O_{19} single crystal with a single straight domain wall along the center of the bridge by low-temperature scanning laser microscopy. We have visualized domain wall superconductivity (DWS) close to the critical temperature of Pb, when the Pb film above the domain wall acts as a superconducting path for the current. The evolution of the DWS signal with temperature and the external-field-driven transition from DWS to reverse domain superconductivity was visualized., 4 pages, 3 figures

At high enough input power in stacks of Bi_2Sr_2CaCu_2O8 intrinsic Josephson junctions a hot spot (a region heated to above the superconducting transition temperature) coexists with regions still in the superconducting state. In the ``cold'' regions cavity resonances can occur, synchronizing the ac Josephson currents and giving rise to strong coherent THz emission. We investigate the interplay of hot spots and standing electromagnetic waves by low temperature scanning laser microscopy and THz emission measurements, using stacks of various geometries. For a rectangular and a arrow-shaped structure we show that the standing wave can be turned on and off in various regions of the stack structure, depending on the hot spot position. We also report on standing wave and hot spot formation in a disk shaped mesa structure.

We have realized multifacet Josephson junctions with periodically alternating critical current density (MJJs) using superconductor-insulator-ferromagnet-superconductor heterostructures. We show that anomalous features of critical current vs. applied magnetic field, observed also for other types of MJJs, are caused by a non-uniform flux density (parallel to the barrier) resulting from screening currents in the electrodes in the presence of a (parasitic) off-plane field component., Comment: submitted to PRB

We present a detailed analysis of the dependence of the critical current I-c on an in-plane magnetic field B of 0, pi, and 0-pi superconductor-insulator-ferromagnet-superconductor Josephson junctions. I-c(B) of the 0 and the pi junction closely follows a Fraunhofer pattern, indicating a homogeneous critical current density j(c)(x). The maximum of I-c(B) is slightly shifted along the field axis, pointing to a small remanent in-plane magnetization of the F-layer along the field axis. I-c(B) of the 0-pi junction exhibits the characteristic central minimum. I-c, however, has a finite value here, due to an asymmetry of j(c) in the 0 and the pi part. In addition, this I-c(B) exhibits asymmetric maxima and bumped minima. To explain these features in detail, flux penetration being different in the 0 part and the pi part needs to be taken into account. We discuss this asymmetry in relation to the magnetic properties of the F-layer and the fabrication technique used to produce the 0-pi junctions.

Heteroepitaxially grown bilayers of ferromagnetic La$_{0.7}$Ca$_{0.3}$MnO$_3$ (LCMO) on top of superconducting YBa$_2$Cu$_3$O$_7$ (YBCO) thin films were investigated by focusing on electric transport properties as well as on magnetism and orbital occupation at the interface. Transport measurements on YBCO single layers and on YBCO/LCMO bilayers, with different YBCO thickness $d_Y$ and constant LCMO thickness $d_L=50$\,nm, show a significant reduction of the superconducting transition temperature $T_c$ only for $d_Y, 8 Figures

Heteroepitaxially grown bilayers of ferromagnetic La 0.7 Ca 0.3 MnO 3 (LCMO) on top of superconducting YBa 2 Cu 3 O 7 (YBCO) thin films were investigated by focusing on electric transport properties as well as on magnetism and orbital occupation at the interface. Transport measurements on YBCO single layers and on YBCO/LCMO bilayers, with different YBCO thickness d Y and constant LCMO thickness d L =50 nm, show a significant reduction in the superconducting transition temperature T c only for d Y

We have realized multifacet Josephson junctions with periodically alternating critical current density (MJJs) using superconductor-insulator-ferromagnet-superconductor heterostructures. We show that anomalous features of critical current vs applied magnetic field, observed also for other types of MJJs, are caused by a nonuniform flux density (parallel to the barrier) resulting from screening currents in the electrodes in the presence of a (parasitic) off-plane field component.

