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Optically detected magnetic resonance (ODMR) spectroscopy of defect-rich semiconductors is being increasingly exploited for realising a variety of practical quantum sensing devices. A prime example is the on-going development of compact magnetometers based on the nitrogen-vacancy (NV) defect in diamond for the remote sensing of magnetic signals with high accuracy and sensitivity. In these applications, the ODMR-active material is integrated into the overall apparatus to form a self-contained sensor. However, some emerging applications require the sensing material to be in physical contact with an external object of interest, thus requiring an independent readout device. Here we present an ODMR-meter, a compact device specially designed to allow convenient, contactless monitoring of ODMR in a target object, and demonstrate its application to temperature monitoring with NV defects. Our prototype is composed of a handheld readout head (integrating all the necessary optical components and a microwave antenna) and a control box connected to a laptop computer, all made primarily from commercial off-the-shelf components. We test our device using an NV-rich bulk diamond as the object, demonstrate a temperature sensitivity of $10~{\rm mK}/\sqrt{\rm Hz}$ in static conditions, and demonstrate the feasibility of handheld operation. The limitations to measurement speed, sensitivity and accuracy are discussed. The presented device may find immediate use in medical and industrial applications where accurate thermometry is required, and can be extended to magnetic field measurements.

Widefield quantum microscopy based on nitrogen-vacancy (NV) centres in diamond has emerged as a powerful technique for quantitative mapping of magnetic fields with a sub-micron resolution. However, the accuracy of the technique has not been characterised in detail so far. Here we show that optical aberrations in the imaging system may cause large systematic errors in the measured quantity beyond trivial blurring. We introduce a simple theoretical framework to model these effects, which extends the concept of a point spread function to the domain of spectral imaging. Using this model, the magnetic field imaging of test magnetic samples is simulated under various scenarios, and the resulting errors quantified. We then apply the model to previously published data, show that apparent magnetic anomalies can be explained by the presence of optical aberrations, and demonstrate a post-processing technique to retrieve the source quantity with improved accuracy. This work presents a guide to predict and mitigate aberration induced artefacts in quantitative NV-based widefield imaging and in spectral imaging more generally., Comment: The following article has been submitted to/accepted by AVS Quantum Science. After it is published, it will be found at https://publishing.aip.org/resources/librarians/products/journals/

Interest in van der Waals materials often stems from a desire to miniaturise existing technologies by exploiting their intrinsic layered structure to create near atomically-thin components that do not suffer from surface defects. One appealing property is easily-switchable yet robust magnetic order, a quality only sparsely demonstrated in the case of in-plane anisotropy. In this work, we use widefield nitrogen-vacancy (NV) center magnetic imaging to measure the properties of individual flakes of CuCrP$_2$S$_6$, a multiferroic van der Waals magnet known to exhibit weak easy-plane anisotropy in the bulk. We chart the crossover between in-plane ferromagnetism in thin flakes down to the trilayer, and the bulk behaviour dominated by a low-field spin-flop transition. Further, by exploiting the directional dependence of NV center magnetometry, we are able to observe an instance of a predominantly out-of-plane ferromagetic phase near zero field, in contradiction with expectation and previous experiments on the bulk material. We attribute this to the presence of surface anisotropies arising from the sample preparation process or exposure to the ambient environment, which is expected to have more general implications for a broader class of weakly anisotropic van der Waals magnets., Comment: 24 pages, 20 figures (including supplementary information)

Magnetic imaging with nitrogen-vacancy centers in diamond, also known as quantum diamond microscopy, has emerged as a useful technique for the spatial mapping of charge currents in solid-state devices. In this work, we investigate an application to photovoltaic (PV) devices, where the currents are induced by light. We develop a widefield nitrogen-vacancy microscope that allows independent stimulus and measurement of the PV device, and test our system on a range of prototype crystalline silicon PV devices. We first demonstrate micrometer-scale vector magnetic field imaging of custom PV devices illuminated by a focused laser spot, revealing the internal current paths in both short-circuit and open-circuit conditions. We then demonstrate time-resolved imaging of photocurrents in an interdigitated back-contact solar cell, detecting current build-up and subsequent decay near the illumination point with microsecond resolution. This work presents a versatile and accessible analysis platform that may find distinct application in research on emerging PV technologies., Comment: 14 pages, 9 figures

