nan Ariando

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.

Origin of the Two-Dimensional Electron Gas at LaAlO3=SrTiO3 Interfaces: The Role of Oxygen Vacancies and Electronic Reconstruction

The relative importance of atomic defects and electron transfer in explaining conductivity at the crystalline LaAlO3/SrTiO3 interface has been a topic of debate. Metallic interfaces with similar electronic properties produced by amorphous oxide overlayers on SrTiO3 have called in question the original polarization catastrophe model. We resolve the issue by a comprehensive comparison of (100)-oriented SrTiO3 substrates with crystalline and amorphous overlayers of LaAlO3 of different thicknesses prepared under different oxygen pressures. For both types of overlayers, there is a critical thickness for the appearance of conductivity, but its value is always 4 unit cells (around 1.6 nm) for the oxygen-annealed crystalline case, whereas in the amorphous case, the critical thickness could be varied in the range 0.5 to 6 nm according to the deposition conditions. Subsequent ion milling of the overlayer restores the insulating state for the oxygen-annealed crystalline heterostructures but not for the amorphous ones. Oxygen post-annealing removes the oxygen vacancies, and the interfaces become insulating in the amorphous case. However, the interfaces with a crystalline overlayer remain conducting with reduced carrier density. These results demonstrate that oxygen vacancies are the dominant source of mobile carriers when the LaAlO3 overlayer is amorphous, while both oxygen vacancies and polarization catastrophe contribute to the interface conductivity in unannealed crystalline LaAlO3/SrTiO3 heterostructures, and the polarization catastrophe alone accounts for the conductivity in oxygen-annealed crystalline LaAlO3/SrTiO3 heterostructures. Furthermore, we find that the crystallinity of the LaAlO3 layer is crucial for the polarization catastrophe mechanism in the case of crystalline LaAlO3 overlayers.

Room temperature ferromagnetism in partially hydrogenated epitaxial graphene

We report room temperature ferromagnetism in partially hydrogenated epitaxial graphene grown on 4HSiC(0001). The presence of ferromagnetism was confirmed by superconducting quantum interference devices measurements. Synchrotron-based near-edge x-ray absorption fine structure and high resolution electron energy loss spectroscopy measurements have been used to investigate the hydrogenation mechanism on the epitaxial graphene and the origin of room temperature ferromagnetism. The partial hydrogenation induces the formation of unpaired electrons in graphene, which together with the remnant delocalized π bonding network, can explain the observed ferromagnetism in partially hydrogenated epitaxial graphene.

Oxygen electrocatalysis on (001)-oriented manganese perovskite films: Mn valency and charge transfer at the nanoscale

We report that the oxygen reduction reaction (ORR) activities of (001)-oriented manganese perovskite films decrease from 10 to 1 nm by more than an order of magnitude, which can be attributed to the barrier associated with interfacial band bending that impedes electron transfer to the electrolyte, and reduction of Mn3+ due to charge transfer from the Nb:SrTiO3 substrate. Furthermore, we show by substitution in La1−x(Ca,Sr)xMnO3 that Mn3+, not Mn4+, is the active valence state for ORR.

Metal-insulator transition in SrTiO(3-x) thin films induced by frozen-out carriers.

Z. Q. Liu, D. P. Leusink, X. Wang, W. M. Lü, K. Gopinadhan, A. Annadi, Y. L. Zhao, X. H. Huang, S. W. Zeng, Z. Huang, A. Srivastava, S. Dhar, T. Venkatesan, and Ariando1,2∗ NUSNNI-Nanocore, Department of Physics, Department of Electrical and Computer Engineering, National University of Singapore, Singapore and Faculty of Science and Technology and MESA Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands (Dated: January 13, 2013)

Highly Active Epitaxial La(1-x)Sr(x)MnO3 Surfaces for the Oxygen Reduction Reaction: Role of Charge Transfer.

Most studies of oxide catalysts for the oxygen reduction reaction (ORR) use oxide powder, where the heterogeneity of exposed surfaces and the composite nature of electrodes limit fundamental understanding of the reaction mechanism. We present the ORR activity of epitaxially oriented La(1-x)Sr(x)MnO3 surfaces and investigate, by varying Sr substitution, the relationship between the role of charge transfer and catalytic activity in an alkaline environment. The activity is greatest for La(1-x)Sr(x)MnO3 with 33% Sr, containing mixed Mn(3+/4+), and the (110) and (111) orientations display comparable activities to that of the (001). Electrochemical measurements using the facile redox couple [Fe(CN)6](3-/4-) illustrate that increasing ORR activity trends with faster charge-transfer kinetics, indicating the importance of facile charge transfer at the oxide/water interface and mixed Mn valence in promoting ORR kinetics.