We present experimental studies of high quality underdamped 0, pi, and 0-pi ferromagnetic Josephson tunnel junctions of intermediate length L (lambda_J < L < 5 lambda_J, where lambda_J is the Josephson penetration depth). The junctions are fabricated as Nb/Al_2O_3/Cu_40Ni_60/Nb Superconductor-Insulator-Ferromagnet-Superconductor heterostructures. Using microwave spectroscopy, we have investigated the eigenfrequencies of 0, pi, and 0-pi Josephson junctions in the temperature range 1.9K...320mK. Harmonic, subharmonic and superharmonic pumping is observed in experiment, and the experimental data are compared with numerical simulations. Escape rate measurements without applied microwaves at temperatures T down to 20mK show that the width of the switching current histogram decreases with temperature and saturates below T=150mK. We analyze our data in the framework of the short junction model. The differences between experimental data and theoretical predictions are discussed., submitted to RPB

We present a detailed analysis of the dependence of the critical current I_c on the magnetic field B of 0, Pi, and 0-Pi superconductor-insulator-ferromagnet-superconductor Josephson junctions. I_c(B) of the 0 and Pi junction closely follows a Fraunhofer pattern, indicating a homogeneous critical current density j_c(x). The maximum of I_c(B) is slightly shifted along the field axis, pointing to a small remanent in-plane magnetization of the F-layer along the field axis. I_c(B) of the 0-Pi junction exhibits the characteristic central minimum. I_c however has a finite value here, due to an asymmetry of j_c in the 0 and Pi part. In addition, this I_c(B) exhibits asymmetric maxima and bumped minima. To explain these features in detail, flux penetration being different in the 0 part and the Pi part needs to be taken into account. We discuss this asymmetry in relation to the magnetic properties of the F-layer and the fabrication technique used to produce the 0-Pi junctions.

We consider a fractional Josephson vortex in an infinitely long 0-kappa Josephson junction. A uniform bias current applied to the junction exerts a Lorentz force acting on a vortex. When the bias current becomes equal to the critical (or depinning) current, the Lorentz force tears away an integer fluxon and the junction switches to the resistive state. In the presence of thermal and quantum fluctuations this escape process takes place with finite probability already at subcritical values of the bias current. We analyze the escape of a fractional vortex by mapping the Josephson phase dynamics to the dynamics of a single particle in a metastable potential and derive the effective parameters of this potential. This allows us to predict the behavior of the escape rate as a function of the topological charge of the vortex.

We consider a symmetric 0-pi Josephson junction of length $L$, which classically can be in one of two degenerate ground states up or down, corresponding to supercurrents circulating clockwise or counterclockwise around the 0-pi boundary. When the length $L$ of the junction becomes smaller than the Josephson penetration depth $\lambda_J$, the system can switch from one state to the other due to thermal fluctuations or quantum tunneling. We map this problem to the dynamics of a single particle in a periodic double well potential and estimate parameters for which macroscopic quantum coherence may be observed. We conclude that this system is not very promising to build a qubit because (a) it requires very low temperatures to reach the quantum regime, (b) its tiny flux is hard to read out and (c) it is very sensitive to the asymmetries between the 0 and pi parts of the junction., Comment: submitted to PRB

We present experimental studies of static and dynamic properties of 0, pi and 0-pi superconductor-insulator-ferromagnet-superconductor (SIFS) Josephson junctions of small and intermediate length. In the underdamped limit these junctions exhibit a rich dynamical behavior such as resonant steps on the current-voltage characteristics. Varying the experimental conditions, zero field steps, Fiske steps and Shapiro steps are observed with a high resolution. A strong signature of the 0-pi Josephson junction is demonstrated by measuring the critical current as a function of two components (B_x, B_y) of an in-plane magnetic field. The experimental observation of a half-integer zero field step in 0-pi SIFS junctions is presented., Comment: 9 pages, 7 figures, 1 table

Recently, it has been shown that large stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8 emit synchronous THz radiation, the synchronization presumably triggered by a cavity resonance. To investigate this effect we use Low Temperature Scanning Laser Microscopy to image electric field distributions. Apart from verifying the appearance of cavity modes at low bias we find that, in a high input power regime, standing-wave patterns are created through interactions with a hot spot, possibly pointing to a new mode of generating synchronized radiation in intrinsic Josephson junction stacks., Comment: 6 pages, 5 figures, supplementary information included

