Spin Dynamics of a Solid-State Qubit in Proximity to a Superconductor.

A broad effort is underway to understand and harness the interaction between superconductors and spin-active color centers with an eye on hybrid quantum devices and novel imaging modalities of superconducting materials. Most work, however, overlooks the interplay between either system and the environment created by the color center host. Here we use a diamond scanning probe to investigate the spin dynamics of a single nitrogen-vacancy (NV) center proximal to a superconducting film. We find that the presence of the superconductor increases the NV spin coherence lifetime, a phenomenon we tentatively rationalize as a change in the electric noise due to a superconductor-induced redistribution of charge carriers near induced redistribution of charge carriers near the NV. We then build on these findings to demonstrate transverse-relaxation-time-weighted imaging of the superconductor film. These results shed light on the dynamics governing the spin coherence of shallow NVs, and promise opportunities for new forms of noise spectroscopy and imaging of superconductors.

The frequency-dependent AC photoresistance behavior of ZnO thin films grown on different sapphire substrates.

Zinc oxide (ZnO) thin films were grown by pulsed layer deposition under an N2 atmosphere at low pressures on a- and r-plane sapphire substrates. Structural studies using X-ray diffraction confirmed that all films had a wurtzite phase. ZnO thin films on a- and r-plane sapphire have grown with orientations along the [0002] and [112[combining macron]0] directions, respectively. Room temperature photoluminescence measurements indicate that the presence of native point defects (interstitial zinc, oxygen vacancies, oxygen antisites and zinc vacancies) is more preponderant for ZnO thin films grown on the r-plane sapphire substrate than the sample grown on the a-plane sapphire substrate. Room temperature impedance spectroscopy measurements were performed in an alternating current frequency range from 40 to 105 Hz in the dark and under normal light. An unusual positive photoresistance effect is observed at frequencies above 100 kHz, which we suggest to be due to intrinsic defects present in the ZnO thin films. Furthermore, an analysis of the optical time response revealed that the film grown on the r-plane sapphire substrate responds faster (characteristic relaxation times for τ1, τ2 and τ3 of 0.05, 0.26 and 6.00 min, respectively) than the film grown on the a-plane sapphire substrate (characteristic relaxation times for τ1, τ2 and τ3 of 0.10, 0.73 and 4.02 min, respectively).

Detection of Defect-Induced Magnetism in Low-Dimensional ZnO Structures by Magnetophotocurrent.

The detection of defect-induced magnetic order in single low-dimensional oxide structures is in general difficult because of the relatively small yield of magnetically ordered regions. In this work, the effect of an external magnetic field on the transient photocurrent measured after light irradiation on different ZnO samples at room temperature is studied. It has been found that a magnetic field produces a change in the relaxation rate of the transient photocurrent only in magnetically ordered ZnO samples. This rate can decrease or increase with field, depending on whether the magnetically ordered region is in the bulk or only at the surface of the ZnO sample. The phenomenon reported here is of importance for the development of magneto-optical low-dimensional oxides devices and provides a new guideline for the detection of magnetic order in low-dimensional magnetic semiconductors.

Electrical properties of ZnO single nanowires.

We have investigated the electrical resistance R(T) of ZnO nanowires of ≈ 400 nm diameter as a function of temperature, between 30 K and 300 K, and frequency in the range 40 Hz to 30 MHz. The measurements were done on the as-prepared and after low-energy proton implantation at room temperature. The temperature dependence of the resistance of the wire, before proton implantation, can be well described by two processes in parallel. One process is the fluctuation induced tunneling conductance (FITC) and the other the usual thermally activated process. The existence of a tunneling conductance was also observed in the current-voltage ([Formula: see text]) results, and can be well described by the FITC model. Impedance spectroscopy measurements in the as-prepared state and at room temperature, indicate and support the idea of two contributions of these two transport processes in the nanowires. Electron backscatter diffraction confirms the existence of different crystalline regions. After the implantation of H(+) a third thermally activated process is found that can be explained by taking into account the impurity band splitting due to proton implantation.

Conducting behavior of chalcopyrite-type CuGaS2 crystals under visible light.

