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arXiv:1907.12601 · Bulletin of the American Physical Society

Imaging Nematic Transitions in Iron-Pnictide Superconductors with a Quantum Gas

Fan Yang, Stephen Taylor, Stephen D. Edkins, Johanna C. Palmstrom, I. R. Fisher, Benjamin Lev

Published Jul 29, 2019• 1 citation
arXivHTMLPDF

Abstract

The SQCRAMscope is a recently realized Scanning Quantum CRyogenic Atom Microscope that utilizes an atomic Bose-Einstein condensate to measure magnetic fields emanating from solid-state samples. The quantum sensor does so with unprecedented DC sensitivity at micron resolution from room-to-cryogenic temperatures. An additional advantage of the SQCRAMscope is the preservation of optical access to the sample: Magnetometry imaging of, e.g., electron transport may be performed in concert with other imaging techniques. This multimodal imaging capability can be brought to bear with great effect in the study of nematicity in iron-pnictide high-temperature superconductors, where the relationship between electronic and structural symmetry-breaking resulting in a nematic phase is under debate. Here, we combine the SQCRAMscope with an in situ microscope that measures optical birefringence near the surface. This enables simultaneous and spatially resolved detection of both bulk and near-surface manifestations of nematicity via transport and structural deformation channels, respectively. By performing the first local measurement of emergent resistivity anisotropy in iron pnictides, we observe sharp, nearly concurrent transport and structural transitions. More broadly, these measurements demonstrate the SQCRAMscope's ability to reveal important insights into the physics of complex quantum materials.

DOI
10.48550/arxiv.1907.12601

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The SQCRAMscope is a recently realized Scanning Quantum CRyogenic Atom Microscope that utilizes an atomic Bose-Einstein condensate to measure magnetic fields emanating from solid-state samples. The quantum sensor does so with unprecedented DC sensitivity at micron resolution from room-to-cryogenic temperatures. An additional advantage of the SQCRAMscope is the preservation of optical access to the sample: Magnetometry imaging of, e.g., electron transport may be performed in concert with other imaging techniques. This multimodal imaging capability can be brought to bear with great effect in the study of nematicity in iron-pnictide high-temperature superconductors, where the relationship between electronic and structural symmetry-breaking resulting in a nematic phase is under debate. Here, we combine the SQCRAMscope with an in situ microscope that measures optical birefringence near the surface. This enables simultaneous and spatially resolved detection of both bulk and near-surface manifestations of nematicity via transport and structural deformation channels, respectively. By performing the first local measurement of emergent resistivity anisotropy in iron pnictides, we observe sharp, nearly concurrent transport and structural transitions. More broadly, these measurements demonstrate the SQCRAMscope's ability to reveal important insights into the physics of complex quantum materials.
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The Scanning Quantum Cryogenic Atom Microscope (SQCRAMscope) is a quantum sensor in which a quasi-1D quantum gas images electromagnetic fields emitted from a nearby sample. We report improvements to the microscope. Cryogen usage is reduced by replacing the liquid cryostat with a closed-cycle system and modified cold finger, and cryogenic cooling is enhanced by adding a radiation shield. The minimum accessible sample temperature is reduced from 35~K to 5.7~K while maintaining low sample vibrations. A new sample mount is easier to exchange, and quantum gas preparation is streamlined.
Magnetotransport underlines many important phenomena in condensed matter physics, such as the Hall effect and magnetoresistance (MR) effect. Thus far, most magnetotransport studies are based on bulk resistance measurements without direct access to microscopic details of the spatial transport pattern. Here, we report nanoscale imaging of magnetotransport using a scanning single-spin quantum magnetometer, which is demonstrated in a graphene-metal hybrid device at room temperature. By visualizing the current flow at elevated magnetic fields (~0.5 T), we directly observe the Lorentz deflection of current near the graphene-metal interface, which is a hallmark of magnetotransport. Combining the local current distribution with global resistance measurements, we reveal that transport properties of the hybrid are governed by a complex interplay of intrinsic MR around the Dirac cone, carrier hydrodynamics, interface resistance, and the nanoscale device geometry. Furthermore, accessing the local transport pattern across the interface enables quantitative mapping of spatial variations in contact resistance, which is commonly present in electronic devices made from two-dimensional materials yet non-trivial to characterize. Our work demonstrates the potential of nanoscale current imaging techniques for studying complex electronic transport phenomena that are difficult to probe by resistance-based measurements.
People have been measuring magnetic fields since 200 BC, giving rise to the field of magnetometry. Recently there has been a drive to measure smaller fields with better resolution, resulting in the production of many different types of magnetometers, each with their own unique features. These magnetometers can have a diverse range of applications such as, GNSS free navigation, biomedical sensing, space and weather measurements, and material physics. When designing a magnetomoter there is a trade off between high sensitivity to magnetic fields and high spatial resolution. Currently, there is a unoccupied niche in direct current (DC) magnetometry, that requires a magnetic field sensitivity in the picotesla range and micron scale spatial resolution. Here, I introduce the Bose-Einstein condensate (BEC), which is a macroscopic quantum object, ideal for probing magnetic fields on the nanotesla to picotesla range, with micron scale resolution. This thesis discusses the use of a BEC microscope (BEC-M) to characterise mate- rials with novel applications in nanotechnology by producing 2D current density maps. To produce the atomic probe for microscopy, a BEC in the quasi-1D regime is carefully prepared, requiring an ultra high vacuum (UHV) environment along with custom designed trapping structures, to optically and magnetically confine the atoms. The sensitivity and resolution of the BEC-M makes it an ideal tool for characterising electrical properties of structures on the micron scale such as carbon nanotube (CNT) bundles and biological samples. Current network configuration in samples such as these, can be mapped via a raster scanning procedure. A measurement was made to identify current pathways in CNTs by measuring the atomic distribution of the ultra cold cloud and converting these images to magnetic field and current density maps.