Quantum microscopes based on solid-state spin quantum sensors have recently emerged as powerful tools for probing material properties and physical processes in regimes not accessible to classical sensors, especially on the nanoscale. Such microscopes have already found utility in a variety of problems, from imaging magnetism and charge transport in nanoscale devices, to mapping remanent magnetic fields from ancient rocks and biological organisms. However, applications of quantum microscopes have so far relied on sensors hosted in a rigid, three-dimensional crystal, typically diamond, which limits their ability to closely interact with the sample under study. Here we demonstrate a versatile and robust quantum microscope using quantum sensors embedded within a thin layer of a van der Waals (vdW) material, hexagonal boron nitride (hBN). To showcase the capabilities of this platform, we assemble several active vdW heterostructures, with an hBN layer acting as the quantum sensor. We demonstrate time-resolved, simultaneous temperature and magnetic imaging near the Curie temperature of a vdW ferromagnet as well as apply this unique microscope to map out charge currents and Joule heating in graphene. By enabling intimate proximity between sensor and sample, potentially down to a single atomic layer, the hBN quantum sensor represents a paradigm shift for nanoscale quantum sensing and microscopy. Moreover, given the ubiquitous use of hBN in modern materials and condensed matter physics research, we expect our technique to find rapid and broad adoption in these fields, further motivated by the prospect of performing in-situ chemical analysis and noise spectroscopy using advanced quantum sensing protocols.

The widefield diamond nitrogen-vacancy (NV) microscope is a powerful instrument for imaging magnetic fields. However, a key limitation impeding its wider adoption is its complex operation, in part due to the difficulty of precisely interfacing the sensor and sample to achieve optimum spatial resolution. Here we demonstrate a solution to this interfacing problem that is both practical and reliably minimizes NV-sample standoff. We built a compact widefield NV microscope which incorporates an integrated widefield diamond probe with full position and angular control, and developed a systematic alignment procedure based on optical interference fringes. Using this platform, we imaged an ultrathin (1 nm) magnetic film test sample, and conducted a detailed study of the spatial resolution. We reproducibly achieved an estimated NV-sample standoff (and hence spatial resolution) of at most $\sim2~\mu$m across a $\sim0.5$ mm field of view. Guided by these results, we suggest future improvements for approaching the optical diffraction limit. This work is a step towards realizing a widefield NV microscope suitable for routine high-throughput mapping of magnetic fields.

A dense layer of nitrogen-vacancy (NV) centers near the surface of a diamond can be interrogated in a widefield optical microscope to produce spatially resolved maps of local quantities such as magnetic field, electric field and lattice strain, providing potentially valuable information about a sample or device placed in proximity. Since the first experimental realization of such a widefield NV microscope in 2010, the technology has seen rapid development and demonstration of applications in various areas across condensed matter physics, geoscience and biology. This Perspective analyzes the strengths and shortcomings of widefield NV microscopy in order to identify the most promising applications and guide future development. We begin with a brief review of quantum sensing with ensembles of NV centers, and the experimental implementation of widefield NV microscopy. We then compare this technology to alternative microscopy techniques commonly employed to probe magnetic materials and charge flow distributions. Current limitations in spatial resolution, measurement accuracy, magnetic sensitivity, operating conditions and ease of use, are discussed. Finally, we identify the technological advances that solve the aforementioned limitations, and argue that their implementation would result in a practical, accessible, high-throughput widefield NV microscope., Comment: 22 pages, 8 figures

Through the combination of transmission electron microscopy analysis of the deformed microstructure and molecular dynamics computer simulations of the deformation processes, the mechanisms of plastic strain recovery in bulk AgCu eutectic with either incoherent twin or cube-on-cube interfaces between the Ag and Cu layers and a bilayer thickness of 500 nm have been revealed. The character of the incoherent twin interfaces changed uniquely after dynamic compressive loading for samples that exhibited plastic strain recovery and was found to drive the recovery, which is due to dislocation retraction and rearrangement of the interfaces. The magnitude of the recovery decreased with increasing strain as dislocation tangles and dislocation cell structures formed. No change in the orientation relationship was found at cube-on-cube interfaces and these exhibited a lesser amount of plastic strain recovery in the simulations and none experimentally in samples with larger layer thicknesses with predominantly cube-on-cube interfaces. Molecular dynamics computer simulations verified the importance of the change in the incoherent twin interface structure as the driving force for dislocation annihilation at the interfaces and the plastic strain recovery.