We investigate in-plane quasiparticle tunneling across thin film grain boundary junctions (GBJs) of the electron-doped cuprate La$_{2-x}$Ce$_{x}$CuO$_4$ in magnetic fields up to $B=16 $T, perpendicular to the CuO$_2$ layers. The differential conductance in the superconducting state shows a zero bias conductance peak (ZBCP) due to zero energy surface Andreev bound states. With increasing temperature $T$, the ZBCP vanishes at the critical temperature $T_c\approx29 $K if B=0, and at $T=12 $K for B=16 T. As the ZBCP is related to the macroscopic phase coherence of the superconducting state, we argue that the disappearance of the ZBCP at a field $B_{ZBCP}(T)$ must occur below the upper critical field $B_{c2}(T)$ of the superconductor. We find $B_{ZBCP}(0) \approx 25 $T which is at least a factor of 2.5 higher than previous estimates of $B_{c2}(0)$., 4 pages, 4 figures

Several recent experiments revealed a change of the sign of the first harmonic in the current-phase relation of Josephson junctions (JJ) based on novel superconductors, e.g., d-wave based or JJ with ferromagnetic barrier. In this situation the role of the second harmonic becomes dominant and it determines the scenario of a 0-pi transition. We discuss different mechanisms of the second harmonic generation and its sign. If the second harmonic is negative the 0-pi transition becomes continuous and the realization of the so-called varphi junction is possible. We study the unusual properties of such a novel JJ and analyze the possible experimental techniques for their observation., submitted to PRB

We report on Josephson and quasiparticle tunneling in YBa2Cu3O7-x(YBCO)/Au/Nb ramp junctions of several geometries. Macroscopically, tunneling is studied in the ab-plane of YBCO either in the (100) or (010) direction, or in the (110) direction. These junctions have a stable and macroscopically well defined geometry. This allows systematic investigations of both quasiparticle and Josephson tunneling over a wide range of temperatures and magnetic fields. With Nb superconducting, the proximity gap induced in the Au layer appears in the quasiparticle conductance spectra as well defined coherence peaks and a dip at the center of a broadened zero-bias conductance peak (ZBCP). The voltage position of the coherence peaks varies with Au layer thickness. As we increase the temperature or an applied magnetic field both the coherence peaks and the dip get suppressed and the ZBCP fully develops, while states are conserved. With Nb in the normal state the ZBCP is observed up to about 77 K and is almost unaffected by an increasing field up to 7 T. The measurements are consistent with a convolution of density of states with broadened Andreev bound states formed at the YBCO/Au/Nb junction interfaces. Since junctions with different geometries are fabricated on the same substrate under the same conditions one expects to extract reliable tunneling information that is crystallographic direction sensitive. In high contrast to Josephson tunneling, however, the quasiparticle conductance spectra are crystallographic orientation insensitive: independent whether the tunneling occurs in the (100) or (110) directions, a pronounced ZBCP is always observed, consistent with microscopic roughness of the junction interfaces. Qualitatively, all these particularities regarding quasiparticle spectra hold regardless whether the YBCO thin film is twinned or untwinned. This suggests that the formation of Andreev bound states is, to a first approximation, insensitive to twinning.

We fabricated high quality Nb/Al2O3/Ni0.6Cu0.4/Nb superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Using a ferromagnetic layer with a steplike thickness, we obtain a 0-pi junction, with equal lengths and critical currents of 0 and pi parts. The ground state of our 330 mu m (1.3 lambda(J)) long junction corresponds to a spontaneous vortex of supercurrent pinned at the 0-pi step and carrying similar to 6.7% of the magnetic flux quantum Phi(0). The dependence of the critical current on the applied magnetic field shows a clear minimum in the vicinity of zero field.

