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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. We verify the appearance of cavity modes at low bias and in the high input-power regime we find that 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.

We fabricated high quality Nb/Al 2 O 3 /Ni 0.6 Cu 0.4 /Nb superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Using a ferromagnetic layer with a step-like thickness, we obtain a 0-π junction, with equal lengths and critical currents of 0 and π parts. The ground state of our 330 µm (1.3λ J ) long junction corresponds to a spontaneous vortex of supercurrent pinned at the 0-π step and carrying ∼ 6.7% of the magnetic flux quantum Φ 0 . The dependence of the critical current on the applied magnetic field shows a clear minimum in the vicinity of zero field. PACS numbers: 74.50.+r, In his classical paper[1] Brian Josephson predicted that the supercurrent through a Josephson junction (JJ) is given by I s = I c sin(µ). Here, µ is the Josephson phase (the difference of phases of the quantum mechanical wave functions describing the superconducting condensate in the electrodes), and I c > 0 is the critical current (maximum supercurrent that one can pass through the JJ). When one passes no current (I s = 0), the Josephson phase µ = 0 corresponds to the minimum of energy (ground state). The solution µ = π corresponds to the energy maximum and is unstable. Later it was suggested that using a ferromagnetic barrier one can realize JJs where. Such junctions obviously have µ = π in the ground state and, therefore, are called π JJs. The solution µ = 0 corresponds to the energy maximum and is unstable.π JJs were recently realized using superconductor-ferromagnet-superconductor (SFS) [3,4,5,6], superconductorinsulator-ferromagnet-superconductor (SIFS) [7] and other [8] technologies. In these junctions the sign of the critical current and, therefore, the phase µ (0 or π) in the ground state, depends on the thickness d F of the ferromagnetic layer and on temperature T [9]. π JJs may substantially improve parameters of various classical and quantum electronic circuits [10,11,12,13,14,15]. To use π JJs not only as a "phase battery", but also as an active (switching) element in various circuits it is important to have a rather high characteristic voltage V c (defined e.g. as V at I = 1.2I c ) and low damping. For example, for classical single flux quantum logic circuits V c defines the speed of operation. For qubits the value of a quasi-particle resistance R qp at V = 0 should be high enough since it defines the decoherence time of the circuits. Both high values of R qp and V c can be achieved by using tunnel SIFS JJs rather than SFS JJs. The dissipation in SIFS JJs decreases exponentially at low temperatures [16], thus, making SIFS technology an appropriate candidate for creating low decoherence quantum circuits, e.g., π qubits. [13,14,15].Actually, the most interesting situation is when one half of the JJ (x < 0) behaves as a 0 JJ, and the other half (x > 0) as a π JJ (a 0-π JJ) [17]: In the symmetric case (equal critical currents and lengths of 0 and π parts) the ground state of such a 0-π JJ corresponds to a spontaneously formed vortex of supercurrent circulating around the 0-π boundary, generating magnetic...

Microscopic studies of superconductors and their vortices play a pivotal role in understanding the mechanisms underlying superconductivity. Local measurements of penetration depths or magnetic stray fields enable access to fundamental aspects such as nanoscale variations in superfluid densities or the order parameter symmetry of superconductors. However, experimental tools that offer quantitative, nanoscale magnetometry and operate over large ranges of temperature and magnetic fields are still lacking. Here, we demonstrate the first operation of a cryogenic scanning quantum sensor in the form of a single nitrogen-vacancy electronic spin in diamond, which is capable of overcoming these existing limitations. To demonstrate the power of our approach, we perform quantitative, nanoscale magnetic imaging of Pearl vortices in the cuprate superconductor YBa2Cu3O7-δ. With a sensor-to-sample distance of ∼10 nm, we observe striking deviations from the prevalent monopole approximation in our vortex stray-field images, and find excellent quantitative agreement with Pearl's analytic model. Our experiments provide a non-invasive and unambiguous determination of the system's local penetration depth and are readily extended to higher temperatures and magnetic fields. These results demonstrate the potential of quantitative quantum sensors in benchmarking microscopic models of complex electronic systems and open the door for further exploration of strongly correlated electron physics using scanning nitrogen-vacancy magnetometry.