Admixtures to d-Wave Gap Symmetry in Untwinned YBa2Cu3O7 Superconducting Films Measured by Angle-Resolved Electron Tunneling

We report on an ab anisotropy of Jc parallel b/Jc parallel a approximately/= 1.8 IcRn parallelb/IcRn parallel a approximately/= 1.2 and in ramp-edge junctions between untwinned YBa2Cu3O7 and s-wave Nb. For these junctions, the angle theta with the YBa2Cu3O7 crystal b axis is varied as a single parameter. The RnA(theta) dependence presents twofold symmetry. The minima in IcRn at theta approximately/= 50 degrees suggest a real s-wave subdominant component and negligible d(xy)-wave or imaginary s-wave admixtures. The IcRn(theta) dependence is well fitted by 83% dx2-y2-, 15% isotropic s-, and 2% anisotropic s-wave order parameter symmetry, consistent with deltab/deltaa approximately/= 1.5.

Flip-flopping fractional flux quanta.

The d-wave pairing symmetry in high–critical temperature superconductors makes it possible to realize superconducting rings with built-in π phase shifts. Such rings have a twofold degenerate ground state that is characterized by the spontaneous generation of fractional magnetic flux quanta with either up or down polarity. We have incorporated π phase–biased superconducting rings in a logic circuit, a flip-flop, in which the fractional flux polarity is controllably toggled by applying single flux quantum pulses at the input channel. The integration of p rings into conventional rapid single flux quantum logic as natural two-state devices should alleviate the need for bias current lines, improve device symmetry, and enhance the operation margins.

Unexpected Anisotropic Two Dimensional Electron Gas at the LaAlO3/SrTiO3 (110) Interface

The observation of a high-mobility two-dimensional electron gas between two insulating complex oxides, especially LaAlO3/SrTiO3, has enhanced the potential of oxides for electronics. The occurrence of this conductivity is believed to be driven by polarization discontinuity, leading to an electronic reconstruction. In this scenario, the crystal orientation has an important role and no conductivity would be expected, for example, for the interface between LaAlO3 and (110)-oriented SrTiO3, which should not have a polarization discontinuity. Here we report the observation of unexpected conductivity at the LaAlO3/SrTiO3 interface prepared on (110)-oriented SrTiO3, with a LaAlO3-layer thickness-dependent metal-insulator transition. Density functional theory calculation reveals that electronic reconstruction, and thus conductivity, is still possible at this (110) interface by considering the energetically favourable (110) interface structure, that is, buckled TiO2/LaO, in which the polarization discontinuity is still present. The conductivity was further found to be strongly anisotropic along the different crystallographic directions with potential for anisotropic superconductivity and magnetism, leading to possible new physics and applications.

Imaging and control of ferromagnetism in LaMnO3/SrTiO3 heterostructures

Control of magnetism in heterostructures The interface between two different materials in a heterostructure can exhibit properties unique to either of the two materials alone. A well-known example is a conducting gas that forms when LaAlO3 is grown on SrTiO3, but only if the LaAlO3 layer is at least four unit cells thick. Wang et al. report a similarly abrupt magnetic transition in a heterostructure formed by another oxide (LaMnO3) on the same SrTiO3 substrate. Even though bulk LaMnO3 is an antiferromagnet, when six or more unit-cell layers of it were deposited on SrTiO3, it behaved like a ferromagnet. Science, this issue p. 716 Superconducting quantum interference device magnetometry is used to observe a magnetic transition in an oxide heterostructure. Oxide heterostructures often exhibit unusual physical properties that are absent in the constituent bulk materials. Here, we report an atomically sharp transition to a ferromagnetic phase when polar antiferromagnetic LaMnO3 (001) films are grown on SrTiO3 substrates. For a thickness of six unit cells or more, the LaMnO3 film abruptly becomes ferromagnetic over its entire area, which is visualized by scanning superconducting quantum interference device microscopy. The transition is explained in terms of electronic reconstruction originating from the polar nature of the LaMnO3 (001) films. Our results demonstrate that functionalities can be engineered in oxide films that are only a few atomic layers thick.