The production factor, or broad band averaged cross-section, for solar wind charge-exchange with hydrogen producing emission in the ROSAT 1/4 keV (R12) band is $3.8\pm0.2\times10^{-20}$ count degree$^{-2}$ cm$^4$. This value is derived from a comparison of the Long-Term (background) Enhancements in the ROSAT All-Sky Survey with magnetohysdrodynamic simulations of the magnetosheath. This value is 1.8 to 4.5 times higher than values derived from limited atomic data, suggesting that those values may be missing a large number of faint lines. This production factor is important for deriving the exact amount of 1/4 keV band flux that is due to the Local Hot Bubble, for planning future observations in the 1/4 keV band, and for evaluating proposals for remote sensing of the magnetosheath. The same method cannot be applied to the 3/4 keV band as that band, being composed primarily of the oxygen lines, is far more sensitive to the detailed abundances and ionization balance in the solar wind. We also show, incidentally, that recent efforts to correlate XMM-Newton observing geometry with magnetosheath solar wind charge-exchange emission in the oxygen lines have been, quite literally, misguided. Simulations of the inner heliosphere show that broader efforts to correlate heliospheric solar wind charge-exchange with local solar wind parameters are unlikely to produce useful results., Comment: 19 pages, 16 figures, submitted to ApJ

The Solar neighborhood is the closest and most easily studied sample of the Galactic interstellar medium, an understanding of which is essential for models of star formation and galaxy evolution. Observations of an unexpectedly intense diffuse flux of easily-absorbed 1/4 keV X rays, coupled with the discovery that interstellar space within ~100 pc of the Sun is almost completely devoid of cool absorbing gas led to a picture of a "local cavity" filled with X-ray emitting hot gas dubbed the local hot bubble. This model was recently upset by suggestions that the emission could instead be produced readily within the solar system by heavy solar wind ions charge exchanging with neutral H and He in interplanetary space, potentially removing the major piece of evidence for the existence of million-degree gas within the Galactic disk. Here we report results showing that the total solar wind charge exchange contribution is 40% +/- 5% (stat) +/- 5% (sys) of the 1/4 keV flux in the Galactic plane. The fact that the measured flux is not dominated by charge exchange supports the notion of a million-degree hot bubble of order 100 pc extent surrounding the Sun., Comment: 9 pages, 6 figures, 1 table. The final version of the paper is available on Nature

The Diffuse X-rays from the Local galaxy (DXL) mission is an approved sounding rocket project with a first launch scheduled around December 2012. Its goal is to identify and separate the X-ray emission generated by solar wind charge exchange from that of the local hot bubble to improve our understanding of both. With 1,000 cm2 proportional counters and grasp of about 10 cm2 sr both in the 1/4 and 3/4 keV bands, DXL will achieve in a 5-minute flight what cannot be achieved by current and future X-ray satellites., Comment: 15 Pages, 5 figures. Accepted for publication on Experimental Astronomy

Both heliophysics and planetary physics seek to understand the complex nature of the solar wind's interaction with solar system obstacles like Earth's magnetosphere, the ionospheres of Venus and Mars, and comets. Studies with this objective are frequently conducted with the help of single or multipoint in situ electromagnetic field and particle observations, guided by the predictions of both local and global numerical simulations, and placed in context by observations from far and extreme ultraviolet (FUV, EUV), hard X-ray, and energetic neutral atom imagers (ENA). Each proposed interaction mechanism (e.g., steady or transient magnetic reconnection, local or global magnetic reconnection, ion pick-up, or the Kelvin- Helmholtz instability) generates diagnostic plasma density structures. The significance of each mechanism to the overall interaction (as measured in terms of atmospheric/ionospheric loss at comets, Venus, and Mars or global magnetospheric/ionospheric convection at Earth) remains to be determined but can be evaluated on the basis of how often the density signatures that it generates are observed as a function of solar wind conditions. This paper reviews efforts to image the diagnostic plasma density structures in the soft (low energy, 0.1-2.0 keV) X-rays produced when high charge state solar wind ions exchange electrons with the exospheric neutrals surrounding solar system obstacles.