Horst Rogalla

Quasi-ideal strontium titanate crystal surfaces through formation of stontium hydroxide

In recent years, well-defined and nearly perfect single crystal surfaces of oxide perovskites have become increasingly important. A single terminated surface is a prerequisite for reproducible thin film growth and fundamental growth studies. In this work, atomic and lateral force microscopy have been used to display different terminations of SrTiO3. We observe hydroxylation of the topmost SrO layer after immersion of SrTiO3 in water, which is used to enhance the etch-selectivity of SrO relative to TiO2 in a buffered HF solution. We reproducibly obtain perfect and single terminated surfaces, irrespective of the initial state of polished surfaces and the pH value of the HF solution. This approach to the problem might be used for a variety of multi-component oxide single crystals. True two-dimensional reflection high-energy electron diffraction intensity oscillations are observed during homo epitaxial growth using pulsed laser deposition on these surfaces.

In-situ monitoring during pulsed laser deposition of complex oxides using reflection high energy electron diffraction under high oxygen pressure

A suitable in situ monitoring technique for growth of thin films is reflection high energy electron diffraction (RHEED). Deposition techniques, like pulsed laser deposition (PLD) and sputter deposition, used for fabrication of complex oxide thin films use relatively high oxygen pressures (up to 100 Pa) and are, therefore, not compatible with ultrahigh vacuum RHEED equipment. We have developed a RHEED system which can be used for growth monitoring during the deposition of complex oxides at standard PLD conditions. We are able to increase the deposition pressure up to 50 Pa using a two-stage differential pumping system. Clear RHEED patterns are observable at these high pressures. The applicability of this system is demonstrated with the study of homoepitaxial growth of SrTiO3 as well as the heteroepitaxial growth of YBa2Cu3O7-δ on SrTiO3. Intensity oscillations of the RHEED reflections, indicating two-dimensional growth, are observed up to several tens of nanometers film thickness in both cases

Multiband model for tunneling in MgB2 junctions

A theoretical model for quasiparticle and Josephson tunneling in multiband superconductors is developed and applied to MgB2-based junctions. The gap functions in different bands in MgB2 are obtained from an extended Eliashberg formalism, using the results of band structure calculations. The temperature and angle dependencies of MgB2 tunneling spectra and the Josephson critical current are calculated. The conditions for observing one or two gaps are given. We argue that the model may help to settle the current debate concerning two-band superconductivity in MgB2.

Controlled preparation of all high-Tc SNS-type edge junctions and DC SQUIDs

High-Tc SNS-type Josephson junctions and DC SQUIDs were successfully fabricated using hetero-epitaxially grown multilayers of YBa2Cu3Ox and PrBa2Cu3Ox. These layers are c-axis oriented and hence edges of the multilayers give rise to a current flow in the ab-plane between the electrodes of a Josephson junction. The necessary structuring was done by Ar ion beam etching. The individual junctions exhibit a supercurrent up to 80 K. The IcRn-product of these junctions usually has a lower limit of 8 mV at 4.2 K. Voltage modulation of the first DC SQUIDs can be observed up to 66 K. Details on the fabrication and measurements are presented.

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.

YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox Josephson ramp junctions

A detailed study of the fabrication process, current voltage (I‐V) characteristics, and Josephson and normal‐state properties of the YBa2Cu3Ox(YBCO)/PrBa2Cu3Ox(PBCO)/YBCO ramp junctions is presented. The I‐V characteristics can be well described by the resistively shunted junction model. It was found that the critical current Ic and the normal‐state conductance 1/Rn scale linearly with the junction area, whereas Ic, the excess current Iex, and IcRn products decrease with increasing barrier thickness. These junctions with cross‐sectional area A have a good controllability, low capacitance, and high values of IcRn and RnA products. The coherence length ξn of the PBCO barrier is estimated to be between 5 and 8 nm. As unambiguous evidence of the Josephson behavior, the microwave response as a function of the microwave power as well as the modulations of critical current Ic(H) with applied magnetic field are shown. A modulation depth of more than 95% has been observed. Small proximity effect parameters and jun...

Growth mode transition from layer by layer to step flow during the growth of heteroepitaxial SrRuO3 on (001) SrTiO3

We have observed the growth mode transition from two-dimensional (2D) layer-by-layer to step-flow in the earliest stage growth of heteroepitaxial SrRuO3 thin films on TiO2-terminated (001) SrTiO3 substrates by in situ high pressure reflective high energy electron diffraction (RHEED) and atomic-force microscopy. There is no RHEED intensity recovery after each laser pulse in the first oscillation when the growth mode is 2D layer-by-layer. On the other hand, it is getting more pronounced in the second oscillation, and finally reaches a dynamic steady state in which the growth mode is completely changed into the step-flow mode. The origin of the growth mode transition can be attributed to a change in the mobility of adatoms and switching the surface termination layer from the substrate to the film. SrRuO3 thin films with an atomically smooth surface grown by atomic layer control can be used in oxide multilayered heterostructure devices.

Superconducting Mg-B films by pulsed-laser deposition in an in situ two-step process using multicomponent targets

Superconducting thin films have been prepared in an insitu two-step process, using the Mg–B plasma generated by pulsed-laser ablation. The target was composed of a mixture of Mg and MgB2 powders to compensate for the volatility of Mg and, therefore, to ensure a high Mg content in the film. The films were deposited at temperatures ranging from room temperature to 300 °C followed by a low-pressure insitu annealing procedure. Various substrates have been used and diverse ways to increase the Mg content into the film were applied. The films show a sharp transition in the resistance and have a zero resistance transition temperature of 22–24 K.of 22-24 K.

Superconducting quantum interference device based on MgB2 nanobridges

The superconductor MgB2, with a transition temperature of 39 K, has significant potential for future electronics. An essential step is the achievement of Josephson circuits, of which the superconducting quantum interference device ~SQUID! is the most important. Here, we report Josephson quantum interference in superconducting MgB2 thin films. Modulation voltages of up to 30 mV are observed in an all-MgB2 SQUID, based on focused-ion-beam patterned nanobridges. These bridges, with a length scale ,100 nm, have outstanding critical current densities of 73106 A/cm2 at 4.2 K.

Characteristics of advanced YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox edge type junctions

This letter describes the scaling behavior and Josephson properties of improved YBCO/PBCO/YBCO edge‐type junctions. The critical current, normal‐state resistance, and IcRn product scale with barrier thickness and junction area. The coherence length of the PBCO barrier is estimated to be between 5 and 8 nm. As unambiguous evidence of the Josephson behavior, the microwave response as a function of microwave power, as well as the current modulation with applied magnetic field, have been studied: well‐developed Shapiro steps at 10 GHz have been observed, and the modulation of Ic(H) shows Fraunhofer‐like behavior with 95% Ic suppression.