C. M. Muirhead

Properties of superconducting planar resonators at millikelvin temperatures

Planar superconducting resonators are now being increasingly used at mK temperatures in a number of novel applications. However, they are also interesting devices in their own right. We have experimentally investigated three types of niobium resonators fabricated on sapphire and SiO2/Si substrates. They all exhibit a non-trivial temperature dependence of the centre frequency and quality factor. Our results are consistent with the idea that the resonators are affected by the presence of two-level fluctuators in the substrate.

Magnetic field tuning of coplanar waveguide resonators

We describe measurements on microwave coplanar resonators designed for quantum bit experiments. Resonators have been patterned onto sapphire and silicon substrates, and quality factors in excess of a million have been observed. The resonant frequency shows a high sensitivity to magnetic field applied perpendicular to the plane of the film, with a quadratic dependence for the fundamental, second, and third harmonics. Frequency shift of hundreds of linewidths can be obtained with no change in the quality factor.

Effect of spin-polarized injection on the mixed state of YBa2Cu3O7−δ

We report measurements of the magnetic moment of YBa2Cu3O7−δ films when subjected to injection of short pulses of spin-polarized current from the colossal magnetoresistance (CMR) manganites La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3, and unpolarized current from the nonmagnetic perovskite LaNiO3(LNO). We study relaxation from the Bean critical state, with and without current pulses. For pulses longer than 100 ms, the effect of injection is similar for all samples and is due to heating. Below 1 ms, we see a clear difference between the CMR and LNO samples and attribute this to the spin polarization of the current injected from the CMR materials.

Superconductivity in thin-film YBa2Cu3O7−δ/La0.7Ca0.3MnO3 bilayers

We report the influence of the magnetic state of thin films of La0.7Ca0.3MnO3 (LCMO) on the superconducting order parameter in LCMO/YBa2Cu3O7−δ (YBCO) thin-film bilayers. We find that the number density of Cooper pairs is enhanced at the coercive field of the LCMO layer relative to that where the magnetic moment is saturated. This effect is at most ∼10% and is only observable within a few degrees of the superconducting transition temperature. Experiments with thin SrTiO3 layers between the LCMO and YBCO layers provide strong evidence that the effect is a consequence of the spin-polarized nature of the electrons at the interface, rather than a direct magnetic effect.

Materials aspects of laser-ablation-deposited cuprate∕manganate bilayers for spin-polarized injection devices

We report studies of cuprate∕manganate bilayers deposited onto SrTiO3 substrates by laser ablation. A significant difference in the properties of the individual layers is observed depending on the material deposition sequence. The material which is on the bottom of the multilayer shows worse properties, e.g., a critical temperature of ∼60K for YBa2Cu3O7−δ (YBCO) covered by La2∕3Ca1∕3MnO3 (LCMO) as opposed to a value of ∼90K when YBCO is on the top. Similarly, the LCMO film has its metal-insulator transition temperature shifted below the Curie temperature when the YBCO is on top. We argue that the effect is not due to degraded crystal quality, cation substitution, or an electronic interaction, but results from a reduction in the oxygen content of the underlying layer. We find that this reduction is a feature of the complete LCMO layer and is not associated with the constituent elements. We discuss possible explanations for this effect.

Spin-polarized injection into YBaCu3O7 grain boundary junctions

YBa2Cu3O7/SrTiO3/La0.67Ca0.33MnO3 trilayer thin films grown on bicrystal substrates were patterned into a structure that allows the injection of spin-polarized current into the region of a grain boundary junction (GBJ). The transfer length LT was determined to ensure that injection was over the entire device width, in contrast to previously reported experiments. Currents up to some 100 times the junction critical current I0 and area density 108 A m−2, have been injected into the GBJ region by using an offset current method. The I0(B) characteristics of the GBJ under injection suggest that the suppression of I0 can be explained by a combination of heating and self-field effects, without any need to invoke the spin-polarized nature of the injected current. We find no evidence of direct injection into the junction.

Critical current control in a superconducting thin film with a ferromagnetic layer

We report suppression of the critical current of thin film YBa/sub 2/Cu/sub 3/O/sub 7/ layers by altering the magnetization of a ferromagnetic La/sub 0.67/Ca/sub 0.33/MnO/sub 3/ overlayer. We provide evidence that the critical current suppression is related to the spin-polarized state of the magnetic layer, rather than a direct magnetic influence. We argue that the spin diffusion length in the superconducting layer is small compared with the typical domain size in the ferromagnetic layer in its demagnetized state.

Magnetic and superconducting properties of CMR/HTS multilayers for spin injection devices

Abstract We report substantial improvement in the critical current in HTS thin film due to the influence of colossal magnetoresistance (CMR) films in YBa 2 Cu 3 O x -based multilayers for spin injection devices. The effect of the CMR strongly depends on the thickness of the HTS, CMR and the intermediate insulator layers. We argue that the improvement in critical current may be due to the self-injection of the spin polarized electrons from the CMR to HTS, opposite to that normally observed in current driven spin injection devices.

Properties of high-quality coplanar waveguide resonators for QIP and detector applications

We have fabricated and characterized λ/2 thin-film coplanar waveguide resonators made from niobium deposited on sapphire and SiO2/Si substrates. The samples have been characterized at temperatures down to 30 mK. In this work we discuss two important properties of these resonators: The tunability of the centre frequency using a weak applied magnetic field, and the RF-power dependence of the quality factor.

Numerical simulations of a flux qubit coupled to a high quality resonator

Circuit-QED - the electrical analogue of cavity quantum electrodynamics in atomic physics- is an emerging field of research. By coupling superconducting qubits to an on-chip high-quality resonator it is possible to study a number of effects that are of interest for fundamental tests of quantum mechanics and also have potential applications in quantum information processing, metrology and communication. We have performed computer simulations of a system comprising a flux qubit strongly coupled to a high-quality coplanar waveguide resonator driven by an external microwave source. Using realistic values for the various parameters we have studied, among other things, the transmittance of the cavity and the spectrum of the emitted radiation.