We report on Josephson and quasiparticle tunneling in YBa2Cu3O7-x(YBCO)/Au/Nb ramp junctions of several geometries. Macroscopically, tunneling is studied in the ab-plane of YBCO either in the (100) or (010) direction, or in the (110) direction. These junctions have a stable and macroscopically well defined geometry. This allows systematic investigations of both quasiparticle and Josephson tunneling over a wide range of temperatures and magnetic fields. With Nb superconducting, the proximity gap induced in the Au layer appears in the quasiparticle conductance spectra as well defined coherence peaks and a dip at the center of a broadened zero-bias conductance peak (ZBCP). The voltage position of the coherence peaks varies with Au layer thickness. As we increase the temperature or an applied magnetic field both the coherence peaks and the dip get suppressed and the ZBCP fully develops, while states are conserved. With Nb in the normal state the ZBCP is observed up to about 77 K and is almost unaffected by an increasing field up to 7 T. The measurements are consistent with a convolution of density of states with broadened Andreev bound states formed at the YBCO/Au/Nb junction interfaces. Since junctions with different geometries are fabricated on the same substrate under the same conditions one expects to extract reliable tunneling information that is crystallographic direction sensitive. In high contrast to Josephson tunneling, however, the quasiparticle conductance spectra are crystallographic orientation insensitive: independent whether the tunneling occurs in the (100) or (110) directions, a pronounced ZBCP is always observed, consistent with microscopic roughness of the junction interfaces. Qualitatively, all these particularities regarding quasiparticle spectra hold regardless whether the YBCO thin film is twinned or untwinned. This suggests that the formation of Andreev bound states is, to a first approximation, insensitive to twinning.

The magnetic flux noise induced by vortices in thin superconducting films is studied. Rigid vortices in thin films as well as pancake vortices in the pancake gas regime are addressed. The vortex dynamics is described by a Feynman path integral which fully accounts for the balance between vortex entropy and vortex energetics. We find that vortex pair creation (annihilation) in the presence of pinning is a natural source for noise with a 1/f spectrum., 4 pages, 1 figure; submitted to Physical Review

We investigate 3-junction SQUIDs which show voltage rectification if biased with an ac current drive with zero mean value. The Josephson phase across the SQUID experiences an effective ratchet potential, and the device acts as an efficient rocking ratchet, as demonstrated experimentally for adiabatic and nonadiabatic drive frequencies. For high-frequency drives the rectified voltage is quantized due to synchronization of the phase dynamics with the external drive. The experimental data are in excellent agreement with numerical simulations including thermal fluctuations., 5 pages, 4 figures -- Fig.4 revised

We report on Josephson and quasiparticle tunneling in YBa2Cu3O7-x(YBCO)/Au/Nb ramp junctions of several geometries. Macroscopically, tunneling occurs in the ab-plane of YBCO either in the (100) and (010) direction, or in the (110) direction. These junctions have a stable and macroscopically well defined geometry. This allows systematic investigations of both quasiparticle and Josephson tunneling over a wide range of temperature and magnetic field. With Nb superconducting, its gap appears in the quasiparticle conductance spectra as Nb coherence peaks and a dip at the center of a broadened zero-bias conductance peak (ZBCP). As we increase the temperature or an applied magnetic field both the Nb coherence peaks and the dip get suppressed and the ZBCP fully develops, while states are conserved. With Nb in the normal state the ZBCP is observed up to about 77 K and is almost unaffected by an increasing field up to 7 T. The measurements are consistent with a convolution of density of states with broadened Andreev bound states formed at the YBCO/Au/Nb junction interfaces. Since junctions with different geometries are fabricated on the same substrate under the same conditions one expects to extract reliable tunneling information that is crystallographic direction sensitive. In high contrast to Josephson tunneling, however, the quasiparticle conductance spectra are crystallographic orientation insensitive: independent whether the tunneling occurs in the (100) or (110) directions, a pronounced ZBCP is always observed, consistent with microscopic roughness of the junction interfaces. Qualitatively, all these particularities regarding quasiparticle spectra hold regardless whether the YBCO thin film is twinned or untwinned., 13 pages, 10 figures

