Subcellular-sized, ultraflexible electrodes form seamless integration with the living brain and afford chronically reliable recording.

The detection of ensembles of spins under ambient conditions has revolutionized the biological, chemical and physical sciences through magnetic resonance imaging 1 and nuclear magnetic resonance 2,3 . Pushing sensing capabilities to the individual-spin level would enable unprecedented applications such as single-molecule structural imaging; however, the weak magnetic fields from single spins are undetectable by conventional far-field resonance techniques 4 . In recent years, there has been a considerable effort to develop nanoscale scanning magnetometers 5-8 , which are able to measure fewer spins by bringing the sensor in close proximity to its target. The most sensitive of these magnetometers generally require low temperatures for operation, but the ability to measure under ambient conditions (standard temperature and pressure) is critical for many imaging applications, particularly in biological systems. Here we demonstrate detection and nanoscale imaging of the magnetic field from an initialized single electron spin under ambient conditions using a scanning nitrogen-vacancy magnetometer. Real-space, quantitative magnetic-field images are obtained by deterministically scanning our nitrogen-vacancy magnetometer 50 nm above a target electron spin, while measuring the local magnetic field using dynamically decoupled magnetometry protocols. We discuss how this single-spin detection enables the study of a variety of room-temperature phenomena in condensed-matter physics with an unprecedented combination of spatial resolution and spin sensitivity.So far, the magnetic fields from single electron spins have been imaged only under extreme conditions (ultralow temperatures and high vacuum) 9 . Magnetometers based on negatively charged nitrogen-vacancy (NV) centres in diamond have been proposed as sensors capable of measuring individual spins 10-13 because they can be initialized and read-out optically 14 and have long coherence times 15 , even under ambient conditions. Moreover, because NV centres are atomic in size, they offer significant advantages in magnetic resolution and sensing capabilities if they can be brought in close proximity of targets to be measured. Recent advances in diamond nanofabrication have allowed for the creation of robust scanning probes that host individual NV centres within roughly 25 nm of their tips 16 . Here, we employ such a scanning NV centre to image the magnetic dipole field of a single target electron spin.Our scanning NV magnetometer (Fig. 1a) consists of a combined confocal and atomic force microscope as previously described 17 , which hosts a sensing NV centre embedded in a diamond nanopillar scanning probe tip 16 . The sensor NV's spin state is initialized optically and read out through spin-dependent fluorescence, and its position relative to the sample is controlled through atomic-force feedback between the tip and sample. Microwaves are used to coherently manipulate the sensor NV spin. Magnetic sensing is achieved by measuring the NV spin's optically detected electron spin reso...

. Although vortex matter has been studied extensively 1,6,7 , the static and dynamic properties of an individual vortex have not. Here, we use magnetic force microscopy (MFM) to image and manipulate individual vortices in a detwinned YBa 2 Cu 3 O 6.991 single crystal, directly measuring the interaction of a moving vortex with the local disorder potential. We find an unexpected and marked enhancement of the response of a vortex to pulling when we wiggle it transversely. In addition, we find enhanced vortex pinning anisotropy that suggests clustering of oxygen vacancies in our sample and demonstrates the power of MFM to probe vortex structure and microscopic defects that cause pinning.

Abstract

We use magnetic force microscopy ͑MFM͒ to measure the local penetration depth in Ba͑Fe 0.95 Co 0.05 ͒ 2 As 2 single crystals and use scanning superconducting quantum interference device susceptometry to measure its temperature variation down to 0.4 K. We observe that superfluid density s over the full temperature range is well described by a clean two-band fully gapped model. We demonstrate that MFM can measure the important and hard-to-determine absolute value of , as well as obtain its temperature dependence and spatial homogeneity. We find s to be uniform on the submicron scale despite the highly disordered vortex pinning. The magnetic penetration depth , one of the two fundamental length scales in superconductors, 1 characterizes many fundamental properties. It evaluates the phase stiffness of the superconducting state by the temperature T max ϰ 1 / 2 at which phase order would disappear. 2 It also determines the superfluid density s =1/ 2 , the number of electrons in the superconducting phase. However, its absolute value is notoriously difficult to measure, especially in samples that may have either intrinsic or extrinsic inhomogeneity. In this Rapid Communication, we will report a technique to measure by magnetic force microscopy ͑MFM͒. The advantage of using local probes over bulk techniques is that they allow us to study the sample homogeneity. We implement this technique to determine s in a iron-pnictide superconductor Ba͑Fe 0.95 Co 0.05 ͒ 2 As 2 .Iron-pnictide superconductors have been under extensive study since their recent discovery.3 The high transition temperature, 4 the proximity to a magnetic state, 5-7 and the existence of multiple conducting bands 8,9 combine to make it difficult and interesting to resolve key issues such as the superconducting order-parameter ͑OP͒ symmetry, 10,11 the pairing mechanism 12 and the role of impurities and inhomogeneity.13 Those problems can be studied by measuring s . When the gap has nodes, s ͑T͒ varies as a power law in T at low T, as demonstrated in YB 2 Cu 3 O 7−␦ , 14,15 while a fully gapped OP gives a low-T exponential dependence. 16 Since it is difficult to determine , its temperature variation ⌬͑T͒ϵ͑T͒ − ͑0͒ is often measured, which follows the same temperature dependence as s at low T. Sometimes this approach is sufficient, e.g., linear ⌬ in clean LaFePO over a wide temperature range provides strong evidence of wellformed line nodes. 17,18 However, in the Ba-122 family, a steep power law ⌬ was obtained in the Co-doped compounds 19 while an exponential s was measured in the K-doped materials. 20 The question waiting for clarification is whether different dopants lead to different OP structure. ⌬ measurement cannot infer OP symmetry except for T Ӷ T c , but for multiband pnictides, the low-T regime may be dominated by the small-gap regions of the Fermi surface and may be altered by interband impurity scattering. 21 It is thus important to measure the absolute value of to determine s over the full temperature range.In this Rapid Communication, we measure th...