We report on THz emission measurements and low temperature scanning laser imaging of Bi2Sr2CaCu2O8 intrinsic Josephson junction stacks. Coherent emission is observed at large dc input power, where a hot spot and a standing wave, formed in the "cold" part of the stack, coexist. By changing bias current and bath temperature, the emission frequency can be varied by more than 40%; the variation matches the Josephson-frequency variation with voltage. The linewidth of radiation is much smaller than expected from a purely cavity-induced synchronization. Thus, an additional mechanism seems to play a role. Some scenarios, related to the presence of the hot spot, are discussed.

We fabricated high quality Nb/Al2O3/Ni0.6Cu0.4/Nb superconductor-insulator-ferromagnetsuperconductor Josephson tunnel junctions. Depending on the thickness of the ferromagnetic Ni0.6Cu0.4 layer and on the ambient temperature, the junctions were in the 0 or π ground state. All junctions have homogeneous interfaces showing almost perfect Fraunhofer patterns. The Al2O3 tunnel barrier allows to achieve rather low damping, which is desired for many experiments especially in the quantum domain. The McCumber parameter βc increases exponentially with decreasing temperature and reaches βc ≈ 700 at T = 2.11 K. The critical current density in the π state was up to 5 A/cm 2 at T = 2.11 K, resulting in a Josephson penetration depth λJ as low as 160 µm. Experimentally determined junction parameters are well described by theory taking into account spin-flip scattering in the Ni0.6Cu0.4 layer and different transparencies of the interfaces. , which is self-biased and well decoupled from the environment, one needs to use high quality π JJs with high resistance (to avoid decoherence) and reasonably high critical current density j c (to have the Josephson energy E J ≫ k B T for junction sizes of few microns or below). High j c is also required to keep the Josephson plasma frequency ω p ∝ √ j c , which plays the role of an attempt frequency in the quantum tunneling problem, on the level of a few GHz.The concept of π JJs was introduced long ago[5, 6], but only recently superconductor-ferromagnet-superconductor (SFS) π JJs were realized [7,8]. Unfortunately SFS π JJs are highly overdamped and cannot be used for applications where low dissipation is required. The obvious way to decrease damping is to make a SFS-like tunnel junction, i.e. a superconductor-insulator-ferromagnet-superconductor (SIFS) junction. Due to the presence of the tunnel barrier the critical current I c in SIFS is lower than in SFS, but both the resistance R (at I I c ) and the I c R product are much higher. Moreover, the value of I c and R can be tuned by changing the thickness d I of the insulator (tunnel barrier).

Several recent experiments revealed a change of the sign of the first harmonic in the current-phase relation of Josephson junctions (JJs) based on novel superconductors, e.g., d-wave based JJs or JJ with ferromagnetic barrier. In this situation the role of the second harmonic can become dominant; in this case, it determines the scenario of a 0-π transition. We discuss different mechanisms of the second harmonic generation and its sign. If the second harmonic is negative, the 0-π transition becomes continuous and the realization of a so-called ϕ junction is possible. We study the unusual properties of such a novel JJ such as critical currents, magnetic field penetration, plasma gap, and microwave response. We also analyze the possible experimental techniques for their observation.

We propose, implement, and test experimentally long Josephson 0-pi junctions fabricated using conventional Nb-AlOx-Nb technology. We show that by using a pair of current injectors one can create an arbitrary discontinuity of the Josephson phase and, in particular, a pi discontinuity, just as in d-wave/s-wave or in d-wave/d-wave junctions, and study fractional Josephson vortices which spontaneously appear. Moreover, using such junctions, we can investigate the dynamics of the fractional vortices-a domain which is not yet available for natural 0-pi junctions due to their inherently high damping. We observe half-integer zero-field steps which appear on the current-voltage characteristics due to the hopping of semifluxons.