H.J.H. Smilde

Ordering and manipulation of the magnetic moments in large-scale superconducting pi-loop arrays

The phase of the macroscopic electron-pair wavefunction in a superconductor can vary only by multiples of 2π when going around a closed contour. This results in quantization of magnetic flux, one of the most striking demonstrations of quantum phase coherence in superconductors. By using superconductors with unconventional pairing symmetry, or by incorporating π-Josephson junctions, a phase shift of π can be introduced in such loops. Under appropriate conditions, this phase shift results in doubly degenerate time-reversed ground states, which are characterized by the spontaneous generation of half quanta of magnetic flux, with magnitude 1/2 Φ0(Φ0 = h/2e = 2.07 × 10-15 Wb) (ref. 7). Until now, it has only been possible to generate individual half flux quanta. Here we report the realization of large-scale coupled π-loop arrays based on YBa2Cu3O7-Au-Nb Josephson contacts. Scanning SQUID (superconducting quantum interference device) microscopy has been used to study the ordering of half flux quanta in these structures. The possibility of manipulating the polarities of individual half flux quanta is also demonstrated. These π-loop arrays are of interest as model systems for studying magnetic phenomena—including frustration effects—in Ising antiferromagnets. Furthermore, studies of coupled π-loops can be useful for designing quantum computers based on flux-qubits with viable quantum error correction capabilities.

Superconducting thin films of MgB2 on Si by pulsed laser deposition

Superconducting thin MgB2 films have been prepared using pulsed-laser deposition. We have studied the influences of deposition conditions such as pressure and temperature, the substrate-material, and annealing-procedures. Various approaches have been pursued to obtain the right Mg content in the film during ablation and annealing. Special care has been taken to avoid oxidation of Mg in the laser plasma and deposited film, by optimizing the background pressure of Ar gas in the deposition chamber. The annealing procedure was found to be the most critical to obtain superconducting films.

Enhanced transparency ramp-type Josephson contacts through interlayer deposition

A thin interlayer is incorporated in ramp-type Josephson junctions to obtain an increased transparency. The interlayer restores the surface damaged by ion milling and has the advantage of an all in situ barrier deposition between two superconductors, leading to clean and well-defined interfaces. The method has been applied to Josephson junctions between high (YBa2Cu3O7–) and low temperature (Nb) superconductors, separated by a Au barrier. Transmission electron microscopy images of these junctions reveal crystalline YBa2Cu3O7– up to the interface with the Au barrier. The junctions have improved critical current density values exceeding 20 kA/cm2, normal state resistances of 3×10–8 cm2 and IcRn products of 0.7 mV at 4.2 K. Furthermore, the junction properties can be controlled by varying the Au barrier thickness.

Antiferromagnetic ordering in arrays of superconducting pi-rings

We report experiments in which continuous YBa2Cu3O7?delta-Nb junctions with internal sign changes in the Josephson coupling, as well as one-dimensional (1D) and two-dimensional (2D) arrays of YBa2Cu3O7?delta-Nb pi-rings, are cooled through the superconducting transition temperature of the Nb in various magnetic fields. These systems have degenerate ground states with either clockwise or counter-clockwise spontaneous circulating supercurrents. The final flux state of each facet corner in the junctions and each ring in the arrays was determined using scanning superconducting quantum interference device (SQUID) microscopy. In the continuous junctions, fabricated with facets alternating between alignment parallel to a [100] axis of the YBCO and rotated 90° to that axis, half-fluxon Josephson vortices order strongly into an arrangement with alternating signs of their magnetic flux. We demonstrate that this ordering is driven by phase coupling and model the cooling process with a numerical solution of the Sine-Gordon equation. The 2D ring arrays couple to each other through the magnetic flux generated by the spontaneous supercurrents. Using pi-rings for the 2D flux coupling experiments eliminates one source of static disorder seen in similar experiments using conventional superconducting rings [Davidovic et al., Phys. Rev. Lett. 76, 815 (1996)], since pi-rings have doubly degenerate ground states in the absence of an applied field. Although anti-ferromagnetic ordering occurs, with larger negative bond orders than previously reported for arrays of conventional rings, ordering over more than a few lattice spacings is never observed in the 2D arrays, even in geometries without geometric frustration. Monte Carlo simulations of the 2D array cooling process are presented and compared with the experiment.