We observe a zero-bias conductance peak (ZBCP) in the ab-plane quasiparticle tunneling spectra of thin film grain-boundary Josephson junctions made of the electron-doped cuprate superconductor La2-xCexCuO4-y. An applied magnetic field reduces the spectral weight around zero energy and shifts it nonlinearly to higher energies consistent with a Doppler shift of the Andreev bound states (ABS) energy. For all magnetic fields the ZBCP appears simultaneously with the onset of superconductivity. These observations strongly suggest that the ZBCP results from the formation of ABS at the junction interfaces, and, consequently, that there is a sign change in the symmetry of the superconducting order parameter of this compound consistent with a d-wave symmetry.

We investigate theoretically the eigenmodes and the stability of one and two arbitrary fractional vortices pinned at one and two $\kappa$-phase discontinuities in a long Josephson junction. In the particular case of a single $\kappa$-discontinuity, a vortex is spontaneously created and pinned at the boundary between the 0 and $\kappa$-regions. In this work we show that only two of four possible vortices are stable. A single vortex has an oscillatory eigenmode with a frequency within the plasma gap. We calculate this eigenfrequency as a function of the fractional flux carried by a vortex. For the case of two vortices, pinned at two $\kappa$-discontinuities situated at some distance $a$ from each other, splitting of the eigenfrequencies occur. We calculate this splitting numerically as a function of $a$ for different possible ground states. We also discuss the presence of a critical distance below which two antiferromagnetically ordered vortices form a strongly coupled ``vortex molecule'' that behaves as a single object and has only one eigenmode., Comment: submitted to Phys. Rev. B ()

We have measured interlayer current transport in small sized RuSr2GdCu2O8 single crystals. We find a clear intrinsic Josephson effect showing that the material acts as a natural superconductor-insulator-ferromagnet-insulator-superconductor superlattice. So far, we detected no unconventional behavior due to the magnetism of the RuO2 layers., 4 pages, 5 figures, to appear in Phys. Rev. Lett

We used low-temperature scanning electron microscopy (LTSEM) for imaging quantized magnetic flux (vortices) in direct current (dc) superconducting quantum interference devices (SQUIDs) with approximately 1 micron spatial resolution at temperature T=77 K in a controllable magnetic field up to 20 microTesla. We demonstrate that LTSEM allows to image the spatial distribution of vortices in a YBa2Cu3O7/SrTiO3/YBa2Cu3O7 multilayer thin-film structure consisting of a dc SQUID washer with an integrated input coil on top. Simultaneously, we are able to measure the low-frequency noise of the sample under test, which allows to correlate the spatial distribution of vortices with low-frequency noise in the SQUID., 3 pages, 3 figures, to be published in Appl. Phys. Lett. 06/04/2001; a pdf-file is also available at http://www.pit.physik.uni-tuebingen.de/Kleiner/papers/2001/straub01_apl.pdf

A quantitative analysis of a microscopic model for the intrinsic Josephson effect in high-temperature superconductors based on interlayer tunneling is presented. The pairing correlations in the CuO_2-planes are modelled by a 2-D Hubbard-model with attractive interaction, a model which accounts well for some of the observed features such as the short planar coherence length. The stack of Hubbard planes is arranged on a torus, which is threaded by a magnetic flux. The current perpendicular to the planes is calculated as a function of applied flux (i. e. the phase), and -- after careful elimination of finite-size effects due to single-particle tunneling -- found to display a sinusoidal field dependence in accordance with interlayer Josephson tunneling. Studies of the temperature dependence of the supercurrent reveal at best a mild elevation of the Josephson transition temperature compared to the planar Kosterlitz-Thouless temperature. These and other results on the dependence of the model parameters are compared with a standard BCS evaluation., Comment: 11 pages, Rev-Tex, 8 figures

We report on angle-resolved photoemission and superconductor-insulator-superconductor tunneling measurements of the normal and superconducting states of Bi2Sr2CaCu2O8+x single crystal samples and Bi2Sr2CaCu2O8+x - native oxide- lead (S-I-S) tunnel junctions.