We have used spin-polarized neutron reflectometry to investigate the magnetization profile of superlattices composed of ferromagnetic Gd and superconducting Nb layers. We have observed a partial suppression of ferromagnetic (F) order of Gd layers in [Gd(dF )/Nb(25nm)]12 superlattices below the superconducting (S) transition of the Nb layers. The amplitude of the suppression decreases with increasing dF . By analyzing the neutron spin asymmetry we conclude that the observed effect has an electromagnetic origin -the proximity-coupled S layers screen out the external magnetic field and thus suppress the F response of the Gd layers inside the structure. Our investigation demonstrates the considerable influence of electromagnetic effects on the magnetic properties of S/F systems.Artificial heterostructures with alternating superconducting (S) and ferromagnetic (F) layers are currently attracting great attention due to a diverse set of proximity effects [1-5], including the Larkin-Ovchinnikov-Fulde-Ferrell phase, π-phase superconductivity and triplet pairing. These effects show how ferromagnetism influences the superconducting properties of the S/F heterostructures. Converse proximity effects in which superconductivity influences ferromagnetism have received less attention. Such magnetic proximity effects are expected in systems where the F and S transition temperatures, T F and T c , are comparable, which is the case for heterostructures of cuprate high-T c superconductors and ferromagnetic manganates [6][7][8][9], and for some bulk compounds [10][11][12]. However, because of the chemical and electronic complexity of these materials, simple model systems for magnetic proximity effects are highly desirable.
Microscopic studies on thin film superconductors play an important role for probing non-equilibrium phase transitions and revealing dynamics at the nanoscale. However, magnetic sensors with nanometer scale spatial and picosecond temporal resolution are essential for exploring these. Here, we present an all-optical, microwave-free method that utilizes the negatively charged nitrogen-vacancy (NV) center in diamond as a non-invasive quantum sensor and enables the spatial detection of the Meissner state in a superconducting thin film. We place an NV implanted diamond membrane on a 20nm thick superconducting La2−xSrxCuO4 (LSCO) thin film with Tc of 34K. The strong B-field dependence of the NV photoluminescence allows us to investigate the Meissner screening in LSCO under an externally applied magnetic field of 4.2mT in a non-resonant manner. The magnetic field profile along the LSCO thin film can be reproduced using Brandt’s analytical model, revealing a critical current density jc of 1.4×108A/cm2. Our work can be potentially extended further with a combination of optical pump probe spectroscopy for the local detection of time-resolved dynamical phenomena in nanomagnetic materials.
We present a methodology based on ex-situ (post-growth) electrochemistry to control the oxygen concentration in thin films of the superconducting oxide La2CuO4+y grown epitaxially on substrates of isostructural LaSrAlO4. The superconducting transition temperature, which depends on the oxygen concentration, can be tuned by adjusting the pH level of the base solution used for the electrochemical reaction. As our main finding, we demonstrate that the dopant oxygens can either occupy the interstitial layer in an orientationally disordered state or organize into a crystalline phase via a mechanism in which dopant oxygens are inserted into the substrate, changing the lattice symmetry of both the substrate and the epitaxial film. We discuss this mechanism, and describe the resulting methodology as a platform to be explored in thin films of other transition metal oxides. arXiv:1907.01079v1 [cond-mat.mtrl-sci] 1 Jul 2019
Quantum defects in diamonds have been studied as a promising resource for quantum science. The subtractive fabrication process for improving photon collection efficiency often require excessive milling time that can adversely affect the fabrication accuracy. We designed and fabricated a Fresnel-type solid immersion lens using the focused ion beam. For a 5.8 µm-deep Nitrogen-vacancy (NV-) center, the milling time was highly reduced (1/3 compared to a hemispherical structure), while retaining high photon collection efficiency (> 2.24 compared to a flat surface). In numerical simulation, this benefit of the proposed structure is expected for a wide range of milling depths.
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