Millimeter size high quality crystals of CuGaS2 were grown by chemical vapor transport. The highly ordered chalcopyrite structure is confirmed by X-ray diffraction and Raman spectroscopy. According to energy dispersive X-ray spectroscopy the composition of the crystals is very close to the formula CuGaS2. Room temperature photoluminescence measurements indicate the presence of an emission peak at about 2.36 eV that can be related to a donor-acceptor pair transition. The electrical resistance as a function of temperature is very well described by the Mott variable range hopping mechanism. Room temperature complex impedance spectroscopy measurements were performed in the alternating current frequency range from 40 to 10(7) Hz in the dark and under normal light. According to the impedance spectroscopy data the experimental results can be well described by two circuits in series, corresponding to bulk and grain boundary contributions. An unusual positive photoresistance effect is observed in the frequency range between 3 and 30 kHz, which we suggest to be due to intrinsic defects present in the CuGaS2 crystal.

On the Localization of Persistent Currents Due to Trapped Magnetic Flux at the Stacking Faults of Graphite at Room Temperature.

Granular superconductivity at high temperatures in graphite can emerge at certain two-dimensional (2D) stacking faults (SFs) between regions with twisted (around the c-axis) or untwisted crystalline regions with Bernal (ABA…) and/or rhombohedral (ABCABCA…) stacking order. One way to observe experimentally such 2D superconductivity is to measure the frozen magnetic flux produced by a permanent current loop that remains after removing an external magnetic field applied normal to the SFs. Magnetic force microscopy was used to localize and characterize such a permanent current path found in one natural graphite sample out of ∼50 measured graphite samples of different origins. The position of the current path drifts with time and roughly follows a logarithmic time dependence similar to the one for flux creep in type II superconductors. We demonstrate that a ≃10 nm deep scratch on the sample surface at the position of the current path causes a change in its location. A further scratch was enough to irreversibly destroy the remanent state of the sample at room temperature. Our studies clarify some of the reasons for the difficulties of finding a trapped flux in a remanent state at room temperature in graphite samples with SFs.

Nanometers-Thick Ferromagnetic Surface Produced by Laser Cutting of Diamond.

In this work, we demonstrate that cutting diamond crystals with a laser (532 nm wavelength, 0.5 mJ energy, 200 ns pulse duration at 15 kHz) produced a ≲20 nm thick surface layer with magnetic order at room temperature. We measured the magnetic moment of five natural and six CVD diamond crystals of different sizes, nitrogen contents and surface orientations with a SQUID magnetometer. A robust ferromagnetic response at 300 K was observed only for crystals that were cut with the laser along the (100) surface orientation. The magnetic signals were much weaker for the (110) and negligible for the (111) orientations. We attribute the magnetic order to the disordered graphite layer produced by the laser at the diamond surface. The ferromagnetic signal vanished after chemical etching or after moderate temperature annealing. The obtained results indicate that laser treatment of diamond may pave the way to create ferromagnetic spots at its surface.

Publisher Correction: Atomic-scale engineering of ferroelectric-ferromagnetic interfaces of epitaxial perovskite films for functional properties.

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Atomic-scale engineering of ferroelectric-ferromagnetic interfaces of epitaxial perovskite films for functional properties.

Besides epitaxial mismatch that can be accommodated by lattice distortions and/or octahedral rotations, ferroelectric-ferromagnetic interfaces are affected by symmetry mismatch and subsequent magnetic ordering. Here, we have investigated La0.67 Sr0.33 MnO3 (LSMO) samples with varying underlying unit cells (uc) of BaTiO3 (BTO) layer on (001) and (110) oriented substrates in order to elucidate the role of symmetry mismatch. Lattice mismatch for 3 uc of BTO and symmetry mismatch for 10 uc of BTO, both associated with local MnO6 octahedral distortions of the (001) LSMO within the first few uc, are revealed by scanning transmission electron microscopy. Interestingly, we find exchange bias along the in-plane [110]/[100] directions only for the (001) oriented samples. Polarized neutron reflectivity measurements confirm the existence of a layer with zero net moment only within (001) oriented samples. First principle density functional calculations show that even though the bulk ground state of LSMO is ferromagnetic, a large lattice constant together with an excess of La can stabilize an antiferromagnetic LaMnO3-type phase at the interface region and explain the experimentally observed exchange bias. Atomic scale tuning of MnO6 octahedra can thus be made possible via symmetry mismatch at heteroepitaxial interfaces. This aspect can act as a vital parameter for structure-driven control of physical properties.