Phase-Sensitive order parameter symmetry test experiments utilizing Nd2-xCexCuO4-y/Nb zigzag junctions

Phase-sensitive order parameter symmetry test experiments are presented on the electron-doped high-T(c) cuprate Nd(2-x)Ce(x)CuO(4-y). These experiments have been conducted using zigzag-shaped thin film Josephson structures, in which the Nd(2-x)Ce(x)CuO(4-y) is connected to the low-T(c) superconductor Nb via an Au barrier layer. For the optimally doped as well as for the overdoped Nd(2-x)Ce(x)CuO(4-y), a clear predominant d(x2-y2)-wave behavior is observed at T=4.2 K. Both compounds were also investigated at T=1.6 K, presenting no indications for a change to a predominant s-wave symmetry with decreasing temperature.

Selective epitaxial growth for YBCO thin films

A novel selective epitaxial growth (SEG) technique for (YBCO) thin films is presented. The method involves the deposition of a thin (about 10 nm) metal layer, in the desired pattern, on a substrate before the deposition of the superconducting thin film. During growth the metal reacts with the YBCO, forming locally an insulating compound. Best results are obtained with Ti or W, yielding structures with sharp boundaries and creating insulating areas with high resistivities. The technique has been analysed for the titanium case. It was found that during the YBCO growth the titanium layer reacts with the YBCO to form an amorphous Ba-Ti-O compound. The YBCO cannot grow epitaxially on top of this layer, and a mainly amorphous YBCO film with insulating characteristics results. The resistivity of the insulating parts has been investigated as a function of the layer thicknesses of both the titanium and the YBCO. Both increasing the YBCO layer or a slight decrease of the titanium layer thickness yields a strong decrease in the resistivity. The SEG technique has been successfully applied to create submicron patterns, without any sign of degradation. Bridge structures with widths down to 200 nm could be prepared that still showed a value of 89 K and values in excess of at 77 K.

Bistable superconducting quantum interference device with built-in switchable π∕2 phase shift

Superconducting quantum interference devices (SQUIDs) have been realized with a built-in π∕2 phase shift of the superconducting wave function. Its design is inspired by the phase-shifter concept, first proposed by Ioffe et al. [Nature 398, 679 (1999)] as the key element in a solid-state qubit configuration. It exploits the dx2−y2 order parameter symmetry in the high-Tc cuprates in multilayer structures based on YBa2Cu3O7-Nb ramp-type Josephson contacts. By reversing the sign of the persistent circulating current in the phase-shifting element, the π∕2 SQUID can be controllably switched between two energetically degenerate ground states, with complementary magnetic flux dependencies.

Realization and properties of ramp-type YBa2Cu3O7−δ/Au/Nb junctions

All-thin-film ramp type Josephson junctions between YBa2Cu3O7−δ and Nb have been fabricated. This procedure allows connections between high-Tc and low-Tc superconductors at different crystal sides of the high-Tc superconductor on one chip, which is of great interest for novel phase devices. A thin Au layer is incorporated as a chemical barrier to avoid oxygen transfer from the YBa2Cu3O7−δ to the Nb. Critical current densities up to 600 A/cm2 are obtained at T=4.2 K, with typical RnA values of 0.8 μΩ cm2. The variation of the magnetic field dependence of the critical current with the angle between the junction barrier and the YBa2Cu3O7−δ crystal axes is explained by considering a predominant dx2−y2 order parameter symmetry of the YBa2Cu3O7−δ. The successful fabrication of these junctions allows the implementation of novel superconducting electronics, such as complementary Josephson circuitry or proposed qubit concepts, using the unconventional order parameter symmetry of the high-Tc superconductor.

Observation of Andreev bound states in YBa2Cu3O7-x/Au/Nb ramp-type Josephson junctions

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.

Josephson coupling in untwinned YBa2Cu3O7-x/Nb d-wave junctions

Theory predicts that d-wave superconductivity induces a significant second harmonic J2 in the Josephson current, as a result of zero-energy Andreev states (ZES) formed at the junction interface. Consequently, anomalies such as half-integer Shapiro steps should be observed. Both ZES formation and J2 are expected to be highly anisotropic as we change the tunneling orientation in the ab plane reaching their maximum for tunneling close to [110] direction and their minimum for [100] or [010] directions. We performed experiments on junctions between untwinned d-wave YBa2Cu3O7-x and Nb and found clear evidence of ZES formation for all 72 different tunneling directions in the ab plane investigated. However, in contrast to the theoretical predictions, we found no trace of half-integer Shapiro steps. That suggests J2 is insignificantly small compared to the first Josephson harmonic for all of the orientations. We believe that microscopic scale roughness, or diffusive reflection at a scale that is much smaller than the Fermi wavelength, dramatically suppresses J2 due to scattering processes. Our findings suggest that YBa2Cu3O7-x/Nb d-wave junctions have a purely sinusoidal current-phase relation which is essential to take into consideration for their implementation as qubits or π-junctions in digital circuits.