On the suppression of the sidelobes of the supercurrent in small Josephson tunnel junctions

The critical currents of Nb/Al, Al-oxide, Al/Nb tunnel junctions of various shapes have been measured as a function of the applied magnetic field. For the square junction and for some special shapes like the diamond, ¿1 + cosine¿ and quartic junctions the Ic(B) pattern falls off theoretically as 1/Bn, with n respectively equal to 1, 2, 3 and 4. In general the measurements are in good agreement with the theoretical predictions. For the "1 + cosine" and quartic shapes we found a sidelobe suppression that is even larger than that obtained in theory. For the quartic junction the first sidelobe is only 0.3% of the zero-field current. An Ic(B) modulation with a small, only slowly decreasing amplitude is observed for the diamond, "1 + cosine", and quartic junctions, that can be explained by rounding of the sharp edges of the junction shapes, due to the fabrication process

X-ray response of tunnel junctions with a trapping layer

The use of trapping layers in superconductive tunnel junctions may drastically improve their functioning as X-ray detectors. Information about these trapping layers can be obtained from I/V-curves and X-ray spectra. The application of a magnetic field causes a substantial reduction of the bandgap in the trapping layer

Visualizing the Coupling between Red and Blue Stark States Using Photoionization Microscopy

In nonhydrogenic atoms in a dc electric field, the finite size of the ionic core introduces a coupling between quasibound Stark states that leads to avoided crossings between states that would otherwise cross. Near an avoided crossing, the interacting states may have decay amplitudes that cancel each other, decoupling one of the states from the ionization continuum. This well- known interference narrowing effect, observed as a strongly electric field- dependent decrease in the ionization rate, was previously observed in several atoms. Here we use photoionization microscopy to visualize interference narrowing in helium atoms, thereby explicitly revealing the mechanism by which Stark states decay. The interference narrowing allows measurements of the nodal patterns of red Stark states, which are otherwise not observable due to their intrinsic short lifetime

Direct determination of the sign of the NO dipole moment.

We report a novel approach for determining the sign of permanent dipole moments, using nitric oxide [NO(v=0)] as an example. State-selected NO (j=|m|=|Ω=1/2) molecules are focused using a hexapole and oriented in a strong dc electric field. The angular distributions of ionic fragments resulting from extreme ultraviolet single-photon and multiphoton dissociative ionization at 400 and 800 nm are measured and indicate that the dipole moment is negative (corresponding to N-O+). The experiments thus rule out an error in the sign of the dipole of NO as the possible source of a remarkable discrepancy between previous theoretical and experimental work on orientation effects in bimolecular collisions involving oriented NO. © 2007 The American Physical Society

Controlling the quantum stereodynamics of ultracold bimolecular reactions

Chemical reaction rates often depend strongly on stereodynamics, namely the orientation and movement of molecules in three-dimensional space. An ultracold molecular gas, with a temperature below 1 uK, provides a highly unusual regime for chemistry, where polar molecules can easily be oriented using an external electric field and where, moreover, the motion of two colliding molecules is strictly quantized. Recently, atom-exchange reactions were observed in a trapped ultracold gas of KRb molecules. In an external electric field, these exothermic and barrierless bimolecular reactions, KRb+KRb -> K2+Rb2, occur at a rate that rises steeply with increasing dipole moment. Here we show that the quantum stereodynamics of the ultracold collisions can be exploited to suppress the bimolecular chemical reaction rate by nearly two orders of magnitude. We use an optical lattice trap to confine the fermionic polar molecules in a quasi-two-dimensional, pancake-like geometry, with the dipoles oriented along the tight confinement direction. With the combination of sufficiently tight confinement and Fermi statistics of the molecules, two polar molecules can approach each other only in a "side-by-side" collision, where the chemical reaction rate is suppressed by the repulsive dipole-dipole interaction. We show that the suppression of the bimolecular reaction rate requires quantum-state control of both the internal and external degrees of freedom of the molecules. The suppression of chemical reactions for polar molecules in a quasi-two-dimensional trap opens the way for investigation of a dipolar molecular quantum gas. Because of the strong, long-range character of the dipole-dipole interactions, such a gas brings fundamentally new abilities to quantum-gas-based studies of strongly correlated many-body physics, where quantum phase transitions and new states of matter can emerge.Comment: 19 pages, 4 figure

The importance of non-LTE models for the interpretation of observations of interstellar NO

Context. The ALMA and Herschel missions promise to deliver data of high spatial and spectral resolution at far-infrared and submillimeter wavelengths. Modeling these data will require the knowledge of accurate radiative and collisional rates for species of astrophysical interest. Aims. We calculate the rotational excitation rate coefficients of NO in collisions with He and check the validity of the LTE approach in interpreting observations of rotational lines of NO. Methods. State-to-state rate coefficients between the 360 lowest hyperfine levels of NO were calculated using the M(J) randomizing limit method for temperatures from 10 K to 350 K. We performed calculations of the excitation of NO using the new rate coefficients using a large velocity gradient (LVG) radiative transfer code. Results. The critical densities of the lines are found to be at greater than or similar to 10(15) cm(-3). The low dipole moment of NO ensures that the line emission is optically thin up to column densities of similar to 10(15) cm(-2). Lines in the ground (Omega = 1/2) state are readily detectable in typical conditions (N(NO) = 10(13) cm(-2); T = 10-30 K), whereas lines in the excited Omega = 3/2 state are observed only in warm (T > 50 K) regions with higher column densities (N(NO) = 10(14) cm(-2)). Line ratios of NO may well be used to constrain the ambient temperature and/or density. Conclusions. The new rate coefficients will help significantly in interpreting NO emission lines observed with current and future telescopes, and enable this molecule to become a powerful astrophysical tool

Josephson Coupling and Fiske Dynamics in Ferromagnetic Tunnel Junctions

We report on the fabrication of Nb/AlO_x/Pd_{0.82}Ni_{0.18}/Nb superconductor/insulator/ferromagnetic metal/superconductor (SIFS) Josephson junctions with high critical current densities, large normal resistance times area products, high quality factors, and very good spatial uniformity. For these junctions a transition from 0- to \pi-coupling is observed for a thickness d_F ~ 6 nm of the ferromagnetic Pd_{0.82}Ni_{0.18} interlayer. The magnetic field dependence of the \pi-coupled junctions demonstrates good spatial homogeneity of the tunneling barrier and ferromagnetic interlayer. Magnetic characterization shows that the Pd_{0.82}Ni_{0.18} has an out-of-plane anisotropy and large saturation magnetization, indicating negligible dead layers at the interfaces. A careful analysis of Fiske modes provides information on the junction quality factor and the relevant damping mechanisms up to about 400 GHz. Whereas losses due to quasiparticle tunneling dominate at low frequencies, the damping is dominated by the finite surface resistance of the junction electrodes at high frequencies. High quality factors of up to 30 around 200 GHz have been achieved. Our analysis shows that the fabricated junctions are promising for applications in superconducting quantum circuits or quantum tunneling experiments.Comment: 15 pages, 9 figure